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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 6189

Network Working Group                                      P. Zimmermann
Internet-Draft                                             Zfone Project
Intended status: Informational                          A. Johnston, Ed.
Expires: May 30, 2009                                              Avaya
                                                               J. Callas
                                                         PGP Corporation
                                                       November 26, 2008


             ZRTP: Media Path Key Agreement for Secure RTP
                      draft-zimmermann-avt-zrtp-11

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on May 30, 2009.

Abstract

   This document defines ZRTP, a protocol for media path Diffie-Hellman
   exchange to agree on a session key and parameters for establishing
   Secure Real-time Transport Protocol (SRTP) sessions.  The ZRTP
   protocol is media path keying because it is multiplexed on the same
   port as RTP and does not require support in the signaling protocol.
   ZRTP does not assume a Public Key Infrastructure (PKI) or require the
   complexity of certificates in end devices.  For the media session,
   ZRTP provides confidentiality, protection against man-in-the-middle
   (MiTM) attacks, and, in cases where the signaling protocol provides



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   end-to-end integrity protection, authentication.  ZRTP can utilize a
   Session Description Protocol (SDP) attribute to provide discovery and
   authentication through the signaling channel.  To provide best effort
   SRTP, ZRTP utilizes normal RTP/AVP profiles.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Key Agreement Modes  . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  Diffie-Hellman Mode Overview . . . . . . . . . . . . .  7
       3.1.2.  Multistream Mode Overview  . . . . . . . . . . . . . .  9
       3.1.3.  Preshared Mode Overview  . . . . . . . . . . . . . . .  9
   4.  Protocol Description . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.1.1.  Protocol Version Negotiation . . . . . . . . . . . . . 11
     4.2.  Commit Contention  . . . . . . . . . . . . . . . . . . . . 13
     4.3.  Matching Shared Secret Determination . . . . . . . . . . . 13
       4.3.1.  Responder Behavior . . . . . . . . . . . . . . . . . . 15
       4.3.2.  Initiator Behavior . . . . . . . . . . . . . . . . . . 16
       4.3.3.  Handling a Shared Secret Cache Mismatch  . . . . . . . 16
     4.4.  DH and non-DH key agreements . . . . . . . . . . . . . . . 18
       4.4.1.  Diffie-Hellman Mode  . . . . . . . . . . . . . . . . . 18
         4.4.1.1.  Hash Commitment in Diffie-Hellman Mode . . . . . . 18
         4.4.1.2.  Responder Behavior in Diffie-Hellman Mode  . . . . 19
         4.4.1.3.  Initiator Behavior in Diffie-Hellman Mode  . . . . 20
         4.4.1.4.  Shared Secret Calculation for DH Mode  . . . . . . 20
       4.4.2.  Multistream Mode . . . . . . . . . . . . . . . . . . . 22
         4.4.2.1.  Commitment in Multistream Mode . . . . . . . . . . 22
         4.4.2.2.  Shared Secret Calculation for Multistream Mode . . 23
       4.4.3.  Preshared Mode . . . . . . . . . . . . . . . . . . . . 24
         4.4.3.1.  Commitment in Preshared Mode . . . . . . . . . . . 25
         4.4.3.2.  Initiator Behavior in Preshared Mode . . . . . . . 25
         4.4.3.3.  Responder Behavior in Preshared Mode . . . . . . . 26
         4.4.3.4.  Shared Secret Calculation for Preshared Mode . . . 26
     4.5.  Key Derivations  . . . . . . . . . . . . . . . . . . . . . 27
       4.5.1.  The ZRTP Key Derivation Function . . . . . . . . . . . 27
       4.5.2.  Deriving keys via the ZRTP KDF . . . . . . . . . . . . 28
     4.6.  Confirmation . . . . . . . . . . . . . . . . . . . . . . . 29
       4.6.1.  Updating the Cache of Shared Secrets . . . . . . . . . 30
     4.7.  Termination  . . . . . . . . . . . . . . . . . . . . . . . 31
       4.7.1.  Termination via Error message  . . . . . . . . . . . . 31
       4.7.2.  Termination via GoClear message  . . . . . . . . . . . 31
         4.7.2.1.  Key Destruction for GoClear message  . . . . . . . 33
       4.7.3.  Key Destruction at Termination . . . . . . . . . . . . 33
     4.8.  Random Number Generation . . . . . . . . . . . . . . . . . 34



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     4.9.  ZID and Cache Operation  . . . . . . . . . . . . . . . . . 34
       4.9.1.  Cacheless implementations  . . . . . . . . . . . . . . 35
   5.  ZRTP Messages  . . . . . . . . . . . . . . . . . . . . . . . . 36
     5.1.  ZRTP Message Formats . . . . . . . . . . . . . . . . . . . 37
       5.1.1.  Message Type Block . . . . . . . . . . . . . . . . . . 37
       5.1.2.  Hash Type Block  . . . . . . . . . . . . . . . . . . . 38
         5.1.2.1.  Implicit Hash and HMAC algorithm . . . . . . . . . 39
       5.1.3.  Cipher Type Block  . . . . . . . . . . . . . . . . . . 39
       5.1.4.  Auth Tag Block . . . . . . . . . . . . . . . . . . . . 40
       5.1.5.  Key Agreement Type Block . . . . . . . . . . . . . . . 40
       5.1.6.  SAS Type Block . . . . . . . . . . . . . . . . . . . . 42
       5.1.7.  Signature Type Block . . . . . . . . . . . . . . . . . 43
     5.2.  Hello message  . . . . . . . . . . . . . . . . . . . . . . 43
     5.3.  HelloACK message . . . . . . . . . . . . . . . . . . . . . 45
     5.4.  Commit message . . . . . . . . . . . . . . . . . . . . . . 45
     5.5.  DHPart1 message  . . . . . . . . . . . . . . . . . . . . . 48
     5.6.  DHPart2 message  . . . . . . . . . . . . . . . . . . . . . 50
     5.7.  Confirm1 and Confirm2 messages . . . . . . . . . . . . . . 52
     5.8.  Conf2ACK message . . . . . . . . . . . . . . . . . . . . . 53
     5.9.  Error message  . . . . . . . . . . . . . . . . . . . . . . 54
     5.10. ErrorACK message . . . . . . . . . . . . . . . . . . . . . 55
     5.11. GoClear message  . . . . . . . . . . . . . . . . . . . . . 56
     5.12. ClearACK message . . . . . . . . . . . . . . . . . . . . . 56
     5.13. SASrelay message . . . . . . . . . . . . . . . . . . . . . 57
     5.14. RelayACK message . . . . . . . . . . . . . . . . . . . . . 59
   6.  Retransmissions  . . . . . . . . . . . . . . . . . . . . . . . 60
   7.  Short Authentication String  . . . . . . . . . . . . . . . . . 61
     7.1.  SAS Verified Flag  . . . . . . . . . . . . . . . . . . . . 62
     7.2.  Signing the SAS  . . . . . . . . . . . . . . . . . . . . . 64
     7.3.  Relaying the SAS through a PBX . . . . . . . . . . . . . . 64
       7.3.1.  PBX Enrollment and the PBX Enrollment Flag . . . . . . 66
   8.  Signaling Interactions . . . . . . . . . . . . . . . . . . . . 67
     8.1.  Binding the media stream to the signaling layer via
           the Hello Hash . . . . . . . . . . . . . . . . . . . . . . 68
       8.1.1.  Integrity-protected signaling enables
               integrity-protected DH exchange  . . . . . . . . . . . 70
     8.2.  Deriving the SRTP secret (srtps) from the signaling
           layer  . . . . . . . . . . . . . . . . . . . . . . . . . . 71
     8.3.  Codec Selection for Secure Media . . . . . . . . . . . . . 72
   9.  False ZRTP Packet Rejection  . . . . . . . . . . . . . . . . . 72
   10. Intermediary ZRTP Devices  . . . . . . . . . . . . . . . . . . 74
   11. The ZRTP Disclosure flag . . . . . . . . . . . . . . . . . . . 76
     11.1. Guidelines on Proper Implementation of the Disclosure
           Flag . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
   12. RTP Header Extension Flag for ZRTP . . . . . . . . . . . . . . 78
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 79
   14. Appendix - Media Security Requirements . . . . . . . . . . . . 79
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 81



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     15.1. Self-healing Key Continuity Feature  . . . . . . . . . . . 84
   16. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 85
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 86
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 86
     17.2. Informative References . . . . . . . . . . . . . . . . . . 87
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 90
   Intellectual Property and Copyright Statements . . . . . . . . . . 91












































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1.  Introduction

   ZRTP is a key agreement protocol which performs Diffie-Hellman key
   exchange during call setup in the media path, and is transported over
   the same port as the Real-time Transport Protocol (RTP) [RFC3550]
   media stream which has been established using a signaling protocol
   such as Session Initiation Protocol (SIP) [RFC3261].  This generates
   a shared secret which is then used to generate keys and salt for a
   Secure RTP (SRTP) [RFC3711] session.  ZRTP borrows ideas from PGPfone
   [pgpfone].  A reference implementation of ZRTP is available as Zfone
   [zfone].

   The ZRTP protocol has some nice cryptographic features lacking in
   many other approaches to media session encryption.  Although it uses
   a public key algorithm, it does not rely on a public key
   infrastructure (PKI).  In fact, it does not use persistent public
   keys at all.  It uses ephemeral Diffie-Hellman (DH) with hash
   commitment, and allows the detection of man-in-the-middle (MiTM)
   attacks by displaying a short authentication string (SAS) for the
   users to read and verbally compare over the phone.  It has Perfect
   Forward Secrecy, meaning the keys are destroyed at the end of the
   call, which precludes retroactively compromising the call by future
   disclosures of key material.  But even if the users are too lazy to
   bother with short authentication strings, we still get reasonable
   authentication against a MiTM attack, based on a form of key
   continuity.  It does this by caching some key material to use in the
   next call, to be mixed in with the next call's DH shared secret,
   giving it key continuity properties analogous to SSH.  All this is
   done without reliance on a PKI, key certification, trust models,
   certificate authorities, or key management complexity that bedevils
   the email encryption world.  It also does not rely on SIP signaling
   for the key management, and in fact does not rely on any servers at
   all.  It performs its key agreements and key management in a purely
   peer-to-peer manner over the RTP packet stream.

   In cases where the short authentication string (SAS) cannot be
   verbally compared by two human users, the SAS can be authenticated by
   exchanging an optional signature over the SAS (described in
   Section 7.2).

   ZRTP can be used and discovered without being declared or indicated
   in the signaling path.  This provides a best effort SRTP capability.
   Also, this reduces the complexity of implementations and minimizes
   interdependency between the signaling and media layers.  However,
   when ZRTP is indicated in the signaling via the zrtp-hash SDP
   attribute, ZRTP has additional useful properties.  By sending a hash
   of the ZRTP Hello message in the signaling, ZRTP provides a useful
   binding between the signaling and media paths, which is explained in



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   Section 8.1.  When this is done through a signaling path that has
   end-to-end integrity protection, the DH exchange is automatically
   protected from a MiTM attack, which is explained in Section 8.1.1.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 and
   indicate requirement levels for compliant implementations [RFC2119].


3.  Overview

   This section provides a description of how ZRTP works.  This
   description is non-normative in nature but is included to build
   understanding of the protocol.

   ZRTP is negotiated the same way a conventional RTP session is
   negotiated in an offer/answer exchange using the standard AVP/RTP
   profile.  The ZRTP protocol begins after two endpoints have utilized
   a signaling protocol such as SIP and are ready to exchange media.  If
   ICE [I-D.ietf-mmusic-ice] is being used, ZRTP begins after ICE has
   completed its connectivity checks.

   ZRTP is multiplexed on the same ports as RTP.  It uses a unique
   header that makes it clearly differentiable from RTP or STUN.

   In environments in which sending ZRTP packets to non-ZRTP endpoints
   might cause problems and signaling path discovery is not an option,
   ZRTP endpoints can include the RTP header extension flag for ZRTP in
   normal RTP packets sent at the start of a session as a probe to
   discover if the other endpoint supports ZRTP.  If the flag is
   received from the other endpoint, ZRTP messages can then be
   exchanged.

   A ZRTP endpoint initiates the exchange by sending a ZRTP Hello
   message to the other endpoint.  The purpose of the Hello message is
   to confirm the endpoint supports the protocol and to see what
   algorithms the two ZRTP endpoints have in common.

   The Hello message contains the SRTP configuration options, and the
   ZID.  Each instance of ZRTP has a unique 96-bit random ZRTP ID or ZID
   that is generated once at installation time.  ZIDs are discovered
   during the Hello message exchange.  The received ZID is used to look
   up retained shared secrets from previous ZRTP sessions with the
   endpoint.



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   A response to a ZRTP Hello message is a ZRTP HelloACK message.  The
   HelloACK message simply acknowledges receipt of the Hello.  Since RTP
   commonly uses best effort UDP transport, ZRTP has retransmission
   timers in case of lost datagrams.  There are two timers, both with
   exponential backoff mechanisms.  One timer is used for
   retransmissions of Hello messages and the other is used for
   retransmissions of all other messages after receipt of a HelloACK.

   If an integrity protected signaling channel is available, a hash of
   the Hello message can be sent.  This allows rejection of false
   injected ZRTP Hello messages by an attacker.

   Hello and other ZRTP messages also contain a hash image that is used
   to link the messages together.  This allows rejection of false
   injected ZRTP messages during an exchange.

3.1.  Key Agreement Modes

   After both endpoints exchange Hello and HelloACK messages, the key
   agreement exchange can begin with the ZRTP Commit message.  ZRTP
   supports a number of key agreement modes including both Diffie-
   Hellman and non-Diffie-Hellman modes as described in the following
   sections.

   The Commit message may be sent immediately after both endpoints have
   completed the Hello/HelloAck discovery handshake.  Or it may be
   deferred until later in the call, after the participants engage in
   some unencrypted conversation.  The Commit message may be manually
   activated by a user interface element, such as a GO SECURE button,
   which becomes enabled after the Hello/HelloAck discovery phase.  This
   emulates the user experience of a number of secure phones in the PSTN
   world [comsec].  However, it is expected that most simple ZRTP user
   agents will omit such buttons and proceed directly to secure mode by
   sending a Commit message immediately after the Hello/HelloAck
   handshake.

3.1.1.  Diffie-Hellman Mode Overview

   An example ZRTP call flow is shown in Figure 1 below.  Note that the
   order of the Hello/HelloACK exchanges in F1/F2 and F3/F4 may be
   reversed.  That is, either Alice or Bob might send the first Hello
   message.  Note that the endpoint which sends the Commit message is
   considered the initiator of the ZRTP session and drives the key
   agreement exchange.  The Diffie-Hellman public values are exchanged
   in the DHPart1 and DHPart2 messages.  SRTP keys and salts are then
   calculated.





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   Alice                                                Bob
     |                                                   |
     |      Alice and Bob establish a media session.     |
     |         They initiate ZRTP on media ports         |
     |                                                   |
     | F1 Hello (version, options, Alice's ZID)          |
     |-------------------------------------------------->|
     |                                       HelloACK F2 |
     |<--------------------------------------------------|
     |            Hello (version, options, Bob's ZID) F3 |
     |<--------------------------------------------------|
     | F4 HelloACK                                       |
     |-------------------------------------------------->|
     |                                                   |
     |             Bob acts as the initiator             |
     |                                                   |
     |               Commit (Bob's ZID, options, hvi) F5 |
     |<--------------------------------------------------|
     | F6 DHPart1 (pvr, shared secret hashes)            |
     |-------------------------------------------------->|
     |            DHPart2 (pvi, shared secret hashes) F7 |
     |<--------------------------------------------------|
     |                                                   |
     |     Alice and Bob generate SRTP session key.      |
     |                                                   |
     | F8 Confirm1 (HMAC, D,A,V,E flags, sig)            |
     |-------------------------------------------------->|
     |            Confirm2 (HMAC, D,A,V,E flags, sig) F9 |
     |<--------------------------------------------------|
     | F10 Conf2ACK                                      |
     |-------------------------------------------------->|
     |                    SRTP begins                    |
     |<=================================================>|
     |                                                   |


           Figure 1: Establishment of an SRTP session using ZRTP

   ZRTP authentication uses a Short Authentication String (SAS) which is
   ideally displayed for the human user.  Alternatively, the SAS can be
   authenticated by exchanging an OPTIONAL digital signature (sig) over
   the short authentication string in the Confirm1 or Confirm2 messages
   (described in Section 7.2).

   The ZRTP Confirm1 and Confirm2 messages are sent for a number of
   reasons, not the least of which is they confirm that all the key
   agreement calculations were successful and thus the encryption will
   work.  They also carry other information such as the Disclosure flag



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   (D), the Allow Clear flag (A), the SAS Verified flag (V), and the PBX
   Enrollment flag (E).  All flags are encrypted to shield them from a
   passive observer.

3.1.2.  Multistream Mode Overview

   Multistream mode is an alternative key agreement method when two
   endpoints have an established SRTP media stream between them and
   hence an active ZRTP Session key.  ZRTP can derive multiple SRTP keys
   from a single DH exchange.  For example, an established secure voice
   call that adds a video stream must use Multistream mode to quickly
   initiate the video stream without a second DH exchange.

   When Multistream mode is indicated in the Commit message, a call flow
   similar to Figure 1 is used, but no DH calculation is performed by
   either endpoint and the DHPart1 and DHPart2 messages are omitted.
   The Confirm1, Confirm2, and Conf2ACK messages are still sent.  Since
   the cache is not affected during this mode, multiple Multistream ZRTP
   exchanges can be performed in parallel between two endpoints.

   When adding additional media streams to an existing call, only
   Multistream mode is used.  Only one DH operation is performed, just
   for the first media stream.

3.1.3.  Preshared Mode Overview

   In the Preshared Mode, endpoints can skip the DH calculation if they
   have a shared secret from a previous ZRTP session.  Preshared mode is
   indicated in the Commit message and results in the same call flow as
   Multistream mode.  The principal difference between Multistream mode
   and Preshared mode is that Preshared mode uses a previously cached
   shared secret, rs1, instead of an active ZRTP Session key as the
   initial keying material.

   This mode could be useful for slow processor endpoints so that a DH
   calculation does not need to be performed every session.  Or, this
   mode could be used to rapidly re-establish an earlier session that
   was recently torn down or interrupted without the need to perform
   another DH calculation.

   Preshared mode has forward secrecy properties.  If a phone's cache is
   captured by an opponent, the cached shared secrets cannot be used to
   recover earlier encrypted calls, because the shared secrets are
   replaced with new ones in each new call, as in DH mode.  However, the
   captured secrets can be used by a passive wiretapper in the media
   path to decrypt the next call, if the next call is in Preshared mode.
   This differs from DH mode, which requires an active MiTM wiretapper
   to exploit captured secrets in the next call.  However, if the next



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   call is missed by the wiretapper, he cannot wiretap any further
   calls.  It thus preserves most of the self-healing properties
   (Section 15.1) of key continuity enjoyed by DH mode.


4.  Protocol Description

   This section begins the normative description of the protocol.

   ZRTP MUST be multiplexed on the same ports as the RTP media packets.

   To support best effort encryption from the Media Security
   Requirements [I-D.ietf-sip-media-security-requirements], ZRTP uses
   normal RTP/AVP profile (AVP) media lines in the initial offer/answer
   exchange.  The ZRTP SDP attribute a=zrtp-hash defined in Section 8
   SHOULD be used in all offers and answers to indicate support for the
   ZRTP protocol.  The Secure RTP/AVP (SAVP) profile MAY be used in
   subsequent offer/answer exchanges after a successful ZRTP exchange
   has resulted in an SRTP session, or if it is known the other endpoint
   supports this profile.

      The use of the RTP/SAVP profile has caused failures in negotiating
      best effort SRTP due to the limitations on negotiating profiles
      using SDP.  This is why ZRTP supports the RTP/AVP profile and
      includes its own discovery mechanisms.

   In all key agreement modes, the initiator SHOULD NOT send RTP media
   after sending the Commit message, and MUST NOT send SRTP media before
   receiving either the Conf2ACK or the first SRTP media (with a valid
   SRTP auth tag) from the responder.  The responder SHOULD NOT send RTP
   media after receiving the Commit message, and MUST NOT send SRTP
   media before receiving the Confirm2 message.

4.1.  Discovery

   During the ZRTP discovery phase, a ZRTP endpoint discovers if the
   other endpoint supports ZRTP and the supported algorithms and
   options.  This information is transported in a Hello message,
   described in Section 5.2.

   ZRTP endpoints SHOULD include the SDP attribute a=zrtp-hash in offers
   and answers, as defined in Section 8.  ZRTP MAY use an RTP [RFC3550]
   extension field as a flag to indicate support for the ZRTP protocol
   in RTP packets as described in Section 12.

   The Hello message includes the ZRTP version, hash type, cipher type,
   authentication method and tag length, key agreement type, and Short
   Authentication String (SAS) algorithms that are supported.  The Hello



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   message also includes a hash image as described in Section 9.  In
   addition, each endpoint sends and discovers ZIDs.  The received ZID
   is used later in the protocol as an index into a cache of shared
   secrets that were previously negotiated and retained between the two
   parties.

   A Hello message can be sent at any time, but is usually sent at the
   start of an RTP session to determine if the other endpoint supports
   ZRTP, and also if the SRTP implementations are compatible.  A Hello
   message is retransmitted using timer T1 and an exponential backoff
   mechanism detailed in Section 6 until the receipt of a HelloACK
   message or a Commit message.

   The use of the a=zrtp-hash SDP attribute to authenticate the Hello
   message is described in Section 8.1.

4.1.1.  Protocol Version Negotiation

   This specification defines ZRTP version 1.00.  Since new versions of
   ZRTP may be developed in the future, this specification defines a
   protocol version negotiation in this section.

   Each party declares what version of the ZRTP protocol they support
   via the version field in the Hello message (Section 5.2).  If both
   parties have the same version number in their Hello messages, they
   can proceed with the rest of the protocol.  To facilitate both
   parties reaching this state of protocol version agreement in their
   Hello messages, ZRTP should use information provided in the signaling
   layer, if available.  If a ZRTP endpoint supports more than one
   version of the protocol, it SHOULD declare them all in a list of SIP
   SDP a=zrtp-hash attributes (defined in Section 8), listing separate
   hashes, with separate ZRTP version numbers in each item in the list.

   Both parties should inspect the list of ZRTP version numbers supplied
   by the other party in the SIP SDP a=zrtp-hash attributes.  Both
   parties should choose the highest version number that appear in both
   parties' list of a=zrtp-hash version numbers, and use that version
   for their Hello messages.  If both parties use the SIP signaling in
   this manner, their initial Hello messages will have the same ZRTP
   version number, provided they both have at least one supported
   protocol version in common.  Before the ZRTP key agreement can
   proceed, an endpoint MUST have sent and received Hellos with the same
   protocol version.

   It is best if the signaling layer is used to negotiate the protocol
   version number.  However, the a=zrtp-hash SDP attribute is not always
   present in the SIP packet, as explained in Section 8.1.  In the
   absence of any guidance from the signaling layer, an endpoint MUST



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   send the highest supported version in initial Hello messages.  If the
   two parties send different protocol version numbers in their Hello
   messages, they can reach agreement to use a common version, if one
   exists.  They iteratively apply the following rules until they both
   have matching version fields in their Hello messages and the key
   agreement can proceed:

   o  If an endpoint receives a Hello message with an unsupported
      version number that is higher than the endpoint's current Hello
      message version, the received Hello message MUST be ignored.  The
      endpoint continues to retransmit Hello messages on the standard
      retry schedule (Section 6).
   o  If an endpoint receives a Hello message with a version number that
      is lower than the endpoint's current Hello message, and the
      endpoint supports a version that is less than or equal to the
      received version number, the endpoint MUST stop retransmitting the
      old version number and MUST start sending a new Hello message with
      the highest supported version number that is less than or equal to
      the received version number.
   o  If an endpoint receives a Hello message with an unsupported
      version number that is lower than the endpoint's current Hello
      message, the endpoint MUST send an Error message (Section 5.9)
      indicating failure to support this ZRTP version.

   The above comparisons are iterated until the version numbers match,
   or until it exits on a failure to match.

      For example, assume that Alice supports protocol version 1.00 and
      2.00, and Bob supports version 1.00 and 1.10.  Alice initially
      sends a Hello with version 2.00, and Bob initially sends a Hello
      with version 1.10.  Bob ignores Alice's 2.00 Hello and continues
      to send his 1.10 Hello.  Alice detects that Bob does not support
      2.00 and she stops sending her 2.00 Hellos and starts sending a
      stream of 1.00 Hellos.  Bob sees the 1.00 Hello from Alice and
      stops sending his 1.10 Hellos and switches to sending 1.00 Hellos.
      At that point, they have converged on using version 1.00 and the
      protocol proceeds on that basis.

   When comparing protocol versions, a ZRTP endpoint MUST include only
   the first three octets of the version field in the comparison.  The
   final octet is ignored, because it is not significant for
   interoperability.  For example, "1.0 ", "1.00", "1.01", or "1.0a" are
   all regarded as a version match, because they would all be
   interoperable versions.

   Changes in protocol version numbers are expected be infrequent after
   version 1.00.  Supporting multiple versions adds code complexity and
   may introduce security weaknesses in the implementation.  The old



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   adage about keeping it simple applies especially to implementing
   security protocols.  Endpoints SHOULD NOT support protocol versions
   earlier than version 1.00.

4.2.  Commit Contention

   After both parties have received compatible Hello messages, a Commit
   message (Section 5.4) can be sent to begin the ZRTP key exchange.
   The endpoint that sends the Commit is known as the initiator, while
   the receiver of the Commit is known as the responder.

   If both sides send Commit messages initiating a secure session at the
   same time the following rules are used to break the tie:

   o  If one Commit is for a DH mode while the other is for Preshared
      mode, then the Preshared Commit MUST be discarded and the DH
      Commit proceeds.
   o  If the two Commits are both Preshared mode, and one party has set
      the MiTM (M) flag in the Hello message and the other has not, the
      Commit message from the party who set the (M) flag MUST be
      discarded, and the one who has not set the (M) flag becomes the
      initiator, regardless of the nonce values.  In other words, for
      Preshared mode, the phone is the initiator and the PBX is the
      responder.
   o  If the two Commits are either both DH modes or both non-DH modes,
      then the Commit message with the lowest hvi value (for DH
      Commits), or lowest nonce value (for non-DH Commits), MUST be
      discarded and the other side is the initiator, and the protocol
      proceeds with the initiator's Commit.  The two hvi or nonce values
      are compared as large unsigned integers in network byte order.

   If one Commit is for Multistream mode while the other is for non-
   Multistream (DH or Preshared) mode, a software error has occurred and
   the ZRTP negotiation should be terminated.  This should never occur
   because of the constraints on Multistream mode described in
   Section 4.4.2.

   In the event that Commit messages are sent by both ZRTP endpoints at
   the same time, but are received in different media streams, the same
   resolution rules apply as if they were received on the same stream.
   The media stream in which the Commit will proceed through the ZRTP
   exchange while the media stream with the discarded Commit must wait
   for the completion of the other ZRTP exchange.

4.3.  Matching Shared Secret Determination

   The following sections describe how ZRTP endpoints generate and/or
   use the set of shared secrets s1, auxsecret, and pbxsecret through



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   the exchange of the DHPart1 and DHPart2 messages.  This doesn't cover
   the Diffie-Hellman calculations.  It only covers the method whereby
   the two parties determine if they already have shared secrets in
   common in their caches.

   Each ZRTP endpoint maintains a long-term cache of shared secrets that
   it has previously negotiated with the other party.  The ZID of the
   other party, received in the other party's Hello message, is used as
   an index into this cache to find the set of shared secrets, if any
   exist.  This cache entry may contain previously retained shared
   secrets, rs1 and rs2, which give ZRTP its key continuity features.
   If the other party is a PBX, the cache may also contain a trusted
   MiTM PBX shared secret, called pbxsecret, defined in Section 7.3.1.

   The DHPart1 and DHPart2 messages contain a list of hashes of these
   shared secrets to allow the two endpoints to compare the hashes with
   what they have in their caches to detect whether the two sides share
   any secrets that can be used in the calculation of the session key.
   The use of this shared secret cache is described in Section 4.9.

   If no secret of a given type is available, a random value is
   generated and used for that secret to ensure a mismatch in the hash
   comparisons in the DHPart1 and DHPart2 messages.  This prevents an
   eavesdropper from knowing which types of shared secrets are available
   between the endpoints.

   Section 4.3.1 and Section 4.3.2 both refer to the auxiliary shared
   secret auxsecret.  The auxsecret shared secret may be defined by the
   VoIP user agent out-of-band from the ZRTP protocol.  In some cases it
   may be provided by the signaling layer as srtps, which is defined in
   Section 8.2.  If it is not provided by the signaling layer, the
   auxsecret shared secret may be manually provisioned in other
   application-specific ways that are out-of-band, such as computed from
   a hashed pass phrase by prior agreement between the two parties.  Or
   it may be a family key used by an institution that the two parties
   both belong to.  It is a generalized mechanism for providing a shared
   secret that is agreed to between the two parties out of scope of the
   ZRTP protocol.  It is expected that most typical ZRTP endpoints will
   rarely use auxsecret.

   For both the initiator and the responder, the shared secrets s1, s2,
   and s3 will be calculated so that they can all be used later to
   calculate s0 in Section 4.4.1.4.  Here is how s1, s2, and s3 are
   calculated by both parties:

   The shared secret s1 will be either the initiator's rs1 or the
   initiator's rs2, depending on which of them can be found in the
   responder's cache.  If the initiator's rs1 matches the responder's



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   rs1 or rs2, then s1 MUST be set to the initiator's rs1.  If and only
   if that match fails, then if the initiator's rs2 matches the
   responder's rs1 or rs2, then s1 MUST be set to the initiator's rs2.
   If that match also fails, then s1 MUST be set to null.  The
   complexity of the s1 calculation is to recover from any loss of cache
   sync from an earlier aborted session, due to the Byzantine Generals'
   Problem [Byzantine].

   The shared secret s2 MUST be set to the value of auxsecret if and
   only if both parties have matching values for auxsecret, as
   determined by comparing the hashes of auxsecret sent in the DH
   messages.  If they don't match, s2 MUST be set to null.

   The shared secret s3 MUST be set to the value of pbxsecret if and
   only if both parties have matching values for pbxsecret, as
   determined by comparing the hashes of pbxsecret sent in the DH
   messages.  If they don't match, s3 MUST be set to null.

   If s1, s2, or s3 have null values, they are assumed to have a zero
   length for the purposes of hashing them later during the s0
   calculation in Section 4.4.1.4.

   The comparison of hashes of rs1, rs2, auxsecret, and pbxsecret is
   described in the next sections.

4.3.1.  Responder Behavior

   The responder calculates an HMAC keyed hash using the first retained
   shared secret, rs1, as the key on the string "Responder" which
   generates a retained secret ID, rs1IDr, which is truncated to the
   leftmost 64 bits.  HMACs are calculated in a similar way for
   additional shared secrets:

      rs1IDr = HMAC(rs1, "Responder")
      rs2IDr = HMAC(rs2, "Responder")
      auxsecretIDr = HMAC(auxsecret, "Responder")
      pbxsecretIDr = HMAC(pbxsecret, "Responder")

   The set of keyed hashes (HMACs) of shared secrets are included by the
   responder in the DHPart1 message.

   The HMACs of the possible shared secrets received in the DHPart2 can
   be compared against the HMACs of the local set of possible shared
   secrets.  From these comparisons, s1, s2, and s3 are calculated per
   the methods described above in Section 4.3.  The expected HMAC values
   of the shared secrets are calculated (using the string "Initiator"
   instead of "Responder") as in Section 4.3.2 and compared to the HMACs
   received in the DHPart2 message.  The secrets corresponding to



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   matching HMACs are kept while the secrets corresponding to the non-
   matching ones are replaced with a null, which is assumed to have a
   zero length for the purposes of hashing them later.  The resulting
   s1, s2, and s3 values are used later to calculate s0 in
   Section 4.4.1.4.

4.3.2.  Initiator Behavior

   The initiator calculates an HMAC keyed hash using the first retained
   shared secret, rs1, as the key on the string "Initiator" which
   generates a retained secret ID, rs1IDi, which is truncated to the
   leftmost 64 bits.  HMACs are calculated in a similar way for
   additional shared secrets:

      rs1IDi = HMAC(rs1, "Initiator")
      rs2IDi = HMAC(rs2, "Initiator")
      auxsecretIDi = HMAC(auxsecret, "Initiator")
      pbxsecretIDi = HMAC(pbxsecret, "Initiator")

   These HMACs of shared secrets are included by the initiator in the
   DHPart2 message.

   The initiator then calculates the set of secret IDs that are expected
   to be received from the responder in the DHPart1 message by
   substituting the string "Responder" instead of "Initiator" as in
   Section 4.3.1.

   The HMACs of the possible shared secrets received are compared
   against the HMACs of the local set of possible shared secrets.  From
   these comparisons, s1, s2, and s3 are calculated per the methods
   described above in Section 4.3.  The secrets corresponding to
   matching HMACs are kept while the secrets corresponding to the non-
   matching ones are replaced with a null, which is assumed to have a
   zero length for the purposes of hashing them later.  The resulting
   s1, s2, and s3 values are used later to calculate s0 in
   Section 4.4.1.4.

   For example, consider two ZRTP endpoints who share secrets rs1 and
   pbxsecret (defined in Section 7.3.1).  During the comparison, rs1ID
   and pbxsecretID will match but auxsecretID will not.  As a result, s1
   = rs1, s2 will be null, and s3 = pbxsecret.

4.3.3.  Handling a Shared Secret Cache Mismatch

   A shared secret cache mismatch is defined to mean that we expected a
   cache match because rs1 exists in our local cache, but we computed a
   null value for s1 (per the method described in Section 4.3).




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   If one party has a cached shared secret and the other party does not,
   this indicates one of two possible situations.  Either there is a
   man-in-the-middle (MiTM) attack, or one of the legitimate parties has
   lost their cached shared secret by some mishap.  Perhaps they
   inadvertently deleted their cache, or their cache was lost or
   disrupted due to restoring their disk from an earlier backup copy.
   The party that has the surviving cache entry can easily detect that a
   cache mismatch has occurred, because they expect their own cached
   secret to match the other party's cached secret, but it does not
   match.  It is possible for both parties to detect this condition if
   both parties have surviving cached secrets that have fallen out of
   sync, due perhaps to one party restoring from a disk backup.

   If either party discovers a cache mismatch, the user agent who makes
   this discovery must treat this as a possible security event and MUST
   alert their own user that there is a heightened risk of a MiTM
   attack, and that the user should verbally compare the SAS with the
   other party to ascertain that no MiTM attack has occurred.  If a
   cache mismatch is detected and it is not possible to compare the SAS,
   either because the user interface does not support it or because one
   or both endpoints are unmanned devices, and no other SAS comparison
   mechanism is available, the session MAY be terminated.

   The session need not be terminated on a cache mismatch event if the
   mechanism described in Section 8.1.1 is available, which allows
   authentication of the DH exchange without human assistance.  Or if
   any mechanism is available to determine if the SAS matches.  This
   would require either circumstances that allow human verbal
   comparisons of the SAS, or by using the OPTIONAL digital signature
   feature on the SAS hash, as described in Section 7.2.  Even if the
   user interface does not permit an SAS comparison, the human user MUST
   be warned, and may elect to proceed with the call at their own risk.

   Here is a non-normative example of a cache-mismatch alert message
   from a ZRTP user agent (specifically, Zfone [zfone]), designed for a
   desktop PC graphical user interface environment.  It is by no means
   required that the alert be this detailed:

      "We expected the other party to have a shared secret cached from a
      previous call, but they don't have it.  This may mean your partner
      simply lost his cache of shared secrets, but it could also mean
      someone is trying to wiretap you.  To resolve this question you
      must check the authentication string with your partner.  If it
      doesn't match, it indicates the presence of a wiretapper."
      If the alert is rendered by a robot voice instead of a GUI,
      brevity may be more important: "Something's wrong.  You must check
      the authentication string with your partner.  If it doesn't match,
      it indicates the presence of a wiretapper."



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4.4.  DH and non-DH key agreements

   The next step is the generation of a secret for deriving SRTP keying
   material.  ZRTP uses Diffie-Hellman and two non-Diffie-Hellman modes,
   described in the following sections.

4.4.1.  Diffie-Hellman Mode

   The purpose of the Diffie-Hellman (either Finite Field Diffie-Hellman
   or Elliptic Curve Diffie-Hellman) exchange is for the two ZRTP
   endpoints to generate a new shared secret, s0.  In addition, the
   endpoints discover if they have any cached or previously stored
   shared secrets in common, and uses them as part of the calculation of
   the session keys.

   Because the DH exchange affects the state of the retained shared
   secret cache, only one in-process ZRTP DH exchange may occur at a
   time between two ZRTP endpoints.  Otherwise, race conditions and
   cache integrity problems will result.  When multiple media streams
   are established in parallel between the same pair of ZRTP endpoints
   (determined by the ZIDs in the Hello Messages), only one can be
   processed.  Once that exchange completes with Confirm2 and Conf2ACK
   messages, another ZRTP DH exchange can begin.  This constraint does
   not apply when Multistream mode key agreement is used since the
   cached shared secrets are not affected.

4.4.1.1.  Hash Commitment in Diffie-Hellman Mode

   From the intersection of the algorithms in the sent and received
   Hello messages, the initiator chooses a hash, cipher, auth tag, key
   agreement type, and SAS type to be used.

   A Diffie-Hellman mode is selected by setting the Key Agreement Type
   to one of the DH or ECDH values in Table 5 in the Commit.  In this
   mode, the key agreement begins with the initiator choosing a fresh
   random Diffie-Hellman (DH) secret value (svi) based on the chosen key
   agreement type value, and computing the public value.  (Note that to
   speed up processing, this computation can be done in advance.)  For
   guidance on generating random numbers, see Section 4.8.  The value
   for the DH generator g, the DH prime p, and the length of the DH
   secret value, svi, are defined in Section 5.1.5.

      pvi = g^svi mod p

   where g and p are determined by the key agreement type value.  The
   pvi value is formatted as a big-endian octet string, fixed to the
   width of the DH prime, and leading zeros MUST NOT be truncated.




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   The hash commitment is performed by the initiator of the ZRTP
   exchange.  The hash value of the initiator, hvi, includes a hash of
   the entire DHPart2 message as shown in Figure 9 (which includes the
   Diffie-Hellman public value, pvi), and the responder's Hello message:

      hvi = hash(initiator's DHPart2 message | responder's Hello
      message)

   Note that the Hello message includes the fields shown in Figure 3.

   The information from the responder's Hello message is included in the
   hash calculation to prevent a bid-down attack by modification of the
   responder's Hello message.

   The initiator sends hvi in the Commit message.

   The use of hash commitment in the DH exchange constrains the attacker
   to only one guess to generate the correct short authentication string
   (SAS) (Section 7) in his attack, which means the SAS can be quite
   short.  A 16-bit SAS, for example, provides the attacker only one
   chance out of 65536 of not being detected.

4.4.1.2.  Responder Behavior in Diffie-Hellman Mode

   Upon receipt of the Commit message, the responder generates its own
   fresh random DH secret value, svr, and computes the public value.
   (Note that to speed up processing, this computation can be done in
   advance.)  For guidance on random number generation, see Section 4.8.
   The value for the DH generator g, the DH prime p, and the length of
   the DH secret value, svr, are defined in Section 5.1.5.

      pvr = g^svr mod p

   The pvr value is formatted as a big-endian octet string, fixed to the
   width of the DH prime, and leading zeros MUST NOT be truncated.

   Upon receipt of the DHPart2 message, the responder checks that the
   initiator's public DH value is not equal to 1 or p-1.  An attacker
   might inject a false DHPart2 packet with a value of 1 or p-1 for
   g^svi mod p, which would cause a disastrously weak final DH result to
   be computed.  If pvi is 1 or p-1, the user should be alerted of the
   attack and the protocol exchange MUST be terminated.  Otherwise, the
   responder computes its own value for the hash commitment using the
   public DH value (pvi) received in the DHPart2 packet and its Hello
   packet and compares the result with the hvi received in the Commit
   packet.  If they are different, a MiTM attack is taking place and the
   user is alerted and the protocol exchange terminated.




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   The responder then calculates the Diffie-Hellman result:

      DHResult = pvi^svr mod p

4.4.1.3.  Initiator Behavior in Diffie-Hellman Mode

   Upon receipt of the DHPart1 message, the initiator checks that the
   responder's public DH value is not equal to 1 or p-1.  An attacker
   might inject a false DHPart1 packet with a value of 1 or p-1 for
   g^svr mod p, which would cause a disastrously weak final DH result to
   be computed.  If pvr is 1 or p-1, the user should be alerted of the
   attack and the protocol exchange MUST be terminated.

   The initiator then sends a DHPart2 message containing the initiator's
   public DH value and the set of calculated shared secret IDs as
   defined in Section 4.3.2.

   The initiator calculates the same Diffie-Hellman result using:

      DHResult = pvr^svi mod p

4.4.1.4.  Shared Secret Calculation for DH Mode

   A hash of the received and sent ZRTP messages in the current ZRTP
   exchange in the following order is calculated by both parties:

      total_hash = hash(Hello of responder | Commit | DHPart1 | DHPart2)

   Note that only the ZRTP messages (Figure 3, Figure 5, Figure 8, and
   Figure 9), not the entire ZRTP packets, are included in the
   total_hash.

   For both the initiator and responder, the DHResult is formatted as a
   big-endian octet string, fixed to the width of the DH prime, and
   leading zeros MUST NOT be truncated.  For example, for a 3072-bit p,
   DHResult would be a 384 octet value, with the first octet the most
   significant.

   The calculation of the final shared secret, s0, is in compliance with
   the recommendations in sections 5.8.1 and 6.1.2.1 of NIST SP 800-56A
   [SP800-56A].  This is done by hashing a concatenation of a number of
   items, including the DHResult, the ZID's of the initiator (ZIDi) and
   the responder (ZIDr), the total_hash, and the set of non-null shared
   secrets as described in Section 4.3.

   In section 5.8.1 of NIST SP 800-56A [SP800-56A], NIST requires
   certain parameters to be hashed together in a particular order, which
   NIST refers to as: Z, AlgorithmID, PartyUInfo, PartyVInfo,



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   SuppPubInfo, and SuppPrivInfo.  In our implementation, our DHResult
   corresponds to Z, "ZRTP-HMAC-KDF" corresponds to AlgorithmID, our
   ZIDi and ZIDr correspond to PartyUInfo and PartyVInfo, our total_hash
   corresponds to SuppPubInfo, and the set of three shared secrets s1,
   s2, and s3 corresponds to SuppPrivInfo.  NIST also requires a 32-bit
   big-endian integer counter to be included in the hash each time the
   hash is computed, which we have set to the fixed value of 1, because
   we only compute the hash once.  NIST refers to the final hash output
   as DerivedKeyingMaterial, which corresponds to our s0 in this
   calculation.

      s0 = hash( counter | DHResult | "ZRTP-HMAC-KDF" | ZIDi | ZIDr |
      total_hash | len(s1) | s1 | len(s2) | s2 | len(s3) | s3 )

   Note that temporary values s1, s2, and s3 were calculated per the
   methods described above in Section 4.3, and they are erased from
   memory immediately after they are used to calculate s0.

   The length of the DHResult field was implicitly agreed to by the
   negotiated DH prime size.  The length of total_hash is implicitly
   determined by the negotiated hash algorithm.  All of the explicit
   length fields, len(), in the above hash are 32-bit big-endian
   integers, giving the length in octets of the field that follows.
   Some members of the set of shared secrets (s1, s2, and s3) may have
   lengths of zero if they are null (not shared), and are each preceded
   by a 4-octet length field.  For example, if s2 is null, len(s2) is
   0x00000000, and s2 itself would be absent from the hash calculation,
   which means len(s3) would immediately follow len(s2).  While
   inclusion of ZIDi and ZIDr may be redundant, because they are
   implicitly included in the total_hash, we explicitly include them
   here to follow NIST SP800-56A. The string "ZRTP-HMAC-KDF" (not null-
   terminated) identifies what purpose the resulting s0 will be used
   for, which is to serve as the master key for the ZRTP HMAC-based key
   derivation function (KDF) defined in Section 4.5.1 and used in
   Section 4.5.2.

   A ZRTP Session Key is derived from s0 via the ZRTP key derivation
   function (Section 4.5.1) which then allows the ZRTP Multistream mode
   to be used to generate SRTP key and salt pairs for additional
   concurrent media streams between this pair of ZRTP endpoints.  If a
   ZRTP Session Key has already been generated between this pair of
   endpoints and is available, no new ZRTP Session Key is calculated.

      ZRTPSess = KDF(s0, "ZRTP Session Key", negotiated hash length)

   The ZRTPSess key is kept for the duration of the call signaling
   session between the two ZRTP endpoints.  That is, if there are two
   separate calls between the endpoints (in SIP terms, separate SIP



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   dialogs), then a ZRTP Session Key MUST NOT be used across the two
   call signaling sessions.  ZRTPSess MUST be destroyed no later than
   the end of the call signaling session.

   The two endpoints proceed with key derivations as described in
   Section 4.5.2, now that there is a defined s0 and ZRTPSess key.

4.4.2.  Multistream Mode

   The Multistream key agreement mode can be used to generate SRTP keys
   and salts for additional media streams established between a pair of
   endpoints.  Multistream mode cannot be used unless there is an active
   SRTP session established between the endpoints which means a ZRTP
   Session key is active.  This ZRTP Session key can be used to generate
   keys and salts without performing another DH calculation.  In this
   mode, the retained shared secret cache is not used or updated.  As a
   result, multiple ZRTP Multistream mode exchanges can be processed in
   parallel between two endpoints.

   Multistream mode is also used to resume a secure call that has gone
   clear using a GoClear message as described in Section 4.7.2.1.

   When adding additional media streams to an existing call, Multistream
   mode MUST be used.  The first media stream MUST use either DH mode or
   Preshared mode.  Only one DH exchange or Preshared exchange is
   performed, just for the first media stream.  The DH exchange or
   Preshared exchange MUST be completed for the first media stream
   before Multistream mode is used to add any other media streams.

4.4.2.1.  Commitment in Multistream Mode

   Multistream mode is selected by the initiator setting the Key
   Agreement Type to "Mult" in the Commit message (Figure 6).  The
   Cipher Type, Auth Tag Length, and Hash in Multistream mode SHOULD be
   set by the initiator to the same as the values as in the initial DH
   Mode Commit.  The SAS Type is ignored as there is no SAS
   authentication in this mode.

      Note: This requirement is needed since some endpoints cannot
      support different SRTP algorithms for different media streams.
      However, in the case of Multstream mode being used to go secure
      after a GoClear, the requirement to use the same SRTP algorithms
      is relaxed if there are no other active SRTP sessions.

   In place of hvi in the Commit, a random nonce of length 4-words (16
   octets) is chosen.  Its value MUST be unique for all nonce values
   chosen for active ZRTP sessions between a pair of endpoints.  If a
   Commit is received with a reused nonce value, the ZRTP exchange MUST



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   be immediately terminated.

      Note: Since the nonce is used to calculate different SRTP key and
      salt pairs for each media stream, a duplication will result in the
      same key and salt being generated for the two media streams, which
      would have disastrous security consequences.

   If a Commit is received selecting Multistream mode, but the responder
   does not have a ZRTP Session Key available, the exchange MUST be
   terminated.  Otherwise, the responder proceeds to the next section on
   Shared Secret Calculation, Section 4.4.2.2.

   If both sides send Multistream Commit messages at the same time, the
   contention is resolved and the initiator/responder roles are settled
   according to Section 4.2, and the protocol proceeds.

   In Multistream mode, both the DHPart1 and DHPart2 messages are
   skipped.  After receiving the Commit message from the initiator, the
   responder sends the Confirm1 message after calculating this stream's
   SRTP keys, as described below.

4.4.2.2.  Shared Secret Calculation for Multistream Mode

   A hash of the received and sent ZRTP messages in the current ZRTP
   exchange for the current media stream is calculated:

      total_hash = hash(Hello of responder | Commit )

   This refers to the Hello and Commit messages for the current media
   stream which is using Multistream mode, not the original media stream
   that included a full DH key agreement.  Note that only the ZRTP
   messages (Figure 3 and Figure 6), not the entire ZRTP packets, are
   included in the hash.

   The SRTP keys and salts for the initiator and responder are
   calculated using the ZRTP Session Key ZRTPSess and the nonce from the
   Commit message.  The nonce from the Commit message is implicitly
   included in the total_hash, which hashed the entire Commit message
   and the other party's Hello message.  For the n-th media stream, s0n
   is derived from ZRTPSess via the ZRTP key derivation function
   (Section 4.5.1):

      s0n = KDF(ZRTPSess, total_hash, negotiated hash length)

   Note that the responder's Hello message, included in the total_hash,
   includes some unique nonce-derived material of its own (the H3 hash
   image), thereby ensuring that each of the two parties can
   unilaterally force the resulting s0n shared secret to be unique for



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   each media stream, even if one party by some error fails to produce a
   unique nonce.  Note also that the ZRTPSess key is derived from
   material that also includes a different and more inclusive total_hash
   from the entire packet sequence that performed the original DH
   exchange for the first media stream in this ZRTP session.

   At this point in Multistream mode, the two endpoints begin key
   derivations as described in Section 4.5.2 using s0n in place of s0 in
   the key derivation formulas for this media stream.

4.4.3.  Preshared Mode

   The Preshared key agreement mode can be used to generate SRTP keys
   and salts without a DH calculation, instead relying on a shared
   secret from previous DH calculations between the endpoints.

   This key agreement mode is useful to rapidly re-establish a secure
   session between two parties who have recently started and ended a
   secure session that has already performed a DH key agreement, without
   performing another lengthy DH calculation, which may be desirable on
   slow processors in resource-limited environments.  Preshared mode
   MUST NOT be used for adding additional media streams to an existing
   call.  Multistream mode MUST be used for this purpose.

   In the most severe resource-limited environments, Preshared mode may
   be useful with processors that cannot perform a DH calculation in an
   ergonomically acceptable time limit.  Shared key material may be
   manually provisioned between two such endpoints in advance and still
   allow a limited subset of functionality.  Such a "better than
   nothing" implementation would have to be regarded as non-compliant
   with the ZRTP specification, but it could interoperate in Preshared
   (and if applicable, Multistream) mode with a compliant ZRTP endpoint.

   Because Preshared mode affects the state of the retained shared
   secret cache, only one in-process ZRTP Preshared exchange may occur
   at a time between two ZRTP endpoints.  This rule is explained in more
   detail in Section 4.4.1, and applies for the same reasons as in DH
   mode.

   Preshared mode MUST NOT be used for establishing a second media
   stream.  Multistream mode is designed for that.

   Preshared mode is only included in this specification to meet the
   R-REUSE requirement in the Media Security Requirements
   [I-D.ietf-sip-media-security-requirements] document.  A series of
   preshared-keyed calls between two ZRTP endpoints should use a DH key
   exchange periodically.  Preshared mode is only used if a cached
   shared secret has been established in an earlier session by a DH



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   exchange, as discussed in Section 4.9.

4.4.3.1.  Commitment in Preshared Mode

   Preshared mode is selected by setting the Key Agreement Type to
   Preshared in the Commit message.  This results in the same call flow
   as Multistream mode.  The principal difference between Multistream
   mode and Preshared mode is that Preshared mode uses a previously
   cached shared secret, rs1, instead of an active ZRTP Session key,
   ZRTPSess, as the initial keying material.

   Because Preshared mode depends on having a reliable shared secret in
   its cache, it is RECOMMENDED that Preshared mode only be used when
   the SAS Verified flag has been previously set.

4.4.3.2.  Initiator Behavior in Preshared Mode

   The Commit message (Figure 7) is sent by the initiator of the ZRTP
   exchange.  From the intersection of the algorithms in the sent and
   received Hello messages, the initiator chooses a hash, cipher, auth
   tag, key agreement type, and SAS type to be used.

   To assemble a Preshared commit, we must first construct a temporary
   preshared_key, which is constructed from one of several possible
   combinations of cached key material, depending on what is available
   in the shared secret cache.  If rs1 is not available in the
   initiator's cache, then Preshared mode MUST NOT be used.

      preshared_key = hash( len(rs1) | rs1 | len(auxsecret) | auxsecret
      | len(pbxsecret) | pbxsecret )

   All of the explicit length fields, len(), in the above hash are 32-
   bit big-endian integers, giving the length in octets of the field
   that follows.  Some members of the set of shared secrets (rs1,
   auxsecret, and pbxsecret) may have lengths of zero if they are null
   (not available), and are each preceded by a 4-octet length field.
   For example, if auxsecret is null, len(auxsecret) is 0x00000000, and
   auxsecret itself would be absent from the hash calculation, which
   means len(pbxsecret) would immediately follow len(auxsecret).

   In place of hvi in the Commit message, two smaller fields are
   inserted by the initiator:

      - A random nonce of length 4-words (16 octets).
      - A keyID = HMAC(preshared_key, "Prsh") truncated to 64 bits.






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4.4.3.3.  Responder Behavior in Preshared Mode

   The responder uses the received keyID to search for matching key
   material in its cache.  It does this by computing a preshared_key
   value and keyID value using the same formula as the initiator,
   depending on what is available in the responder's local cache.  If
   the locally computed keyID does not match the received keyID in the
   Commit, the responder recomputes a new preshared_key and keyID from a
   different subset of shared keys from the cache, dropping auxsecret or
   pbxsecret or both from the hash calculation, until a matching
   preshared_key is found or it runs out of possibilities.  Note that
   rs2 is not included in the process.

   If it finds the appropriate matching shared key material, it is used
   to derive s0 and a new ZRTPSess key, as described in the next section
   on Shared Secret Calculation, Section 4.4.3.4.

   If the responder determines that it does not have a cached shared
   secret from a previous DH exchange, or it fails to match the keyID
   hash from the initiator with any combination of its shared keys, it
   SHOULD respond with its own DH Commit message.  This would reverse
   the roles and the responder would become the initiator, because the
   DH Commit must always "trump" the Preshared Commit message as
   described in Section 4.2.  The key exchange would then proceeds using
   DH mode.  However, if a severely resource-limited responder lacks the
   computing resources to respond in a reasonable time with a DH Commit,
   it MAY respond with a ZRTP Error message (Section 5.9) indicating
   that no shared secret is available.

   If both sides send Preshared Commit messages initiating a secure
   session at the same time, the contention is resolved and the
   initiator/responder roles are settled according to Section 4.2, and
   the protocol proceeds.

   In Preshared mode, both the DHPart1 and DHPart2 messages are skipped.
   After receiving the Commit message from the initiator, the responder
   sends the Confirm1 message after calculating this stream's SRTP keys,
   as described below.

4.4.3.4.  Shared Secret Calculation for Preshared Mode

   A hash of the received and sent ZRTP messages in the current ZRTP
   exchange for the current media stream is calculated:

      total_hash = hash(Hello of responder | Commit )

   Note that only the ZRTP messages (Figure 3 and Figure 7), not the
   entire ZRTP packets, are included in the hash.  The nonce from the



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   Commit message is implicitly included in the total_hash, which hashed
   the entire Commit message and the other party's Hello message.  Next,
   the preshared_key is used to derive s0 and ZRTPSess, via the ZRTP key
   derivation function (Section 4.5.1):

      s0 = KDF(preshared_key, total_hash, negotiated hash length)
      ZRTPSess = KDF(s0, "ZRTP Session Key", negotiated hash length)

   The preshared_key MUST be erased as soon as it has been used to
   calculate s0 and ZRTPSess.  The ZRTPSess key allows the later use of
   Multistream mode for adding additional media streams to this session.

   Note that the responder's Hello message, included in the total_hash,
   includes some unique nonce-derived material of its own (the H3 hash
   image), thereby ensuring that each of the two parties can
   unilaterally force the resulting s0 shared secret to be unique for
   each media stream, even if one party by some error fails to produce a
   unique nonce.

      Note: Since the nonce is used to calculate different SRTP key and
      salt pairs for each media stream, a duplication will result in the
      same key and salt being generated for the two media streams, which
      would have disastrous security consequences.

   At this point in Preshared mode, the two endpoints begin key
   derivations as described in Section 4.5.2, now that there is a
   defined s0 and ZRTPSess key.

4.5.  Key Derivations

4.5.1.  The ZRTP Key Derivation Function

   To derive keys from a shared secret, ZRTP uses an HMAC-based key
   derivation function, or KDF.  It is used throughout Section 4.5.2 and
   in other sections.

   The ZRTP KDF is defined in this manner:

      KDF(KI, Label, L) returns HMAC(KI, Label), truncated to L bits

   The HMAC function for the KDF is based on the negotiated hash
   algorithm defined in Section 5.1.2.  The HMAC in the KDF is keyed by
   KI, which is a secret key derivation key that is unknown to the
   wiretapper (for example, s0), and the HMAC is computed on a string
   (Label) that need not be a secret.  The output of the KDF is
   truncated to the leftmost L bits, where L is not to exceed the length
   of the output of the HMAC.  If SHA-256 is the negotiated hash
   algorithm, the HMAC would be HMAC-SHA-256, thus the maximum value of



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   L would be 256, the negotiated hash length.

   The ZRTP KDF is designed to provide key separation, which is a
   security requirement for the cryptographic keys derived from the same
   key derivation key.  The keys shall be separate in the sense that the
   compromise of some derived keys will not degrade the security
   strength of any of the other derived keys.

   This KDF was designed before the publication of NIST SP 800-108
   [SP800-108], but has the same security properties as the KDF in the
   NIST document, when used within the confines of the ZRTP protocol.
   The ZRTP KDF is similar to the NIST KDF using the HMAC-based
   pseudorandom function in counter mode, with only a single iteration
   of the counter.  The ZRTP KDF never has to generate more than 256
   bits of output key material, so only a single invocation of the HMAC
   function is needed.

   The ZRTP KDF is not to be confused with the SRTP KDF defined in
   [RFC3711].

4.5.2.  Deriving keys via the ZRTP KDF

   The following calculations derive a set of keys from s0.  For the
   original media stream that calculated s0 from the DH exchange, s0
   means the original s0.  For any additional media streams that were
   activated in Multistream mode, s0 means s0n, for the n-th media
   stream.  It is also assumed that the ZRTPSess key has been defined.

   Subkeys are not drawn directly from s0, as done in NIST SP800-56A. To
   enhance key separation, ZRTP uses s0 to key an HMAC-based Key
   Derivation Function (Section 4.5.1).

   Separate SRTP master keys and master salts are derived for use in
   each direction for each media stream.  Unless otherwise specified,
   ZRTP uses SRTP with no MKI, 32 bit authentication using HMAC-SHA1,
   AES-CM 128 or 256 bit key length, 112 bit session salt key length,
   2^48 key derivation rate, and SRTP prefix length 0.

   The ZRTP initiator encrypts and the ZRTP responder decrypts packets
   by using srtpkeyi and srtpsalti, while the ZRTP responder encrypts
   and the ZRTP initiator decrypts packets by using srtpkeyr and
   srtpsaltr.  These are generated by:

      srtpkeyi = KDF(s0, "Initiator SRTP master key", negotiated AES key
      length)
      srtpsalti = KDF(s0, "Initiator SRTP master salt", 112)





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      srtpkeyr = KDF(s0, "Responder SRTP master key", negotiated AES key
      length)
      srtpsaltr = KDF(s0, "Responder SRTP master salt", 112)

   The SRTP key and salt values are truncated (taking the leftmost bits)
   to the length determined by the chosen SRTP algorithm.

   The HMAC keys are the same length as the output of the underlying
   hash function in the KDF, and are thus generated without truncation
   by:

      hmackeyi = KDF(s0, "Initiator HMAC key", negotiated hash length)
      hmackeyr = KDF(s0, "Responder HMAC key", negotiated hash length)
      Note that these HMAC keys are used only by ZRTP and not by SRTP.
      Note: Different HMAC keys are needed for the initiator and the
      responder to ensure that GoClear messages in each direction are
      unique and can not be cached by an attacker and reflected back to
      the endpoint.

   ZRTP keys are generated for the initiator and responder to use to
   encrypt the Confirm1 and Confirm2 messages.  They are truncated to
   the same size as the negotiated SRTP key size.

      zrtpkeyi = KDF(s0, "Initiator ZRTP key", negotiated AES key
      length)
      zrtpkeyr = KDF(s0, "Responder ZRTP key", negotiated AES key
      length)

   All key material is destroyed as soon as it is no longer needed, no
   later than the end of the call. s0 is erased in Section 4.6.1, and
   the rest of the session key material is erased in Section 4.7.2.1 and
   Section 4.7.3.

   The Short Authentication String (SAS) value is calculated from the
   HMAC of a fixed string, keyed with the ZRTPSess key derived from the
   DH key agreement.  This means the same SAS is used for all media
   streams which are derived from a single DH key agreement in a ZRTP
   session.

      sashash = KDF(ZRTPSess, "SAS", negotiated hash length)
      sasvalue = sashash [truncated to leftmost 32 bits]

4.6.  Confirmation

   The Confirm1 and Confirm2 messages (Figure 10) contain the cache
   expiration interval (defined in Section 4.9) for the newly generated
   retained shared secret.  The flagoctet is an 8 bit unsigned integer
   made up of these flags: the PBX Enrollment flag (E) defined in



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   Section 7.3.1, SAS Verified flag (V) defined in Section 7.1, Allow
   Clear flag (A) defined in Section 4.7.2, and Disclosure flag (D)
   defined in Section 11.

      flagoctet = (E * 2^3) + (V * 2^2) + (A * 2^1) + (D * 2^0)

   Part of the Confirm1 and Confirm2 messages are encrypted using full-
   block Cipher Feedback Mode, and contain a 128-bit random CFB
   Initialization Vector (IV).  The Confirm1 and Confirm2 messages also
   contain an HMAC covering the encrypted part of the Confirm1 or
   Confirm2 message which includes a string of zeros, the signature
   length, flag octet, cache expiration interval, signature type block
   (if present) and signature block (Section 7.2) (if present).  For the
   responder:

      hmac = HMAC(hmackeyr, encrypted part of Confirm1)

   For the initiator:

      hmac = HMAC(hmackeyi, encrypted part of Confirm2)

   The hmackeyi and hmackeyr keys are computed in Section 4.5.2.

   The exchange is completed when the responder sends either the
   Conf2ACK message or the responder's first SRTP media packet (with a
   valid SRTP auth tag).  The initiator MUST treat the first valid SRTP
   media from the responder as equivalent to receiving a Conf2ACK.  The
   responder may respond to Confirm2 with either SRTP media or Conf2ACK,
   or both, in whichever order the responder chooses (or whichever order
   the "cloud" chooses to deliver them).

4.6.1.  Updating the Cache of Shared Secrets

   After receiving the Confirm messages, both parties must now update
   their retained shared secret rs1 in their respective caches, provided
   the following conditions hold:

      1) This key exchange is either DH or Preshared mode, not
      Multistream mode, which does not update the cache.
      2) Depending on the values of the cache expiration intervals that
      are received in the two Confirm messages, there are some scenarios
      that do not update the cache, as explained in Section 4.9.
      3) The responder MUST receive the initiator's Confirm2 message
      before updating the responder's cache.
      4) The initiator MUST receive either the responder's Conf2ACK
      message or the responder's SRTP media (with a valid SRTP auth tag)
      before updating the initiator's cache.




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   For DH mode only, before updating the retained shared secret rs1 in
   the cache, each party first discards their old rs2 and copies their
   old rs1 to rs2.  The old rs1 is saved to rs2 because of the risk of
   session interruption after one party has updated his own rs1 but
   before the other party has enough information to update her own rs1.
   If that happens, they may regain cache sync in the next session by
   using rs2 (per Section 4.3).  This mitigates the well-known Byzantine
   Generals' Problem [Byzantine].  The old rs1 value is not saved in
   Preshared mode.

   For DH mode and Preshared mode, both parties compute a new rs1 value
   from s0 via the ZRTP key derivation function (Section 4.5.1):

      rs1 = KDF(s0, "retained secret", negotiated hash length)

   After s0 is used to derive the new rs1, it MUST be erased.  Even if
   rs1 is not updated (in the case of Multistream mode), s0 MUST still
   be destroyed.

4.7.  Termination

   A ZRTP session is normally terminated at the end of a call, but it
   may be terminated early by either the Error message or the GoClear
   message.

4.7.1.  Termination via Error message

   The Error message (Section 5.9) is used to terminate an in-progress
   ZRTP exchange due to an error.  The Error message contains an integer
   Error Code for debugging purposes.  The termination of a ZRTP key
   agreement exchange results in no updates to the cached shared secrets
   and deletion of all crypto context.

   The ZRTP Session key, ZRTPSess, is only deleted if the ZRTP session
   in which it was generated and all ZRTP sessions which are using it
   are terminated.

4.7.2.  Termination via GoClear message

   The GoClear message (Section 5.11) is used to switch from SRTP to
   RTP, usually because the user has chosen to do that by pressing a
   button.  The GoClear uses an HMAC of the Message Type Block sent in
   the GoClear Message computed with the hmackey derived from the shared
   secret.  This HMAC is truncated to the leftmost 64 bits.  When sent
   by the initiator:






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      clear_hmac = HMAC(hmackeyi, "GoClear ")

   When sent by the responder:

      clear_hmac = HMAC(hmackeyr, "GoClear ")

   A GoClear message which does not receive a ClearACK response must be
   resent.  If a GoClear message is received with a bad HMAC, it must be
   ignored, and no ClearACK is sent.

   A ZRTP endpoint MAY choose to accept GoClear messages after the
   session has switched to SRTP, allowing the session to revert to RTP.
   This is indicated in the Confirm1 or Confirm2 messages (Figure 10) by
   setting the Allow Clear flag (A).  If an endpoint sets the Allow
   Clear (A) flag in their Confirm message, it indicates that they
   support receiving GoClear messages.

   A ZRTP endpoint that receives a GoClear MUST authenticate the message
   by checking the clear_hmac.  If the message authenticates, the
   endpoint stops sending SRTP packets, and generates a ClearACK in
   response.  It MUST also delete all the crypto key material for all
   the SRTP media streams, as defined in Section 4.7.2.1.

   Until confirmation from the user is received (e.g. clicking a button,
   pressing a DTMF key, etc.), the ZRTP endpoint MUST NOT resume sending
   RTP packets.  The endpoint then renders to the user an indication
   that the media session has switched to clear mode, and waits for
   confirmation from the user.  This blocks the flow of sensitive
   discourse until the user is forced to take notice that he's no longer
   protected by encryption.  To prevent pinholes from closing or NAT
   bindings from expiring, the ClearACK message MAY be resent at regular
   intervals (e.g. every 5 seconds) while waiting for confirmation from
   the user.  After confirmation of the notification is received from
   the user, the sending of RTP packets may begin.

   After sending a GoClear message, the ZRTP endpoint stops sending SRTP
   packets.  When a ClearACK is received, the ZRTP endpoint deletes the
   crypto context for the SRTP session, as defined in Section 4.7.2.1,
   and may then resume sending RTP packets.

   In the event a ClearACK is not received before the retransmissions of
   GoClear are exhausted, the key material is deleted, as defined in
   Section 4.7.2.1.

   After the users have transitioned from SRTP media back to RTP media
   (clear mode), they may decide later to return to secure mode by
   manual activation, usually by pressing a GO SECURE button.  In that
   case, a new secure session is initiated by the party that presses the



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   button, by sending a new Commit packet, leadng to a new session key
   negotiation.  It is not necessary to send another Hello packet, as
   the two parties have already done that at the start of the call and
   thus have already discovered each other's ZRTP capabilities.  It is
   possible for users to toggle back and forth between clear and secure
   modes multiple times in the same call, just as they could in the old
   days of secure PSTN phones.

4.7.2.1.  Key Destruction for GoClear message

   All SRTP session key material MUST be erased by the receiver of the
   GoClear message upon receiving a properly authenticated GoClear.  The
   same key destruction MUST be done by the sender of GoClear message,
   upon receiving the ClearACK.

   In particular, the destroyed key material includes the SRTP session
   keys and salts, SRTP master keys and salts, and all material
   sufficient to reconstruct the SRTP keys and salts, including ZRTPSess
   (s0 should have been destroyed earlier, in Section 4.6.1).  All key
   material that would have been erased at the end of the SIP session
   MUST be erased.  However, ZRTPSess is destroyed in a manner different
   from the other key material.  Both parties replace ZRTPSess with a
   hash of itself, without truncation:

      ZRTPSess = hash(ZRTPSess)

   This meets the requirements of Perfect Forward Secrecy (PFS), but
   preserves a new version of ZRTPSess, so that the user can later re-
   initiate secure mode during the same call without performing another
   Diffie-Hellman calculation using Multistream mode which requires and
   assumes the existence of ZRTPSess with the same value at both ZRTP
   endpoints.  A new key negotiation after a GoClear SHOULD use a
   Multistream Commit message.

      Note: Multistream mode is preferred over a Diffie-Hellman mode
      since this does not require the generation of a new hash chain and
      a new signaling exchange to exchange new hash values.

   Later, at the end of the entire call, ZRTPSess is finally destroyed
   along with the other key material, as described in Section 4.7.3.

4.7.3.  Key Destruction at Termination

   All SRTP session key material MUST be erased by both parties at the
   end of the call.  In particular, the destroyed key material includes
   the SRTP session keys and salts, SRTP master keys and salts, and all
   material sufficient to reconstruct the SRTP keys and salts, including
   ZRTPSess and s0 (although s0 should have been destroyed earlier, in



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   Section 4.6.1).  The only exceptions are the cached shared secrets
   needed for future calls, including rs1, rs2, and pbxsecret.

4.8.  Random Number Generation

   The ZRTP protocol uses random numbers for cryptographic key material,
   notably for the DH secret exponents and nonces, which must be freshly
   generated with each session.  Whenever a random number is needed, all
   of the following criteria must be satisfied:

   Random numbers MUST be freshly generated, meaning that it must not
   have been used in a previous calculation.

   When generating a random number k of L bits in length, k MUST be
   chosen with equal probability from the range of [1 < k < 2^L].

   It MUST be derived from a physical entropy source, such as RF noise,
   acoustic noise, thermal noise, high resolution timings of
   environmental events, or other unpredictable physical sources of
   entropy.  For a detailed explanation of cryptographic grade random
   numbers and guidance for collecting suitable entropy, see RFC 4086
   [RFC4086] and Chapter 10 of Practical Cryptography [Ferguson].  The
   raw entropy must be distilled and processed through a deterministic
   random bit generator (DRBG).  Examples of DRBGs may be found in NIST
   SP 800-90 [SP800-90], and in [Ferguson].  Failure to use true entropy
   from the physical environment as a basis for generating random
   cryptographic key material would lead to a disastrous loss of
   security.

4.9.  ZID and Cache Operation

   Each instance of ZRTP has a unique 96-bit random ZRTP ID or ZID that
   is generated once at installation time.  It is used to look up
   retained shared secrets in a local cache.  A single global ZID for a
   single installation is the simplest way to implement ZIDs.  However,
   it is specifically not precluded for an implementation to use
   multiple ZIDs, up to the limit of a separate one per callee.  This
   then turns it into a long-lived "association ID" that does not apply
   to any other associations between a different pair of parties.  It is
   a goal of this protocol to permit both options to interoperate
   freely.

   Each time a new s0 is calculated, a new retained shared secret rs1 is
   generated and stored in the cache, indexed by the ZID of the other
   endpoint.  This cache updating is described in Section 4.6.1.  For
   the new retained shared secret, each endpoint chooses a cache
   expiration value which is an unsigned 32 bit integer of the number of
   seconds that this secret should be retained in the cache.  The time



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   interval is relative to when the Confirm1 message is sent or
   received.

   The cache intervals are exchanged in the Confirm1 and Confirm2
   messages (Figure 10).  The actual cache interval used by both
   endpoints is the minimum of the values from the Confirm1 and Confirm2
   messages.  A value of 0 seconds means the newly-computed shared
   secret SHOULD NOT be stored in the cache, and if a cache entry
   already exists from an earlier call, the stored cache interval should
   be set to 0.  This means if either Confirm message contains a null
   cache expiration interval, and there is no cache entry already
   defined, no new cache entry is created.  A value of 0xffffffff means
   the secret should be cached indefinitely and is the recommended
   value.  If the ZRTP exchange is Multistream Mode, the field in the
   Confirm1 and Confirm2 is set to 0xffffffff and ignored, and the cache
   is not updated.

   The expiration interval need not be used to force the deletion of a
   shared secret from the cache when the interval has expired.  It just
   means the shared secret MAY be deleted from that cache at any point
   after the interval has expired without causing the other party to
   note it as an unexpected security event when the next key negotiation
   occurs between the same two parties.  This means there need not be
   perfectly synchronized deletion of expired secrets from the two
   caches, and makes it easy to avoid a race condition that might
   otherwise be caused by clock skew.

   If the expiration interval is not properly agreed to by both
   endpoints, it may later result in false alarms of MiTM attacks, due
   to apparent cache mismatches (Section 4.3.3).

4.9.1.  Cacheless implementations

   It is possible to implement a simplified but nonetheless useful
   profile of the ZRTP protocol that does not support any caching of
   shared secrets.  In this case the cache expiration interval should
   always be set to zero, and the SAS Verified (V) flag (Section 7.1)
   should always be set to false.  The users would have to rely
   exclusively on the verbal SAS comparison for every call.  That is,
   unless MiTM protection is provided by the mechanisms in Section 8.1.1
   or Section 7.2, which introduce their own forms of complexity.

   If caching of shared secrets is not supported, it would sacrifice the
   key continuity features, as well as Preshared mode (Section 4.4.3).
   There would also be no PBX trusted MiTM (Section 7.3) features,
   including the PBX security enrollment (Section 7.3.1) mechanism.





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5.  ZRTP Messages

   All ZRTP messages use the message format defined in Figure 2.  All
   word lengths referenced in this specification are 32 bits or 4
   octets.  All integer fields are carried in network byte order, that
   is, most significant byte (octet) first, commonly known as big-
   endian.

     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Not Used (set to zero) |         Sequence Number       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  ZRTP Magic Cookie (0x5a525450)               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Source Identifier                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |           ZRTP Message (length depends on Message Type)       |
    |                            . . .                              |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          CRC (1 word)                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: ZRTP Packet Format

   The Sequence Number is a count that is incremented for each ZRTP
   packet sent.  The count is initialized to a random value.  This is
   useful in estimating ZRTP packet loss and also detecting when ZRTP
   packets arrive out of sequence.

   The ZRTP Magic Cookie is a 32 bit string that uniquely identifies a
   ZRTP packet, and has the value 0x5a525450.

   Source Identifier is the SSRC number of the RTP stream that this ZRTP
   packet relates to.  For cases of forking or forwarding, RTP and hence
   ZRTP may arrive at the same port from several different sources -
   each of these sources will have a different SSRC and may initiate an
   independent ZRTP protocol session.

   This format is clearly identifiable as non-RTP due to the first two
   bits being zero which looks like RTP version 0, which is not a valid
   RTP version number.  It is clearly distinguishable from STUN since
   the magic cookies are different.  The 12 not used bits are set to
   zero and MUST be ignored when received.

   The ZRTP Messages are defined in Figure 3 to Figure 17 and are of
   variable length.



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   The ZRTP protocol uses a 32 bit CRC checksum in each ZRTP packet as
   defined in RFC 3309 [RFC3309] to detect transmission errors.  ZRTP
   packets are typically transported by UDP, which carries its own
   built-in 16-bit checksum for integrity, but ZRTP does not rely on it.
   This is because of the effect of an undetected transmission error in
   a ZRTP message.  For example, an undetected error in the DH exchange
   could appear to be an active man-in-the-middle attack.  The
   psychological effects of a false announcement of this by ZRTP clients
   can not be overstated.  The probability of such a false alarm hinges
   on a mere 16-bit checksum that usually protects UDP packets, so more
   error detection is needed.  For these reasons, this belt-and-
   suspenders approach is used to minimize the chance of a transmission
   error affecting the ZRTP key agreement.

   The CRC is calculated across the entire ZRTP packet shown in
   Figure 2, including the ZRTP Header and the ZRTP Message, but not
   including the CRC field.  If a ZRTP message fails the CRC check, it
   is silently discarded.

5.1.  ZRTP Message Formats

   ZRTP messages are designed to simplify endpoint parsing requirements
   and to reduce the opportunities for buffer overflow attacks (a good
   goal of any security extension should be to not introduce new attack
   vectors).

   ZRTP uses 8 octets (2 words) blocks to encode Message Type. 4 octets
   (1 word) blocks are used to encode Hash Type, Cipher Type, and Key
   Agreement Type, and Authentication Tag. The values in the blocks are
   ASCII strings which are extended with spaces (0x20) to make them the
   desired length.  Currently defined block values are listed in Tables
   1-6 below.

   Additional block values may be defined and used.

   ZRTP uses this ASCII encoding to simplify debugging and make it
   "Wireshark (Ethereal) friendly".

5.1.1.  Message Type Block

   Currently 14 Message Type Blocks are defined - they represent the set
   of ZRTP message primitives.  ZRTP endpoints MUST support the Hello,
   HelloACK, Commit, DHPart1, DHPart2, Confirm1, Confirm2, Conf2ACK,
   SASrelay, RelayACK, Error and ErrorACK message types.  ZRTP endpoints
   MAY support the GoClear and ClearACK messages.  Additional messages
   may be defined in extensions to ZRTP.





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    Message Type Block   |  Meaning
    ---------------------------------------------------
    "Hello   "           |  Hello Message
    ---------------------------------------------------
    "HelloACK"           |  HelloACK Message
    ---------------------------------------------------
    "Commit  "           |  Commit Message
    ---------------------------------------------------
    "DHPart1 "           |  DHPart1 Message
    ---------------------------------------------------
    "DHPart2 "           |  DHPart2 Message
    ---------------------------------------------------
    "Confirm1"           |  Confirm1 Message
    ---------------------------------------------------
    "Confirm2"           |  Confirm2 Message
    ---------------------------------------------------
    "Conf2ACK"           |  Conf2ACK Message
    ---------------------------------------------------
    "Error   "           |  Error Message
    ---------------------------------------------------
    "ErrorACK"           |  ErrorACK Message
    ---------------------------------------------------
    "GoClear "           |  GoClear Message
    ---------------------------------------------------
    "ClearACK"           |  ClearACK Message
    ---------------------------------------------------
    "SASrelay"           |  SASrelay Message
    ---------------------------------------------------
    "RelayACK"           |  RelayACK Message
    ---------------------------------------------------

    Table 1. Message Type Block Values

5.1.2.  Hash Type Block

   Only one Hash Type is currently defined, SHA-256 [FIPS-180-2], and
   all ZRTP endpoints MUST support this hash.  Additional Hash Types can
   be registered and used, such as the NIST SHA-3 hash [SHA-3] when it
   becomes available.  Note that the Hash Type refers to the hash
   algorithm that will be used throughout the ZRTP key exchange, not the
   hash algorithm to be used in the SRTP Authentication Tag.

   ZRTP makes use of HMAC message authentication codes based on the
   negotiated Hash Type.  The HMAC function is defined in [FIPS-198-1].
   Test vectors for HMAC-SHA-256 may be found in [RFC4231].  The HMAC
   function based on the negotiated Hash Type is also used in the ZRTP
   key derivation function (Section 4.5.1).




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    Hash Type Block      |  Meaning
    ---------------------------------------------------
    "S256"               |  SHA-256 Hash defined in FIPS 180-2
    ---------------------------------------------------

    Table 2. Hash Type Block Values

   All hashes and HMACs used throughout the ZRTP protocol will use the
   negotiated Hash Type, except for the special cases noted in
   Section 5.1.2.1.

5.1.2.1.  Implicit Hash and HMAC algorithm

   While most of the HMACs used in ZRTP are defined by the negotiated
   Hash Type (Section 5.1.2), some hashes and HMACs must be precomputed
   prior to negotiations, and thus cannot have their algorithms
   negotiated during the ZRTP exchange.  They are implicitly
   predetermined to use SHA-256 [FIPS-180-2] and HMAC-SHA-256.

   These are the hashes and HMACs that MUST use the Implicit hash and
   HMAC algorithm:

      The hash chain H0-H3 defined in Section 9.
      The HMACs that are keyed by this hash chain, as defined in
      Section 8.1.1.
      The Hello Hash in the a=zrtp-hash attribute defined in
      Section 8.1.

   ZRTP defines a method for negotiating different ZRTP protocol
   versions (Section 4.1.1).  SHA-256 is the Implicit Hash for ZRTP
   protocol version 1.00.  Future ZRTP protocol versions may, if
   appropriate, use another hash algorithm as the Implicit Hash, such as
   the NIST SHA-3 hash [SHA-3] when it becomes available.  For example,
   a future SIP packet may list two a=zrtp-hash SDP attributes, one
   based on SHA-256 for ZRTP version 1.00, and another based on SHA-3
   for ZRTP version 2.00.

5.1.3.  Cipher Type Block

   All ZRTP endpoints MUST support AES-128 (AES1) and MAY support AES-
   256 (AES3) or other Cipher Types.  The choice of the AES key length
   is coupled to the Key Agreement type, as explained in Section 5.1.5.

   The use of AES-128 in SRTP is defined by [RFC3711].  The use of AES-
   256 in SRTP is defined by [I-D.ietf-avt-srtp-big-aes].






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     Cipher Type Block    |  Meaning
    ---------------------------------------------------
    "AES1"                |  AES-CM with 128 bit keys
                          |  as defined in RFC 3711
    ---------------------------------------------------
    "AES3"                |  AES-CM with 256 bit keys
                          |
    ---------------------------------------------------

    Table 3. Cipher Type Block Values

5.1.4.  Auth Tag Block

   All ZRTP endpoints MUST support HMAC-SHA1 authentication, 32 bit and
   80 bit length tags as defined in [RFC3711].

    Auth Tag Block        |  Meaning
    ---------------------------------------------------
    "HS32"                |  HMAC-SHA1 32 bit authentication
                          |  tag as defined in RFC 3711
    ---------------------------------------------------
    "HS80"                |  HMAC-SHA1 80 bit authentication
                          |  tag as defined in RFC 3711
    ---------------------------------------------------

    Table 4. Auth Tag Values

5.1.5.  Key Agreement Type Block

   All ZRTP endpoints MUST support DH3k, SHOULD support Preshared, and
   MAY support EC25, EC38, EC52, and DH2k.

   If a ZRTP endpoint supports multiple concurrent media streams, such
   as audio and video, it MUST support Multistream (Section 4.4.2) mode.
   Also, if a ZRTP endpoint supports the GoClear message
   (Section 4.7.2), it SHOULD support Multistream, to be used if the two
   parties choose to return to the secure state after going Clear (as
   explained in Section 4.7.2.1).

   For Finite Field Diffie-Hellman, ZRTP endpoints MUST use the DH
   parameters defined in RFC 3526 [RFC3526], as follows.  DH3k uses the
   3072-bit MODP group.  DH2k uses the 2048-bit MODP group.  The DH
   generator g is 2.  The random Diffie-Hellman secret exponent SHOULD
   be twice as long as the AES key length.  If AES-128 is used, the DH
   secret value SHOULD be 256 bits long.  If AES-256 is used, the secret
   value SHOULD be 512 bits long.

   If Elliptic Curve DH is used, the ECDH algorithm and key generation



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   is from NIST SP 800-56A [SP800-56A].  The curves used are from NSA
   Suite B [NSA-Suite-B], which uses the same curves as ECDSA defined by
   FIPS 186-3 [FIPS-186-3], and can also be found in RFC 4753 [RFC4753],
   sections 3.1 through 3.3.  The validation procedures are from NIST SP
   800-56A [SP800-56A] section 5.6.2.6, method 3, ECC Partial
   Validation.  Both the X and Y coordinates of the point on the curve
   are sent, in the first and second half of the ECDH public value,
   respectively.

   The choice of AES key length is coupled to the choice of key
   agreement type.  If either EC38 or EC52 is chosen as the key
   agreement, AES-256 (AES3) SHOULD be used.  If DH3K or EC25 is chosen,
   either AES-128 (AES1) or AES-256 (AES3) MAY be used.

   DH2k is intended only for low power applications, and may be used
   with AES-128.  DH2k is not recommended for high security
   applications.  Its security can be augmented by implementing the key
   continuity features (Section 15.1).

   ECDH-521 is not recommended for low power environments, due to
   inconvenient computational delays.  Note that ECDH-521 is not part of
   NSA Suite B.

   ZRTP also defines two non-DH modes, Multistream and Preshared, in
   which the SRTP key is derived from a shared secret and some nonce
   material.

   Table 5 lists the pv length in words and DHPart1 and DHPart2 message
   length in words for each Key Agreement Type Block.






















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    Key Agreement |  pv   | message | Meaning
    Type Block    | words |  words  |
    ---------------------------------------------------
    "DH3k"        |   96  |   117   |  DH mode with p=3072 bit prime
                  |       |         |  as defined in RFC 3526
    ---------------------------------------------------
    "Prsh"        |    -  |     -   |  Preshared Non-DH mode
                  |       |         |
    ---------------------------------------------------
    "Mult"        |    -  |     -   |  Multistream Non-DH mode
                  |       |         |
    ---------------------------------------------------
    "EC25"        |   16  |    37   |  Elliptic Curve DH, P-256
                  |       |         |  per RFC 4753, section 3.1
    ---------------------------------------------------
    "EC38"        |   24  |    45   |  Elliptic Curve DH, P-384
                  |       |         |  per RFC 4753, section 3.2
    ---------------------------------------------------
    "EC52"        |   33  |    54   |  Elliptic Curve DH, P-521
                  |       |         |  per RFC 4753, section 3.3
     ---------------------------------------------------
    "DH2k"        |   64  |    85   |  DH mode with p=2048 bit prime
                  |       |         |  as defined in RFC 3526
   ---------------------------------------------------

    Table 5. Key Agreement Type Block Values

5.1.6.  SAS Type Block

   The SAS Type determines how the SAS is rendered to the user so that
   the user may verbally compare it with his partner over the voice
   channel.  This allows detection of a man-in-the-middle (MiTM) attack.

   All ZRTP endpoints MUST support the base32 and MAY support the
   base256 rendering schemes for the Short Authentication String, and
   other SAS rendering schemes.  The ZRTP SAS rendering schemes are
   described in Section 7.

     SAS Type Block       |  Meaning
    ---------------------------------------------------
    "B32 "                |  Short Authentication String using
                          |  base32 encoding
    ---------------------------------------------------
    "B256"                |  Short Authentication String using
                          |  base256 encoding (PGP Word List)
    ---------------------------------------------------

    Table 6. SAS Type Block Values



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5.1.7.  Signature Type Block

   The signature type block is a 4 octet (1 word) block used to
   represent the signature algorithm discussed in Section 7.2.
   Suggested signature algorithms and key lengths are a future subject
   of standardization.

5.2.  Hello message

   The Hello message has the format shown in Figure 3.  The Hello ZRTP
   message begins with the preamble value 0x505a then a 16 bit length in
   32 bit words.  This length includes only the ZRTP message (including
   the preamble and the length) but not the ZRTP header or CRC.

   Next is the Message Type Block and a 4 character string containing
   the version (ver) of the ZRTP protocol which is "1.00" for this
   specification.  Next is the Client Identifier string (cid) which is 4
   words long and identifies the vendor and release of the ZRTP
   software.  The 256-bit hash image H3 is defined in Section 9.  The
   next parameter is the ZID, the 96 bit long unique identifier for the
   ZRTP endpoint.

   The next four bits contains flag bits.  The MiTM flag (M) is a
   Boolean that is set to true if and only if this Hello message is sent
   from a device, usually a PBX, that has the capability to send an
   SASrelay message (Section 5.13).  The Passive flag (P) is a Boolean
   normally set to False.  A ZRTP endpoint which is configured to never
   initiate secure sessions is regarded as passive, and would set the P
   bit to True.  The next 8 bits are unused and SHOULD be set to zero
   when sent and MUST be ignored on receipt.

   Next is a list of supported Hash algorithms, Cipher algorithms, SRTP
   Auth Tag types, Key Agreement types, and SAS types.  The number of
   listed algorithms are listed for each type: hc=hash count, cc=cipher
   count, ac=auth tag count, kc=key agreement count, and sc=sas count.
   The values for these algorithms are defined in Tables 2, 3, 4, 5, and
   6.  A count of zero means that only the mandatory to implement
   algorithms are supported.  Mandatory algorithms MAY be included in
   the list.  The order of the list indicates the preferences of the
   endpoint.  If a mandatory algorithm is not included in the list, it
   is added to the end of the list for preference.

      Note: Implementers are encouraged to keep these algorithm lists
      small - the list does not need to include every cipher and hash
      supported, just the ones the endpoint would prefer to use for this
      ZRTP exchange.

   The 64-bit HMAC at the end of the message is computed across the



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   whole message, not including the HMAC.  The HMAC key is the sender's
   H2 (defined in Section 9), and thus the HMAC cannot be checked by the
   receiving party until the sender's H2 value is known to the receiving
   party later in the protocol.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|             length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Message Type Block="Hello   " (2 words)            |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   version="1.00" (1 word)                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                Client Identifier (4 words)                    |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                   Hash image H3 (8 words)                     |
    |                             . . .                             |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                         ZID  (3 words)                        |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|0|M|P| unused (zeros)|  hc   |  cc   |  ac   |  kc   |  sc   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 hash algorthms (0 to 7 values)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               cipher algorthms (0 to 7 values)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  auth tag types (0 to 7 values)               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               key agreement types (0 to 7 values)             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    SAS types (0 to 7 values)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         HMAC (2 words)                        |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Hello message format





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5.3.  HelloACK message

   The HelloACK message is used to stop retransmissions of a Hello
   message.  A HelloACK is sent regardless if the version number in the
   Hello is supported or the algorithm list supported.  The receipt of a
   HelloACK stops retransmission of the Hello message.  The format is
   shown in the Figure below.  Note that a Commit message can be sent in
   place of a HelloACK by an Initiator.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|         length=3 words        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Message Type Block="HelloACK" (2 words)          |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: HelloACK message format

5.4.  Commit message

   The Commit message is sent to initiate the key agreement process
   after both sides have received a Hello message, which means it can
   only be sent after receiving both a Hello message and a HelloACK
   message.  There are three subtypes of Commit messages, whose formats
   are shown in Figure 5, Figure 6, and Figure 7.

   The Commit message contains the Message Type Block, then the 256-bit
   hash image H2 which is defined in Section 9.  The next parameter is
   the initiator's ZID, the 96 bit long unique identifier for the ZRTP
   endpoint.

   Next is a list of algorithms selected by the initiator (hash, cipher,
   auth tag type, key agreement, sas type).  For a DH Commit, the hash
   value hvi is a hash of the DHPart2 of the Initiator and the
   Responder's Hello message, as explained in Section 4.4.1.1.

   The 64-bit HMAC at the end of the message is computed across the
   whole message, not including the HMAC.  The HMAC key is the sender's
   H1 (defined in Section 9), and thus the HMAC cannot be checked by the
   receiving party until the sender's H1 value is known to the receiving
   party later in the protocol.








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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=29 words        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="Commit  " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                   Hash image H2 (8 words)                     |
       |                             . . .                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                         ZID  (3 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       hash algorihm                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      cipher algorihm                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       auth tag type                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     key agreement type                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SAS type                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                       hvi (8 words)                           |
       |                           . . .                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 5: DH Commit message format














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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=25 words        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="Commit  " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                   Hash image H2 (8 words)                     |
       |                             . . .                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                         ZID  (3 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       hash algorihm                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      cipher algorihm                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       auth tag type                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  key agreement type = "Mult"                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SAS type                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                       nonce (4 words)                         |
       |                           . . .                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 6: Multistream Commit message format














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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=27 words        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="Commit  " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                   Hash image H2 (8 words)                     |
       |                             . . .                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                         ZID  (3 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       hash algorihm                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      cipher algorihm                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       auth tag type                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   key agreement type = "Prsh"                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         SAS type                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                       nonce (4 words)                         |
       |                           . . .                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        keyID (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7: Preshared Commit message format

5.5.  DHPart1 message

   The DHPart1 message begins the DH exchange.  The format is shown in
   Figure 8 below.  The DHPart1 message is sent by the Responder if a
   valid Commit message is received from the Initiator.  The length of
   the pvr value and the length of the DHPart1 message depends on the
   Key Agreement Type chosen.  This information is contained in Table 5.



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   Note that for both Multistream and Preshared modes, no DHPart1 or
   DHPart2 message will be sent.

   The 256-bit hash image H1 is defined in Section 9.

   The next four parameters are HMACs of potential shared secrets used
   in generating the ZRTP secret.  The first two, rs1IDr and rs2IDr, are
   the HMACs of the responder's two retained shared secrets, truncated
   to 64 bits.  Next is auxsecretIDr, the HMAC of the responder's
   auxsecret (defined in Section 4.3), truncated to 64 bits.  The last
   parameter is the HMAC of the trusted MiTM PBX shared secret
   pbxsecret, defined in Section 7.3.1.  The Message format for the
   DHPart1 message is shown in Figure 8.

   The 64-bit HMAC at the end of the message is computed across the
   whole message, not including the HMAC.  The HMAC key is the sender's
   H0 (defined in Section 9), and thus the HMAC cannot be checked by the
   receiving party until the sender's H0 value is known to the receiving
   party later in the protocol.
































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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|   length=depends on KA Type   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="DHPart1 " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                   Hash image H1 (8 words)                     |
       |                             . . .                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        rs1IDr (2 words)                       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        rs2IDr (2 words)                       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     auxsecretIDr (2 words)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     pbxsecretIDr (2 words)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                  pvr (length depends on KA Type)              |
       |                               . . .                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 8: DHPart1 message format

5.6.  DHPart2 message

   The DHPart2 message completes the DH exchange.  A DHPart2 message is
   sent by the Initiator if a valid DHPart1 message is received from the
   Responder.  The length of the pvr value and the length of the DHPart2
   message depends on the Key Agreement Type chosen.  This information
   is contained in Table 5.  Note that for both Multistream and
   Preshared modes, no DHPart1 or DHPart2 message will be sent.

   The 256-bit hash image H1 is defined in Section 9.

   The next four parameters are HMACs of potential shared secrets used



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   in generating the ZRTP secret.  The first two, rs1IDi and rs2IDi, are
   the HMACs of the initiator's two retained shared secrets, truncated
   to 64 bits.  Next is auxsecretIDi, the HMAC of the initiator's
   auxsecret (defined in Section 4.3), truncated to 64 bits.  The last
   parameter is the HMAC of the trusted MiTM PBX shared secret
   pbxsecret, defined in Section 7.3.1.  The message format for the
   DHPart2 message is shown in Figure 9.

   The 64-bit HMAC at the end of the message is computed across the
   whole message, not including the HMAC.  The HMAC key is the sender's
   H0 (defined in Section 9), and thus the HMAC cannot be checked by the
   receiving party until the sender's H0 value is known to the receiving
   party later in the protocol.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|   length=depends on KA Type   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="DHPart2 " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                   Hash image H1 (8 words)                     |
       |                             . . .                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        rs1IDi (2 words)                       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        rs2IDi (2 words)                       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     auxsecretIDi (2 words)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     pbxsecretIDi (2 words)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                  pvi (length depends on KA Type)              |
       |                               . . .                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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                     Figure 9: DHPart2 message format

5.7.  Confirm1 and Confirm2 messages

   The Confirm1 message is sent by the Responder in response to a valid
   DHPart2 message after the SRTP session key and parameters have been
   negotiated.  The Confirm2 message is sent by the Initiator in
   response to a Confirm1 message.  The format is shown in Figure 10
   below.  The message contains the Message Type Block "Confirm1" or
   "Confirm2".  Next is the HMAC, a keyed hash over encrypted part of
   the message (shown enclosed by "===" in Figure 10).  This HMAC is
   keyed and computed according to Section 4.6.  The next 16 octets
   contain the CFB Initialization Vector.  The rest of the message is
   encrypted using CFB and protected by the HMAC.

   The first field inside the encrypted region is the hash pre-image H0,
   which is defined in detail in Section 9.

   The next 15 bits are not used and SHOULD be set to zero when sent and
   MUST be ignored when received in Confirm1 or Confirm2 messages.

   The next 9 bits contain the signature length.  If no SAS signature
   (described in Section 7.2) is present, all bits are set to zero.  The
   signature length is in words and includes the signature type block.
   If the calculated signature octet count is not a multiple of 4, zeros
   are added to pad it out to a word boundary.  If no signature block is
   present, the overall length of the Confirm1 or Confirm2 Message will
   be set to 19 words.

   The next 8 bits are used for flags.  Undefined flags are set to zero
   and ignored.  Four flags are currently defined.  The PBX Enrollment
   flag (E) is a Boolean bit defined in Section 7.3.1.  The SAS Verified
   flag (V) is a Boolean bit defined in Section 7.1.  The Allow Clear
   flag (A) is a Boolean bit defined in Section 4.7.2.  The Disclosure
   Flag (D) is a Boolean bit defined in Section 11.  The cache
   expiration interval is defined in Section 4.9.

   If the signature length (in words) is non-zero, a signature type
   block will be present along with a signature block.  Next is the
   signature block.  The signature block includes the key used to
   generate the signature (Section 7.2).

   CFB [SP800-38A] mode is applied with a feedback length of 128-bits, a
   full cipher block, and the final block is truncated to match the
   exact length of the encrypted data.  The CFB Initialization Vector is
   a 128 bit random nonce.  The block cipher algorithm and the key size
   is the same as what was negotiated for the media encryption.  CFB is
   used to encrypt the part of the Confirm1 message beginning after the



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   CFB IV to the end of the message (the encrypted region is enclosed by
   "======" in Figure 10).

   The responder uses the zrtpkeyr to encrypt the Confirm1 message.  The
   initiator uses the zrtpkeyi to encrypt the Confirm2 message.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|         length=variable       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Message Type Block="Confirm1" or "Confirm2" (2 words)    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                CFB Initialization Vector (4 words)            |
       |                                                               |
       |                                                               |
       +===============================================================+
       |                                                               |
       |                 Hash pre-image H0 (8 words)                   |
       |                             . . .                             |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Unused (15 bits of zeros)   | sig len (9 bits)|0 0 0 0|E|V|A|D|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              cache expiration interval (1 word)               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      optional signature type block (1 word if present)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |           optional signature block (variable length)          |
       |                            . . .                              |
       |                                                               |
       |                                                               |
       +===============================================================+

              Figure 10: Confirm1 and Confirm2 message format

5.8.  Conf2ACK message

   The Conf2ACK message is sent by the Responder in response to a valid
   Confirm2 message.  The message format for the Conf2ACK is shown in
   the Figure below.  The receipt of a Conf2ACK stops retransmission of
   the Confirm2 message.  Note that the first SRTP media (with a valid



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   SRTP auth tag) from the responder also stops retransmission of the
   Confirm2 message.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|         length=3 words        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="Conf2ACK" (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 11: Conf2ACK message format

5.9.  Error message

   The Error message is sent to terminate an in-process ZRTP key
   agreement exchange due to an error.  The format is shown in the
   Figure below.  The use of the Error message is described in
   Section 4.7.1.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=4 words         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="Error   " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Integer Error Code (1 word)                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 12: Error message format

   Defined hexadecimal values for the Error Code are listed in Table 7.















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   Error Code |  Meaning
   -----------------------------------------------------------
    0x10      | Malformed packet (CRC OK, but wrong structure)
   -----------------------------------------------------------
    0x20      | Critical software error
   -----------------------------------------------------------
    0x30      | Unsupported ZRTP version
   -----------------------------------------------------------
    0x40      | Hello components mismatch
   -----------------------------------------------------------
    0x51      | Hash type not supported
   -----------------------------------------------------------
    0x52      | Cipher type not supported
   -----------------------------------------------------------
    0x53      | Public key exchange not supported
   -----------------------------------------------------------
    0x54      | SRTP auth. tag not supported
   -----------------------------------------------------------
    0x55      | SAS scheme not supported
   -----------------------------------------------------------
    0x56      | No shared secret available, DH mode required
   -----------------------------------------------------------
    0x61      | DH Error: bad pvi or pvr ( == 1, 0, or p-1)
   -----------------------------------------------------------
    0x62      | DH Error: hvi != hashed data
   -----------------------------------------------------------
    0x63      | Received relayed SAS from untrusted MiTM
   -----------------------------------------------------------
    0x70      | Auth. Error: Bad Confirm pkt HMAC
   -----------------------------------------------------------
    0x80      | Nonce reuse
   -----------------------------------------------------------
    0x90      | Equal ZIDs in Hello
   -----------------------------------------------------------
    0xA0      | Service unavailable
   -----------------------------------------------------------
    0x100     | GoClear packet received, but not allowed
   -----------------------------------------------------------

   Table 7. ZRTP Error Codes

5.10.  ErrorACK message

   The ErrorACK message is sent in response to an Error message.  The
   receipt of an ErrorACK stops retransmission of the Error message.
   The format is shown in the Figure below.





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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=3 words         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="ErrorACK" (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 13: ErrorAck message format

5.11.  GoClear message

   Support for the GoClear message is OPTIONAL in the protocol, and it
   is sent to switch from SRTP to RTP.  The format is shown in the
   Figure below.  The clear_hmac is used to authenticate the GoClear
   message so that bogus GoClear messages introduced by an attacker can
   be detected and discarded.  The use of GoClear is described in
   Section 4.7.2.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=5 words         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="GoClear " (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       clear_hmac (2 words)                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 14: GoClear message format

5.12.  ClearACK message

   Support for the ClearACK message is OPTIONAL in the protocol, and it
   is sent to acknowledge receipt of a GoClear.  A ClearACK is only sent
   if the clear_hmac from the GoClear message is authenticated.
   Otherwise, no response is returned.  The format is shown in the
   Figure below.










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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|        length=3 words         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="ClearACK" (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 15: ClearAck message format

5.13.  SASrelay message

   The SASrelay message is sent by a trusted Man in The Middle (MiTM),
   most often a PBX.  It is not sent as a response to a packet, but is
   sent as a self-initiated packet by the trusted MiTM.  It can only be
   sent after the rest of the ZRTP key negotiations have completed,
   after the Confirm packets and their ACKs.  It can only be sent after
   the trusted MiTM has finished key negotiations with the other party,
   because it is the other party's SAS that is being relayed.  It is
   sent with retry logic until a RelayACK message (Section 5.14) is
   received or the retry schedule has been exhausted.

   If a device, usually a PBX, sends an SASrelay message, it MUST have
   previously declared itself as a MiTM device by setting the MiTM (M)
   flag in the Hello message (Section 5.2).  If the receiver of the
   SASrelay message did not previously receive a Hello message with the
   MiTM (M) flag set, the Relayed SAS SHOULD NOT be rendered.  A
   RelayACK is still sent, but no Error message is sent.

   The SASrelay message format is shown in Figure 16 below.  The message
   contains the Message Type Block "SASrelay".  Next is the HMAC, a
   keyed hash over encrypted part of the message (shown enclosed by
   "===" in Figure 16).  This HMAC is keyed the same way as the HMAC in
   the Confirm messages (see Section 4.6).  The next 16 octets contain
   the CFB Initialization Vector.  The rest of the message is encrypted
   using CFB and protected by the HMAC.

   The next 15 bits are not used and SHOULD be set to zero when sent and
   MUST be ignored when received in SASrelay messages.

   The next 9 bits contain the signature length.  The trusted MiTM MAY
   compute a digital signature on the SAS hash, as described in
   Section 7.2, using a persistant signing key owned by the trusted
   MiTM.  If no SAS signature is present, all bits are set to zero.  The
   signature length is in words and includes the signature type block.
   If the calculated signature octet count is not a multiple of 4, zeros
   are added to pad it out to a word boundary.  If no signature block is



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   present, the overall length of the SASrelay Message will be set to 12
   words.

   The next 8 bits are used for flags.  Undefined flags are set to zero
   and ignored.  Three flags are currently defined.  The Disclosure Flag
   (D) is a Boolean bit defined in Section 11.  The Allow Clear flag (A)
   is a Boolean bit defined in Section 4.7.2.  The SAS Verified flag (V)
   is a Boolean bit defined in Section 7.1.  These flags are updated
   values to the same flags provided earlier in the Confirm packet, but
   they are updated to reflect the new flag information relayed by the
   PBX from the other party.

   The next 32 bit word contains the rendering scheme for the relayed
   sasvalue, which will be the same rendering scheme used by the other
   party on the other side of the trusted MiTM.  Section 7.3 describes
   how the PBX determines whether the ZRTP client regards the PBX as a
   trusted MiTM.  If the PBX determines that the ZRTP client trusts the
   PBX, the next 32 bit word contains the binary sasvalue relayed from
   the other party.  If this SASrelay packet is being sent to a ZRTP
   client that does not trust this MiTM, the next 32 bit word will be
   ignored by the recipient and should be set to zero by the PBX.

   If the signature length (in words) is non-zero, a signature type
   block will be present along with a signature block.  Next is the
   signature block.  The signature block includes the key used to
   generate the signature (Section 7.2).

   CFB [SP800-38A] mode is applied with a feedback length of 128-bits, a
   full cipher block, and the final block is truncated to match the
   exact length of the encrypted data.  The CFB Initialization Vector is
   a 128 bit random nonce.  The block cipher algorithm and the key size
   is the same as what was negotiated for the media encryption.  CFB is
   used to encrypt the part of the SASrelay message beginning after the
   CFB IV to the end of the message (the encrypted region is enclosed by
   "======" in Figure 16).

   Depending on whether the trusted MiTM had taken the role of the
   initiator or the responder during the ZRTP key negotiation, the
   SASrelay message is encrypted with zrtpkeyi or zrtpkeyr.












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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|         length=variable       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Message Type Block="SASrelay" (2 words)           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         HMAC (2 words)                        |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                CFB Initialization Vector (4 words)            |
       |                                                               |
       |                                                               |
       +===============================================================+
       | Unused (15 bits of zeros)   | sig len (9 bits)|0 0 0 0|0|V|A|D|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         rendering scheme of relayed sasvalue (1 word)         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Trusted MiTM relayed sasvalue (1 word)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      optional signature type block (1 word if present)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |           optional signature block (variable length)          |
       |                            . . .                              |
       |                                                               |
       |                                                               |
       +===============================================================+

                    Figure 16: SASrelay message format

5.14.  RelayACK message

   The RelayACK message is sent in response to a valid SASrelay message.
   The message format for the RelayACK is shown in the Figure below.
   The receipt of a RelayACK stops retransmission of the SASrelay
   message.












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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|         length=3 words        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Message Type Block="RelayACK" (2 words)          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 17: RelayACK message format


6.  Retransmissions

   ZRTP uses two retransmission timers T1 and T2.  T1 is used for
   retransmission of Hello messages, when the support of ZRTP by the
   other endpoint may not be known.  T2 is used in retransmissions of
   all the other ZRTP messages.

   All message retransmissions MUST be identical to the initial message
   including nonces, public values, etc; otherwise, hashes of the
   message sequences may not agree.

   Practical experience has shown that RTP packet loss at the start of
   an RTP session can be extremely high.  Since the entire ZRTP message
   exchange occurs during this period, the defined retransmission scheme
   is defined to be aggressive.  Since ZRTP packets with the exception
   of the DHPart1 and DHPart2 messages are small, this should have
   minimal effect on overall bandwidth utilization of the media session.

   ZRTP endpoints MUST NOT exceed the bandwidth of the resulting media
   session as determined by the offer/answer exchange in the signaling
   layer.

   Hello ZRTP messages are retransmitted at an interval that starts at
   T1 seconds and doubles after every retransmission, capping at 200ms.
   T1 has a recommended initial value of 50 ms.  A Hello message is
   retransmitted 20 times before giving up, which means the entire retry
   schedule for Hello messages is exhausted after 3.75 seconds (50 + 100
   + 18*200 ms).  Retransmission of a Hello ends upon receipt of a
   HelloACK or Commit message.

   The post-Hello ZRTP messages are retransmitted only by the session
   initiator - that is, only Commit, DHPart2, and Confirm2 are
   retransmitted if the corresponding message from the responder,
   DHPart1, Confirm1, and Conf2ACK, are not received.  Note that the
   Confirm2 message retransmission can also be stopped by receiving the
   first SRTP media (with a valid SRTP auth tag) from the responder.



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   The GoClear, Error, and SASrelay messages may be initiated and
   retransmitted by either party, and responded to by the other party,
   regardless of which party is the overall session initiator.  They are
   retransmitted if the corresponding response message ClearACK,
   ErrorACK, and RelayACK, are not received.

   Non-Hello ZRTP messages are retransmitted at an interval that starts
   at T2 seconds and doubles after every retransmission, capping at
   600ms.  T2 has a recommended initial value of 150 ms.  Each non-Hello
   message is retransmitted 10 times before giving up, which means the
   entire retry schedule is exhausted after 5.25 seconds (150 + 300 +
   8*600 ms).  Only the initiator performs retransmissions.  Each
   message has a response message that stops retransmissions, as shown
   below in Table 8.  The higher values of T2 means that retransmissions
   will likely only occur with packet loss.

   These recommended retransmission intervals are designed for a typical
   broadband Internet connection.  In some high latency communication
   channels, such as those provided by some mobile phone environments or
   geostationary satellites, the initial value for the T1 or T2
   retransmission timer should be increased to be no less than the round
   trip time provided by the communications channel.  It should take
   into account the time required to transmit the entire message and the
   entire reply.

       Message      Acknowledgement Message
       -------      -----------------------
       Hello        HelloACK or Commit
       Commit       DHPart1 or Confirm1
       DHPart2      Confirm1
       Confirm2     Conf2ACK or SRTP media
       GoClear      ClearACK
       Error        ErrorACK
       SASrelay     RelayACK

      Table 8. Retransmitted ZRTP Messages and Responses


7.  Short Authentication String

   This section will discuss the implementation of the Short
   Authentication String, or SAS in ZRTP.  The SAS can be verbally
   verified by the human users reading the string aloud, or by
   validating an OPTIONAL digital signature (described in Section 7.2)
   exchanged in the Confirm1 or Confirm2 messages.

   The use of hash commitment in the DH exchange (Section 4.4.1.1)
   constrains the attacker to only one guess to generate the correct SAS



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   in his attack, which means the SAS can be quite short.  A 16-bit SAS,
   for example, provides the attacker only one chance out of 65536 of
   not being detected.

   The rendering of the SAS value to the user depends on the SAS Type
   agreed upon in the Commit message.  For the SAS Type of base32, the
   leftmost 20 bits of the 32-bit sasvalue are rendered as a form of
   base32 encoding known as z-base-32 [z-base-32].  The purpose of
   z-base-32 is to represent arbitrary sequences of octets in a form
   that is as convenient as possible for human users to manipulate.  As
   a result, the choice of characters is slightly different from base32
   as defined in RFC 3548.  The leftmost 20 bits of the sasvalue results
   in four base32 characters which are rendered to both ZRTP endpoints.
   For the SAS Type of base256, the leftmost 16 bits of the 32-bit
   sasvalue are rendered using the PGP Wordlist [pgpwordlist]
   [Juola1][Juola2].  Other SAS Types may be defined to render the SAS
   value in other ways.

   The SAS SHOULD be rendered to the user for authentication.

   The SAS is not treated as a secret value, but it must be compared to
   see if it matches at both ends of the communications channel.  The
   two users read it aloud to their partners to see if it matches.  This
   allows detection of a man-in-the-middle (MiTM) attack.

   There is only one SAS value computed per call.  That is the SAS value
   for the first media stream established, which computes the ZRTPSess
   key, using DH mode.  The ZRTPSess key is used to compute the SAS, as
   well as the SRTP session keys for each additional media stream in
   Multistream mode.  This SAS applies to all media streams for the same
   call.

7.1.  SAS Verified Flag

   The SAS Verified flag (V) is set based on the user indicating that
   SAS comparison has been successfully performed.  The SAS Verified
   flag is exchanged securely in the Confirm1 and Confirm2 messages
   (Figure 10) of the next session.  In other words, each party sends
   the SAS Verified flag from the previous session in the Confirm
   message of the current session.  It is perfectly reasonable to have a
   ZRTP endpoint that never sets the SAS Verified flag, because it would
   require adding complexity to the user interface to allow the user to
   set it.  The SAS Verified flag is not required to be set, but if it
   is available to the client software, it allows for the possibility
   that the client software could render to the user that the SAS verify
   procedure was carried out in a previous session.

   Regardless of whether there is a user interface element to allow the



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   user to set the SAS Verified flag, it is worth caching a shared
   secret, because doing so reduces opportunities for an attacker in the
   next call.

   If at any time the users carry out the SAS comparison procedure, and
   it actually fails to match, then this means there is a very
   resourceful man-in-the-middle.  If this is the first call, the MiTM
   was there on the first call, which is impressive enough.  If it
   happens in a later call, it also means the MiTM must also know the
   cached shared secret, because you could not have carried out any
   voice traffic at all unless the session key was correctly computed
   and is also known to the attacker.  This implies the MiTM must have
   been present in all the previous sessions, since the initial
   establishment of the first shared secret.  This is indeed a
   resourceful attacker.  It also means that if at any time he ceases
   his participation as a MiTM on one of your calls, the protocol will
   detect that the cached shared secret is no longer valid -- because it
   was really two different shared secrets all along, one of them
   between Alice and the attacker, and the other between the attacker
   and Bob. The continuity of the cached shared secrets make it possible
   for us to detect the MiTM when he inserts himself into the ongoing
   relationship, as well as when he leaves.  Also, if the attacker tries
   to stay with a long lineage of calls, but fails to execute a DH MiTM
   attack for even one missed call, he is permanently excluded.  He can
   no longer resynchronize with the chain of cached shared secrets.

   Some sort of user interface element (maybe a checkbox) is needed to
   allow the user to tell the software the SAS verify was successful,
   causing the software to set the SAS Verified flag (V), which
   (together with our cached shared secret) obviates the need to perform
   the SAS procedure in the next call.  An additional user interface
   element can be provided to let the user tell the software he detected
   an actual SAS mismatch, which indicates a MiTM attack.  The software
   can then take appropriate action, clearing the SAS Verified flag, and
   erase the cached shared secret from this session.  It is up to the
   implementer to decide if this added user interface complexity is
   warranted.

   If the SAS matches, it means there is no MiTM, which also implies it
   is now safe to trust a cached shared secret for later calls.  If
   inattentive users don't bother to check the SAS, it means we don't
   know whether there is or is not a MiTM, so even if we do establish a
   new cached shared secret, there is a risk that our potential attacker
   may have a subsequent opportunity to continue inserting himself in
   the call, until we finally get around to checking the SAS.  If the
   SAS matches, it means no attacker was present for any previous
   session since we started propagating cached shared secrets, because
   this session and all the previous sessions were also authenticated



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   with a continuous lineage of shared secrets.

7.2.  Signing the SAS

   In some applications, it may be hard to arrange for two human users
   to verbally compare the SAS.  To handle these cases, ZRTP allows for
   an OPTIONAL signature feature, which allows the SAS to be checked
   without human participation.  The SAS MAY be signed and the signature
   sent inside the Confirm1, Confirm2 (Figure 10), or SASrelay
   (Figure 16) messages.  The signature algorithm, length of the
   signature and the key used to create the signature are all sent along
   with the signature.  The key types and signature algorithms are for
   future study.  The signature is calculated over the entire SAS hash
   result (sashash) that was truncated down to derive the sasvalue.  The
   signatures exchanged in the encrypted Confirm1, Confirm2, or SASrelay
   messages MAY be used to authenticate the ZRTP exchange.

7.3.  Relaying the SAS through a PBX

   ZRTP is designed to use end-to-end encryption.  The two parties'
   verbal comparison of the short authentication string (SAS) depends on
   this assumption.  But in some PBX environments, such as Asterisk,
   there are usage scenarios that have the PBX acting as a trusted man-
   in-the-middle (MiTM), which means there are two back-to-back ZRTP
   connections with separate session keys and separate SAS's.

   For example, imagine that Bob has a ZRTP-enabled VoIP phone that has
   been registered with his company's PBX, so that it is regarded as an
   extension of the PBX.  Alice, whose phone is not associated with the
   PBX, might dial the PBX from the outside, and a ZRTP connection is
   negotiated between her phone and the PBX.  She then selects Bob's
   extension from the company directory in the PBX.  The PBX makes a
   call to Bob's phone (which might be offsite, many miles away from the
   PBX through the Internet) and a separate ZRTP connection is
   negotiated between the PBX and Bob's phone.  The two ZRTP sessions
   have different session keys and different SAS's, which would render
   the SAS useless for verbal comparison between Alice and Bob. They
   might even mistakenly believe that a wiretapper is present because of
   the SAS mismatch, causing undue alarm.

   ZRTP has a mechanism for solving this problem by having the PBX relay
   the Alice/PBX SAS to Bob, sending it through to Bob in a special
   SASrelay packet as defined in Section 5.13, which is sent after the
   PBX/Bob ZRTP negotiation is complete, after the Confirm packets.
   Only the PBX, acting as a special trusted MiTM (trusted by the
   recipient of the SAS relay packet), will relay the SAS.  The SASrelay
   packet protects the relayed SAS from tampering via an included HMAC,
   similar to how the Confirm packet is protected.  Bob's ZRTP-enabled



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   phone accepts the relayed SAS for rendering only because Bob's phone
   had previously been configured to trust the PBX.  This special
   trusted relationship with the PBX can be established through a
   special security enrollment procedure.  After that enrollment
   procedure, the PBX is treated by Bob as a special trusted MiTM.  This
   results in Alice's SAS being rendered to Bob, so that Alice and Bob
   may verbally compare them and thus prevent a MiTM attack by any other
   untrusted MiTM.

   A real bad-guy MiTM cannot exploit this protocol feature to mount a
   MiTM attack and relay Alice's SAS to Bob, because Bob has not
   previously carried out a special registration ritual with the bad
   guy.  The relayed SAS would not be rendered by Bob's phone, because
   it did not come from a trusted PBX.  The recognition of the special
   trust relationship is achieved with the prior establishment of a
   special shared secret between Bob and his PBX, which is called
   pbxsecret (defined in Section 7.3.1), also known as the trusted MiTM
   key.

   The trusted MiTM key can be stored in a special cache at the time of
   the initial enrollment (which is carried out only once for Bob's
   phone), and Bob's phone associates this key with the ZID of the PBX,
   while the PBX associates it with the ZID of Bob's phone.  After the
   enrollment has established and stored this trusted MiTM key, it can
   be detected during subsequent ZRTP call negotiations between the PBX
   and Bob's phone, because the PBX and the phone MUST pass the hash of
   the trusted MiTM key in the DH packet.  It is then used as part of
   the key agreement to calculate s0.

   During a key agreement with two other ZRTP endpoints, the PBX may
   have a shared trusted MiTM key with both endpoints, only one
   endpoint, or neither endpoint.  If the PBX has a shared trusted MiTM
   key with neither endpoint, the PBX SHOULD NOT relay the SAS.  If the
   PBX has a shared trusted MiTM key with only one endpoint, the PBX
   SHOULD relay the SAS from one party the other by sending an SASrelay
   message to the endpoint that it shares a trusted MiTM key.  If the
   PBX has a shared trusted MiTM key with both endpoints, the PBX SHOULD
   relay the SAS from one party the other by sending an SASrelay message
   to only one of the endpoints.

      Note: In the case of sharing trusted MiTM key with both endpoints,
      it does not matter which endpoint receives the relayed SAS as long
      as only one endpoint receives it.

   The PBX can determine whether it is trusted by the ZRTP user agent of
   the caller or callee.  The presence of a shared trusted MiTM key in
   the key negotiation sequence indicates that the phone has been
   enrolled with this PBX and therefore trusts it to act as a trusted



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   MiTM.  The PBX SHOULD relay the SAS from the other party in this
   case.

   The relayed SAS fields contain the SAS rendering type and the binary
   32-bit sasvalue.  The receiver absolutely MUST NOT render the relayed
   SAS if it does not come from a specially trusted ZRTP endpoint.  The
   security of the ZRTP protocol depends on not rendering a relayed SAS
   from an untrusted MiTM, because it may be relayed by a MiTM attacker.
   See the SASrelay packet definition (Figure 16) for further details.

   To ensure that both Alice and Bob will use the same SAS rendering
   scheme after the keys are negotiated, the PBX also sends the SASrelay
   message to the unenrolled party (which does not regard this PBX as a
   trusted MiTM), conveying the SAS rendering scheme, but not the SAS
   value, which it sets to zero.  The unenrolled party will ignore the
   relayed SAS field, but will use the specified SAS rendering scheme.

   The next section describes the initial enrollment procedure that
   establishes a special shared secret between the PBX and Bob's phone,
   a trusted MiTM key, so that the phone will learn to recognize the PBX
   as a trusted MiTM.

7.3.1.  PBX Enrollment and the PBX Enrollment Flag

   Both the PBX and the endpoint need to know when enrollment is taking
   place.  One way of doing this is to setup an enrollment extension on
   the PBX which a newly configured endpoint would call and establish a
   ZRTP session.  The PBX would then play audio media that offers the
   user an opportunity to configure his phone to trust this PBX as a
   trusted MiTM.  The PBX calculates and stores the trusted MiTM shared
   secret in its cache and associates it with this phone, indexed by the
   phone's ZID.  The trusted MiTM PBX shared secret is derived from
   ZRTPSess via the ZRTP key derivation function (Section 4.5.1) in this
   manner:

      pbxsecret = KDF(ZRTPSess, "Trusted MiTM key", negotiated hash
      length)

   The PBX signals the enrollment process by setting the PBX Enrollment
   flag (E) in the Confirm message (Figure 10).  This flag is used to
   trigger the ZRTP endpoint's user interface to prompt the user if they
   want to trust this PBX and calculate and store the pbxsecret in the
   cache.  If the user decides to respond by activating the appropriate
   user interface element (a menu item, checkbox, or button), his ZRTP
   user agent calculates pbxsecret using the same formula and saves it
   in a special cache entry associated with this PBX.

   If the user elects not to enroll, perhaps because he dialed a wrong



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   number or does not yet feel comfortable with this PBX, he can simply
   hang up and not save the pbxsecret in his cache.  The PBX will have
   it saved in the PBX cache, but that will do no harm.  The SASrelay
   scheme does not depend on the PBX trusting the phone.  It only
   depends on the phone trusting the PBX.  It is the phone (the user)
   who is at risk if the PBX abuses its MiTM privileges.

   An endpoint MUST NOT store the pbxsecret in the cache without
   explicit user authorization.

   After this enrollment process, the PBX and the ZRTP-enabled phone
   both share a secret that enables the phone to recognize the PBX as a
   trusted MiTM in future calls.  This means that when a future call
   from an outside ZRTP-enabled caller is relayed through the PBX to
   this phone, the phone will render a relayed SAS from the PBX.  If the
   SASrelay packet comes from a MiTM which does not know the pbxsecret,
   the phone treats it as a "bad guy" MiTM, and refuses to render the
   relayed SAS.  Regardless of which party initiates any future phone
   calls through the PBX, the enrolled phone or the outside phone, the
   PBX will relay the SAS to the enrolled phone.

   There are other ways that ZRTP user agents can be configured to trust
   a PBX.  Perhaps the pbxsecret can be configured into the phone by
   some automated provisioning process in large IT environments.  This
   specification does not require that products be configured solely by
   this enrollment process.  Any process that results in a pbxsecret to
   be computed and shared between the PBX and the phone will suffice.
   This is one such method that has been shown to work.


8.  Signaling Interactions

   This section discusses how ZRTP, SIP, and SDP work together.

   Note that ZRTP may be implemented without coupling with the SIP
   signaling.  For example, ZRTP can be implemented as a "bump in the
   wire" or as a "bump in the stack" in which RTP sent by the SIP UA is
   converted to ZRTP.  In these cases, the SIP UA will have no knowledge
   of ZRTP.  As a result, the signaling path discovery mechanisms
   introduced in this section should not be definitive - they are a
   hint.  Despite the absence of an indication of ZRTP support in an
   offer or answer, a ZRTP endpoint SHOULD still send Hello messages.

   ZRTP endpoints which have control over the signaling path include a
   ZRTP SDP attributes in their SDP offers and answers.  The ZRTP
   attribute, a=zrtp-hash is used to indicate support for ZRTP and to
   convey a hash of the Hello message.  The hash is computed according
   to Section 8.1.



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   Aside from the advantages described in Section 8.1, there are a
   number of potential uses for this attribute.  It is useful when
   signaling elements would like to know when ZRTP may be utilized by
   endpoints.  It is also useful if endpoints support multiple methods
   of SRTP key management.  The ZRTP attribute can be used to ensure
   that these key management approaches work together instead of against
   each other.  For example, if only one endpoint supports ZRTP but both
   support another method to key SRTP, then the other method will be
   used instead.  When used in parallel, an SRTP secret carried in an
   a=keymgt [RFC4567] or a=crypto [RFC4568] attribute can be used as a
   shared secret for the srtps computation defined in Section 8.2.  The
   ZRTP attribute is also used to signal to an intermediary ZRTP device
   not to act as a ZRTP endpoint, as discussed in Section 10.

   The a=zrtp-hash attribute can only be included in the SDP at the
   media level since Hello messages sent in different media streams will
   have unique hashes.

   The ABNF for the ZRTP attribute is as follows:

        zrtp-attribute   = "a=zrtp-hash:" zrtp-version zrtp-hash-value

        zrtp-version     = token

        zrtp-hash-value  = 1*(HEXDIG)

   Example of the ZRTP attribute in an initial SDP offer or answer used
   at the session level:

   v=0
   o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com
   s=
   c=IN IP4 client.biloxi.example.com
   t=0 0
   m=audio 3456 RTP/AVP 97 33
   a=rtpmap:97 iLBC/8000
   a=rtpmap:33 no-op/8000
   a=zrtp-hash:1.00 fe30efd02423cb054e50efd0248742ac7a52c8f91bc2df881ae642c371ba46df

8.1.  Binding the media stream to the signaling layer via the Hello Hash

   It is desirable to tie the media stream to the signaling channel to
   prevent a third party from inserting false media packets.  If the
   signaling layer contains information that ties it to the media
   stream, false media streams can be rejected.

   To accomplish this, a 256-bit hash (using the hash algorithm defined
   in Section 5.1.2.1) is computed across the entire ZRTP Hello message



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   (as shown in Figure 3).  This hash image is made available to the
   signaling layer, where it is transmitted as a hexadecimal value in
   the SIP channel using the SDP attribute, a=zrtp-hash defined in this
   specification.  Each media stream (audio or video) will have a
   separate Hello packet, and thus will require a separate a=zrtp-hash
   in an SDP attribute.  The recipient of the SIP/SDP message can then
   use this hash image to detect and reject false Hello packets in the
   media channel, as well as identify which media stream is associated
   with this SIP call.  Each Hello packet hashes uniquely, because it
   contains the H3 field derived from a random nonce, defined in
   Section 9.

   The Hello Hash as an SDP attribute is an OPTIONAL feature, because
   some ZRTP endpoints do not have the ability to add SDP attributes to
   the signaling.  For example, if ZRTP is implemented in a hardware
   bump-in-the-wire device, it might only have the ability to modify the
   media packets, not the SIP packets, especially if the SIP packets are
   integrity protected and thus cannot be modified on the wire.  If the
   SDP has no hash image of the ZRTP Hello message, the recipient's ZRTP
   user agent cannot check it, and thus will not be able to reject Hello
   messages based on this hash.

   After the Hello Hash is used to properly identify the ZRTP Hello
   message as belonging to this particular SIP call, the rest of the
   ZRTP message sequence is protected from false packet injection by
   other protection mechanisms.  For example, the use of the total_hash
   in the shared secret calculation, and also the hash chaining
   mechanism defined in Section 9.

   An attacker who controls only the signaling layer, such as an
   uncooperative VoIP service provider, may be able to deny service by
   corrupting the hash of the Hello message in the SDP attribute, which
   would force ZRTP to reject perfectly good Hello messages.  If there
   is reason to believe this is happening, the ZRTP endpoint MAY allow
   Hello messages to be accepted that do not match the hash image in the
   SDP attribute.

   Even in the absence of SIP integrity protection, the inclusion of the
   a=zrtp-hash SDP attribute, when coupled with the hash chaining
   mechanism defined in Section 9, meets the R-ASSOC requirement in the
   Media Security Requirements
   [I-D.ietf-sip-media-security-requirements], which requires:

      "...a mechanism for associating key management messages with both
      the signaling traffic that initiated the session and with
      protected media traffic.  Allowing such an association also allows
      the SDP offerer to avoid performing CPU-consuming operations
      (e.g., Diffie-Hellman or public key operations) with attackers



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      that have not seen the signaling messages."

   The a=zrtp-hash SDP attribute becomes especially useful if the SDP is
   integrity-protected end-to-end by SIP Identity (RFC 4474) [RFC4474]
   or better still, Dan Wing's SIP Identity using Media Path
   [I-D.wing-sip-identity-media].  This leads to an ability to stop MiTM
   attacks independent of ZRTP's SAS mechanism, as explained in
   Section 8.1.1 below.

8.1.1.  Integrity-protected signaling enables integrity-protected DH
        exchange

   If and only if the signaling path and the SDP is protected by some
   form of end-to-end integrity protection, such as one of the
   abovementioned mechanisms, so that it can guarantee delivery of the
   a=zrtp-hash attribute without any tampering by a third party, and if
   there is good reason to trust the signaling layer to protect the
   interests of the end user, it is possible to authenticate the key
   exchange and prevent a MiTM attack.  This can be done without
   requiring the users to verbally compare the SAS, by using the hash
   chaining mechanism defined in Section 9 to provide a series of HMAC
   keys that protect the entire ZRTP key exchange.  Thus, an end-to-end
   integrity-protected signaling layer automatically enables an
   integrity-protected Diffie-Hellman exchange in ZRTP, which in turn
   means immunity from a MiTM attack.  Here's how it works.

   The integrity-protected SIP SDP contains a hash commitment to the
   entire Hello message.  The Hello message contains H3, which provides
   a hash commitment for the rest of the hash chain H0-H2 (Section 9).
   The Hello message is protected by a 64-bit HMAC, keyed by H2.  The
   Commit message is protected by a 64-bit HMAC keyed by H1.  The
   DHPart1 or DHPart2 messages are protected by a 64-bit HMAC keyed by
   H0.  The HMAC protecting the Confirm messages are computed by a
   different HMAC key derived from the resulting key agreement.  Each
   message's HMAC is checked when the HMAC key is received in the next
   message.  If a bad HMAC is discovered, it MUST be treated as a
   security exception indicating a MiTM attack, perhaps by logging or
   alerting the user, and MUST NOT be treated as a random error.  Random
   errors are already discovered and quietly rejected by bad CRCs
   (Figure 2).

   The Hello message must be assembled before any hash algorithms are
   negotiated, so an implicit predetermined hash algorthm and HMAC
   algorthm (both defined in Section 5.1.2.1) must be used.  All of the
   aforementioned HMACs keyed by the hashes in the aforementioned hash
   chain MUST be computed with the HMAC algorithm defined in
   Section 5.1.2.1, with the HMAC truncated to 64 bits.




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   The Media Security Requirements
   [I-D.ietf-sip-media-security-requirements] R-EXISTING requirement can
   be fully met by leveraging a certificate-backed PKI in the signaling
   layer to integrity-protect the delivery of the a=zrtp-hash SDP
   attribute.  This would thereby protect ZRTP against a MiTM attack,
   without requiring the user to check the SAS, without adding any
   explicit signatures or signature keys to the ZRTP key exchange, and
   without any extra public key operations or extra packets.

   Without an end-to-end integrity protection mechanism in the signaling
   layer to guarantee delivery of the a=zrtp-hash SDP attribute without
   modification by a third party, these HMACs alone will not prevent a
   MiTM attack.  In that case, ZRTP's built-in SAS mechanism will still
   have to be used to authenticate the key exchange.  At the time of
   this writing, very few deployed VoIP clients offer a fully
   implemented SIP stack that provides end-to-end integrity protection
   for the delivery of SDP attributes.  Also, end-to-end signaling
   integrity becomes more problematic if E.164 numbers [RFC3824] are
   used in SIP.  Thus, real-world implementations of ZRTP endpoints will
   continue to depend on SAS authentication for quite some time.  Even
   after there is widespread availability of SIP user agents that offer
   integrity protected delivery of SDP attributes, many users will still
   be faced with the fact that the signaling path may be controlled by
   institutions that do not have the best interests of the end user in
   mind.  In those cases, SAS authentication will remain the gold
   standard for the prudent user.

   Even without SIP integrity protection, the Media Security
   Requirements [I-D.ietf-sip-media-security-requirements] R-ACT-ACT
   requirement can be met by ZRTP's SAS mechanism.  Although ZRTP may
   benefit from an integrity-protected SIP layer, it is fortunate that
   ZRTP's self-contained MiTM defenses do not actually require an
   integrity-protected SIP layer.  ZRTP can bypass the delays and
   problems that SIP integrity faces, such as E.164 number usage, and
   the complexity of building and maintaining a PKI.

   In contrast, DTLS-SRTP [I-D.ietf-avt-dtls-srtp] appears to depend
   heavily on end-to-end integrity protection in the SIP layer.
   Further, DTLS-SRTP must bear the additional cost of a signature
   calculation of its own, in addition to the signature calculation the
   SIP layer uses to achieve its integrity protection.  ZRTP needs no
   signature calculation of its own to leverage the signature
   calculation carried out in the SIP layer.

8.2.  Deriving the SRTP secret (srtps) from the signaling layer

   The shared secret calculations defined in Section 4.3 make use of the
   SRTP secret (srtps), if it is provided by the signaling layer.



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   It is desirable for only one SRTP key negotiation protocol to be
   used, and that protocol should be ZRTP.  But in the event the
   signaling layer negotiates its own SRTP master key and salt, using
   the SDES [RFC4568] or [RFC4567], it can be passed from the signaling
   to the ZRTP layer and mixed into ZRTP's own shared secret
   calculations, without compromising security by creating a dependency
   on the signaling for media encryption.

   ZRTP computes srtps from the SRTP master key and salt parameters
   provided by the signaling layer in this manner:

      srtps = hash(SRTP master key | SRTP master salt)

   It is expected that the srtps parameter will be rarely computed or
   used in typical ZRTP endpoints, because it is likely and desirable
   that ZRTP will be the sole means of negotiating SRTP keys, needing no
   help from SDES [RFC4568] or [RFC4567].  If srtps is computed, it will
   be stored in the auxiliary shared secret auxsecret, defined in
   Section 4.3, and used in Section 4.3.1 and Section 4.3.2.

8.3.  Codec Selection for Secure Media

   Codec selection is negotiated in the signaling layer.  If the
   signaling layer determines that ZRTP is supported by both endpoints,
   this should provide guidance in codec selection to avoid variable
   bit-rate (VBR) codecs that leak information.

   When voice is compressed with a VBR codec, the packet lengths vary
   depending on the types of sounds being compressed.  This leaks a lot
   of information about the content even if the packets are encrypted,
   regardless of what encryption protocol is used [Wright1].  It is
   RECOMMENDED that VBR codecs be avoided in encrypted calls.  It is not
   a problem if the codec adapts the bit rate to the available channel
   bandwidth.  The vulnerable codecs are the ones that change their bit
   rate depending on the type of sound being compressed.

   It also appears that voice activity detection (VAD) leaks information
   about the content of the conversation, but to a lesser extent than
   VBR.  This effect can be ameliorated by lengthening the VAD hangover
   time by about 1 to 2 seconds, if this is feasible in your
   application.  This is a topic that requires further study.


9.  False ZRTP Packet Rejection

   An attacker who is not in the media path may attempt to inject false
   ZRTP protocol packets, possibly to effect a denial of service attack,
   or to inject his own media stream into the call.  VoIP by its nature



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   invites various forms of denial of service attacks and requires
   protocol features to reject such attacks.  While bogus SRTP packets
   may be easily rejected via the SRTP auth tag field, that can only be
   applied after a key agreement is completed.  During the ZRTP key
   negotiation phase, other false packet rejection mechanisms are
   needed.  One such mechanism is the use of the total_hash in the final
   shared secret calculation, but that can only detect false packets
   after performing the computationally expensive Diffie-Hellman
   calculation.

   The VoIP developer community expects to see a lot of denial of
   service attacks, especially from attackers who are not in the media
   path.  Such an attacker might inject false ZRTP packets to force a
   ZRTP endpoint to engage in an endless series of pointless and
   expensive DH calculations.  To detect and reject false packets
   cheaply and rapidly as soon as they are received, ZRTP uses a hash
   chain, which is a series of successive hash images.  Before each
   session, the following values are computed:

      H0 = 256-bit random nonce (different for each party)
      H1 = hash (H0)
      H2 = hash (H1)
      H3 = hash (H2)

   The hash chain MUST use the hash algorithm defined in
   Section 5.1.2.1.  Each 256-bit hash image is the pre-image of the
   next, and the sequence of images is sent in reverse order in the ZRTP
   packet sequence.  The hash image H3 is sent in the Hello packet, H2
   is sent in the Commit packet, H1 is sent in the DHPart1 or DHPart2
   packets, and H0 is sent in the Confirm1 or Confirm2 packets.  The
   initial random H0 nonces that each party generates MUST be
   unpredictable to an attacker and unique within a ZRTP call, which
   thereby forces the derived hash images H1-H3 to also be unique and
   unpredictable.

   The recipient checks if the packet has the correct hash pre-image, by
   hashing it and comparing the result with the hash image for the
   preceding packet.  Packets which contain an incorrect hash pre-image
   MUST NOT be used by the recipient, but MAY be processed as security
   exceptions, perhaps by logging or alerting the user.  As long as
   these bogus packets are not used, and correct packets are still being
   received, the protocol SHOULD be allowed to run to completion,
   thereby rendering ineffective this denial of service attack.

   Because these hash images alone do not protect the rest of the
   contents of the packet they reside in, this scheme assumes the
   attacker cannot modify the packet contents from a legitimate party,
   which is a reasonable assumption for an attacker who is not in the



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   media path.  This covers an important range of denial-of-service
   attacks.  For dealing with the remaining set of attacks that involve
   packet modification, other mechanisms are used, such as the
   total_hash in the final shared secret calculation, and the hash
   commitment in the Commit packet.

   False Hello packets may be detected and rejected by the mechanism
   defined in Section 8.1.  This mechanism requires that each Hello
   packet be unique, and the inclusion of the H3 hash image meets that
   requirement.

   If and only if an integrity-protected signaling channel is available,
   this hash chaining scheme can be used to key HMACs to authenticate
   the entire ZRTP key exchange, and thereby prevent a MiTM attack,
   without relying on the users verbally comparing the SAS.  See
   Section 8.1.1 for details.

   Some ZRTP user agents allow the user to manually switch to clear mode
   (via the GoClear packet) in the middle of a secure call, and then
   later initiate secure mode again.  Many consumer client products will
   omit this feature, but those that allow it may return to secure mode
   again in the same media stream.  Although the same chain of hash
   images will be re-used and thus rendered ineffective the second time,
   no real harm is done because the new SRTP session keys will be
   derived in part from a cached shared secret, which was safely
   protected from the MiTM in the previous DH exchange earlier in the
   same call.


10.  Intermediary ZRTP Devices

   This section discusses the operation of a ZRTP endpoint which is
   actually an intermediary.  For example, consider a device which
   proxies both signaling and media between endpoints.  There are three
   possible ways in which such a device could support ZRTP.

   An intermediary device can act transparently to the ZRTP protocol.
   To do this, a device MUST pass RTP header extensions and payloads (to
   allow the ZRTP Flag) and non-RTP protocols multiplexed on the same
   port as RTP (to allow ZRTP and STUN).  This is the RECOMMENDED
   behavior for intermediaries as ZRTP and SRTP are best when done end-
   to-end.

   An intermediary device could implement the ZRTP protocol and act as a
   ZRTP endpoint on behalf of non-ZRTP endpoints behind the intermediary
   device.  The intermediary could determine on a call-by-call basis
   whether the endpoint behind it supports ZRTP based on the presence or
   absence of the ZRTP SDP attribute flag (a=zrtp-hash).  For non-ZRTP



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   endpoints, the intermediary device could act as the ZRTP endpoint
   using its own ZID and cache.  This approach SHOULD only be used when
   there is some other security method protecting the confidentiality of
   the media between the intermediary and the inside endpoint, such as
   IPSec or physical security.

   The third mode, which is NOT RECOMMENDED, is for the intermediary
   device to attempt to back-to-back the ZRTP protocol.  The only
   exception to this case is where the intermediary device is a trusted
   element providing services to one of the endpoints - e.g. a Private
   Branch Exchange or PBX.  In this mode, the intermediary would attempt
   to act as a ZRTP endpoint towards both endpoints of the media
   session.  This approach MUST NOT be used except as described in
   Section 7.3 as it will always result in a detected man-in-the-middle
   attack and will generate alarms on both endpoints and likely result
   in the immediate termination of the session.

   In cases where centralized media mixing is taking place, the SAS will
   not match when compared by the humans.  However, this situation is
   known in the SIP signaling by the presence of the isfocus feature tag
   [RFC4579].  As a result, when the isfocus feature tag is present, the
   DH exchange can be authenticated by the mechanism defined in
   Section 8.1.1 or by validating signatures (Section 7.2) in the
   Confirm or SASrelay messages.  For example, consider a audio
   conference call with three participants Alice, Bob, and Carol hosted
   on a conference bridge in Dallas.  There will be three ZRTP encrypted
   media streams, one encrypted stream between each participant and
   Dallas.  Each will have a different SAS.  Each participant will be
   able to validate their SAS with the conference bridge by using
   signatures optionally present in the Confirm messages (described in
   Section 7.2).  Or, if the signaling path has end-to-end integrity
   protection, each DH exchange will have automatic MiTM protection by
   using the mechanism in Section 8.1.1.

   SIP feature tags can also be used to detect if a session is
   established with an automaton such as an IVR, voicemail system, or
   speech recognition system.  The display of SAS strings to users
   should be disabled in these cases.

   It is possible that an intermediary device acting as a ZRTP endpoint
   might still receive ZRTP Hello and other messages from the inside
   endpoint.  This could occur if there is another inline ZRTP device
   which does not include the ZRTP SDP attribute flag.  An intermediary
   acting as a ZRTP endpoint receiving ZRTP Hello and other messages
   from the inside endpoint MUST NOT pass these ZRTP messages.






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11.  The ZRTP Disclosure flag

   There are no back doors defined in the ZRTP protocol specification.
   The designers of ZRTP would like to discourage back doors in ZRTP-
   enabled products.  However, despite the lack of back doors in the
   actual ZRTP protocol, it must be recognized that a ZRTP implementer
   might still deliberately create a rogue ZRTP-enabled product that
   implements a back door outside the scope of the ZRTP protocol.  For
   example, they could create a product that discloses the SRTP session
   key generated using ZRTP out-of-band to a third party.  They may even
   have a legitimate business reason to do this for some customers.

   For example, some environments have a need to monitor or record
   calls, such as stock brokerage houses who want to discourage insider
   trading, or special high security environments with special needs to
   monitor their own phone calls.  We've all experienced automated
   messages telling us that "This call may be monitored for quality
   assurance".  A ZRTP endpoint in such an environment might
   unilaterally disclose the session key to someone monitoring the call.
   ZRTP-enabled products that perform such out-of-band disclosures of
   the session key can undermine public confidence in the ZRTP protocol,
   unless we do everything we can in the protocol to alert the other
   user that this is happening.

   If one of the parties is using a product that is designed to disclose
   their session key, ZRTP requires them to confess this fact to the
   other party through a protocol message to the other party's ZRTP
   client, which can properly alert that user, perhaps by rendering it
   in a graphical user interface.  The disclosing party does this by
   sending a Disclosure flag (D) in Confirm1 and Confirm2 messages as
   described in Section 5.7.

   Note that the intention here is to have the Disclosure flag identify
   products that are designed to disclose their session keys, not to
   identify which particular calls are compromised on a call-by-call
   basis.  This is an important legal distinction, because most
   government sanctioned wiretap regulations require a VoIP service
   provider to not reveal which particular calls are wiretapped.  But
   there is nothing illegal about revealing that a product is designed
   to be wiretap-friendly.  The ZRTP protocol mandates that such a
   product "out" itself.

   You might be using a ZRTP-enabled product with no back doors, but if
   your own graphical user interface tells you the call is (mostly)
   secure, except that the other party is using a product that is
   designed in such a way that it may have disclosed the session key for
   monitoring purposes, you might ask him what brand of secure telephone
   he is using, and make a mental note not to purchase that brand



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   yourself.  If we create a protocol environment that requires such
   back-doored phones to confess their nature, word will spread quickly,
   and the "invisible hand" of the free market will act.  The free
   market has effectively dealt with this in the past.

   Of course, a ZRTP implementer can lie about his product having a back
   door, but the ZRTP standard mandates that ZRTP-compliant products
   MUST adhere to the requirement that a back door be confessed by
   sending the Disclosure flag to the other party.

   There will be inevitable comparisons to Steve Bellovin's 2003 April
   fool's joke, when he submitted RFC 3514 [RFC3514] which defined the
   "Evil bit" in the IPV4 header, for packets with "evil intent".  But
   we submit that a similar idea can actually have some merit for
   securing VoIP.  Sure, one can always imagine that some implementer
   will not be fazed by the rules and will lie, but they would have lied
   anyway even without the Disclosure flag.  There are good reasons to
   believe that it will improve the overall percentage of
   implementations that at least tell us if they put a back door in
   their products, and may even get some of them to decide not to put in
   a back door at all.  From a civic hygiene perspective, we are better
   off with having the Disclosure flag in the protocol.

   If an endpoint stores or logs SRTP keys or information that can be
   used to reconstruct or recover SRTP keys after they are no longer in
   use (i.e. the session is active), or otherwise discloses or passes
   SRTP keys or information that can be used to reconstruct or recover
   SRTP keys to another application or device, the Disclosure flag D
   MUST be set in the Confirm1 or Confirm2 message.

11.1.  Guidelines on Proper Implementation of the Disclosure Flag

   Some implementers have asked for guidance on implementing the
   Disclosure Flag.  Some people have incorrectly thought that a
   connection secured with ZRTP cannot be used in a call center, with
   voluntary voice recording, or even with a voicemail system.
   Similarly, some potential users of ZRTP have over considered the
   protection that ZRTP can give them.  These guidelines clarify both
   concerns.

   The ZRTP Disclosure Flag only governs the ZRTP/SRTP stream itself.
   It does not govern the underlying RTP media stream, nor the actual
   media itself.  Consequently, a PBX that uses ZRTP may provide
   conference calls, call monitoring, call recording, voicemail, or
   other PBX features and still say that it does not disclose the ZRTP
   key material.  A video system may provide DVR features and still say
   that it does not disclose the ZRTP key material.  The ZRTP Disclosure
   Flag, when not set, means only that the ZRTP cryptographic key



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   material stays within the bounds of the ZRTP subsystem.

   If an application has a need to disclose the ZRTP cryptographic key
   material, the easiest way to comply with the protocol is to set the
   flag to the proper value.  The next easiest way is to overestimate
   disclosure.  For example, a call center that commonly records calls
   might choose to set the disclosure flag even though all recording is
   an analog recording of a call (and thus outside the ZRTP scope)
   because it sets an expectation with clients that their calls might be
   recorded.

   Note also that the ZRTP Disclosure Flag does not require an
   implementation to preclude hacking or malware.  Malware that leaks
   ZRTP cryptographic key material does not create a liability for the
   implementor from non-compliance with the ZRTP specification.

   A user of ZRTP should note that ZRTP is not a panacea against
   unauthorized recording.  ZRTP does not and cannot protect against an
   untrustworthy partner who holds a microphone up to the speaker.  It
   does not protect against someone else being in the room.  It does not
   protect against analog wiretaps in the phone or in the room.  It does
   not mean your partner has not been hacked with spyware.  It does not
   mean that the software has no flaws.  It means that the ZRTP
   subsystem is not knowingly leaking ZRTP cryptographic key material.


12.  RTP Header Extension Flag for ZRTP

   This specification defines a new RTP header extension used only for
   discovery of support for ZRTP.  No ZRTP data is transported in the
   extension.  When used, the X bit is set in the RTP header to indicate
   the presence of the RTP header extension.

   Section 5.3.1 in RFC 3550 [RFC3550] defines the format of an RTP
   Header extension.  The Header extension is appended to the RTP
   header.  The first 16 bits are an identifier for the header
   extension, and the following 16 bits are length of the extension
   header in 32 bit words.  The ZRTP flag RTP header extension has the
   value of 0x505A and a length of 0.  The format of the header
   extension is as shown in the Figure below.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 18: RTP Extension header format for ZRTP Flag



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   ZRTP endpoints MAY include the ZRTP Flag in RTP packets sent at the
   start of a session.  For example, an endpoint may decide to include
   the flag in the first 2 seconds of RTP packets sent.  The inclusion
   of the flag MAY be ended if a ZRTP message (such as Hello) is
   received.


13.  IANA Considerations

   This specification defines a new SDP [RFC4566] attribute in
   Section 8.

   Contact name:          Philip Zimmermann <prz@mit.edu>

   Attribute name:        "zrtp-hash".

   Type of attribute:     Media level.

   Subject to charset:    Not.

   Purpose of attribute:  The 'zrtp-hash' indicates that a UA supports the
                          ZRTP protocol and provides a hash of the ZRTP Hello
                          message.  The ZRTP protocol version number is also
                          specified.

   Allowed attribute values:  Hex.


14.  Appendix - Media Security Requirements

   This section discuses how ZRTP meets all RTP security requirements
   discussed in the Media Security Requirements
   [I-D.ietf-sip-media-security-requirements] document without any
   dependencies on other protocols or extensions, unlike DTLS-SRTP
   [I-D.ietf-avt-dtls-srtp] which requires additional protocols and
   mechanisms.

      R-FORK-RETARGET is met since ZRTP is a media path key agreement
      protocol.

      R-DISTINCT is met since ZRTP uses ZIDs and allows multiple
      independent ZRTP exchanges to proceed.

      R-REUSE is met using the Multistream and Preshared modes.

      R-AVOID-CLIPPING is met since ZRTP is a media path key agreement
      protocol




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      R-RTP-VALID is met since the ZRTP packet format does not pass the
      RTP validity check

      R-ASSOC is met using the a=zrtp-hash SDP attribute in INVITEs and
      responses.

      R-NEGOTIATE is met using the Commit message.

      R-PSTN is met since ZRTP can be implemented in Gateways.

      R-PFS is met using ZRTP Diffie-Hellman key agreement methods.

      R-COMPUTE is met using the Hello/Commit ZRTP exchange.

      R-CERTS is met using the optional signature field in ZRTP Confirm
      messages.

      R-FIPS is met since ZRTP uses algorithms that allow FIPS
      certification.

      R-DOS is met since ZRTP does not introduce any new denial of
      service attacks.

      R-EXISTING is met since ZRTP can support the use of certificates
      or keys.

      R-AGILITY is met since the set of hash, cipher, authentication tag
      length, key agreement method, SAS type, and signature type can all
      be extended and negotiated.

      R-DOWNGRADE is met since ZRTP has protection against downgrade
      attacks.

      R-PASS-MEDIA is met since ZRTP prevents a passive adversary with
      access to the media path from gaining access to keying material
      used to protect SRTP media packets.

      R-PASS-SIG is met since ZRTP prevents a passive adversary with
      access to the signaling path from gaining access to keying
      material used to protect SRTP media packets.

      R-SIG-MEDIA is met using the a=zrtp-hash SDP attribute in INVITEs
      and responses.

      R-ID-BINDING is met using the a=zrtp-hash SDP attribute.





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      R-ACT-ACT is met using the a=zrtp-hash SDP attribute in INVITEs
      and responses.

      R-BEST-SECURE is met since ZRTP utilizes the RTP/AVP profile and
      hence best effort SRTP in every case.

      R-OTHER-SIGNALING is met since ZRTP can utilize modes in which
      there is no dependency on the signaling path.

      R-RECORDING is met using the ZRTP Disclosure flag.

      R-TRANSCODER is met if the transcoder operates as a trusted MitM
      (i.e. a PBX).

      R-ALLOW-RTP is met due to ZRTP's best effort encryption.


15.  Security Considerations

   This document is all about securely keying SRTP sessions.  As such,
   security is discussed in every section.

   Most secure phones rely on a Diffie-Hellman exchange to agree on a
   common session key.  But since DH is susceptible to a man-in-the-
   middle (MiTM) attack, it is common practice to provide a way to
   authenticate the DH exchange.  In some military systems, this is done
   by depending on digital signatures backed by a centrally-managed PKI.
   A decade of industry experience has shown that deploying centrally
   managed PKIs can be a painful and often futile experience.  PKIs are
   just too messy, and require too much activation energy to get them
   started.  Setting up a PKI requires somebody to run it, which is not
   practical for an equipment provider.  A service provider like a
   carrier might venture down this path, but even then you have to deal
   with cross-carrier authentication, certificate revocation lists, and
   other complexities.  It is much simpler to avoid PKIs altogether,
   especially when developing secure commercial products.  It is
   therefore more common for commercial secure phones in the PSTN world
   to augment the DH exchange with a Short Authentication String (SAS)
   combined with a hash commitment at the start of the key exchange, to
   shorten the length of SAS material that must be read aloud.  No PKI
   is required for this approach to authenticating the DH exchange.  The
   AT&T TSD 3600, Eric Blossom's COMSEC secure phones [comsec], PGPfone
   [pgpfone], and CryptoPhone [cryptophone] are all examples of products
   that took this simpler lightweight approach.

   The main problem with this approach is inattentive users who may not
   execute the voice authentication procedure, or unattended secure



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   phone calls to answering machines that cannot execute it.

   Additionally, some people worry about voice spoofing.  But it is a
   mistake to think this is simply an exercise in voice impersonation
   (perhaps this could be called the "Rich Little" attack).  Although
   there are digital signal processing techniques for changing a
   person's voice, that does not mean a man-in-the-middle attacker can
   safely break into a phone conversation and inject his own short
   authentication string (SAS) at just the right moment.  He doesn't
   know exactly when or in what manner the users will choose to read
   aloud the SAS, or in what context they will bring it up or say it, or
   even which of the two speakers will say it, or if indeed they both
   will say it.  In addition, some methods of rendering the SAS involve
   using a list of words such as the PGP word list[Juola2], in a manner
   analogous to how pilots use the NATO phonetic alphabet to convey
   information.  This can make it even more complicated for the
   attacker, because these words can be worked into the conversation in
   unpredictable ways.  Remember that the attacker places a very high
   value on not being detected, and if he makes a mistake, he doesn't
   get to do it over.  Some people have raised the question that even if
   the attacker lacks voice impersonation capabilities, it may be unsafe
   for people who don't know each other's voices to depend on the SAS
   procedure.  This is not as much of a problem as it seems, because it
   isn't necessary that they recognize each other by their voice, it is
   only necessary that they detect that the voice used for the SAS
   procedure matches the voice in the rest of the phone conversation.

   A popular and field-proven approach is used by SSH (Secure Shell)
   [RFC4251], which Peter Gutmann likes to call the "baby duck" security
   model.  SSH establishes a relationship by exchanging public keys in
   the initial session, when we assume no attacker is present, and this
   makes it possible to authenticate all subsequent sessions.  A
   successful MiTM attacker has to have been present in all sessions all
   the way back to the first one, which is assumed to be difficult for
   the attacker.  ZRTP's key continuity features are actually better
   than SSH, at least for VoIP, for reasons described in Section 15.1.
   All this is accomplished without resorting to a centrally-managed
   PKI.

   We use an analogous baby duck security model to authenticate the DH
   exchange in ZRTP.  We don't need to exchange persistent public keys,
   we can simply cache a shared secret and re-use it to authenticate a
   long series of DH exchanges for secure phone calls over a long period
   of time.  If we read aloud just one SAS, and then cache a shared
   secret for later calls to use for authentication, no new voice
   authentication rituals need to be executed.  We just have to remember
   we did one already.




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   If one party ever loses this cached shared secret, it is no longer
   available for authentication of DH exchanges.  This cache mismatch
   situation is easy to detect by the party that still has a surviving
   shared secret cache entry.  If it fails to match, either there is a
   MiTM attack or one side has lost their shared secret cache entry.
   The user agent that discovers the cache mismatch must alert the user
   that a cache mismatch has been detected, and that he must do a verbal
   comparison of the SAS to distinguish if the mismatch is because of a
   MiTM attack or because of the other party losing her cache.  From
   that point on, the two parties start over with a new cached shared
   secret.  Then they can go back to omitting the voice authentication
   on later calls.

   A particularly compelling reason why this approach is attractive is
   that SAS is easiest to implement when a graphical user interface or
   some sort of display is available, which raises the question of what
   to do when a display is less conveniently available.  For example,
   some devices that implement ZRTP might have a graphical user
   interface that is only visible through a web browser, such as a PBX
   or some other nearby device that implements ZRTP as a "bump-in-the-
   wire".  If we take an approach that greatly reduces the need for a
   SAS in each and every call, we can operate in products without a
   graphical user interface with greater ease.  Then the SAS can be
   compared less frequently through a web browser, or it might even be
   presented as needed to the local user through a locally generated
   voice prompt, which the local user hears and verbally repeats and
   compares with the remote party.  Using a voice prompt in this way is
   purely for the local ZRTP user agent to render the SAS to the local
   user, and is not to be confused with the verbal comparison of the SAS
   between two human users.

   It is a good idea to force your opponent to have to solve multiple
   problems in order to mount a successful attack.  Some examples of
   widely differing problems we might like to present him with are:
   Stealing a shared secret from one of the parties, being present on
   the very first session and every subsequent session to carry out an
   active MiTM attack, and solving the discrete log problem.  We want to
   force the opponent to solve more than one of these problems to
   succeed.

   ZRTP can use different kinds of shared secrets.  Each type of shared
   secret is determined by a different method.  All of the shared
   secrets are hashed together to form a session key to encrypt the
   call.  An attacker must defeat all of the methods in order to
   determine the session key.

   First, there is the shared secret determined entirely by a Diffie-
   Hellman key agreement.  It changes with every call, based on random



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   numbers.  An attacker may attempt a classic DH MiTM attack on this
   secret, but we can protect against this by displaying and reading
   aloud an SAS, combined with adding a hash commitment at the beginning
   of the DH exchange.

   Second, there is an evolving shared secret, or ongoing shared secret
   that is automatically changed and refreshed and cached with every new
   session.  We will call this the cached shared secret, or sometimes
   the retained shared secret.  Each new image of this ongoing secret is
   a non-invertable function of its previous value and the new secret
   derived by the new DH agreement.  It is possible that no cached
   shared secret is available, because there were no previous sessions
   to inherit this value from, or because one side loses its cache.

   There are other approaches for key agreement for SRTP that compute a
   shared secret using information in the signaling.  For example,
   [RFC4567] describes how to carry a MIKEY (Multimedia Internet KEYing)
   [RFC3830] payload in SDP [RFC4566].  Or RFC 4568 (SDES) [RFC4568]
   describes directly carrying SRTP keying and configuration information
   in SDP.  ZRTP does not rely on the signaling to compute a shared
   secret, but if a client does produce a shared secret via the
   signaling, and makes it available to the ZRTP protocol, ZRTP can make
   use of this shared secret to augment the list of shared secrets that
   will be hashed together to form a session key.  This way, any
   security weaknesses that might compromise the shared secret
   contributed by the signaling will not harm the final resulting
   session key.

   The shared secret provided by the signaling (if available), the
   shared secret computed by DH, and the cached shared secret are all
   hashed together to compute the session key for a call.  If the cached
   shared secret is not available, it is omitted from the hash
   computation.  If the signaling provides no shared secret, it is also
   omitted from the hash computation.

   No DH MiTM attack can succeed if the ongoing shared secret is
   available to the two parties, but not to the attacker.  This is
   because the attacker cannot compute a common session key with either
   party without knowing the cached secret component, even if he
   correctly executes a classic DH MiTM attack.

15.1.  Self-healing Key Continuity Feature

   The key continuity features of ZRTP are analogous to those provided
   by SSH (Secure Shell) [RFC4251], but they differ in one respect.  SSH
   caches public signature keys that never change, and uses a permanent
   private signature key that must be guarded from disclosure.  If
   someone steals your SSH private signature key, they can impersonate



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   you in all future sessions and mount a successful MiTM attack any
   time they want.

   ZRTP caches symmetric key material used to compute secret session
   keys, and these values change with each session.  If someone steals
   your ZRTP shared secret cache, they only get one chance to mount a
   MiTM attack, in the very next session.  If they miss that chance, the
   retained shared secret is refreshed with a new value, and the window
   of vulnerability heals itself, which means they are locked out of any
   future opportunities to mount a MiTM attack.  This gives ZRTP a
   "self-healing" feature if any cached key material is compromised.

   A MiTM attacker must always be in the media path.  This presents a
   significant operational burden for the attacker in many VoIP usage
   scenarios, because being in the media path for every call is often
   harder than being in the signaling path.  This will likely create
   coverage gaps in the attacker's opportunities to mount a MiTM attack.
   ZRTP's self-healing key continuity features are better than SSH at
   exploiting any temporary gaps in MiTM attack coverage.  Thus, ZRTP
   quickly recovers from any disclosure of cached key material.

   The infamous Debian OpenSSL weak key vulnerability [dsa-1571]
   (discovered and patched in May 2008) offers a real-world example of
   why ZRTP's self-healing scheme is a good way to do key continuity.
   The Debian bug resulted in the production of a lot of weak SSH (and
   TLS/SSL) keys, which continued to compromise security even after the
   bug had been patched.  In contrast, ZRTP's key continuity scheme adds
   new entropy to the cached key material with every call, so old
   deficiencies in entropy are washed away with each new session.

   It should be noted that the addition of shared secret entropy from
   previous sessions can extend the strength of the new session key to
   AES-256 levels, even if the new session uses Diffie-Hellman keys no
   larger than DH-3072 or ECDH-256, provided the cached shared secrets
   were initially established when the wiretapper was not present.  This
   is why AES-256 MAY be used with the smaller DH key sizes in
   Section 5.1.5.

   Caching shared symmetric key material is also less CPU intensive
   compared with using digital signatures, which may be important for
   low-power mobile platforms.


16.  Acknowledgments

   The authors would like to thank Bryce Wilcox-O'Hearn and Colin Plumb
   for their contributions to the design of this protocol, and to thank
   Hal Finney, Viktor Krikun, Werner Dittmann, Jon Peterson, Dan Wing,



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   Sagar Pai, Colin Perkins, David McGrew, and Roni Even for their
   helpful comments and suggestions.

   The use of hash chains to key HMACs in ZRTP is similar to Adrian
   Perrig's TESLA protocol [TESLA].


17.  References

17.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
              Diffie-Hellman groups for Internet Key Exchange (IKE)",
              RFC 3526, May 2003.

   [RFC3309]  Stone, J., Stewart, R., and D. Otis, "Stream Control
              Transmission Protocol (SCTP) Checksum Change", RFC 3309,
              September 2002.

   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
              RFC 4231, December 2005.

   [SP800-90]
              Barker, E. and J. Kelsey, "Recommendation for Random
              Number Generation Using Deterministic Random Bit
              Generators", NIST Special Publication 800-90 (Revised)
              March 2007.

   [SP800-56A]
              Barker, E., Johnson, D., and M. Smid, "Recommendation for
              Pair-Wise Key Establishment Schemes Using Discrete
              Logarithm Cryptography", NIST Special Publication 800-
              56A Revision 1, March 2007.

   [FIPS-180-2]
              "Secure Hash Signature Standard (SHS)", NIST FIPS PUB 180-



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              2 August 2002.

   [FIPS-198-1]
              "The Keyed-Hash Message Authentication Code (HMAC)", NIST
              FIPS PUB 198-1 July 2008.

   [NSA-Suite-B]
              "Fact Sheet NSA Suite B Cryptography", NSA Information
              Assurance Directorate Fact Sheet NSA Suite B.

   [RFC4753]  Fu, D. and J. Solinas, "ECP Groups For IKE and IKEv2",
              RFC 4753, January 2007.

   [FIPS-186-3]
              "Digital Signature Standard (DSS)", NIST FIPS PUB 186-
              3 Draft, November 2008.

   [SP800-38A]
              Dworkin, M., "Recommendation for Block Cipher: Methods and
              Techniques", NIST Special Publication 800-38A 2001
              Edition.

   [z-base-32]
              Wilcox, B., "Human-oriented base-32 encoding",
              http://zooko.com/repos/z-base-32/base32/DESIGN .

   [pgpwordlist]
              "PGP Words", http://en.wikipedia.org/wiki/PGP_Words .

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

17.2.  Informative References

   [I-D.ietf-sip-media-security-requirements]
              Wing, D., Fries, S., Tschofenig, H., and F. Audet,
              "Requirements and Analysis of Media Security Management
              Protocols", draft-ietf-sip-media-security-requirements-08
              (work in progress), October 2008.

   [SP800-108]
              Chen, L., "Recommendation for Key Derivation Using
              Pseudorandom Functions", NIST Special Publication 800-
              108 November 2008.

   [Ferguson]
              Ferguson, N. and B. Schneier, "Practical Cryptography",
              Wiley Publishing 2003.



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   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [Juola1]   Juola, P. and P. Zimmermann, "Whole-Word Phonetic
              Distances and the PGPfone Alphabet", Proceedings of the
              International Conference of Spoken Language Processing
              (ICSLP-96) 1996.

   [Juola2]   Juola, P., "Isolated Word Confusion Metrics and the
              PGPfone Alphabet", Proceedings of New Methods in Language
              Processing 1996.

   [pgpfone]  Zimmermann, P., "PGPfone",
              http://philzimmermann.com/docs/pgpfone10b7.pdf .

   [zfone]    Zimmermann, P., "Zfone",
              http://www.philzimmermann.com/zfone .

   [Byzantine]
              "The Two Generals' Problem",
              http://en.wikipedia.org/wiki/Two_Generals%27_Problem .

   [TESLA]    Perrig, A., Canetti, R., Tygar, J., and D. Song, "The
              TESLA Broadcast Authentication Protocol", http://
              www.ece.cmu.edu/~adrian/projects/tesla-cryptobytes/
              tesla-cryptobytes.pdf .

   [SHA-3]    "Cryptographic Hash Algorithm Competition", NIST Computer
              Security Resource Center Cryptographic Hash Project.

   [comsec]   Blossom, E., "The VP1 Protocol for Voice Privacy Devices
              Version 1.2", http://www.comsec.com/vp1-protocol.pdf .

   [cryptophone]
              "CryptoPhone", http://www.cryptophone.de/ .

   [Wright1]  Wright, C., Ballard, L., Coull, S., Monrose, F., and G.
              Masson, "Spot me if you can: Uncovering spoken phrases in
              encrypted VoIP conversations", Proceedings of the 2008
              IEEE Symposium on Security and Privacy 2008.

   [dsa-1571]
              "Debian Security Advisory - OpenSSL predictable random
              number generator",
              http://www.debian.org/security/2008/dsa-1571 .

   [I-D.ietf-avt-srtp-big-aes]
              McGrew, D., "The use of AES-192 and AES-256 in Secure



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              RTP", http://www1.tools.ietf.org/html/
              draft-ietf-avt-srtp-big-aes .

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, July 2006.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC3514]  Bellovin, S., "The Security Flag in the IPv4 Header",
              RFC 3514, April 1 2003.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, August 2006.

   [I-D.wing-sip-identity-media]
              Wing, D. and H. Kaplan, "SIP Identity using Media Path",
              draft-wing-sip-identity-media-02 (work in progress),
              February 2008.

   [RFC3824]  Peterson, J., Liu, H., Yu, J., and B. Campbell, "Using
              E.164 numbers with the Session Initiation Protocol (SIP)",



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              RFC 3824, June 2004.

   [I-D.ietf-avt-dtls-srtp]
              McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",
              draft-ietf-avt-dtls-srtp-06 (work in progress),
              October 2008.


Authors' Addresses

   Philip Zimmermann
   Zfone Project

   Email: prz@mit.edu


   Alan Johnston (editor)
   Avaya
   St. Louis, MO  63124

   Email: alan@sipstation.com


   Jon Callas
   PGP Corporation

   Email: jon@pgp.com






















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