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Internet Engineering Task Force                                 J. Arkko
MSEC Working Group                                            E. Carrara
INTERNET-DRAFT                                               F. Lindholm
Expires: December 2003                                        M. Naslund
                                                              K. Norrman
                                                                Ericsson
                                                              June, 2003




                      MIKEY: Multimedia Internet KEYing
                       <draft-ietf-msec-mikey-07.txt>


   Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/lid-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html



Abstract

   Security protocols for real-time multimedia applications have started
   to appear. This has brought forward the need for a key management
   solution to support these protocols.

   This document describes a key management scheme that can be used for
   real-time applications (both for peer-to-peer communication and group
   communication), and shows how it may work together with protocols
   such as SIP and RTSP. In particular, its use to support the Secure
   Real-time Transport Protocol, [SRTP], is described in detail.





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TABLE OF CONTENTS

   1. Introduction.....................................................3
   1.1. Existing solutions.............................................4
   1.2. Notational Conventions.........................................4
   1.3. Definitions....................................................4
   1.4. Abbreviations..................................................5
   1.5. Outline........................................................6
   2. Basic Overview...................................................6
   2.1. Scenarios......................................................6
   2.2. Design Goals...................................................7
   2.3. System Overview................................................8
   2.4. Relation to GKMARCH............................................9
   3. Basic Key Transport and Exchange Methods.........................9
   3.1. Pre-shared key................................................11
   3.2. Public-key encryption.........................................12
   3.3. Diffie-Hellman key exchange...................................13
   4. Key Management..................................................14
   4.1. Key Calculation...............................................14
   4.1.1. Assumptions.................................................14
   4.1.2. Notation....................................................15
   4.1.3. PRF Description.............................................15
   4.1.4. Generating keys from TGK....................................16
   4.1.5. Generating keys from an envelope/pre-shared key.............16
   4.2 Pre-defined Transforms and Timestamp Formats...................17
   4.2.1 Hash functions...............................................17
   4.2.2 Pseudo random number generator and PRF.......................17
   4.2.3 Key data transport encryption................................17
   4.2.4 MAC and Verification Message function........................18
   4.2.5 Envelope Key encryption......................................18
   4.2.6 Digital Signatures...........................................18
   4.2.7 Diffie-Hellman Groups........................................18
   4.2.8. Timestamps..................................................18
   4.2.9. Adding new parameters to MIKEY..............................19
   4.3. Certificates, Policies and Authorization......................19
   4.3.1. Certificate handling........................................19
   4.3.2. Authorization...............................................20
   4.3.3. Data Policies...............................................21
   4.4. Retrieving the Data SA........................................21
   4.5. TGK re-keying and CSB updating................................21
   5. Behavior and message handling...................................23
   5.1. General.......................................................23
   5.1.1. Capability Discovery........................................23
   5.1.2. Error Handling..............................................23
   5.2. Creating a message............................................24
   5.3. Parsing a message.............................................25
   5.4. Replay handling and timestamp usage...........................26
   5.5. Reliability...................................................28
   6. Payload Encoding................................................28
   6.1. Common header payload (HDR)...................................28
   6.1.1. SRTP ID.....................................................31



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   6.2. Key data transport payload (KEMAC)............................31
   6.3. Envelope data payload (PKE)...................................33
   6.4. DH data payload (DH)..........................................33
   6.5. Signature payload (SIGN)......................................34
   6.6. Timestamp payload (T).........................................35
   6.7. ID payload (ID) / Certificate payload (CERT)..................35
   6.8. Cert hash payload (CHASH).....................................36
   6.9. Ver msg payload (V)...........................................37
   6.10. Security Policy payload (SP).................................37
   6.10.1. SRTP policy................................................38
   6.11. RAND payload (RAND)..........................................39
   6.12. Error payload (ERR)..........................................40
   6.13. Key data sub-payload.........................................40
   6.14. Key validity data............................................42
   6.15. General Extension Payload....................................43
   7. Integration with session establishment protocols................43
   7.1. SDP integration...............................................43
   7.2. MIKEY within SIP..............................................44
   7.3. MIKEY with RTSP...............................................45
   7.4. MIKEY Interface...............................................45
   8. Groups..........................................................46
   8.1. Simple one-to-many............................................47
   8.2. Small-size interactive group..................................47
   9. Security Considerations.........................................48
   9.1. General.......................................................48
   9.2. Key lifetime..................................................50
   9.3. Timestamps....................................................50
   9.4. Identity protection...........................................51
   9.5. Denial of Service.............................................51
   9.6. Session establishment.........................................51
   10. IANA considerations............................................52
   11. Conclusions....................................................54
   12. Acknowledgments................................................54
   13. Author's Addresses.............................................54
   14. References.....................................................55
   14.1. Normative References.........................................55
   14.2. Informative References.......................................56
   Appendix A. - MIKEY - SRTP relation................................57
   Change History since -06 draft.....................................58

1. Introduction

   There has recently been work to define a security protocol for the
   protection of real-time applications running over RTP, [SRTP].
   However, a security protocol needs a key management solution to
   exchange keys and related security parameters. There are some
   fundamental properties that such a key management scheme has to
   fulfill with respect to the kind of real-time applications (such as
   streaming, unicast, groups, multicast) and also with respect to
   heterogeneous (mix of wired and wireless) networks.




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   This document describes a key management solution that addresses
   multimedia scenarios (e.g. SIP calls and RTSP sessions). The focus is
   on how to set up key management for secure multimedia sessions such
   that requirements in a heterogeneous environment are fulfilled.

1.1. Existing solutions

   There is work done in IETF to develop key management schemes. For
   example, IKE [IKE] is a widely accepted unicast scheme for IPsec, and
   the MSEC WG is developing other schemes, addressed to group
   communication [GDOI, GSAKMP]. For reasons discussed below, there is
   however a need for a scheme with low latency, suitable for demanding
   cases such as real-time data over heterogeneous networks, and small
   interactive groups.

1.2. Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119.

1.3. Definitions

   Crypto Session (CS): uni- or bi-directional data stream(s), protected
   by a single instance of a security protocol. E.g. when SRTP is used,
   the Crypto Session will often contain two streams, an RTP stream and
   the corresponding RTCP which are both protected by a single SRTP
   Cryptographic Context, i.e. share key and the bulk of security
   parameters in the SRTP Cryptographic Context (default behavior in
   [SRTP]). In the case of IPsec, a Crypto Session would represent an
   instantiation of an IPsec SA. A Crypto Session can be viewed as a
   Data SA (as defined in [GKMARCH]) and could therefore be mapped to
   other security protocols if needed.

   Crypto Session Bundle (CSB): collection of one or more Crypto
   Sessions, which can have common TEK Generation Keys and security
   parameters.

   Crypto Session ID: unique identifier for the Crypto Session within an
   CSB.

   Crypto Session Bundle ID: unique identifier for the CSB.

   TEK Generation Key (TGK): a bit-string agreed upon by two or more
   parties, associated with CSB. From the TEK Generation Key, Traffic-
   encrypting Keys can then be generated without need of further
   communication.

   Traffic-encrypting Key (TEK): the key used by the security protocol
   to protect the crypto session (this key may be used directly by the




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   security protocol or may be used to derive further keys depending on
   the security protocol). The TEKs are derived from the CSB's TGK.

   TGK re-keying: the process of re-negotiating/updating the TGK (and
   consequently future TEK(s)).

   Initiator: the Initiator of the key management protocol, not
   necessarily the Initiator of the communication.

   Responder: the Responder in the key management protocol.

   Data SA: information for the security protocol, including a TEK and a
   set of parameters/policies.

   PRF(k,x):  a keyed pseudo-random function.
   E(k,m):    encryption of m with the key k.
   PKx:       the public key of x
   []         an optional piece of information
   {}         denotes zero or more occurrences
   ||         concatenation
   |          OR (selection operator)
   ^          exponentiation
   XOR        exclusive or

   Bit and byte ordering: throughout the document bits and bytes are as
   usual indexed from left to right, with the leftmost bits/bytes being
   the most significant.

1.4. Abbreviations

   AES    Advanced Encryption Standard
   CM     Counter Mode (as defined in [SRTP])
   CS     Crypto Session
   CSB    Crypto Session Bundle
   DH     Diffie-Hellman
   DoS    Denial of Service
   MAC    Message Authentication Code
   MIKEY  Multimedia Internet KEYing
   PK     Public-Key
   PSK    Pre-Shared key
   RTP    Real-time Transport Protocol
   RTSP   Real Time Streaming Protocol
   SDP    Session Description Protocol
   SIP    Session Initiation Protocol
   SRTP   Secure RTP
   TEK    Traffic-encrypting key
   TGK    TEK Generation Key







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1.5. Outline

   Section 2 describes the basic scenarios and the design goals for
   which MIKEY is intended. It also gives a brief overview of the entire
   solution and its relation to the group key management architecture
   [GKMARCH].

   The basic key transport/exchange mechanisms are explained in detail
   in Section 3. The key derivation, and other general key management
   procedures are described in Section 4.

   Section 5 describes the expected behavior of the involved parties.
   This also includes message creation and parsing.

   All definitions of the payloads in MIKEY are described in Section 6.

   As MIKEY can be carried in SDP over SIP or RTSP, Section 7 describes
   how to integrate and use MIKEY in these scenarios.

   Section 8 focuses on how MIKEY is used in group scenarios.

   The Security Considerations section (Section 9), gives a deeper
   explanation of important security related topics.

2. Basic Overview

2.1. Scenarios

   MIKEY is mainly intended to be used for peer-to-peer, simple one-to-
   many, and small-size (interactive) groups. One of the main multimedia
   scenarios considered when designing MIKEY has been the conversational
   multimedia scenario, where users may interact and communicate in
   real-time. In these scenarios it can be expected that peers set up
   multimedia sessions between each other, where a multimedia session
   may consist of one or more secured multimedia streams (e.g. SRTP
   streams).

   peer-to-peer/         many-to-many           many-to-many
    simple one-to-many           (distributed)          (centralized)
              ++++        ++++          ++++     ++++           ++++
              |. |        |A |          |B |     |A |----   ----|B |
            --| ++++      |  |----------|  |     |  |    \ /    |  |
   ++++    /  ++|. |      ++++          ++++     ++++    (S)    ++++
   |A |---------| ++++       \          /                 |
   |  |    \    ++|B |        \        /                  |
   ++++     \-----|  |         \ ++++ /                  ++++
                  ++++          \|C |/                   |C |
                                 |  |                    |  |
                                 ++++                    ++++





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   Figure 2.1: Examples of the four scenarios: peer-to-peer, simple one-
   to-many, many-to-many without centralized server (also denoted as
   small interactive group), and many-to-many with a centralized server.

   We identify in the following some typical scenarios which involve the
   multimedia applications we are dealing with (see also Figure 2.1).

   a) peer-to-peer (unicast), e.g. a SIP-based [SIP] call between two
   parties where it may be desirable that the security is either set up
   by mutual agreement or that each party sets up the security for its
   own outgoing streams.

   b) simple one-to-many (multicast), e.g. real-time presentations,
   where the sender is in charge of setting up the security.

   c) many-to-many, without a centralized control unit, e.g. for small-
   size interactive groups where each party may set up the security for
   its own outgoing media. Two basic models may be used here. In the
   first model, the initiator of the group acts as the group server (and
   is the only one authorized to include new members). In the second
   model, authorization information to include new members can be
   delegated to other participants.

   d) many-to-many, with a centralized control unit, e.g. for larger
   groups with some kind of Group Controller that sets up the security.

   The key management solutions may be different in the above scenarios.
   When designing MIKEY, the main focus has been on case a, b, and c.
   For scenario c, only the first model is covered by this document.

2.2. Design Goals

   The key management protocol is designed to have the following
   characteristics:

   * End-to-end security. Only the participants involved in the
   communication have access to the generated key(s).

   * Simplicity.

   * Efficiency. Designed to have:
     - low bandwidth consumption,
     - low computational workload,
     - small code size, and
     - minimal number of roundtrips.

   * Tunneling. Possibility to "tunnel"/integrate MIKEY in session
   establishment protocols (e.g. SIP and RTSP).

   * Independent of any specific security functionality of the
   underlying transport.



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2.3. System Overview

   One objective of MIKEY is to produce a Data security protocol SA
   (Data SA), including a traffic-encrypting key (TEK), which is derived
   from a TEK Generation Key (TGK) and used as the input to the security
   protocol.

   MIKEY supports the possibility to establish keys and parameters for
   more than one security protocol at the same time. The concept of
   Crypto Session Bundle (CSB) is used to denote a collection of one or
   more Crypto Sessions that can have common TEK Generation Keys and
   security parameters, but which obtain distinct TEKs from MIKEY.

   The procedure of setting up a CSB and creating a TEK (and Data SA),
   is done in accordance with Figure 2.2:

   1. A set of security parameters and TEK Generation Key(s) (TGK) are
   agreed upon for the Crypto Session Bundle (this is done by one of the
   three alternative key transport/exchange mechanisms, see Section 3).

   2. The TGK(s) is used to derive (in a cryptographically secure way) a
   TEK for each Crypto Session.

   3. The TEK, together with the security protocol policy parameters
   represent the Data SA, which is used as the input to the Security
   Protocol.

            +-----------------+
            |       CSB       |
            |  Key transport  |                      (see Section 3)
            |    /exchange    |
            +-----------------+
                     |      :
                     | TGK  :
                     v      :
               +----------+ :
       CS ID ->|   TEK    | : Security protocol      (see Section 4)
               |derivation| : parameters (policies)
               +----------+:
                  TEK |     :
                      v     v
                      Data SA
                        |
                        v
               +-------------------+
               |  Crypto Session   |
               |(Security Protocol)|
               +-------------------+

   Figure 2.2: Overview of the key management procedure.



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   The security protocol can then either use the TEK directly, or, if
   supported, derive further session keys from the TEK (e.g. see SRTP
   [SRTP]). It is however up to the security protocol to define how the
   TEK is used.

   MIKEY can be used to update TEKs and the Crypto Sessions in a current
   Crypto Session Bundle (see Section 4.5). This is done by executing
   the transport/exchange phase once again to obtain a new TGK (and
   consequently derive new TEKs) or to update some other specific Crypto
   Session parameters.

2.4. Relation to GKMARCH

   The Group key management architecture (GKMARCH) [GKMARCH] describes a
   general architecture for group key management protocols. MIKEY is a
   part of this architecture, and can be used as a so called
   Registration protocol. The main entities involved in the architecture
   are a group controller/key server (GCKS), the receiver(s), and the
   sender(s).

   In MIKEY, the sender could act as GCKS and push down keys to the
   receiver(s).

   Note that e.g., in a SIP-initiated call, the sender may also be a
   receiver. As MIKEY addresses small interactive groups, a member may
   dynamically change between being a sender and receiver (or being both
   simultaneously).


3. Basic Key Transport and Exchange Methods

   The following sub-sections define three different methods to
   transport/establish a TEK Generation Key (TGK): with the use of a
   pre-shared key, public-key encryption, and Diffie-Hellman (DH) key
   exchange. In the following we for simplicity assume unicast
   communication. In addition to the TGK, a random "nonce", denoted
   RAND, is also transported. In all three cases, the TGK and RAND
   values are then used to derive TEKs as described in Section 4.1.4.

   The pre-shared key method and the public-key method are both based on
   key transport mechanisms, where the actual TGK is pushed (securely)
   to the recipient(s). In the Diffie-Hellman method, the actual TGK is
   instead derived from the Diffie-Hellman values exchanged between the
   peers.

   The pre-shared case is, by far, the most efficient way to handle the
   key transport due to the use of symmetric cryptography only. This
   approach has also the advantage that only a small amount of data has
   to be exchanged. Of course, the problematic issue is scalability as
   it is not always feasible to share individual keys with a large group



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   of peers. Therefore, this case mainly addresses scenarios such as
   server-to-client and also those cases where the public-key modes have
   already been used thus allowing to  "cache" a symmetric key (see
   below and Section 3.2).

   Public-key cryptography can be used to create a scalable system. A
   disadvantage with this approach is that it is more resource consuming
   than the pre-shared key approach. Another disadvantage is that in
   most cases a PKI (Public Key Infrastructure) is needed to handle the
   distribution of public keys. Of course, it is possible to use public
   keys as pre-shared keys (e.g. by using self-signed certificates). It
   should also be noted that, as mentioned above, this method may be
   used to establish a "cached" symmetric key that later can be used to
   establish subsequent TGKs by using the pre-shared key method (hence,
   the subsequent request can be executed more efficiently).

   The Diffie-Hellman (DH) key agreement method has in general a higher
   resource consumption (both computationally and in bandwidth) than the
   previous ones. However, it has the advantage of providing perfect
   forward secrecy (PFS) and flexibility by allowing implementation in
   several different finite groups.

   Note that by using the DH method, the two involved parties will
   generate a unique random key (which neither of the parties are likely
   to significantly affect the outcome of). Therefore, it is not
   possible to use this DH method to establish a group TEK (as the
   different parties in the group would end up with different TEKs). It
   is not the intention of the DH method to work in this scenario, but
   be a good alternative in the special peer-to-peer case.

   The following general notation is used:

   HDR:  The general MIKEY header, which includes MIKEY CSB related data
   (e.g. CSB ID) and information mapping to the specific security
   protocol used. See Section 6.1 for payload definition.

   T:    The timestamp. See Section 6.6 for payload definition and also
   Section 5.4 for other timestamp related information.

   IDx:  The identity of x. See Section 6.7 for payload definition.

   RAND: Random/pseudorandom byte-string, which is always included in
   the first message from the Initiator. It is not included in update
   messages of a CSB. See Section 6.11 for payload definition.

   SP:   The security policies for the data security protocol. See
   Section 6.10 for payload definition.







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3.1. Pre-shared key

   In this method, the pre-shared secret key, s, is used to derive key
   material for both the encryption (encr_key) and the integrity
   protection (auth_key) as described in Section 4.1.5. The encryption
   and authentication transforms are described in Section 4.2.


   Initiator                                 Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi],
        {SP}, KEMAC          --->
                                         R_MESSAGE =
                            [<---]       HDR, T, [IDr], V


   The main objective of the Initiator's message is to transport one or
   more TGKs and a set of data protocol parameters to the Responder in a
   secure manner. As the verification message from the Responder is
   optional, the Initiator indicates in the HDR whether it requires a
   verification message or not from the Responder.

   KEMAC = E(encr_key, {TGK}) || MAC

   The KEMAC payload contains a set of encrypted sub-payloads and a MAC.
   Each sub-payload includes a, by the Initiator, randomly and
   independently chosen TGK (and possible other related parameters,
   e.g., the key lifetime). The MAC is a Message Authentication Code
   covering the entire MIKEY message (with the exception of the MAC
   field itself) using the authentication key, auth_key. See Section 6.2
   for payload definition and Section 5.2 for exact definition of the
   MAC calculation.

   The main objective of the verification message from the Responder is
   to obtain mutual authentication. The verification, V, is a MAC
   computed over the Responder's entire message (with the exception of
   the Verification MAC field), the timestamp (that was included in the
   Initiator's message), and the two parties identities, using the
   authentication key. See also Section 5.2 for the exact definition of
   the Verification MAC calculation and Section 6.9 for payload
   definition.

   The ID fields are optional. However, leaving out an ID can only be
   done when it can be expected that the peer already knows the other
   party's ID (otherwise it cannot look up the pre-shared key). This
   could e.g. be the case if the ID is extracted from SIP.







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3.2. Public-key encryption


   Initiator                                          Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi], {SP},
       [CHASH], KEMAC, PKE, SIGNi    --->
                                                   R_MESSAGE =
                                      [<---]       HDR, T, [IDr], V

   The main objective of the Initiator's message is to transport one or
   more TGKs and a set of data protocol parameters to the Responder in a
   secure manner. This is done using an envelope approach where the TGKs
   are encrypted (and integrity protected) with keys derived from a
   randomly/pseudorandomly chosen "envelope key". The envelope key is
   sent to the Responder encrypted with the public key of the Responder.
   The KEMAC contains a set of encrypted sub-payloads and a MAC:

   KEMAC = E(encr_key, IDi || {TGK}) || MAC

   The first sub-payload is the identity of the Initiator (not a
   certificate, but generally the same ID as the one specified in the
   certificate). Each of the following sub-payloads includes a, by the
   Initiator, randomly and independently chosen TGK (and possible other
   related parameters, e.g., the key lifetime). The encrypted part is
   then followed by a MAC, which is calculated over the KEMAC payload
   with exception of the MAC field. The encr_key and the auth_key are
   derived from the envelope key, env_key, see Section 4.1.5. See also
   Section 6.2 for payload definition.

   The PKE contains the encrypted envelope key: PKE = E(PKr, env_key).
   It is encrypted using the Responder's public key. If the Responder
   posses several public keys, the Initiator can use CHASH to indicate
   the key used.

   The SIGNi is a signature covering the entire MIKEY message (excluding
   SIGN itself), using the Initiator's signature key (see also Section
   5.2 for the exact definition).

   The main objective of the verification message from the Responder is
   to obtain mutual authentication. As the verification message V from
   the Responder is optional, the Initiator indicates in the HDR whether
   it requires a verification message or not from the Responder. V is
   calculated in the same way as for the one in the pre-shared key mode
   (see also Section 5.2 for the exact definition). See Section 6.9 for
   payload definition.

   Note that there will be one encrypted IDi and possibly also one
   unencrypted IDi. The encrypted one is together with the MAC used as a
   countermeasure for certain man-in-the-middle attacks (while the



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   unencrypted is always useful for the Responder to immediately
   identify the Initiator). The encrypted IDi MUST always be verified to
   be equal with the expected IDi.

   It is possible to cache the envelope key, so that it can be used as a
   pre-shared key. It is not recommended to cache this key indefinitely
   (however it is up to the local policy to decide this). This function
   may be very convenient during the life-time of a Crypto Session
   Bundle, if a new crypto session needs to be added (or an expired one
   removed). Then, the pre-shared key can be used, instead of the public
   keys (see also Section 4.5). If the Initiator indicates that the
   envelope key should be cached, the key is at least to be cached
   during the life-time of the entire CSB.

   The ID fields and certificate are optional. However, leaving out an
   ID (or certificate) can only be done when it can be expected that the
   peer already knows the other party's ID (or can obtain the
   certificate in some other manner). This could e.g. be the case if the
   ID is extracted from SIP.

   For certificate handling, authorization and policies, see
   Section 4.3.

3.3. Diffie-Hellman key exchange

   For a fixed, agreed upon, cyclic group, (G,*), we let g denote a
   generator for this group. Choices for the parameters are given in
   Section 4.2.7. The other transforms below are described in Section
   4.2.

   With this method only one key is created, i.e. the DH-key, which is
   used as the TGK. This implies that this method cannot be used to
   create group keys, but only be used to create single peer-to-peer
   keys. This method is OPTIONAL to implement.

   Initiator                                          Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi],
        {SP}, DHi, SIGNi       --->
                                     R_MESSAGE =
                               <---  HDR, T, [IDr|CERTr], IDi,
                                          DHr, DHi, SIGNr


   The main objective of the Initiator's message is to, in a secure way,
   provide the Responder with its DH value (i.e., DHi = g^(xi), where xi
   MUST be randomly/pseudorandomly and secretly chosen) and a set of
   data protocol parameters.





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   The SIGNi is a signature covering the Initiator's MIKEY message,
   I_MESSAGE, using the Initiator's signature key.

   The main objective of the Responder's message is to, in a secure way,
   provide the Initiator with its own DH value (i.e., DHr = g^(xr),
   where xr MUST be randomly/pseudorandomly and secretly chosen).

   The SIGNr is a signature covering the Responder's MIKEY message,
   R_MESSAGE, using the Responder's signature key (see also Section 5.2
   for the exact definition).

   The group parameters (e.g., the group G, the generator g, etc) are a
   set of parameters chosen by the Initiator and signaled to the
   responder. Both parties calculate the TGK, g^(xi*xr) from the
   exchanged DH-values.

   Note that this approach does not require that the Initiator has to
   posses any of the responder's certificates before the setup. Instead,
   it is sufficient that the responder includes it's signing certificate
   in the response.

   The ID fields and certificate are optional. However, leaving out an
   ID (or certificate) can only be done when it can be expected that the
   peer already knows the other party's ID (or can obtain the
   certificate in some other manner). This could e.g. be the case if the
   ID is extracted from SIP.

   For certificate handling, authorization and policies, see
   Section 4.3.

4. Selected Key Management Functions

4.1. Key Calculation

   We define in the following a general method (pseudo random function)
   to derive one or more keys from a "master" key. This method is used
   to derive:

   * TEKs from a TGK and the RAND value,

   * encryption, authentication, or salting key from a pre-shared/
   envelope key and the RAND value.

4.1.1. Assumptions

   We assume that the following parameters are in place:

   csb_id: Crypto Session Bundle ID (32-bits unsigned integer)
   cs_id:  The Crypto Session ID (8-bits unsigned integer)
   RAND:   An (at least) 128-bit (pseudo)random bit-string sent by the
            Initiator in the initial exchange.



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   The key derivation method has the following input parameters:

   inkey:      the input key to the derivation function.
   inkey_len:  the length in bits of the input key.
   label:      a specific label, dependent on the type of the key to be
               derived, the RAND, and the session IDs.
   outkey_len: desired length in bits of the output key.

   The key derivation method has the following output:

   outkey: the output key of desired length.

4.1.2. Notation

   Let HMAC be the SHA1 based message authentication function, see
   [HMAC,SHA1]. Similar to [TLS], define:


      P (s, label, m) = HMAC (s, A_1 || label) ||
                        HMAC (s, A_2 || label) || ...
                        HMAC (s, A_m || label)
   where

      A_0 = label,
      A_i = HMAC (s, A_(i-1)).

   While SHA-1 is the default, HMAC using other hash function MAY be
   used, see Section 4.2.2.

4.1.3. PRF Description

   The following procedure describes a pseudo-random function, denoted
   PRF(inkey,label), applied to compute the output key, outkey:

   * let n = inkey_len / 512, rounded up to the nearest integer
   * split the inkey into n blocks, inkey = s_1 || ... || s_n, where all
      s_i, except possibly s_n, are 512 bits each
   * let m = outkey_len / 160,  rounded up to the nearest integer

   If another hash function than SHA1 is used, "512" and "160" SHALL be
   replaced by the appropriate input/output block-sizes of that
   function.

   Then, the output key, outkey, is obtained as the outkey_len most
   significant bits of

   PRF(inkey, label) = P(s_1, label, m) XOR P(s_2, label, m) XOR ...
                       XOR P(s_n, label, m).





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4.1.4. Generating keys from TGK

   The key derivation method should be executed with the following
   parameters to generate a TEK:

   inkey:      TGK
   inkey_len:  bit length of TGK
   label:      0x2AD01C64 || cs_id || csb_id || RAND
   outkey_len: bit length of the output TEK.

   If the security protocol does not support key derivation for
   authentication and encryption itself from the TEK, separate
   authentication and encryption keys MAY directly be created for the
   security protocol by replacing 0x2AD01C64 with 0x1B5C7973 and
   0x15798CEF respectively, and outkey_len by the desired key-length(s)
   in each case.

   A salt key can be derived from the TGK as well. This is done by using
   the constant 0x39A2C14B.

   Note that the 32-bit constant integers (i.e. 0x2AD01C64 or the one
   replacing it) are taken from the decimal digits of e (i.e.
   2.7182...), and where each constant consist of nine decimals digits
   (e.g. the first nine decimal digits 718281828 = 0x2AD01C64). The
   strings of nine decimal digits are not chosen at random, but as
   consecutive "chunks" from the decimal digits of e.

4.1.5. Generating keys from an envelope/pre-shared key

   This derivation is to form the symmetric encryption key (and salting
   key) for the encryption of the TGK in the pre-shared key and public
   key methods. This is also used to derive the symmetric key used for
   the message authentication code in these messages (and the
   corresponding verification messages). Hence, this derivation is
   needed in order to get different keys for the encryption and the MAC
   (and in the case of the pre-shared key, it will result in fresh key
   material for each new CSB).

   inkey:      the envelope key or the pre-shared key
   inkey_len:  the bit length of inkey
   label:      0x150533E1 || 0xFF || csb_id || RAND (for encryption key)
               or
               0x2D22AC75 || 0xFF || csb_id || RAND (for auth. key)
               or
               0x29B88916 || 0xFF || csb_id || RAND (for salting key)

   outkey_len: desired bit length of the
                authentication/encryption/salting key.






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4.2 Pre-defined Transforms and Timestamp Formats

   This section identifies standard transforms for MIKEY. The following
   transforms are mandatory to implement and support in the respective
   case. New transforms can be added in the future (see Section 4.2.9
   for further guidelines).

4.2.1 Hash functions

   In MIKEY, SHA-1 is the default hash function that is mandatory to
   implement.

4.2.2 Pseudo random number generator and PRF

   A cryptographically secure random or pseudo random number generator
   MUST be used for the generation of the keying material and nonces,
   e.g. [BMGL]. However, it is implementation specific which one to use
   (as the choice will not affect the interoperability).

   For the key derivations, the PRF specified in Section 4.1, using SHA-
   1 is mandatory to implement. This PRF MAY be extended by using SHA-
   256, SHA-384, or SHA-512, instead of SHA-1. However, it is not
   mandatory to support these.

4.2.3 Key data transport encryption

   The default and mandatory-to-implement key transport encryption is
   AES in counter mode, as defined in [SRTP], using a 128-bit key as
   derived in Section 4.1.5, and using initialization vector

   IV = [S XOR (0x0000 || CSB ID || T)] || 0x0000,

   where S is a 112-bit salting key, also derived as in Section 4.1.5,
   and where T is the timestamp sent by the Initiator.

   Note: this restricts the maximum size that can be encrypted to 2^23
   bits, which is still enough for all practical purposes.

   The NULL encryption algorithm (i.e., no encryption) can be used (but
   is not mandatory to implement). Note that this MUST NOT be used
   unless the underlying protocols can guarantee the security. The main
   reason for including this is for certain specific SIP scenarios,
   where SDP is protected end-to-end. For this scenario, MIKEY MAY be
   used with the pre-shared key method and the NULL encryption and
   authentication algorithm while relying on the security of SIP. Use
   this option with caution!

   The AES key wrap function [AESKW] is included as an optional (but not
   mandatory to implement) method. If the key wrap function is used, the
   NULL MAC is RECOMMENDED to be used when using the public key method
   (NOT the pre-shared key method) as the key wrap itself will provide



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   integrity of the keys. The 128-bit key and a 64-bit salt, S, are
   derived in accordance to Section 4.1.5 and the key wrap IV is then
   set to S.

4.2.4 MAC and Verification Message function

   MIKEY uses a 160-bit authentication tag, generated by HMAC with SHA-1
   as the mandatory to implement method, see [HMAC]. Authentication keys
   are derived according to Section 4.1.5. Note that the authentication
   key size MUST be equal to the size of the hash function's output
   (e.g. for HMAC-SHA-1, a 160-bit authentication key is used).

   The NULL authentication algorithm (i.e., no MAC) can be used together
   with the NULL encryption algorithm (but is not mandatory to
   implement). Note that this MUST NOT be used unless the underlying
   protocols can guarantee the security. The main reason for including
   this is for certain specific SIP scenarios, where SDP is protected
   end-to-end. For this scenario, MIKEY MAY be used with the pre-shared
   key method and the NULL encryption and authentication algorithm while
   relying on the security of SIP. Use this option with caution!

4.2.5 Envelope Key encryption

   The public key encryption algorithm applied is defined by, and
   dependent on the certificate used.

4.2.6 Digital Signatures

   The signature algorithm applied is defined by, and dependent on the
   certificate used.

4.2.7 Diffie-Hellman Groups

   The Diffie-Hellman key exchange uses OAKLEY 5 [OAKLEY] as mandatory
   to implement. Both OAKLEY 1 and OAKLEY 2 MAY be used (but these are
   not mandatory to implement).

   See Section 4.2.9 for the guidelines to specify a new DH Group to be
   used within MIKEY.

4.2.8. Timestamps

   The timestamp is as defined in NTP [NTP], i.e. a 64-bit number in
   seconds relative to 0h on 1 January 1900. An implementation MUST be
   aware of (and take into account) the fact that the counter will
   overflow approximately every 136th year. It is RECOMMENDED that the
   time is always specified in UTC.







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4.2.9. Adding new parameters to MIKEY

   There are two different parameter sets that can be added to MIKEY.
   The first is a set of MIKEY transforms (needed for the exchange
   itself), and the second is the data security protocol policies/
   parameters.

   New transforms and parameters SHALL be added by registering (with
   IANA) a new number for the concerned payload, and also if necessary,
   document how the new transform/parameter is used. Sometimes it might
   be enough to point to an already specified document for the usage,
   e.g., when adding a new already standardized hash function.

   In the case of adding a new DH group, the group needs to be specified
   in a companion RFC (it is RECOMMENDED that the specified group uses
   the same format as used in [OAKLEY]). A number can then be assigned
   by IANA for such a group to be used in MIKEY.

   When adding support for a new data security protocol, the following
   MUST be specified:

   * A map sub payload (see Section 6.1). This is used to be able to map
   a crypto session to the right instance of the data security protocol
   and possibly also to provide individual parameters for each data
   security protocol.

   * a policy payload, i.e., specification of parameters and supported
   values.

   * general guidelines of usage.


4.3. Certificates, Policies and Authorization

4.3.1. Certificate handling

   Certificate handling may involve a number of additional tasks not
   shown here, and effect the inclusion of certain parts of the message
   (c.f. [X.509]). The following observations can, however, be made:

   * The Initiator typically has to find the certificate of the
      Responder in order to send the first message. If the Initiator
      does not have the Responder's certificate already, this may
      involve one or more roundtrips to a central directory agent.

   * It will be possible for the Initiator to omit its own certificate
      and rely on the Responder getting this certificate using other
      means. However, we recommend doing this, only when it is
      reasonable to expect that the Responder has cached the certificate
      from a previous connection. Otherwise accessing the certificate
      would mean additional roundtrips for the Responder as well.



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   * Verification of the certificates using Certificate Revocation Lists
      (CRLs) [X.509] or protocols such as OCSP [OCSP] may be necessary.
      All parties in a MIKEY exchange should have a local policy which
      dictates whether such checks are made, how they are made, and how
      often they are made. Note that performing the checks may imply
      additional messaging.


4.3.2. Authorization

   In general, there are two different models for making authorization
   decisions for both the Initiator and the Responder, in the context of
   the applications targeted by MIKEY:

   * Specific peer-peer configuration. The user has configured the
      application to trust a specific peer.

      When pre-shared secrets are used, this is pretty much the only
      available scheme. Typically, the configuration/entering of the
      pre-shared secret is taken to mean that authorization is implied.

      In some cases one could use this also with public keys, e.g. if
      two peers exchange keys offline and configure them to be used for
      the purpose of running MIKEY.

   * Trusted root. The user accepts all peers that can prove to have a
      certificate issued by a specific CA. The granularity of
      authorization decisions is not very precise in this method.

      In order to make this method possible, all participants in the
      MIKEY protocol need to configure one or more trusted roots. The
      participants also need to be capable of performing certificate
      chain validation, and possibly transfer more than a single
      certificate in the MIKEY messages (see also Section 6.7).

   In practice, a combination of both mentioned methods might be
   advantageous. Also, the possibility for a user to explicitly exclude
   a specific peer (or sub tree) in a trust chain might be needed.

   These authorization policies address the MIKEY scenarios a-c of
   Section 2.1, where the Initiator acts as the group owner and who is
   also the only one that can invite others. This implies that for each
   Responder, the distributed keys MUST NOT be re-distributed to other
   parties.

   In a many-to-many situation, where the group control functions are
   distributed (and/or where it is possible to delegate the group
   control function to others), there MUST exist means to distribute
   authorization information about who may be added to the group.




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   However, it is out of scope for this document to specify how this
   should be done.

   For any broader communication situation, an external authorization
   infrastructure may be used (following the assumptions of [GKMARCH]).

4.3.3. Data Policies

   Included in the message exchange, policies for the Data security
   protocol are transmitted. The policies are defined in a separate
   payload and are specific to the security protocol (see also Section
   6.10). Together with the keys, the validity period of these can also
   be specified. This can be done e.g., with an SPI (or SRTP MKI) or
   with an Interval (e.g. a sequence number interval for SRTP),
   depending on the security protocol.

   New parameters can be added to a policy by documenting how they
   should be interpreted by MIKEY and also by registering new values in
   the appropriate name space. If a completely new policy is needed, see
   Section 4.2.9 for guidelines.

4.4. Retrieving the Data SA

   The retrieval of a Data SA will depend on the security protocol as
   different security protocols will have different characteristics.
   When adding support for a security protocol to MIKEY, some interface
   of how the security protocol retrieves the Data SA from MIKEY MUST be
   specified (together with policies that can be negotiated etc.).

   For SRTP the SSRC (see [SRTP]) is one of the parameters used to
   retrieve the Data SA. However, the SSRC is not sufficient. For the
   retrieval of the Data SA from MIKEY, it is RECOMMENDED that the MIKEY
   implementation supports a lookup using destination network address
   and port together with SSRC. Note that MIKEY does not send network
   addresses or ports. One reason for this is that they may not be known
   in advance, as well as if a NAT exists in-between, problems may
   arise. When SIP or RTSP is used, the local view of the destination
   address and port can be obtained from either SIP or RTSP. MIKEY can
   then use these addresses as the index for the Data SA lookup.

4.5. TGK re-keying and CSB updating

   MIKEY provides the means to update the CSB (e.g. transporting a new
   TGK/TEK or adding a new Crypto Session to the CSB). The updating of
   the CSB is done by the Initiator and performed by executing MIKEY
   again e.g. before a TEK expires, or when a new Crypto Session is
   added to the CSB. Note that MIKEY does not provide re-keying in the
   GKMARCH sense, only updating of the keys by normal unicast messages.

   When MIKEY is executed again to update the CSB, it is not necessary
   to include certificates and other information that was provided in



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   the first exchange, i.e. all payloads that are static or optional to
   include may be left out (see Figure 4.1).

   The new message exchange uses the same CSB ID as the initial
   exchange, but a new timestamp. A new RAND is NOT included in the
   message exchange (the RAND will only have affect in the Initial
   exchange). New Crypto Sessions are added if desired in the update
   message. Note that a MIKEY update message does not need to contain
   new keying material (i.e., new TGK). In this case the crypto session
   continues to use the previously established keying material, while
   updating the new information.

   As explained in Section 3.2, the envelope key can be "cached" as a
   pre-shared key (this is indicated by the Initiator in the first
   message sent). If so, the update message is a pre-shared key message
   (with the cached envelope key as the pre-shared key), i.e., it MUST
   NOT be a public key message. If the public key message is used, but
   the envelope key is not cached, the Initiator MUST provide a new
   encrypted envelope key that can be used in the verification message.
   However, the Initiator does not need to provide any other keys.

   Figure 4.1 visualizes the update messages that can be sent, including
   the optional parts. The big difference from the original message is
   mainly that it is optional to include TGKs (or DH values in the DH
   method). See also Section 3 for more details of the specific methods.

   By definition, a Crypto Session Bundle can contain several Crypto
   Sessions. A problem that then might occur is to synchronize the TGK
   re-keying if an SPI (or similar functionality, e.g., MKI) is not
   used. It is therefore recommended that an SPI or MKI is used, if more
   than one Crypto Session is used.


   Initiator                                       Responder

   Pre-shared key method:

   I_MESSAGE =
   HDR, T, [IDi], {SP}, KEMAC          --->
                                               R_MESSAGE =
                                      [<---]   HDR, T, [IDr], V

   Public key method:

   I_MESSAGE =
   HDR, T, [IDi|CERTi], {SP}, [CHASH],
        [KEMAC], PKE, SIGNi           --->
                                               R_MESSAGE =
                                      [<---]   HDR, T, [IDr], V





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   DH method:

   I_MESSAGE =
   HDR, T, [IDi|CERTi], {SP},
        [DHi], SIGNi                  --->
                                            R_MESSAGE =
                                      <---  HDR, T, [IDr|CERTr], IDi,
                                                 [DHr, DHi], SIGNr

   Figure 4.1: Update messages.

   Note that for the DH method, if the Initiator includes the DHi
   payload, then the responder MUST include DHr, DHi. If the initiator
   does not include DHi, the responder MUST NOT include DHr, DHi.

5. Behavior and message handling

   Each message that is sent by the Initiator or the Responder, is built
   by a set of payloads. This section describes how messages are created
   and also when they can be used.

5.1. General

5.1.1. Capability Discovery

   The initiator indicates the security policy to use (i.e. in terms of
   security protocol algorithms etc). If the Responder does not support
   it (for some reason), the Responder can together with an error
   message (indicating that it does not support the parameters), send
   back its own capabilities (negotiation) to let the Initiator choose a
   common set of parameters. This is done by including one or more
   security policy payloads. Multiple attributes can be provided in
   sequence in the response. This is done to reduce the number of
   roundtrips as much as possible (i.e. in most cases, where the policy
   is accepted the first time, one roundtrip is enough). If the
   Responder does not accept the offer, the Initiator must go out with a
   new MIKEY message.

   If the Responder is not willing/capable to provide security or the
   parties simply cannot agree, it is up to the parties' policies how to
   behave, i.e. accept an insecure communication or reject it.

   Note that it is not the intention of this protocol to have a very
   broad variety of options, as it is assumed that it should not be too
   common that an offer is denied.

5.1.2. Error Handling

   All errors due to the key management protocol SHOULD be reported to
   the peer(s) by an error message. The Initiator SHOULD therefore
   always be prepared to receive such message back from the Responder.



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   If the Responder does not support the set of parameters suggested by
   the Initiator, the error message SHOULD include the supported
   parameters (see also Section 5.1.2).

   The error message should be formed as:

   HDR, T, {ERR}, [V|SIGNr]

   Note that if the failure is due to the inability to authenticate the
   peer, the error message is OPTIONAL, and does not need to be
   authenticated. It is up to the local policy how to treat this kind of
   messages. However, if a signed error message in response to a failed
   authentication is returned this can be used for DoS purposes.
   Similarly, an unauthenticated error message could be sent to the
   Initiator in order to fool her to tear down the CSB. It is highly
   RECOMMENDED that the local policy takes this into consideration. One
   advice would be not to authenticate such an error message, and when
   receiving an unauthenticated error message only see it as a
   recommendation of what may have gone wrong.

5.2. Creating a message

   To create a MIKEY message, a Common header payload is first created.
   This payload is then followed, depending on the message type, by a
   set of information payloads (e.g. DH-value payload, Signature
   payload, Security Protocol payload). The defined payloads and the
   exact encoding of each payload are described in Section 6.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  version      !  data type    ! next payload  !               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...            +
   ~                   Common Header...                            ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! next payload  !   Payload 1 ...                               !
   +-+-+-+-+-+-+-+-+                                               +
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                             :                                 :
   :                             :                                 :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! next payload  !   Payload x ...                               !
   +-+-+-+-+-+-+-+-+                                               +
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                   MAC/Signature                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Figure 5.1. MIKEY payload message example. Note that the payloads are
   byte aligned and not 32-bit aligned.


   The process of generating a MIKEY message consists of the following
   steps:

   * Create an initial MIKEY message starting with the Common header
   payload.

   * Concatenate necessary payloads to the MIKEY message (see the
   exchange definitions for payloads that may be included and
   recommended order).

   * As a last step (for messages that must be authenticated, this also
   include the verification message), create and concatenate the
   MAC/signature payload without the MAC/signature field filled in (if a
   Next payload field is included in this payload, it is set to Last
   payload).

   * Calculate the MAC/signature over the entire MIKEY message, except
   the MAC/Signature field, and add the MAC/signature in the field. In
   the case of the verification message, the Identity_i || Identity_r ||
   Timestamp MUST follow directly after the MIKEY message in the
   Verification MAC calculation. Note that the identities and the
   timestamp that are added are identical to those transported in the ID
   and T payloads.

   In the public key case, the Key data transport payload is generated
   by concatenating the IDi with the TGKs. This is then encrypted and
   placed in the data field. The MAC is calculated over the entire Key
   data transport payload except the MAC field. Before calculating the
   MAC, the Next payload field is set to zero.

   Note that all messages from the Initiator MUST use a unique
   timestamp. The Responder does not create a new timestamp, but uses
   the timestamp used by the Initiator.

5.3. Parsing a message

   In general, parsing of a MIKEY message is done by extracting payload
   by payload and checking that no errors occur (the exact procedure is
   implementation specific). However, for the Responder, it is
   RECOMMENDED that the following procedure is followed:

   * Extract the Timestamp and check that it is within the allowable
   clock skew (if not, discard the message). Also check the replay cache
   so that the message is not replayed (see also Section 5.4). If the
   message is replayed, discard it.





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   * Extract ID and authentication algorithm (if not included, assume
   the default one).

   * Verify the MAC/signature.

   * If the authentication is not successful, an Auth failure Error
   message is possibly sent to the Initiator (if SIP is used, this is
   signaled to SIP as a rejection of the offer). The message is then
   discarded from further processing. See also Section 5.1.2 for
   treatment of errors.

   * If the authentication is successful, the message is processed and
   also added to the replay cache. How it is processed is implementation
   specific. Note also that it is only successfully authenticated
   messages that are stored in the replay cache.

   * If any unsupported parameters or errors occur during the
   processing, these are reported to the Initiator by sending an error
   message. The processing is then aborted. The error message can also
   include payloads to describe the supported parameters. If SIP is
   used, this is signaled to SIP as a rejection of the offer (see also
   Section 7.2).

   * If the processing was successful and if needed, a verification/
   response message is created and sent to the Initiator.

5.4. Replay handling and timestamp usage

   MIKEY does not use a challenge-response mechanism for replay handling
   instead timestamps are used. This requires that the clocks are
   synchronized. The required synchronization is dependent on the number
   of messages that can be cached (note though, that the replay cache
   only contain messages that has been successfully authenticated). If
   we could assume an unlimited cache, the terminals would not need to
   be synchronized at all (as the cache could then contain all previous
   messages). However, if there are restrictions on the size of the
   replay cache, the clocks will need to be synchronized to some extent.
   In short, one can in general say that it is a tradeoff between the
   size of the replay cache and the required synchronization.

   Timestamp usage prevents against replay attacks under the following
   assumptions:

   * Each host have a clock which is at least "loosely synchronized" to
   the clocks of the other hosts.

   * If the clocks are to be synchronized over the network, a secure
   network clock synchronization protocol is used.






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   * Each Responder utilizes a replay cache in order to remember the
   successfully authenticated messages presented within an allowable
   clock skew (which is set by the local policy).

   * Replayed and outdated messages, i.e., messages that can be found in
   the replay cache or which have an outdated timestamp, are discarded
   and not processed.

   * If the host loses track of the incoming requests (e.g. due to
   overload), it rejects all incoming requests until the clock skew
   interval has passed.

   In a client-server scenario, servers may be the entities that will
   have the highest workload. It is therefore RECOMMENDED that the
   servers are the Initiators of MIKEY. This will result in that the
   servers will not need to manage any significant replay cache as they
   will refuse all incoming messages that are not a response to an
   already (by the server) sent message.

   In general, a client may not expect a very high load of incoming
   messages and may therefore allow the degree of looseness to be on the
   order of several minutes to hours. If a (D)DoS attack is launched and
   the replay cache grows too large, MIKEY MAY dynamically decrease the
   looseness so that the replay cache becomes manageable (however, note
   that such (D)DoS can only be performed by peers that can authenticate
   themselves). Another, way to treat the problem is of course to reject
   all incoming messages from the specific peer(s) that executes the
   attack.

   The maximum number of messages that a client will need to cache may
   vary depending on the capacity of the client itself and the network,
   but also the number of expected messages should be taken into
   account.

   For example, assume that we can at most spend 6kB on a replay cache.
   Assume further that we need to store 30 bytes for each incoming
   authenticated message (the hash of the message is 20 bytes). This
   implies that it is possible to cache approximately 204 messages. If
   the expected number of messages per minute can be estimated, the
   clock skew can easily be calculated. E.g., in a SIP scenario where
   the client is expected in the most extreme case to receive 10 calls
   per minute, the clock skew needed is then approximately 20 minutes.
   In a not so extreme setting, where one could expect an incoming call
   every 5th minute, this would result in a clock skew on the order of
   16.5 hours (approx 1000 minutes).

   Consider a very extreme case, where the maximum number of incoming
   messages are assumed to be on the order of 120 messages per minute,
   and a requirement that the clock skew is on the order of 10 minutes,
   a 48kB replay cache would be required.




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   Hence, one can note that the required clock skew will depend very
   much on the setting in which MIKEY is used. One recommendation is to
   fix a size for the replay cache, and let the allowable clock skew be
   large (the initial clock skew can be set depending on the application
   in which it is used). As the replay cache grows, the clock skew is
   decreased depending on how many percent of the replay cache that are
   used. Note that this is locally handled, which will not require
   interaction with the peer (even though it may indirectly affect the
   peer). Exactly how to implement such functionality is however out of
   the scope of this document and considered implementation specific.

   In case of a DoS attack, the client will most likely be able to
   handle the replay cache. A bigger problem will probably be to process
   the messages (verify signatures/MACs), due to the computational
   workload this implies.

5.5. Reliability

   If MIKEY is sent on an unreliable transport, the basic processing
   applied to ensure protocol reliability is the following.

   The transmitting entity (Initiator or Responder) MUST:

   * Set a timer and initialize a retry counter

   * If the timer expires, the message is resent and the retry counter
   is decreased.

   * If the retry counter reaches zero (0), the event MAY be logged in
   the appropriate system audit file.

6. Payload Encoding

   This section describes in detail all the payloads. For all encoding,
   Network byte order is always used.

6.1. Common header payload (HDR)

   The Common header payload MUST always be present as the first payload
   in each message. The common header includes general description of
   the exchange message.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  version      !  data type    ! next payload  !V! PRF func    !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                         CSB ID                                !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! #CS           ! CS ID map type! CS ID map info                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The common header contains the following information:

   * version: the version number of MIKEY.

     version = 1


   * data type: describes the type of message (e.g. public-key transport
   message, verification message, error message).

     Data type     | Value | Comment
     --------------------------------------
     Pre-shared    |     0 | Initiator's pre-shared key message
     PSK ver msg   |     1 | Verification message of a Pre-shared
                   |       | key message
     Public key    |     2 | Initiator's public-key transport message
     PK ver msg    |     3 | Verification message of a public-key
                   |       | message
     D-H init      |     4 | Initiator's DH exchange message
     D-H resp      |     5 | Responder's DH exchange message
     Error         |     6 | Error message


   * next payload: identifies the payload that is added after this
   payload.

     Next payload  | Value | Section
     ------------------------------
     Last payload  |     0 | -
     KEMAC         |     1 | 6.2
     PKE           |     2 | 6.3
     DH            |     3 | 6.4
     SIGN          |     4 | 6.5
     T             |     5 | 6.6
     ID            |     6 | 6.7
     CERT          |     7 | 6.7
     CHASH         |     8 | 6.8
     V             |     9 | 6.9
     SP            |    10 | 6.10
     RAND          |    11 | 6.11
     ERR           |    12 | 6.12
     Key data      |    20 | 6.13
     General Ext.  |    21 | 6.15

   Note that some of the payloads cannot possibly come right after the
   header (such as "Last payload", "Signature", etc.). However, the Next
   payload field is generic for all payloads. Therefore, a value is
   allocated for each payload.





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   * V: flag to indicate whether a verification message is expected or
   not (this has only meaning when it is set by the Initiator).

     V = 0  ==> no response expected
     V = 1  ==> response expected


   * PRF func: Indicates the PRF function that has been/will be used for
   key derivation etc.

     PRF func      | Value | Comments
     --------------------------------------------------------
     MIKEY-1       |     0 | Mandatory, Default (see Section 4.1.2-3)
     MIKEY-256     |     1 | (as MIKEY-1 but using a HMAC with SHA256)
     MIKEY-384     |     2 | (as MIKEY-1 but using a HMAC with SHA384)
     MIKEY-512     |     3 | (as MIKEY-1 but using a HMAC with SHA512)


   * CSB ID: A 32-bit integer to identify the CSB. It is RECOMMENDED
   that it is chosen at random by the Initiator. This ID MUST be unique
   between each Initiator-Responder pair, i.e., not globally unique. An
   Initiator MUST check for collisions when choosing the ID (if the
   Initiator already has one or more established CSB with the
   Responder). The Responder uses the same CSB ID in the response.


   * #CS: Indicates the number of Crypto Sessions that will be handled.
   Note that even though it is possible to use 255 CSs, it is not likely
   that a CSB will include this many CSs. The integer 0 is interpreted
   as no CS included. This may be the case in an initial setup message.


   * CS ID map type: specifies the method to uniquely map Crypto
   Sessions to the security protocol sessions.

     CS ID map type | Value
     -----------------------
     SRTP-ID        |     0


   * CS ID map info: Identifies the crypto session(s) that the SA should
   be created for. The currently defined map type is the SRTP-ID
   (defined in Section 6.1.1).











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6.1.1. SRTP ID

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Policy_no_1   ! SSRC_1                                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ SSRC_1 (cont) ! ROC_1                                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ ROC_1 (cont)  ! Policy_no_2   ! SSRC_2                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ SSRC_2 (cont)                 ! ROC_2                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ ROC_2 (cont)                  !                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
   :                               :                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Policy_no_#CS !           SSRC_#CS                            ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~SSRC_#CS (cont)!           ROC_#CS                             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ ROC_#CS (cont)!
   +-+-+-+-+-+-+-+-+

   * Policy_no_i: The policy applied for the stream with SSRC_i. The
   same policy may apply for all CSs.

   * SSRC_i: specifies the SSRC that MUST be used for the i-th SRTP
   stream. Note that it is the sender of the streams who chooses the
   SSRC. Therefore, it might be that the Initiator of MIKEY can not fill
   in all fields. In this case, SSRCs that are not chosen by the
   Initiator are set to zero and the Responder fills in these field in
   the response message. It is in general RECOMMENDED or required to use
   unique SSRCs (both to avoid RTP SSRC collision, and from an SRTP
   perspective, to avoid two-time pad problems if the same TEK is used
   for more than one stream).

   * ROC_i: Current rollover counter used in SRTP. If the SRTP session
   has not started, this field is set to 0. This field is used to be
   able for a member to join and synchronize to an already started
   stream.

   NOTE: The stream using SSRC_i will also have Crypto Session ID equal
   to no i (NOT to the SSRC).

6.2. Key data transport payload (KEMAC)

   The Key data transport payload contains encrypted Key data payloads
   (see Section 6.13 for definition of Key data payloads). It may
   contain one or more Key data payloads each including a TGK. The last
   Key data payload has its Next payload field set to Last payload. For



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   an update message (see also Section 4.5), it is allowed to skip the
   Key data payloads (which will result in that the Encr data len is
   equal to 0).

   If the transport method used is the pre-shared key method, this Key
   data transport payload is the last payload in the message (note that
   the Next payload field is set to Last payload). The MAC is then
   calculated over the entire MIKEY message (as described in Section
   5.2).

   If the transport method used is the public-key method, the
   Initiator's identity is added in the encrypted data. This is done by
   adding the ID payload as the first payload, which then are followed
   by the Key data payloads. Note that for an update message, the ID is
   still sent encrypted to the Responder (this is to avoid certain re-
   direction attacks) even though no Key data payloads is added after.

   The MAC field is in the public-key case calculated only over the Key
   data transport payload except the MAC field and where the Next
   payload field has been set to zero (see also Section 5.2).

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! Encr alg      ! Encr data len                 !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                        Encr data                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Mac alg       !        MAC                                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload (see Section 6.1 for defined values).

   * Encr alg: The encryption algorithm used to encrypt the TGK.

     Encr alg      | Value | Comments
     -------------------------------------------
     AES-CM-128    |     1 | Mandatory (as defined in Section 4.2.3,
   using a 128-bit key)
     NULL          |     2 | Very restricted usage, see Section 4.2.3!
     AES-KW-128    |     3 | Optional (AES Key Wrap, see also Section
   4.2.3)

   * Encr len: Length of encrypted part (in bytes).

   * Encr data: The encrypted TGK sub-payloads (see Section 6.13).

   * MAC alg specifies the authentication algorithm used.





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     MAC alg       | Value | Comments
     --------------------------------------
     HMAC-SHA1-160 |     0 | Mandatory (see Section 4.2.4)
     NULL          |     1 | Very restricted usage, see Section 4.2.4!

   * MAC: The message authentication code of the entire message.

6.3. Envelope data payload (PKE)

   The Envelope data payload contains the encrypted envelope key that is
   used in the public-key transport to protect the data in the Key data
   transport payload. The encryption algorithm used is implicit from the
   certificate/public key used.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  ! C ! Data len                  ! Data          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload.

   * C: Envelope key cache indicator (see also Section 3.2, for more
   information of the usage).

     Cache type    | Value | Comments
     --------------------------------------
     No cache      |     0 | The envelope key MUST NOT be cached
     Cache         |     1 | The envelope key MUST be cached
     Cache for CSB |     2 | The envelope key MUST be cached, but only
                   |       | to be used for the specific CSB.

   * Data len: The length of the data field (in bytes).

   * Data: The encrypted envelope key (if nothing else stated in the
   certificate, padding and formatting is done according to RSA/PKCS#1
   if RSA is used).

6.4. DH data payload (DH)

   The DH data payload carries the DH-value and indicates the DH-group
   used. Notice that in this sub-section "MANDATORY" is conditioned upon
   DH being supported at all.
                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! DH-Group      !  DH-value                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Reserv! KV    ! KV data (optional)                            ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   * next payload: identifies the payload that is added after this
   payload.

   * DH-Group: identifies the DH group used.

     DH-Group      | Value | Comments
     --------------------------------------
     OAKLEY 5      |     0 | Mandatory
     OAKLEY 1      |     1 |
     OAKLEY 2      |     2 |

   * DH-value: The public DH-value (the length is implicit from the
   group used).

   * KV: Indicates the type of key validity period specified. This may
   be done by using an SPI (alternatively an MKI) or by providing an
   interval in which the key is valid (e.g. in the latter case, for SRTP
   this will be the index range where the key is valid). See Section
   6.13 for pre-defined values.

   * KV data: This includes either the SPI/MKI or an interval (see
   Section 6.14). If KV is NULL, this field is not included.

6.5. Signature payload (SIGN)

   The Signature payload carries the signature and its related data. The
   signature payload is always the last payload in the PK transport and
   DH exchange messages. The signature algorithm used is implicit from
   the certificate/public key used.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! S type| Signature len         ! Signature                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * S type: Indicates the signature algorithm applied by signer.

     S type        | Value | Comments
     -------------------------------------
     RSA/PKCS#1/1.5|     0 | Denotes a PKCS #1 version 1.5 signature
     RSA/PSS       |     1 | Denotes a PSS signature

   * Signature len: The length of the signature field (in bytes).

   * Signature: The signature (if nothing else stated in the
   certificate, padding and formatting is done according to RSA/PKCS#1
   if RSA is used).





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6.6. Timestamp payload (T)

   The timestamp payload carries the timestamp information.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   TS type     ! TS value                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload. If no more payload follows, it MUST be set to Last payload.
   See Section 6.1 for values.

   * TS type: specifies the timestamp type used.

     TS type       | Value | Comments
     -------------------------------------
     NTP-UTC       |     0 | Mandatory (64-bits)
     NTP           |     1 | Mandatory (64-bits)
     COUNTER       |     2 | Optional (32-bits)

   * TS-value: The timestamp value of the specified TS type.

6.7. ID payload (ID) / Certificate payload (CERT)

   Note that the ID payload and the Certificate payload are two
   completely different payloads (having different payload identifiers).
   However, as they share the same payload structure they are described
   in the same section.

   The ID payload carries a uniquely-defined identifier.

   The certificate payload contains an indicator of the certificate
   provided as well as the certificate data. If a certificate chain are
   to be provided, each certificate in the chain should be included in a
   separate CERT payload.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! ID/Cert Type  ! ID/Cert len                   !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                       ID/Certificate Data                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload. See Section 6.1 for values.

   If the payload is an ID payload the following values applies for the
   ID type field:



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   * ID Type: specifies the identifier type used.

     ID Type       | Value | Comments
     ----------------------------------------------
     NAI           |     0 | Mandatory (see [NAI])
     URI           |     1 | Mandatory (see [URI])

   If the payload is an Certificate payload the following values applies
   for the Cert type field:

   * Cert Type: specifies the certificate type used.

     Cert Type     | Value | Comments
     ----------------------------------------------
     X.509v3       |     0 | Mandatory
     X.509v3 URL   |     1 | plain ASCII URL to the location of the Cert
     X.509v3 Sign  |     2 | Mandatory (used for signatures only)
     X.509v3 Encr  |     3 | Mandatory (used for encryption only)

   * ID/Cert len: The length of the ID or Certificate field (in bytes).

   * ID/Certificate: The ID or Certificate data. The X.509 [X.509]
   certificates are included as a bytes string using DER encoding as
   specified in X.509.

6.8. Cert hash payload (CHASH)

   The Cert hash payload contains the hash of the certificate used.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  ! Hash func     ! Hash                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload.

   * Hash func: Indicates the hash function that is used (see also
   Section 4.2.1).

     Hash func     | Value
     ----------------------
     SHA-1         |     0  Mandatory
     SHA256        |     1
     SHA384        |     2
     SHA512        |     3
     MD5           |     4





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   * Hash: The hash data. Note: the hash length is implicit from the
   hash function used.

6.9. Ver msg payload (V)

   The Ver msg payload contains the calculated verification message in
   the pre-shared key and the public-key transport methods. Note that
   the MAC is calculated over the entire MIKEY message as well as the
   IDs and Timestamp (see also Section 5.2).

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  ! Auth alg      ! Ver data                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload. If no more payload follows, it is set to Last payload. See
   Section 6.1 for values.

   * Auth alg: specifies the MAC algorithm used for the verification
   message. See Section 6.2 for defined (MAC field) for defined values.

   * Ver data: The verification message data. Note: the length is
   implicit from the authentication algorithm used.

6.10. Security Policy payload (SP)

   The Security Policy payload defines a set of policies that applies to
   a specific security protocol.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! Policy no     ! Prot type     ! Policy param  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~ length (cont) ! Policy param                                  ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * Next payload: identifies the payload that is added after this
   payload. See Section 6.1 for values.

   * Policy no: Each security policy payload must be given a distinct
   number for the current MIKEY session by the local peer. This number
   is used to be able to map a crypto session to a specific policy (see
   also Section 6.1.1).

   * Prot type: defines the security protocol.






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     Prot type     | Value |
     ---------------------------
     SRTP          |     0 |

   * Policy param length: defines the total length of the policy
   parameters for the specific security protocol.

   * Policy param: defines the policy for the specific security
   protocol.

   The Policy param part is built up by a set of Type/Length/Value
   fields. For each security protocol, a set of possible types/values
   that can be negotiated are defined.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Type          ! Length        ! Value                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * Type: specifies the type of the parameter.

   * Length: specifies the length of the Value field (in bytes).

   * Value: specifies the value of the parameter.

6.10.1. SRTP policy

   This policy specifies the parameters for SRTP and SRTCP. The
   types/values that can be negotiated are defined by the following
   table:

     Type | Meaning                     | Possible values
     ----------------------------------------------------
        0 | Encryption algorithm        | see below
        1 | Session Encr. key length    | depends on cipher used
        2 | Authentication algorithm    | see below
        3 | Session Auth. key length    | depends on MAC used
        4 | Session Salt key length     | see [SRTP] for recommendations
        5 | SRTP Pseudo Random Function | see below
        6 | Key derivation rate         | see [SRTP] for recommendations
        7 | SRTP encryption off/on      | 0 if off, 1 if on
        8 | SRTCP encryption off/on     | 0 if off, 1 if on
        9 | senderÆs FEC order          | see below
       10 | SRTP authentication off/on  | 0 if off, 1 if on
       11 | Authentication tag length   | in bytes
       12 | SRTP prefix length          | in bytes

   Note that if a Type/Value is not set, the default one is used
   (according to SRTPs own criteria).




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   For the Encryption algorithm, it is enough with a one byte length and
   the currently defined possible Values are:

     SRTP encr alg | Value
     ---------------------
     NULL          |     0
     AES-CM        |     1
     AES-F8        |     2

   where AES-CM is AES in CM and AES-F8 is AES in f8 mode [SRTP].

   For the Authentication algorithm, it is enough with a one byte length
   and the currently define possible Values are:

     SRTP auth alg | Value
     ---------------------
     NULL          |     0
     HMAC-SHA1     |     1

   For the SRTP pseudo random function, it is also enough with a one
   byte length and the currently define possible Values are:

     SRTP PRF      | Value
     ---------------------
     AES-CM        |     0

   If FEC is used at the same time as SRTP is used, MIKEY can negotiate
   the order in which these should be applied at the sender side.

     FEC order     | Value | Comments
     --------------------------------
     FEC-SRTP      |     0 | First FEC, then SRTP
     SRTP-FEC      |     1 | First SRTP, then FEC

6.11. RAND payload (RAND)

   The RAND payload consist of a (pseudo)random bit-string. The RAND
   MUST be independently generated per CSB (note that the if a CSB has
   several members, the Initiator MUST use the same RAND to all the
   members).

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! RAND len      ! RAND                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * Next payload: identifies the payload that is added after this
   payload.

   * RAND len: Length of the RAND (in bytes). SHOULD be at least 16.



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   * RAND: a (pseudo)randomly chosen bit-string.

6.12. Error payload (ERR)

   The Error payload is used to specify the error(s) that may have
   occurred.
                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! Error no      !           Reserved            !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * next payload: identifies the payload that is added after this
   payload. If no more payload follows, it is set to Last payload. See
   Section 6.1 for values.

   * Error no indicates the type of error that was encountered.

     Error no      | Value | Comment
     -------------------------------------------------------
     Auth failure  |     0 | Authentication failure
     Invalid TS    |     1 | Invalid timestamp
     Invalid PRF   |     2 | PRF function not supported
     Invalid MAC   |     3 | MAC algorithm not supported
     Invalid EA    |     4 | Encryption algorithm not supported
     Invalid HA    |     5 | Hash function not supported
     Invalid DH    |     6 | DH group not supported
     Invalid ID    |     7 | ID not supported
     Invalid Cert  |     8 | Certificate not supported
     Invalid SP    |     9 | SP type not supported
     Invalid SPpar |    10 | SP parameters not supported

6.13. Key data sub-payload

   The Key data payload contains TGKs. The Key data payloads are never
   included in clear, but as an encrypted part of the Key data transport
   payload.
                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Next Payload ! Type  ! KV    ! Key data len                  !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                         Key data                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Salt len (optional)           ! Salt data (optional)          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                        KV data (optional)                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   * Next payload: identifies the payload that is added after this
   payload.

   * Type: Indicates the type of the key included in the payload. Note
   that generally TEKs are not sent directly, but a TGK, which is then
   used to derive the TEK (or TEKs if there are several crypto sessions)
   as described in Section 4.1.4.

     Type     | Value | Comments
     ---------------------------------------
     TGK      |     0 | A TGK (used to derive TEKs from)
     TGK+SALT |     1 | A TGK + a salt key are included
     TEK      |     2 | A plain TEK
     TEK+SALT |     3 | A plain TEK + a salt key are included

   Note that the possibility to include a TEK (instead of using the TGK)
   is provided. However, if this is used, the TEK can generally not be
   shared between more than one Crypto Session. The recommended use of a
   TEK instead of a TGK is when pre-encrypted material exists and
   therefore, the TEK must be known in advance.

   * KV: Indicates the type of key validity period specified. This may
   be done by using an SPI (or MKI in the case of [SRTP]) or by
   providing an interval in which the key is valid (e.g., in the latter
   case, for SRTP this will be the index range where the key is valid).

     KV            | Value | Comments
     -------------------------------------------
     Null          |     0 | No specific usage rule (e.g. a TEK
                   |       | that has no specific lifetime)
     SPI           |     1 | The key is associated with the SPI/MKI
     Interval      |     2 | The key has a start and expiration time
                   |       | (e.g. an SRTP TEK)

   Note that when NULL is specified, any SPI or Interval is valid. For
   an Interval this means that the key is valid from the first observed
   sequence number until the key is replaced (or the security protocol
   is shutdown).

   * Key data len: The length of the Key data field (in bytes). Note
   that the sum of the overall length of all the Key data sub-payloads
   contained in a single Key data transport payload (KEMAC) MUST be such
   that the KEMAC payload does not exceed a length of 2^16 bytes (total
   length of KEMAC, see Section 6.2).

   * Key data: The TGK or TEK data.

   * Salt len: The salt key length in bytes. Note that this field is
   only included if the salt is specified in the Type-field.





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   * Salt data: The salt key data. Note that this field is only included
   if the salt is specified in the Type-field. (For SRTP, this is the
   so-called master salt.)

   * KV data: This includes either the SPI or an interval (see Section
   6.14). If KV is NULL, this field is not included.

6.14. Key validity data

   The Key validity data is not a standalone payload, but part of either
   the Key data payload (see Section 6.13) or the DH payload (see
   Section 6.4). The Key validity data gives a guideline of when the key
   should be used. This can be done, using an SPI/MKI or a lifetime
   range.

   SPI/MKI
                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! SPI Length    ! SPI                                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * SPI Length: The length of the SPI (or MKI) in bytes.

   * SPI: The SPI (or MKI) value.

   Interval
                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! VF Length     ! Valid from                                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! VT Length     ! Valid to (expires)                            ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * VF Length: Length of the Valid From field in bytes.

   * Valid From: Sequence number, index, timestamp, or other start value
   that the security protocol uses to identify the start position of the
   key usage.

   * VT Length: Length of the Valid To field in bytes.

   * Valid to: Sequence number, index, timestamp, or other expiration
   value that the security protocol can use to identify the expiration
   of the key usage.

   Note that for SRTP usage, the key validity period for a TGK should be
   specified with either an interval, where the VF/VT length is equal to
   6 bytes (i.e., the size of the index), or, with an MKI. It is
   RECOMMENDED that if more than one SRTP stream is sharing the same



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   keys and key update/re-keying is desired, this is handled using MKI
   rather than the From-To method.

6.15. General Extension Payload

   The General extensions payload is included to allow possible
   extensions to MIKEY without the need to define a complete new payload
   each time. This payload can be used in any MIKEY message and is part
   of the authenticated/signed data part.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next payload  ! Type          ! Length                        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Data                                                          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   * Next payload: identifies the payload that is added after this
   payload.

   * Type: identifies the type of the general payload.

     Type      | Value | Comments
     ---------------------------------------
     Vendor ID |     0 | Vendor specific byte string
     SDP IDs   |     1 | List of SDP key mgmt IDs (see also Section 7.1)

   * Length: the length in bytes of the Data field.

   * Data: the general payload data.


7. Integration with session establishment protocols

   This section describes how MIKEY should be integrated with SDP, SIP
   and RTSP. It is based on [KMASDP], which describes extensions to
   SIP/SDP and RTSP to carry key management protocol information.

7.1. SDP integration

   SDP descriptions [SDP] can be carried by several protocols, such as
   SIP and RTSP. Both SIP and RTSP often use SDP to describe the media
   sessions. Therefore, it is also convenient to be able to integrate
   the key management in the session description it is supposed to
   protect. [KMASDP] describes attributes that should be used by a key
   management protocol that is integrated in SDP. We refer to [KMASDP]
   for both definitions and examples. Note that MIKEY SHALL use the name
   "mikey" as a protocol identifier in SDP and RTSP. The key management
   data that is placed in SDP or RTSP MUST be base64 encoded.




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   Following the requirements in [KMASDP] (to avoid certain types of
   bidding down attacks when more than one key management protocol is
   offered within SDP), MIKEY SHALL specify how to authenticate the list
   of identifiers in SDP. In MIKEY, the list of protocol identifiers
   MUST be input to MIKEY by SDP with ";" separated identifiers. All the
   offered protocol identifiers MUST be included, in the same order as
   they appear in the corresponding SDP description; if only MIKEY is
   offered, the only protocol identifier to be included SHALL be
   "mikey". MIKEY MUST place this list in a General Extension Payload
   (of type "SDP IDs") which then automatically will be integrity
   protected/signed. The receiver can then match the list in the General
   Extension Payload with the list included in SDP and SHOULD (according
   to policy) if they differ, or if integrity/signature verification
   fails, reject the offer. Further description can be found in
   [KMASDP].

7.2. MIKEY within SIP

   In e.g., a basic SIP call (see Figure 7.1.), SIP (Session Initiation
   Protocol, [SIP]) is used as a session establishment protocol between
   two or more parties. In general an offer is made, whereby it is
   either accepted or rejected by the answerer. SIP complies to the
   offer/answer model [OFFANS], to which MIKEY over SIP MUST be
   compliant with as well.


                          ---------           ---------
                          |AÆs SIP| <.......> |BÆs SIP|
                          |Server | SIP/MIKEY |Server |
                          ---------           ---------
                               ^                ^
                               .                .
             ++++    SIP/MIKEY .                .   SIP/MIKEY   ++++
             |  | <.............                ..............> |  |
             |  |                                               |  |
             ++++ <-------------------------------------------> ++++
                                      SRTP

   Fig 7.1.: SIP-based call example. The two parties uses MIKEY over SIP
   to set up an SRTP stream between A and B.


   The SIP offerer will be the MIKEY Initiator and the SIP answerer will
   be the MIKEY Responder. This implies that in the offer, the MIKEY
   Initiator's message is included, and in the answer to the offer, the
   MIKEY Responder's message is included.

   If the MIKEY part of the offer is not accepted, a MIKEY error message
   is provided in the answer (following Section 5.1.2). The MIKEY
   implementation signals to the SIP implementation whether the MIKEY
   message was an acceptable offer or not.



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   It may be assumed that the offerer knows the identity of the
   answerer. However, unless the InitiatorÆs identity can be derived
   from SIP itself, the Initiator (caller) MUST provide the identity to
   the callee. It is RECOMMENDED to use the same identity for both SIP
   and MIKEY.

   Updating of the CSB (e.g. TEK update) is only supposed to be seen as
   a new offer. Note that it might not be necessary to send all
   information, such as the certificate, due to the already established
   call (see also Section 4.5).

7.3. MIKEY with RTSP

   The Real Time Streaming Protocol (RTSP) [RTSP] is used to control
   media streaming from a server. The media session is typically
   obtained via an SDP description, received by a DESCRIBE message, or
   by other means (e.g., HTTP). To be able to pass the MIKEY messages in
   RTSP messages which does not contain an SDP description, the RTSP
   KeyMgmt header (defined in [KMASDP]) is used. This header includes
   basically the same fields as the SDP extensions. As for SDP, "mikey"
   is used as the protocol identifier.

   In an RTSP scenario, the RTSP server and the MIKEY Initiator will be
   the same entity. The Initiator/RTSP server includes the MIKEY message
   in an SDP description. When responding to this, the client uses the
   defined RTSP header to send back the answer (included in the SETUP
   message).

   Note that it is the server that will be the Initiator of MIKEY in
   this case. This has some advantages. First, the server will always be
   able to choose the key for the content it distributes. Secondly, it
   will then have the possibility to use the same key for the same
   content that are streamed/sent to more than one client.

   To be able to have a server-initiated CSB update procedure, the
   ANNOUNCE message is used to send the updated MIKEY material. Note
   that the ANNOUNCE method has the ability to send SDP descriptions to
   update previous ones (i.e., it is not required to use the RTSP
   KeyMgmt header from server to client).

7.4. MIKEY Interface

   The SDP, SIP, and RTSP processing is defined in [KMASDP]. However, it
   is necessary that MIKEY can work properly with these protocols. This
   subsection describes some aspects which implementers SHOULD consider.
   If the MIKEY implementation is separate from the SDP/SIP/RTSP, an
   application programming interface (API) between MIKEY and these
   protocols is needed with certain functionality (however, exactly what
   it looks like is implementation dependent).




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   Implementers of MIKEY are RECOMMENDED to consider providing at least
   the following functionality:

   * the possibility for MIKEY to receive information about the sessions
   negotiated. This is to some extent implementation dependent. But it
   is RECOMMENDED that, in the case of SRTP streams, the number of SRTP
   streams are included (and the direction of these). The destination
   addresses and ports is also RECOMMENDED to be provided to MIKEY.

   * the possibility for MIKEY to receive incoming MIKEY messages and
   return a status code from/to the SIP/RTSP application.

   * the possibility for the SIP or RTSP applications to receive
   information from MIKEY. This would typically include the receiving of
   the CSB ID or the SSRCs for SRTP. It is also RECOMMENDED that extra
   information about errors can be received.

   * the possibility for the SIP or RTSP application to receive outgoing
   MIKEY messages.

   * the possibility to tear down a MIKEY CSB (e.g. if the SIP session
   is closed, the CSB SHOULD also be closed).

   Note that if a CSB has already been established, it is still valid
   for the SIP or RTSP implementation to request a new message from
   MIKEY, e.g. when a new offer is issued. MIKEY SHOULD then send an
   update message to the Responder (see also Section 4.5).

8. Groups

   What has been discussed up to now is not limited to single peer-to-
   peer communication (except for the DH method), but can be used to
   distribute group keys for small-size interactive groups and simple
   one-to-many scenarios. This section describes how MIKEY is used in a
   group scenario (though, see also Section 4.3 for issues related to
   authorization).


















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8.1. Simple one-to-many

                            ++++
                            |S |
                            |  |
                            ++++
                              |
                      --------+-------------- - -
                      |       |      |
                      v       v      v
                    ++++    ++++   ++++
                    |A |    |B |   |C |
                    |  |    |  |   |  |
                    ++++    ++++   ++++

   Figure 8.1. Simple one-to-many scenario.

   In the simple one-to-many  scenario, a server is streaming to a small
   group of clients. RTSP or SIP is used for the registration and the
   key management set up. The streaming server acts as the Initiator of
   MIKEY. In this scenario the pre-shared key or public key transport
   mechanism will be appropriate to use to transport the same TGK to all
   the clients (which will result in common TEKs for the group).

   Note, if the same TGK/TEK(s) should be used by all the group members,
   the streaming server MUST specify the same CSB_ID and CS_ID(s) for
   the session to all the group members.

8.2. Small-size interactive group

   As described in the overview section, for small-size interactive
   groups, one may expect that each client will be in charge for setting
   up the security for its outgoing streams. In these scenarios, the
   pre-shared key or the public-key transport method is used.

                       ++++          ++++
                       |A | -------> |B |
                       |  | <------- |  |
                       ++++          ++++
                        ^ |          | ^
                        | |          | |
                        | |   ++++   | |
                        | --->|C |<--- |
                        ------|  |------
                              ++++

   Figure 8.2. Small-size group without centralized controller.


   One scenario may then be that the client sets up a three-part call,
   using SIP. Due to the small size of the group, unicast SRTP is used



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   between the clients. Each client sets up the security for its
   outgoing stream(s) to the others.

   As for the simple one-to-many case, the streaming client specifies
   the same CSB_ID and CS_ID(s) for its outgoing sessions if the same
   TGK/TEK(s) is used for all the group members.


9. Security Considerations

9.1. General

   Key management protocols based on timestamps/counters and one-
   roundtrip key transport have previously been standardized in e.g.,
   ISO [ISO1, ISO2]. The general security of these types of protocols
   can be found in various literature and articles, c.f. [HAC, AKE,
   LOA].

   No chain is stronger than its weakest link. If a given level of
   protection is wanted, then the cryptographic functions protecting the
   keys during transport/exchange MUST offer a security at least
   corresponding to that level.

   For instance, if a security against attacks with complexity 2^96 is
   wanted, then one should choose a secure symmetric cipher supporting
   at least 96 bit keys (128 bits may be a practical choice) for the
   actual media protection, and a key transport mechanism that provides
   equivalent protection, e.g. MIKEY's pre-shared key transport with 128
   bit TGK, or, RSA with 1024 bit keys (which according to [LV]
   corresponds to the desired 96 bit level, with some margin).

   In summary, key size for the key-exchange mechanism MUST be weighed
   against the size of the exchanged TGK so that it offers at least the
   required level. For efficiency reasons, one SHOULD also avoid a
   security overkill, e.g. by not using a public key transport with
   public keys giving a security level that is orders of magnitude
   higher than length of the transported TGK. We refer to [LV] for
   concrete key size recommendations.

   Moreover, if the TGKs are not random (or pseudorandom), a brute force
   search may be facilitated, again lowering the effective key size.
   Therefore, care MUST be taken when designing the (pseudo) random
   generators for TGK generation, see [FIPS][RAND].

   For the selection of the hash function, SHA-1 with 160-bit output is
   the default one. In general, hash sizes should be twice the "security
   level", indicating that SHA1-256, [SHA256], should be used for the
   default 128-bit level. However, due to the real-time aspects in the
   scenarios we are treating, hash size slightly below 256 are
   acceptable as the normal "existential" collision probabilities would
   be of secondary importance.



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   In a Crypto Session Bundle, the Crypto Sessions can share the same
   TGK as discussed earlier. From a security point of view, the
   criterion to be satisfied is that the encryption of the individual
   Crypto Sessions are performed "independently". In MIKEY this is
   accomplished by having unique Crypto Session identifiers (see also
   Section 4.1) and a TEK derivation method that provides
   cryptographically independent TEKs to distinct Crypto Sessions
   (within the Crypto Session Bundle), regardless of the security
   protocol used.

   Specifically, the key derivations, as specified in Section 4.1, are
   implemented by a pseudo-random function. The one used here is a
   simplified version of that used in TLS [TLS]. Here, only one single
   hash function is used, whereas TLS uses two different functions. This
   choice is motivated by the high confidence in the SHA-1 hash
   function, and, by efficiency and simplicity of design (complexity
   does not imply security). Indeed, as shown in [DBJ], if one of the
   two hashes is severely broken, the TLS PRF is actually less secure
   than if a single hash had been used on the whole key, as is done in
   MIKEY.

   In the pre-shared key and public-key schemes, the TGK is generated by
   a single party (Initiator). This makes MIKEY somewhat more sensitive
   if the Initiator uses a bad random number generator. It should also
   be noted that neither the pre-shared nor the public-key scheme
   provides perfect forward secrecy. If mutual contribution or perfect
   forward secrecy is desired, the Diffie-Hellman method is to be used.

   Forward/backward security: if the TGK is exposed, all TEKs generated
   from it are compromised. However, under the assumption that the
   derivation function is a pseudo-random function, disclosure of an
   individual TEK does not compromise other (previous or later) TEKs
   derived from the same TGK. The Diffie-Hellman mode can be considered
   by cautious users as it is the only one that supports so called
   perfect forward secrecy (PFS). This is in contrast to a compromise of
   the pre-shared key (or the secret key of the public key mode), where
   future sessions and recorded session from the past are then also
   compromised.
   The use of random nonces (RANDs) in the key derivation is of utmost
   importance to counter off-line pre-computation attacks. Note however
   that update messages re-use the old RAND. This means that the total
   effective key entropy (relative to pre-computation attacks) for k
   consecutive key updates, assuming the TGKs and RAND are each n bits
   long, is about L = n*(k+1)/2 bits, compared to the theoretical
   maximum of n*k bits. In other words, a 2^L work effort MAY enable an
   attacker to get all k n-bit keys, which is better than brute force
   (except when k = 1). While this might seem as a defect, first note
   that for proper choice of n, the 2^L complexity of the attack is way
   out of reach. Moreover, the fact that more than one key can be
   compromised in a single attack is inherent to the key exchange



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   problematic. Consider for instance a user who, using say a fixed
   1024-bit RSA key, exchanges keys and communicates during one or two
   years life-time of the public key. Breaking this single RSA key will
   enable access to all exchanged keys and consequently the entire
   communication of that user over the whole period.

   All the pre-defined transforms in MIKEY use state-of-the-art
   algorithms that has undergone large amounts of public evaluation. One
   of the reasons to use AES-CM from SRTP [SRTP] is to have the
   possibility to limit the overall number of different encryption modes
   and algorithms, at the same time that it offers a high level of
   security.

9.2. Key lifetime

   Even if the lifetime of a TGK (or TEK) is not specified, it MUST be
   taken into account that the encryption transform in the underlying
   security protocol can in some way degenerate after a certain amount
   of encrypted data. It is not possible to here state general key life-
   time bounds, universally applicable; each security protocol should
   define such maximum amount and trigger a re-keying procedure before
   the "exhaustion" of the key. E.g., according to SRTP [SRTP] the TEK
   MUST be changed at least every 2^48 SRTP packet (i.e. every time the
   ROC || SEQ no in SRTP wraps).

   Still, the following can be said as a rule of thumb. If the security
   protocol uses an "ideal" b-bit block cipher (in CBC mode, counter
   mode, or a feedback mode, e.g OFB, with full b-bit feedback),
   degenerate behavior in the crypto stream, possibly useful for an
   attacker, is (with constant probability) expected to occur after a
   total of roughly 2^(b/2) encrypted b-bit blocks (using random IVs).
   For security margin, re-keying MUST be triggered well in advance
   compared to the above bound. See [BDJR] for more details.

   For use of a dedicated stream cipher, we refer to the analysis and
   documentation of said cipher in each specific case.

9.3. Timestamps

   The use of timestamps instead of challenge-response requires the
   systems to have synchronized clocks. Of course, if two clients are
   not synchronized, they will have difficulties with setting up the
   security. The current timestamp based solution has been selected to
   allow a maximum of one roundtrip (i.e., two messages), but still
   provide a reasonable replay protection. A (secure) challenge-response
   based version would require at least three messages. For a detailed
   description of the timestamp and replay handling in MIKEY, see
   Section 5.4.

   Practical experiences of Kerberos and other timestamp based systems
   indicate that it is not always necessary to synchronize the terminals



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   over the network. Manual configuration could be a feasible
   alternative in many cases (especially in scenarios where the degree
   of looseness is high). However, the choice must be carefully based
   with respect to the usage scenario.

9.4. Identity protection

   Identity protection was not a main design goal for MIKEY. Such
   feature will add more complexity to the protocol and was therefore
   chosen not to be included. As MIKEY is anyway proposed to be
   transported over e.g. SIP, the identity may be exposed by this.
   However, if the transporting protocol is secured and also provides
   identity protection, MIKEY might inherit the same feature. How this
   should be done is for future study.

9.5. Denial of Service

   This protocol is resistant to Denial of Service attacks in the sense
   that a Responder does not construct any state (at the key management
   protocol level) before it has authenticated the Initiator. However,
   this protocol, like many others, is open to attacks that use spoofed
   IP addresses to create a large number of fake requests. This may
   e.g., be solved by letting the protocol transporting MIKEY do an IP
   address validity test. For example, the SIP protocol can provide this
   using the anonymous authentication challenge mechanism (specified in
   Section 22.1 of [SIP]).

   As also discussed in Section 5.4, the tradeoff between time
   synchronization and the size of the replay cache, may be affected in
   case of e.g., a flooding type of DoS attack. However, if the
   recommendations of using a dynamic size of the replay cache are
   followed, it is believed that the client will in most cases be able
   to handle the replay cache. Of course, as the replay cache decreases
   in size, the required time synchronization is more restricted.
   However, a bigger problem during such attack would probably be to
   process the messages (e.g., verify signatures/MACs), due to the
   computational workload this implies.

9.6. Session establishment

   It should be noted that if the session establishment protocol is
   insecure there may be attacks on this that will have indirect
   security implications on the secured media streams. This however only
   applies to groups (and is not specific to MIKEY). The threat is that
   one group member may re-direct a stream from one group member to
   another. This will have the same implication as when a member tries
   to impersonate another member, e.g. by changing its IP address. If
   this is seen as a problem, it is RECOMMENDED that a Source Origin
   Authentication (SOA) scheme (e.g., digital signatures) is applied to
   the security protocol.




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   Re-direction of streams can of course be done even if it is not a
   group. However, the effect will not be the same compared to a group
   where impersonation can be done if SOA is not used. Instead, re-
   direction will only deny the receiver the possibility to receive (or
   just delay) the data.

10. IANA considerations

   This document defines several new name spaces associated with the
   MIKEY payloads. This section summarizes the name spaces for which
   IANA is requested to manage the allocation of values. A new port is
   required for MIKEY for stand alone use (the assignment should be for
   UDP, TCP, and SCTP).

   IANA is requested to record the pre-defined values defined in the
   given sections for each name space. IANA is also requested to manage
   the definition of additional values in the future. Unless explicitly
   stated otherwise, values in the range 0-240 for each name space
   should be approved by the process of IETF consensus and values in the
   range 241-255 are reserved for Private Use.

   The name spaces for the following fields in the Common header payload
   (from Section 6.1) are requested to be managed by IANA:

   * version

   * data type

   * Next payload

   * PRF func. This name space is between 0-127 where values between 0-
   111 should be approved by the process of IETF consensus and values
   between 112-127 are reserved for Private Use.

   * CS ID map type

   The name spaces for the following fields in the Key data transport
   payload (from Section 6.2) are requested to be managed by IANA:

   * Encr alg

   * MAC alg

   The name spaces for the following fields in the DH data payload (from
   Section 6.4) are requested to be managed by IANA:

   * DH-Group

   The name spaces for the following fields in the Timestamp payload
   (from Section 6.6) are requested to be managed by IANA:




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   * TS type

   The name spaces for the following fields in the ID payload and the
   Certificate payload (from Section 6.7) are requested to be managed by
   IANA:

   * ID type

   * Cert type

   The name spaces for the following fields in the Cert hash payload
   (from Section 6.8) are requested to be managed by IANA:

   * Hash func

   The name spaces for the following fields in the Security policy
   payload (from Section 6.10) are requested to be managed by IANA:

   * Prot type

   From Section 6.10.1.

   * SRTP Type

   * SRTP encr alg

   * SRTP auth alg

   * SRTP PRF

   * FEC order

   The name spaces for the following fields in the Error payload (from
   Section 6.12) are requested to be managed by IANA:

   * Error no

   The name spaces for the following fields in the Key data payload
   (from Section 6.13) are requested to be managed by IANA:

   * Type. This name space is between 0-16 which should be approved by
   the process of IETF consensus.

   * KV. This name space is between 0-16 which should be approved by the
   process of IETF consensus.

   The name spaces for the following fields in the General Extensions
   payload (from Section 6.15) are requested to be managed by IANA:

   * Type




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11. Conclusions

   Work for securing real-time applications have started to appear. This
   has brought forward the need for a key management solution to support
   the security protocol. The key management has to fulfil requirements,
   which make it suitable in the context of conversational multimedia in
   a heterogeneous environment and small interactive groups. MIKEY is
   designed to fulfill such requirements and optimized so that it also
   may be integrated in other protocols such as SIP and RTSP.

   MIKEY is designed to be used in scenarios for peer-to-peer
   communication, simple one-to-many, and for small-size interactive
   groups without a centralized group server.


12. Acknowledgments

   The authors would like to thank Mark Baugher, Ran Canetti, Martin
   Euchner, Steffen Fries, Peter Barany, Russ Housley, Pasi Ahonen (with
   his group), Rolf Blom, and Magnus Westerlund, for their valuable
   feedback.


13. Author's Addresses

     Jari Arkko
     Ericsson
     02420 Jorvas             Phone:  +358 40 5079256
     Finland                  Email:  jari.arkko@ericsson.com

     Elisabetta Carrara
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 50877040
     Sweden                   EMail:  elisabetta.carrara@era.ericsson.se

     Fredrik Lindholm
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 58531705
     Sweden                   EMail:  fredrik.lindholm@era.ericsson.se

     Mats Naslund
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 58533739
     Sweden                   EMail:  mats.naslund@era.ericsson.se

     Karl Norrman
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 4044502
     Sweden                   EMail:  karl.norrman@era.ericsson.se





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14. References

14.1. Normative References

   [AES] Advanced Encryption Standard (AES), Federal Information
   Processing Standard Publications (FIPS PUBS) 197, November 2001.

   [HMAC] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-Hashing
   for Message Authentication", RFC 2104, February 1997.

   [KMASDP] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
   Norrman, K., "Key Management Extensions for SDP and RTSP", Internet
   Draft, Work in Progress (MMUSIC WG).

   [NAI] Aboba, B. and Beadles, M., "The Network Access Identifier",
   IETF, RFC 2486, January 1999.

   [OAKLEY] Orman, H., "The Oakley Key Determination Protocol", RFC
   2412, November 1998.

   [OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with
   SDP", Internet Draft, IETF, Work in progress (MMUSIC).

   [RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time
   Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [SDP] Handley, M., and Jacobson, V., "Session Description Protocol
   (SDP), IETF, RFC2327

   [SHA1] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
   http://csrc.nist.gov/fips/fip180-1.ps

   [SIP] Rosenberg, J. et al, "SIP: Session Initiation Protocol", IETF,
   RFC3261.

   [SRTP] Baugher, M., Blom, R., Carrara, E., McGrew, D., Naslund, M,
   Norrman, K., and Oran, D., "The Secure Real Time Transport Protocol",
   Internet Draft, IETF, Work in Progress (AVT WG).

   [URI] Berners-Lee. T., Fielding, R., Masinter, L., "Uniform Resource
   Identifiers (URI): Generic Syntax", IETF, RFC 2396.

   [X.509] Housley, R., Polk, W., Ford, W., and Solo, D., "Internet
   X.509 Public Key Infrastructure Certificate and Certificate
   Revocation List (CRL) Profile", IETF, RFC 3280.

   [AESKW] Schaad, J., Housley R., "Advanced Encryption Standard (AES)
   Key Wrap Algorithm", IETF, RFC 3394.






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14.2. Informative References

   [AKE] Canetti, R. and Krawczyk, H., "Analysis of Key-Exchange
   Protocols and their use for Building Secure Channels", Eurocrypt
   2001, LNCS 2054, pp. 453-474, 2001.

   [BDJR] Bellare, M., Desai, A., Jokipii, E., and Rogaway, P., "A
   Concrete Analysis of Symmetric Encryption: Analysis of the DES Modes
   of Operation", in Proceedings of the 38th Symposium on Foundations of
   Computer Science, IEEE, 1997, pp. 394-403.

   [BMGL] Hastad, J. and Naslund, M.: "Practical Construction and
   Analysis of Pseduo-randomness Primitives", Proceedings of
   AsiacryptÆ01, Lecture Notes in Computer Science vol 2248, pp. 442-
   459.

   [DBJ] Johnson, D.B., "Theoretical Security Concerns with TLS use of
   MD5", Contribution to ANSI X9F1 WG, 2001.

   [FIPS] "Security Requirements for Cryptographic Modules", Federal
   Information Processing Standard Publications (FIPS PUBS) 140-2,
   December 2002.

   [GKMARCH] Baugher, M., Canetti, R., Dondeti, L., and Lindholm, F.,
   "Group Key Management Architecture", Internet Draft, Work in Progress
   (MSEC WG).

   [GDOI] Baugher, M., Hardjono, T., Harney, H., Weis, B., "The Group
   Domain of Interpretation", Internet Draft, Work in Progress (MSEC
   WG).

   [GSAKMP] Harney, H., Colegrove, A., Harder, E., Meth, U., Fleischer,
   R., "Group Secure Association Key Management Protocol", Internet
   Draft, Work in Progress (MSEC WG).

   [HAC] Menezes, A., van Oorschot, P., and Vanstone, S., "Handbook of
   Applied Cryptography", CRC press, 1996.

   [IKE] Harkins, D. and Carrel, D., "The Internet Key Exchange (IKE)",
   RFC 2409, November 1998.

   [ISO1] ISO/IEC 9798-3: 1997, Information technology - Security
   techniques - Entity authentication - Part 3: Mechanisms using digital
   signature techniques.

   [ISO2] ISO/IEC 11770-3: 1997, Information technology - Security
   techniques - Key management - Part 3: Mechanisms using digital
   signature techniques.

   [LOA] Burrows, Abadi, and Needham, ôA logic of authenticationö, ACM
   Transactions on Computer Systems 8 No.1 (Feb. 1990), 18-36.



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   [LV] Lenstra, A. K., and Verheul, E. R., "Suggesting Key Sizes for
   Cryptosystems", http://www.cryptosavvy.com/suggestions.htm

   [NTP] Mills, D., "Network Time Protocol (Version 3) specification,
   implementation and analysis", RFC 1305, March 1992.

   [OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and Adams
   C., "X.509 Internet Public Key Infrastructure Online Certificate
   Status Protocol - OCSP", IETF, RFC 2560.

   [PKCS1] PKCS #1 - RSA Cryptography Standard,
   http://www.rsalabs.com/pkcs/pkcs-1/

   [RAND] Eastlake, D., Schiller, J., and Crocker, S., "Randomness
   Requirements for Security", Internet Draft, Work in Progress.

   [RSA] Rivest, R., Shamir, A., and Adleman, L. "A Method for Obtaining
   Digital Signatures and Public-Key Cryptosystems". Communications of
   the ACM. Vol.21. No.2. pp.120-126. 1978.

   [SHA256] NIST, "Description of SHA-256, SHA-384, and SHA-512",
   http://csrc.nist.gov/encryption/shs/sha256-384-512.pdf

   [TLS] Dierks, T. and Allen, C., "The TLS Protocol - Version 1.0",
   IETF, RFC 2246.


Appendix A. - MIKEY - SRTP relation

   The terminology in MIKEY differs from the one used in SRTP as MIKEY
   needs to be more general. Therefore it might be hard to see the
   relations between keys and parameters generated in MIKEY and the ones
   used by SRTP. This section provides some hints on their relation.

   MIKEY            | SRTP
   -------------------------------------------------
   Crypto Session   | SRTP stream
   Data SA          | input to SRTP's crypto context
   TEK              | SRTP master key

   The Data SA is built up by a TEK and the security policy exchanged.
   SRTP may use a MKI to index the TEK. The TEK is then derived from the
   TGK that have the corresponding MKI.










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Revision History since -06 draft

   This section summarizes the most important changes in the document
   since the last version.

   - Section 2.5. "Existing Solutions" moved to Section 1.1.
   - Clarifications of scenarios (Section 2.1).

   - Number of clarifications in section 3.
   - Certificate discussions in Section 3.2 moved to new Section 4.3.1.

   - How to use AES Key wrap added to Section 4.2.3.
   - How to add a new DH group added to section 4.2.9.
   - New section about Certificate handling, policies and authorization
   added (replaces the old Section 4.3).

   - Examples in section 5.4 updated.

   - AES Key Wrap added as an optional mode in Section 6.2.
   - Signature type field added in signature payload (Section 6.5).

   - More rational added to the security considerations (Section 9).


   This Internet-Draft expires in December 2003.





























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