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Internet Engineering Task Force                                A. Bittau
Internet-Draft                                                  D. Boneh
Intended status: Standards Track                              M. Hamburg
Expires: August 18, 2014                             Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                                Q. Slack
                                                     Stanford University
                                                       February 14, 2014


           Cryptographic protection of TCP Streams (tcpcrypt)
                     draft-bittau-tcp-crypt-04.txt

Abstract

   This document presents tcpcrypt, a TCP extension for
   cryptographically protecting TCP segments.  Tcpcrypt maintains the
   confidentiality of data transmitted in TCP segments against a passive
   eavesdropper.  It protects connections against denial-of-service
   attacks involving desynchronizing of sequence numbers, and when
   enabled, against forged RST segments.  Finally, applications that
   perform authentication can obtain end-to-end confidentiality and
   integrity guarantees by tying authentication to tcpcrypt Session ID
   values.

   The extension defines two new TCP options, CRYPT and MAC, which are
   designed to provide compatible interworking with TCPs that do not
   implement tcpcrypt.  The CRYPT option allows hosts to negotiate the
   use of tcpcrypt and establish shared secret encryption keys.  The MAC
   option carries a message authentication code with which hosts can
   verify the integrity of transmitted TCP segments.  Tcpcrypt is
   designed to require relatively low overhead, particularly at servers,
   so as to be useful even in the case of servers accepting many TCP
   connections per second.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months



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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



















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Table of Contents

   1.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Idealized protocol . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Stages of the protocol . . . . . . . . . . . . . . . . . .  5
       3.1.1.  The setup phase  . . . . . . . . . . . . . . . . . . .  6
       3.1.2.  The ENCRYPTING state . . . . . . . . . . . . . . . . .  6
       3.1.3.  The DISABLED state . . . . . . . . . . . . . . . . . .  7
     3.2.  Cryptographic algorithms . . . . . . . . . . . . . . . . .  7
     3.3.  "C" and "S" roles  . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Key exchange protocol  . . . . . . . . . . . . . . . . . .  9
     3.5.  Data encryption and authentication . . . . . . . . . . . . 11
     3.6.  Authenticated Sequence Mode (ASM)  . . . . . . . . . . . . 12
       3.6.1.  ASM-Encrypt  . . . . . . . . . . . . . . . . . . . . . 14
       3.6.2.  ASM-Decrypt  . . . . . . . . . . . . . . . . . . . . . 15
       3.6.3.  ASM-Update . . . . . . . . . . . . . . . . . . . . . . 15
     3.7.  Re-keying  . . . . . . . . . . . . . . . . . . . . . . . . 16
     3.8.  Session caching  . . . . . . . . . . . . . . . . . . . . . 16
       3.8.1.  Session caching control  . . . . . . . . . . . . . . . 17
   4.  Extensions to TCP  . . . . . . . . . . . . . . . . . . . . . . 17
     4.1.  Protocol states  . . . . . . . . . . . . . . . . . . . . . 18
     4.2.  Role negotiation . . . . . . . . . . . . . . . . . . . . . 22
       4.2.1.  Simultaneous open  . . . . . . . . . . . . . . . . . . 23
     4.3.  The TCP CRYPT option . . . . . . . . . . . . . . . . . . . 24
       4.3.1.  The HELLO suboption  . . . . . . . . . . . . . . . . . 27
       4.3.2.  The DECLINE suboption  . . . . . . . . . . . . . . . . 28
       4.3.3.  The NEXTK1 and NEXTK2 suboptions . . . . . . . . . . . 28
       4.3.4.  The PKCONF suboption . . . . . . . . . . . . . . . . . 30
       4.3.5.  The UNKNOWN suboption  . . . . . . . . . . . . . . . . 31
       4.3.6.  The SYNCOOKIE and ACKCOOKIE suboptions . . . . . . . . 31
       4.3.7.  The SYNC_REQ and SYNC_OK suboptions  . . . . . . . . . 32
       4.3.8.  The REKEY and REKEYSTREAM suboptions . . . . . . . . . 34
       4.3.9.  The INIT1 and INIT2 suboptions . . . . . . . . . . . . 36
       4.3.10. The IV suboption . . . . . . . . . . . . . . . . . . . 38
     4.4.  The TCP MAC option . . . . . . . . . . . . . . . . . . . . 39
   5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     5.1.  Example 1: Normal handshake  . . . . . . . . . . . . . . . 41
     5.2.  Example 2: Normal handshake with SYN cookie  . . . . . . . 41
     5.3.  Example 3: tcpcrypt unsupported  . . . . . . . . . . . . . 42
     5.4.  Example 4: Reusing established state . . . . . . . . . . . 42
     5.5.  Example 5: Decline of state reuse  . . . . . . . . . . . . 42
     5.6.  Example 6: Reversal of client and server roles . . . . . . 42
   6.  API extensions . . . . . . . . . . . . . . . . . . . . . . . . 43
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 45
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 48
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48



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     10.1. Normative References . . . . . . . . . . . . . . . . . . . 48
     10.2. Informative References . . . . . . . . . . . . . . . . . . 49
   Appendix A.  Protocol constant values  . . . . . . . . . . . . . . 50
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 50















































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1.  Requirements Language

   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 [RFC2119].


2.  Introduction

   This document describes tcpcrypt, an extension to TCP for
   cryptographic protection of session data.  Tcpcrypt was designed to
   meet the following goals:

   o  Maintain confidentiality of communications against a passive
      adversary.  Ensure that an adversary must actively intercept and
      modify the traffic to eavesdrop, either by re-encrypting all
      traffic or by forcing a downgrade to an unencrypted session.

   o  Minimize computational cost, particularly on servers.

   o  Provide interfaces to higher-level software to facilitate end-to-
      end security, either in the application level protocol or after
      the fact.  (E.g., client and server log session IDs and can
      compare them after the fact; if there was no tampering or
      eavesdropping, the IDs will match.)

   o  Be compatible with further extensions that allow authenticated
      resumption of TCP connections when either end changes IP address.

   o  Facilitate multipath TCP [RFC6824] by identifying a TCP stream
      with a session ID independent of IP addresses and port numbers.

   o  Provide for incremental deployment and graceful fallback, even in
      the presence of NATs and other middleboxes that might remove
      unknown options, and traffic normalizers.


3.  Idealized protocol

   This section describes the tcpcrypt protocol at an abstract level,
   without reference to particular cryptographic algorithms or data
   encodings.  Readers who simply wish to see the key exchange protocol
   should skip to Section 3.4.

3.1.  Stages of the protocol

   A tcpcrypt endpoint goes through multiple stages.  It begins in a
   setup phase and ends up in one of two states, ENCRYPTING or DISABLED,



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   before applications may send or receive data.  The ENCRYPTING and
   DISABLED states are definitive and mutually exclusive; an endpoint
   that has been in one of the two states MUST NOT ever enter the other,
   nor ever re-enter the setup phase.

3.1.1.  The setup phase

   The setup phase negotiates use of the tcpcrypt extension.  During
   this phase, two hosts agree on a suite of cryptographic algorithms
   and establish shared secret session keys.

   The setup phase uses the Data portion of TCP segments to exchange
   cryptographic keys.  Implementations MUST NOT include application
   data in TCP segments during setup and MUST NOT allow applications to
   read or write data.  System calls MUST behave the same as for TCP
   connections that have not yet entered the ESTABLISHED state; calls to
   read and write SHOULD block or return temporary errors, while calls
   to poll or select SHOULD consider connections not ready.

   When setup succeeds, tcpcrypt enters the ENCRYPTING state.
   Importantly, a successful setup also produces an important value
   called the _Session ID_.  The Session ID is tied to the negotiated
   algorithms and cryptographic keys, and is unique over all time with
   overwhelming probability.

   Operating systems MUST make the Session ID available to applications.
   To prevent man-in-the-middle attacks, applications MAY authenticate
   the session ID through any protocol that ensures both endpoints of a
   connection have the same value.  Applications MAY alternatively just
   log Session IDs so as to enable attack detection after the fact
   through comparison of the values logged at both ends.

   The setup phase can also fail for various reasons, in which case
   tcpcrypt enters the DISABLED state.

   Applications MAY test whether setup succeeded by querying the
   operating system for the Session ID.  Requests for the Session ID
   MUST return an error when tcpcrypt is not in the ENCRYPTING state.
   Applications SHOULD authenticate the returned Session ID.
   Applications relying on tcpcrypt for security SHOULD authenticate the
   Session ID and SHOULD treat unauthenticated Session IDs the same as
   connections in the DISABLED state.

3.1.2.  The ENCRYPTING state

   When the setup phase succeeds, tcpcrypt enters the ENCRYPTING state.
   Once in this state, applications may read and write data with the
   expected semantics of TCP connections.



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   In the ENCRYPTING state, a host MUST encrypt the Data portion of all
   TCP segments transmitted and MUST include a Message Authentication
   Code (MAC) in all segments transmitted.  A host MUST furthermore
   ignore any TCP segments received without the RST bit set, unless
   those segments also contain a valid MAC option.

   A host SHOULD accept RST segments without valid MACs by default.
   However, the application SHOULD be allowed to force unMACed RST
   segments to be dropped by enabling the TCP_CRYPT_RSTCHK option on the
   connection.

   Once in the ENCRYPTING state, an endpoint MUST NOT directly or
   indirectly transition to the DISABLED state under any circumstances.

3.1.3.  The DISABLED state

   When setup fails, tcpcrypt enters the DISABLED state.  In this case,
   the host MUST continue just as TCP would without tcpcrypt, unless
   network conditions would cause a plain TCP connection to fail as
   well.  Entering the DISABLED state prohibits the endpoint from ever
   entering the ENCRYPTING state.

   An implementation MUST behave identically to ordinary TCP in the
   DISABLED state, except that the first segment transmitted after
   entering the DISABLED state MAY include a TCP CRYPT option with a
   DECLINE suboption (and optionally other suboptions such as UNKNOWN)
   to indicate that tcpcrypt is supported but not enabled.
   Section 4.3.2 describes how this is done.

   Operating systems MUST allow applications to turn off tcpcrypt by
   setting the state to DISABLED before opening a connection.  An active
   opener with tcpcrypt disabled MUST behave identically to an
   implementation of TCP without tcpcrypt.  A passive opener with
   tcpcrypt disabled MUST also behave like normal TCP, except that it
   MAY optionally respond to SYN segments containing a CRYPT option with
   SYN-ACK segments containing a DECLINE suboption, so as to indicate
   that tcpcrypt is supported but not enabled.

3.2.  Cryptographic algorithms

   The setup phase employs three types of cryptographic algorithms:

   o  A _public key cipher_ is used with a short-lived public key to
      exchange (or agree upon) a random, shared secret.

   o  An _extract function_ is used to generate a pseudo-random key from
      some initial keying material, typically the output of the public
      key chipher.  The notation Extract (S, IKM) denotes the output of



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      the extract function with salt S and initial keying material IKM.

   o  A _collision-resistant pseudo-random function (CPRF)_ is used to
      generate multiple cryptographic keys from a pseudo-random key,
      typically the output of the extract function.  We use the notation
      CPRF (K, TAG, L) to designate the output of L bytes of the pseudo-
      random function identified by key K on TAG.  A collision-resistant
      function is one on which, for sufficiently large L, an attacker
      cannot find two distinct inputs K_1, TAG_1 and K_2, TAG_2 such
      that CPRF (K_1, TAG_1, L) = CPRF (K_2, TAG_2, L).  Collision
      resistance is important to assure the uniqueness of Session IDs,
      which are generated using the CPRF.

   The Extract and CPRF functions used by default are the Extract and
   Expand functions of HKDF [RFC5869].  These are defined as follows:

            HKDF-Extract(salt, IKM) -> PRK
                PRK = HMAC-Hash(salt, IKM)

            HKDF-Expand(PRK, TAG, L) -> OKM
               T(0) = empty string (zero length)
               T(1) = HMAC-Hash(PRK, T(0) | TAG | 0x01)
               T(2) = HMAC-Hash(PRK, T(1) | TAG | 0x02)
               T(3) = HMAC-Hash(PRK, T(2) | TAG | 0x03)
               ...

               OKM  = first L octets of T(1) | T(2) | T(3) | ...

   The symbol | denotes concatenation, and the counter concatenated with
   TAG is a single octet.

   Because the public key cipher, the extract function, and the expand
   function all make use of cryptographic hashes in their constructions,
   the three algorithms are negotiated as a unit employing a single hash
   function.  For example, the OAEP+-RSA [RFC2437] cipher, which uses a
   SHA-256-based mask-generation function, is coupled with HKDF
   [RFC5869] using HMAC-SHA256 [RFC2104].

   The encrypting phase employs an _authenticated encryption mode_ to
   encrypt all application data.  This mode authenticates both
   application data and most of the TCP header (excepting header fields
   commonly modified by middleboxes).

   Note that public key generation, public key encryption, and shared
   secret generation all require randomness.  Other tcpcrypt functions
   may also require randomness depending on the algorithms and modes of
   operation selected.  A weak pseudo-random generator at either host
   will defeat tcpcrypt's security.  Thus, any host implementing



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   tcpcrypt MUST have a cryptographically secure source of randomness or
   pseudo-randomness.

3.3.  "C" and "S" roles

   Tcpcrypt transforms a single pseudo-random key (PRK) into
   cryptographic session keys for each direction.  Doing so requires an
   asymmetry in the protocol, as the key derivation function must be
   perturbed differently to generate different keys in each direction.
   Tcpcrypt includes other asymmetries in the roles of the two hosts,
   such as the process of negotiating algorithms (e.g., proposing vs.
   selecting cipher suites).

   We use the terms "C" and "S" to denote the distinct roles of the two
   hosts in tcpcrypt's setup phase.  In the case of key transport, "C"
   is the host that supplies a public key, while "S" is the host that
   encrypts a pre-master secret with the key belonging to "C".  Which
   role a host plays can have performance implications, because for some
   public key algorithms encryption is much faster than decryption.  For
   instance, on a machine at the time of writing, encryption with a
   2,048-bit RSA-3 key costs 82 microseconds, while decryption costs
   10 milliseconds.

   Because servers often need to establish connections at a faster rate
   than clients, and because servers are often passive openers, by
   default the passive opener plays the "S" role.  However, operating
   systems MUST provide a mechanism for the passive opener to reverse
   roles and play the "C" role, as discussed in Section 4.2.

3.4.  Key exchange protocol

   Every machine C has a short-lived public encryption key or key
   agreement parameter, PK_C, which gets refreshed periodically and
   SHOULD NOT ever be written to persistent storage.

   When a host C connects to S, the two engage in the following
   protocol:

                 C -> S:  HELLO
                 S -> C:  PKCONF, pub-cipher-list
                 C -> S:  INIT1, sym-cipher-list, N_C, PK_C
                 S -> C:  INIT2, sym-cipher, KX_S

   The parameters are defined as follows:

   o  pub-cipher-list: a list of public key ciphers and parameters
      acceptable to S. These are defined in Figure 3.




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   o  sym-cipher-list: a list of symmetric cipher suites acceptable to
      C. These are specified in Table 6.

   o  N_C: Nonce chosen at random by C.

   o  PK_C: C's public key or key agreement parameter.

   o  sym-cipher: the symmetric cipher selected by S.

   o  KX_S: key exchange information produced by S. KX_S will depend on
      whether key transport is being done (e.g., RSA) or key agreement
      (e.g., Diffie-Hellman).  KX_S is defined in Table 1.

        +----------------+-----------------+----------------------+
        | Cipher         | KX_S            | PMS                  |
        +----------------+-----------------+----------------------+
        | OAEP+-RSA exp3 | ENC (PK_C, R_S) | R_S                  |
        | ECDHE          | N_S, PK_S       | key-agreement-output |
        +----------------+-----------------+----------------------+

    ENC (PK_C, R_S) denotes an encryption of R_S with public key PK_C.
   R_S and N_S are random values chosen by S. Their lengths are defined
    in Figure 3.  PK_S is S's key agreement parameter.  PMS is the Pre
           Master Secret from which keys are ultimately derived.

                                  Table 1

   The two sides then compute a pseudo-random key (PRK) from which all
   session keys are derived as follows:

           param    := { pub-cipher-list, sym-cipher-list, sym-cipher }
           PRK      := Extract (N_C, { param, PK_C, KX_S, PMS })

   A series of "session secrets" and corresponding Session IDs are then
   computed as follows:

                  ss[0] := PRK
                  ss[i] := CPRF (ss[i-1], CONST_NEXTK, K_LEN)

                 SID[i] := CPRF (ss[i], CONST_SESSID, K_LEN)

   The value ss[0] is used to generate all key material for the current
   connection.  SID[0] is the session ID for the current connection, and
   will with overwhelming probability be unique for each individual TCP
   connection.  The most computationally expensive part of the key
   exchange protocol is the public key cipher.  The values of ss[i] for
   i > 0 can be used to avoid public key cryptography when establishing
   subsequent connections between the same two hosts, as described in



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   Section 3.8.  The TAG values are constants defined in Table 7.  The
   K_LEN values along with nonce sizes are negotiated, and are specified
   in Figure 3.

   Given a session secret, ss, the two sides compute a series of master
   keys as follows:

                  mk[0] := CPRF (ss, CONST_REKEY, K_LEN)
                  mk[i] := CPRF (mk[i-1], CONST_REKEY, K_LEN)

   Finally, each master key mk is used to generate keys for
   authenticated encryption for the "S" and "C" roles.  Key k_cs is used
   by "C" to encrypt and "S" to decrypt, while k_sc is used by "S" to
   encrypt and "C" to decrypt.

                   k_cs := CPRF (mk, CONST_KEY_C, ae_len)
                   k_sc := CPRF (mk, CONST_KEY_S, ae_len)

   tcpcrypt does not use HKDF directly for key derivation because it
   requires multiple expand steps with different keys.  This is needed
   for forward secrecy so that ss[n] can be forgotten once a session is
   established, and mk[n] can be forgotten once a session is rekeyed.

   There is no key confirmation step in tcpcrypt.  This is not required
   since in tcpcrypt's threat model, a connection to an adversary can be
   made and so keys need not be verified.  If an erroneous key
   negotiation that yields two different keys occurs, all subsequent
   packets will be dropped due to an incorrect MAC, causing the TCP
   connection to hang.  This is not a threat because in plain TCP, an
   active attacker could have modified sequence and ack numbers to hang
   the connection anyway.

3.5.  Data encryption and authentication

   tcpcrypt encrypts and authenticates all application data.  It also
   authenticates some parts of the TCP header.  There are several TCP-
   specific constraints with regards to authenticated encryption that
   tcpcrypt must meet for performance and compatibility with
   middleboxes:

   o  The ciphertext for a particular byte position in tcpcrypt's
      sequence must never change, even if reencryption occurs after
      coalescing and retransmission.  This is because a middlebox may
      discard a changed payload on retransmission.

   o  Authentication must occur only on fields not modified by
      middleboxes.  In particular, port numbers must not be
      authenticated, and sequence and ack numbers must be authenticated



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      according to an offset from the initial sequence number, because
      these can be modulated by a middlebox.

   o  An efficient mechanism is needed for recomputing the
      authentication tag when only the ack numbers change.  For example,
      on retransmissions, the authenticated encryption authentication
      tag can be efficiently updated without having to recompute the tag
      on the entire packet payload.

   Authenticated encryption modes such as GCM do not meet these
   criteria.  For example, even with identical plaintext, ciphertext
   values depend on the byte position at which one starts encrypting a
   segment.  Hence two small segments will appear to have different
   content from their coalesced counterpart; middleboxes might drop such
   coalesced retransmissions after falsely detecting subterfuge attacks.
   Furthermore, existing authenticated encryption modes do not allow
   efficient updating of the authentication tag when only small parts of
   the data have changed.  A new mode is needed to meet all these
   constraints, and we introduce _Authenticated Sequence Mode_ (ASM) in
   Section 3.6 as a solution.

   ASM takes three parameters: a cipher, a MAC and an ACK MAC.  At a
   high-level, the cipher is used to encrypt the TCP payload in counter
   mode, using a counter derived from TCP's sequence number.  The MAC
   covers the ciphertext and parts of the TCP header.  The ACK MAC
   covers the ACK numbers and is XORed with the previously computed MAC
   to produce the authenticated encryption authentication tag.  This tag
   can be quickly updated if only the ACK numbers have changed.  This
   approach is principled because ACK messages are conceptually separate
   from data packets, so MACing them separately is appropriate.  In TCP,
   ACKs are piggybacked to data segments merely as an optimization.

   XORing two PRF-based MACs together was shown secure by Katz and
   Lindell [aggregate-macs].

3.6.  Authenticated Sequence Mode (ASM)

   ASM is parameterized by a cipher, MAC and ACK MAC.  The operations
   supported by ASM are:












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   ASM-Encrypt (PRK, Seq, Message, Assoc-Data, Up-Data) ->
                    (Ciphertext, Auth-Tag)

   ASM-Decrypt (PRK, Seq, Cipher-Text, Assoc-Data, Up-Data, Auth-Tag) ->
                    { (Valid, Message)  OR
                      (Invalid, )
                    }

   ASM-Update (PRK, Up-Data-Prev, Up-Data-New, Auth-Tag-Prev) ->
                    Auth-Tag


   The arguments and return values are:

   o  _PRK_ a pseudo-random key.

   o  _Seq_ the byte position in the stream of Message or Cipher-Text.
      In tcpcrypt, this is an extended version of TCP's sequence number.

   o  _Message_ the Message to encrypt.  In tcpcrypt, this is TCP's
      payload.

   o  _Assoc-Data_ the associated data to be MACed but not encrypted.
      In tcpcrypt, this contains parts of the TCP header.

   o  _Up-Data_ the updatable data to be MACed but not encrypted, that
      can also be efficiently updated and reMACed.  In tcpcrypt, this
      will cover an extended version of TCP's ACK numbers.

   o  _Ciphertext_ the encrypted version of Message.

   o  _Auth-Tag_ the authenticated encryption authentication tag.  In
      tcpcrypt, this will be the MAC option.

   ASM-Decrypt either returns the Valid or Invalid constants, depending
   on whether the authentication tag can be verified successfully or
   not.  For Valid inputs, the Message is returned as well.

   The PRK supplied to ASM is expanded into keys used for individual
   operation as follows:

              k_enc := CPRF (PRK, CONST_KEY_ENC, cipher-key-len)
              k_mac := CPRF (PRK, CONST_KEY_MAC, mac-key-len)
              k_ack := CPRF (PRK, CONST_KEY_ACK, ack-mac-key-len)

   The next sections describe ASM operations in detail.





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3.6.1.  ASM-Encrypt

   The interface to encrypt is as follows:

          ASM-Encrypt (PRK, Seq, Message, Assoc-Data, Up-Data) ->
                           (Ciphertext, Auth-Tag)

   Keys (denoted by k_*) are derived from PRK as explained in
   Section 3.6.

   The following steps occur:

   1.  Message is encrypted to produce Ciphertext using the cipher in
       counter mode.  Seq is the counter and k_enc is the key.  When
       encrypting Seq, its value must always be a multiple of the
       cipher's block size.  In the event that the message does not
       begin on an even block boundary, Seq must be rounded down,
       encrypted, and leading bytes of its encryption discarded.

   2.  The MAC is run over the concatenation of Ciphertext and Assoc-
       Data to produce MAC1, using k_mac as the key.

   3.  The ACK MAC is run over Up-Data to produce MAC2, using k_ack as
       the key.

   4.  MAC1 and MAC2 are XORed to produce Auth-Tag.

   Using AES-128 as an example, encryption in counter mode using Seq as
   the counter happens as follows.

   o  Compute B = Seq - (Seq % 16).

   o  Let B* = 0^{128-|B|} | B be B in network (big-endian) byte order
      with enough 0 bits pre-pended to make B* exactly 128 bits long.

   o  Let C = ENC-AES (ke, B*).

   o  Discard the first (Seq-B) bytes on C and begin byte-by-byte XORing
      the remaining portion with the message.

   If AES-128 is used as the ACK MAC, the Ack number (64-bit extended,
   offset from ISN) is first padded on the left with enough zeros to
   produce a 128-bit big-endian value.  The number is then encrypted
   using AES.







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3.6.2.  ASM-Decrypt

   The interface to decrypt is as follows:

   ASM-Decrypt (PRK, Seq, Cipher-Text, Assoc-Data, Up-Data, Auth-Tag) ->
                    { (Valid, Message)  OR
                      (Invalid, )

   Keys (denoted by k_*) are derived from PRK as explained in
   Section 3.6.

   The following steps occur:

   1.  The MAC is run over the concatenation of Ciphertext and Assoc-
       Data to produce MAC1, using k_mac as the key.

   2.  The ACK MAC is run over Up-Data to produce MAC2, using k_ack as
       the key.

   3.  MAC1 and MAC2 are XORed and compared to Auth-Tag.  If different,
       the process stops and the constant Invalid is returned along with
       no message.  Otherwise the process continues.

   4.  Ciphertext is decrypted to produce Message using the cipher in
       counter mode.  Seq is the counter and k_enc is the key.  The
       Valid constant is returned along with Message.

3.6.3.  ASM-Update

   The interface to update the authenticated encryption authentication
   tag is as follows:

       ASM-Update (PRK, Up-Data-Prev, Up-Data-New, Auth-Tag-Prev) ->
                        Auth-Tag

   Keys (denoted by k_*) are derived from PRK as explained in
   Section 3.6.

   The following steps occur:

   1.  The ACK MAC is run over Up-Data-Prev using k_ack to produce MAC2-
       Prev.

   2.  MAC2-Prev is XORed with Auth-Tag-Prev to produce MAC1.

   3.  The ACK MAC is run over Up-Data to produce MAC2, using k_ack as
       the key.




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   4.  MAC1 and MAC2 are XORed to produce Auth-Tag.

3.7.  Re-keying

   We refer to the two encryption keys (k_cs, k_sc) as a _key set_.  We
   refer to the key set generated by mk[i] as the key set with
   _generation number_ i within a session.  Initially, the two hosts use
   the key set with generation number 0.

   Either host may decide to evolve the encryption key at one or more
   points within a session, by incrementing the generation number of its
   transmit keys.  When switching keys to generation j, a host must
   label the segments it transmits with a REKEY option containing j, so
   that the recipient host knows to check the MAC and decrypt the
   segment using the new keyset:

                   A -> B:  REKEY<j>, MAC<...>, Data<...>

   Upon receiving a REKEY<j> segment, a recipient using transmit keys
   from a generation less than j must also update its transmit keys and
   start including a REKEY<j> option in all of its segments.  A host
   must continue transmitting REKEY options until all segments with
   other generation numbers have been processed at both ends.

   Implementations MUST always transmit and retransmit identical
   ciphertext Data bytes for the same TCP sequence numbers.  Thus, a
   retransmitted segment MUST always use the same keyset as the original
   segment.  Hosts MUST NOT combine segments that were encrypted with
   different keysets.

   Implementations SHOULD delete older-generation keys from memory once
   they have received all segments they will need to decrypt with the
   old keys and received acknowledgments for all segments they might
   need to retransmit.

3.8.  Session caching

   When two hosts have already negotiated session secret ss[i-1], they
   can establish a new connection without public key operations using
   ss[i].  The four-message protocol of Section 3.4 is replaced by:

                          A -> B:  NEXTK1, SID[i]
                          B -> A:  NEXTK2

   Which symmetric keys a host uses for transmitted segments is
   determined by its role in the original session ss[0].  It does not
   depend on which host is the passive opener in the current session.
   If A had the "C" role in the first session, then A uses k_cs for



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   sending segments and k_sc for receiving.  Otherwise, if A had the "S"
   role originally, it uses k_sc and k_cs, respectively.  B similarly
   uses the transmit keys that correspond to its role in the original
   session.

   After using ss[i] to compute mk[0], implementations SHOULD compute
   and cache ss[i+1] for possible use by a later session, then erase
   ss[i] from memory.  Hosts SHOULD keep ss[i+1] around for a period of
   time until it is used or the memory needs to be reclaimed.  Hosts
   SHOULD NOT write a cached ss[i+1] value to non-volatile storage.

   It is an implementation-specific issue as to how long ss[i+1] should
   be retained if it is unused.  If the passive opener times it out
   before the active opener does, the only cost is the additional twelve
   bytes to send NEXTK1 for the next connection.  The behavior then
   falls back to a normal public-key handshake.

3.8.1.  Session caching control

   Implementations MUST allow applications to control session caching by
   setting the following option:

   TCP_CRYPT_CACHE_FLUSH  When set on a TCP endpoint that is in the
      ENCRYPTING state, this option causes the operating system to flush
      from memory the cached ss[i+1] (or ss[i+1+n] if other connections
      have already been established).  When set on an endpoint that is
      in the setup phase, causes any cached ss[i] that would have been
      used to be flushed from memory.  In either case, future
      connections will have to undertake another round of the public key
      protocol in Section 3.4.  Applications SHOULD set
      TCP_CRYPT_CACHE_FLUSH whenever authentication of the session ID
      fails.


4.  Extensions to TCP

   The tcpcrypt extension adds two new kinds of option: CRYPT, and MAC.
   Both are described in this section.  During the setup phase, all TCP
   segments MUST have the CRYPT option.  In the ENCRYPTING state, all
   segments MUST have the MAC option and may include the CRYPT option
   for various purposes such as re-keying or keep-alive probes.

   The idealized protocol of the previous section must be embedded in
   the TCP handshake.  Unfortunately, since the maximum TCP header size
   is 60 bytes and the basic TCP header fields require 20 bytes, there
   are at most 40 option payload bytes available, which is not enough to
   hold the INIT1 and INIT2 messages.  Tcpcrypt therefore uses the Data
   portion of TCP segments (after the SYN exchanges) to send the body of



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   these messages.

   Operating systems MUST keep track of which phase a data segment
   belongs to, and MUST only deliver data to applications from segments
   that are processed in the ENCRYPTING or DISABLED states.

4.1.  Protocol states

   The setup phase is divided into six states: CLOSED, NEXTK-SENT,
   HELLO-SENT, C-MODE, LISTEN, and S-MODE.  Together with the ENCRYPTING
   and DISABLED states already discussed, this means a tcpcrypt endpoint
   can be in one of eight states.

   In addition to tcpcrypt's state, each endpoint will also be in one of
   the 11 TCP states described in the TCP protocol specification
   [RFC0793].  Not all pairs of states are valid.  Table 2 shows which
   TCP states an endpoint can be in for each tcpcrypt state.

   +-------------+--------------------------+--------------------------+
   | Tcpcrypt    | TCP states for an active | TCP states for a passive |
   | state       | opener                   | opener                   |
   +-------------+--------------------------+--------------------------+
   | CLOSED      | CLOSED                   | CLOSED                   |
   | NEXTK-SENT  | SYN-SENT                 | n/a                      |
   | HELLO-SENT  | SYN-SENT                 | SYN-RCVD                 |
   | C-MODE      | ESTABLISHED, FIN-WAIT-1  | ESTABLISHED, FIN-WAIT-1  |
   | LISTEN      | n/a                      | LISTEN                   |
   | S-MODE      | (SYN-RCVD), ESTABLISHED  | SYN-RCVD                 |
   | ENCRYPTING  | (SYN-RCVD), ESTABLISHED+ | SYN-RCVD, ESTABLISHED+   |
   | DISABLED    | any                      | any                      |
   +-------------+--------------------------+--------------------------+

     Valid tcpcrypt and TCP state combinations.  States in parentheses
   occur only with simultaneous open.  ESTABLISHED+ means ESTABLISHED or
    any later state (FIN-WAIT-1, FIN-WAIT-2, CLOSING, TIME-WAIT, CLOSE-
                            WAIT, or LAST-ACK).

                                  Table 2

   Figure 1 shows how tcpcrypt transitions between states.  Each
   transition is labeled by events that may trigger the transition above
   the line, and an action the local host is permitted to take in
   response below the line. "snd" and "rcv" denote sending and receiving
   segments, respectively. "any" means any possible event. "internal"
   means any possible event except for receiving a segment (i.e., timers
   and system calls). "drop" means discarding the last received segment
   and preventing it from having any effect on TCP's state. "mac" means
   any valid TCP action, including no action, except that any segments



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   transmitted must be encrypted and contain a valid TCP MAC option. "x"
   indicates that a host sends no segments when taking a transition.

   A segment is described as "F/Op".  F specifies constraints on the
   control bits of the TCP header, as follows:

                   +----+------------------------------+
                   | F  | Meaning                      |
                   +----+------------------------------+
                   | S  | SYN=1, ACK=0, FIN=0, RST=0   |
                   | SA | SYN=1, ACK=1, FIN=0, RST=0   |
                   | A  | SYN=0, ACK=1, FIN=0, RST=0   |
                   | S? | SYN=1, ACK=any, FIN=0, RST=0 |
                   | ?A | SYN=any, ACK=1, FIN=0, RST=0 |
                   | R  | RST=1                        |
                   | *  | any                          |
                   +----+------------------------------+

   Op designates message types in the abstract protocol, which also
   correspond to particular suboptions of the TCP CRYPT option,
   described in Section 4.3, or "MAC" for a valid TCP MAC option, as
   described in Section 4.4.  A segment with SYN=1 and ACK=0 that
   contains the NEXTK1 suboption will also explicitly or implicitly
   contain the HELLO suboption; such a segment matches event constraints
   on either option--e.g., it matches any of the "rcv S/HELLO", "rcv
   S?/HELLO", and "rcv S/NEXTK1" events.  An empty Op matches any
   segment with the appropriate control bits.  A segment MUST contain
   the TCP MAC option if and only if Op is "MAC".

   The "drop" transitions from NEXTK-SENT and HELLO-SENT to HELLO-SENT
   change TCP slightly by ignoring a segment and preventing a TCP
   transition from SYN-SENT to SYN-RCVD that would otherwise occur
   during simultaneous open.  Therefore, these transitions SHOULD be
   disabled by default.  They MAY be enabled on one side by an
   application that wishes to enable tcpcrypt on simultaneous open, as
   discussed in Section 4.2.1.















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           active OPEN               passive OPEN
           ------------ +----------+ ------------ +----------+
           snd S/NEXTK1 |  CLOSED  | x            |  LISTEN  |
    +-------------------|          |------------->|          |---------+
    |                   +----------+              +----------+         |
    |              +---+  |active OPEN              | |                |
    |   rcv S/HELLO|   |  |-----------   rcv S/HELLO| |    rcv S/NEXTK1|
    |   -----------|   |  |snd S/HELLO  ------------| |   -------------|
    V          drop|   V  V             snd SA/HELLO| |   snd SA/NEXTK2|
   +----------+    | +----------+                   | |                |
   |  NEXTK-  |___/ \|  HELLO-  |<------------------+ |                |
   |   SENT   |      |   SENT   |                     |rcv S/HELLO     |
   +----------+      +----------+                     |-------------   |
    | | |                | |rcv S?/HELLO              |snd SA/PKCONF   |
    | | |rcv S?/HELLO    | |-------------             V                |
    | | |-------------   | |snd ?A/PKCONF    +----------+              |
    | | |snd ?A/PKCONF   | +---------------->|  S-MODE  |              |
    | | +----------------|------------------>|          |              |
    | +----------------+ |                   +----------+              |
    |     rcv SA/PKCONF| |rcv ?A/PKCONF           |                    |
    |     -------------| |-------------           |rcv A/INIT1         |
    |       snd A/INIT1| |snd A/INIT1             |-----------         |
    |                  V V                        |snd A/INIT2         |
    |               +----------+                  |                    |
    |rcv SA/NEXTK2  |  C-MODE  |            +---+ | +---+              |
    |-------------  |          |     rcv */ |   | | |   |internal      |
    |snd A/MAC      +----------+     -------|   | | |   |or rcv */MAC  |
    |  == or ==        |rcv A/INIT2     drop|   | | |   |or rcv R/     |
    |rcv S/NEXTK1      |-----------         |   V V V   |------------  |
    |------------      |x                  +----------+ |mac           |
    |snd SA/NEXTK2     +------------------>|ENCRYPTING|-+              |
    +------------------------------------->|          |<---------------+
                                           +----------+

    State diagram for tcpcrypt.  Transitions to DISABLED and CLOSED are
                                not shown.

                                 Figure 1

   Any segment that would be discarded by TCP (e.g., for being out of
   window) MUST also be ignored by tcpcrypt.  However, certain segments
   that might otherwise be accepted by TCP MUST be dropped by tcpcrypt
   and prevented from affecting TCP's state.

   Except for these drop actions, tcpcrypt MUST abide by the TCP
   protocol specification [RFC0793].  Thus, any segment transmitted by a
   host MUST be permitted by the TCP specification in addition to
   matching either a transition in Figure 1 or one of the transitions to



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   DISABLED or CLOSED described below.  In particular, a host MUST NOT
   acknowledge an INIT1 segment unless either the acknowledgment
   contains an INIT2 or the host transitions to DISABLED.

   Various events cause transitions to DISABLED from states other than
   ENCRYPTING.  In particular:

   o  Operating systems MUST provide a mechanism for applications to
      transition to DISABLED from the CLOSED and LISTEN states.

   o  A host in the setup phase MUST transition to DISABLED upon
      receiving any segment without a TCP CRYPT option.

   o  A host in the setup phase MUST transition to DISABLED upon
      receiving any segment with the FIN or RST control bit set.

   o  A host in the setup phase MUST transition to DISABLED upon sending
      a segment with the FIN bit set.  (As discussed below, however, a
      host MUST NOT send a FIN segment from the C-MODE state.)

   Other specific conditions cause a transition to DISABLED and are
   discussed in the sections that follow.

   CLOSED is a pseudo-state representing a connection that does not
   exist.  A tcpcrypt connection's lifetime is identical to that of its
   associated TCP connection.  Thus, tcpcrypt transitions to CLOSED
   exactly when TCP transitions to CLOSED.

   A host MUST NOT send a FIN segment from the C-MODE state.  The reason
   is that the remote side can be in the ENCRYPTING state and would thus
   require the segment to contain a valid MAC, yet a host in C-MODE
   cannot compute the necessary encryption keys before receiving the
   INIT2 segment.

   If a CLOSE happens in C-MODE, a host MUST delay sending a FIN segment
   until receiving an ACK for its INIT1 segment.  If the remote host is
   in ENCRYPTING, the ACK segment will contain INIT2 and the local host
   can transition to ENCRYPTING before sending the FIN.  If the remote
   host is not in ENCRYPTING, the ACK will not contain INIT2, and thus
   the local host can transition to DISABLED before sending the FIN.

   If a CLOSE happens in C-MODE, an implementation MAY delay processing
   the CLOSE event and entering the TCP FIN-WAIT-1 state until sending
   the FIN.  If it does not, the implementation MUST ensure all relevant
   timers correspond to the time of transmission of the FIN segment, not
   the time of entry into the FIN-WAIT-1 state.

   The only valid tcpcrypt state transition from ENCRYPTING is to



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   CLOSED, which occurs only when TCP transitions to CLOSED. tcpcrypt
   per-se cannot cause TCP to transition to CLOSED.

4.2.  Role negotiation

   A passive opener receiving an S/HELLO segment may choose to play the
   "S" role (by transitioning to S-MODE) or the "C" role (by
   transitioning to HELLO-SENT).  An active opener may accept the role
   not chosen by the passive opener, or may instead disable tcpcrypt.
   During simultaneous open, one endpoint must choose the "C" role while
   the other chooses the "S" role.  Operating systems MUST allow
   applications to guide these choices on a per-connection basis.

   Applications SHOULD be able to exert this control by setting a per-
   connection _CMODE disposition_, which can take on one of the
   following five values:

   TCP_CRYPT_CMODE_DEFAULT  This disposition SHOULD be the default.  A
      passive opener will only play the "S" role, but an active opener
      can play either the "C" or the "S" role.  Simultaneous open
      without session caching will cause tcpcrypt to be disabled unless
      the remote host has set the TCP_CMODE_ALWAYS[_NK] disposition.

   TCP_CRYPT_CMODE_ALWAYS

   TCP_CRYPT_CMODE_ALWAYS_NK  With this disposition, a host will only
      play the "C" role.  The _NK version additionally prevents the use
      of session caching if the session was originally established in
      the "S" role.

   TCP_CRYPT_CMODE_NEVER

   TCP_CRYPT_CMODE_NEVER_NK  With this disposition, a host will only
      play the "S" role.  The _NK version additionally prevents the use
      of session caching if the session was originally established in
      the "C" role.

   The CMODE disposition prohibits certain state transitions, as
   summarized in Table 3.  If an event occurs for which all valid
   transitions in Figure 1 are prohibited, a host MUST transition to
   DISABLED.  Operating systems MAY add additional CMODE dispositions,
   for instance to force or prohibit session caching.









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        +-----------------------------+---------------------------+
        |           CMODE disposition | Prohibited transitions    |
        +-----------------------------+---------------------------+
        |     TCP_CRYPT_CMODE_DEFAULT | LISTEN --> HELLO-SENT     |
        |                             | HELLO-SENT --> HELLO-SENT |
        |                             | NEXTK-SENT --> HELLO-SENT |
        |                             |                           |
        | TCP_CRYPT_CMODE_ALWAYS[_NK] | any --> S-MODE            |
        |                             |                           |
        |  TCP_CRYPT_CMODE_NEVER[_NK] | LISTEN --> HELLO-SENT     |
        |                             | HELLO-SENT --> HELLO-SENT |
        |                             | NEXTK-SENT --> HELLO-SENT |
        |                             | any --> C-MODE            |
        +-----------------------------+---------------------------+

          State transitions prohibited by each CMODE disposition

                                  Table 3

4.2.1.  Simultaneous open

   During simultaneous open, two ends of a TCP connection are both
   active openers.  If both hosts attempt to use session caching by
   simultaneously transmitting S/NEXTK1 segments, and if both transmit
   the same session ID, then both may reply with SA/NEXTK2 segments and
   immediately enter the ENCRYPTING state.  In this case, the host that
   played "C" when the session was initially negotiated MUST use the
   symmetric encryption keys for "C" (i.e., encrypt with k_cs, decrypt
   with k_sc), while the host that initially played "S" uses the "S"
   keys for the new connection.

   If both hosts in a simultaneous open do not attempt to use session
   caching, or if the two hosts use incompatible Session IDs, then they
   MUST engage in public-key-based key negotiation to use tcpcrypt.
   Doing so requires one host to play the "C" role and the other to play
   the "S" role.  With the TCP_CRYPT_CMODE_DEFAULT disposition, these
   roles are usually determined by the passive opener choosing the "S"
   role.  With no passive opener, both active openers will end up in
   S-MODE, then transition to DISABLED upon receiving an unexpected
   PKCONF.

   Simultaneous open can work with key negotiation if exactly one of the
   two hosts selects the TCP_CRYPT_CMODE_ALWAYS disposition.  This host
   will then drop S/HELLO segments and remain in C-MODE while the other
   host transitions to S-MODE.  Applications SHOULD NOT set
   TCP_CRYPT_CMODE_ALWAYS on both sides of a simultaneous open, as this
   will result in tcpcrypt being disabled.  The reception of two
   simultaneous HELLO (or NEXTK) messages will disable tcpcrypt because



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   it is not explicit as to who is playing the "C" or "S" role.

4.3.  The TCP CRYPT option

   A CRYPT option has the following format:

                  Byte     0       1       2         N
                       +-------+-------+-------...-------+
                       | Kind= |Length=|    Suboptions   |
                       |  OPT1 |   N   |                 |
                       +-------+-------+-------...-------+

                        Format of TCP CRYPT option

   Kind is always OPT1.  Length is the total length of the option,
   including the two bytes used for Kind and Length.  These first two
   bytes are then followed by zero or more suboptions.  Suboptions
   determine the meaning of the TCP CRYPT option.  When a TCP header
   contains more than one CRYPT option, a host MUST interpret them the
   same as if all the suboptions appeared in a single CRYPT option.
   This makes tcpcrypt options future-proof as new suboptions can be
   placed in a separate CRYPT option, which can be ignored if not
   understood, while other CRYPT options can still be processed.

   Each suboption begins with an Opcode byte.  The specific format of
   the option depends on the two most significant bits of the Opcode.

   Suboptions with opcodes from 0x00 to 0x3f contain no data other than
   the single opcode byte:

   bit  0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |   Opcode =    |
       |0 0 x x x x x x|
       +-+-+-+-+-+-+-+-+

   Hosts MUST ignore any opcodes of this format that they do not
   recognize.

   Suboptions with opcodes from 0x40 to 0x7f contain an opcode, a length
   field, and data bytes.

        0                   1
   bit  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------...
       |    Opcode =   |   Length =    |     N-2 bytes
       |0 1 x x x x x x|       N       |  of suboption data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------...



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   Hosts MUST ignore any opcodes of this format that they do not
   recognize.

   Suboptions with opcodes from 0x80 to 0xbf contain zero or more bytes
   of data whose length depends on the opcode.  These suboptions can be
   either fixed length or variable length; implementations that
   understand these opcodes will known which they are; if the suboption
   is fixed length the implementation will know the length; otherwise it
   will know where to look for the length field.

   bit  0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+-------...
       |    Opcode =   | data
       |1 0 x x x x x x|
       +-+-+-+-+-+-+-+-+-------...

   If a host sees an unknown opcode in this range, it MUST ignore the
   suboption and all subsequent suboptions in the same TCP CRYPT option.
   However, if more than one CRYPT option appears in the TCP header, the
   host MUST continue processing suboptions from the next TCP CRYPT
   option.  Skipping suboptions in the TCP CRYPT option applies only to
   this option range since the length of the suboption cannot be
   determined by the receiver.  In other cases, where the length is
   known, the receiver skips to the next suboption.

   Suboptions with opcodes from 0xc0 to 0xff also contain an opcode-
   specific length of data.  As before, these suboptions can be either
   fixed length or variable length.  However, suboptions in this range
   are classed as mandatory as far as the protocol is concerned.
   However, they are not MANDATORY to implement unless otherwise stated,
   as discussed below.

   bit  0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+-------...
       |    Opcode =   | data
       |1 1 x x x x x x|
       +-+-+-+-+-+-+-+-+-------...

   Should a host encounter an unknown opcode greater than or equal to
   0xc0 during the setup phase of the protocol, the host MUST transition
   to the DISABLED state.  It SHOULD respond with both a DECLINE
   suboption and an UNKNOWN suboption specifying the opcode of the
   unknown mandatory suboption, after which the host MUST NOT send any
   further CRYPT options.

   Should a host encounter an unknown opcode greater than or equal to
   0xc0 while in the ENCRYPTING state, the host MUST respond with an
   UNKNOWN suboption specifying the opcode of the unknown mandatory



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   suboption, and should ensure the session continues with the same
   encryption and authentication state as it had before the segment was
   received.  This may require ignoring other suboptions within the same
   message, or reverting any half-negotiated state.

   Table 4 summarizes the opcodes discussed in this document.  It is
   MANDATORY that all implementations support every opcode in this
   table.  Each opcode is listed with the length in bytes of the
   suboption (including the opcode byte), or * for variable-length
   suboptions.  The last column specifies in which of the (S)etup phase,
   (E)NCRYPTING state, and (D)ISABLED state an opcode may be used.  A
   host MUST NOT send an option unless it is in one of the stages
   indicated by this column.

             +-------+--------+---------------------+--------+
             | Value | Length | Name                | Stages |
             +-------+--------+---------------------+--------+
             |  0x01 |      1 | HELLO               | S      |
             |  0x02 |      1 | HELLO-app-support   | S      |
             |  0x03 |      1 | HELLO-app-mandatory | S      |
             |  0x04 |      1 | DECLINE             | SD     |
             |  0x05 |      1 | NEXTK2              | S      |
             |  0x06 |      1 | NEXTK2-app-support  | S      |
             |  0x07 |      1 | INIT1               | S      |
             |  0x08 |      1 | INIT2               | S      |
             |  0x41 |      * | PKCONF              | S      |
             |  0x42 |      * | PKCONF-app-support  | S      |
             |  0x43 |      * | UNKNOWN             | SED    |
             |  0x44 |      * | SYNCOOKIE           | S      |
             |  0x45 |      * | ACKCOOKIE           | SED    |
             |  0x80 |      5 | SYNC_REQ            | E      |
             |  0x81 |      5 | SYNC_OK             | E      |
             |  0x82 |      2 | REKEY               | E      |
             |  0x83 |      6 | REKEYSTREAM         | E      |
             |  0x84 |     10 | NEXTK1              | S      |
             |  0x85 |      * | IV                  | E      |
             +-------+--------+---------------------+--------+

              Opcodes for suboptions of the TCP CRYPT option.

                                  Table 4

   If a TCP segment (sent by an active opener) has the SYN flag set, the
   ACK flag clear, and one or more TCP CRYPT options, there is an
   implicit HELLO suboption even if that suboption does not appear in
   the segment.  In particular, when such a SYN segment contains a
   single, empty, two-byte TCP CRYPT option, the passive opener MUST
   interpret that option as equivalent to the three-byte TCP option



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   composed of bytes OPT1, 3, 1 (Kind = OPT1, Length = 3, Suboption =
   HELLO).

   A host MUST enter the DISABLED state if, during the setup phase, it
   receives a segment containing neither a TCP CRYPT nor a TCP MAC
   option.  This is for robustness against middleboxes that strip
   options.  A host MUST also enter DISABLED if, during the setup phase,
   it receives a DECLINE suboption or any unrecognized suboption with
   opcode greater than or equal to 0xc0.  The DECLINE option is the
   preferred way for a host to refuse tcpcrypt.  A host MAY also choose
   reply without a TCP CRYPT option to disable tcpcrypt.  Once a host
   has entered DISABLED, it MUST NOT include the MAC option in any
   transmitted segment.  The host MAY include a CRYPT option in the next
   segment transmitted, but only if the segment also contains the
   DECLINE suboption.  All subsequently transmitted packets MUST NOT
   contain the CRYPT option.

4.3.1.  The HELLO suboption

   The HELLO dataless suboption MUST only appear in a segment with the
   SYN control bit set.  It is used by an active opener to indicate
   interest in using tcpcrypt for a connection, and by a passive opener
   to indicate that the passive opener wishes to play the "C" role.

   The initial SYN segment from an active opener wishing to use tcpcrypt
   MUST contain a TCP CRYPT option with either an explicit or an
   implicit HELLO suboption.

   After receiving a SYN segment with the HELLO suboption, a passive
   opener MUST respond in one of three ways:

   o  To continue setting up tcpcrypt and play the "S" role, the passive
      opener MUST respond with a PKCONF suboption in the SYN-ACK segment
      and transition to S-MODE.

   o  To continue setting up tcpcrypt and play the "C" role, the passive
      opener MUST respond with a HELLO suboption in the SYN-ACK segment
      and transition to HELLO-SENT.

   o  To continue without tcpcrypt, the passive opener MUST respond with
      either no CRYPT option or the DECLINE suboption in the SYN-ACK
      segment, then transition to the DISABLED state.

   An active opener receiving HELLO in a SYN-ACK segment must either
   transition to S-MODE and respond with a PKCONF suboption, or
   transition to DISABLED.

   There are three variants of the HELLO option used for application-



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   level authentication, each encoded differently as shown in Table 4.
   The variants are: a plain HELLO where the application is not
   tcpcrypt-aware (but the kernel is), an "application supported" HELLO
   where the application is tcpcrypt-aware and is advertising the fact,
   and a "application mandatory" HELLO where the application requires
   the remote application to support tcpcrypt otherwise the connection
   MUST revert to plain TCP.  The application supported HELLO can be
   used, for example, when implementing HTTP digest authentication - an
   application can check whether the peer's application is tcpcrypt
   aware and proceed to authenticate tcpcrypt's session ID over HTTP,
   otherwise reverting to standard HTTP digest authentication.  The
   application mandatory HELLO can be used, for example, when
   implementing an SSL library that attempts tcpcrypt but reverts to SSL
   if the peer's SSL library does not support tcpcrypt.  The application
   mandatory HELLO avoids double encrypting (SSL-over-tcpcrypt) since
   the connection will revert to plain TCP if the remote SSL library is
   not tcpcrypt-ware.

4.3.2.  The DECLINE suboption

   The DECLINE dataless suboption is sent by a host to indicate that the
   host will not enable tcpcrypt on a connection.  If a host is in the
   DISABLED state or transitioning to the DISABLED state, and the host
   transmits a segment containing a CRYPT option, then the segment MUST
   contain the DECLINE suboption.

   A passive opener SHOULD send a DECLINE suboption in response to a
   HELLO suboption or NEXTK1 suboption in a received SYN segment if it
   supports tcpcrypt but does not wish to engage in encryption for this
   particular session.

   Implementations MUST NOT send segments containing the DECLINE
   suboption from the C-MODE or ENCRYPTING states.

4.3.3.  The NEXTK1 and NEXTK2 suboptions

   The NEXTK1 suboption MUST only appear in a segment with the SYN
   control bit set and the ACK bit clear.  It is used by the active
   opener to initiate a TCP session without the overhead of public key
   cryptography.  The new session key is derived from a previously
   negotiated session secret, as described in Section 3.8.

   The suboption is always 10 bytes in length; the data contains the
   first nine bytes of SID[i] and is used to to start the session with
   session secret ss[i].  The format of the suboption is:






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                   Byte     0       1       2       3
                        +-------+-------+-------+-------+
                      0 |Opcode |       Bytes 0-2       |
                        | 0x84  |       of SID[i]       |
                        +-------+-------+-------+-------+
                      4 |           Bytes 3-6           |
                        |           of SID[i]           |
                        +-------+-------+-------+-------+
                      8 |   Bytes 7-8   |
                        |   of SID[i]   |
                        +-------+-------+

                      Format of the NEXTK1 suboption

   The active opener MUST use the lowest value of i that has not already
   appeared in a NEXTK1 segment exchanged with the same host and for the
   same pre-session seed.

   If the passive opener recognizes SID[i] and knows ss[i], it SHOULD
   respond with a segment containing the dataless NEXTK2 suboption.  The
   NEXTK2 option MUST only appear in a segment with both the SYN and ACK
   bits set.

   If the passive opener does not recognize SID[i], or SID[i] is not
   valid or has already been used, the passive opener SHOULD respond
   with a PKCONF or HELLO option and continue key negotiation as usual.

   When two hosts have previously negotiated a tcpcrypt session, either
   host may use the NEXTK1 option regardless of which host was the
   active opener or played the "C" role in the previous session.
   However, a given host must either encrypt with k_cs for all sessions
   derived from the same pre-session seed, or k_sc.  Thus, which keys a
   host uses to send segments depends only whether the host played the
   "C" or "S" role in the initial session that used ss[0]; it is not
   affected by which host was the active opener transmitting the SYN
   segment containing a NEXTK1 suboption.

   A host MUST reject a NEXTK1 message if it has previously sent or
   received one with the same SID[i].  In the event that two hosts
   simultaneously send SYN segments to each other with the same SID[i],
   but the two segments are not part of a simultaneous open, both
   connections will have to revert to public key cryptography.  To avoid
   this limitation, implementations MAY chose to implement session
   caching such that a given pre-session key is only good for either
   passive or active opens at the same host, not both.

   In the case of simultaneous open, two hosts that simultaneously send
   SYN packets with NEXTK1 and the same SID[i] may establish a



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   connection, as described in Section 4.2.1.

4.3.4.  The PKCONF suboption

   The PKCONF option has the following format:

                  Byte     0       1       2         N
                       +-------+-------+-------...-------+
                       |Opcode=|Length=|    Algorithm    |
                       |  0x41 |   N   |   Specifiers    |
                       +-------+-------+-------...-------+

                      Format of the PKCONF suboption

   The suboption data, whose length (N-2) must be divisible by 3,
   contains one or more 3-byte algorithm specifiers of the following
   form:

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

   Format of algorithm specifier within PKCONF.  Fields starting with 1
     are reserved for future use by algorithm identifiers longer than
                               three bytes.

   The algorithm identifier specifies a number of parameters, defined in
   Figure 3.

   Hosts MUST implement OAEP+-RSA3 and ECDHE-P256 and ECDHE-P512.

   Servers demanding utmost performance SHOULD use RSA because the RSA
   encrypt operation is must faster than Diffie-Hellman operations,
   resulting in a higher connection rate.

   Depending on the encoding of the PKCONF suboption (see Table 4), it
   can indicate whether "S's" application is tcpcrypt-aware or not.  For
   the "C" role, the encoding of the HELLO suboption does this.  This
   mechanism can be used for bootstrapping application-level
   authentication without requiring probing in upper layer protocols to
   check for support (which may not be possible).  The application
   controls these encodings via the TCP_CRYPT_SUPPORT socket option.







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4.3.5.  The UNKNOWN suboption

   The UNKNOWN option has the following format:

               Byte     0       1       2              N
                    +-------+-------+-------........-------+
                    |Opcode=|Length=| N-2 unknown one-byte |
                    |  0x42 |   N   |   opcodes received   |
                    +-------+-------+-------........-------+

                      Format of the UNKNOWN suboption

   This suboption is sent in response to an unknown suboption that has
   been received.  The contents of the option are a complete list of the
   mandatory suboption opcodes from the received packet that were not
   understood.  Note that this option is only sent once, in the next
   packet that the host sends.  This means that it is reliable when sent
   in a SYN-ACK, but unreliable otherwise.  Any mechanism sending new
   mandatory attributes must take this into account.  If multiple
   packets, each containing unknown options, are received before an
   UNKNOWN suboption can be sent, the options list MUST contain the
   union of the two sets.  The order of the opcode list is not
   significant.

   If a host receives an unknown option, it SHOULD reply with the
   UNKNOWN suboption to notify the other side.  If the host transitions
   to DISABLED as a result of the unknown option, then the host MUST
   also include the DECLINE suboption if it sends an UNKNOWN suboption
   (or more generally if it includes a CRYPT option in the next packet).

   As a special case, if PKCONF (0x41) or INIT1 (0x06) appears in the
   unknown opcode list, it does not mean the sender does not understand
   the option (since these options are MANDATORY).  Instead, it means
   the sender does not implement any of the algorithms specified in the
   PKCONF or INIT1 message.  In either case, the segment must also
   contain a DECLINE suboption.

4.3.6.  The SYNCOOKIE and ACKCOOKIE suboptions

   A passive opener MAY include the SYNCOOKIE suboption in a segment
   with both the SYN and ACK flags set.  SYNCOOKIE allows a server to be
   stateless until the TCP handshake has completed.  It has the
   following format:








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                  Byte     0       1       2         N
                       +-------+-------+-------...-------+
                       |Opcode=|Length=|  N-2 bytes of   |
                       |  0x43 |   N   |   opaque data   |
                       +-------+-------+-------...-------+

                     Format of the SYNCOOKIE suboption

   The data is opaque as far as the protocol is concerned; it is
   entirely up to implementations how to make use of this suboption to
   hold state.  It is OPTIONAL to send a SYNCOOKIE, but MANDATORY to
   understand and respond to them.

   The ACKCOOKIE suboption echoes the contents of a SYNCOOKIE; it MUST
   be sent in a packet acknowledging receipt of a packet containing a
   SYNCOOKIE, and MUST NOT be sent in any other packet.  It has the
   following format:

                 Byte     0       1       2              N
                      +-------+-------+-------...-------+
                      |Opcode=|Length=|  N-2 bytes of   |
                      |  0x44 |   N   | SYNCOOKIE data  |
                      +-------+-------+-------...-------+

                     Format of the ACKCOOKIE suboption

   Servers that rely on suboption data from ACKCOOKIE to reconstruct
   session state SHOULD embed a cryptographically strong message
   authentication code within the SYNCOOKIE data so as to be able to
   reject forged ACKCOOKIE suboptions.

   Though an implementation MUST NOT send a SYNCOOKIE in any context
   except the SYN-ACK packet returned by a passive opener,
   implementations SHOULD accept SYNCOOKIEs in other contexts and reply
   with the appropriate ACKCOOKIE if possible.

4.3.7.  The SYNC_REQ and SYNC_OK suboptions

   Many hosts implement TCP Keep-Alives [RFC1122] as an option for
   applications to ensure that the other end of a TCP connection still
   exists even when there is no data to be sent.  A TCP Keep-Alive
   segment carries a sequence number one prior to the beginning of the
   send window, and may carry one byte of "garbage" data.  Such a
   segment causes the remote side to send an acknowledgment.

   Unfortunately, Keep-Alive acknowledgments might not contain unique
   data.  Hence, an old but cryptographically valid acknowledgment could
   be replayed by an attacker to prolong the existence of a session at



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   one host after the other end of the connection no longer exists.
   (Such an attack might prevent a process with sensitive data from
   exiting, giving an attacker more time to compromise a host and
   extract the sensitive data.)

   The TCP Timestamps Option (TSopt) [RFC1323] could alternatively have
   been used to make Keep-Alives unique.  However, because some
   middleboxes change the value of TSopt in packets, tcpcrypt does not
   protect the contents of the TCP TSopt option.  Hence the SYNC_REQ and
   SYNC_OK suboptions allow the cryptographically protected TCP CRYPT
   option to contain unique data.

   The SYNC_REQ suboption is always 5 bytes, and has the following
   format:

               Byte     0       1       2       3       4
                    +-------+-------+-------+-------+-------+
                    |Opcode=|             Clock             |
                    | 0x80  |                               |
                    +-------+-------+-------+-------+-------+

                     Format of the SYNC_REQ suboption

   Clock is a 32-bit non-decreasing value.  A host MUST increment Clock
   at least once for every interval in which it sends a Keep-Alive.
   Implementations that support TSopt MAY chose to use the same value
   for Clock that they would put in the TSval field of the TCP TSopt.
   However, implementations SHOULD "fuzz" any system clocks used to
   avoid disclosing either when a host was last rebooted or at what rate
   the hardware clock drifts.

   A host that receives a SYNC_REQ suboption MUST reply with a SYNC_OK
   suboption, which is always five bytes and has the following format:

               Byte     0       1       2       3       4
                    +-------+-------+-------+-------+-------+
                    |Opcode=|        Received-Clock         |
                    | 0x81  |                               |
                    +-------+-------+-------+-------+-------+

                      Format of the SYNC_OK suboption

   The value of Received-Clock depends on the values of the Clock fields
   in SYNC_REQ messages a host has received.  A host must set Received-
   Clock to a value at least as high as the most recently received
   Clock, but no higher than the highest Clock value received this
   session.  If a host delays acknowledgment of multiple packets with
   SYNC_REQ suboptions, it SHOULD send a single SYNC_OK with Received-



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   Clock set to the highest Clock in the packets it is acknowledging.

   Because middleboxes sometimes "correct" inconsistent retransmissions,
   Keep-Alive segments with one byte of garbage data MUST use the same
   ciphertext byte as previously transmitted for that sequence number.
   Otherwise, a middlebox might change the byte back to its value in the
   original transmission, causing the cryptographic MAC to fail.

4.3.8.  The REKEY and REKEYSTREAM suboptions

   The REKEY and REKEYSTREAM suboptions are used to evolve encryption
   keys.  Exactly one of the two options is valid with any given
   symmetric encryption algorithm and mode.  Generally block ciphers
   will use REKEY while stream ciphers use REKEYSTREAM.  We refer to a
   segment containing either option as a REKEY segment.

   REKEY allows hosts to wipe from memory keys that could decrypt
   previously transmitted segments.  It also allows the use of message
   authentication codes that are only secure up to a fixed number of
   messages.  However, implementations MUST work in the presence of
   middleboxes that "correct" inconsistent data retransmissions.  Hence,
   the value of ciphertext bytes must be the same in the original
   transmission and all retransmissions of a particular sequence number.
   This means a host MUST always use the same encryption key when
   transmitting or retransmitting the same range of sequence numbers.
   Re-keying only affects data transmitted in the future.  Moreover,
   segments encrypted with different keysets MUST NOT be combined in
   retransmissions.

   When switching keys, the REKEY suboption specifies which key set has
   been used to encrypt and integrity-protect the current segment.  The
   suboption is always two bytes, and has the following format:

                           Byte     0       1
                                +-------+-------+
                                |Opcode=|KeyLSB |
                                | 0x83  |       |
                                +-------+-------+

                       Format of the REKEY suboption

   KeyLSB is the generation number of the keys used to encrypt and MAC
   the current segment, modulo 256.  REKEYSTREAM is the same as REKEY
   but includes the TCP Sequence Number offset at which the key change
   took effect, for cases in which decryption requires knowing how many
   bytes have been encrypted thus far with a key.  To interoperate with
   middleboxes that rewrite sequence numbers, offsets from the Initial
   Sequence Number (ISN) are used instead of TCP sequence numbers



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   throughout tcpcrypt.  The same occurs when dealing with
   acknowledgement numbers.

           Byte     0       1       2       3       4       5
                +-------+-------+-------+-------+-------+-------+
                |Opcode=|KeyLSB |        Sequence Number Offset |
                | 0x83  |       |               from ISN        |
                +-------+-------+-------+-------+-------+-------+

                    Format of the REKEYSTREAM suboption

   A host MAY use REKEY to increment the session key generation number
   beyond the highest generation it knows the other side to be using.
   We call this process _initiating_ re-keying.  When one host initiates
   re-keying, the other host MUST increment its key generation number to
   match, as described below (unless the other host has also
   simultaneously initiated re-keying).

   A host MAY initiate re-keying by including a REKEY suboption in a
   _syncable_ segment.  A syncable segment is one that either contains
   data, or is acknowledgment-only but contains a SYNC_REQ suboption
   with a fresh Clock value--i.e., higher than any Clock value it has
   previously transmitted.  We say a syncable segment is _synced_ when
   the transmitter knows the remote side has received it and all
   previous sequence numbers.  A data segment is synced when the
   transmitter receives a cumulative acknowledgment for its sequence
   number (a Selective Acknowledgment [RFC2018] is insufficient).  An
   acknowledgment-only segment is synced when the sender receives an
   acknowledgment for its sequence number and a SYNC_OK with a high
   enough Clock number.

   A host MUST NOT initiate re-keying with an acknowledgment-only
   segment that has either no SYNC_REQ suboption or a SYNC_REQ with an
   old Clock value, because such a segment is not syncable.  A host MUST
   NOT initiate re-keying with any KeyLSB other than its current key
   number plus one modulo 256.

   When a host receives a segment containing a REKEY suboption, it MUST
   proceed as follows:

   1.  The receiver computes RECEIVE-KEY-NUMBER to be the closest
       integer to its own transmit key number that also equals KeyLSB
       modulo 256.  If no number is closest (because KeyLSB is exactly
       128 away from the transmit number modulo 256), the receiver MUST
       discard the segment.  If RECEIVE-KEY-NUMBER is negative, the
       receiver MUST also discard the segment.





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   2.  The receiver MUST authenticate and decrypt the segment using the
       receive keys with generation number RECEIVE-KEY-NUMBER.  The
       receiver MUST discard the packet as usual if the MAC is invalid.


   3.  If RECEIVE-KEY-NUMBER is greater than the receiver's current
       transmit key number, the receiver must wait to receive all
       sequence numbers prior to the REKEY segment's.  Once it receives
       segments covering all these missing sequence numbers (if any), it
       MUST increase its transmit number to RECEIVE-KEY-NUMBER and
       transmit a REKEY suboption.  If the receiver has gotten multiple
       REKEY segments with different KeyLSB values, it MUST increase its
       transmit key number to the highest RECEIVE-KEY-NUMBER of any
       segment for which it is not missing prior sequence numbers.

   After sending a REKEY (whether initiating re-keying or just
   responding), a host MUST continue to send REKEY in all subsequent
   segments until at least one of the following holds:

   o  One of the REKEY segments the host transmitted for its current
      transmit key number was syncable, and it has been synced.

   o  The host receives a cumulative acknowledgment for one of its REKEY
      segments with the current transmit key number, and the cumulative
      acknowledgment is in a segment encrypted with the new key but not
      containing a REKEY suboption.

   A host SHOULD erase old keys from memory once the above requirements
   are met.

   A host MUST NOT initiate re-keying if it initiated a re-keying less
   than 60 seconds ago and has not transmitted at least 1 Megabyte
   (increased its sequence number by 1,048,576) since the last re-
   keying.  A host MUST NOT initiate re-keying if it has outstanding
   unacknowledged REKEY segments for key numbers that are 127 or more
   below the current key.  A host SHOULD not initiate more than one
   concurrent re-key operation if it has no data to send.

4.3.9.  The INIT1 and INIT2 suboptions

   The INIT1 dataless suboption indicates that the Data portion of the
   TCP segment contains the following data structure:









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                   Byte     0       1       2       3
                        +-------+-------+-------+-------+
                        |         # bytes INIT1         |
                        +-------+-------+-------+-------+
                        |  INIT1_MAGIC  | # auth enc alg|
                        +-------+-------+-------+-------+
                        |# bytes of N_C |# byte of PK_C |
                        +-------+-------+-------+-------+
                        |   authenticated encryption    |
                        :          algorithms           :
                        +-------+-------+-------+-------+
                        |              N_C              |
                        :                               :
                        +-------+-------+-------+-------+
                        |   0   |      type of K_C      |
                        +-------+-------+-------+-------+
                        |             PK_C              |
                        :                               :
                        +-------+-------+-------+-------+

   The INIT1_MAGIC is specified in Table 7.  The following values for
   authenticated sequence mode (ASM) encryption algorithms are defined:

   The first entry is mandatory and MUST be supported by all
   implementations.  The sequence number for ASM mode is TCP's extended
   64-bit sequence number offset from the ISN.

   The value "type of PK_C" must be one of the public key specifiers
   included earlier in the other host's PKCONF message.

   The INIT2 dataless suboption indicates that the Data portion of the
   TCP segment contains the following data structure:

                   Byte     0       1       2       3
                        +-------+-------+-------+-------+
                        |         # byte INIT2          |
                        +-------+-------+-------+-------+
                        |  INIT2_MAGIC  |#byte kmaterial|
                        +-------+-------+-------+-------+
                        |    symmetric cipher suite     |
                        +-------+-------+-------+-------+
                        |           key material        |
                        :                               :
                        +-------+-------+-------+-------+

                       Format of the INIT2 suboption

                                 Figure 2



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   The INIT2_MAGIC is specified in Table 7.  The symmetric cipher suite
   is one selected by the host transmitting the INIT2 segment, which
   will be playing the "S" role.  The key material depends on the public
   key cipher selected, as described in Section 3.4.  When ECDHE is
   used, key material is encoded as follows:

                     Byte     0    1       N         M
                          +-------+---...---+---...---+
                          |# byte |   N_S   |  PK_S   |
                          |  N_S  |         |         |
                          +-------+---...---+---...---+

   Hosts MUST set the TCP PSH control bits on INIT1 and INIT2 segments.
   Implementations MUST NOT set the TCP FIN control bit on INIT
   segments.

4.3.10.  The IV suboption

   The IV suboption is used to hold an initialization vector (IV) when
   the negotiated encryption mode requires an initialization vector to
   be transmitted with packets.  It MUST NOT be included in transmitted
   packets except in the ENCRYPTING state when the negotiated encryption
   mode requires IVs.  When the negotiated encryption mode does require
   IVs, all segments transmitted in ENCRYPTING mode MUST contain an IV
   suboption.

   The IV suboption has the following format:

                      Byte     0       1         N
                           +-------+-------...-------+
                           |Opcode=| Initialization  |
                           |  0x85 |     Vector      |
                           +-------+-------...-------+

                        Format of the IV suboption

   The length N of the IV is determined by the encryption algorithm and
   mode negotiated.

   As discussed in Section 4.3.8, a host MUST always transmit the same
   ciphertext byte in retransmissions of a particular sequence number.
   Thus, retransmitted segments must use the same IV each time.
   Moreover, previously transmitted segments MUST NOT be combined on
   retransmission if their IVs would prevent the ciphertext bytes from
   remaining the same as in the original transmission.






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4.4.  The TCP MAC option

   The MAC option is used to authenticate a TCP segment.  Once a host
   has entered the encrypting phase for a session, the HOST must include
   a TCP MAC option in all segments it sends.  Furthermore, once in the
   encrypting phase, a host MUST ignore any segments it receives that do
   not have a valid MAC option, except for segments with the RST bit set
   if the application has not requested cryptographic verification of
   RST segments.

   The length of the MAC option is determined by the symmetric message
   authentication code selected.  The format of the MAC option is:

                   Byte     0       1       2      N+1
                        +-------+-------+------...------+
                        | Kind  | Len=  |    N-byte     |
                        | OPT2  |  2+N  |      MAC      |
                        +-------+-------+------...------+

                         Format of TCP MAC option

   The MAC is the authentication tag as output from autheticated
   encryption.  Apart from payload, two headers are included in the
   authenticated encryption process: a pseudo-header structure we call
   Assoc-Data, and an acknowledgment structure we call Up-Data.  The
   format of Assoc-Data is as follows:

                   Byte     0       1       2       3
                        +-------+-------+-------+-------+
                      0 |    0x8000     |    length     |
                        +-------+-------+-------+-------+
                      4 |  off  | flags |    window     |
                        +-------+-------+---------------+
                      8 |    0x0000     |      urg      |
                        +-------+-------+-------+-------+
                     12 |        seqno offset hi        |
                        +-------+-------+-------+-------+
                     16 |         seqno offset          |
                        +-------+-------+-------+-------+
                     20 |            options            |
                        :                               :
                        +-------+-------+-------+-------+

                         Assoc-Data data structure

   The fields of Assoc-Data are defined as follows:





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   length
      Total size of the TCP segment from the start of the TCP header to
      the end of the IP datagram.

   off
      Byte 12 of the TCP header (Data Offset)

   flags
      Byte 13 of the TCP header (Control Bits)

   window
      Bytes 14-15 of the TCP header (Window)

   urg
      Bytes 18-19 of the TCP header (Urgent Pointer)

   seqno offset hi
      Number of times the seqno offset field has wrapped from 0xffffff
      -> 0

   seqno offset
      The low 32 bits of the sequence number offset (the Sequence Number
      in the TCP header - ISN)

   options
      These are bytes 20-off of the TCP header.  However, where the
      TSOPT (8), Skeeter (16), Bubba (17), MD5 (19), and MAC (OPT2)
      options appear, their contents (all but the kind and length bytes)
      are replaced with all zeroes.

   The format of the Up-Data structure is as follows:

                   Byte     0       1       2       3
                        +-------+-------+-------+-------+
                      0 |        ackno offset hi        |
                        +-------+-------+-------+-------+
                      4 |         ackno offset          |
                        +-------+-------+-------+-------+

                          Up-Data data structure

   The fields of Up-Data are defined as follows:

   ackno offset hi  The number of times ackno offset hi has wrapped from
      0xffffff -> 0.






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   ackno offset  The lower 32 bits of the acknowledgement number offset
      from the remote end's ISN (TCP's acknowledgement header - ISN
      received).

   The two structures, Assoc-Data and Up-Data, are used in ASM mode to
   calculate the TCP MAC option.


5.  Examples

   To illustrate these suboptions, consider the following series of ways
   in which a TCP connection may be established from host A to host B.
   We use notation S for SYN-only packet, SA for SYN-ACK packet, and A
   for packets with the ACK bit but not SYN bit.  These examples are not
   normative.

5.1.  Example 1: Normal handshake

   (1) A -> B: S  CRYPT<>
   (2) B -> A: SA CRYPT<PKCONF<0x200,0x201>>
   (3) A -> B: A  data<INIT1...>
   (4) B -> A: A  data<INIT2...>
   (5) A -> B: A  MAC<m> data<...>

   (1) A indicates interest in using tcpcrypt.  In (2), the server
   indicates willingness to use ECDHE with curves P256 and P512.
   Messages (3) and (4) complete the INIT1 and INIT2 key exchange
   messages described above, which are embedded in the data portion of
   the TCP segment. (5) From this point on, all messages are encrypted
   and their integrity protected by a MAC option.

5.2.  Example 2: Normal handshake with SYN cookie

   (1) A -> B: S  CRYPT<>
   (2) B -> A: SA CRYPT<PKCONF<0x200,0x201>, SYNCOOKIE<val>>
   (3) A -> B: A  CRYPT<ACKCOOKIE<val>> data<INIT1...>
   (4) B -> A: A  data<INIT2...>
   (5) B -> A: A  MAC<m> data<...>

   Same as previous example, except the server sends the client a SYN
   cookie value, which the client must echo in (3).  Here also the
   application level protocol begins by B transmitting data, while in
   the previous example, A was the first to transmit application-level
   data.







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5.3.  Example 3: tcpcrypt unsupported

   (1) A -> B: S  CRYPT<>
   (2) B -> A: SA
   (3) A -> A: A

   (1) A indicates interest in using tcpcrypt. (2) B does not support
   tcpcrypt, or a middle box strips out the CRYPT TCP option. (3) the
   client completes a normal three-way handshake, and tcpcrypt is not
   enabled for the connection.

5.4.  Example 4: Reusing established state

   (1) A -> B: S  CRYPT<NEXTK1<ID>>
   (2) B -> A: SA CRYPT<NEXTK2>
   (3) A -> A: A  MAC<m>

   (1) A indicates interest in using tcpcrypt with a session key derived
   from an existing key, to avoid the use of public key cryptography for
   the new session. (2) B supports tcpcrypt, has ID in its session ID
   cache, and is willing to proceed with session caching. (3) the client
   completes tcpcrypt's handshake within TCP's three-way handshake and
   tcpcrypt is enabled for the connection.

5.5.  Example 5: Decline of state reuse

   (1) A -> B: S  CRYPT<NEXTK1<ID>>
   (2) B -> A: SA CRYPT<PKCONF<1, 4, 16>>
   (3) A -> B: A  data<INIT1...>
   (4) B -> A: A  data<INIT2...>
   (5) A -> B: A  MAC<m> data<...>

   A wishes to use a key derived from a previous session key, but B does
   not recognize the session ID or has flushed it from its cache.
   Therefore, session establishment proceeds as in the first connection,
   using public key cryptography to negotiate a new series of session
   secrets (ss[i] values).

5.6.  Example 6: Reversal of client and server roles

   (1) A -> B: S  CRYPT<>
   (2) B -> A: SA CRYPT<HELLO>
   (3) A -> B: A  CRYPT<PKCONF<0x100>>
   (4) B -> A: A  data<INIT1...>
   (5) A -> B: A  data<INIT2...>
   (6) B -> A: A  MAC<m> data<...>

   Here the passive opener, B, wishes to play the role of the decryptor



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   using RSA.  By sending a HELLO suboption, B causes A to switch roles,
   so that now A is "S" and B plays the role of "C".


6.  API extensions

   The getsockopt call should have new options for IPPROTO_TCP:

      TCP_CRYPT_SESSID -> returns the session ID and MUST return an
      error if tcpcrypt is in not in the ENCRYPTING state (e.g., because
      it has transitioned to DISABLED).

      TCP_CRYPT_CMODE -> returns 1 if the local host played the "C" role
      in session key negotiation, 0 otherwise.

      TCP_CRYPT_PUBKEY_LOCAL -> When the local host played the "C" role,
      returns the hosts public key, PK_C. When the local host played the
      "S" role, returns PK_S if KX_S supports such a value or returns an
      error otherwise.  Hosts MAY return an error after transmitting the
      first application-level payload bytes (so as to reclaim the memory
      used to store keys).

      TCP_CRYPT_PUBKEY_PEER -> Analogous to TCP_CRYPT_PUBKEY_LOCAL with
      the roles reversed.  (Returns PK_C when the local host played the
      "S" role, and PK_S, if applicable, when the local host played the
      "C" role.)

      TCP_CRYPT_CONF -> returns the four-byte authenticated encryption
      algorithm in use by the connection (as specified in Table 6).  In
      addition, implementations SHOULD provide the three-byte public key
      cipher (Figure 3) initially used to negotiate the session keys, as
      well as the public key length for algorithms with variable key
      sizes (e.g., OAEP+-RSA3).

      TCP_CRYPT_SUPPORT -> returns 1 if the remote application is
      tcpcrypt-aware, as indicated by the remote host's use of a HELLO-
      app-support, HELLO-app-mandatory, or PKCONF-app-support CRYPT
      suboption (see Table 4).

   The setsockopt call should have:

      TCP_CRYPT_CACHE_FLUSH -> setting this option to non-zero wipes
      cached session keys.  Useful if application-level authentication
      discovers a man in the middle attack, to prevent the next
      connection from using NEXTK.

   The following options should be readable and writable with getsockopt
   and setsockopt:



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      TCP_CRYPT_ENABLE -> one bit, enables or disables tcpcrypt
      extension on an unconnected (listening or new) socket.

      TCP_CRYPT_SECURST -> one bit, means ignore unauthenticated RST
      packets for this connection when set to 1.

      TCP_CRYPT_CMODE_{DEFAULT,NEVER,ALWAYS}[_NK] -> As described in
      Section 4.2.

      TCP_CRYPT_PKCONF -> set of allowed public key algorithms and CPRFs
      this host advertises in CRYPT PKCONF suboptions.

      TCP_CRYPT_CCONF -> set of allowed symmetric ciphers and message
      authentication codes this host advertises in CRYPT INIT1 segments.

      TCP_CRYPT_SCONF -> order of preference of symmetric ciphers.

      TCP_CRYPT_SUPPORT -> set to 1 if the application is tcpcrypt-
      aware. set to 2 if the application is tcpcrypt-aware and wishes to
      enter the DISABLED state if the remote application is not
      tcpcrypt-aware.  An active opener SHOULD set the default value to
      0 for each new connection.  A passive opener SHOULD use a default
      value to 0 for each port, but SHOULD inherit the value of the
      listening socket for accepted connections.  The behavior for each
      value is as follows:

      When set to 0  The host MUST transition to the DISABLED state upon
         receiving a HELLO-app-mandatory option.  The host MUST NOT send
         the HELLO-app-support, HELLO-app-mandatory, NEXTK2-app-support,
         or PKCONF-app-support options.

      When set to 1  The "C" role host MUST use HELLO-app-support in
         place of the HELLO option, while the "S" role host MUST use the
         "PKCONF-app-support" in place of the "PKCONF" option.  Either
         role must use NEXTK2-app-support in place of NEXTK2.

      When set to 2  The "C" role host MUST use HELLO-app-mandatory
         option in place of the HELLO option, while the "S" role host
         MUST use "PKCONF-app-support" in place of the "PKCONF" option.
         Either role must use NEXTK2-app-support in place of NEXTK2.
         Either host MUST transition to DISABLED upon receipt of a HELLO
         or PKCONF option, but MUST proceed as usual in response to
         HELLO-app-support, HELLO-app-mandatory, and PKCONF-app-support.

   Finally, system administrators must be able to set the following
   system-wide parameters:





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   o  Default TCP_CRYPT_ENABLE value

   o  Default TCP_CRYPT_PKCONF value

   o  Default TCP_CRYPT_CCONF value

   o  Default TCP_CRYPT_SCONF value

   o  Types, key lengths, and regeneration intervals of local host's
      short-lived public keys

   The session ID can be used for end-to-end security.  For instance,
   applications might sign the session ID with public keys to
   authenticate their ends of a connection.  Because session IDs are not
   secret, servers can sign them in batches to amortize the cost of the
   signature over multiple connections.  Alternatively, DSA signatures
   are cheaper to compute than to verify, so might be a good way for
   servers to authenticate themselves.  A voice application could
   display the session ID on both parties' screens, and if they confirm
   by voice that they have the same ID, then the conversation is secure.

   Because the public key may change less often than once a session, it
   may alternatively be useful for the local end of a connection to
   authenticate itself by signing the local host's public key instead of
   the session ID.


7.  Acknowledgments

   This work was funded by gifts from Intel (to Brad Karp) and from
   Google, and by NSF award CNS-0716806 (A Clean-Slate Infrastructure
   for Information Flow Control).


8.  IANA Considerations

   The following numbers need assignment by IANA:

   o  New TCP option kind number for CRYPT

   o  New TCP option kind number for MAC

   A new registry entitled "tcpcrypt CRYPT suboptions" needs to be
   maintained by IANA as per the following table.







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                      +---------------------+-------+
                      | Symbol              | Value |
                      +---------------------+-------+
                      | HELLO               |  0x01 |
                      | HELLO-app-support   |  0x02 |
                      | HELLO-app-mandatory |  0x03 |
                      | DECLINE             |  0x04 |
                      | NEXTK2              |  0x05 |
                      | NEXTK2-app-support  |  0x06 |
                      | INIT1               |  0x07 |
                      | INIT2               |  0x08 |
                      | PKCONF              |  0x41 |
                      | PKCONF-app-support  |  0x42 |
                      | UNKNOWN             |  0x43 |
                      | SYNCOOKIE           |  0x44 |
                      | ACKCOOKIE           |  0x45 |
                      | SYNC_REQ            |  0x80 |
                      | SYNC_OK             |  0x81 |
                      | REKEY               |  0x82 |
                      | REKEYSTREAM         |  0x83 |
                      | NEXTK1              |  0x84 |
                      | IV                  |  0x85 |
                      +---------------------+-------+

                           TCP CRYPT suboptions.

                                  Table 5

   A "tcpcrypt Algorithm Identifiers" registry needs to be maintained by
   IANA as per the following table.





















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       +-----------------------------------------------------------+
       | Algorithm Identifier                           |    Value |
       +------------------------------------------------+----------+
       | Cipher: OAEP+-RSA with exponent 3              | 0x000100 |
       | Extract: HKDF-Extract-SHA256                   |          |
       | CPRF: HKDF-Expand-SHA256                       |          |
       | N_C len: 32 bytes                              |          |
       | R_S len: 48 bytes                              |          |
       | K_LEN: 32 bytes                                |          |
       +------------------------------------------------+----------+
       | Cipher: ECDHE-P256                             | 0x000200 |
       | Extract: HKDF-Extract-SHA256                   |          |
       | CPRF: HKDF-Expand-SHA256                       |          |
       | N_C len: 32 bytes                              |          |
       | N_S len: 32 bytes                              |          |
       | K_LEN: 32 bytes                                |          |
       +------------------------------------------------+----------+
       | Cipher: ECDHE-P512                             | 0x000201 |
       | Extract: HKDF-Extract-SHA256                   |          |
       | CPRF: HKDF-Expand-SHA256                       |          |
       | N_C len: 32 bytes                              |          |
       | N_S len: 32 bytes                              |          |
       | K_LEN: 32 bytes                                |          |
       +------------------------------------------------+----------+

                     TCP CRYPT algorithm identifiers.

                                 Figure 3

   A "tcpcrypt authenticated encryption algorithms" registry needs to be
   maintained by IANA as per the following table.

      +------------------------------------------------+------------+
      | Authenticated Encryption                       |      value |
      +------------------------------------------------+------------+
      | AES-128 ASM mode HMAC-SHA2-128 AES-128 ACK MAC | 0x00000100 |
      | AES-128 ASM mode Poly1305-AES AES-128 ACK MAC  | 0x00000200 |
      | AES-128 ASM mode CMAC-AES-128 AES-128 ACK MAC  | 0x00000300 |
      +------------------------------------------------+------------+

              TCP CRYPT authenticated encryption algorithms.

                                  Table 6








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9.  Security Considerations

   Tcpcrypt guarantees that no man-in-the-middle attacks occurred if
   Session IDs match on both ends of a connection, unless the attacker
   has broken the underlying cryptographic primitives (e.g., RSA).  A
   proof has been published [tcpcrypt].

   If the application performs no authentication, then there are no
   guarantees against active attackers.  Session IDs can be logged on
   both ends and man-in-the-middle attacks can be detected after the
   fact by comparing Session IDs offline.

   Session IDs are not confidential.

   Tcpcrypt can be downgraded to regular TCP during the connection setup
   phase by removing any of the CRYPT options.  The downgrade, and
   absence of protection, can of course be detected by the application
   as no Session ID will be returned.

   By default tcpcrypt does not protect against RST packet injection.
   The connection must be configured with TCP_CRYPT_RSTCHK enabled to
   protect against malicious (unMACed) RSTs.

   tcpcrypt uses short-lived keys to provide some forward secrecy.  If a
   key is compromised all connections (new and cached) derived from that
   key will be compromised.  The life of these keys should be kept to a
   minimum for stronger protection.  A life of less than two minutes is
   recommended.  Keys can be generated as frequently as practical, for
   example when servers have idle CPU time.  For ECDHE-based key
   agreement, a new key can be chosen for each connection.

   In the 4-way handshake, tcpcrypt does not have a key confirmation
   step.  Hence, an active attacker can cause a connection to hang,
   though this is possible even without tcpcrypt by altering sequence
   and ack numbers.

   Attackers cannot force passive openers to move forward in their
   session caching chain without guessing the content of the NEXTK1
   option, which will be hard without key knowledge.


10.  References

10.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.




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   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

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

   [RFC2437]  Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
              Specifications Version 2.0", RFC 2437, October 1998.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869, May 2010.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

10.2.  Informative References

   [I-D.narten-iana-considerations-rfc2434bis]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs",
              draft-narten-iana-considerations-rfc2434bis-09 (work in
              progress), March 2008.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [aggregate-macs]
              Katz, J. and A. Lindell, "Aggregate Message Authentication
              Codes", Topics in Cryptology - CT-RSA , 2008.

   [tcpcrypt]
              Bittau, A., Hamburg, M., Handley, M., Mazieres, D., and D.
              Boneh, "The case for ubiquitous transport-level
              encryption", USENIX Security , 2010.





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Appendix A.  Protocol constant values

                        +--------+---------------+
                        |  Value | Name          |
                        +--------+---------------+
                        |  0x01  | CONST_NEXTK   |
                        |  0x02  | CONST_SESSID  |
                        |  0x03  | CONST_REKEY   |
                        |  0x04  | CONST_KEY_C   |
                        |  0x05  | CONST_KEY_S   |
                        |  0x06  | CONST_KEY_ENC |
                        |  0x07  | CONST_KEY_MAC |
                        |  0x08  | CONST_KEY_ACK |
                        | 0x2911 | INIT1_MAGIC   |
                        | 0x8310 | INIT2_MAGIC   |
                        +--------+---------------+

                            Protocol constants.

                                  Table 7


Authors' Addresses

   Andrea Bittau
   Stanford University
   Department of Computer Science
   353 Serra Mall, Room 288
   Stanford, CA  94305
   US

   Phone: +1 650 723 8777
   Email: bittau@cs.stanford.edu


   Dan Boneh
   Stanford University
   Department of Computer Science
   353 Serra Mall, Room 475
   Stanford, CA  94305
   US

   Phone: +1 650 725 3897
   Email: dabo@cs.stanford.edu







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   Mike Hamburg
   Stanford University
   Department of Computer Science
   353 Serra Mall, Room 475
   Stanford, CA  94305
   US

   Phone: +1 650 725 3897
   Email: mike@shiftleft.org


   Mark Handley
   University College London
   Department of Computer Science
   University College London
   Gower St.
   London  WC1E 6BT
   UK

   Phone: +44 20 7679 7296
   Email: M.Handley@cs.ucl.ac.uk


   David Mazieres
   Stanford University
   Department of Computer Science
   353 Serra Mall, Room 290
   Stanford, CA  94305
   US

   Phone: +1 415 490 9451
   Email: dm@uun.org


   Quinn Slack
   Stanford University
   Department of Computer Science
   353 Serra Mall, Room 288
   Stanford, CA  94305
   US

   Phone: +1 650 723 8777
   Email: sqs@cs.stanford.edu








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