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Cryptographic protection of TCP Streams (tcpcrypt)
draft-bittau-tcpinc-tcpcrypt-02

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Authors Andrea Bittau, Dan Boneh , Daniel B. Giffin , Mike Hamburg , Mark J. Handley , David Mazieres , Quinn Slack
Last updated 2015-04-23
Replaced by draft-ietf-tcpinc-tcpcrypt, RFC 8548
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draft-bittau-tcpinc-tcpcrypt-02
Internet Engineering Task Force                                A. Bittau
Internet-Draft                                                  D. Boneh
Intended status: Standards Track                               D. Giffin
Expires: October 25, 2015                                     M. Hamburg
                                                     Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                                Q. Slack
                                                     Stanford University
                                                          April 23, 2015

           Cryptographic protection of TCP Streams (tcpcrypt)
                  draft-bittau-tcpinc-tcpcrypt-02.txt

Abstract

   This document presents tcpcrypt, a TCP extension for
   cryptographically protecting TCP connections.  Tcpcrypt maintains the
   confidentiality of data transmitted in TCP connections against a
   passive eavesdropper.  Additionally, applications that perform
   authentication can obtain end-to-end confidentiality and integrity
   guarantees by tying authentication to tcpcrypt Session ID values.

   The extension defines a new TCP option, CRYPT, which is 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.  These keys
   are then used with an authenticated-encryption mode to protect both
   the confidentiality and the integrity of transmitted application
   data.  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
   and may be updated, replaced, or obsoleted by other documents at any

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   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 October 25, 2015.

Copyright Notice

   Copyright (c) 2015 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   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
   modifications of such material outside the IETF Standards Process.
   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.

Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Idealized protocol  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Stages of the protocol  . . . . . . . . . . . . . . . . .   5
       3.1.1.  The setup phase . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  The ENCRYPTING state  . . . . . . . . . . . . . . . .   6
       3.1.3.  The DISABLED state  . . . . . . . . . . . . . . . . .   6
     3.2.  Cryptographic algorithms  . . . . . . . . . . . . . . . .   6
     3.3.  "C" and "S" roles . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Protocol negotiation  . . . . . . . . . . . . . . . . . .   8
     3.5.  Key exchange protocol . . . . . . . . . . . . . . . . . .   8
     3.6.  Data encryption and authentication  . . . . . . . . . . .  11
     3.7.  Re-keying . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.8.  Session caching . . . . . . . . . . . . . . . . . . . . .  12

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       3.8.1.  Session caching    control  . . . . . . . . . . . . .  13
   4.  Extensions to TCP . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Protocol states . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Role negotiation  . . . . . . . . . . . . . . . . . . . .  18
       4.2.1.  Simultaneous open . . . . . . . . . . . . . . . . . .  19
     4.3.  The TCP CRYPT option  . . . . . . . . . . . . . . . . . .  20
       4.3.1.  The HELLO suboption . . . . . . . . . . . . . . . . .  23
       4.3.2.  The DECLINE suboption . . . . . . . . . . . . . . . .  24
       4.3.3.  The NEXTK1 and NEXTK2 suboptions  . . . . . . . . . .  24
       4.3.4.  The PKCONF suboption  . . . . . . . . . . . . . . . .  26
       4.3.5.  The UNKNOWN suboption . . . . . . . . . . . . . . . .  27
       4.3.6.  The SYNCOOKIE and ACKCOOKIE suboptions  . . . . . . .  28
     4.4.  Messages in the TCP datastream  . . . . . . . . . . . . .  29
       4.4.1.  Datatypes and encodings . . . . . . . . . . . . . . .  29
         4.4.1.1.  Primitive and derived types . . . . . . . . . . .  29
         4.4.1.2.  Type definition . . . . . . . . . . . . . . . . .  30
         4.4.1.3.  Type declarations . . . . . . . . . . . . . . . .  30
         4.4.1.4.  Example . . . . . . . . . . . . . . . . . . . . .  30
         4.4.1.5.  Adaptive encoding of natural numbers  . . . . . .  30
       4.4.2.  Key-exchange messages . . . . . . . . . . . . . . . .  31
       4.4.3.  ENCRYPTING-phase messages . . . . . . . . . . . . . .  32
         4.4.3.1.  Frames  . . . . . . . . . . . . . . . . . . . . .  32
         4.4.3.2.  Frame security  . . . . . . . . . . . . . . . . .  33
         4.4.3.3.  Application data messages . . . . . . . . . . . .  35
         4.4.3.4.  Keep-alive and synchronization messages . . . . .  35
         4.4.3.5.  Re-keying messages  . . . . . . . . . . . . . . .  37
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  39
     5.1.  Example 1: Normal handshake . . . . . . . . . . . . . . .  39
     5.2.  Example 2: Normal handshake with SYN cookie . . . . . . .  39
     5.3.  Example 3: tcpcrypt unsupported . . . . . . . . . . . . .  39
     5.4.  Example 4: Reusing established state  . . . . . . . . . .  40
     5.5.  Example 5: Decline of state reuse . . . . . . . . . . . .  40
     5.6.  Example 6: Reversal of client and server roles  . . . . .  40
   6.  API extensions  . . . . . . . . . . . . . . . . . . . . . . .  40
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  43
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  43
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  45
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  46
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  46
     10.2.  Informative References . . . . . . . . . . . . . . . . .  46
   Appendix A.  Protocol constant values . . . . . . . . . . . . . .  47
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

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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  Permit "opportunistic" protection: that is, allow for a host
      configuration that employs the protocol whenever possible, without
      requiring the involvement of user applications.

   o  Provide interfaces to higher-level software to facilitate end-to-
      end security.  For example, exposing a session ID to an
      application would allow it to compare this value with its peer
      (after the fact, or via some real-time mechanism) and thereby
      detect any active modification of the connection.

   o  Minimize computational cost, particularly on servers.

   o  Operate independently of IP addresses, making it possible to
      authenticate resumed TCP connections even 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 extention-negotiation
   and key-exchange protocols should skip to Section 3.4.

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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,
   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

   In the setup phase, two hosts negotiate use of the tcpcrypt extension
   and then, if it will be used, agree on a suite of cryptographic
   algorithms and establish secret session keys.

   After establishing that both hosts are willing to perform the
   tcpcrypt protocol, 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.  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.  Alternatively, Applications MAY
   simply log Session IDs so as to enable attack detection after the
   fact, by comparing the values logged at each end.

   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.

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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.

   In the ENCRYPTING state, a host MUST employ the authenticated-
   encryption protocol described in Section 4.4.3 to protect all
   transmitted application data.

   Once it has entered the ENCRYPTING state, an endpoint MUST NOT ever
   transition, directly or indirectly, to the DISABLED state.

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 cipher.  The notation Extract (S, IKM) denotes the output of
      the extract function with salt S and initial keying material IKM.

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   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, CONST, L) to designate the output of L bytes of the
      pseudo-random function identified by key K on CONST.  A collision-
      resistant function is one on which, for sufficiently large L, an
      attacker cannot find two distinct inputs K_1, CONST_1 and K_2,
      CONST_2 such that CPRF (K_1, CONST_1, L) = CPRF (K_2, CONST_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, CONST, L) -> OKM
              T(0) = empty string (zero length)
              T(1) = HMAC-Hash(PRK, T(0) | CONST | 0x01)
              T(2) = HMAC-Hash(PRK, T(1) | CONST | 0x02)
              T(3) = HMAC-Hash(PRK, T(2) | CONST | 0x03)
              ...

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

   The symbol | denotes concatenation, and the counter concatenated with
   CONST 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 and integrity-protect all application data.

   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 compromise tcpcrypt's security.  Thus, any host implementing
   tcpcrypt MUST have a cryptographically-secure source of randomness or
   pseudo-randomness.

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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 is over two orders of magnitude faster than
   decryption.

   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,
   implementations MUST provide a mechanism for the passive opener to
   reverse roles and play the "C" role, as discussed in Section 4.2.

3.4.  Protocol negotiation

   When a host C connects to S, the two use the TCP option TCPCRYPT to
   negotiate the use of the tcpcrypt extension.  If the server is
   willing, the following exchange will occur:

                     C -> S:  HELLO
                     S -> C:  PKCONF, pub-cipher-list

   The pub-cipher-list value is a list of public-key ciphers and
   parameters acceptable to S.  These are defined in Figure 2.

   If S is not willing to engage in tcpcrypt, it may respond with
   DECLINE instead; if it does not implement tcpcrypt, it will fail to
   respond to the HELLO at all.  In either case, C will immediately
   transition to the DISABLED state.

3.5.  Key exchange protocol

   If the protocol negotiation above indicates that both sides wish to
   employ tcpcrypt, they will then use the TCP datastream to continue
   with this key-exchange protocol:

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

   The parameters are defined as follows:

   o  sym-cipher-list: a list of symmetric ciphers (AEAD algorithms)
      acceptable to C.  These are specified in Table 6.

   o  N_C: a nonce chosen at random by C.

   o  pub-cipher: a choice from the pub-cipher-list received in protocol
      negotiation.  Determines the type of PK_C.

   o  PK_C: C's public key or key agreement parameter.  This is a short-
      lived value that SHOULD be refreshed periodically and SHOULD NOT
      ever be written to persistent storage.

   o  sym-cipher: the symmetric cipher (AEAD algorithm) 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 2.  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  := { num-pub-ciphers, pub-cipher-list, init1, init2 }
         PRK    := Extract (N_C, { param, PMS })

   Here num-pub-ciphers is a single octet specifying how many three-byte
   algorithm specifiers were provided by the "S" host in a PKCONF
   suboption (described in Section 4.3.4).  pub-cipher-list is this many
   three-byte specifiers, taken from the body of the PKCONF suboption.

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   init1 and init2 are the encodings of the Init1 and Init2 messages
   described in Section 4.4.2.

   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
   Section 3.8.  The CONST values are constants defined in Table 7.  The
   K_LEN values and nonce sizes are negotiated, and are specified in
   Figure 2.

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

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

   Where flags is a single octet from 0x0 to 0x3, computed as follows:

            bit  0 1 2 3 4 5 6 7
                +-+-+-+-+-+-+-+-+
                |0 0 0 0 0 0 s c|
                +-+-+-+-+-+-+-+-+

   Here bit "s" is set when the "S" mode host has indicated application-
   level support for tcpcrypt.  The "c" bit is set when the "C" mode
   host has indicated application-level support for tcpcrypt.  Both bits
   are 0 by default unless the application has enabled the
   TCP_CRYPT_SUPPORT option described in Section 6.

   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_keylen)
   k_sc := CPRF (mk, CONST_KEY_S, ae_keylen)

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   The ae_keylen depends on the authenticated-encryption algorithm
   selected, and is given under "Key Length" in Table 6.

   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 because tcpcrypt's threat model includes the possibility of
   a connection to an adversary.  If key negotiation is compromised and
   yields two different keys, all subsequent frames will be ignored due
   to a failed integrity check, causing the application's connection to
   hang.  This is not a new threat because in plain TCP, an active
   attacker could have modified sequence and acknowledgement numbers to
   hang the connection anyway.

3.6.  Data encryption and authentication

   Following key exchange, all further communication in a tcpcrypt-
   enabled connection is carried out within delimited data-frames that
   are encrypted and authenticated using the agreed keys.

   This protection is provided via algorithms for Authenticated
   Encryption with Associated Data (AEAD).  The particular algorithms
   that may be used are listed in Table 6.  One algorithm is selected
   during the negotiation described in Section 3.5.

   With reference to the "AEAD Interface" described in Section 2 of
   [RFC5116], tcpcrypt invokes the AEAD algorithm with the secret key K
   set to k_cs or k_sc, according to the host's role as described in
   Section 3.5.

   Further, tcpcrypt provides inputs of the following types to the
   authenticated encryption operation, where the datatypes and their
   values are defined in Section 4.4.3:

       N: FrameNonce
       A: AssocData
       P: PlainText

   The output of the encryption operation, C, is transmitted as the
   frame's "ciphertext" value.

   When a frame is received, tcpcrypt reconstructs the N and A values
   from data sent in the clear, and provides these and the ciphertext
   value to the the authenticated decryption operation.  The output of
   this operation is either P, a value of type "Plaintext", or the

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   special symbol FAIL.  In the latter case, the implementation may
   either ignore the frame or terminate the connection.

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 frames it transmits with a REKEY message containing j, so
   that the recipient host knows to decrypt the application data using
   the new keyset.

   Upon receiving a REKEY<j> message, a recipient using transmit keys
   from a generation less than j must also update its transmit keys and
   start including a REKEY<j> message in its outgoing frames.  A host
   must continue transmitting REKEY messages until its peer acknowledges
   the switch to the new keys.

   Implementations SHOULD delete older-generation keys from memory once
   they have received all frames they will need to decrypt with the old
   keys and have encrypted all outgoing frames under the old keys.

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 3.5 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
   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

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   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 ten
   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 3.5.  Applications SHOULD set
      TCP_CRYPT_CACHE_FLUSH whenever authentication of the session ID
      fails.

4.  Extensions to TCP

   The tcpcrypt extension adds a new kind of option, CRYPT.  During the
   setup phase, all TCP segments MUST have the CRYPT option.

   The idealized protocol of the previous section is performed partly
   with TCP options during the handshake, and partly in the Data portion
   of TCP segments (after the SYN exchanges).  In particular, the
   negotiation of the tcpcrypt extension is embedded in the handshake,
   and successful negotiation then allows all further communications to
   be framed in the TCP datastream: the INIT1 and INIT2 messages
   accomplish the exchange of keys, and are followed by encrypted
   application data.

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

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   [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.  "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.  "enc" means any valid TCP
   action, including no action, except that any segments transmitted
   must contain encrypted frames of application data.  "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:

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                   +----+------------------------------+
                   | 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 if Op is "Frame" it refers to a data
   portion that either contains successfully-authenticated framed data
   or else contains incomplete frames which must be buffered before
   further processing.  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.

   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/         +----------+     -------|   | | |   |or rcv */Frame|
    |  == or ==        |rcv A/INIT2     drop|   | | |   |or rcv R/     |
    |rcv S/NEXTK1      |-----------         |   V V V   |------------  |
    |------------      |x                  +----------+ |enc           |
    |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.

   The only valid tcpcrypt state transition from ENCRYPTING is to
   CLOSED, which occurs only when TCP transitions to CLOSED.  tcpcrypt
   per se cannot cause TCP to transition to CLOSED.

   If a CLOSE happens in the ENCRYPTING state, a host MUST send the
   ApplicationFin message before transitioning 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 receipt of an ApplicationFin message to securely signal the
   end of data, 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 ApplicationFin along with
   the FIN flag.  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 flag.

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   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.

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

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   simultaneous HELLO (or NEXTK) messages will disable tcpcrypt because
   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   |
        | CRYPT |   N   |                 |
        +-------+-------+-------...-------+

                        Format of TCP CRYPT option

   Kind is always CRYPT.  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.

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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------...

   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.  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

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   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
   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      |
             |  0x41 |      * | PKCONF              | S      |
             |  0x42 |      * | PKCONF-app-support  | S      |
             |  0x43 |      * | UNKNOWN             | SED    |
             |  0x44 |      * | SYNCOOKIE           | S      |
             |  0x45 |      * | ACKCOOKIE           | SED    |
             |  0x84 |     10 | NEXTK1              | S      |
             +-------+--------+---------------------+--------+

              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 CRYPT, 3, 1 (Kind = CRYPT, Length = 3, Suboption =
   HELLO).

   A host MUST enter the DISABLED state if, during the setup phase, it
   receives a segment containing no TCP CRYPT 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 to reply without a TCP CRYPT
   option to disable tcpcrypt.  Once a host has entered DISABLED, 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.

   We now precisely specify the format of each suboption.  In the
   sections that follow, all multi-byte values are encoded in big-endian
   format.

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.

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

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 choose 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.

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   In the case of simultaneous open, two hosts that simultaneously send
   SYN packets with NEXTK1 and the same SID[i] may establish a
   connection, as described in Section 4.2.1.

4.3.4.  The PKCONF suboption

   The PKCONF option has one of the following two formats:

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

   Byte     0       1       2         N
        +-------+-------+-------...-------+
        |Opcode=|Length=|    Algorithm    |
        |  0x42 |   N   |   Specifiers    |
        +-------+-------+-------...-------+

                      Formats of the PKCONF suboption

   The two are treated identically by tcpcrypt, except that opcode 0x42
   (PKCONF-app-support) signals that the application on the sending host
   has set the TCP_CRYPT_SUPPORT option to non-zero, and hence the
   receiving host should return 1 for the TCP_CRYPT_PEER_SUPPORT socket
   option, as discussed in Section 6.

   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 2.

   Hosts MUST implement OAEP+-RSA3 and ECDHE-P256 and ECDHE-P521, but
   MAY by default disable certain algorithms and key sizes.  In
   particular, implementations SHOULD disable larger RSA keys (Algorithm

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   identifiers 0x102-0x103) by default unless such larger keys and
   ciphertexts can fit into a single TCP segment.

   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.

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 unrecognized 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 unrecognized 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 unrecognized option, it SHOULD reply with the
   UNKNOWN suboption to notify the other side.  If the host transitions
   to DISABLED as a result of the unrecognized 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
   unrecognized opcode list, it does not mean the sender does not

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   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:

   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.

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4.4.  Messages in the TCP datastream

   If the use of tcpcrypt is negotiated between two hosts, they may then
   begin sending messages in the TCP datastream.  These messages may or
   may not share boundaries with the TCP segments that transport them.
   We first provide a simple language for describing the data that must
   be exchanged, and then detail the messages used for exchanging keys
   and for transmitting encrypted application data.

4.4.1.  Datatypes and encodings

   This document uses various datatypes to describe classes of values
   and the manner of encoding them for transmission in the TCP
   datastream.

4.4.1.1.  Primitive and derived types

   Primitive types include:

   o  "Byte", an octet.  Its encoding is simply its value.

   o  "Natural", a natural number between 0 and 2^60 - 1, inclusive.
      Its encoding is described in Section 4.4.1.5.

   Derived types include:

   o  A _tuple_ of two component types, written "type1, type2".  Its
      encoding is the concatenation of the encodings of its components.

   o  A _vector_ containing multiple values of one element type.  It may
      have arbitrary length, written "type[]", or a static length "n",
      written "type[n]".  Its encoding is the concatenation of the
      encodings of its components.

   o  A _union_ of two alternative types, written "type1 | type2".  Its
      encoding is the encoding of either of its types.

   o  An _encapsulation_, written "{ type }".  Its encoding is the same
      as that of "Natural, type", where the "Natural" value gives the
      length of the encoding of the following "type" value.

   o  A _constant-bytes_ type, written as an all-capitalized word.  This
      singleton type, a sub-type of "Byte[]", includes only the value
      assigned to its name in Appendix A.

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4.4.1.2.  Type definition

   The notation "type-name ::= type-expr" defines the name "type-name"
   to represent the type expressed by "type-expr".  Type names are
   always written in mixed case, with first letter capitalized.

4.4.1.3.  Type declarations

   The notation "type-expr value-name" declares "value-name" to be a
   value of type "type-expr".  It is used to distinguish particular
   values for discussion.  The declaration as a whole simply represents
   the type "type-expr".

4.4.1.4.  Example

   A datatype used in several places in this document is a vector of
   encapsulated messages; e.g.:

       Messages ::= {Message}[]

   If "Message" is a union type and thus may have variable length, the
   vector "Messages" may nevertheless be decoded completely without
   necessarily knowing how to decode every possible "Message" variant
   type, as the encapsulation prepends each message with its length.

   This allows legacy implementations to operate insensitively to
   extensions which add variants to message types.

4.4.1.5.  Adaptive encoding of natural numbers

   The "adaptive encoding" is a variable-length encoding of the integers
   between zero and 2^60 - 1, inclusive.

   The length of the encoding depends on the values of the initial bits
   of its first byte, b0:

             Bit  0 1 2 3     8
                 +-+-+-+-+-...-+
        b0 =     |i j k l      |
                 +-+-+-+-+-...-+

   The table below gives, for certain patterns of the initial bits in
   b0, the total length of the encoding and the manner of computing the
   encoded value from b0 and the following bytes b1, b2, ...:

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         i j k l   len             value                max value
       +---------+-----+------------------------------+------------+
       | 0 - - - |  1  | b0                           |       127  |
       | 1 0 - - |  2  | nbo2(b0 & 0x3f, b1)          |  2^14 - 1  |
       | 1 1 0 - |  4  | nbo4(b0 & 0x1f, b1, b2, b3)  |  2^29 - 1  |
       | 1 1 1 0 |  8  | nbo8(b0 & 0x0f, b1, ..., b7) |  2^60 - 1  |
       +---------+-----+------------------------------+------------+

   Above, an eight-bit byte represents an unsigned integer in the
   traditional manner, and the '&' operator performs the bitwise logical
   AND of its arguments.  The function "nbo2" computes an integer from
   its two arguments using network byte-order, that is:

       nbo2(a, b) = (a * 2^8) + b

   Similarly, the functions "nbo4" and "nbo8" compute integers from
   series of 4 and 8 bytes, respectively, in network byte-order.

4.4.2.  Key-exchange messages

   Before application data may be sent, the INIT1 and INIT2 messages
   must be exchanged to negotiate session keys.

   The key-exchange messages use constants to identify cryptographic
   algorithms.  A "PubCipher" is a three-byte identifier for a public-
   key suite as specified in Figure 2:

       PubCipher ::= Byte[3]

   A "SymCipher" is a four-byte symmetric algorithm specifier from
   Table 6:

       SymCipher ::= Byte[4]

   The INIT1 message has the following type:

       Init1 ::= INIT1_MAGIC, PubCipher     pub-cipher,
                              {SymCipher[]} sym-cipher-list,
                              Byte[]        n_c,
                              Byte[]        pk_c

   The "pub-cipher" value is a selection among the entries of "pub-
   cipher-list" from the received PKCONF suboption, and determines both
   the length of "n_c" and the type of "pk_c".

   The INIT2 message has the following type:

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       Init2 ::= INIT2_MAGIC, SymCipher   sym-cipher,
                              KeyMaterial kx_s

   The "sym-cipher" value is a selection among the entries of "sym-
   cipher-list" from the received INIT1 message.

   The type of the key material in "kx_s" depends on the public key
   cipher selected, as described in Section 3.5.

       KeyMaterial ::= KeyMaterialECDHE | KeyMaterialOAEP

   When ECDHE is used, the key material is encoded as follows:

       KeyMaterialECDHE ::= Byte[] n_s,
                            Byte[] pk_s

   The length of "n_s" depends on "pub-cipher" and is given in Figure 2.
   The "pk_s" value occupies the remainder of the Init2 message.  When
   OAEP+-RSA exp3 is used, "kx_s" is simply a ciphertext in big-endian
   format:

       KeyMaterialOAEP ::= Byte[]

4.4.3.  ENCRYPTING-phase messages

   Once key-exchange succeeds and a host enters the ENCRYPTING phase, it
   may send frames of application data in the TCP datastream.  Within
   these frames, the application data and some other control messages
   are encrypted; further, they are integrity-protected along with any
   other control messages that are sent in the clear.

4.4.3.1.  Frames

   A "Frame" comprises a header containing "public" control messages
   that are sent in the clear, together with a ciphertext portion
   containing the encryption of any application data or private control
   data needing to be sent:

       Frame ::= FRAME, { {Header}  header,
                          Byte[]    ciphertext
                        }

   The header contains a vector of "PublicMessage" values:

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       Header ::= {PublicMessage}[]

       PublicMessage ::= ApplicationOffset
                       | ApplicationFin
                       | Rekey
                       | NonceClock

   These public-message types are described below in Section 4.4.3.3,
   Section 4.4.3.5, and Section 4.4.3.2.

   A frame's ciphertext contains the encryption of a "PlainText" value:

       PlainText ::= {PrivateMessage}[]

       PrivateMessage ::= ApplicationData
                        | Urgent
                        | SyncReq
                        | SyncOk

   These private-message types are described in Section 4.4.3.3 and
   Section 4.4.3.4.

4.4.3.2.  Frame security

   The frame's ciphertext value incorporates an "authentication tag"
   that protects the integrity of both the application data and the
   header; the decryption operation will fail if integrity has been
   compromised.

   If an implementation receives a frame that fails to decrypt, it MUST
   ignore all public messages it did not need to process during the
   decryption attempt.  It MAY either terminate the connection, or
   ignore the frame and attempt further processing.

   An "AssocData" value is related to a frame and is authenticated as
   described in Section 3.6, but is not transmitted.  It contains the
   frame's header:

       AssocData ::= ASSOC_DATA, Header

   The header may contain a "NonceClock" message, which provides a value
   that has never before been transmitted by the sending host in a
   distinct frame of this session:

       NonceClock ::= FRAME_NONCE_CLOCK, Natural clock

   A host MUST NOT ever transmit two distinct frames with the same
   "clock" value.  A sending host SHOULD begin a session sending frames

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   with the value set to zero, and increment the value at least once for
   each frame sent.  In any case, it MUST use values small enough to
   prevent a "NonceClock" encoding greater than 12 bytes.

   A "FrameNonce" value is related to a frame and is used as an input to
   the encryption and decryption operations, but is not transmitted:

       FrameNonce ::= FrameNonceClock | FrameNonceOffset

   If a frame contains a "NonceClock" in its header, then "FrameNonce"
   takes this form:

       FrameNonceClock ::= NonceClock, Byte[] padding

   Above, the "NonceClock" value is the same as the one in the frame
   header, and "padding" is enough zero-valued bytes to extend the
   encoding of the "FrameNonceClock" to exactly 12 bytes.

   If a frame contains no "NonceClock" message, then the "FrameNonce"
   value instead takes this form:

       FrameNonceOffset ::= ApplicationOffset, Byte[] padding

   Above, the "ApplicationOffset" is the same as the one in the frame
   header, and "padding" is enough zero-valued bytes to extend the
   encoding of the "FrameNonceOffset" to exactly 12 bytes.

   If a frame contains neither a "NonceClock" message nor an
   "ApplicationOffset" message, then the "FrameNonce" value is undefined
   and a receiving host MUST treat the frame the same as a frame whose
   decryption fails.

   When constructing an outgoing frame, a host MUST include a
   "NonceClock" message if the frame will contain no application data;
   that is, if it contains no "ApplicationData" message or contains an
   "ApplicationData" message with empty data.  Furthermore, it MUST NOT
   ever transmit more than one frame containing application data for a
   particular "ApplicationOffset" value, unless the frames are
   identical.

   The above requirements ensure that no two, distinct frames with the
   same "FrameNonce" value will ever be encrypted with the same key, in
   order to preserve the security properties of the authenticated
   encryption algorithm.

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4.4.3.3.  Application data messages

   The following message transmits a portion of the application
   datastream.

       ApplicationData ::= APPLICATION_DATA, Byte[] data

   An implementation MUST NOT include more than one "ApplicationData" in
   any frame.  For any frame which does contain one, it MUST also
   include an "ApplicationOffset" message, described below.

   The following message provides an offset into the application
   datastream:

       ApplicationOffset ::= APPLICATION_OFFSET, Natural offset

   When it occurs in a frame containing an "ApplicationData", it
   indicates the location of that portion of data in the datastream.  It
   may also occur in a frame with no application data.  Its role in re-
   keying is described in Section 4.4.3.5.

   The following message indicates that the application data contained
   in this frame lies at the end of the application datastream:

       ApplicationFin ::= APPLICATION_FIN

   An "ApplicationFin" message MUST be accompanied in the same frame by
   an "ApplicationOffset" message.  These messages declare that the
   offset of the end of the datastream is the offset in the
   "ApplicationOffset" message plus the length of data in the
   accompanying "ApplicationData" message, or plus zero of there is
   none.

   The URGENT message declares that the application data in this frame
   is "urgent", and that it belongs at the given offset into the
   (unframed) stream of application data.

       Urgent ::= URGENT, Natural offset

4.4.3.4.  Keep-alive and synchronization messages

   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.

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   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
   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.

   Instead, tcpcrypt uses the SYNC_REQ and SYNC_OK messages, protected
   by the enclosing frame's integrity protection, to probe connection
   liveness.  This mechanism also provides a limited form of
   acknowledgement that is used in re-keying, as described in
   Section 4.4.3.5.  These messages may be exchanged even when there is
   no application data to send.

   The SYNC_REQ message has the following type:

       SyncReq ::= SYNC_REQ, Natural clock

   The "clock" value is a non-decreasing number.  A host MUST increment
   "clock" at least once for every interval in which it sends a SYNC_REQ
   message.  Implementations that support TSopt MAY choose 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 message MUST reply with a SYNC_OK
   message, which has the following type:

       SyncOk ::= SYNC_OK, Natural received-clock

   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 receives multiple frames with
   SYNC_REQ messages before it sends frames in the opposite direction,
   it SHOULD send a single SYNC_OK with "received-clock" set to the
   highest "clock" in the frames it has received.

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4.4.3.5.  Re-keying messages

   During a tcpcrypt session, the set of keys used to perform
   authenticated encryption may be changed.  This allows hosts to wipe
   from memory keys that could decrypt previously-transmitted frames.
   It also allows the use of message authentication codes that can
   safely protect only a limited number of messages.

   The REKEY message is used while switching keys, and specifies which
   generation of keys has been used to encrypt and integrity-protect the
   current frame.  The message has the following format:

       Rekey ::= REKEY, Byte keyLSB

   The byte "keyLSB" is the generation number of the keys used for the
   current frame, modulo 256.

   Any frame containing a REKEY message MUST also contain an
   APPLICATION_OFFSET message, even if it contains no application data.
   The offset indicates the point at which data will be protected with
   the new key.

   Once a host sends a REKEY message with a particular generation number
   and data offset, it MUST NOT use any previous generation of keys to
   encrypt frames carrying data at that offset or greater.

   A host MAY use REKEY to increment the 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 MUST NOT initiate re-keying with any KeyLSB other than its
   current key number plus one modulo 256.

   When initiating re-keying, a host MUST include, in the same frame as
   the REKEY message, a SYNC_REQ message with a fresh "clock" value;
   i.e., higher than any "clock" value it has previously transmitted.

   We say that an outgoing frame is _acknowledged_ when the transmitter
   knows the remote side has received it: specifically, when the frame
   contained a SYNC_REQ with value "clock" and the transmitter later
   receives a SYNC_OK with a "received-clock" value at least as high as
   "clock".

   When a host receives a frame containing a REKEY message, it MUST
   proceed as follows:

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   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 frame.  If RECEIVE-KEY-NUMBER is negative, the
       receiver MUST also discard the frame.

   2.  The receiver MUST authenticate and decrypt the frame using the
       receive keys with generation number RECEIVE-KEY-NUMBER.  The
       receiver MUST discard the frame as usual if decryption fails.

   3.  If RECEIVE-KEY-NUMBER is greater than the receiver's current
       transmit key number, the receiver must wait to receive all frames
       with application data that precede the APPLICATION_OFFSET in the
       REKEY frame.  Once it receives frames covering all this missing
       data (if any), it MUST increase its transmit number to RECEIVE-
       KEY-NUMBER and transmit a REKEY message.  If the receiver has
       gotten multiple REKEY frames with different "keyLSB" values, it
       MUST increase its transmit key number to the highest RECEIVE-KEY-
       NUMBER of any frame for which it is not missing prior application
       data.

   After sending a REKEY (whether initiating re-keying or just
   responding), a host MUST continue to send REKEY in all subsequent
   frames until one of those frames is acknowledged.  This requirement
   allows tcpcrypt implementations to safely decrypt incoming frames
   out-of-order: any received frame that uses a new generation of keys
   will contain a REKEY message indicating the generation number, and
   frames with application data later in the stream can then be assumed
   to use this new generation.

   A host SHOULD erase old transmit keys from memory once it has
   finished encrypting any outgoing frames with those keys, and old
   receive keys once it has decrypted all incoming application data
   prior to the offset of any REKEY message it has received.

   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 application-data offset by 1,048,576) since the last
   re-keying.  A host MUST NOT initiate re-keying if it has outstanding
   unacknowledged frames with REKEY messages 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.

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5.  Examples

   To illustrate the use of the CRYPT option in establishing a tcpcrypt
   session, consider the following 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  data<Frame,...>

   (1) A indicates interest in using tcpcrypt.  In (2), the server
   indicates willingness to use ECDHE with curves P256 and P521.
   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 integrity-protected inside frames, which may or may not align
   with segment boundaries.

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,Frame,...>
   (5) A -> B: A  data<Frame,...>

   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 with B transmitting data, while in
   the previous example, A was the first to transmit application-level
   data.

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.

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5.4.  Example 4: Reusing established state

   (1) A -> B: S  CRYPT<NEXTK1<ID>>
   (2) B -> A: SA CRYPT<NEXTK2>
   (3) A -> B: A  data<Frame,...>

   (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<0x200,0x201>>
   (3) A -> B: A  data<Init1>
   (4) B -> A: A  data<Init2>
   (5) A -> B: A  data<Frame,...>

   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,Frame,...>

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

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

      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 2) 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_PEER_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).

      TCP_CRYPT_FIN_RCVD -> returns 1 if an ApplicationFin message (see
      Section 4.4.3.1) has been received in this connection.

   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:

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

      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 INIT1 messages.

      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 of
      0 for each new connection.  A passive opener SHOULD use a default
      value of 0 for each port, but SHOULD inherit the value of the

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      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:

   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.

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7.  Acknowledgments

   This work was funded by gifts from Intel (to Brad Karp) and from
   Google, by NSF award CNS-0716806 (A Clean-Slate Infrastructure for
   Information Flow Control), and by DARPA CRASH under contract
   #N66001-10-2-4088.

8.  IANA Considerations

   The following numbers need assignment by IANA:

   o  New TCP option kind number for CRYPT

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

                      +---------------------+-------+
                      | Symbol              | Value |
                      +---------------------+-------+
                      | HELLO               |  0x01 |
                      | HELLO-app-support   |  0x02 |
                      | HELLO-app-mandatory |  0x03 |
                      | DECLINE             |  0x04 |
                      | NEXTK2              |  0x05 |
                      | NEXTK2-app-support  |  0x06 |
                      | PKCONF              |  0x41 |
                      | PKCONF-app-support  |  0x42 |
                      | UNKNOWN             |  0x43 |
                      | SYNCOOKIE           |  0x44 |
                      | ACKCOOKIE           |  0x45 |
                      | NEXTK1              |  0x84 |
                      +---------------------+-------+

                           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              |          |
   |            min/max key size 2048/4096 bits ... | 0x000100 |
   |            min/max key size 4096/8192 bits ... | 0x000101 |
   |            min/max key size 8192/16384 bits .. | 0x000102 |
   |            min     key size 16384 bits ....... | 0x000103 |
   | 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-P521                             | 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 2

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

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              +------------------+------------+------------+
              | AEAD Algorithm   | Key Length | sym-cipher |
              +------------------+------------+------------+
              | AEAD_AES_128_GCM | 16 bytes   | 0x00000100 |
              | AEAD_AES_256_GCM | 32 bytes   | 0x00000200 |
              +------------------+------------+------------+

     Authenticated-encryption algorithms corresponding to 4-byte sym-
   cipher specifiers in INIT1 and INIT2 messages.  The use of encryption
        is described in Section 3.6.  The algorithms are defined in
                                [RFC5116].

                                  Table 6

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.

   tcpcrypt is not robust to the injection of FIN or RST packets.  These
   will force the closure of the connection, but applications may probe
   the operating system to determine whether an authenticated end-of-
   stream has been signaled, thus avoiding semantic truncation attacks.

   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,

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   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.

   [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.

   [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.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", January 2008.

   [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

   [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      |
                    |    0x18    | FRAME              |
                    |    0x20    | FRAME_NONCE_CLOCK  |
                    |    0x21    | FRAME_NONCE_OFFSET |
                    |    0x22    | ASSOC_DATA         |
                    |    0x23    | APPLICATION_DATA   |
                    |    0x24    | APPLICATION_OFFSET |
                    |    0x25    | APPLICATION_FIN    |
                    |    0x26    | URGENT             |
                    |    0x27    | SYNC_REQ           |
                    |    0x28    | SYNC_OK            |
                    |    0x29    | REKEY              |
                    | 0x15101a0e | INIT1_MAGIC        |
                    | 0x097105e0 | 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

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   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

   Daniel B. Giffin
   Stanford University
   Department of Computer Science
   353 Serra Mall, Room 288
   Stanford, CA  94305
   US

   Email: dbg@scs.stanford.edu

   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

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   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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