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Versions: (draft-bittau-tcpinc-tcpcrypt) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 8548

Network Working Group                                          A. Bittau
Internet-Draft                                                    Google
Intended status: Experimental                                   D. Boneh
Expires: August 19, 2017                                       D. Giffin
                                                              M. Hamburg
                                                     Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                                Q. Slack
                                                     Stanford University
                                                                E. Smith
                                                       Kestrel Institute
                                                       February 15, 2017

           Cryptographic protection of TCP Streams (tcpcrypt)


   This document specifies tcpcrypt, a TCP encryption protocol designed
   for use in conjunction with the TCP Encryption Negotiation Option
   (TCP-ENO) [I-D.ietf-tcpinc-tcpeno].  Tcpcrypt coexists with
   middleboxes by tolerating resegmentation, NATs, and other
   manipulations of the TCP header.  The protocol is self-contained and
   specifically tailored to TCP implementations, which often reside in
   kernels or other environments in which large external software
   dependencies can be undesirable.  Because the size of TCP options is
   limited, the protocol requires one additional one-way message latency
   to perform key exchange before application data may be transmitted.
   However, this cost can be avoided between two hosts that have
   recently established a previous tcpcrypt connection.

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
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on August 19, 2017.

Copyright Notice

   Copyright (c) 2017 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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   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Requirements language . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Encryption protocol . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Cryptographic algorithms  . . . . . . . . . . . . . . . .   4
     3.2.  Protocol negotiation  . . . . . . . . . . . . . . . . . .   5
     3.3.  Key exchange  . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Session ID  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Session caching . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  Data encryption and authentication  . . . . . . . . . . .  11
     3.7.  TCP header protection . . . . . . . . . . . . . . . . . .  12
     3.8.  Re-keying . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.9.  Keep-alive  . . . . . . . . . . . . . . . . . . . . . . .  13
   4.  Encodings . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Key exchange messages . . . . . . . . . . . . . . . . . .  14
     4.2.  Application frames  . . . . . . . . . . . . . . . . . . .  16
       4.2.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.2.  Associated data . . . . . . . . . . . . . . . . . . .  17

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       4.2.3.  Frame nonce . . . . . . . . . . . . . . . . . . . . .  17
   5.  Key agreement schemes . . . . . . . . . . . . . . . . . . . .  18
   6.  AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  20
   9.  Design notes  . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Asymmetric roles  . . . . . . . . . . . . . . . . . . . .  21
     9.2.  Verified liveness . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Protocol constant values . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [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  Meet the requirements of the TCP Encryption Negotiation Option
      (TCP-ENO) [I-D.ietf-tcpinc-tcpeno] for protecting connection data.

   o  Be amenable to small, self-contained implementations inside TCP

   o  Minimize additional latency at connection startup.

   o  As much as possible, prevent connection failure in the presence of
      NATs and other middleboxes that might normalize traffic or
      otherwise manipulate TCP segments.

   o  Operate independently of IP addresses, making it possible to
      authenticate resumed sessions efficiently even when either end
      changes IP address.

3.  Encryption protocol

   This section describes the tcpcrypt protocol at an abstract level.
   The concrete format of all messages is specified in Section 4.

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3.1.  Cryptographic algorithms

   Setting up a tcpcrypt connection employs three types of cryptographic

   o  A _key agreement scheme_ is used with a short-lived public key to
      agree upon a shared secret.

   o  An _extract function_ is used to generate a pseudo-random key from
      some initial keying material, typically the output of the key
      agreement scheme.  The notation Extract(S, IKM) denotes the output
      of the extract function with salt S and initial keying material

   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.

   The Extract and CPRF functions used by default are the Extract and
   Expand functions of HKDF [RFC5869].  These are defined as follows in
   terms of the PRF "HMAC-Hash(key, value)" for a negotiated "Hash"

           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) | ...

       Figure 1: The symbol | denotes concatenation, and the counter
           concatenated to the right of CONST is a single octet.

   Lastly, once tcpcrypt has been successfully set up and encryption
   keys have been derived, an algorithm for Authenticated Encryption
   with Associated Data (AEAD) is used to protect the confidentiality
   and integrity of all transmitted application data.  AEAD algorithms
   use a single key to encrypt their input data and also to generate a
   cryptographic tag to accompany the resulting ciphertext; when
   decryption is performed, the tag allows authentication of the
   encrypted data and of optional, associated plaintext data.

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3.2.  Protocol negotiation

   Tcpcrypt depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to negotiate
   whether encryption will be enabled for a connection, and also which
   key agreement scheme to use.  TCP-ENO negotiates the use of a
   particular TCP encryption protocol or _TEP_ by including protocol
   identifiers in ENO suboptions.  This document associates four TEP
   identifiers with the tcpcrypt protocol, as listed in Table 1.  Each
   identifier indicates the use of a particular key-agreement scheme.
   Future standards may associate additional identifiers with tcpcrypt.

   An active opener that wishes to negotiate the use of tcpcrypt
   includes an ENO option in its SYN segment.  That option includes
   suboptions with tcpcrypt TEP identifiers indicating the key-agreement
   schemes it is willing to enable.  The active opener MAY additionally
   include suboptions indicating support for encryption protocols other
   than tcpcrypt, as well as global suboptions as specified by TCP-ENO.

   If a passive opener receives an ENO option including tcpcrypt TEPs it
   supports, it MAY then attach an ENO option to its SYN-ACK segment,
   including _solely_ the TEP it wishes to enable.

   To establish distinct roles for the two hosts in each connection,
   tcpcrypt depends on the role-negotiation mechanism of TCP-ENO.  As
   one result of the negotiation process, TCP-ENO assigns hosts unique
   roles abstractly called "A" at one end of the connection and "B" at
   the other.  Generally, an active opener plays the "A" role and a
   passive opener plays the "B" role; but in the case of simultaneous
   open, an additional mechanism breaks the symmetry and assigns
   different roles to the two hosts.  This document adopts the terms
   "host A" and "host B" to identify each end of a connection uniquely,
   following TCP-ENO's designation.

   ENO suboptions include a flag "v" which indicates the presence of
   associated, variable-length data.  In order to propose fresh key
   agreement with a particular tcpcrypt TEP, a host sends a one-byte
   suboption containing the TEP identifier and "v = 0".  In order to
   propose session resumption (described further below) with a
   particular TEP, a host sends a variable-length suboption containing
   the TEP identifier, the flag "v = 1", and an identifier for a session
   previously negotiated with the same host and the same TEP.

   Once two hosts have exchanged SYN segments, TCP-ENO defines the
   _negotiated TEP_ to be the last valid TEP identifier in the SYN
   segment of host B (that is, the passive opener in the absence of
   simultaneous open) that also occurs in that of host A.  If there is
   no such TEP, hosts MUST disable TCP-ENO and tcpcrypt.

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   If the negotiated TEP was sent by host B with "v = 0", it means that
   fresh key agreement will be performed as described below in
   Section 3.3.  If it had "v = 1", the key-exchange messages will be
   omitted in favor of determining keys via session-caching as described
   in Section 3.5, and protected application data may immediately be
   sent as detailed in Section 3.6.

   Note that the negotiated TEP is determined without reference to the
   "v" bits in ENO suboptions, so if host A offers resumption with a
   particular TEP and host B replies with a non-resumption suboption
   with the same TEP, that may become the negotiated TEP and fresh key
   agreement will be performed.  That is, sending a resumption suboption
   also implies willingness to perform fresh key agreement with the
   indicated TEP.

   As required by TCP-ENO, once a host has both sent and received an ACK
   segment containing a valid ENO option, encryption MUST be enabled and
   plaintext application data MUST NOT ever be exchanged on the
   connection.  If the negotiated TEP is among those listed in Table 1,
   a host MUST follow the protocol described in this document.

3.3.  Key exchange

   Following successful negotiation of a tcpcrypt TEP, all further
   signaling is performed in the Data portion of TCP segments.  Except
   when resumption was negotiated (described below in Section 3.5), the
   two hosts perform key exchange through two messages, "Init1" and
   "Init2", at the start of the data streams of host A and host B,
   respectively.  These messages may span multiple TCP segments and need
   not end at a segment boundary.  However, the segment containing the
   last byte of an "Init1" or "Init2" message SHOULD have TCP's PSH bit

   The key exchange protocol, in abstract, proceeds as follows:

       A -> B:  Init1 = { INIT1_MAGIC, sym-cipher-list, N_A, PK_A }
       B -> A:  Init2 = { INIT2_MAGIC, sym-cipher, N_B, PK_B }

   The concrete format of these messages is specified in Section 4.1.

   The parameters are defined as follows:

   o  "INIT1_MAGIC", "INIT2_MAGIC": constants defined in Table 3.

   o  "sym-cipher-list": a list of symmetric ciphers (AEAD algorithms)
      acceptable to host A.  These are specified in Table 2.

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   o  "sym-cipher": the symmetric cipher selected by host B from the
      "sym-cipher-list" sent by host A.

   o  "N_A", "N_B": nonces chosen at random by hosts A and B,

   o  "PK_A", "PK_B": ephemeral public keys for hosts A and B,
      respectively.  These, as well as their corresponding private keys,
      are short-lived values that SHOULD be refreshed periodically.  The
      private keys SHOULD NOT ever be written to persistent storage.

   The ephemeral secret ("ES") is the result of the key-agreement
   algorithm (see Section 5) indicated by the negotiated TEP.  The
   inputs to the algorithm are the local host's ephemeral private key
   and the remote host's ephemeral public key.  For example, host A
   would compute "ES" using its own private key (not transmitted) and
   host B's public key, "PK_B".

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

          PRK = Extract(N_A, eno-transcript | Init1 | Init2 | ES)

   Above, "|" denotes concatenation; "eno-transcript" is the protocol-
   negotiation transcript defined in TCP-ENO; and "Init1" and "Init2"
   are the transmitted encodings of the messages described in
   Section 4.1.

   A series of "session secrets" are then computed from "PRK" as

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

   The value "ss[0]" is used to generate all key material for the
   current connection.  The values "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 3.5.
   The "CONST_*" values are constants defined in Table 3.  The length
   "K_LEN" depends on the tcpcrypt TEP in use, and is specified in
   Section 5.

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

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

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   The particular master key in use is advanced as described in
   Section 3.8.

   Finally, each master key "mk" is used to generate keys for
   authenticated encryption for the "A" and "B" roles.  Key "k_ab" is
   used by host A to encrypt and host B to decrypt, while "k_ba" is used
   by host B to encrypt and host A to decrypt.

                  k_ab = CPRF(mk, CONST_KEY_A, ae_keylen)
                  k_ba = CPRF(mk, CONST_KEY_B, ae_keylen)

   The value "ae_keylen" depends on the authenticated-encryption
   algorithm selected, and is given under "Key Length" in Table 2.

   After host B sends "Init2" or host A receives it, that host may
   immediately begin transmitting protected application data as
   described in Section 3.6.

   If host A receives "Init2" with a "sym-cipher" value that was not
   present in the "sym-cipher-list" it previously transmitted in
   "Init1", it MUST abort the connection and raise an error condition
   distinct from the end-of-file condition.

3.4.  Session ID

   TCP-ENO requires each TEP to define a _session ID_ value that
   uniquely identifies each encrypted connection.

   As required, a tcpcrypt session ID begins with the negotiated TEP
   identifier along with the "v" bit as transmitted by host B.  The
   remainder of the ID is derived from the session secret, as follows:

        session_id[i] = TEP-byte | CPRF(ss[i], CONST_SESSID, K_LEN)

   Again, the length "K_LEN" depends on the TEP, and is specified in
   Section 5.

3.5.  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]".  A host signals willingness to resume with a particular
   session secret by sending a SYN segment with a resumption suboption:
   that is, an ENO suboption containing the negotiated TEP identifier
   from the original session and part of an identifier for the session.

   The resumption identifier is calculated from a session secret "ss[i]"
   as follows:

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                 resume[i] = CPRF(ss[i], CONST_RESUME, 18)

   To name a session for resumption, a host sends either the first or
   second half of the resumption identifier, according to the role it
   played in the original session with secret "ss[0]".

   A host that originally played role A and wishes to resume from a
   cached session sends a suboption with the first half of the
   resumption identifier:

       byte     0        1                  9      (10 bytes total)
            |  TEP-  |      resume[i]{0..8}      |
            |  byte  |                           |

    Figure 2: Resumption suboption sent when original role was A.  The
          TEP-byte contains a tcpcrypt TEP identifier and v = 1.

   Similarly, a host that originally played role B sends a suboption
   with the second half of the resumption identifier:

       byte     0        1                  9      (10 bytes total)
            |  TEP-  |      resume[i]{9..17}     |
            |  byte  |                           |

    Figure 3: Resumption suboption sent when original role was B.  The
          TEP-byte contains a tcpcrypt TEP identifier and v = 1.

   If a passive opener recognizes the identifier-half in a resumption
   suboption it has received and knows "ss[i]", it SHOULD (with
   exceptions specified below) agree to resume from the cached session
   by sending its own resumption suboption, which will contain the other
   half of the identifier.

   If it does not agree to resumption with a particular TEP, the passive
   opener may either request fresh key exchange by responding with a
   non-resumption suboption using the same TEP, or else respond to any
   other received suboption.

   If an active opener receives a resumption suboption for a particular
   TEP and the received identifier-half does not match the "resume[i]"
   value whose other half it previously sent in a resumption suboption
   for the same TEP, it MUST ignore that suboption.  In the typical case
   that this was the only ENO suboption received, this means the host

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   MUST disable TCP-ENO and tcpcrypt: that is, it MUST NOT send any more
   ENO options and MUST NOT encrypt the connection.

   When a host concludes that TCP-ENO negotiation has succeeded for some
   TEP that was received in a resumption suboption, it MUST then enable
   encryption with that TEP, using the cached session secret, as
   described in Section 3.6.

   The session ID (Section 3.4) is constructed in the same way for
   resumed sessions as it is for fresh ones.  In this case the first
   byte will always have "v = 1".  The remainder of the ID is derived
   from the cached session secret.

   In the case of simultaneous open where TCP-ENO is able to establish
   asymmetric roles, two hosts that simultaneously send SYN segments
   with compatible resumption suboptions may resume the associated

   In a particular SYN segment, a host SHOULD NOT send more than one
   resumption suboption, and MUST NOT send more than one resumption
   suboption with the same TEP identifier.  But in addition to any
   resumption suboptions, an active opener MAY include non-resumption
   suboptions describing other key-agreement schemes it supports (in
   addition to that indicated by the TEP in the resumption suboption).

   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 retain "ss[i+1]" 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.

   When proposing resumption, the active opener MUST use the lowest
   value of "i" that has not already been used (successfully or not) to
   negotiate resumption with the same host and for the same pre-session
   key "ss[0]".

   A host MUST NOT resume with a session secret if it has ever
   successfully negotiated resumption in the past, in either role, with
   the same secret.  In the event that two hosts simultaneously send SYN
   segments to each other that propose resumption with the same session
   secret but the two segments are not part of a simultaneous open, both
   connections will have to revert to fresh key-exchange.  To avoid this
   limitation, implementations MAY choose to implement session caching
   such that a given pre-session key "ss[0]" is only used for either
   passive or active opens at the same host, not both.

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   When two hosts have previously negotiated a tcpcrypt session, either
   host may initiate session resumption regardless of which host was the
   active opener or played the "A" role in the previous session.

   However, a given host must either encrypt with "k_ab" for all
   sessions derived from the same pre-session key "ss[0]", or with
   "k_ba".  Thus, which keys a host uses to send segments is not
   affected by the role it plays in the current connection: it depends
   only on whether the host played the "A" or "B" role in the initial

   Implementations that perform session caching MUST provide a means for
   applications to control session caching, including flushing cached
   session secrets associated with an ESTABLISHED connection or
   disabling the use of caching for a particular connection.

3.6.  Data encryption and authentication

   Following key exchange (or its omission via session caching), all
   further communication in a tcpcrypt-enabled connection is carried out
   within delimited _application 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 2.  One algorithm is selected
   during the negotiation described in Section 3.3.

   The format of an application frame is specified in Section 4.2.  A
   sending host breaks its stream of application data into a series of
   chunks.  Each chunk is placed in the "data" portion of a "plaintext"
   value, which is then encrypted to yield a frame's "ciphertext" field.
   Chunks must be small enough that the ciphertext (whose length depends
   on the AEAD cipher used, and is generally slightly longer than the
   plaintext) has length less than 2^16 bytes.

   An "associated data" value (see Section 4.2.2) is constructed for the
   frame.  It contains the frame's "control" field and the length of the

   A "frame nonce" value (see Section 4.2.3) is also constructed for the
   frame but not explicitly transmitted.  It contains an "offset" field
   whose integer value is the zero-indexed byte offset of the beginning
   of the current application frame in the underlying TCP datastream.
   (That is, the offset in the framing stream, not the plaintext
   application stream.)  Because it is strictly necessary for the
   security of the AEAD algorithm, an implementation MUST NOT ever
   transmit distinct frames with the same nonce value under the same

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   encryption key.  In particular, a retransmitted TCP segment MUST
   contain the same payload bytes for the same TCP sequence numbers, and
   a host MUST NOT transmit more than 2^64 bytes in the underlying TCP
   datastream (which would cause the "offset" field to wrap) before re-

   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_ab or k_ba, according to the host's role as described in
   Section 3.3.  The plaintext value serves as "P", the associated data
   as "A", and the frame nonce as "N".  The output of the encryption
   operation, "C", is transmitted in the frame's "ciphertext" field.

   When a frame is received, tcpcrypt reconstructs the associated data
   and frame nonce values (the former contains only data sent in the
   clear, and the latter is implicit in the TCP stream), and provides
   these and the ciphertext value to the the AEAD decryption operation.
   The output of this operation is either a plaintext value "P" or the
   special symbol FAIL.  In the latter case, the implementation MUST
   either ignore the frame or abort the connection; but if it aborts,
   the implementation MUST raise an error condition distinct from the
   end-of-file condition.

3.7.  TCP header protection

   The "ciphertext" field of the application frame contains protected
   versions of certain TCP header values.

   When "URGp" is set, the "urgent" value indicates an offset from the
   current frame's beginning offset; the sum of these offsets gives the
   index of the last byte of urgent data in the application datastream.

   When "FINp" is set, it indicates that the sender will send no more
   application data after this frame.  When the TCP FIN flag differs
   from "FINp", a receiving host MUST either ignore the segment
   altogether or abort the connection and raise an error condition
   distinct from the end-of-file condition.

3.8.  Re-keying

   Re-keying allows hosts to wipe from memory keys that could decrypt
   previously transmitted segments.  It also allows the use of AEAD
   ciphers that can securely encrypt only a bounded number of messages
   under a given key.

   We refer to the two encryption keys (k_ab, k_ba) 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.  Each host maintains a

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   _local generation number_ that determines which key-set it uses to
   encrypt outgoing frames, and a _remote generation number_ equal to
   the highest generation used in frames received from its peer.
   Initially, these two values are set to zero.

   A host MAY increment its local generation number beyond the remote
   generation number it has recorded.  We call this action _initiating

   When a host has incremented its local generation number and uses the
   new key-set for the first time to encrypt an outgoing frame, it MUST
   set "rekey = 1" for that frame.  It MUST set this field to zero in
   all other cases.

   When a host receives a frame with "rekey = 1", it increments its
   record of the remote generation number.  If the remote generation
   number is now greater than the local generation number, the receiver
   MUST immediately increment its local generation number to match and
   send a frame (with empty application data if necessary) with "rekey =

   A host SHOULD NOT initiate more than one concurrent re-key operation
   if it has no data to send; that is, it should not initiate re-keying
   with an empty application frame more than once while its record of
   the remote generation number is less than its own.

   When retransmitting, implementations must always transmit the same
   bytes for the same TCP sequence numbers.  Thus, a frame in a
   retransmitted segment MUST always be encrypted with the same key as
   when it was originally transmitted.

   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.

3.9.  Keep-alive

   Instead of using TCP Keep-Alives to verify that the remote endpoint
   is still responsive, tcpcrypt implementations SHOULD employ the re-
   keying mechanism for this purpose, as follows.  When necessary, a
   host SHOULD probe the liveness of its peer by initiating re-keying
   and transmitting a new frame immediately (with empty application data
   if necessary).

   As described in Section 3.8, a host receiving a frame encrypted under
   a generation number greater than its own MUST increment its own
   generation number and immediately transmit a new frame (with zero-
   length application data, if necessary).

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   Implementations MAY use TCP Keep-Alives for purposes that do not
   require endpoint authentication, as discussed in Section 9.2.

4.  Encodings

   This section provides byte-level encodings for values transmitted or
   computed by the protocol.

4.1.  Key exchange messages

   The "Init1" message has the following encoding:

       byte   0       1       2       3
          |          INIT1_MAGIC          |
          |                               |

                  4        5      6       7
              |          message_len          |
              |              = M              |

              |nciphers|sym-   |sym-   |         |sym-   |
              | =K+1   |cipher0|cipher1|         |cipherK|

                 K + 10                    K + 10 + N_A_LEN
                  |                         |
                  v                         v
              |           N_A           |          PK_A           |
              |                         |                         |

                                  M - 1
              |          ignored        |
              |                         |

   The constant "INIT1_MAGIC" is defined in Table 3.  The four-byte
   field "message_len" gives the length of the entire "Init1" message,
   encoded as a big-endian integer.  The "nciphers" field contains an
   integer value that specifies the number of one-byte symmetric-cipher

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   identifiers that follow.  The "sym-cipher" bytes identify
   cryptographic algorithms in Table 2.  The length "N_A_LEN" and the
   length of "PK_A" are both determined by the negotiated key-agreement
   scheme, as described in Section 5.

   When sending "Init1", implementations of this protocol MUST omit the
   field "ignored"; that is, they must construct the message such that
   its end, as determined by "message_len", coincides with the end of
   the field "PK_A".  When receiving "Init1", however, implementations
   MUST permit and ignore any bytes following "PK_A".

   The "Init2" message has the following encoding:

       byte   0       1       2       3
          |          INIT2_MAGIC          |
          |                               |

                  4        5      6       7       8
              |          message_len          |sym-   |
              |              = M              |cipher |

                  9                        9 + N_B_LEN
                  |                         |
                  v                         v
              |           N_B           |          PK_B           |
              |                         |                         |

                                  M - 1
              |          ignored        |
              |                         |

   The constant "INIT2_MAGIC" is defined in Table 3.  The four-byte
   field "message_len" gives the length of the entire "Init2" message,
   encoded as a big-endian integer.  The "sym-cipher" value is a
   selection from the symmetric-cipher identifiers in the previously-
   received "Init1" message.  The length "N_B_LEN" and the length of
   "PK_B" are both determined by the negotiated key-agreement scheme, as
   described in Section 5.

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   When sending "Init2", implementations of this protocol MUST omit the
   field "ignored"; that is, they must construct the message such that
   its end, as determined by "message_len", coincides with the end of
   the "PK_B" field.  When receiving "Init2", however, implementations
   MUST permit and ignore any bytes following "PK_B".

4.2.  Application frames

   An _application frame_ comprises a control byte and a length-prefixed
   ciphertext value:

          byte   0       1       2       3               clen+2
             |control|      clen     |        ciphertext       |

   The field "clen" is an integer in big-endian format and gives the
   length of the "ciphertext" field.

   The byte "control" has this structure:

                  bit     7                 1       0
                      |          cres           | rekey |

   The seven-bit field "cres" is reserved; implementations MUST set
   these bits to zero when sending, and MUST ignore them when receiving.

   The use of the "rekey" field is described in Section 3.8.

4.2.1.  Plaintext

   The "ciphertext" field is the result of applying the negotiated
   authenticated-encryption algorithm to a "plaintext" value, which has
   one of these two formats:

          byte   0       1               plen-1
             | flags |           data          |

          byte   0       1       2       3              plen-1
             | flags |    urgent     |          data           |

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   (Note that "clen" in the previous section will generally be greater
   than "plen", as the ciphertext produced by the authenticated-
   encryption scheme must both encrypt the application data and provide
   a way to verify its integrity.)

   The "flags" byte has this structure:

               bit    7    6    5    4    3    2    1    0
                   |            fres             |URGp|FINp|

   The six-bit value "fres" is reserved; implementations MUST set these
   six bits to zero when sending, and MUST ignore them when receiving.

   When the "URGp" bit is set, it indicates that the "urgent" field is
   present, and thus that the plaintext value has the second structure
   variant above; otherwise the first variant is used.

   The meaning of "urgent" and of the flag bits is described in
   Section 3.7.

4.2.2.  Associated data

   An application frame's "associated data" (which is supplied to the
   AEAD algorithm when decrypting the ciphertext and verifying the
   frame's integrity) has this format:

                       byte   0       1       2
                          |control|     clen      |

   It contains the same values as the frame's "control" and "clen"

4.2.3.  Frame nonce

   Lastly, a "frame nonce" (provided as input to the AEAD algorithm) has
   this format:

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                      0 |     FRAME_NONCE_MAGIC     |
                      4 |                           |
                        +           offset          +
                      8 |                           |

   The 4-byte magic constant is defined in Table 3.  The 8-byte "offset"
   field contains an integer in big-endian format.  Its value is
   specified in Section 3.6.

5.  Key agreement schemes

   The TEP negotiated via TCP-ENO may indicate the use of one of the
   key-agreement schemes named in Table 1.  For example,
   "TCPCRYPT_ECDHE_P256" names the tcpcrypt protocol with key-agreement
   scheme ECDHE-P256.

   All schemes listed there use HKDF-Expand-SHA256 as the CPRF, and
   these lengths for nonces and session keys:

                             N_A_LEN: 32 bytes
                             N_B_LEN: 32 bytes
                             K_LEN:   32 bytes

   Key-agreement schemes ECDHE-P256 and ECDHE-P521 employ the ECSVDP-DH
   secret value derivation primitive defined in [ieee1363].  The named
   curves are defined in [nist-dss].  When the public-key values "PK_A"
   and "PK_B" are transmitted as described in Section 4.1, they are
   encoded with the "Elliptic Curve Point to Octet String Conversion
   Primitive" described in Section E.2.3 of [ieee1363], and are prefixed
   by a two-byte length in big-endian format:

              byte   0       1       2               L - 1
                 |   pubkey_len  |          pubkey         |
                 |      = L      |                         |

   Implementations SHOULD encode these "pubkey" values in "compressed
   format", and MUST accept values encoded in "compressed",
   "uncompressed" or "hybrid" formats.

   Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 use the
   functions X25519 and X448, respectively, to perform the Diffie-Helman
   protocol as described in [RFC7748].  When using these ciphers,

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   public-key values "PK_A" and "PK_B" are transmitted directly with no
   length prefix: 32 bytes for Curve25519, and 56 bytes for Curve448.

   A tcpcrypt implementation MUST support at least the schemes
   ECDHE-P256 and ECDHE-P521, although system administrators need not
   enable them.

6.  AEAD algorithms

   Specifiers and key-lengths for AEAD algorithms are given in Table 2.
   The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM" are
   specified in [RFC5116].  The algorithm "AEAD_CHACHA20_POLY1305" is
   specified in [RFC7539].

7.  IANA considerations

   Tcpcrypt's TEP identifiers will need to be incorporated in IANA's
   TCP-ENO encryption protocol identifier registry, as follows:

                   |  cs  | Spec name                 |
                   | 0x21 | TCPCRYPT_ECDHE_P256       |
                   | 0x22 | TCPCRYPT_ECDHE_P521       |
                   | 0x23 | TCPCRYPT_ECDHE_Curve25519 |
                   | 0x24 | TCPCRYPT_ECDHE_Curve448   |

              Table 1: TEP identifiers for use with tcpcrypt

   A "tcpcrypt AEAD parameter" registry needs to be maintained by IANA
   as in the following table.  The use of encryption is described in
   Section 3.6.

           | AEAD Algorithm         | Key Length | sym-cipher |
           | AEAD_AES_128_GCM       | 16 bytes   |       0x01 |
           | AEAD_AES_256_GCM       | 32 bytes   |       0x02 |
           | AEAD_CHACHA20_POLY1305 | 32 bytes   |       0x10 |

    Table 2: Authenticated-encryption algorithms corresponding to sym-
              cipher specifiers in Init1 and Init2 messages.

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8.  Security considerations

   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.  Many of tcpcrypt's
   cryptographic functions require random input, and thus any host
   implementing tcpcrypt MUST have access to a cryptographically-secure
   source of randomness or pseudo-randomness.

   Most implementations will rely on system-wide pseudo-random
   generators seeded from hardware events and a seed carried over from
   the previous boot.  Once a pseudo-random generator has been properly
   seeded, it can generate effectively arbitrary amounts of pseudo-
   random data.  However, until a pseudo-random generator has been
   seeded with sufficient entropy, not only will tcpcrypt be insecure,
   it will reveal information that further weakens the security of the
   pseudo-random generator, potentially harming other applications.  As
   required by TCP-ENO, implementations MUST NOT send ENO options unless
   they have access to an adequate source of randomness.

   The cipher-suites specified in this document all use HMAC-SHA256 to
   implement the collision-resistant pseudo-random function denoted by
   "CPRF".  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

   All of the security considerations of TCP-ENO apply to tcpcrypt.  In
   particular, tcpcrypt does not protect against active eavesdroppers
   unless applications authenticate the session ID.  If it can be
   established that the session IDs computed at each end of the
   connection match, then tcpcrypt guarantees that no man-in-the-middle
   attacks occurred unless the attacker has broken the underlying
   cryptographic primitives (e.g., ECDH).  A proof of this property for
   an earlier version of the protocol has been published [tcpcrypt].

   To gain middlebox compatibility, tcpcrypt does not protect TCP
   headers.  Hence, the protocol is vulnerable to denial-of-service from
   off-path attackers just as plain TCP is.  Possible attacks include
   desynchronizing the underlying TCP stream, injecting RST or FIN
   segments, and forging rekey bits.  These attacks will cause a
   tcpcrypt connection to hang or fail with an error, but not in any
   circumstance where plain TCP could continue uncorrupted.
   Implementations MUST give higher-level software a way to distinguish

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   such errors from a clean end-of-stream (indicated by an authenticated
   "FINp" bit) so that applications can avoid semantic truncation

   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 failed integrity checks, 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.

   Tcpcrypt uses short-lived public keys to provide forward secrecy.
   All currently specified key agreement schemes involve ECDHE-based key
   agreement, meaning a new key can be efficiently computed for each
   connection.  If implementations reuse these parameters, they SHOULD
   limit the lifetime of the private parameters, ideally to no more than
   two minutes.

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

9.  Design notes

9.1.  Asymmetric roles

   Tcpcrypt transforms a shared 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).

9.2.  Verified liveness

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

   Unfortunately, tcpcrypt cannot cryptographically verify Keep-Alive
   acknowledgments.  Hence, an attacker could prolong the existence of a
   session at one host after the other end of the connection no longer

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   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.)

   Thus, tcpcrypt specifies a way to stimulate the remote host to send
   verifiably fresh and authentic data, described in Section 3.9.

   The TCP keep-alive mechanism has also been used for its effects on
   intermediate nodes in the network, such as preventing flow state from
   expiring at NAT boxes or firewalls.  As these purposes do not require
   the authentication of endpoints, implementations may safely
   accomplish them using either the existing TCP keep-alive mechanism or
   tcpcrypt's verified keep-alive mechanism.

10.  Acknowledgments

   We are grateful for contributions, help, discussions, and feedback
   from the TCPINC working group, including Marcelo Bagnulo, David
   Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav
   Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose.

   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); by DARPA CRASH under contract
   #N66001-10-2-4088; and by the Stanford Secure Internet of Things

11.  References

11.1.  Normative References

              Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
              D., and E. Smith, "TCP-ENO: Encryption Negotiation
              Option", draft-ietf-tcpinc-tcpeno-07 (work in progress),
              January 2017.

              "IEEE Standard Specifications for Public-Key Cryptography
              (IEEE Std 1363-2000)", 2000.

              "Digital Signature Standard, FIPS 186-2", 2000.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <http://www.rfc-editor.org/info/rfc7748>.

11.2.  Informative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

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

Appendix A.  Protocol constant values

                    | Value      | Name              |
                    | 0x01       | CONST_NEXTK       |
                    | 0x02       | CONST_SESSID      |
                    | 0x03       | CONST_REKEY       |
                    | 0x04       | CONST_KEY_A       |
                    | 0x05       | CONST_KEY_B       |
                    | 0x06       | CONST_RESUME      |
                    | 0x15101a0e | INIT1_MAGIC       |
                    | 0x097105e0 | INIT2_MAGIC       |
                    | 0x44415441 | FRAME_NONCE_MAGIC |

                        Table 3: Protocol constants

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Authors' Addresses

   Andrea Bittau
   345 Spear Street
   San Francisco, CA  94105

   Email: bittau@google.com

   Dan Boneh
   Stanford University
   353 Serra Mall, Room 475
   Stanford, CA  94305

   Email: dabo@cs.stanford.edu

   Daniel B. Giffin
   Stanford University
   353 Serra Mall, Room 288
   Stanford, CA  94305

   Email: dbg@scs.stanford.edu

   Mike Hamburg
   Stanford University
   353 Serra Mall, Room 475
   Stanford, CA  94305

   Email: mike@shiftleft.org

   Mark Handley
   University College London
   Gower St.
   London  WC1E 6BT

   Email: M.Handley@cs.ucl.ac.uk

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   David Mazieres
   Stanford University
   353 Serra Mall, Room 290
   Stanford, CA  94305

   Email: dm@uun.org

   Quinn Slack
   Stanford University
   353 Serra Mall, Room 288
   Stanford, CA  94305

   Email: sqs@cs.stanford.edu

   Eric W. Smith
   Kestrel Institute
   3260 Hillview Avenue
   Palo Alto, CA  94304

   Email: eric.smith@kestrel.edu

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