[Search] [pdf|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04                                 Standards Track
Network Working Group                                          A. Bittau
Internet-Draft                                                  D. Boneh
Intended status: Standards Track                               D. Giffin
Expires: April 19, 2016                                       M. Hamburg
                                                     Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                                Q. Slack
                                                     Stanford University
                                                                E. Smith
                                                       Kestrel Institute
                                                        October 17, 2015

           Cryptographic protection of TCP Streams (tcpcrypt)


   This document specifies tcpcrypt, a cryptographic protocol that
   protects TCP payload data and is negotiated by means of 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 of option
   size restrictions, the protocol requires one additional one-way
   message latency to perform key exchange.  However, this cost is
   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."

Bittau, et al.           Expires April 19, 2016                 [Page 1]

Internet-Draft                  tcpcrypt                    October 2015

   This Internet-Draft will expire on April 19, 2016.

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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 . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Encryption protocol . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Cryptographic algorithms  . . . . . . . . . . . . . . . .   4
     3.2.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Protocol negotiation  . . . . . . . . . . . . . . . . . .   5
     3.4.  Key exchange  . . . . . . . . . . . . . . . . . . . . . .   6
     3.5.  Session caching . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  Data encryption and authentication  . . . . . . . . . . .  10
     3.7.  TCP header protection . . . . . . . . . . . . . . . . . .  11
     3.8.  Re-keying . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.9.  Keep-alive  . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Encodings . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Key exchange messages . . . . . . . . . . . . . . . . . .  13
     4.2.  Application frames  . . . . . . . . . . . . . . . . . . .  15
       4.2.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.2.  Associated data . . . . . . . . . . . . . . . . . . .  17

Bittau, et al.           Expires April 19, 2016                 [Page 2]

Internet-Draft                  tcpcrypt                    October 2015

       4.2.3.  Frame nonce . . . . . . . . . . . . . . . . . . . . .  17
   5.  API extensions  . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Key agreement schemes . . . . . . . . . . . . . . . . . . . .  18
   7.  AEAD algorithms . . . . . . . . . . . . . . . . . . . . . . .  20
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   10. Security considerations . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Protocol constant values . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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  Avoid unnecessary round trips.

   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 TCP connections even when either end changes
      IP address.

3.  Encryption protocol

   This section describes the tcpcrypt protocol at an abstract level, so
   as to provide an overview and facilitate analysis.  The next section
   specifies the byte formats of all messages.

Bittau, et al.           Expires April 19, 2016                 [Page 3]

Internet-Draft                  tcpcrypt                    October 2015

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.  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 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 with CONST is a single octet.

   Once tcpcrypt has been successfully set up, we say the connection
   moves to an ENCRYPTING phase, where it employs an _authenticated

Bittau, et al.           Expires April 19, 2016                 [Page 4]

Internet-Draft                  tcpcrypt                    October 2015

   encryption mode_ to encrypt and integrity-protect all application

   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

3.2.  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).

   To establish roles for the hosts, tcpcrypt depends on TCP-ENO
   [I-D.ietf-tcpinc-tcpeno].  As part 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, though an
   additional mechanism breaks the symmetry of simultaneous open.  This
   document adopts the terms "A" and "B" to identify each end of a
   connection uniquely, following TCP-ENO's designation.

3.3.  Protocol negotiation

   Tcpcrypt also depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to
   negotiate the use of tcpcrypt and a particular key agreement scheme.
   TCP-ENO negotiates an _encryption spec_ by means of suboptions
   embedded in SYN segments.  Each suboption is identified by a byte
   consisting of a seven-bit _encryption spec identifier_ value, "cs",
   and a one-bit additional data indicator, "v".  This document reserves
   and associates four "cs" values with tcpcrypt, as listed in Table 1;
   future standards can associate additional values with tcpcrypt.

   A TCP connection MUST employ tcpcrypt and transition to the
   ENCRYPTING phase when and only when:

   1.  The TCP-ENO negotiated spec contains a "cs" value associated with
       tcpcrypt, and

   2.  The presence of variable-length data matches the suboption usage.

Bittau, et al.           Expires April 19, 2016                 [Page 5]

Internet-Draft                  tcpcrypt                    October 2015

   Specifically, when the "cs" value is "TCPCRYPT_RESUME", whose use is
   described in Section 3.5, there MUST be associated data (i.e., "v"
   MUST be 1).  For all other "cs" values specified in this document,
   there MUST NOT be additional suboption data (i.e., "v" MUST be 0).
   Future "cs" values associated with tcpcrypt might or might not
   specify the use of associated data.  Tcpcrypt implementations MUST
   ignore suboptions whose "cs" and "v" values do not agree as specified
   in this paragraph.

   In normal usage, an active opener that wishes to negotiate the use of
   tcpcrypt will include an ENO option in its SYN segment; that option
   will include the tcpcrypt suboptions corresponding to the key-
   agreement schemes it is willing to enable, and possibly also a
   resumption suboption.  The active opener MAY additionally include
   suboptions indicating support for encryption protocols other than
   tcpcrypt, as well as other general options as specified by TCP-ENO.

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

   Once two hosts have exchanged SYN segments, the _negotiated spec_ is
   the last spec 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 spec, hosts MUST disable TCP-
   ENO and tcpcrypt.

3.4.  Key exchange

   Following successful negotiation of a tcpcrypt spec, all further
   signaling is performed in the Data portion of TCP segments.  If the
   negotiated spec is not TCPCRYPT_RESUME, the two hosts perform key
   exchange through two messages, INIT1 and INIT2, at the start of host
   A's and host B's data streams, respectively.  INIT1 or INIT2 can 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 set.

   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 format of these messages is specified in detail in Section 4.1.

   The parameters are defined as follows:

Bittau, et al.           Expires April 19, 2016                 [Page 6]

Internet-Draft                  tcpcrypt                    October 2015

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

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

   o  N_A, N_B: nonces chosen at random by A and B, respectively.

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

   The pre-master secret (PMS) is defined to be the result of the key-
   agreement algorithm whose inputs are the local host's ephemeral
   private key and the remote host's ephemeral public key.  For example,
   host A would compute PMS 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:

                param  := { eno-transcript, init1, init2 }
                PRK    := Extract (N_A, { param, PMS })

   Above, "eno-transcript" is the protocol-negotiation transcript
   defined in TCP-ENO; "init1" and "init2" are the transmitted encodings
   of the INIT1 and INIT2 messages described in Section 4.1.

   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 3.5.  The CONST values are constants defined in Table 3.  The
   K_LEN values depend on the tcpcrypt spec in use, and are specified in
   Figure 3.

Bittau, et al.           Expires April 19, 2016                 [Page 7]

Internet-Draft                  tcpcrypt                    October 2015

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

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

   Finally, each master key mk is used to generate keys for
   authenticated encryption for the "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 ae_keylen value depends on the authenticated-encryption algorithm
   selected, and is given under "Key Length" in Table 2.

   HKDF is not used directly for key derivation because tcpcrypt
   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
   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.

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 wishing to request this facility will include in its
   SYN segment an ENO option whose last suboption contains the spec
   identifier TCPCRYPT_RESUME:

                byte     0        1                  8
                     | Opt =  |       SID[i]{0..7}        |
                     | resume |                           |

        Figure 2: ENO suboption used to initiate session resumption

Bittau, et al.           Expires April 19, 2016                 [Page 8]

Internet-Draft                  tcpcrypt                    October 2015

   Above, the "resume" value is the byte whose lower 7 bits are
   TCPCRYPT_RESUME and whose top bit "v" is 1 (indicating variable-
   length data follows).  The remainder of the suboption is filled with
   the first eight bytes of the Session ID SID[i].

   A host SHOULD also include ENO suboptions describing the key-
   agreement schemes it supports in addition to a resume suboption, so
   as to fall back to full key exchange in the event that session
   resumption fails.

   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 the role it plays in the current session.  For example, if
   a host had the "A" role in the first session, then it uses k_ab for
   sending segments and k_ba for receiving.

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

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

   The active opener MUST use the lowest value of "i" that has not
   already appeared in a resumption suboption 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 an ENO option containing a dataless resumption
   suboption; that is, the suboption whose "cs" value is TCPCRYPT_RESUME
   and whose "v" bit is zero.

   If the passive opener does not recognize SID[i], or SID[i] is not
   valid or has already been used, the passive opener SHOULD inspect any
   other ENO suboptions in hopes of negotiating a fresh key exchange as
   described in Section 3.4.

   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 seed, or k_ba.  Thus, which keys a
   host uses to send segments depends only whether the host played the

Bittau, et al.           Expires April 19, 2016                 [Page 9]

Internet-Draft                  tcpcrypt                    October 2015

   "A" or "B" 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 resumption suboption.

   A host MUST ignore a resumption suboption 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.

   In the case of simultaneous open where TCP-ENO is able to establish
   asymmetric roles, two hosts that simultaneously send SYN segments
   with resumption suboptions containing the same SID[i] may resume the
   associated session.

   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, 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 3.4.

   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 frame's
   "plaintext" value, which is then encrypted to yield the frame's
   "ciphertext" field.  Chunks must be small enough that the ciphertext
   (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), containing an "offset" field
   whose integer value is the byte-offset of the beginning of the

Bittau, et al.           Expires April 19, 2016                [Page 10]

Internet-Draft                  tcpcrypt                    October 2015

   current application frame in the underlying TCP datastream.  (That
   is, the offset in the framing stream, not the plaintext application
   stream.)  As the security of the AEAD algorithm depends on this nonce
   being used to encrypt at most one distinct plaintext value, an
   implementation MUST NOT ever transmit distinct frames at the same
   location in the underlying TCP datastream.

   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 3.4.  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 "P", a plaintext value, or the
   special symbol FAIL.  In the latter case, the implementation MAY
   either ignore the frame or terminate the connection.

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.  A receiver MUST ignore the TCP
   FIN flag and instead wait for "FINp" to signal to the local
   application that the stream is complete.

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
   _current generation number_ that it uses to encrypt outgoing frames.
   Initially, the two hosts have current generation number 0.

Bittau, et al.           Expires April 19, 2016                [Page 11]

Internet-Draft                  tcpcrypt                    October 2015

   When a host has just incremented its current generation number and
   has used the new key-set for the first time to encrypt an outgoing
   frame, it MUST set the frame's "rekey" field (see Section 4.2) to 1.
   It MUST set this field to zero in all other cases.

   A host MAY increment its generation number beyond the highest
   generation it knows the other side to be using.  We call this action
   _initiating re-keying_.

   A host SHOULD NOT initiate more than one concurrent re-key operation
   if it has no data to send.

   On receipt, a host increments its record of the remote host's current
   generation number if and only if the "rekey" field is set to 1.

   If a received frame's generation number is greater than the
   receiver's current generation number, the receiver MUST immediately
   increment its current generation number to match.  After incrementing
   its generation number, if the receiver does not have any application
   data to send, it MUST send an empty application frame with the
   "rekey" field set to 1.

   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

   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
   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.)

Bittau, et al.           Expires April 19, 2016                [Page 12]

Internet-Draft                  tcpcrypt                    October 2015

   Instead of TCP Keep-Alives, tcpcrypt implementations SHOULD employ
   the re-keying mechanism to stimulate the remote host to send
   verifiably fresh and authentic data.  When required, a host SHOULD
   probe the liveness of its peer by initiating re-keying as described
   in Section 3.8, and then transmitting a new frame (with zero-length
   application data if necessary).  A host receiving a frame whose key
   generation number is greater than its current generation number MUST
   increment its current generation number and MUST immediately transmit
   a new frame (with zero-length application data, if necessary).

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:

Bittau, et al.           Expires April 19, 2016                [Page 13]

Internet-Draft                  tcpcrypt                    October 2015

       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
   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 shown in Figure 3.

   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 PK_A field.  When receiving INIT1, however, implementations MUST
   permit and ignore any bytes following PK_A.

   The INIT2 message has the following encoding:

Bittau, et al.           Expires April 19, 2016                [Page 14]

Internet-Draft                  tcpcrypt                    October 2015

       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 shown in
   Figure 3.

   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:

Bittau, et al.           Expires April 19, 2016                [Page 15]

Internet-Draft                  tcpcrypt                    October 2015

          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           |

   (Note that "clen" will generally be greater than "plen", as the
   authenticated-encryption scheme attaches an integrity "tag" to the
   encrypted input.)

   The "flags" byte has this structure:

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

Bittau, et al.           Expires April 19, 2016                [Page 16]

Internet-Draft                  tcpcrypt                    October 2015

   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:

                      0 | 0x44 | 0x41 | 0x54 | 0x41 |
                      4 |                           |
                        +           offset          +
                      8 |                           |

   The 8-byte "offset" field contains an integer in big-endian format.
   Its value is specified in Section 3.6.

5.  API extensions

   Applications aware of tcpcrypt will need an API for interacting with
   the protocol.  They can do so if implementations provide the
   recommended API for TCP-ENO.  This section recommends several
   additions to that API, described in the style of socket options.
   However, these recommendations are non-normative:

Bittau, et al.           Expires April 19, 2016                [Page 17]

Internet-Draft                  tcpcrypt                    October 2015

   The following options is read-only:

   TCP_CRYPT_CONF:  Returns the one-byte authenticated encryption
      algorithm in use by the connection (as specified in Table 2).

   The following option is write-only:

   TCP_CRYPT_CACHE_FLUSH:  Setting this option to non-zero wipes cached
      session keys as specified in Section 3.5.  Useful if application-
      level authentication discovers a man in the middle attack, to
      prevent the next connection from using session caching.

   The following options should be readable and writable:

   TCP_CRYPT_ACONF:  Set of allowed symmetric ciphers and message
      authentication codes this host advertises in INIT1 messages.

   TCP_CRYPT_BCONF:  Order of preference of symmetric ciphers.

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

   o  Default TCP_CRYPT_ACONF value

   o  Default TCP_CRYPT_BCONF value

   o  Types, key lengths, and regeneration intervals of local host's
      short-lived public keys for implementations that do not use fresh
      ECDH parameters for each connection.

6.  Key agreement schemes

   The encryption spec negotiated via TCP-ENO may indicate the use of
   one of these key-agreement schemes:

Bittau, et al.           Expires April 19, 2016                [Page 18]

Internet-Draft                  tcpcrypt                    October 2015

     |   Encryption spec (cs)    |     Key-agreement scheme         |
     | TCPCRYPT_ECDHE_P256       | Cipher: ECDHE-P256               |
     |                           | Extract: HKDF-Extract-SHA256     |
     |                           | CPRF: HKDF-Expand-SHA256         |
     |                           | N_A_LEN: 32 bytes                |
     |                           | N_B_LEN: 32 bytes                |
     |                           | K_LEN: 32 bytes                  |
     | TCPCRYPT_ECDHE_P521       | Cipher: ECDHE-P521               |
     |                           | Extract: HKDF-Extract-SHA256     |
     |                           | CPRF: HKDF-Expand-SHA256         |
     |                           | N_A_LEN: 32 bytes                |
     |                           | N_B_LEN: 32 bytes                |
     |                           | K_LEN: 32 bytes                  |
     | TCPCRYPT_ECDHE_Curve25519 | Cipher: ECDHE-Curve25519         |
     |                           | Extract: HKDF-Extract-SHA256     |
     |                           | CPRF: HKDF-Expand-SHA256         |
     |                           | N_A_LEN: 32 bytes                |
     |                           | N_B_LEN: 32 bytes                |
     |                           | K_LEN: 32 bytes                  |

                      Figure 3: Key agreement schemes

   Ciphers 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.

   The ECDHE-Curve25519 cipher uses the X25519 function described in
   [I-D.irtf-cfrg-curves].  When using this cipher, public-key values

Bittau, et al.           Expires April 19, 2016                [Page 19]

Internet-Draft                  tcpcrypt                    October 2015

   PK_A and PK_B are transmitted directly as 32-byte values (with no
   length prefix).

   A tcpcrypt implementation MUST support at least the schemes
   TCPCRYPT_ECDHE_P256 and TCPCRYPT_ECDHE_P521, although system
   administrators need not enable them.

7.  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

8.  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

9.  IANA Considerations

   Tcpcrypt's spec identifiers ("cs" values) will need to be added to
   IANA's ENO suboption registry, as follows:

   |  cs  | Spec name                 | Meaning                        |
   | 0x20 | TCPCRYPT_RESUME           | tcpcrypt session resumption    |
   | 0x21 | TCPCRYPT_ECDHE_P256       | tcpcrypt with ECDHE-P256       |
   | 0x22 | TCPCRYPT_ECDHE_P521       | tcpcrypt with ECDHE-P521       |
   | 0x23 | TCPCRYPT_ECDHE_Curve25519 | tcpcrypt with ECDHE-Curve25519 |

                 Table 1: cs values for use with tcpcrypt

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

Bittau, et al.           Expires April 19, 2016                [Page 20]

Internet-Draft                  tcpcrypt                    October 2015

           | 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.

10.  Security considerations

   It is worth reiterating just how crucial both the quality and
   quantity of randomness are to tcpcrypt's security.  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.  In the
   absence of secure hardware random generators, implementations MUST
   disable tcpcrypt after rebooting until the pseudo-random generator
   has been reseeded (usually by a bootup script) or sufficient entropy
   has been gathered.

   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., ECDH).  A
   proof has been published [tcpcrypt].

   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.

   To gain middlebox compatibility, tcpcrypt does not protect TCP
   headers.  Hence, the protocol is vulnerable to denial-of-service from
   off-path attackers.  Possible attacks include desynchronizing the
   underlying TCP stream, injecting RST packets, and forging or
   suppressing rekey bits.  These attacks will cause a tcpcrypt
   connection to hang or fail with an error.  Implementations MUST give
   higher-level software a way to distinguish such errors from a clean
   end-of-stream (indicated by an authenticated "FINp" bit) so that
   applications can avoid semantic truncation attacks.

Bittau, et al.           Expires April 19, 2016                [Page 21]

Internet-Draft                  tcpcrypt                    October 2015

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

   Tcpcrypt uses short-lived public key parameters to provide forward
   secrecy.  All currently specified key agreement schemes involve
   ECDHE-based key agreement, meaning a new key can be chosen 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
   suboption, which will be hard without key knowledge.

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-00 (work in progress),
              September 2015.

              Langley, A. and M. Hamburg, "Elliptic Curves for
              Security", draft-irtf-cfrg-curves-10 (work in progress),
              October 2015.

              "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,

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

Bittau, et al.           Expires April 19, 2016                [Page 22]

Internet-Draft                  tcpcrypt                    October 2015

   [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,

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  |
                       | 0x15101a0e | INIT1_MAGIC  |
                       | 0x097105e0 | INIT2_MAGIC  |

                        Table 3: Protocol constants

Authors' Addresses

   Andrea Bittau
   Stanford University
   353 Serra Mall, Room 288
   Stanford, CA  94305

   Email: bittau@cs.stanford.edu

Bittau, et al.           Expires April 19, 2016                [Page 23]

Internet-Draft                  tcpcrypt                    October 2015

   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

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

   Email: dm@uun.org

Bittau, et al.           Expires April 19, 2016                [Page 24]

Internet-Draft                  tcpcrypt                    October 2015

   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

Bittau, et al.           Expires April 19, 2016                [Page 25]