Cryptographic protection of TCP Streams (tcpcrypt)
draft-ietf-tcpinc-tcpcrypt-11

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Last updated 2017-11-30 (latest revision 2017-11-29)
Replaces draft-bittau-tcpinc-tcpcrypt
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Network Working Group                                          A. Bittau
Internet-Draft                                                    Google
Intended status: Experimental                                  D. Giffin
Expires: June 3, 2018                                Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                     Stanford University
                                                                Q. Slack
                                                             Sourcegraph
                                                                E. Smith
                                                       Kestrel Institute
                                                       November 30, 2017

           Cryptographic protection of TCP Streams (tcpcrypt)
                     draft-ietf-tcpinc-tcpcrypt-11

Abstract

   This document specifies tcpcrypt, a TCP encryption protocol designed
   for use in conjunction with the TCP Encryption Negotiation Option
   (TCP-ENO).  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 June 3, 2018.

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
   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.

Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Encryption Protocol . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Cryptographic Algorithms  . . . . . . . . . . . . . . . .   3
     3.2.  Protocol Negotiation  . . . . . . . . . . . . . . . . . .   5
     3.3.  Key Exchange  . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Session ID  . . . . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Session Resumption  . . . . . . . . . . . . . . . . . . .   9
     3.6.  Data Encryption and Authentication  . . . . . . . . . . .  12
     3.7.  TCP Header Protection . . . . . . . . . . . . . . . . . .  14
     3.8.  Re-Keying . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.9.  Keep-Alive  . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  Encodings . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  Key-Exchange Messages . . . . . . . . . . . . . . . . . .  16
     4.2.  Encryption Frames . . . . . . . . . . . . . . . . . . . .  18
       4.2.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . .  19
       4.2.2.  Associated Data . . . . . . . . . . . . . . . . . . .  20
       4.2.3.  Frame ID  . . . . . . . . . . . . . . . . . . . . . .  20
     4.3.  Constant Values . . . . . . . . . . . . . . . . . . . . .  20
   5.  Key-Agreement Schemes . . . . . . . . . . . . . . . . . . . .  21
   6.  AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . .  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
     8.1.  Asymmetric Roles  . . . . . . . . . . . . . . . . . . . .  25
     8.2.  Verified Liveness . . . . . . . . . . . . . . . . . . . .  26
     8.3.  Mandatory Key-Agreement Schemes . . . . . . . . . . . . .  26
   9.  Experiments . . . . . . . . . . . . . . . . . . . . . . . . .  27
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  28
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  28

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   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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
      stacks.

   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.

   A companion document [I-D.ietf-tcpinc-api] describes recommended
   interfaces for configuring certain parameters of this protocol.

3.  Encryption Protocol

   This section describes the operation of the tcpcrypt protocol.  The
   wire format of all messages is specified in Section 4.

3.1.  Cryptographic Algorithms

   Setting up a tcpcrypt connection employs three types of cryptographic
   algorithms:

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   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
      (PRK) 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 IKM.

   o  A _collision-resistant pseudo-random function (CPRF)_ is used to
      generate multiple cryptographic keys from a pseudo-random key,
      typically the output of the extract function.  The CPRF produces
      an arbitrary amount of Output Keying Material (OKM), and we use
      the notation CPRF(K, CONST, L) to designate the first L bytes of
      the OKM produced by the CPRF when parameterized by key K and the
      constant CONST.

   The Extract and CPRF functions used by the tcpcrypt variants defined
   in this document are the Extract and Expand functions of HKDF
   [RFC5869], which is built on HMAC [RFC2104].  These are defined as
   follows in terms of the function "HMAC-Hash(key, value)" for a
   negotiated "Hash" function such as SHA-256; the symbol "|" denotes
   concatenation, and the counter concatenated to the right of CONST
   occupies a single octet.

           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) | ...
              where L < 255*OutputLength(Hash)

             Figure 1: HKDF functions used for key derivation

   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 4 in
   Section 7.  Each identifier indicates the use of a particular key-
   agreement scheme, with an associated CPRF and length parameters.
   Future standards may associate additional TEP identifiers with
   tcpcrypt, following the assignment policy specified by TCP-ENO.

   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 a
   distinct role to each host.  TCP-ENO uses the terms "host A" and
   "host B" to identify each end of a connection uniquely, and this
   document employs those terms in the same way.

   An ENO suboption includes 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 derived from
   a session secret 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

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   simultaneous open) that also occurs in that of host A.  If there is
   no such TEP, hosts MUST disable TCP-ENO and tcpcrypt.

   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-resumption 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 4,
   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 MUST have TCP's push flag
   (PSH) set.

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

       A -> B:  Init1 = { INIT1_MAGIC, sym_cipher_list, N_A, Pub_A }
       B -> A:  Init2 = { INIT2_MAGIC, sym_cipher, N_B, Pub_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 Section 4.3.

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   o  "sym_cipher_list": a list of identifiers of symmetric ciphers
      (AEAD algorithms) acceptable to host A.  These are specified in
      Table 5 in Section 7.

   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,
      respectively.

   o  "Pub_A", "Pub_B": ephemeral public keys for hosts A and B,
      respectively.  These, as well as their corresponding private keys,
      are short-lived values that MUST be refreshed as frequently as
      practically possible.  The private keys SHOULD NOT ever be written
      to persistent storage.  The security risks associated with the
      storage of these keys are discussed in Section 8.

   If a host receives an ephemeral public key from its peer and a
   required key-validation step fails (see Section 5), it MUST abort the
   connection and raise an error condition distinct from the end-of-file
   condition.

   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, "Pub_B".

   The two sides then compute a pseudo-random key "PRK", from which all
   session secrets 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 Section 4.8 of
   [I-D.ietf-tcpinc-tcpeno]; 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
   follows:

                 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

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   connections between the same two hosts, as described in Section 3.5.
   The "CONST_*" values are constants defined in Section 4.3.  The
   length "K_LEN" depends on the tcpcrypt TEP in use, and is specified
   in Section 5.

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

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

   The process of advancing through the series of master keys is
   described in Section 3.8.

   Finally, each master key "mk[j]" is used to generate traffic keys for
   protecting application data using authenticated encryption:

       k_ab[j] = CPRF(mk[j], CONST_KEY_A, ae_key_len + ae_nonce_len)
       k_ba[j] = CPRF(mk[j], CONST_KEY_B, ae_key_len + ae_nonce_len)

   In the first session derived from fresh key-agreement, traffic keys
   "k_ab[j]" are used by host A to encrypt and host B to decrypt, while
   keys "k_ba[j]" are used by host B to encrypt and host A to decrypt.
   In a resumed session, as described more thoroughly below in
   Section 3.5, each host uses the keys in the same way as it did in the
   original session, regardless of its role in the current session: for
   example, if a host played role "A" in the first session, it will use
   keys "k_ab[j]" to encrypt in each derived session.

   The values "ae_key_len" and "ae_nonce_len" depend on the
   authenticated-encryption algorithm selected, and are given in Table 3
   in Section 6.  The algorithm uses the first "ae_key_len" bytes of
   each traffic key as an authenticated-encryption key, and the
   following "ae_nonce_len" bytes as a "nonce randomizer".

   Implementations SHOULD provide an interface allowing the user to
   specify, for a particular connection, the set of AEAD algorithms to
   advertize in "sym_cipher_list" (when playing role "A") and also the
   order of preference to use when selecting an algorithm from those
   offered (when playing role "B").  A companion document
   [I-D.ietf-tcpinc-api] describes recommended interfaces for this
   purpose.

   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.

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   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.

   Throughout this document, to "abort the connection" means to issue
   the "Abort" command as described in [RFC0793], Section 3.8.  That is,
   the TCP connection is destroyed, RESET is transmitted, and the local
   user is alerted to the abort event.

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 byte transmitted
   by host B that contains the negotiated TEP identifier along with the
   "v" bit.  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 Resumption

   If two hosts have previously negotiated a session with secret
   "ss[i-1]", they can establish a new connection without public-key
   operations using "ss[i]", the next session secret in the sequence
   derived from the original PRK.

   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 whose value is the negotiated TEP identifier of the
   previous session concatenated with half of the "resumption
   identifier" for the new session.

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

                 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]".

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   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 receives a resumption suboption containing an
   identifier-half that names a session secret that it has cached and
   the subobtion's TEP matches the TEP used in the previous session, 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.  Otherwise, it MUST NOT
   agree to resumption.

   If a passive opener does not agree to resumption with a particular
   TEP, it 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 a passive opener receives an ENO suboption with a TEP identifier
   and "v = 1", but the suboption data does not have length 9 bytes, it
   MUST behave as if the same TEP had been sent with "v = 0".  That is,
   the suboption MUST be interpreted as an offer to negotiate fresh key
   exchange with that TEP.

   If an active opener sends a resumption suboption with a particular
   TEP and the appropriate half of a resumption identifier and then, in
   the same TCP handshake, receives a resumption suboption with the same
   TEP and an identifier-half that does not match that resumption

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   identifier, it MUST ignore that suboption.  In the typical case that
   this was the only ENO suboption received, this means the host 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
   session.

   In a particular SYN segment, a host SHOULD NOT send more than one
   resumption suboption (because this consumes TCP option space and is
   unlikely to be a useful practice), 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 TEPs it supports (in addition
   to the TEP in the resumption suboption).

   After using the session secret "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 any session secrets 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 original
   session secret "ss[0]".

   A session secret may not be used to secure more than one TCP
   connection.  To prevent this, a host MUST NOT resume with a session
   secret if it has ever enabled encryption in the past with the same
   secret, in either role.  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

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   session resumption such that all session secrets derived from a given
   "ss[0]" are used for either passive or active opens at the same host,
   not both.

   If two hosts have previously negotiated a tcpcrypt session, either
   host may later 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 keys "k_ab[j]" for all
   sessions derived from the same original session secret "ss[0]", or
   with keys "k_ba[j]".  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 session.

   Implementations that cache session secrets MUST provide a means for
   applications to control that caching.  In particular, when an
   application requests a new TCP connection, it must be able to specify
   two policies for the duration of the connection: 1) that resumption
   requests will be ignored, and thus fresh key exchange will be
   required; and 2) that no session secrets will be cached.  (These
   policies may be specified independently or as a unit.)  And for an
   established connection, an application must be able to cause any
   cache state that was used in or resulted from establishing the
   connection to be flushed.  A companion document [I-D.ietf-tcpinc-api]
   describes recommended interfaces for this purpose.

3.6.  Data Encryption and Authentication

   Following key exchange (or its omission via session resumption), all
   further communication in a tcpcrypt-enabled connection is carried out
   within delimited _encryption 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 5 in Section 7.  Additional
   algorithms may be specified according to the policy in that section.
   One algorithm is selected during the negotiation described in
   Section 3.3.  The lengths "ae_key_len" and "ae_nonce_len" associated
   with each algorithm are found in Table 3 in Section 6, together with
   requirements for which algorithms must be implemented.

   The format of an encryption 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

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   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
   ciphertext.

   A "frame ID" value (see Section 4.2.3) is also constructed for the
   frame, but not explicitly transmitted.  It contains a 64-bit "offset"
   field whose integer value is the zero-indexed byte offset of the
   beginning of the current encryption frame in the underlying TCP
   datastream.  (That is, the offset in the framing stream, not the
   plaintext application stream.)  The offset is then left-padded with
   zero-valued bytes to form a value of length "ae_nonce_len".  Because
   it is strictly necessary for the security of the AEAD algorithms
   specified in this document, an implementation MUST NOT ever transmit
   distinct frames with the same frame ID value under the same
   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-
   keying as decribed in Section 3.8.

   With reference to the "AEAD Interface" described in Section 2 of
   [RFC5116], tcpcrypt invokes the AEAD algorithm with values taken from
   the traffic key "k_ab[j]" or "k_ba[j]" for some "j", according to the
   host's role as described in Section 3.3.

   First, the traffic key is divided into two parts:

                                      ae_key_len + ae_nonce_len - 1
                                                       |
        byte  0                    ae_key_len          |
              |                           |            |
              v                           v            v
            +----+----+--...--+----+----+----+--...--+----+
            |             K             |        NR       |
            +----+----+--...--+----+----+----+--...--+----+

            \_____________________________________________/
                             traffic key

   The first "ae_key_len" bytes of the traffic key provide the AEAD key
   "K", while the remaining "ae_nonce_len" bytes provide a "nonce
   randomizer" value "NR".  The frame ID is then combined via bitwise
   exclusive-or with the nonce randomizer to yield "N", the AEAD nonce
   for the frame:

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                            N = frame_ID xor NR

   The plaintext value serves as "P", and the associated data as "A".
   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 ID values (the former contains only data sent in the clear,
   and the latter is implicit in the TCP stream), computes the nonce "N"
   as above, and provides these and the ciphertext value to 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 SHOULD abort the connection and raise an error
   condition distinct from the end-of-file condition.  But if none of
   the TCP segment(s) containing the frame have been acknowledged and
   retransmission could potentially result in a valid frame, an
   implementation MAY instead drop these segments (and "renege" if they
   have been SACKed, according to [RFC2018] Section 8).

3.7.  TCP Header Protection

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

   When the "URGp" bit 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.

   A sender MUST set the "FINp" bit on the last frame it sends in the
   connection (unless it aborts the connection), and MUST NOT set "FINp"
   on any other frame.

   TCP sets the FIN flag when a sender has no more data, which with
   tcpcrypt means setting FIN on the segment containing the last byte of
   the last frame.  However, a receiver MUST report the end-of-file
   condition to the connection's local user when and only when it
   receives a frame with the "FINp" bit set.  If a host receives a
   segment with the TCP FIN flag set but the received datastream
   including this segment does not contain a frame with "FINp" set, the
   host SHOULD abort the connection and raise an error condition
   distinct from the end-of-file condition.  But if there are
   unacknowledged segments whose retransmission could potentially result
   in a valid frame, the host MAY instead drop the segment with the TCP
   FIN flag set (and "renege" if it has been SACKed, according to
   [RFC2018] Section 8).

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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.

   As described above in Section 3.3, a master key "mk[j]" is used to
   generate two encryption keys "k_ab[j]" and "k_ba[j]".  We refer to
   these as a _key-set_ with _generation number_ "j".  Each host
   maintains a _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 generation numbers 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
   re-keying_.

   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.
   Moreover, if the receiver has not yet transmitted a segment with the
   FIN flag set, it MUST immediately send a frame (with empty
   application data if necessary) with "rekey = 1".

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

   Note that when parts of the datastream are retransmitted, TCP
   requires that implementations always send the same data bytes for the
   same TCP sequence numbers.  Thus, frame data in retransmitted
   segments must be encrypted with the same key as when it was first
   transmitted, regardless of the current local generation number.

   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.

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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 (if it has not already transmitted a segment
   with FIN set) immediately transmit a new frame (with zero-length
   application data if necessary).

   Implementations MAY use TCP Keep-Alives for purposes that do not
   require endpoint authentication, as discussed in Section 8.2.

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:

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       byte   0       1       2       3
          +-------+-------+-------+-------+
          |          INIT1_MAGIC          |
          |                               |
          +-------+-------+-------+-------+

                  4        5      6       7
              +-------+-------+-------+-------+
              |          message_len          |
              |              = M              |
              +-------+-------+-------+-------+

                  8
              +--------+-----+----+-----+----+---...---+-----+-----+
              |nciphers|sym_      |sym_      |         |sym_       |
              | = K    |cipher[0] |cipher[1] |         |cipher[K-1]|
              +--------+-----+----+-----+----+---...---+-----+-----+

               2*K + 9                     2*K + 9 + N_A_LEN
                  |                         |
                  v                         v
              +-------+---...---+-------+-------+---...---+-------+
              |           N_A           |          Pub_A          |
              |                         |                         |
              +-------+---...---+-------+-------+---...---+-------+

                                  M - 1
              +-------+---...---+-------+
              |         ignored         |
              |                         |
              +-------+---...---+-------+

   The constant "INIT1_MAGIC" is defined in Section 4.3.  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 two-byte symmetric-cipher
   identifiers that follow.  The "sym_cipher[i]" identifiers indicate
   cryptographic algorithms in Table 5 in Section 7.  The length
   "N_A_LEN" and the length of "Pub_A" are both determined by the
   negotiated TEP, as described in Section 5.

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

   The "Init2" message has the following encoding:

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       byte   0       1       2       3
          +-------+-------+-------+-------+
          |          INIT2_MAGIC          |
          |                               |
          +-------+-------+-------+-------+

                  4        5      6       7       8       9
              +-------+-------+-------+-------+-------+-------+
              |          message_len          |  sym_cipher   |
              |              = M              |               |
              +-------+-------+-------+-------+-------+-------+

                  10                      10 + N_B_LEN
                  |                         |
                  v                         v
              +-------+---...---+-------+-------+---...---+-------+
              |           N_B           |          Pub_B          |
              |                         |                         |
              +-------+---...---+-------+-------+---...---+-------+

                                  M - 1
              +-------+---...---+-------+
              |          ignored        |
              |                         |
              +-------+---...---+-------+

   The constant "INIT2_MAGIC" is defined in Section 4.3.  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
   "Pub_B" are both determined by the negotiated TEP, as described in
   Section 5.

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

4.2.  Encryption Frames

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

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          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" 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|
                   +----+----+----+----+----+----+----+----+

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   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 encryption 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"
   fields.

4.2.3.  Frame ID

   Lastly, a "frame ID" (used to construct the nonce for the AEAD
   algorithm) has this format:

          byte  0            ae_nonce_len - 8    ae_nonce_len - 1
                |                   |             |
                v                   v             v
             +-----+--...--+-----+-----+--...--+-----+
             |  0  |       |  0  |       offset      |
             +-----+--...--+-----+-----+--...--+-----+

   The 8-byte "offset" field contains an integer in big-endian format.
   Its value is specified in Section 3.6.  Zero-valued bytes are
   prepended to the "offset" field to form a structure of length
   "ae_nonce_len".

4.3.  Constant Values

   The table below defines values for the constants used in the
   protocol.

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                       +------------+--------------+
                       | 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  |
                       +------------+--------------+

               Table 1: Constant values used in the protocol

5.  Key-Agreement Schemes

   The TEP negotiated via TCP-ENO indicates the use of one of the key-
   agreement schemes named in Table 4 in Section 7.  For example,
   "TCPCRYPT_ECDHE_P256" names the tcpcrypt protocol using ECDHE-P256
   together with the CPRF and length parameters specified below.

   All the TEPs specified in this document require the use of HKDF-
   Expand-SHA256 as the CPRF, and these lengths for nonces and session
   secrets:

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

   If future documents assign additional TEPs for use with tcpcrypt,
   they may specify different values for the lengths above.  Note that
   the minimum session ID length required by TCP-ENO, together with the
   way tcpcrypt constructs session IDs, implies that "K_LEN" must have
   length at least 32 bytes.

   Key-agreement schemes ECDHE-P256 and ECDHE-P521 employ the ECSVDP-DH
   secret value derivation primitive defined in [IEEE-1363].  The named
   curves are defined in [NIST-DSS].  When the public-key values "Pub_A"
   and "Pub_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 [IEEE-1363], and are
   prefixed by a two-byte length in big-endian format:

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

   Implementations MUST encode these "pubkey" values in "compressed
   format".  Implementations MUST validate these "pubkey" values
   according to the algorithm in [IEEE-1363] Section A.16.10.

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

   Implementations are required to implement certain TEPs, according to
   Table 2 below.  Note that system administrators may configure which
   TEPs a host will negotiate, independent of these requirements.

                +-------------+---------------------------+
                | Requirement | TEP                       |
                +-------------+---------------------------+
                | MUST        | TCPCRYPT_ECDHE_Curve25519 |
                | SHOULD      | TCPCRYPT_ECDHE_Curve448   |
                | MAY         | TCPCRYPT_ECDHE_P256       |
                | MAY         | TCPCRYPT_ECDHE_P521       |
                +-------------+---------------------------+

             Table 2: Requirements for implementation of TEPs

6.  AEAD Algorithms

   This document uses "sym-cipher" identifiers in the messages "Init1"
   and "Init2" (see Section 3.3) to negotiate the use of AEAD
   algorithms; the values of these identifiers are given in Table 5 in
   Section 7.  The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM"
   are specified in [RFC5116].  The algorithm "AEAD_CHACHA20_POLY1305"
   is specified in [RFC7539].

   Implementations are required to support certain AEAD algorithms
   according to Table 3 below.  Note that system administrators may
   configure which algorithms a host will negotiate, independent of
   these requirements.

   Lastly, this document uses the lengths "ae_key_len" and
   "ae_nonce_len" to specify aspects of encryption and data formats.

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   These values depend on the negotiated AEAD algorithm, also according
   to the table below.

   +------------------------+-------------+------------+--------------+
   | AEAD Algorithm         | Requirement | ae_key_len | ae_nonce_len |
   +------------------------+-------------+------------+--------------+
   | AEAD_AES_128_GCM       | MUST        | 16 bytes   | 12 bytes     |
   | AEAD_AES_256_GCM       | SHOULD      | 32 bytes   | 12 bytes     |
   | AEAD_CHACHA20_POLY1305 | SHOULD      | 32 bytes   | 12 bytes     |
   +------------------------+-------------+------------+--------------+

         Table 3: Requirement and lengths for each AEAD algorithm

7.  IANA Considerations

   For use with TCP-ENO's negotiation mechanism, tcpcrypt's TEP
   identifiers will need to be incorporated in IANA's "TCP encryption
   protocol identifiers" registry under the "Transmission Control
   Protocol (TCP) Parameters" registry, as in Table 4 below.  The
   various key-agreement schemes used by these tcpcrypt variants are
   defined in Section 5.

             +-------+---------------------------+-----------+
             | Value | Meaning                   | Reference |
             +-------+---------------------------+-----------+
             | 0x21  | TCPCRYPT_ECDHE_P256       | [RFC-TBD] |
             | 0x22  | TCPCRYPT_ECDHE_P521       | [RFC-TBD] |
             | 0x23  | TCPCRYPT_ECDHE_Curve25519 | [RFC-TBD] |
             | 0x24  | TCPCRYPT_ECDHE_Curve448   | [RFC-TBD] |
             +-------+---------------------------+-----------+

              Table 4: TEP identifiers for use with tcpcrypt

   In Section 6, this document defines the use of several AEAD
   algorithms for encrypting application data.  To name these
   algorithms, the tcpcrypt protocol uses two-byte identifiers in the
   range 0x0001 to 0xFFFF inclusive, for which IANA is to maintain a new
   "tcpcrypt AEAD Algorithm" registry under the "Transmission Control
   Protocol (TCP) Parameters" registry.  The initial values for this
   registry are given in Table 5 below.  Future assignments are to be
   made upon satisfying either of two policies defined in [RFC8126]:
   "IETF Review" or (for non-IETF stream specifications) "Expert Review
   with RFC Required."  IANA will furthermore provide early allocation
   [RFC7120] to facilitate testing before RFCs are finalized.

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         +--------+------------------------+---------------------+
         | Value  | AEAD Algorithm         | Reference           |
         +--------+------------------------+---------------------+
         | 0x0001 | AEAD_AES_128_GCM       | [RFC-TBD] Section 6 |
         | 0x0002 | AEAD_AES_256_GCM       | [RFC-TBD] Section 6 |
         | 0x0010 | AEAD_CHACHA20_POLY1305 | [RFC-TBD] Section 6 |
         +--------+------------------------+---------------------+

    Table 5: Authenticated-encryption algorithms for use with tcpcrypt

8.  Security Considerations

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

   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.
   That is, once an implementation removes these keys from memory, a
   compromise of the system will not provide any means to derive the
   session secrets for past connections.  All currently-specified key
   agreement schemes involve ECDHE-based key agreement, meaning a new
   key-pair can be efficiently computed for each connection.  If

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   implementations reuse these parameters, they MUST limit the lifetime
   of the private parameters as far as practical in order to minimize
   the number of past connections that are vulnerable.  Of course,
   placing private keys in persistent storage introduces severe risks
   that they may not be destroyed reliably and in a timely fashion, and
   SHOULD be avoided at all costs.

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

   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 for 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.

   Lastly, 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.
   Recommendations on how to achieve this may be found in [RFC4086].

   Most implementations will rely on a device's pseudo-random generator,
   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.

8.1.  Asymmetric Roles

   Tcpcrypt transforms a shared pseudo-random key (PRK) into
   cryptographic traffic 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).

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8.2.  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
   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.)

   To counter this threat, 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.

8.3.  Mandatory Key-Agreement Schemes

   This document mandates that tcpcrypt implementations provide support
   for at least one key-agreement scheme: ECDHE using Curve25519.  This
   choice of a single mandatory algorithm is the result of a difficult
   tradeoff between cryptographic diversity and the ease and security of
   actual deployment.

   The IETF's appraisal of best current practice on this matter
   [RFC7696] says, "Ideally, two independent sets of mandatory-to-
   implement algorithms will be specified, allowing for a primary suite
   and a secondary suite.  This approach ensures that the secondary
   suite is widely deployed if a flaw is found in the primary one."

   To meet that ideal, it might appear natural to also mandate ECDHE
   using P-256, as this scheme is well-studied, widely implemented, and
   sufficiently different from the Curve25519-based scheme that it is
   unlikely they will both suffer from a single (non-quantum)
   cryptanalytic advance.

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   However, implementing the Diffie-Hellman function using NIST elliptic
   curves (including those specified for use with tcpcrypt, P-256 and
   P-521) appears to be very difficult to achieve without introducing
   vulnerability to side-channel attacks [NIST-fail].  Although well-
   trusted implementations are available as part of large cryptographic
   libraries, these may be difficult to extract for use in operating-
   system kernels where tcpcrypt is usually best implemented.  In
   contrast, the characteristics of Curve25519 together with its recent
   popularity has led to many safe and efficient implementations,
   including some that fit naturally into the kernel environment.

   [RFC7696] insists that, "The selected algorithms need to be resistant
   to side-channel attacks and also meet the performance, power, and
   code size requirements on a wide variety of platforms."  On this
   principle, tcpcrypt excludes the NIST curves from the set of
   mandatory-to-implement key-agreement algorithms.

   Lastly, this document encourages (via SHOULD) support for key-
   agreement with Curve448 as this scheme appears likely to admit safe
   and efficient implementations; but it does not absolutely require
   such support, as well-proven implementations may not yet be
   available.

9.  Experiments

   Some experience will be required to determine whether the tcpcrypt
   protocol can be deployed safely and successfully across the diverse
   environments of the global internet.

   Safety means that TCP implementations that support tcpcrypt are able
   to communicate reliably in all the same settings as they would
   without tcpcrypt.  As described in [I-D.ietf-tcpinc-tcpeno]
   Section 9, this property can be subverted if middleboxes strip ENO
   options from non-SYN segments after allowing them in SYN segments; or
   if the particular communication patterns of tcpcrypt offend the
   policies of middleboxes doing deep-packet-inspection.

   Success, in addition to safety, means that hosts which implement
   tcpcrypt actually enable encryption when they connect to each other.
   This property depends on the network's treatment of the TCP-ENO
   handshake, and can be subverted if middleboxes merely strip unknown
   TCP options or if they terminate TCP connections and relay data back
   and forth unencrypted.

   Ease of implementation will be a further challenge to deployment.
   Because tcpcrypt requires encryption operations on frames that may
   span TCP segments, kernel implementations are forced to buffer
   segments in different ways than are necessary for plain TCP.  More

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   implementation experience will show how much additional code
   complexity is required in various operating systems, and what kind of
   performance effects can be expected.

10.  Acknowledgments

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

   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
   Project.

11.  Contributors

   Dan Boneh and Michael Hamburg were co-authors of the draft that
   became this document.

12.  References

12.1.  Normative References

   [I-D.ietf-tcpinc-tcpeno]
              Bittau, A., Giffin, D., Handley, M., Mazieres, D., and E.
              Smith, "TCP-ENO: Encryption Negotiation Option", draft-
              ietf-tcpinc-tcpeno-13 (work in progress), November 2017.

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

   [NIST-DSS]
              NIST, "FIPS PUB 186-4: Digital Signature Standard (DSS)",
              2013.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996, <https://www.rfc-
              editor.org/info/rfc2018>.

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   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997, <https://www.rfc-
              editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010, <https://www.rfc-
              editor.org/info/rfc5869>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
              <https://www.rfc-editor.org/info/rfc7539>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [I-D.ietf-tcpinc-api]
              Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
              D., and E. Smith, "Interface Extensions for TCP-ENO and
              tcpcrypt", draft-ietf-tcpinc-api-05 (work in progress),
              September 2017.

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   [NIST-fail]
              Bernstein, D. and T. Lange, "Failures in NIST's ECC
              standards", 2016, <https://cr.yp.to/newelliptic/nistecc-
              20160106.pdf>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989, <https://www.rfc-
              editor.org/info/rfc1122>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005, <https://www.rfc-
              editor.org/info/rfc4086>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

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

Authors' Addresses

   Andrea Bittau
   Google
   345 Spear Street
   San Francisco, CA  94105
   US

   Email: bittau@google.com

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

   Email: dbg@scs.stanford.edu

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

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

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

   Email: dm@uun.org

   Quinn Slack
   Sourcegraph
   121 2nd St Ste 200
   San Francisco, CA  94105
   US

   Email: sqs@sourcegraph.com

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

   Email: eric.smith@kestrel.edu

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