Cryptographic Protection of TCP Streams (tcpcrypt)

Internet Engineering Task Force (IETF)                         A. Bittau
Request for Comments: 8548                                        Google
Category: Experimental                                         D. Giffin
ISSN: 2070-1721                                      Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                     Stanford University
                                                                Q. Slack
                                                             Sourcegraph
                                                                E. Smith
                                                       Kestrel Institute
                                                                May 2019


           Cryptographic Protection of TCP Streams (tcpcrypt)

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 can be transmitted.  However, the extra latency can
   be avoided between two hosts that have recently established a
   previous tcpcrypt connection.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8548.

Copyright Notice

   Copyright (c) 2019 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
   (https://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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Encryption Protocol . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Cryptographic Algorithms  . . . . . . . . . . . . . . . .   4
     3.2.  Protocol Negotiation  . . . . . . . . . . . . . . . . . .   6
     3.3.  Key Exchange  . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Session ID  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Session Resumption  . . . . . . . . . . . . . . . . . . .  10
     3.6.  Data Encryption and Authentication  . . . . . . . . . . .  14
     3.7.  TCP Header Protection . . . . . . . . . . . . . . . . . .  16
     3.8.  Rekeying  . . . . . . . . . . . . . . . . . . . . . . . .  16
     3.9.  Keep-Alive  . . . . . . . . . . . . . . . . . . . . . . .  17
   4.  Encodings . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  Key-Exchange Messages . . . . . . . . . . . . . . . . . .  18
     4.2.  Encryption Frames . . . . . . . . . . . . . . . . . . . .  20
       4.2.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . .  20
       4.2.2.  Associated Data . . . . . . . . . . . . . . . . . . .  21
       4.2.3.  Frame ID  . . . . . . . . . . . . . . . . . . . . . .  21
     4.3.  Constant Values . . . . . . . . . . . . . . . . . . . . .  22
   5.  Key-Agreement Schemes . . . . . . . . . . . . . . . . . . . .  22
   6.  AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . .  24
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     8.1.  Asymmetric Roles  . . . . . . . . . . . . . . . . . . . .  27
     8.2.  Verified Liveness . . . . . . . . . . . . . . . . . . . .  27
     8.3.  Mandatory Key-Agreement Schemes . . . . . . . . . . . . .  27
   9.  Experiments . . . . . . . . . . . . . . . . . . . . . . . . .  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     10.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  31
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  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) [RFC8547] 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 [TCPINC-API] describes recommended interfaces
   for configuring certain parameters of this protocol.

2.  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.

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:

   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 produced by 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 the HMAC-
   based Key Derivation Function (HKDF) [RFC5869], which is built on
   Keyed-Hashing for Message Authentication (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.

3.2.  Protocol Negotiation

   Tcpcrypt depends on TCP-ENO [RFC8547] to negotiate whether encryption
   will be enabled for a connection as well as which key-agreement
   scheme to use.  TCP-ENO negotiates the use of a particular TCP
   encryption protocol (TEP) by including protocol identifiers in ENO
   suboptions.  This document associates four TEP identifiers with the
   tcpcrypt protocol as listed in Table 4 of Section 7.  Each identifier
   indicates the use of a particular key-agreement scheme, with an
   associated CPRF and length parameter.  Future standards can 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
   that 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; 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, an identifier derived from a
   session secret previously negotiated with the same host and the same
   TEP, and a nonce.

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

   If the negotiated TEP was sent by host B with v = 0, it means that
   fresh key agreement will be performed as described in Section 3.3.
   If, on the other hand, host B sent the TEP with v = 1 and both hosts
   sent appropriate resumption identifiers in their suboption data, then
   the key-exchange messages will be omitted in favor of determining
   keys via session resumption as described in Section 3.5.  With
   session resumption, protected application data MAY be sent
   immediately 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 could become the negotiated TEP, in which
   case 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 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.

   o  sym_cipher_list: A list of identifiers of symmetric ciphers (AEAD
      algorithms) acceptable to host A.  These are specified in Table 5
      of 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 frequently.  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 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 [RFC8547], and Init1
   and Init2 are the transmitted encodings of the messages described in
   Section 4.1.

   A series of session secrets are 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 are used by session
   resumption 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
   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 | sn[i], 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.  The values represented by sn[i] are
   session nonces.  For the initial session with i = 0, the session
   nonce is zero bytes long.  The values for subsequent sessions are
   derived from fresh connection data as described in Section 3.5.

   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 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 of Section 6.
   The algorithm uses the first ae_key_len bytes of each traffic key as
   an authenticated-encryption key, and it uses 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
   advertise 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 [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.

   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 Section 3.8 of [RFC793].  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.

   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 and session
   nonce, as follows:

    session_id[i] = TEP-byte | CPRF(ss[i], CONST_SESSID | sn[i], 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 its willingness to resume with a particular session
   secret by sending a SYN segment with a resumption suboption, i.e., an
   ENO suboption containing the negotiated TEP identifier of the
   previous session, half of the resumption identifier for the new
   session, and a resumption nonce.

   The resumption nonce MUST have a minimum length of zero bytes and
   maximum length of eight bytes.  The value MUST be chosen randomly or
   using a mechanism that guarantees uniqueness even in the face of
   virtual-machine cloning or other re-execution of the same session.
   An attacker who can force either side of a connection to reuse a
   session secret with the same nonce will completely break the security
   of tcpcrypt.  Reuse of session secrets is possible in the event of
   virtual-machine cloning or reuse of system-level hibernation state.
   Implementations SHOULD provide an API through which to set the
   resumption nonce length and MUST default to eight bytes if they
   cannot prohibit the reuse of session secrets.

   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].

   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
              +------+------+--...--+------+------+--...--+------+
              | TEP- |   resume[i]{0..8}   |       nonce_a       |
              | byte |                     |                     |
              +------+------+--...--+------+------+--...--+------+

      Figure 2: Resumption suboption sent when original role was "A".

   The TEP-byte contains a tcpcrypt TEP identifier and v = 1.  The nonce
   value MUST have length between 0 and 8 bytes.

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

         byte     0      1             9      10
              +------+------+--...--+------+------+--...--+------+
              | TEP- |   resume[i]{9..17}  |       nonce_b       |
              | byte |                     |                     |
              +------+------+--...--+------+------+--...--+------+

      Figure 3: Resumption suboption sent when original role was "B".

   The TEP-byte contains a tcpcrypt TEP identifier and v = 1.  The nonce
   value MUST have length between 0 and 8 bytes.

   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 TEP suboption.

   If a passive opener receives an ENO suboption with a TEP identifier
   and v = 1, but the suboption data is less than 9 bytes in length, 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, it receives a resumption suboption with the
   same TEP and an identifier-half that does not match that resumption
   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; 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.  To do
   this, it first constructs sn[i] as follows:

                         sn[i] = nonce_a | nonce_b

   Master keys are then computed from s[i] and sn[i] as described in
   Section 3.3 as well as from application data encrypted 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 and the session nonce that was generated
   during resumption.

   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 it 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 and then erase ss[i] from memory.  Hosts MAY 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 given session secret ss[i] MUST 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 with both segments not
   part of a simultaneous open, both connections would need to revert to
   fresh key exchange.  To avoid this limitation, implementations MAY
   choose to implement 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 have a way to
   specify two policies for the duration of the connection: 1) that
   resumption requests will be ignored, and thus fresh key exchange will
   be necessary; and 2) that no session secrets will be cached.  (These
   policies can be specified independently or as a unit.)  And for an
   established connection, an application MUST have a means to cause any
   cache state that was used in or resulted from establishing the
   connection to be flushed.  A companion document [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-upon keys.

   This protection is provided via algorithms for Authenticated
   Encryption with Associated Data (AEAD).  The permitted algorithms are
   listed in Table 5 of Section 7.  Additional algorithms can be
   specified in the future 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 of Section 6 along 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 field of a plaintext value,
   which is then encrypted to yield a frame's ciphertext field.  Chunks
   MUST be small enough that the ciphertext (whose length depends on the
   AEAD cipher used, and is generally slightly longer than the
   plaintext) has length less than 2^16 bytes.

   An "associated data" value (see Section 4.2.2) is constructed for the
   frame.  It contains the frame's control field and the length of the
   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
   rekeying as described in Section 3.8.

   Keys for AEAD encryption are 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 appropriate traffic key is divided into two
   parts:

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

                      Figure 4: Format of Traffic Key

   With reference to the "AEAD Interface" described in Section 2 of
   [RFC5116], the first ae_key_len bytes of the traffic key provide the
   AEAD key K.  The remaining ae_nonce_len bytes provide a nonce
   randomizer value NR, which is combined via bitwise exclusive-or with
   the frame ID to yield N, the AEAD nonce for the frame:

                            N = frame_ID XOR NR

   The remaining AEAD inputs, P and A, are provided by the frame's
   plaintext value and associated data, respectively.  The output of the
   AEAD 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 selectively acknowledged (SACKed), according to Section 8
   of [RFC2018]).

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 field 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 Section 8 of [RFC2018]).

3.8.  Rekeying

   Rekeying 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 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 both 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
   rekeying".

   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 rekey = 0 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 rekey operation if
   it has no data to send; that is, it MUST NOT initiate rekeying 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.

3.9.  Keep-Alive

   Instead of using TCP keep-alives to verify that the remote endpoint
   is still responsive, tcpcrypt implementations SHOULD employ the
   rekeying mechanism for this purpose, as follows.  When necessary, a
   host SHOULD probe the liveness of its peer by initiating rekeying 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:

       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
   ignored field 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:

       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:

          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 control field 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 both encrypts the application data and provides redundancy
   with which to verify its integrity.)

   The flags field has this structure:

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

   The six-bit field 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 the urgent field 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.

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

   Future documents assigning additional TEPs for use with tcpcrypt
   might specify different values for the lengths above.  Note that the
   minimum session ID length specified 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 Elliptic
   Curve Secret Value Derivation Primitive, Diffie-Hellman version
   (ECSVDP-DH) 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:

              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 Section A.16.10 of [IEEE-1363].

   Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 perform the
   Diffie-Hellman protocol using the functions X25519 and X448,
   respectively.  Implementations SHOULD compute these functions using
   the algorithms described in [RFC7748].  When they do so,
   implementations MUST check whether the computed Diffie-Hellman shared
   secret is the all-zero value and abort if so, as described in
   Section 6 of [RFC7748].  Alternative implementations of these
   functions SHOULD abort when either input forces the shared secret to
   one of a small set of values as discussed in Section 7 of [RFC7748].

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

   Table 2 below specifies the requirement levels of the four TEPs
   specified in this document.  In particular, all implementations of
   tcpcrypt MUST support TCPCRYPT_ECDHE_Curve25519.  However, system
   administrators MAY configure which TEPs a host will negotiate
   independent of these implementation requirements.

                +-------------+---------------------------+
                | Requirement | TEP                       |
                +-------------+---------------------------+
                | REQUIRED    | TCPCRYPT_ECDHE_Curve25519 |
                | RECOMMENDED | TCPCRYPT_ECDHE_Curve448   |
                | OPTIONAL    | TCPCRYPT_ECDHE_P256       |
                | OPTIONAL    | 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
   [RFC8439].

   Implementations MUST support certain AEAD algorithms according to
   Table 3.  Note that system administrators MAY configure which
   algorithms a host will negotiate independently of these requirements.

   Lastly, this document uses the lengths ae_key_len and ae_nonce_len to
   specify aspects of encryption and data formats.  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       | REQUIRED    | 16 bytes   | 12 bytes     |
   | AEAD_AES_256_GCM       | RECOMMENDED | 32 bytes   | 12 bytes     |
   | AEAD_CHACHA20_POLY1305 | RECOMMENDED | 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 have been incorporated in IANA's "TCP Encryption Protocol
   Identifiers" registry under the "Transmission Control Protocol (TCP)
   Parameters" registry, as in Table 4.  The various key-agreement
   schemes used by these tcpcrypt variants are defined in Section 5.

             +-------+---------------------------+-----------+
             | Value | Meaning                   | Reference |
             +-------+---------------------------+-----------+
             | 0x21  | TCPCRYPT_ECDHE_P256       | [RFC8548] |
             | 0x22  | TCPCRYPT_ECDHE_P521       | [RFC8548] |
             | 0x23  | TCPCRYPT_ECDHE_Curve25519 | [RFC8548] |
             | 0x24  | TCPCRYPT_ECDHE_Curve448   | [RFC8548] |
             +-------+---------------------------+-----------+

              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, inclusively, for which IANA maintains a new
   "tcpcrypt AEAD Algorithms" registry under the "Transmission Control
   Protocol (TCP) Parameters" registry.  The initial values for this
   registry are given in Table 5.  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.

        +--------+------------------------+----------------------+
        | Value  | AEAD Algorithm         | Reference            |
        +--------+------------------------+----------------------+
        | 0x0001 | AEAD_AES_128_GCM       | [RFC8548], Section 6 |
        | 0x0002 | AEAD_AES_256_GCM       | [RFC8548], Section 6 |
        | 0x0010 | AEAD_CHACHA20_POLY1305 | [RFC8548], 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 network
   attackers 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., Elliptic Curve Diffie-Hellman (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.  Therefore, the protocol is vulnerable to denial-of-service
   from off-path attackers just as plain TCP is.  Possible attacks
   include desynchronizing the underlying TCP stream, injecting RST or
   FIN segments, and forging rekey bits.  These attacks will cause a
   tcpcrypt connection to hang or fail with an error, but not in any
   circumstance where plain TCP could continue uncorrupted.
   Implementations MUST give higher-level software a way to distinguish
   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 needed
   because tcpcrypt's threat model includes the possibility of a
   connection to an adversary.  If key negotiation is compromised and
   yields two different keys, failed integrity checks on every
   subsequent frame will cause the connection either to hang or to
   abort.  This is not a new threat as an active attacker can achieve
   the same results against a plain TCP connection by injecting RST
   segments or modifying sequence and acknowledgement numbers.

   Tcpcrypt uses short-lived public keys to provide forward secrecy;
   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 key agreement based on Ephemeral Elliptic Curve
   Diffie-Hellman (ECDHE), meaning a new key pair can be efficiently
   computed for each connection.  If implementations reuse these
   parameters, they MUST limit the lifetime of the private parameters as
   far as is 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 will not be
   destroyed reliably and in a timely fashion, and it SHOULD be avoided
   whenever possible.

   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.
   [RFC4086] provides recommendations on how to achieve this.

   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).

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.  Therefore, 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.  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 can 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 support for key agreement with
   Curve448, categorizing it as RECOMMENDED.  Curve448 appears likely to
   admit safe and efficient implementations.  However, support is not
   REQUIRED because existing implementations might not yet be
   sufficiently well proven.

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 Section 9 of [RFC8547], 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 hosts that implement tcpcrypt
   actually enable encryption when connecting to one another.  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 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
   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.  References

10.1.  Normative References

   [IEEE-1363]
              IEEE, "IEEE Standard Specifications for Public-Key
              Cryptography", IEEE Standard 1363-2000,
              DOI 10.1109/IEEESTD.2000.92292.

   [NIST-DSS] National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013.

   [RFC793]   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>.

   [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>.

   [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>.

   [RFC8439]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
              <https://www.rfc-editor.org/info/rfc8439>.

   [RFC8547]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., and
              E. Smith, "TCP-ENO: Encryption Negotiation Option",
              RFC 8547, DOI 10.17487/RFC8547, May 2019,
              <https://www.rfc-editor.org/info/rfc8547>.

10.2.  Informative References

   [NIST-fail]
              Bernstein, D. and T. Lange, "Failures in NIST's ECC
              Standards", January 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 Symposium, August 2010.

   [TCPINC-API]
              Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
              D., and E. Smith, "Interface Extensions for TCP-ENO and
              tcpcrypt", Work in Progress, draft-ietf-tcpinc-api-06,
              June 2018.

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.

Contributors

   Dan Boneh and Michael Hamburg were coauthors of the draft that became
   this document.

Authors' Addresses

   Andrea Bittau
   Google
   345 Spear Street
   San Francisco, CA  94105
   United States of America

   Email: bittau@google.com
   Daniel B. Giffin
   Stanford University
   353 Serra Mall, Room 288
   Stanford, CA  94305
   United States of America

   Email: daniel@beech-grove.net


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

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


   David Mazieres
   Stanford University
   353 Serra Mall, Room 290
   Stanford, CA  94305
   United States of America

   Email: dm@uun.org


   Quinn Slack
   Sourcegraph
   121 2nd St Ste 200
   San Francisco, CA  94105
   United States of America

   Email: sqs@sourcegraph.com


   Eric W. Smith
   Kestrel Institute
   3260 Hillview Avenue
   Palo Alto, CA  94304
   United States of America

   Email: eric.smith@kestrel.edu