    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.

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    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.

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    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

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    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.

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       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.

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    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.

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       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.

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       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)

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       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.

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       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

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       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

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       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

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       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

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       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.

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       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]).

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    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".

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       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.

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    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

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       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.

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       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           |
                 +-------+-------+-------+-------+---...---+-------+

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       (Note that clen in the previous section will generally be greater
       than plen, as the ciphertext produced by the authenticated-encryption
       scheme 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      |
                 +-----+--...--+-----+-----+--...--+-----+

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       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

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       (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

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    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

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       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

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       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.

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    RFC 8548            tcpcrypt: TCP Encryption Protocol           May 2019

    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

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       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.

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    RFC 8548            tcpcrypt: TCP Encryption Protocol           May 2019

       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>.

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    RFC 8548            tcpcrypt: TCP Encryption Protocol           May 2019

       [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>.

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    RFC 8548            tcpcrypt: TCP Encryption Protocol           May 2019

       [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

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    RFC 8548            tcpcrypt: TCP Encryption Protocol           May 2019

       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

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