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rfc:rfc8548

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.

Bittau, et al. Experimental [Page 1] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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.

Bittau, et al. Experimental [Page 2] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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

Bittau, et al. Experimental [Page 3] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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.

Bittau, et al. Experimental [Page 4] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 5] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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.

Bittau, et al. Experimental [Page 6] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 7] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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)

Bittau, et al. Experimental [Page 8] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 9] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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

Bittau, et al. Experimental [Page 10] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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

Bittau, et al. Experimental [Page 11] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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

Bittau, et al. Experimental [Page 12] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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

Bittau, et al. Experimental [Page 13] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 14] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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]).

Bittau, et al. Experimental [Page 15] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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".

Bittau, et al. Experimental [Page 16] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 17] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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

Bittau, et al. Experimental [Page 18] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 19] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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

Bittau, et al. Experimental [Page 20] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

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

Bittau, et al. Experimental [Page 21] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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

Bittau, et al. Experimental [Page 24] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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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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

Bittau, et al. Experimental [Page 27] RFC 8548 tcpcrypt: TCP Encryption Protocol May 2019

 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.

Bittau, et al. Experimental [Page 28] 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>.

Bittau, et al. Experimental [Page 29] 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>.

Bittau, et al. Experimental [Page 30] 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

Bittau, et al. Experimental [Page 31] 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

Bittau, et al. Experimental [Page 32]

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