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Versions: 00 01 02 draft-ietf-tcpinc-tcpeno

Network Working Group                                          A. Bittau
Internet-Draft                                                  D. Boneh
Intended status: Experimental                                  D. Giffin
Expires: February 11, 2016                           Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                     Stanford University
                                                                E. Smith
                                                       Kestrel Institute
                                                         August 10, 2015

                 TCP-ENO: Encryption Negotiation Option


   Despite growing adoption of TLS [RFC5246], a significant fraction of
   TCP traffic on the Internet remains unencrypted.  The persistence of
   unencrypted traffic can be attributed to at least two factors.
   First, some legacy protocols lack a signaling mechanism (such as a
   "STARTTLS" command) by which to convey support for encryption, making
   incremental deployment impossible.  Second, legacy applications
   themselves cannot always be upgraded, requiring a way to implement
   encryption transparently entirely within the transport layer.  The
   TCP Encryption Negotiation Option (TCP-ENO) addresses both of these
   problems through a new TCP option kind providing out-of-band, fully
   backward-compatible negotiation of encryption.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 11, 2016.

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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Requirements language . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The TCP-ENO option  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  TCP-ENO roles . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  TCP-ENO handshake . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Handshake examples  . . . . . . . . . . . . . . . . .   9
     3.3.  General suboptions  . . . . . . . . . . . . . . . . . . .  10
     3.4.  Negotiation transcript  . . . . . . . . . . . . . . . . .  11
   4.  Requirements for encryption specs . . . . . . . . . . . . . .  12
     4.1.  Session IDs . . . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Option kind sharing . . . . . . . . . . . . . . . . . . .  14
   5.  API extensions  . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Experiments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security considerations . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Introduction

   Many applications and protocols running on top of TCP today do not
   encrypt traffic.  This failure to encrypt lowers the bar for certain
   attacks, harming both user privacy and system security.
   Counteracting the problem demands a minimally intrusive, backward-
   compatible mechanism for incrementally deploying encryption.  The TCP
   Encryption Negotiation Option (TCP-ENO) specified in this document
   provides such a mechanism.

   While the need for encryption is immediate, future developments could
   alter trade-offs and change the best approach to TCP-level encryption
   (beyond introducing new cipher suites).  For example:

   o  Increased option space in TCP [I-D.ietf-tcpm-tcp-edo][I-D.briscoe-
      tcpm-inspace-mode-tcpbis][I-D.touch-tcpm-tcp-syn-ext-opt] could
      reduce round trip times and simplify protocols.

   o  API revisions to socket interfaces [RFC3493] could benefit from
      integration with TCP-level encryption, particularly if combined
      with technologies such as DANE [RFC6394].

   o  The forthcoming TLS 1.3 [I-D.ietf-tls-tls13] standard could reach
      more applications given an out-of-band, backward-compatible
      mechanism for enabling encryption.

   o  TCP fast open [RFC7413], as it gains more widespread adoption and
      middlebox acceptance, could potentially benefit from tailored
      encryption support.

   o  Cryptographic developments that either shorten or lengthen the
      minimal key exchange messages required could affect how such
      messages are best encoded in TCP segments.

   Introducing TCP options, extending operating system interfaces to
   support TCP-level encryption, and extending applications to take
   advantage of TCP-level encryption will all require effort.  To the
   greatest extent possible, this effort ought to remain applicable if
   the need arises to change encryption strategies.  To this end, it is
   useful to consider two questions separately:

   1.  How to negotiate the use of encryption at the TCP layer, and

   2.  How to perform encryption at the TCP layer.

   This document addresses question 1 with a new option called TCP-ENO.
   TCP-ENO provides a framework in which two endpoints can agree on one
   among multiple possible TCP encryption _specs_.  For future

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   compatibility, encryption specs can vary widely in terms of wire
   format, use of TCP option space, and integration with the TCP header
   and segmentation.  A companion document, the TCPINC encryption spec,
   addresses question 2.  TCPINC enables TCP-level traffic encryption
   today.  TCP-ENO ensures that the effort invested to deploy TCPINC can
   benefit future encryption specs should a different approach at some
   point be preferable.

   At a lower level, TCP-ENO was designed to achieve the following

   1.  Enable endpoints to negotiate the use of a separately specified
       encryption _spec_.

   2.  Transparently fall back to unencrypted TCP when not supported by
       both endpoints.

   3.  Provide signaling through which applications can better take
       advantage of TCP-level encryption (for instance by improving
       authentication mechanisms in the presence of TCP-level

   4.  Provide a standard negotiation transcript through which specs can
       defend against tampering with TCP-ENO.

   5.  Make parsimonious use of TCP option space.

   6.  Define roles for the two ends of a TCP connection so as to break
       the symmetry of simultaneous open and uniquely name each end of a
       connection for authentication purposes.

3.  The TCP-ENO option

   TCP-ENO is a TCP option used during connection establishment to
   negotiate how to encrypt traffic.  As an option, TCP-ENO can be
   deployed incrementally.  Legacy hosts unaware of the option simply
   ignore it and never send it, causing traffic to fall back to
   unencrypted TCP.  Similarly, middleboxes that strip out unknown
   options including TCP-ENO will downgrade connections to plaintext
   without breaking them.  Of course, downgrading makes TCP-ENO
   vulnerable to active attackers, but appropriately modified
   applications can protect themselves by considering the state of TCP-
   level encryption during authentication, as discussed in Section 7.

   The ENO option takes two forms.  In TCP segments with the SYN flag
   set, it acts as a container for a series of one or more suboptions,
   as shown in Figure 1.  In non-SYN segments, ENO conveys only a single
   bit of information, namely an acknowledgment that the sender received

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   an ENO option in the other host's SYN segment.  (Such acknowledgments
   enable graceful fallback to unencrypted TCP in the event that a
   middlebox strips ENO options in one direction.)  Figure 2 illustrates
   the non-SYN form of the ENO option.  We refer to this form as an ACK-
   only ENO option.  Encryption specs MAY include extra bytes in an ACK-
   only ENO option, but TCP-ENO itself MUST ignore them.  In accordance
   with TCP [RFC0793], the first two bytes of the ENO option always
   consist of the kind (ENO) and the total length of the option.

         byte    0     1     2     3            2+i  3+i ... N-1
              |Kind=|Len= |Opt_0|Opt_1|       |Opt_i|   Opt_i   |
              | ENO |  N  |     |     |       |     |   data    |

    Figure 1: TCP-ENO option in SYN segment (MUST contain at least one

          byte    0     1                0     1     2     N-1
               +-----+-----+          +-----+-----+-----...----+
               |Kind=|Len= |          |Kind=|Len= |  ignored   |
               | ENO |  2  |    OR    | ENO |  N  | by TCP-ENO |
               +-----+-----+          +-----+-----+-----...----+

           Figure 2: ACK-only TCP-ENO option in non-SYN segment

   Every suboption starts with a byte of the form illustrated in
   Figure 3.  The seven-bit value "cs" specifies the meaning of the
   suboption.  Each value of "cs" either specifies general parameters
   (discussed in Section 3.3) or indicates the willingness to use a
   specific encryption spec detailed in a separate document.

       bit   7   6   5   4   3   2   1   0
           | v |            cs             |

           v  - 1 when suboption followed by variable-length data
           cs - 7-bit global configuration option or encryption spec

                    Figure 3: Format of suboption byte

   The high bit "v" in a suboption's first byte specifies whether or not
   the suboption is followed by variable-length data.  If "v" is 0, the
   suboption consists of only the one byte.  If "v" is 1, then the
   suboption is followed by variable-length data.  Suboption data MAY be
   used for session caching, cipher suite negotiation, key exchange, or
   other purposes, as determined by the value of "cs".

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   Every suboption but the last in an ENO option MUST be a one-byte
   suboption (with "v" = 0).  The last suboption MAY be a variable-
   length suboption.  Its length is determined by the total length of
   the TCP option.  In Figure 1, "Opt_i" is the variable-length option;
   its total size is N-(2+i) bytes--one byte for "Opt_i" itself and
   N-(3+i) bytes for additional data.  Multiple suboptions with data may
   be included in a single TCP SYN segment by repeating the ENO option.

   Table 1 summarizes the allocation of values of "cs".  Values under
   0x10 are reserved for _general suboptions_ whose meaning applies
   across encryption specs, as discussed in Section 3.3.  Values greater
   than or equal to 0x20 are are reserved for _spec identifiers_.
   Values in the range 0x10-0x1f are reserved for possible future
   general options.  Implementations MUST ignore all unknown suboptions.

    | cs        | Meaning                                             |
    | 0x00-0x0f | General options (see Section 3.3)                   |
    | 0x10-0x1f | Reserved for possible use by future general options |
    | 0x20-0x7f | Used to designate encryption specs                  |

           Table 1: Allocation of cs bits in TCP-ENO suboptions

3.1.  TCP-ENO roles

   TCP-ENO uses abstract roles to distinguish the two ends of a TCP
   connection: One host plays the "A" role, while the other host plays
   the "B" role.  Following a normal three-way handshake, the active
   opener plays the A role and the passive opener plays the B role.  An
   active opener is a host that sends a SYN segment without the ACK flag
   set (after a "connect" system call on socket-based systems).  A
   passive opener sends a SYN segment with the ACK flag set (after a
   "listen" call on socket-based systems).

   Roles are abstracted from the active/passive opener distinction to
   deal with simultaneous open, in which both hosts are active openers.
   For simultaneous open, the general suboptions discussed in
   Section 3.3 define a tie-breaker bit "b", where the host with "b = 1"
   plays the B role, and the host with "b = 0" plays the A role.  If two
   active openers have the same "b" bit, TCP-ENO fails and reverts to
   unencrypted TCP.

   More precisely, the above role assignment can be reduced to comparing
   a two-bit role _priority_ for each host, shown in Figure 4.  The most
   significant bit, "p", is 1 for a passive opener and 0 for an active
   opener.  The least-significant bit "b" is the tie-breaker bit.  The

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   host with the lower priority assumes the A role; the host with the
   higher priority assumes the B role.  In the event of a tie, TCP-ENO
   fails and MUST continue with unencrypted TCP as if the ENO options
   had not been present in SYN segments.

            bit   1   0
                | p   b |

                p - 0 for active opener, 1 for passive opener
                b - b bit from general suboptions sent by host

                  Figure 4: Role priority of an endpoint

   Encryption specs SHOULD refer to TCP-ENO's A and B roles to specify
   asymmetric behavior by the two hosts.  For the remainder of this
   document, we will use the terms "host A" and "host B" to designate
   the hosts with role A and B respectively in a connection.

3.2.  TCP-ENO handshake

   The TCP-ENO option is intended for use during TCP connection
   establishment.  To enable incremental deployment, a host needs to
   ensure both that the other host supports TCP-ENO and that no
   middlebox has stripped the ENO option from its own TCP segments.  In
   the event that either of these conditions does not hold,
   implementations MUST immediately cease sending TCP-ENO options and
   MUST continue with unencrypted TCP as if the ENO option had not been

   More precisely, for negotiation to succeed, the TCP-ENO option MUST
   be present in the SYN segment sent by each host, so as to indicate
   support for TCP-ENO.  Additionally, the ENO option MUST be present in
   the first ACK segment sent by each host, so as to indicate that no
   middlebox stripped the ENO option from the ACKed SYN.  Depending on
   whether a host is an active or a passive opener, the first ACK
   segment may or may not be the same as the SYN segment.  Specifically:

   o  An active opener begins with a SYN-only segment, and hence must
      send two segments containing ENO options.  The initial SYN-only
      segment MUST contain an ENO option with at least one suboption, as
      pictured in Figure 1.  If ENO succeeds, the active opener's first
      ACK segment MUST subsequently contain an ACK-only ENO option, as
      pictured in Figure 2.

   o  A passive opener's first transmitted segment has both the SYN and
      ACK flags set.  Therefore, a passive opener MUST restrict ENO

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      options to the single SYN-ACK segment it sends and include an ENO
      option of the type shown in Figure 1.

   A spec identifier in one host's SYN segment is _valid_ if it is
   compatible with a suboption in the other host's SYN segment.  Two
   suboptions are _compatible_ when they have the same "cs" value (>=
   0x20) and when the particular combination of "v" bits and suboption
   data in suboptions of the two SYN segments is well-defined by the
   corresponding encryption spec.  Specs MAY allow or disallow any
   combination of values of "v" in the two SYN segments.

   Once the two sides have exchanged SYN segments, the _negotiated spec_
   is the first valid spec identifier in the SYN segment of host B (that
   is, the passive opener in the absence of simultaneous open).  In
   other words, the order of suboptions in host B's SYN segment
   determines spec priority, while the order of suboptions in host A's
   SYN segment has no effect.  Hosts must disable TCP-ENO if there is no
   valid spec in host B's SYN segment.

   When possible, host B SHOULD send only one spec identifier (suboption
   in the range 0x20-0xff), and SHOULD ensure this option is valid.
   However, sending a single valid spec identifier is not required, as
   doing so could be impractical in some cases, such as simultaneous
   open or library-level implementations that can only provide a static
   TCP-ENO option to the kernel.

   A host MUST disable ENO if any of the following conditions holds:

   1.  The host receives a SYN segment without an ENO option,

   2.  The host receives a SYN segment that contains no valid encryption
       specs when paired with the SYN segment that the host has already
       sent or would otherwise have sent,

   3.  The host receives a SYN segment containing general suboptions
       that are incompatible with the SYN segment that it has already
       sent or would otherwise have sent, or

   4.  The first ACK segment received by a host does not contain an ENO

   After disabling ENO, a host MUST NOT transmit any further ENO options
   and MUST fall back to unencrypted TCP.

   Conversely, if a host receives an ACK segment containing an ENO
   option, then encryption MUST be enabled.  From this point the host
   MUST follow the encryption protocol of the negotiated spec and MUST
   NOT present raw TCP payload data to the application.  In particular,

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   data segments MUST contain ciphertext or key agreement messages as
   determined by the negotiated spec, and MUST NOT contain plaintext
   application data.

3.2.1.  Handshake examples

          (1) A -> B:  SYN      ENO<X,Y>
          (2) B -> A:  SYN-ACK  ENO<Y>
          (3) A -> B:  ACK      ENO<>
          [rest of connection encrypted according to spec for Y]

     Figure 5: Three-way handshake with successful TCP-ENO negotiation

   Figure 5 shows a three-way handshake with a successful TCP-ENO
   negotiation.  The two sides agree to follow the encryption spec
   identified by suboption Y.

                (1) A -> B:  SYN      ENO<X,Y>
                (2) B -> A:  SYN-ACK
                (3) A -> B:  ACK
                [rest of connection unencrypted legacy TCP]

       Figure 6: Three-way handshake with failed TCP-ENO negotiation

   Figure 6 shows a failed TCP-ENO negotiation.  The active opener (A)
   indicates support for specs corresponding to suboptions X and Y.
   Unfortunately, at this point one of thee things occurs:

   1.  The passive opener (B) does not support TCP-ENO,

   2.  B supports TCP-ENO, but supports neither of specs X and Y, and so
       does not reply with an ENO option, or

   3.  The network stripped the ENO option out of A's SYN segment, so B
       did not receive it.

   Whichever of the above applies, the connection transparently falls
   back to unencrypted TCP.

        (1) A -> B:  SYN      ENO<X,Y>
        (2) B -> A:  SYN-ACK  ENO<X>    [ENO stripped by middlebox]
        (3) A -> B:  ACK
        [rest of connection unencrypted legacy TCP]

     Figure 7: Failed TCP-ENO negotiation because of network filtering

   Figure 7 Shows another handshake with a failed encryption
   negotiation.  In this case, the passive opener B receives an ENO

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   option from A and replies.  However, the reverse network path from B
   to A strips ENO options.  Hence, A does not receive an ENO option
   from B, disables ENO, and does not include the required ACK-only ENO
   option in its first ACK segment.  The lack of ENO in A's ACK segment
   signals to B that the connection will not be encrypted.  At this
   point, the two hosts proceed with an unencrypted TCP connection.

          (1) A -> B:  SYN  ENO<X,Y>
          (2) B -> A:  SYN  ENO<0x01,Z,Y,X>
          (3) A -> B:  ACK  ENO<>
          (4) B -> A:  ACK  ENO<>
          [rest of connection encrypted according to spec for Y]

      Figure 8: Simultaneous open with successful TCP-ENO negotiation

   Figure 8 shows a successful TCP-ENO negotiation with simultaneous
   open.  Here the first four segments MUST contain an ENO option (the
   first SYN and first ACK from each host).  Note the use of the tie-
   breaker bit in general suboption 0x01 assigns B its role, as
   discussed in Section 3.3.

3.3.  General suboptions

   Suboptions 0x00-0x0f are used for general conditions that apply
   regardless of the negotiated encryption spec.  A TCP segment MUST
   include at most one suboption whose high nibble is 0.  The value of
   the low nibble is interpreted as a bitmask, illustrated in Figure 9.

              bit   7   6   5   4   3   2   1   0
                  | 0   0   0   0   z    aa     b |

                  z  - Zero bit (reserved for future use)
                  aa - Application-aware bits
                  b  - Tie-breaker bit for simultaneous open

                Figure 9: Format of the general option byte

   The fields of the bitmask are interpreted as follows:

   z  The "z" bit is reserved for future revisions of TCP-ENO.  Its
      value MUST be set to zero in sent segments and ignored in received

   aa The two application-aware bits indicate that the application on
      the sending host is aware of TCP-ENO and has been extended to
      alter its behavior in the presence of encrypted TCP.  There are

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      four possible values, as shown in Table 2.  The default, when
      applications have not been modified to take advantage of TCP-ENO,
      MUST be 00.  However, implementations SHOULD provide an API
      through which applications can set the bits to other values and
      query for the other host's application-aware bits.  The values 01
      and 10 (binary) indicate that the application is aware of TCP-ENO.
      The distinction between 01 and 10 is left to the application.

      Value 11 (binary) indicates that an application is aware of TCP-
      ENO and requires application awareness from the other side.  If
      one host sends value 00 and the other host sends 11, then TCP-ENO
      MUST be disabled and fall back to unencrypted TCP.  A possible use
      of value 11 is for applications that perform legacy encryption and
      wish to disable TCP-ENO unless higher-layer encryption can be

     | Value | Meaning                                               |
     |    00 | Application is not aware of TCP-ENO                   |
     |    01 | Application is aware of TCP-ENO                       |
     |    10 | Application is aware of TCP-ENO                       |
     |    11 | Application awareness is mandatory for use of TCP-ENO |

            Table 2: Meaning of the two application-aware bits

   b  This is the tie-breaker bit in role priority, discussed in
      Section 3.1.

   A SYN segment without an explicit general suboption has an implicit
   general suboption of 0x00.

3.4.  Negotiation transcript

   To defend against attacks on encryption negotiation itself,
   encryption specs need a way to reference a transcript of TCP-ENO's
   negotiation.  In particular, an encryption spec MUST fail with high
   probability if its selection resulted from tampering with or forging
   initial SYN segments.

   TCP-ENO defines its negotiation transcript as a packed data structure
   consisting of a series of TCP-ENO options (each including the ENO and
   length bytes, as they appeared in the TCP header).  Specifically, the
   transcript is constructed from the following, in order:

   1.  Every TCP-ENO option in host A's SYN segment, in the order they
       appeared in that SYN segment.

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   2.  A minimal two-byte ENO option, as shown on the left in Figure 2.

   3.  Every TCP-ENO option in host B's SYN segment, in the order they
       appeared in that SYN segment.

   4.  A minimal two-byte ENO option, as shown on the left in Figure 2.

   Note that 2 and 4 merely serve as delimiters to separate the two
   hosts' options from each other and from any data that follows the
   transcript.  Note further that any ignored data in ACK-only ENO
   options does not appear in the transcript.  Because parts 2 and 4 are
   always exactly two bytes and SYN segments MUST NOT contain two-byte
   ENO options, this encoding is unambiguous.

   For the transcript to be well defined, hosts MUST NOT alter ENO
   options in retransmitted segments, except that an active opener MAY
   remove the ENO option altogether from a retransmitted SYN segment and
   disable TCP-ENO.  Such removal could be useful if middleboxes are
   dropping segments with the ENO option.

4.  Requirements for encryption specs

   TCP-ENO was designed to afford encryption spec authors a large amount
   of design flexibility.  Nonetheless, to fit all encryption specs into
   a coherent framework and abstract most of the differences away from
   application writers, all encryption specs claiming ENO "cs" numbers
   MUST satisfy the following properties.

   o  Specs MUST always cryptographically protect both the integrity and
      confidentiality of TCP data streams.

   o  Specs MUST define a session ID whose value identifies the TCP
      connection and, with overwhelming probability, is unique over all
      time if either host correctly obeys the spec.  Section 4.1
      describes the requirements of the session ID in more detail.

   o  Specs MUST NOT permit the negotiation of any encryption algorithms
      with weaker than 128-bit security.

   o  Specs MUST NOT allow the negotiation of null cipher suites, even
      for debugging purposes.  (Implementations MAY support debugging
      modes that allow applications to extract their own session keys.)

   o  Specs MUST NOT allow the negotiation of encryption modes that do
      not provide forward secrecy.

   o  Specs MUST protect and authenticate the end-of-file marker
      traditionally conveyed by TCP's FIN flag when the remote

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      application calls "close" or "shutdown".  However, end-of-file MAY
      be conveyed though a mechanism other than TCP FIN.  Moreover,
      specs MAY permit attacks that cause TCP connections to abort with

   o  Specs MAY disallow the use of TCP urgent data by applications, but
      MUST NOT allow attackers to manipulate the URG flag and urgent
      pointer in ways that are visible to applications.

4.1.  Session IDs

   Each spec MUST define a session ID that uniquely identifies each
   encrypted TCP connection.  Implementations SHOULD expose the session
   ID to applications via an API extension.  Applications that are aware
   of TCP-ENO SHOULD incorporate the session ID value into any
   authentication mechanisms layered over TCP encryption so as to
   authenticate actual TCP endpoints.

   In order to avoid replay attacks and prevent authenticated session
   IDs from being used out of context, session IDs MUST be unique over
   all time with high probability.  This uniqueness property MUST hold
   even if one end of a connection maliciously manipulates the protocol
   in an effort to create duplicate session IDs.  In other words, it
   MUST be infeasible for a host, even by deviating from the encryption
   spec, to establish two TCP connections with the same session ID to
   remote hosts obeying the spec.

   To prevent session IDs from being confused across specs, all session
   IDs begin with the negotiated spec identifier--that is, the first
   valid spec identifier in host B's SYN segment.  If the "v" bit was 1
   in host B's SYN segment, then it is also 1 in the session ID.
   However, only the first byte is included, not the suboption data.
   Figure 10 shows the resulting format.

                 byte    0     1     2        N-1    N
                      | sub-| collision-resistant hash  |
                      | opt | of connection information |

                     Figure 10: Format of a session ID

   Though specs retain considerable flexibility in their definitions of
   the session ID, all session IDs MUST meet certain minimum
   requirements.  In particular:

   o  The session ID MUST be at least 33 bytes (including the one-byte
      suboption), though specs may choose longer session IDs.

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   o  The session ID MUST depend in a collision-resistant way on fresh
      data contributed by both sides of the connection.

   o  The session ID MUST depend in a collision-resistant way on any
      public keys, public Diffie-Hellman parameters, or other asymmetric
      cryptographic parameters that are employed by the encryption spec
      and have corresponding private data that is known by only one side
      of the connection.

   o  The session ID MUST NOT contain any confidential data (such as
      data permitting the derivation of session keys).

   o  The session ID MUST depend on the negotiation transcript specified
      in Section 3.4 in a collision-resistant way.

4.2.  Option kind sharing

   This draft specifically prohibits ENO options from appearing in any
   segments other than the initial SYN and ACK segments of a connection.
   This means any use of the ENO option kind in subsequent segments will
   not conflict with TCP-ENO.  Therefore, encryption specs that require
   TCP option space MAY re-purpose the ENO option kind for use in
   segments after the initial TCP handshake.  As previously stated,
   specs MAY also place data in ACK-only ENO options, as shown on the
   right side of Figure 2.

5.  API extensions

   Implementations SHOULD provide API extensions through which
   applications can query and configure the behavior of TCP-ENO,
   including retrieving session IDs, setting and reading application-
   aware bits, and specifying which specs to negotiate.  The specifics
   of such an API are outside the scope of this document.

6.  Experiments

   This document has experimental status.  One of the primary open
   questions is to what extent middleboxes will permit the use of TCP-
   ENO.  Once TCP-ENO is deployed, we will be in a better position to
   gather data on two types of failure:

   1.  Middleboxes downgrading TCP-ENO connections to unencrypted TCP.
       This can happen if middleboxes strip unknown TCP options or if
       they terminate TCP connections and relay data back and forth.

   2.  Middleboxes causing TCP-ENO connections to fail completely.  This
       can happen if applications perform deep packet inspection and
       start dropping segments that unexpectedly contain ciphertext.

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   The first type of failure is tolerable since TCP-ENO is designed for
   incremental deployment anyway.  The second type of failure is more
   problematic, and, if prevalent, will require the development of
   techniques to avoid and recover from such failures.

7.  Security considerations

   An obvious use case for TCP-ENO is opportunistic encryption.
   However, if applications do not check and verify the session ID, they
   will be open to man-in-the-middle attacks as well as simple downgrade
   attacks in which an attacker strips off the TCP-ENO option.  Hence,
   where possible, applications SHOULD be modified to fold the session
   ID into authentication mechanisms, and SHOULD employ the application-
   aware bits as needed to enable such negotiation in a backward-
   compatible way.

   Because TCP-ENO enables multiple different encryption specs to
   coexist, security could potentially be only as strong as the weakest
   available encryption spec.  For this reason, it is crucial for
   session IDs to depend on the TCP-ENO transcript in a strong way.
   Hence, encryption specs SHOULD compute session IDs using only well-
   studied and conservative hash functions.  Thus, even if an encryption
   spec is broken, and even if people deprecate it instead of disabling
   it, and even if an attacker tampers with ENO options to force
   negotiation of the broken spec, it should still be intractable for
   the attacker to induce identical session IDs at both hosts.

   Implementations MUST not send ENO options unless encryption specs
   have access to a strong source of randomness or pseudo-randomness.
   Without secret unpredictable data at both ends of a connection, it is
   impossible for encryption specs to meet the confidentiality and
   forward secrecy requirements required of them.

8.  IANA Considerations

   A new TCP option kind number needs to be assigned to ENO by IANA.

   In addition, IANA will need to maintain an ENO suboption registry
   mapping suboption "cs" values to encryption specs.

9.  Acknowledgments

   This work was funded by DARPA CRASH under contract #N66001-10-2-4088.

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10.  References

10.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, DOI 10.17487/RFC0793, September 1981,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

10.2.  Informative References

              Briscoe, B., "Inner Space for all TCP Options (Kitchen
              Sink Draft - to be Split Up)", draft-briscoe-tcpm-inspace-
              mode-tcpbis-00 (work in progress), March 2015.

              Touch, J. and W. Eddy, "TCP Extended Data Offset Option",
              draft-ietf-tcpm-tcp-edo-03 (work in progress), April 2015.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-07 (work in progress),
              July 2015.

              Touch, J. and T. Faber, "TCP SYN Extended Option Space
              Using an Out-of-Band Segment", draft-touch-tcpm-tcp-syn-
              ext-opt-02 (work in progress), April 2015.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6", RFC
              3493, DOI 10.17487/RFC3493, February 2003,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
              RFC5246, August 2008,

   [RFC6394]  Barnes, R., "Use Cases and Requirements for DNS-Based
              Authentication of Named Entities (DANE)", RFC 6394, DOI
              10.17487/RFC6394, October 2011,

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   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

Authors' Addresses

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

   Email: bittau@cs.stanford.edu

   Dan Boneh
   Stanford University
   353 Serra Mall, Room 475
   Stanford, CA  94305

   Email: dabo@cs.stanford.edu

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

   Email: dbg@scs.stanford.edu

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

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

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

   Email: dm@uun.org

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

   Email: eric.smith@kestrel.edu

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