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TCP-ENO: Encryption Negotiation Option
RFC 8547

Document Type RFC - Experimental (May 2019)
Authors Andrea Bittau, Daniel B. Giffin , Mark J. Handley , David Mazieres , Eric W. Smith
Last updated 2019-05-22
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Mirja K├╝hlewind
Send notices to (None)
RFC 8547
Internet Engineering Task Force (IETF)                         A. Bittau
Request for Comments: 8547                                        Google
Category: Experimental                                         D. Giffin
ISSN: 2070-1721                                      Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                     Stanford University
                                                                E. Smith
                                                       Kestrel Institute
                                                                May 2019

                 TCP-ENO: Encryption Negotiation Option


   Despite growing adoption of TLS, 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, thus
   making incremental deployment impossible.  Second, legacy
   applications themselves cannot always be upgraded and therefore
   require 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

Status of This Memo

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

   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

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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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Design Goals  . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  TCP-ENO Specification . . . . . . . . . . . . . . . . . . . .   6
     4.1.  ENO Option  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  The Global Suboption  . . . . . . . . . . . . . . . . . .   9
     4.3.  TCP-ENO Roles . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Specifying Suboption Data Length  . . . . . . . . . . . .  11
     4.5.  The Negotiated TEP  . . . . . . . . . . . . . . . . . . .  12
     4.6.  TCP-ENO Handshake . . . . . . . . . . . . . . . . . . . .  13
     4.7.  Data in SYN Segments  . . . . . . . . . . . . . . . . . .  14
     4.8.  Negotiation Transcript  . . . . . . . . . . . . . . . . .  16
   5.  Requirements for TEPs . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Session IDs . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Future Developments . . . . . . . . . . . . . . . . . . . . .  21
   8.  Design Rationale  . . . . . . . . . . . . . . . . . . . . . .  22
     8.1.  Handshake Robustness  . . . . . . . . . . . . . . . . . .  22
     8.2.  Suboption Data  . . . . . . . . . . . . . . . . . . . . .  22
     8.3.  Passive Role Bit  . . . . . . . . . . . . . . . . . . . .  22
     8.4.  Application-Aware Bit . . . . . . . . . . . . . . . . . .  23
     8.5.  Use of ENO Option Kind by TEPs  . . . . . . . . . . . . .  24
     8.6.  Unpredictability of Session IDs . . . . . . . . . . . . .  24
   9.  Experiments . . . . . . . . . . . . . . . . . . . . . . . . .  24
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

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1.  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.

   Introducing TCP options, extending operating system interfaces to
   support TCP-level encryption, and extending applications to take
   advantage of TCP-level encryption all require effort.  To the
   greatest extent possible, the effort invested in realizing TCP-level
   encryption today needs to remain applicable in the future should the
   need arise 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

   2.  How to perform encryption at the TCP layer

   This document addresses question 1 with a new TCP option, ENO.
   TCP-ENO provides a framework in which two endpoints can agree on a
   TCP encryption protocol (TEP) out of multiple possible TEPs.  For
   future compatibility, TEPs can vary widely in terms of wire format,
   use of TCP option space, and integration with the TCP header and
   segmentation.  However, ENO abstracts these differences to ensure the
   introduction of new TEPs can be transparent to applications taking
   advantage of TCP-level encryption.

   Question 2 is addressed by one or more companion TEP specification
   documents.  While current TEPs enable TCP-level traffic encryption
   today, TCP-ENO ensures that the effort invested to deploy today's
   TEPs will additionally benefit future ones.

1.1.  Design Goals

   TCP-ENO was designed to achieve the following goals:

   1.  Enable endpoints to negotiate the use of a separately specified
       TCP encryption protocol (TEP) suitable for either opportunistic
       security [RFC7435] of arbitrary TCP communications or stronger
       security of applications willing to perform endpoint

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

   3.  Provide out-of-band 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.  Define a standard negotiation transcript that TEPs can use to
       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 name
       each end of a connection for encryption or authentication
       purposes even following a symmetric simultaneous open.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  Terminology

   Throughout this document, we use the following terms, several of
   which have more detailed normative descriptions in [RFC793]:

   SYN segment
      A TCP segment in which the SYN flag is set

   ACK segment
      A TCP segment in which the ACK flag is set (which includes most
      segments other than an initial SYN segment)

   Non-SYN segment
      A TCP segment in which the SYN flag is clear

   SYN-only segment
      A TCP segment in which the SYN flag is set but the ACK flag is

   SYN-ACK segment
      A TCP segment in which the SYN and ACK flags are both set

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   Active opener
      A host that initiates a connection by sending a SYN-only segment.
      With the BSD socket API, an active opener calls "connect".  In
      client-server configurations, active openers are typically

   Passive opener
      A host that does not send a SYN-only segment but responds to one
      with a SYN-ACK segment.  With the BSD socket API, passive openers
      call "listen" and "accept", rather than "connect".  In client-
      server configurations, passive openers are typically servers.

   Simultaneous open
      The act of symmetrically establishing a TCP connection between two
      active openers (both of which call "connect" with BSD sockets).
      Each host of a simultaneous open sends both a SYN-only and a SYN-
      ACK segment.  Simultaneous open is less common than asymmetric
      open with one active and one passive opener, but it can be used
      for NAT traversal by peer-to-peer applications [RFC5382].

      A TCP encryption protocol intended for use with TCP-ENO and
      specified in a separate document

   TEP identifier
      A unique 7-bit value in the range 0x20-0x7f that IANA has assigned
      to a TEP

   Negotiated TEP
      The single TEP governing a TCP connection, determined by use of
      the TCP ENO option specified in this document

4.  TCP-ENO Specification

   TCP-ENO extends TCP connection establishment to enable encryption
   opportunistically.  It uses a new TCP option kind [RFC793] to
   negotiate one among multiple possible TCP encryption protocols
   (TEPs).  The negotiation involves hosts exchanging sets of supported
   TEPs, where each TEP is represented by a suboption within a larger
   TCP ENO option in the offering host's SYN segment.

   If TCP-ENO succeeds, it yields the following information:

   o  a negotiated TEP represented by a unique 7-bit TEP identifier,

   o  a few extra bytes of suboption data from each host, if needed by
      the TEP,

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   o  a negotiation transcript with which to mitigate attacks on the
      negotiation itself,

   o  role assignments designating one endpoint "host A" and the other
      endpoint "host B", and

   o  a bit available to higher-layer protocols at each endpoint for
      out-of-band negotiation of updated behavior in the presence of TCP

   If TCP-ENO fails, encryption is disabled and the connection falls
   back to traditional unencrypted TCP.

   The remainder of this section provides the normative description of
   the TCP ENO option and handshake protocol.

4.1.  ENO Option

   TCP-ENO employs an option in the TCP header [RFC793].  Figure 1
   illustrates the high-level format of this option.

         byte    0     1     2             N+1   (N+2 bytes total)
              |Kind=|Len= |                    |
              |  69 | N+2 | contents (N bytes) |

                       Figure 1: The TCP-ENO Option

   The contents of an ENO option can take one of two forms.  A SYN-form
   ENO option, illustrated in Figure 2, appears only in SYN segments.  A
   non-SYN-form ENO option, illustrated in Figure 3, appears only in
   non-SYN segments.  The SYN-form ENO option acts as a container for
   zero or more suboptions, labeled "Opt_0", "Opt_1", ... in Figure 2.
   The non-SYN-form ENO option, by its presence, acts as a one-bit
   acknowledgment, with the actual contents ignored by ENO.  Particular
   TEPs MAY assign additional meaning to the contents of non-SYN-form
   ENO options.  When a negotiated TEP does not assign such meaning, the
   contents of a non-SYN-form ENO option MUST be zero bytes (i.e.,
   N = 0) in sent segments and MUST be ignored in received segments.

         byte    0     1     2     3                     ... N+1
              |Kind=|Len= |Opt_0|Opt_1|       |Opt_i|   Opt_i   |
              |  69 | N+2 |     |     |       |     |   data    |

                       Figure 2: SYN-Form ENO Option

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                      byte   0     1     2     N+1
                          |Kind=|Len= |  ignored   |
                          |  69 | N+2 | by TCP-ENO |

            Figure 3: Non-SYN-Form ENO option, Where N MAY Be 0

   Every suboption starts with a byte of the form illustrated in
   Figure 4.  The high bit "v", when set, introduces suboptions with
   variable-length data.  When v = 0, the byte itself constitutes the
   entirety of the suboption.  The remaining 7-bit value, called "glt",
   takes on various meanings as defined below:

   o  Global configuration data (discussed in Section 4.2)

   o  Suboption data length for the next suboption (discussed in
      Section 4.4)

   o  An offer to use a particular TEP defined in a separate TEP
      specification document

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

          v   - non-zero for use with variable-length suboption data
          glt - Global suboption, Length, or TEP identifier

                Figure 4: Format of Initial Suboption Byte

   Table 1 summarizes the meaning of initial suboption bytes.  Values of
   glt below 0x20 are used for global suboptions and length information
   (the "gl" in "glt"), while those greater than or equal to 0x20 are
   TEP identifiers (the "t").  When v = 0, since the initial suboption
   byte constitutes the entirety of the suboption, all information is
   expressed by the 7-bit glt value, which can be either a global
   suboption or a TEP identifier.  When v = 1, it indicates a suboption
   with variable-length suboption data.  Only TEP identifiers have
   suboption data, not global suboptions.  Therefore, bytes with v = 1
   and glt < 0x20 are not global suboptions but rather length bytes
   governing the length of the next suboption (which MUST be a TEP
   identifier).  In the absence of a length byte, a TEP identifier
   suboption with v = 1 has suboption data extending to the end of the
   TCP option.

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       | glt       | v | Meaning                                   |
       | 0x00-0x1f | 0 | Global suboption (Section 4.2)            |
       | 0x00-0x1f | 1 | Length byte (Section 4.4)                 |
       | 0x20-0x7f | 0 | TEP identifier without suboption data     |
       | 0x20-0x7f | 1 | TEP identifier followed by suboption data |

                  Table 1: Initial Suboption Byte Values

   A SYN segment MUST contain at most one TCP ENO option.  If a SYN
   segment contains more than one ENO option, the receiver MUST behave
   as though the segment contained no ENO options and disable
   encryption.  A TEP MAY specify the use of multiple ENO options in a
   non-SYN segment.  For non-SYN segments, ENO itself only distinguishes
   between the presence or absence of ENO options; multiple ENO options
   are interpreted the same as one.

4.2.  The Global Suboption

   Suboptions 0x00-0x1f are used for global configuration that applies
   regardless of the negotiated TEP.  A TCP SYN segment MUST include at
   most one ENO suboption in this range.  A receiver MUST ignore all but
   the first suboption in this range in any given TCP segment so as to
   anticipate updates to ENO that assign new meaning to bits in
   subsequent global suboptions.  The value of a global suboption byte
   is interpreted as a bit mask, illustrated in Figure 5.

               bit   7   6   5   4   3   2   1   0
                   | 0 | 0 | 0 |z1 |z2 |z3 | a | b |

                   b  - Passive role bit
                   a  - Application-aware bit
                   z* - Zero bits (reserved for future use)

               Figure 5: Format of the Global Suboption Byte

   The fields of the bit mask are interpreted as follows:

      The passive role bit MUST be 1 for all passive openers.  For
      active openers, it MUST default to 0, but implementations MUST
      provide an API through which an application can explicitly set b =
      1 before initiating an active open.  (Manual configuration of "b"
      is only necessary to enable encryption with a simultaneous open

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      and requires prior coordination to ensure exactly one endpoint
      sets b = 1 before connecting.)  See Section 8.3 for further

      Legacy applications can benefit from ENO-specific updates that
      improve endpoint authentication or avoid double encryption.  The
      application-aware bit "a" is an out-of-band signal through which
      higher-layer protocols can enable ENO-specific updates that would
      otherwise not be backwards compatible.  Implementations MUST set
      this bit to zero by default, and MUST provide an API through which
      applications can change the value of the bit as well as examine
      the value of the bit sent by the remote host.  Implementations
      MUST furthermore support a mandatory application-aware mode in
      which TCP-ENO is automatically disabled if the remote host does
      not set a = 1.  See Section 8.4 for further discussion.

   z1, z2, z3
      The "z" bits are reserved for future updates to TCP-ENO.  They
      MUST be set to zero in sent segments and MUST be ignored in
      received segments.

   A SYN segment without an explicit global suboption has an implicit
   global suboption of 0x00.  Because passive openers MUST always set
   b = 1, they cannot rely on this implicit 0x00 byte and MUST include
   an explicit global suboption in their SYN-ACK segments.

4.3.  TCP-ENO Roles

   TCP-ENO uses abstract roles called "A" and "B" to distinguish the two
   ends of a TCP connection.  These roles are determined by the "b" bit
   in the global suboption.  The host that sent an implicit or explicit
   suboption with b = 0 plays the A role.  The host that sent b = 1
   plays the B role.  Because a passive opener MUST set b = 1 and an
   active opener by default has b = 0, the normal case is for the active
   opener to play role A and the passive opener role B.

   Applications performing a simultaneous open, if they desire TCP-level
   encryption, need to arrange for exactly one endpoint to set b = 1
   (despite being an active opener) while the other endpoint keeps the
   default b = 0.  Otherwise, if both sides use the default b = 0 or if
   both sides set b = 1, then TCP-ENO will fail and fall back to
   unencrypted TCP.  Likewise, if an active opener explicitly configures
   b = 1 and connects to a passive opener (which MUST always have
   b = 1), then TCP-ENO will fail and fall back to unencrypted TCP.

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   TEP specifications 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 roles A and B, respectively, in a connection.

4.4.  Specifying Suboption Data Length

   A TEP MAY optionally make use of one or more bytes of suboption data.
   The presence of such data is indicated by setting v = 1 in the
   initial suboption byte (see Figure 4).  A suboption introduced by a
   TEP identifier with v = 1 (i.e., a suboption whose first octet has
   value 0xa0 or higher) extends to the end of the TCP option.  Hence,
   if only one suboption requires data, the most compact way to encode
   it is to place it last in the ENO option, after all other suboptions.
   In Figure 2, for example, the last suboption, Opt_i, has suboption
   data and thus requires v = 1.  However, the suboption data length is
   inferred from the total length of the TCP option.

   When a suboption with data is not last in an ENO option, the sender
   MUST explicitly specify the suboption data length for the receiver to
   know where the next suboption starts.  The sender does so by
   introducing the suboption with a length byte, depicted in Figure 6.
   The length byte encodes a 5-bit value nnnnn.  Adding one to nnnnn
   yields the length of the suboption data (not including the length
   byte or the TEP identifier).  Hence, a length byte can designate
   anywhere from 1 to 32 bytes of suboption data (inclusive).

               bit   7   6   5   4   3   2   1   0
                   | 1   0   0         nnnnn       |

                   nnnnn - 5-bit value encoding (length - 1)

                     Figure 6: Format of a Length Byte

   A suboption preceded by a length byte MUST be a TEP identifier
   (glt >= 0x20) and MUST have v = 1.  Figure 7 shows an example of such
   a suboption.

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       byte    0      1       2      nnnnn+2  (nnnnn+3 bytes total)
            |length| TEP  | suboption data  |
            | byte |ident.| (nnnnn+1 bytes) |

            length byte    - specifies nnnnn
            TEP identifier - MUST have v = 1 and glt >= 0x20
            suboption data - length specified by nnnnn+1

                   Figure 7: Suboption with Length Byte

   A host MUST ignore an ENO option in a SYN segment and MUST disable
   encryption if either of the following apply:

   1.  A length byte indicates that suboption data would extend beyond
       the end of the TCP ENO option.

   2.  A length byte is followed by an octet in the range 0x00-0x9f
       (meaning the following byte has v = 0 or glt < 0x20).

   Because the last suboption in an ENO option is special-cased to have
   its length inferred from the 8-bit TCP option length, it MAY contain
   more than 32 bytes of suboption data.  Other suboptions are limited
   to 32 bytes by the length byte format.  However, the TCP header
   itself can only accommodate a maximum of 40 bytes of options.
   Therefore, regardless of the length byte format, a segment would not
   be able to contain more than one suboption over 32 bytes in size.
   That said, TEPs MAY define the use of multiple suboptions with the
   same TEP identifier in the same SYN segment, providing another way to
   convey over 32 bytes of suboption data even with length bytes.

4.5.  The Negotiated TEP

   A TEP identifier glt (with glt >= 0x20) is valid for a connection
   when all of the following hold:

   1.  Each side has sent a suboption for glt in its SYN-form ENO

   2.  Any suboption data in these glt suboptions is valid according to
       the TEP specification and satisfies any runtime constraints.

   3.  If an ENO option contains multiple suboptions with glt, then such
       repetition is well-defined by the TEP specification.

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   A passive opener (which is always host B) sees the remote host's SYN
   segment before constructing its own SYN-ACK segment.  Therefore, a
   passive opener SHOULD include only one TEP identifier in SYN-ACK
   segments and SHOULD ensure this TEP identifier is valid.  However,
   simultaneous open or implementation considerations can prevent host B
   from offering only one TEP.

   To accommodate scenarios in which host B sends multiple TEP
   identifiers in the SYN-ACK segment, the negotiated TEP is defined as
   the last valid TEP identifier in host B's SYN-form ENO option.  This
   definition means host B specifies TEP suboptions in order of
   increasing priority, while host A does not influence TEP priority.

4.6.  TCP-ENO Handshake

   A host employing TCP-ENO for a connection MUST include an ENO option
   in every TCP segment sent until either encryption is disabled or the
   host receives a non-SYN segment.  In particular, this means an active
   opener MUST include a non-SYN-form ENO option in the third segment of
   a three-way handshake.

   A host MUST disable encryption, refrain from sending any further ENO
   options, and fall back to unencrypted TCP if any of the following

   1.  Any segment it receives up to and including the first received
       ACK segment does not contain an ENO option (or contains an ill-
       formed SYN-form ENO option).

   2.  The SYN segment it receives does not contain a valid TEP

   3.  It receives a SYN segment with an incompatible global suboption.
       (Specifically, "incompatible" means the two hosts set the same
       "b" value, or the connection is in mandatory application-aware
       mode and the remote host set a = 0.)

   Hosts MUST NOT alter SYN-form ENO options in retransmitted segments,
   or between the SYN and SYN-ACK segments of a simultaneous open, with
   two exceptions for an active opener.  First, an active opener MAY
   unilaterally disable ENO (and thus remove the ENO option) between
   retransmissions of a SYN-only segment.  (Such removal could enable
   recovery from middleboxes dropping segments with ENO options.)
   Second, an active opener performing simultaneous open MAY include no
   TCP-ENO option in its SYN-ACK if the received SYN caused it to
   disable encryption according to the above rules (for instance,
   because role negotiation failed).

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   Once a host has both sent and received an ACK segment containing an
   ENO option, encryption MUST be enabled.  Once encryption is enabled,
   hosts MUST follow the specification of the negotiated TEP and MUST
   NOT present raw TCP payload data to the application.  In particular,
   data segments MUST NOT contain plaintext application data, but rather
   ciphertext, key negotiation parameters, or other messages as
   determined by the negotiated TEP.

   A host MAY send a SYN-form ENO option containing zero TEP identifier
   suboptions, which we term a "vacuous" ENO option.  If either host's
   SYN segment contains a vacuous ENO option, it follows that there are
   no valid TEP identifiers for the connection, and therefore the
   connection MUST fall back to unencrypted TCP.  Hosts MAY send vacuous
   ENO options to indicate that ENO is supported but unavailable by
   configuration, or to probe network paths for robustness to ENO
   options.  However, a passive opener MUST NOT send a vacuous ENO
   option in a SYN-ACK segment unless there was an ENO option in the SYN
   segment it received.  Moreover, a passive opener's SYN-form ENO
   option MUST still include a global suboption with b = 1 as discussed
   in Section 4.3.

4.7.  Data in SYN Segments

   TEPs MAY specify the use of data in SYN segments so as to reduce the
   number of round trips required for connection setup.  The meaning of
   data in a SYN segment with an ENO option (a SYN+ENO segment) is
   determined by the last TEP identifier in the ENO option, which we
   term the segment's "SYN TEP".  A SYN+ENO segment MAY of course
   include multiple TEP suboptions, but only the SYN TEP (i.e., the last
   one) specifies how to interpret the SYN segment's data payload.

   A host sending a SYN+ENO segment MUST NOT include data in the segment
   unless the SYN TEP's specification defines the use of such data.
   Furthermore, to avoid conflicting interpretations of SYN data, a
   SYN+ENO segment MUST NOT include a non-empty TCP Fast Open (TFO)
   option [RFC7413].

   Because a host can send SYN data before knowing which if any TEP the
   connection will negotiate, hosts implementing ENO are REQUIRED to
   discard data from SYN+ENO segments when the SYN TEP does not become
   the negotiated TEP.  Hosts are furthermore REQUIRED to discard SYN
   data in cases where another Internet standard specifies a conflicting
   interpretation of SYN data (as would occur when receiving a non-empty
   TFO option).  This requirement applies to hosts that implement ENO
   even when ENO has been disabled by configuration.  However, note that
   discarding SYN data is already common practice [RFC4987] and the new
   requirement applies only to segments containing ENO options.

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   More specifically, a host that implements ENO MUST discard the data
   in a received SYN+ENO segment if any of the following applies:

   o  ENO fails and TEP-indicated encryption is disabled for the

   o  The received segment's SYN TEP is not the negotiated TEP.

   o  The negotiated TEP does not define the use of SYN data.

   o  The SYN segment contains a non-empty TFO option or any other TCP
      option implying a conflicting definition of SYN data.

   A host discarding SYN data in compliance with the above requirement
   MUST NOT acknowledge the sequence number of the discarded data, but
   rather MUST acknowledge the other host's initial sequence number as
   if the received SYN segment contained no data.  Furthermore, after
   discarding SYN data, such a host MUST NOT assume the SYN data will be
   identically retransmitted, and MUST process data only from non-SYN

   If a host sends a SYN+ENO segment with data and receives
   acknowledgment for the data, but the SYN TEP in its transmitted SYN
   segment is not the negotiated TEP (either because a different TEP was
   negotiated or because ENO failed to negotiate encryption), then the
   host MUST abort the TCP connection.  Proceeding in any other fashion
   risks misinterpreted SYN data.

   If a host sends a SYN-only SYN+ENO segment bearing data and
   subsequently receives a SYN-ACK segment without an ENO option, that
   host MUST abort the connection even if the SYN-ACK segment does not
   acknowledge the SYN data.  The issue is that unacknowledged data
   could nonetheless have been cached by the receiver; later
   retransmissions intended to supersede this unacknowledged data could
   fail to do so if the receiver gives precedence to the cached original
   data.  Implementations MAY provide an API call for a non-default mode
   in which unacknowledged SYN data does not cause a connection abort,
   but applications MUST use this mode only when a higher-layer
   integrity check would anyway terminate a garbled connection.

   To avoid unexpected connection aborts, ENO implementations MUST
   disable the use of data in SYN-only segments by default.  Such data
   MAY be enabled by an API command.  In particular, implementations MAY
   provide a per-connection mandatory encryption mode that automatically
   aborts a connection if ENO fails, and they MAY enable SYN data in
   this mode.

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   To satisfy the requirement of the previous paragraph, all TEPs SHOULD
   support a normal mode of operation that avoids data in SYN-only
   segments.  An exception is TEPs intended to be disabled by default.

4.8.  Negotiation Transcript

   To defend against attacks on encryption negotiation itself, a TEP
   MUST, with high probability, fail to establish a working connection
   between two ENO-compliant hosts when SYN-form ENO options have been
   altered in transit.  (Of course, in the absence of endpoint
   authentication, two compliant hosts can each still be connected to a
   man-in-the-middle attacker.)  To detect SYN-form ENO option
   tampering, TEPs MUST reference a transcript of TCP-ENO's negotiation.

   TCP-ENO defines its negotiation transcript as a packed data structure
   consisting of two TCP-ENO options exactly as they appeared in the TCP
   header (including the TCP option kind and TCP option length byte as
   illustrated in Figure 1).  The transcript is constructed from the
   following, in order:

   1.  The TCP-ENO option in host A's SYN segment, including the kind
       and length bytes

   2.  The TCP-ENO option in host B's SYN segment, including the kind
       and length bytes

   Note that because the ENO options in the transcript contain length
   bytes as specified by TCP, the transcript unambiguously delimits A's
   and B's ENO options.

5.  Requirements for TEPs

   TCP-ENO affords TEP specifications a large amount of design
   flexibility.  However, to abstract TEP differences away from
   applications requires fitting them all into a coherent framework.  As
   such, any TEP claiming an ENO TEP identifier MUST satisfy the
   following normative list of properties:

   o  TEPs MUST protect TCP data streams with authenticated encryption.
      (Note that "authenticated encryption" refers only to the form of
      encryption, such as an Authenticated Encryption with Associated
      Data (AEAD) algorithm meeting the requirements of [RFC5116]; it
      does not imply endpoint authentication.)

   o  TEPs 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 TEP.  Section 5.1
      describes the requirements of the session ID in more detail.

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   o  TEPs MUST NOT make data confidentiality dependent on encryption
      algorithms with a security strength [NIST-SP-800-57] of less than
      120 bits.  The number 120 was chosen to accommodate ciphers with
      128-bit keys that lose a few bits of security either to
      particularities of the key schedule or to highly theoretical and
      unrealistic attacks.

   o  TEPs 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  TEPs MUST guarantee the confidentiality of TCP streams without
      assuming the security of any long-lived secrets.  Implementations
      SHOULD provide forward secrecy soon after the close of a TCP
      connection and SHOULD therefore bound the delay between closing a
      connection and erasing any relevant cryptographic secrets.
      (Exceptions to forward secrecy are permissible only at the
      implementation level and only in response to hardware or
      architectural constraints -- e.g., storage that cannot be securely

   o  TEPs MUST protect and authenticate the end-of-file marker conveyed
      by TCP's FIN flag.  In particular, a receiver MUST, with
      overwhelming probability, detect a FIN flag that was set or
      cleared in transit and does not match the sender's intent.  A TEP
      MAY discard a segment with such a corrupted FIN bit or MAY abort
      the connection in response to such a segment.  However, any such
      abort MUST raise an error condition distinct from an authentic
      end-of-file condition.

   o  TEPs MUST prevent corrupted packets from causing urgent data to be
      delivered when none has been sent.  There are several ways to do
      so.  For instance, a TEP MAY cryptographically protect the URG
      flag and urgent pointer alongside ordinary payload data.
      Alternatively, a TEP MAY disable urgent data functionality by
      clearing the URG flag on all received segments and returning
      errors in response to sender-side urgent-data API calls.
      Implementations SHOULD avoid negotiating TEPs that disable urgent
      data by default.  The exception is when applications and protocols
      are known never to send urgent data.

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5.1.  Session IDs

   Each TEP MUST define a session ID that is computable by both
   endpoints and uniquely identifies each encrypted TCP connection.
   Implementations MUST expose the session ID to applications via an API
   extension.  The API extension MUST return an error when no session ID
   is available because ENO has failed to negotiate encryption or
   because no connection is yet established.  Applications that are
   aware of TCP-ENO SHOULD, when practical, authenticate the TCP
   endpoints by incorporating the values of the session ID and TCP-ENO
   role (A or B) into higher-layer authentication mechanisms.

   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 violating the TEP
   specification, to establish two TCP connections with the same session
   ID to remote hosts properly implementing the TEP.

   To prevent session IDs from being confused across TEPs, all session
   IDs begin with the negotiated TEP identifier -- that is, the last
   valid TEP identifier in host B's SYN segment.  Furthermore, this
   initial byte has bit "v" set to the same value that accompanied the
   negotiated TEP identifier in B's SYN segment.  However, only this
   single byte is included, not any suboption data.  Figure 8 shows the
   resulting format.  This format is designed for TEPs to compute unique
   identifiers; it is not intended for application authors to pick apart
   session IDs.  Applications SHOULD treat session IDs as monolithic
   opaque values and SHOULD NOT discard the first byte to shorten
   identifiers.  (An exception is for non-security-relevant purposes,
   such as gathering statistics about negotiated TEPs.)

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

                     Figure 8: Format of a Session ID

   Though TEP specifications retain considerable flexibility in their
   definitions of the session ID, all session IDs MUST meet the
   following normative list of requirements:

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

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   o  The session ID MUST depend, in a collision-resistant way, on all
      of the following (meaning it is computationally infeasible to
      produce collisions of the session ID derivation function unless
      all of the following quantities are identical):

      *  Fresh data contributed by both sides of the connection

      *  Any public keys, public Diffie-Hellman parameters, or other
         public asymmetric cryptographic parameters that are employed by
         the TEP and have corresponding private data that is known by
         only one side of the connection

      *  The negotiation transcript specified in Section 4.8

   o  Unless and until applications disclose information about the
      session ID, all but the first byte MUST be computationally
      indistinguishable from random bytes to a network eavesdropper.

   o  Applications MAY choose to make session IDs public.  Therefore,
      TEPs MUST NOT place any confidential data in the session ID (such
      as data permitting the derivation of session keys).

6.  Examples

   This subsection illustrates the TCP-ENO handshake with a few non-
   normative examples.

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

     Figure 9: Three-Way Handshake with Successful TCP-ENO Negotiation

   Figure 9 shows a three-way handshake with a successful TCP-ENO
   negotiation.  Host A includes two ENO suboptions with TEP identifiers
   X and Y.  Host A does not include an explicit global suboption, which
   means it has an implicit global suboption 0x00 conveying passive role
   bit b = 0.  The two sides agree to follow the TEP 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 10: Three-Way Handshake with Failed TCP-ENO Negotiation

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   Figure 10 shows a failed TCP-ENO negotiation.  The active opener (A)
   indicates support for TEPs corresponding to suboptions X and Y.
   Unfortunately, at this point, one of several things occurs:

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

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

   3.  B supports TCP-ENO but has the connection configured in mandatory
       application-aware mode and thus disables ENO because A's SYN
       segment contains an implicit global suboption with a = 0.

   4.  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<b=1,X> [ENO stripped by middlebox]
       (3) A -> B:  ACK
       [rest of connection unencrypted legacy TCP]

     Figure 11: Failed TCP-ENO Negotiation Because of Option Stripping

   Figure 11 Shows another handshake with a failed encryption
   negotiation.  In this case, the passive opener (B) receives an ENO
   option from A and replies.  However, the reverse network path from B
   to A strips ENO options.  Therefore, A does not receive an ENO option
   from B, it disables ENO, and it does not include a non-SYN-form ENO
   option in segment 3 when ACKing B's SYN.  Had A not disabled
   encryption, Section 4.6 would have required it to include a non-SYN-
   form ENO option in segment 3.  The omission of this option informs B
   that encryption negotiation has failed, after which the two hosts
   proceed with unencrypted TCP.

             (1) A -> B:  SYN      ENO<Y,X>
             (2) B -> A:  SYN      ENO<b=1,X,Y,Z>
             (3) A -> B:  SYN-ACK  ENO<Y,X>
             (4) B -> A:  SYN-ACK  ENO<b=1,X,Y,Z>
             [rest of connection encrypted according to TEP Y]

     Figure 12: Simultaneous Open with Successful TCP-ENO Negotiation

   Figure 12 shows a successful TCP-ENO negotiation with simultaneous
   open.  Here, the first four segments contain a SYN-form ENO option,
   as each side sends both a SYN-only and a SYN-ACK segment.  The ENO

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   option in each host's SYN-ACK is identical to the ENO option in its
   SYN-only segment, as otherwise, connection establishment could not
   recover from the loss of a SYN segment.  The last valid TEP in host
   B's ENO option is Y, so Y is the negotiated TEP.

7.  Future Developments

   TCP-ENO is designed to capitalize on future developments that could
   alter trade-offs and change the best approach to TCP-level encryption
   (beyond introducing new cipher suites).  By way of example, we
   discuss a few such possible developments.

   Various proposals exist to increase the maximum space for options in
   the TCP header.  These proposals are highly experimental --
   particularly those that apply to SYN segments.  Therefore, future
   TEPs are unlikely to benefit from extended SYN option space.  In the
   unlikely event that SYN option space is one day extended, however,
   future TEPs could benefit by embedding key agreement messages
   directly in SYN segments.  Under such usage, the 32-byte limit on
   length bytes could prove insufficient.  This document intentionally
   aborts TCP-ENO if a length byte is followed by an octet in the range
   0x00-0x9f.  If necessary, a future update to this document can define
   a format for larger suboptions by assigning meaning to such currently
   undefined byte sequences.

   New revisions to socket interfaces [RFC3493] could involve library
   calls that simultaneously have access to hostname information and an
   underlying TCP connection.  Such an API enables the possibility of
   authenticating servers transparently to the application, particularly
   in conjunction with technologies such as DNS-Based Authentication of
   Named Entities (DANE) [RFC6394].  An update to TCP-ENO can adopt one
   of the "z" bits in the global suboption to negotiate the use of an
   endpoint authentication protocol before any application use of the
   TCP connection.  Over time, the consequences of failed or missing
   endpoint authentication can gradually be increased from issuing log
   messages to aborting the connection if some as yet unspecified DNS
   record indicates authentication is mandatory.  Through shared library
   updates, such endpoint authentication can potentially be added
   transparently to legacy applications without recompilation.

   TLS can currently only be added to legacy applications whose
   protocols accommodate a STARTTLS command or equivalent.  TCP-ENO,
   because it provides out-of-band signaling, opens the possibility of
   future TLS revisions being generically applicable to any TCP

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8.  Design Rationale

   This section describes some of the design rationale behind TCP-ENO.

8.1.  Handshake Robustness

   Incremental deployment of TCP-ENO depends critically on failure cases
   devolving to unencrypted TCP rather than causing the entire TCP
   connection to fail.

   Because a network path might drop ENO options in one direction only,
   a host needs to know not just that the peer supports encryption, but
   that the peer has received an ENO option.  To this end, ENO disables
   encryption unless it receives an ACK segment bearing an ENO option.
   To stay robust in the face of dropped segments, hosts continue to
   include non-SYN-form ENO options in segments until the point that
   they have received a non-SYN segment from the other side.

   One particularly pernicious middlebox behavior found in the wild is
   load balancers that echo unknown TCP options found in SYN segments
   back to an active opener.  The passive role bit "b" in global
   suboptions ensures encryption will always be disabled under such
   circumstances, as sending back a verbatim copy of an active opener's
   SYN-form ENO option always causes role negotiation to fail.

8.2.  Suboption Data

   TEPs can employ suboption data for session caching, cipher suite
   negotiation, or other purposes.  However, TCP currently limits total
   option space consumed by all options to only 40 bytes, making it
   impractical to have many suboptions with data.  For this reason, ENO
   optimizes the case of a single suboption with data by inferring the
   length of the last suboption from the TCP option length.  Doing so
   saves one byte.

8.3.  Passive Role Bit

   TCP-ENO, TEPs, and applications all have asymmetries that require an
   unambiguous way to identify one of the two connection endpoints.  As
   an example, Section 4.8 specifies that host A's ENO option comes
   before host B's in the negotiation transcript.  As another example,
   an application might need to authenticate one end of a TCP connection
   with a digital signature.  To ensure the signed message cannot be
   interpreted out of context to authenticate the other end, the signed
   message would need to include both the session ID and the local role,
   A or B.

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   A normal TCP three-way handshake involves one active and one passive
   opener.  This asymmetry is captured by the default configuration of
   the "b" bit in the global suboption.  With simultaneous open, both
   hosts are active openers, so TCP-ENO requires that one host
   explicitly configure b = 1.  An alternate design might automatically
   break the symmetry to avoid this need for explicit configuration.
   However, all such designs we considered either lacked robustness or
   consumed precious bytes of SYN option space even in the absence of
   simultaneous open.  (One complicating factor is that TCP does not
   know it is participating in a simultaneous open until after it has
   sent a SYN segment.  Moreover, with packet loss, one host might never
   learn it has participated in a simultaneous open.)

8.4.  Application-Aware Bit

   Applications developed before TCP-ENO can potentially evolve to take
   advantage of TCP-level encryption.  For instance, an application
   designed to run only on trusted networks might leverage TCP-ENO to
   run on untrusted networks, but, importantly, needs to authenticate
   endpoints and session IDs to do so.  In addition to user-visible
   changes such as requesting credentials, this kind of authentication
   functionality requires application-layer protocol changes.  Some
   protocols can accommodate the requisite changes -- for instance, by
   introducing a new verb analogous to STARTTLS, while others cannot do
   so in a backwards-compatible manner.

   The application-aware bit "a" in the global suboption provides a
   means of incrementally deploying enhancements specific to TCP-ENO to
   application-layer protocols that would otherwise lack the necessary
   extensibility.  Software implementing the enhancement always sets a =
   1 in its own global suboption, but only activates the new behavior
   when the other end of the connection also sets a = 1.

   A related issue is that an application might leverage TCP-ENO as a
   replacement for legacy application-layer encryption.  In this
   scenario, if both endpoints support TCP-ENO, then application-layer
   encryption can be disabled in favor of simply authenticating the TCP-
   ENO session ID.  On the other hand, if one endpoint is not aware of
   the new mode of operation specific to TCP-ENO, there is little
   benefit to performing redundant encryption at the TCP layer; data is
   already encrypted once at the application layer, and authentication
   only has meaning with respect to this application-layer encryption.
   The mandatory application-aware mode lets applications avoid double
   encryption in this case: the mode sets a = 1 in the local host's
   global suboption but also disables TCP-ENO entirely in the event that
   the other side has not also set a = 1.

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   Note that the application-aware bit is not needed by applications
   that already support adequate higher-layer encryption such as those
   provided by TLS [RFC8446] or SSH [RFC4253].  To avoid double
   encryption in such cases, it suffices to disable TCP-ENO by
   configuration on any ports with known secure protocols.

8.5.  Use of ENO Option Kind by TEPs

   This document does not specify the use of ENO options beyond the
   first few segments of a connection.  Moreover, it does not specify
   the content of ENO options in non-SYN segments, only their presence.
   As a result, any use of option kind 69 after the SYN exchange does
   not conflict with this document.  In addition, because ENO guarantees
   at most one negotiated TEP per connection, TEPs will not conflict
   with one another or ENO if they use option kind 69 for out-of-band
   signaling in non-SYN segments.

8.6.  Unpredictability of Session IDs

   Section 5.1 specifies that all but the first (TEP identifier) byte of
   a session ID MUST be computationally indistinguishable from random
   bytes to a network eavesdropper.  This property is easy to ensure
   under standard assumptions about cryptographic hash functions.  Such
   unpredictability helps security in a broad range of cases.  For
   example, it makes it possible for applications to use a session ID
   from one connection to authenticate a session ID from another,
   thereby tying the two connections together.  It furthermore helps
   ensure that TEPs do not trivially subvert the 33-byte minimum-length
   requirement for session IDs by padding shorter session IDs with

9.  Experiments

   This document has experimental status because TCP-ENO's viability
   depends on middlebox behavior that can only be determined a
   posteriori.  Specifically, we need to determine 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 middleboxes perform deep packet inspection and
       start dropping segments that unexpectedly contain ciphertext, or

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       if middleboxes strip ENO options from non-SYN segments after
       allowing them in SYN segments.

   Type-1 failures are tolerable since TCP-ENO is designed for
   incremental deployment anyway.  Type-2 failures are more problematic,
   and, if prevalent, will require the development of techniques to
   avoid and recover from such failures.  The experiment will succeed so
   long as we can avoid type-2 failures and find sufficient use cases
   that avoid type-1 failures (possibly along with a gradual path for
   further reducing type-1 failures).

   In addition to the question of basic viability, deploying TCP-ENO
   will allow us to identify and address other potential corner cases or
   relaxations.  For example, does the slight decrease in effective TCP
   segment payload pose a problem to any applications, which would
   require restrictions on how TEPs interpret socket buffer sizes?
   Conversely, can we relax the prohibition on default TEPs that disable
   urgent data?

   A final important metric, related to the pace of deployment and
   incidence of type-1 failures, will be the extent to which
   applications adopt enhancements specific to TCP-ENO for endpoint

10.  Security Considerations

   An obvious use case for TCP-ENO is opportunistic encryption, e.g.,
   encrypting some connections, but only where supported and without any
   kind of endpoint authentication.  Opportunistic encryption provides a
   property known as "opportunistic security" [RFC7435], which protects
   against undetectable large-scale eavesdropping.  However, it does not
   protect against detectable large-scale eavesdropping (for instance,
   if ISPs terminate TCP connections and proxy them or simply downgrade
   connections to unencrypted).  Moreover, opportunistic encryption
   emphatically does not protect against targeted attacks that employ
   trivial spoofing to redirect a specific high-value connection to a
   man-in-the-middle attacker.  Hence, the mere presence of TEP-
   indicated encryption does not suffice for an application to represent
   a connection as secure to the user.

   Achieving stronger security with TCP-ENO requires verifying session
   IDs.  Any application relying on ENO for communication security MUST
   incorporate session IDs into its endpoint authentication.  By way of
   example, an authentication mechanism based on keyed digests (such as
   Digest Access Authentication [RFC7616]) can be extended to include
   the role and session ID in the input of the keyed digest.
   Authentication mechanisms with a notion of channel binding (such as
   Salted Challenge Response Authentication Mechanism (SCRAM) [RFC5802])

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   can be updated to derive a channel binding from the session ID.
   Higher-layer protocols MAY use the application-aware "a" bit to
   negotiate the inclusion of session IDs in authentication even when
   there is no in-band way to carry out such a negotiation.  Because
   there is only one "a" bit, however, a protocol extension that
   specifies use of the "a" bit will likely require a built-in
   versioning or negotiation mechanism to accommodate crypto agility and
   future updates.

   Because TCP-ENO enables multiple different TEPs to coexist, security
   could potentially be only as strong as the weakest available TEP.  In
   particular, if TEPs use a weak hash function to incorporate the TCP-
   ENO transcript into session IDs, then an attacker can undetectably
   tamper with ENO options to force negotiation of a deprecated and
   vulnerable TEP.  To avoid such problems, security reviewers of new
   TEPs SHOULD pay particular attention to the collision resistance of
   hash functions used for session IDs (including the state of
   cryptanalysis and research into possible attacks).  Even if other
   parts of a TEP rely on more esoteric cryptography that turns out to
   be vulnerable, it ought nonetheless to be intractable for an attacker
   to induce identical session IDs at both ends after tampering with ENO
   contents in SYN segments.

   Implementations MUST NOT send ENO options unless they have access to
   an adequate source of randomness [RFC4086].  Without secret
   unpredictable data at both ends of a connection, it is impossible for
   TEPs to achieve confidentiality and forward secrecy.  Because systems
   typically have very little entropy on bootup, implementations might
   need to disable TCP-ENO until after system initialization.

   With a regular three-way handshake (meaning no simultaneous open),
   the non-SYN-form ENO option in an active opener's first ACK segment
   MAY contain N > 0 bytes of TEP-specific data, as shown in Figure 3.
   Such data is not part of the TCP-ENO negotiation transcript and
   therefore MUST be separately authenticated by the TEP.

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11.  IANA Considerations

   This document defines a new TCP option kind for TCP-ENO, assigned a
   value of 69 from the TCP option space.  This value is defined as:

     | Kind | Length | Meaning                          | Reference |
     | 69   | N      | Encryption Negotiation (TCP-ENO) | RFC 8547  |

                     Table 2: TCP Option Kind Numbers

   Early implementations of TCP-ENO and a predecessor TCP encryption
   protocol made unauthorized use of TCP option kind 69.  These earlier
   uses of option 69 are not compatible with TCP-ENO and could disable
   encryption or suffer complete connection failure when interoperating
   with TCP-ENO-compliant hosts.  Hence, legacy use of option 69 MUST be
   disabled on hosts that cannot be upgraded to TCP-ENO.  More recent
   implementations used experimental option 253 per [RFC6994] with
   16-bit ExID 0x454E.  Current and new implementations of TCP-ENO MUST
   use option 69, while any legacy implementations MUST migrate to
   option 69.  Note in particular that Section 4.1 requires at most one
   SYN-form ENO option per segment, which means hosts MUST NOT include
   both option 69 and option 253 with ExID 0x454E in the same TCP

   This document defines a 7-bit glt field in the range of 0x20-0x7f.
   IANA has created and will maintain a new registry titled "TCP
   Encryption Protocol Identifiers" under the "Transmission Control
   Protocol (TCP) Parameters" registry.  Table 3 shows the initial
   contents of this registry.  This document allocates one TEP
   identifier (0x20) for experimental use.  In case the TEP identifier
   space proves too small, identifiers in the range 0x70-0x7f are
   reserved to enable a future update to this document to define
   extended identifier values.  Future assignments are to be made upon
   satisfying either of two policies defined in [RFC8126]: "IETF Review"
   or (for non-IETF stream specifications) "Expert Review with RFC
   Required".  IANA will furthermore provide early allocation [RFC7120]
   to facilitate testing before RFCs are finalized.

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RFC 8547            TCP Encryption Negotiation Option           May 2019

         | Value     | Meaning                      | Reference |
         | 0x20      | Experimental Use             | RFC 8547  |
         | 0x70-0x7f | Reserved for extended values | RFC 8547  |

               Table 3: TCP Encryption Protocol Identifiers

12.  References

12.1.  Normative References

              National Institute of Standards and Technology,
              "Recommendation for Key Management - Part 1: General",
              NIST Special Publication, 800-57, Revision 4,
              DOI 10.6028/NIST.SP.800-57pt1r4, January 2016,

   [RFC793]   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,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <>.

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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12.2.  Informative References

   [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,

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,

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

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,

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

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

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RFC 8547            TCP Encryption Negotiation Option           May 2019

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,


   We are grateful for contributions, help, discussions, and feedback
   from the IETF and its TCPINC Working Group, including Marcelo
   Bagnulo, David Black, Bob Briscoe, Benoit Claise, Spencer Dawkins,
   Jake Holland, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Watson
   Ladd, Kathleen Moriarty, Yoav Nir, Christoph Paasch, Eric Rescorla,
   Adam Roach, Kyle Rose, Michael Scharf, Joe Touch, and Eric Vyncke.
   This work was partially funded by DARPA CRASH and the Stanford Secure
   Internet of Things Project.


   Dan Boneh was a coauthor of the draft that became this document.

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RFC 8547            TCP Encryption Negotiation Option           May 2019

Authors' Addresses

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


   Daniel B. Giffin
   Stanford University
   353 Serra Mall, Room 288
   Stanford, CA  94305
   United States of America


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


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


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


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