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Network Working Group                                          A. Bittau
Internet-Draft                                                    Google
Intended status: Experimental                                   D. Boneh
Expires: April 28, 2017                                        D. Giffin
                                                     Stanford University
                                                              M. Handley
                                               University College London
                                                             D. Mazieres
                                                     Stanford University
                                                                E. Smith
                                                       Kestrel Institute
                                                        October 25, 2016

                 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 April 28, 2017.

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Copyright Notice

   Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Design goals  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  TCP-ENO specification . . . . . . . . . . . . . . . . . . . .   5
     4.1.  ENO option  . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  The global suboption  . . . . . . . . . . . . . . . . . .   9
     4.3.  TCP-ENO roles . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Specifying suboption data length  . . . . . . . . . . . .  10
     4.5.  The negotiated TEP  . . . . . . . . . . . . . . . . . . .  12
     4.6.  TCP-ENO handshake . . . . . . . . . . . . . . . . . . . .  12
     4.7.  Data in SYN segments  . . . . . . . . . . . . . . . . . .  13
     4.8.  Negotiation transcript  . . . . . . . . . . . . . . . . .  15
   5.  Requirements for TEPs . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Session IDs . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   7.  Design rationale  . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Future developments . . . . . . . . . . . . . . . . . . .  19
     7.2.  Handshake robustness  . . . . . . . . . . . . . . . . . .  20
     7.3.  Suboption data  . . . . . . . . . . . . . . . . . . . . .  21
     7.4.  Passive role bit  . . . . . . . . . . . . . . . . . . . .  21
     7.5.  Option kind sharing . . . . . . . . . . . . . . . . . . .  21
   8.  Experiments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  22
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  24
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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1.  Requirements language

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

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.

   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, and

   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 one
   among multiple possible TCP encryption protocols or _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.

2.1.  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 or _TEP_.

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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.  Provide a standard negotiation transcript through which TEPs 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 name
       each end of a connection for encryption or authentication
       purposes even following a symmetric simultaneous open.

3.  Terminology

   We define the following terms, which are used throughout this

   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

   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

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      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 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 to negotiate one
   among multiple possible TCP encryption protocols or 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,

   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 indicating whether or not the application at each end knows
      it is using TCP-ENO.

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   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 [RFC0793].  There are two
   equivalent kinds of ENO option, shown in Figure 1.  Section 10
   specifies which of the two kinds is permissible and/or preferred.

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

   byte    0     1     2     3     4             N+3   (N+4 bytes total)
        |Kind=|Len= |   ExID    |                    |
        | 253 | N+4 | 69  | 78  | contents (N bytes) |

             Figure 1: Two equivalent kinds of TCP-ENO option

   The contents of an ENO option can take one of two forms.  A SYN form,
   illustrated in Figure 2, appears only in SYN segments.  A non-SYN
   form, illustrated in Figure 3, appears only in non-SYN segments.  The
   SYN form of ENO acts as a container for zero or more suboptions,
   labeled "Opt_0", "Opt_1", ... in Figure 2.  The non-SYN form, 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 ENO options.  When a negotiated TEP does not
   assign such meaning, the contents of a non-SYN ENO option MUST be
   zero bytes.

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

   byte    0     1     2     3     4     5                     ... N+3
        |Kind=|Len= |   ExID    |Opt_0|Opt_1|       |Opt_i|   Opt_i   |
        | 253 | N+4 | 69  | 78  |     |     |       |     |   data    |

                         Figure 2: SYN form of ENO

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

                byte   0     1     2     3     4     N+3
                    |Kind=|Len= |   ExID    |  ignored   |
                    | 253 | N+4 | 69  | 78  | by TCP-ENO |

              Figure 3: Non-SYN form of ENO, 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 7-bit value "cs" expresses one of:

   o  Global configuration data (discussed in Section 4.2),

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

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

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      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", the initial
   suboption byte constitutes the entirety of the suboption and all
   information is expressed by the 7-bit "glt" value, which can be
   either a global suboption or TEP identifier.  When "v = 1", it
   indicates a suboption with variable-length suboption data.  Only TEP
   identifiers may have suboption data, not global suboptions.  Hence,
   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 identifer).  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.

       | 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.

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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 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 bitmask,
   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 bitmask 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 set "b = 1" before
      initiating an active open.  (Manual configuration of "b" is
      necessary for simultaneous open.)

      The application-aware bit "a" is an out-of-band signal indicating
      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.  Implementations MUST set this bit to 0 by default,
      and SHOULD 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 SHOULD furthermore support a
      _mandatory_ application-aware mode in which TCP-ENO is
      automatically disabled if the remote host does not set "a = 1".

   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

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   = 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 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"

   If both sides of a connection set "b = 1" (which can happen if the
   active opener misconfigures "b" before calling "connect"), or both
   sides set "b = 0" (which can happen with simultaneous open), then
   TCP-ENO MUST be disabled and the connection MUST fall back to
   unencrypted TCP.

   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 A and B roles, 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).  By default, suboption data
   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.  As an example, in
   Figure 2, the last suboption, "Opt_i", has suboption data and thus
   requires "v = 1"; however, the suboption data length can be 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
   preceding 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).

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               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

       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:

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

   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.  The TCP header itself can
   only accommodate a maximum of 40 bytes of options per segment,
   however, so regardless of the length byte could not fit more than one
   suboption over 32 bytes.  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.

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4.5.  The negotiated TEP

   A TEP identifier "glt" (with "glt >= 0x20") is _valid_ for a
   connection when:

   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.

   The _negotiated TEP_ is 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.

   A passive opener (which is always host B) sees the remote host's SYN
   segment before constructing its own SYN-ACK.  Hence, 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.

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.

   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 a ENO option (or contains an ill-
       formed SYN-form ENO option),

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

   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".)

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   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 be useful
   if middleboxes are dropping segments with the ENO option.)  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).

   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 negotiated TEP.

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 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 option 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 will
   govern a connection, hosts implementing ENO are REQUIRED to discard
   data from SYN+ENO segments when the SYN TEP does not govern the
   connection or when there is any ambiguity over the meaning of the SYN
   data.  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.

   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

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

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

   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 governing the data is
   not the negotiated TEP (either because a different TEP was negotiated
   or because ENO failed to negotiate encryption), then the host MUST
   reset 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 reset the connection even if the SYN-ACK segment does not
   acknowledge the SYN data.  The issue is that unacknowledged data may
   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 reset, but
   applications MUST only use this mode when a higher-layer integrity
   check would anyway terminate a garbled connection.

   To avoid unexpected connection resets, 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
   resets a connection if ENO fails, and MAY enable SYN data in this

   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.

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4.8.  Negotiation transcript

   To defend against attacks on encryption negotiation itself, TEPs need
   a way to reference a transcript of TCP-ENO's negotiation.  In
   particular, a TEP MUST with high probability fail to reach key
   agreement between two honest endpoints if the TEP's selection
   resulted from tampering with the contents of SYN-form ENO options.
   (Of course, in the absence of endpoint authentication, two honest
   endpoints can still each end up talking to a man-in-the-middle
   attacker rather than to each other.)

   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, TCP option length byte, and,
   for option kind 253, the bytes 69 and 78 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.

   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.

   o  TEPs MUST NOT permit the negotiation of any encryption algorithms
      with significantly less than 128-bit security.

   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.)

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   o  TEPs MUST NOT depend on long-lived secrets for data
      confidentiality, as implementations SHOULD provide forward secrecy
      some bounded, short time after the close of a TCP connection.
      (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 high
      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

   o  TEPs MUST prevent corrupted packets from causing urgent data to be
      delivered when none has been sent.  A TEP MAY do so by
      cryptographically protecting 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.

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.  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

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   TEP 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 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.

   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, and

      *  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).

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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.  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

   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 TEPs X and Y, and so
       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 does not set the application-aware bit, or

   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 network filtering

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   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.  Hence, A does not receive an ENO option
   from B, disables ENO, and does not include a non-SYN form ENO option
   when ACKing B's SYN 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<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 MUST contain a SYN-form ENO
   option, as each side sends both a SYN-only and a SYN-ACK segment.
   The ENO 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.  Design rationale

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

7.1.  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 option space in TCP [I-D.ietf-tcp
   yn-ext-opt].  If SYN segments gain large options, it becomes possible
   to fit public keys or Diffie-Hellman parameters into SYN segments.
   Future TEPs can take advantage of this by performing key agreement
   directly within suboption data, both simplifying protocols and
   reducing the number of round trips required for connection setup.

   If TCP gains large SYN option support, the 32-byte limit on length
   bytes may prove problematic.  This draft intentionally aborts TCP-ENO
   if a length byte is followed by an octet in the range 0x00-0x9f.  Any
   document updating TCP's option size limit can also enable larger

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   suboptions by updating this draft to assign 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 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

7.2.  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 may drop ENO options in one direction only, a
   host must 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 must continue
   to include non-SYN form ENO options in segments until such point as
   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.

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7.3.  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.

7.4.  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 not 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.

   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 manually
   configure "b = 1".  An alternate design might automatically break the
   symmetry to avoid this need for manual 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.)

7.5.  Option kind sharing

   This draft 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 TBD (or option kind 253 with ExID
   0x454E) after the SYN exchange does not conflict with this document.
   Because in addition ENO guarantees at most one negotiated TEP per
   connection, TEPs will not conflict with one another or ENO if they
   use ENO's option kind for out-of-band signaling in non-SYN segments.

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8.  Experiments

   This document has experimental status because TCP-ENO's viability
   depends on middlebox behavior that can only be determined _a
   posteriori_.  Specifically, we must 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 applications perform deep packet inspection and
       start dropping segments that unexpectedly contain ciphertext.

   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.

9.  Security considerations

   An obvious use case for TCP-ENO is opportunistic encryption--that is,
   encrypting some connections, but only where supported and without any
   kind of endpoint authentication.  Opportunistic encryption 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.

   Achieving stronger security with TCP-ENO requires verifying session
   IDs.  Any application relying on ENO for communications security MUST
   incorporate session IDs into its endpoint authentication.  By way of
   example, an authentication mechanism based on keyed digests (such
   Digest Access Authentication [RFC7616]) can be extended to include
   the role and session ID in the input of the keyed digest.  To
   preserve backwards compatibility, applications MAY use the
   application-aware bit to negotiate the inclusion of session IDs in

   Because TCP-ENO enables multiple different TEPs to coexist, security
   could potentially be only as strong as the weakest available TEP.  In
   particular, if session IDs do not depend on the TCP-ENO transcript in

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   a strong way, an attacker can undetectably tamper with ENO options to
   force negotiation of a deprecated and vulnerable TEP.  To avoid such
   problems, TEPs MUST compute session IDs using only well-studied and
   conservative hash functions.  That way, even if other parts of a TEP
   are vulnerable, it is still 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
   hence MUST be separately authenticated by the TEP.

10.  IANA Considerations

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

     | Kind | Length | Meaning                          | Reference |
     | TBD  | N      | Encryption Negotiation (TCP-ENO) | [RFC-TBD] |

                          TCP Option Kind Numbers

   Early implementations of TCP-ENO and a predecessor TCP encryption
   protocol made unauthorized use of TCP option kind 69.

   [RFC-editor: please glue the following text to the previous paragraph
   iff TBD == 69, otherwise delete it.]  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.

   [RFC-editor: please glue this to the previous paragraph regardless of
   the value of TBD.]  More recent implementations used experimental
   option 253 per [RFC6994] with 16-bit ExID 0x454E, and SHOULD migrate
   to option TBD by default.

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   This document defines a 7-bit "glt" field in the range of 0x20-0x7f
   for which IANA shall maintain a new sub-registry entitled "TCP
   encryption protocol identifiers" under the "Transmission Control
   Protocol (TCP) Parameters" registry.  The description of this
   registry should be interpreted with respect to the terminology
   defined in [RFC5226].

   The intention is for IANA to grant registration requests for TEP
   identifiers in anticipation of a published RFC.  Hence, a
   Specification is Required.  However, to allow for implementation
   experience, identifiers should be allocated prior to the RFC being
   approved for publication.  A Designated Expert appointed by the IESG
   area director shall approve allocations once it seems more likely
   than not that an RFC will eventually be published.  The Designated
   Expert shall post a request to the TCPINC WG mailing list (or a
   successor designated by the Area Director) for comment and review,
   including an Internet-Draft.  Before a period of 30 days has passed,
   the Designated Expert will either approve or deny the registration
   request and publish a notice of the decision to the TCPINC WG mailing
   list or its successor, as well as informing IANA.  A denial notice
   must be justified by an explanation, and in the cases where it is
   possible, concrete suggestions on how the request can be modified so
   as to become acceptable should be provided.

   The initial values of the TCP-ENO encryption protocol identifier
   registry are shown in Table 2.

    | Value | Meaning                   | Reference                  |
    | 0x20  | Experimental Use          |                            |
    | 0x21  | TCPCRYPT_ECDHE_P256       | [I-D.ietf-tcpinc-tcpcrypt] |
    | 0x22  | TCPCRYPT_ECDHE_P521       | [I-D.ietf-tcpinc-tcpcrypt] |
    | 0x23  | TCPCRYPT_ECDHE_Curve25519 | [I-D.ietf-tcpinc-tcpcrypt] |
    | 0x24  | TCPCRYPT_ECDHE_Curve448   | [I-D.ietf-tcpinc-tcpcrypt] |
    | 0x30  | TCP-Use-TLS               | [I-D.ietf-tcpinc-use-tls]  |

               Table 2: TCP encryption protocol identifiers

11.  Acknowledgments

   We are grateful for contributions, help, discussions, and feedback
   from the TCPINC working group, including Marcelo Bagnulo, David
   Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav
   Nir, Christoph Paasch, Eric Rescorla, Kyle Rose, and Joe Touch.  This
   work was partially funded by DARPA CRASH and the Stanford Secure
   Internet of Things Project.

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12.  References

12.1.  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,

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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,

12.2.  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.

              Bittau, A., Boneh, D., Giffin, D., Hamburg, M., Handley,
              M., Mazieres, D., Slack, Q., and E. Smith, "Cryptographic
              protection of TCP Streams (tcpcrypt)", draft-ietf-tcpinc-
              tcpcrypt-02 (work in progress), July 2016.

              Rescorla, E., "Using TLS to Protect TCP Streams", draft-
              ietf-tcpinc-use-tls-01 (work in progress), May 2016.

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              Touch, J. and W. Eddy, "TCP Extended Data Offset Option",
              draft-ietf-tcpm-tcp-edo-06 (work in progress), June 2016.

              Touch, J. and T. Faber, "TCP SYN Extended Option Space
              Using an Out-of-Band Segment", draft-touch-tcpm-tcp-syn-
              ext-opt-04 (work in progress), April 2016.

   [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,

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

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

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

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

Authors' Addresses

   Andrea Bittau
   345 Spear Street
   San Francisco, CA  94105

   Email: bittau@google.com

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   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

   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

Bittau, et al.           Expires April 28, 2017                [Page 27]

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