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

                    Interface Extensions for TCPINC


   TCP-ENO negotiates encryption at the transport layer.  It also
   defines a few parameters that are intended to be used or configured
   by applications.  This document specifies operating system interfaces
   for access for these TCP-ENO parameters.  We describe the interfaces
   in terms of socket options, the de facto standard API for adjusting
   per-connection behavior in TCP/IP, and sysctl, a popular mechanism
   for setting global defaults.  Operating systems that lack socket or
   sysctl functionality can implement similar interfaces in their native
   frameworks, but should ideally adapt their interfaces from those
   presented in this document.

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
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   Drafts is at

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

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

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  API extensions  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Automatic configuration protocol  . . . . . . . . . . . . . .   6
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Cookie-based authentication . . . . . . . . . . . . . . .   8
     4.2.  Signature-based authentication  . . . . . . . . . . . . .   8
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The TCP Encryption Negotiation Option (TCP-ENO)
   [I-D.bittau-tcpinc-tcpeno] permits hosts to negotiate encryption of a
   TCP connection.  One of TCP-ENO's use cases is to encrypt traffic
   transparently, unbeknownst to legacy applications.  Transparent
   encryption requires no changes to existing APIs.  However, other use
   cases require applications to interact with TCP-ENO.  In particular:

   o  Transparent encryption protects only against passive
      eavesdroppers.  Stronger security requires applications to
      authenticate a _Session ID_ value associated with each encrypted

   o  Applications that have been updated to authenticate Session IDs
      must somehow advertise this fact to peers in a backward-compatible
      way.  TCP-ENO carries a two-bit "application-aware" status for

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      this purpose, but this status is not accessible through existing

   o  Applications employing TCP's simultaneous open feature need a way
      to supply a symmetry-breaking "tie-breaker" bit to TCP-ENO.

   o  System administrators and applications may wish to set and examine
      negotiation preferences, such as which encryption schemes (and
      perhaps versions) to enable and disable.

   o  Applications that perform their own encryption may wish to disable
      TCP-ENO entirely.

   The remainder of this document describes an API through which systems
   can meet the above needs.  The API extensions relate back to
   quantities defined by TCP-ENO.

2.  API extensions

   Application should access TCP-ENO options through the same mechanism
   they use to access other TCP configuration options, such as
   "TCP_NODELAY" [RFC0896].  With the popular sockets API, this
   mechanism consists of two socket options, "getsockopt" and
   "setsockopt", shown in Figure 1.  Socket-based TCP-ENO
   implementations should define a set of new "option_name" values
   accessible at "level" "IPPROTO_TCP" (generally defined as 6, to match
   the IP protocol field).

      int getsockopt(int socket, int level, int option_name,
                     void *option_value, socklen_t *option_len);

      int setsockopt(int socket, int level, int option_name,
                     const void *option_value, socklen_t option_len);

                        Figure 1: Socket option API

   Table 1 summarizes the new "option_name" arguments that TCP-ENO
   introduces to the socket option (or equivalent) system calls.  For
   each option, the table lists whether it is read-only (R) or read-
   write (RW), as well as the type of the option's value.  Read-write
   options, when read, always return the previously successfully written
   value or the default if they have not been written.  Options of type
   "bytes" consist of a variable-length array of bytes, while options of
   type "int" consist of a small integer with the exact range indicated
   in parentheses.  We discuss each option in more detail below.

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                | Option name       | RW | Type           |
                | TCPENO_ENABLED    | RW | int (-1 - 1)   |
                | TCPENO_SESSID     | R  | bytes          |
                | TCPENO_NEGSPEC    | R  | int (32 - 255) |
                | TCPENO_SPECS      | RW | bytes          |
                | TCPENO_SELF_AWARE | RW | int (0 - 3)    |
                | TCPENO_PEER_AWARE | R  | int (0 - 3)    |
                | TCPENO_TIEBREAKER | RW | int (0 - 1)    |
                | TCPENO_ROLE       | R  | int (0 - 1)    |
                | TCPENO_RAW        | RW | bytes          |
                | TCPENO_TRANSCRIPT | R  | bytes          |

             Table 1: Suggested new IPPROTO_TCP socket options

   TCPENO_ENABLED  When set to 0, completely disables TCP-ENO regardless
      of any other socket option settings except "TCPENO_RAW".  When set
      to 1, enables TCP-ENO.  When set to -1 (which should be the
      default), uses a system default value to determine whether or not
      to enable TCP-ENO.  This option must return an error after a SYN
      segment has already been sent.

   TCPENO_SESSID  Returns the session ID of the connection, as defined
      by the encryption spec in use.  This option must return an error
      if encryption is disabled, the connection is not yet established,
      or the transport layer does not implement the negotiated spec.

   TCPENO_NEGSPEC  Returns the negotiated encryption spec for the
      current connection.  As defined by TCP-ENO, the negotiated spec is
      the first valid suboption in the "B" host's SYN segment (without
      any suboption data for variable-length suboptions).  This option
      must return an error if encryption is disabled or the connection
      is not yet established.

   TCPENO_SPECS  Allows the application to specify an ordered list of
      encryption specs different from the system default list.  If the
      list is empty, TCP-ENO is disabled for the connection.  Each byte
      in the list specifies one suboption type from 0x20-0xff.  The list
      contains no suboption data for variable-length suboptions, only
      the one-byte spec identifier.  The order of the list matters only
      for the host playing the "B" role.  Implementations must return an
      error if an application attempts to set this option after the SYN
      segment has been sent.  Implementations should return an error if
      any of the bytes are below 0x20 or are not implemented by the TCP

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   TCPENO_SELF_AWARE  The value is an integer from 0-3, allowing
      applications to specify the "aa" bits in the general suboption
      sent by the host.  When listening on a socket, the value of this
      option applies to each accepted connection.  Implementations must
      return an error if an application attempts to set this option
      after a SYN segment has been sent.

   TCPENO_PEER_AWARE  The value is an integer from 0-3 reporting the
      "aa" bits in the general suboption of the peer's segment.
      Implementations must return an error if an application attempts to
      read this value before the connection is established.

   TCPENO_TIEBREAKER  The value is a bit (0 or 1), indicating the value
      of the "b" bit to set in the host's general suboption.  The "b"
      bit breaks the symmetry of simultaneous open to assign a unique
      role "A" or "B" to each end of the connection.  The host that sets
      the "b" bit assumes the "B" role (which in non-simultaneous open
      is assigned to the passive opener).  Implementations must return
      an error for this options after the SYN segment has already been

   TCPENO_ROLE  The value is a bit (0 or 1).  TCP-ENO defines two roles,
      "A" and "B", for the two ends of a connection.  After a normal
      three-way handshake, the active opener is "A" and the passive
      opener is "B".  Simultaneous open uses the tie-breaker bit to
      assign unique roles.  This option returns 0 when the local host
      has the "A" role, and 1 when the local host has the "B" role.
      This call must return an error before the connection is
      established or if TCP-ENO has failed.

   TCPENO_RAW  This option is for use by library-level implementations
      of encryption specs.  It allows applications to make use of the
      TCP-ENO option, potentially including encryption specs not
      supported by the transport layer, and then entirely bypass any
      TCP-level encryption so as to encrypt above the transport layer.
      The default value of this option is a 0-byte vector, which
      disables RAW mode.  If the option is set to any other value, it
      disables all other socket options described in this section except

      The value of the option is a raw ENO option contents (without the
      kind and length) to be included in the host's SYN segment.  In raw
      mode, the TCP layer considers negotiation successful when the two
      SYN segments both contain a suboption with the same encryption
      spec value "cs" >= 0x20.  For an active opener in raw mode, the
      TCP layer automatically sends a two-byte minimal ENO option when
      negotiation is successful.  Note that raw mode performs no sanity

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      checking on the "v" bits or any suboption data, and hence provides
      slightly less flexibility than a true TCP-level implementation.

   TCPENO_TRANSCRIPT  Returns the negotiation transcript as specified by
      TCP-ENO.  Implementations must return an error if negotiation
      failed or has not yet completed.

   In addition to these per-socket options, implementations should use
   "sysctl" or an equivalent mechanism to allow administrators to
   configure system-wide defaults for "TCPENO_ENABLED" and
   "TCPENO_SPECS".  These parameters should be named "eno_enabled" and
   "eno_specs" and placed alongside most TCP parameters.  For example,
   on BSD derived systems a suitable name would be
   "net.inet.tcp.eno_enabled" and "net.inet.tcp.eno_specs", while on
   Linux more appropriate names would be "net.ipv4.tcp_eno_enabled" and

   Because initial deployment may run into issues with middleboxes or
   incur slowdown for unnecessary double-encryption, implementations
   should also allow ENO to be blacklisted for particular local and
   remote ports, via sysctl on "net.inet.tcp.eno_bad_localport" and
   "net.inet.tcp.eno_bad_remoteport" (or the equivalent under "net.ipv4"
   for linux), both of which consist of a list of TCP port numbers on
   which to disable TCP-ENO by default.  For example the following

              sysctl net.inet.tcp.eno_bad_remoteport=443,993

   would disable ENO encryption on outgoing connections to ports 443 and
   993 (which use application-layer encryption for TLS and IMAP,

   The per-socket "TCPENO_ENABLED" option, if not -1, should override
   both the "eno_enabled" and port-range sysctls.

3.  Automatic configuration protocol

   TCP-ENO is designed to fail by reverting to unencrypted TCP.  Such
   behavior is necessary for incremental deployment, and is no worse
   than the status quo in which there is no TCP-layer encryption.
   However, one outcome worse than the status quo would be to for TCP-
   ENO connections to fail completely where unenecrypted connections
   would work.  Fortunately, if TCP-ENO is not supported by both
   endpoints, or if middleboxes strip the ENO option from packets, then
   implementations simply revert to unencrypted TCP upon receiving a SYN
   or initial ACK segment without an ENO option.  This fallback approach
   also applies to interception proxies [RFC3040], which typically

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   terminate TCP connections and hence will not include ENO in their SYN
   segments if they do not know about it.

   However, given that the goal of TCP-ENO is to encrypt previously
   plaintext traffic, there is always the possibility that a middlebox
   performing deep packet inspection could shut down a connection
   because the ciphertext does not resemble an expected higher-level
   application protocol such as HTTP.  Such middleboxes would cause TCP-
   ENO connections to fail.  Systems may wish to probe the network so as
   to enable TCP-ENO only in places where middleboxes do not induce such

   A precedent for probing middlebox behavior is the STUN protocol
   [RFC5389], which applications use to characterize NAT.  STUN relies
   on having a known, publicly-accessible server beyond any locally
   administered middleboxes.  STUN is typically invoked by applications
   that require peer-to-peer communication to decide whether they can
   accept incoming connections.  For TCP-ENO, which affects all TCP
   connections, it makes more sense to probe for network compatibility
   at the time network interfaces are configured by DHCP [RFC2131],
   stateless address autoconfiguration [RFC4862], or other mechanisms.
   Many DHCP implementation already provide hooks through which such
   probes can be configured.

   Like STUN, TCP-ENO probing requires a known external server running
   an agreed upon protocol.  We suggests using HTTP as the protocol, and
   responding to the path "/tcp-eno/session-id" with a response of type
   "text/plain".  Upon successful TCP-ENO negotiation, servers should
   reply with the string "encrypted " followed by a lower-case
   hexadecimal encoding of the tcpcrypt session ID followed by a
   newline.  For connection on which TCP-ENO fails, the same path should
   return the string "unencrypted\n" (with no session ID).  If such a
   request works with TCP-ENO disabled but hangs or resets with TCP-ENO
   enabled, then TCP-ENO should be disabled for the host.  Otherwise, if
   probes succeed, even if they return "unencrypted", TCP-ENO should be
   enabled (for the possible benefit of local connections), as
   middleboxes may simply be stripping off the option.

   Hosts should perform the above probe twice, using both port 80 and a
   different port, we suggest 8080, on the same server.  Given the
   prevalence of interception proxies on port 80, port 80 may experience
   entirely different failure modes from other ports.  If the port 80
   probe fails, TCP-ENO should be disabled for port 80.  If the other
   probe fails, TCP-ENO should be disabled entirely.

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4.  Examples

   This section provides examples of how applications might authenticate
   session IDs.  Authentication requires exchanging messages over the
   TCP connection, and hence is not backwards compatible with existing
   application protocols.  To fall back to opportunistic encryption in
   the event that both applications have not been updated to
   authenticate the session ID, TCP-ENO provides the application-aware
   bits.  To signal it has been upgraded to support application-level
   authentication, applications should set "TCPENO_SELF_AWARE" to 1
   before opening a connection.  An application should then check that
   "TCPENO_PEER_AWARE" is non-zero before attempting to send
   authenticators that would otherwise be misinterpreted as application

4.1.  Cookie-based authentication

   In cookie-based authentication, a client and server both share a
   cryptographically strong random or pseudo-random secret known as a
   "cookie".  Such a cookie is preferably at least 128 bits long.  To
   authenticate a session ID using a cookie, each computes and sends the
   following value to the other side:

              authenticator = PRF(cookie, role || session-ID)

   Here "PRF" is a psueo-random function such as HMAC-SHA-256 [RFC6234].
   "role" is the byte 0 or 1, as returned by the "TCPENO_ROLE" socket
   options.  "session-ID" is the session ID returned by the
   "TCPENO_SESSID" session ID.  The symbol "||" denotes concatenation.
   Each side must verify that the other side's authenticator is correct.
   Assuming the authenticators are correct, applications can rely on the
   TCP-layer encryption for resistance against active network attackers.

   Note that if the same cookie is used in other contexts besides
   session ID authentication, appropriate domain separation should be
   employed, such as prefixing "role || session-ID" with a unique prefix
   to ensure "authenticator" cannot be used out of context.

4.2.  Signature-based authentication

   In signature-based authentication, one or both endpoints of a
   connection possess a private signature key the public half of which
   is known to or verifiable by the other endpoint.  To authenticate
   itself, the host with a private key computes the following signature:

             authenticator = Sign(PrivKey, role || session-ID)

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   The other end verifies this value using the corresponding public key.
   Whichever side validates an authenticator in this way knows that the
   other side belongs to a host that possesses the appropriate signature

   Once again, if the same signature key is used in other contexts
   besides session ID authentication, appropriate domain separation
   should be employed, such as prefixing "role || session-ID" with a
   unique prefix to ensure "authenticator" cannot be used out of

5.  Security considerations

   The TCP-ENO specification discusses several important security
   considerations that this document incorporates by reference.  The
   most important one, which bears reiterating, is that until and unless
   a session ID has been authenticated, TCP-ENO is vulnerable to an
   active network attacker, through either a downgrade or active man-in-
   the-middle attack.

   Because of this vulnerability to active network attackers, it is
   critical that implementations return appropriate errors for socket
   options when TCP-ENO is not enabled.  Equally critical is that
   applications must never use these socket options without checking for

   Applications with high security requirements that rely on TCP-ENO for
   security must either fail or fallback to application-layer encryption
   if TCP-ENO fails or session IDs authentication fails.

6.  Acknowledgments

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

7.  References

7.1.  Normative References

              Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
              D., and E. Smith, "TCP-ENO: Encryption Negotiation
              Option", draft-bittau-tcpinc-tcpeno-01 (work in progress),
              August 2015.

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7.2.  Informative References

   [RFC0896]  Nagle, J., "Congestion Control in IP/TCP Internetworks",
              RFC 896, DOI 10.17487/RFC0896, January 1984,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, DOI 10.17487/RFC2131, March 1997,

   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", RFC 3040, DOI 10.17487/
              RFC3040, January 2001,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, DOI 10.17487/
              RFC4862, September 2007,

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI
              10.17487/RFC6234, May 2011,

Authors' Addresses

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


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


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


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


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


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


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