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Versions: 00                                                            
Network Working Group                                        N. Williams
Internet-Draft                                              Cryptonector
Intended status: Informational                              July 8, 2013
Expires: January 9, 2014

   Hypertext Transport Protocol (HTTP) Session Continuation: Problem


   One of the most often talked about problems in web security is
   "cookies".  Web cookies are a method of associating requests with
   "sessions" that may have been authenticated somehow.  Cookies are a
   form of bearer token that leave much to be desired.  This document
   describes the session "continuation" problem for the HyperText
   Transport Protocol (HTTP).

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 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 January 9, 2014.

Copyright Notice

   Copyright (c) 2013 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.  Conventions used in this document  . . . . . . . . . . . . .  4
   2.    Requirements . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.  Statelessness  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.    IANA Considerations  . . . . . . . . . . . . . . . . . . . .  8
   4.    Security Considerations  . . . . . . . . . . . . . . . . . .  9
   4.1.  On Origin  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.2.  User Interface Considerations  . . . . . . . . . . . . . . .  9
   5.    Proposed Session Continuation Protocols  . . . . . . . . . . 10
   6.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
   7.    References . . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.1.  Normative References . . . . . . . . . . . . . . . . . . . . 12
   7.2.  Informative References . . . . . . . . . . . . . . . . . . . 12
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   Today most web applications use "cookies" to associate Hypertext
   Transfer Protocol (HTTP) [RFC2616] requests with "sessions".  A
   "session" is a set of related HTTP requests (and responses), where
   the relation is to some request(s) that created the session.  Some
   sessions are created by the act of authenticating a user, in which
   case the primary goal of "sessions" is to avoid having to re-
   authenticate the user on every request.  Other times a session is
   created when a request is received that is not associated with any
   session, in which case the primary purpose of "sessions" may be to
   provide a pseudonymous identifier for an otherwise anonymous user.
   We call the mechanisms by which requests are strung into sessions
   "session continuation".

   "Cookies" are server-assigned bearer tokens -- nothing more, nothing
   less, though some cookies are used just to store things like
   "shopping cart" state.  A bearer token is an octet blob which can be
   presented as-is, possibly repeatedly, to authenticate a user to some
   party; mere possession of the bearer token is sufficient to act on
   the user's behalf to at least one service.  As such they are
   susceptible to theft via passive attacks (eavesdropping) if not
   protected in some other way (e.g., by using Transport Layer Security
   (TLS) [RFC5246]), or via active attacks such as BEAST and CRIME
   [http://www.xors.me/?attachment_id=3727], as well as to leakage in
   various ways [XXX expand].

   We would like a session continuation mechanism to replace or augment
   cookies that has better security semantics than bearer tokens.  In
   particular we would like a system that is not susceptible to theft
   via active attacks like BEAST and CRIME.  We believe that such a
   scheme should use cryptographic algorithms and cryptographic session
   keys, and should be amenable to being keyed by HTTP- and web-
   authentication mechanisms.  A new session continuation mechanism
   should be suitable for use in web and non-web HTTP applications, and
   should work even for unauthenticated sessions.

1.1.  Motivation

   The motivation for this document document is as follows.  We need:

   o  A variable authentication token instead of (or in addition to) web
      cookies, for resistance to BEAST, CRIME, and other adaptive chosen
      plaintext active attacks on TLS.

   o  The ability to explicitly logout and destroy all session state
      even if the session has been compromised, assuming there is no Man
      In The Browser (MITB).  (This feature being optional for stateless

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   o  The ability to manage sessions.  For example, the ability to query
      session state.

   o  The ability to negotiate replay protection.

   o  Cryptographic binding ("channel binding" [RFC5056]) to the lower
      transport layer (TLS, where available), so as to reduce the cost
      of session continuation cryptographic protection.  (That is, once
      a channel is bound to a session then there should be no further
      need to use cryptography for binding requests and responses to the

   o  Cryptographic binding to user authentication mechanisms (e.g.,
      where the authentication mechanism can export a secret value).

   o  The ability to use HTTP/Negotiate [RFC4559] in such a way that a)
      new HTTP(S) connections need not result in re-authentication, b)
      does not strongly bind requests in a single HTTP connection to the
      HTTP/Negotiate authentication that precedes them.

1.2.  Conventions used in this document

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

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2.  Requirements

   Any session continuation scheme to replace or augment cookies MUST
   provide the following functionality:

   1.   Support for authenticated and unauthenticated sessions alike.

   2.   Support for http: and https: both.

   3.   Session continuation must be possible to implement without
        keeping state on the server side (see below), and it must be
        possible to keep some state on the server and some on the

   4.   Resistance to active attacks on HTTPS, e.g., CRIME and BEAST.
        Specifically, an active attack on HTTPS that allows an attacker
        to recover traditional web cookies should not be sufficient to
        recover session continuation state with which to impersonate the
        user to the server.  [NOTE: This should probably NOT be a
        requirement.  Instead we should be happy to note where a
        proposed protocol provides this.]

   5.   Session continuation must be expressed via HTTP headers.

   6.   Session continuation header values must be cryptographically
        difficult for attackers to spoof, and servers must be able to
        validate these values.

   7.   Session continuation header values used with TLS must be
        cryptographically distinct from those used without TLS such that
        no such values taken from HTTP requests sent without TLS can be
        used in HTTP requests with TLS.

   8.   Session continuation must provide protection against man-in-the-
        middle (MITM) attacks when using TLS, at least when coupled with
        (bound to) user and/or server authentication, whether in HTTP or
        at the application layer.  (This is important when using
        anonymous Diffie-Hellman cipher suites for TLS, as well as when
        using server certificates from low-value Public Key
        Infrastructures (PKI).

   9.   Must support explicit session termination ("logout"), initiated
        by the client.  Once a session is logged out there should be no
        way to use it again, even if any session keys are compromised.
        Note that this is not a deployment requirement, just a protocol
        requirement; a fully stateless deployment may not be able to
        implement faithful logout.

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   10.  Must work across all types of proxies, at least those that do
        not destructively modify data nor metadata authenticated by
        session continuation.

   11.  Sessions should be tied to "origins"; multi-origin sessions
        (sharing sessions across servers) are allowed, but there are
        user interface considerations.

   12.  It must be possible to share sessions across multiple servers
        responding to the same origin (e.g., behind a load balancer).
        In particular, server implementations should agree on encrypted
        state cookie formats and/or stored state lookup procedures.

   [[anchor1: Can you move a session from one server to another?  No,
   probably not.  Servers can share sessions, so we need to at least be
   able to scope sessions to sets of servers or DNS sub-domains.  This
   appears to require that sessions have names.  Once we have proper
   session continuation we may well end up needing a mechanism by which
   to authenticate to a service as a user of a given session on a
   foreign service that is "friends" with the first (this too will have
   user interface considerations).]]


   1.  Session continuation SHOULD use proof-of-possession of secret
       session key(s).

   2.  Session continuation header values SHOULD include a
       cryptographically-secure value (indistinguishable from random)
       that can be validated by the server and is hard for attackers to
       guess.  For example, message authentication codes (MAC) using a
       shared session key and applied to all the data (e.g., request/
       response bodies) and metadata (e.g., some or all headers).

   3.  Session continuation header values should be salted with a nonce
       to defeate BEAST- and CRIME-style active attacks.

2.1.  Statelessness

   Session continuation protocols for HTTP MUST allow for stateless
   implementation on the server side, at least when TLS is used.
   Statelessness is not a requirement of deployments; implementations
   SHOULD support both, stateful and stateless servers.  This generally
   means that any state must be encrypted and encoded into a session
   state cookie that is re-sent by the client to the server on every
   request.  The server, of course, must be the one to assign such
   state, and it must use an encryption key known only by the server.

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   Server-side statelessness is NOT REQUIRED in actual deployments, but
   the ability to implement session continuation in a stateless fashion
   on the server side is REQUIRED.

   Note that statelessness implies that there is no way to implement
   replay protection.  In the case of session continuation with TLS this
   is not a concern because TLS itself protects against replays.  But
   when session continuation is used without TLS then statelessness
   really does mean that there can be no replay protection (of course,
   this is also true of web cookies).  Therefore servers that require
   replay protection must either require the use of TLS or must use
   stateful sessions.

   Note also that statelessness makes session logout a no-op on the
   server-side, which generally (see note on probabilistic structures
   below) means that a compromised session can continue to be used even
   after a client attempts to logout.  A session continuation protocol
   MUST allow for storing some state on the server, and some on the
   client, allowing deployments where the only state stored is the
   existence of a session.

   Probabilistic data structures (e.g., adaptive Bloom filters) MAY be
   used to record logouts.  This may require the ability to expire and
   refresh sessions to render the logout system scalable -- in other
   words, HTTP responses MUST be allowed to replace session server state
   stored on the client side.

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3.  IANA Considerations

   This document does not specify any protocols and has no IANA

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4.  Security Considerations

   This document does not specify any protocols and is Informational.
   There are, however, few security considerations to document here.

   We seek to improve security on the web (as well as for non-web HTTP
   applications) by:

   1.  reducing the need for expensive HTTP authentication exchanges
       (e.g., HTTP/Negotiate), thereby removing an obstacle to their

   2.  reducing exposure to session credentials theft via attacks on TLS
       such as BEAST and CRIME;

   3.  reducing exposure to session credentials theft when not using

   4.  introducing a replacement for / augmentation of cookies that will
       give browsers a chance to pursue better security policies.

   As discussed in Section 2.1, there is a security consideration
   regarding session continuation without TLS and with server-side
   statelessness: there can be no replay protection in this case.
   However, this is not a loss of security relative to web cookies.
   Applications must use TLS if they require integrity protection.

4.1.  On Origin

   [[anchor2: We should probably discuss the notion of "origin"

4.2.  User Interface Considerations

   At its simplest, a session continuation protocol should only have an
   impact on how users manage sessions, namely: instead of (or in
   addition to) deleting cookies to destroy sessions, users may see an
   interface by which to logout sessions.

   Any functionality for extending the scope of sessions, or adding
   origins to sessions, may require more extensive user interfaces.

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5.  Proposed Session Continuation Protocols

   o  [I-D.hallambaker-httpsession]

   o  [I-D.draft-williams-websec-session-continue-proto]

   o  <https://github.com/hueniverse/hawk>

   o  OAuth [RFC5849]

   o  and various others (the author begs forgiveness for those left

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6.  Acknowledgements

   The author thanks Yaron Sheffer, Yoav Nir, and Phillip Hallam-Baker,
   all of whom are practically co-authors, and invited to be listed as
   such.  The term "session continuation" is Phillip's.  The motivation,
   requirements and recommendations text is a group effort.

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7.  References

7.1.  Normative References

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

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

7.2.  Informative References

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, June 2006.

              Hallam-Baker, P., "HTTP Session Management",
              draft-hallambaker-httpsession-01 (work in progress),
              May 2013.

              Williams, N., "Hypertext Transport Protocol (HTTP) Session
              Continuation Protocol",
              draft-williams-websec-session-continue-proto-00 (work in
              progress), January 2013.

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Author's Address

   Nicolas Williams
   Cryptonector, LLC

   Email: nico@cryptonector.com

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