Network Working Group                                         D. Balfanz
Internet-Draft                                               R. Hamilton
Expires: December 31, 2013                                    Google Inc
                                                           June 29, 2013

               Transport Layer Security (TLS) Channel IDs


   This document describes a Transport Layer Security (TLS) extension
   for identifying client machines at the TLS layer without using bearer

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   This Internet-Draft will expire on December 31, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Why not client certificates  . . . . . . . . . . . . . . . . .  4
   3.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  6
   4.  Channel ID Client Keys . . . . . . . . . . . . . . . . . . . .  7
   5.  Channel ID Extension . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Channel-Bound Cookies  . . . . . . . . . . . . . . . . . . 12
     7.2.  Channel-Bound OAuth Tokens . . . . . . . . . . . . . . . . 12
   8.  Privacy Considerations . . . . . . . . . . . . . . . . . . . . 13
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 16
   Appendix B.  History of Changes  . . . . . . . . . . . . . . . . . 17
     B.1.  Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Introduction

   Many applications on the Internet use _bearer tokens_ to authenticate
   clients to servers.  The most prominent example is the HTTP-based
   World Wide Web, which overwhelmingly uses HTTP cookies to
   authenticate client requests.  Other examples include OpenID or SAML
   assertions, and OAuth tokens.  All these have in common that the
   _bearer_ of the HTTP cookie or authentication token is granted access
   to a protected resource, regardless of the channel over which the
   token is presented, or who presented it.

   As a result, an adversary that manages to steal a bearer token from a
   client can impersonate that client to services that require the

   This document describes a light-weight mechanism for establishing a
   _cryptographic channel_ between client and server.  A server can
   choose to bind authentication tokens to this channel, thus rendering
   the theft of authentication tokens fruitless - tokens must be sent
   over the channel to which they are bound (i.e., by the client to
   which they were issued) or else they will be ignored.

   This document does not prescribe _how_ authentication tokens are
   bound to the underlying channel.  Rather, it prescribes how a client
   can establish a long-lived channel with a server.  Such a channel
   persists across HTTP requests, TLS connections, and even multiple TLS
   sessions, as long as the same client communicates with the same

   The basic idea is that the client proves, during the TLS handshake,
   possession of a private key.  The corresponding public key becomes
   the "Channel ID" that identifies this TLS connection.  Clients should
   re-use the same private/public key pair across subsequent TLS
   connections to the same server, thus creating TLS connections that
   share the same Channel ID.

   Using private/public key pairs to define a channel (as opposed to,
   say, an HTTP session cookie) has several advantages: One, the
   credential establishing the channel (the private key) is never sent
   from client to server, thus removing it from the reach of
   eavesdroppers in the network.  Two, clients can choose to implement
   cryptographic operations in a secure hardware module, which further
   removes the private key from the reach of eavesdroppers residing on
   the client itself.

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2.  Why not client certificates

   TLS already supports a means of identifying clients without using
   bearer tokens: client certificates.  However, a number of problems
   with using client certificates motivated the development of an

   Most importantly, it's not acceptable for a client identifier to be
   transmitted in the clear, because eavesdroppers in the network could
   use these identifiers to deanonymize TLS connections.  Client
   certificates in TLS, however, are sent unencrypted.  Although we
   could also define a change to the TLS state machine to move the
   client certificates under encryption, such changes eliminate most of
   the benefits of reusing something that's already defined.

   TLS client certificates are also defined to be part of the session
   state.  Even though the key material used for TLS client
   authentication might be protected from theft from compromised clients
   (for example, by employing hardware secure elements on the client),
   TLS session resumption information rarely is.  Because client
   certificates are part of the session state, stolen session resumption
   information gives the attacker something equivalent to a stolen
   client private key.  Our objective, however, is that attackers should
   not be able to give the impression that they can wield a private key
   unless they are actually in control of that private key.

   Client-certificates typically identify a user, while we seek to
   identify machines.  Since they are not, conceptually, mutually
   exclusive and as only a single client certificate can be provided in
   TLS, we don't want to consume that single slot and eliminate the
   possibility of also using existing client certificates.

   Client certificates are implemented in TLS as X.509 certificates and
   we don't wish to require servers to parse arbitrary ASN.1.  ASN.1 is
   a complex encoding that has been the source of several security
   vulnerabilities in the past and typical TLS servers can currently
   avoid doing ASN.1 parsing.

   X.509 certificates always include a signature, which would be a self-
   signature in this case.  Calculating and transmitting the self-
   signature is a waste of computation and network traffic in our use.
   Although we could define a null signature algorithm with an empty
   signature, such deviations from X.509 eliminate many of the benefits
   of reusing something that is already implemented.

   Finally, client certificates trigger significant server-side
   processing by default and often need to be stored in their entirety
   for the duration of the connection.  Since this design is intended to

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   be widely used, it allows servers to retain only a cryptographic hash
   of the client's public key after the handshake completes.

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3.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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4.  Channel ID Client Keys

   For the purpose of this specification, a public key is a point "Q =
   dG" on the P-256 curve [DSS] (where "d" is the ECC private key, and
   "G" is the curve base point).  Clients SHOULD use a separate key pair
   "(d, Q)" for each server they connect to, and generate a new key pair
   if necessary according to appendix B.4 in FIPS-186-3 [DSS].

   A public key "Q" has two affine coordinates "x, y": "Q = (x,y)".  The
   public key "Q" - or, in other words, the pair "x, y" - that a client
   uses for a specific server is that client's Channel ID for that

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5.  Channel ID Extension

   A new extension type ("channel_id(TBD)") is defined and MAY be
   included by the client in its "ClientHello" message.  If, and only
   if, the server sees this extension in the "ClientHello", it MAY
   choose to echo the extension in its "ServerHello".  In both cases,
   the "extension_data" field MUST be empty.

   enum {
     channel_id(TBD), (65535)
   } ExtensionType;

   A new handshake message type ("encrypted_extensions(TBD)") is
   defined.  If the server included a "channel_id" extension in its
   "ServerHello" message, the client MUST verify that the selected
   cipher suite is sufficiently strong.  If the cipher suite provides <
   80-bits of security, the client MUST abort the handshake with a fatal
   "illegal_parameter" alert.  Otherwise, the client MUST send an
   "EncryptedExtensions" message after its "ChangeCipherSpec" and before
   its "Finished" message.

   enum {
     encrypted_extensions(TBD), (65535)
   } HandshakeType;

   Therefore a full handshake with "EncryptedExtensions" has the
   following flow (contrast with section 7.3 of RFC 5246 [RFC5246]):

   Client                                               Server

   ClientHello (ChannelID extension)   -------->
                                         (ChannelID extension)
                                <--------      ServerHelloDone
   Finished                     -------->
                                <--------             Finished
   Application Data             <------->     Application Data

   An abbreviated handshake with "EncryptedExtensions" has the following

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   Client                                                Server

   ClientHello (ChannelID extension)    -------->
                                         (ChannelID extension)
                                 <--------            Finished
   Finished                      -------->
   Application Data              <------->    Application Data

   The "EncryptedExtensions" message contains a series of "Extension"
   structures (see section of RFC 5246 [RFC5246]

   If the server included a "channel_id" extension in its "ServerHello"
   message, the client MUST include, within an EncryptedExtensions
   message, an "Extension" with "extension_type" equal to
   "channel_id(TBD)".  The "extension_data" of which has the following

   struct {
     opaque x[32];
     opaque y[32];
     opaque r[32];
     opaque s[32];
   } ChannelIDExtension;

   The contents of each of "x", "y", "r" and "s" is a 32-byte, big-
   endian number.  The "x" and "y" fields contain the affine coordinates
   of the client's Channel ID Q (i.e., a P-256 [DSS] curve point).  The
   "r" and "s" fields contain an ECDSA [DSS] signature by the
   corresponding private key over this US-ASCII strong (not including
   quotes, and where "\x00" represents an octet containing all zero

   "TLS Channel ID signature\x00"

   followed by hashes of both the client-sent and server-sent handshake
   messages, as seen by the client, prior to the "EncryptedExtensions"

   Unlike many other TLS extensions, this extension does not establish
   properties of the session, only of the connection.  When session
   resumption or session tickets [RFC5077] are used, the previous
   contents of this extension are irrelevant and only the values in the

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   new handshake messages are considered.

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6.  Security Considerations

   There are four classes of attackers against which we consider our
   security guarantees: passive network attackers, active network
   attackers, active network attackers with misissued certificates and
   attackers in possession of the legitimate server's private key.

   First, we wish to guarantee that we don't disclose the Channel ID to
   passive or active network attackers.  We do this by sending a
   constant-length Channel ID under encryption.  However, since the
   Channel ID may be transmitted before the server's Finished message is
   received, it's possible that the server isn't in possession of the
   corresponding private key to the certificate that it presented.  In
   this situation, an active attacker could cause a Channel ID to be
   transmitted under a random key in a cipher suite of their choosing.
   Therefore we limit the permissible cipher suites to those where
   decrypting the message is infeasible.

   Even with this limit, an active attacker can cause the Channel ID to
   be transmitted in a non-forward-secure manner.  Subsequent disclosure
   of the server's private key would allow previously recorded Channel
   IDs to be decrypted.

   Second, we wish to guarantee that none of the first three attackers
   can terminate/hijack a TLS connection and impersonate a Channel ID
   from that connection when connecting to the legitimate server.  We
   assume that TLS provides sufficient security to prevent these
   attackers from being able to hijack the TLS connection.  An active
   attacker illegitimately in possession of a certificate for a server
   can successfully terminate a TLS connection destined for that server
   and decrypt the Channel ID.  However, as the signature covers the
   handshake hashes, and therefore the server's certificate, it wouldn't
   be accepted by the true server.

   Against an attacker with the legitimate server's private key we can
   provide the second guarantee only if the legitimate server uses a
   forward-secret cipher suite, otherwise the attacker can hijack the

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7.  Use Cases

7.1.  Channel-Bound Cookies

   An HTTP application on the server can _channel-bind_ its cookies by
   associating them with the Channel ID of the user-agent that the
   cookies are being set on.  The server MAY then choose to consider
   cookies sent from the user-agent invalid if the Channel ID associated
   with the cookie does not match the Channel ID used by the user-agent
   when it sends the cookie back to the server.

   Such a mismatch could occur when the cookie has been obtained from
   the legitimate user-agent and is now being sent by a client not in
   possession of the legitimate user-agent's Channel ID private key.
   The mismatch can also occur if the legitimate user-agent has changed
   the Channel ID it is using for the server, presumably due to the user
   requesting a Channel ID reset through the user-agent's user interface
   (see Section 8).  Such a user intervention is analogous to the user's
   removal of cookies from the user-agent, but instead of removing
   cookies, the cookies are being rendered invalid (in the eyes of the

7.2.  Channel-Bound OAuth Tokens

   Similarly to cookies, a server may choose to channel-bind OAuth
   tokens (or any other kind of authorization tokens) to the clients to
   which they are issued.  The mechanism on the server remains the same
   (it associates the OAuth token with the client's Channel ID either by
   storing this information in a database, or by suitably encoding the
   information in the OAuth token itself), but the application-level
   protocol may be different: In addition to HTTP, OAuth tokens are used
   in protocols such as IMAP and XMPP.

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8.  Privacy Considerations

   The TLS layer does its part in protecting user privacy by
   transmitting the Channel ID public key under encryption.  Higher
   levels of the stack must ensure that the same Channel ID is not used
   with different servers in such a way as to provide a linkable
   identifier.  For example, a user-agent must use different Channel IDs
   for communicating with different servers.  Because channel-bound
   cookies are an important use case for TLS Channel ID, and cookies can
   be set on top-level domains, it is RECOMMENDED that user-agents use
   the same Channel ID for servers within the same top-level domain, and
   different Channel IDs for different top-level domains.  User-agents
   must also ensure that Channel ID state can be reset by the user in
   the same way as other identifiers, i.e. cookies.

   However, there are some security concerns that could result in the
   disclosure of a client's Channel ID to a network attacker.  This is
   covered in the Security Considerations section.

   Clients that share an IP address can be disambiguated through their
   Channel IDs.  This is analogous to protocols that use cookies (e.g.,
   HTTP), which also allow disambiguation of user-agents behind proxies.

   Channel ID has been designed to provide privacy equivalent to that of
   cookies.  User-agents SHOULD continue to meet this design goal at
   higher layers of the protocol stack.  For example, if a user
   indicates that they would like to block third-party cookies (or if
   the user-agent has some sort of policy around when it blocks third-
   party cookies by default), then the user agent SHOULD NOT use Channel
   ID on third-party connections (or other connections through which the
   user-agent would refuse to send or accept cookies).

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9.  IANA Considerations

   This document requires IANA to update its registry of TLS extensions
   to assign an entry referred to here as "channel_id".

   This document also requires IANA to update its registry of TLS
   handshake types to assign an entry referred to here as

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10.  References

10.1.  Normative References

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

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

   [DSS]      National Institute of Standards and Technology, "FIPS
              186-3: Digital Signature Standard".

10.2.  Informative References

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

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Appendix A.  Acknowledgements

   The following individuals contributed to this specification:

   Dirk Balfanz, Wan-Teh Chang, Ryan Hamilton, Adam Langley, and Mayank

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Appendix B.  History of Changes

B.1.  Version 01

   o  Some clarifications, mostly around the Channel ID and session

   o  Added a section on Use Cases.

   o  Expanded the Privacy Considerations sections to include discussion
      of third-party connections in HTTP user-agents.

   o  Fixed some typos.

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Authors' Addresses

   Dirk Balfanz
   Google Inc


   Ryan Hamilton
   Google Inc


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