Network Working Group                                         J. Salowey
Internet-Draft                                                   H. Zhou
Expires: April 21, 2006                                    Cisco Systems
                                                               P. Eronen
                                                           H. Tschofenig
                                                        October 18, 2005

 Transport Layer Security Session Resumption without Server-Side State

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes a mechanism which enables the Transport Layer
   Security (TLS) server to resume sessions and avoid keeping per-client
   session state.  The TLS server encapsulates the session state into a
   ticket and forwards it to the client.  The client can subsequently

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   resume a session using the obtained ticket.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1   Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.2   SessionTicket TLS extension  . . . . . . . . . . . . . . .  5
     3.3   SessionTicket handshake message  . . . . . . . . . . . . .  5
     3.4   Interaction with TLS session ID  . . . . . . . . . . . . .  6
   4.  Recommended Ticket Construction  . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     5.1   Invalidating Sessions  . . . . . . . . . . . . . . . . . .  9
     5.2   Stolen Tickets . . . . . . . . . . . . . . . . . . . . . .  9
     5.3   Forged Tickets . . . . . . . . . . . . . . . . . . . . . .  9
     5.4   Denial of Service Attacks  . . . . . . . . . . . . . . . .  9
     5.5   Ticket Protection Key Lifetime . . . . . . . . . . . . . .  9
     5.6   Alternate Ticket Formats and Distribution Schemes  . . . . 10
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1   Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2   Informative References . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

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1.  Introduction

   This document defines a way to resume a Transport Layer Security
   (TLS) [RFC2246] session without requiring session-specific state at
   the TLS server.  This mechanism may be used with any TLS ciphersuite.
   The mechanism makes use of TLS extensions defined in [I-D.ietf-tls-
   rfc3546bis] and defines a new TLS message type.

   This mechanism is useful in the following types of situations
      (1) servers that handle a large number of transactions from
      different users
      (2) servers that desire to cache sessions for a long time
      (3) ability to load balance requests across servers
      (4) embedded servers with little memory

2.  Terminology

   Within this document the term 'ticket' refers to a cryptographically
   protected data structure which is created by the server and consumed
   by the server to rebuild session specific state.

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

3.  Protocol

   This specification describes a mechanism to distribute encrypted
   session state information in a ticket from a TLS server to a TLS
   client and for a TLS client to present a ticket to a TLS server to
   resume a session.  Implementations of this specification are expected
   to support both mechanism.  Other specifications can take advantage
   of the session tickets, perhaps specifying alternative means for
   distribution or selection.  For example a separate specification may
   describe an alternate way to distribute a ticket and use the TLS
   extension in this document to resume the session.  This behavior is
   beyond the scope of the document and would need to be described in a
   separate specification.

3.1  Overview

   The client indicates that it supports this mechanism by including a
   SessionTicket TLS extension in the ClientHello message.  The
   extension will be empty if the client does not already possess a
   ticket for the server.  The extension is described in Section 3.2

   If the server wants to use this mechanism, it stores its session
   state (such as ciphersuite and master secret) to a ticket that is

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   encrypted and integrity-protected by a key known only to the server.
   The ticket is distributed to the client using the SessionTicket TLS
   handshake message described in Section 3.3.  This message is sent
   during the TLS handshake before the ChangeCipherSpec message after
   the server has successfully verified the client's Finished message.

         Client                                               Server

         ClientHello                   -------->
        (empty SessionTicket extension)
                                      <--------      ServerHelloDone
         Finished                     -------->
                                      <--------             Finished
         Application Data             <------->     Application Data

   The client caches this ticket along with the master secret and other
   parameters associated with the current session.  When the client
   wishes to resume the session, it includes the ticket in the
   SessionTicket extension within ClientHello message.  The server then
   verifies that the ticket has not been tampered with, decrypts the
   contents, retrieves the session state from the contents of the ticket
   and uses this state to resume the session.  The interaction with the
   TLS Session ID is described in Section 3.4.  If the server
   successfully verifies the client's ticket then it may renew the
   ticket by including a SessionTicket handshake message after the

         (SessionTicket extension)      -------->
                                       <--------             Finished
         Finished                      -------->
         Application Data              <------->     Application Data

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   A recommended ticket format is given in Section 4.  If the server
   cannot or does not want to honor the ticket then it can initiate a
   full handshake with the client.

3.2  SessionTicket TLS extension

   The SessionTicket TLS extension is based on [I-D.ietf-tls-
   rfc3546bis].  The format of the ticket is an opaque structure used to
   carry session specific state information.  This extension is sent in
   the ClientHello.  If the client posses a ticket that it wants to use
   to resume a session then it includes it in the SessionTicket
   extension in the ClientHello.  If the client does not have a ticket
   and it is prepared to receive one in the SessionTicket handshake
   message then it MUST include a zero length ticket in the
   SessionTicket extension.  If the client is not prepared to receive a
   ticket in the SessionTicket handshake message then it MUST NOT
   include a zero length SessionTicket extension.

   If the server fails to verify the ticket then it falls back to
   performing a full handshake.  If the ticket is accepted by the server
   but the handshake fails the client SHOULD delete the ticket.

   The SessionTicket extension has been assigned the number TBD1.  The
   format of the SessionTicket extension is given below.

      struct {
          opaque ticket<0..2^16-1>;
      } SessionTicket;

3.3  SessionTicket handshake message

   This message is sent during the TLS handshake before the
   ChangeCipherSpec message.  This message MUST only be sent if either
   the client included a SessionTicket extension with a zero length
   ticket in the ClientHello or if the client included a ticket that was
   previously issued in a SessionTicket handshake message.  In the case
   of a full handshake, the server MUST verify the client's Finished
   message before sending the ticket.  The client MUST NOT treat the
   ticket as valid until it has verified the server's Finished message.

   If the server successfully verifies the client's ticket then it MAY
   renew the ticket by including a SessionTicket handshake message after
   the ServerHello in the abbreviated handshake.  The client should
   start using the new ticket as soon as possible after it verifies the
   Server's finished message for new connections.  Note that since the
   updated ticket is issued before the handshake completes it is

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   possible that the client may not put the new ticket into use before
   it initiates new connections.  The server MUST NOT assume the client
   actually received the updated ticket until it successfully verifies
   the client's finished message.

   The SessionTicket handshake message has been assigned the number
   TBD2.  The definition of the SessionTicket handshake message is given

      struct {
          HandshakeType msg_type;
          uint24 length;
          select (HandshakeType) {
              case hello_request:       HelloRequest;
              case client_hello:        ClientHello;
              case server_hello:        ServerHello;
              case certificate:         Certificate;
              case server_key_exchange: ServerKeyExchange;
              case certificate_request: CertificateRequest;
              case server_hello_done:   ServerHelloDone;
              case certificate_verify:  CertificateVerify;
              case client_key_exchange: ClientKeyExchange;
              case finished:            Finished;
              case new_session_ticket:  SessionTicket; /* NEW */
          } body;
      } Handshake;

      struct {
          opaque ticket<0..2^16-1>;
      } SessionTicket;

3.4  Interaction with TLS session ID

   If a server is planning on issuing a SessionTicket to a client that
   does not present one it SHOULD include an empty Session ID in the
   ServerHello.  If the server includes a non-empty session ID then it
   is indicating intent to use stateful session resume.  If the client
   receives a SessionTicket from the server then it discards any Session
   ID that was sent in the ServerHello.

   When presenting a ticket the client MAY generate and include a
   Session ID in the TLS ClientHello.  If the server accepts the ticket
   and the Session ID is not empty then it MUST respond with the same
   Session ID present in the ClientHello.  This allows the client to
   easily differentiate when the server is resuming a session or falling

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   back to a full handshake.  Since the client generates a Session ID
   the server MUST NOT rely upon the Session ID having a particular
   value when validating the ticket.  If a ticket is presented by the
   client the server MUST NOT attempt to use the Session ID in the
   ClientHello for stateful session resume.  Alternatively, the client
   MAY include an empty Session ID in the ClientHello.  In this case the
   client ignores the Session ID sent in the ServerHello and must
   determine if the server is resuming a session by the subsequent
   handshake messages.

4.  Recommended Ticket Construction

   This section describes a recommended format and protection for the
   ticket.  Note that the ticket is opaque to the client so the
   structure is not subject to interoperability concerns, so
   implementations may diverge from this format.  If implementations do
   diverge from this format they must take security concerns seriously.
   Clients MUST NOT examine the ticket under the assumption that it
   complies with this document.

   The server uses two different keys, one 128-bit key for AES
   encryption and one 128-bit key for HMAC-SHA1.

   The ticket is structured as follows:

      struct {
          uint32 key_version;
          opaque iv[16]
          opaque encrypted_state<0..2^16-1>;
          opaque mac[20];
      } Ticket;

   Here key_version identifies a particular set of keys.  One
   possibility is to generate new random keys every time the server is
   started, and use the timestamp as the key version.  The same
   mechanisms known from a number of other protocols can be reused for
   this purpose.

   The actual state information in encrypted_state is encrypted using
   128-bit AES in CBC mode with the given IV.  The MAC is calculated
   using HMAC-SHA1 over key_version (4 octets)and IV (16 octets),
   followed by the length of the encrypted_state field (2 octets) and
   its contents (variable length).

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      struct {
          ProtocolVersion protocol_version;
          CipherSuite cipher_suite;
          CompressionMethod compression_method;
          opaque master_secret[48];
          ExampleClientIdentity client_identity;
          uint32 timestamp;
      } StatePlaintext;

      enum {
     } ClientAuthenticationType;

      struct {
          ClientAuthenticationType client_authentication_type;
          select (ClientAuthenticationType) {
              case anonymous: struct {};
              case certificate_based:
                  ASN.1Cert certificate_list<0..2^24-1>;
              case psk:
                  opaque psk_identity<0..2^16-1>;

       } ClientIdentity;

   The structure StatePlaintext stores the TLS session state including
   the master_secret.  The timestamp within this structure allows the
   TLS server to expire tickets.  To cover the authentication and key
   exchange protocols provided by TLS the ClientIdentity structure
   contains the authentication type of the client used in the initial
   exchange (see ClientAuthenticationType).  To offer the TLS server
   with the same capabilities for authentication and authorization a
   certificate list is included in case of public key based
   authentication.  The TLS server is therefore able to inspect a number
   of different attributes within these certificates.  A specific
   implementation might choose to store a subset of this information or
   additional information.  Other authentication mechanism such as
   Kerberos [RFC2712] would require different client identity data.

5.  Security Considerations

   This section addresses security issues related to the usage of a
   ticket.  Tickets must be sufficiently authenticated and encrypted to
   prevent modification or eavesdropping by an attacker.  Several
   attacks described below will be possible if this is not carefully

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   Implementations should take care to ensure that the processing of
   tickets does not increase the chance of denial of serve as described

5.1  Invalidating Sessions

   The TLS specification requires that TLS sessions be invalidated when
   errors occur.  [CSSC] discusses the security implications of this in
   detail.  In the analysis in this paper, failure to invalidate
   sessions does not pose a security risk.  This is because the TLS
   handshake uses a non-reversible function to derive keys for a session
   so information about one session does not provide an advantage to
   attack the master secret or a different session.  If a session
   invalidation scheme is used the implementation should verify the
   integrity of the ticket before using the contents to invalidate a
   session to ensure an attacker cannot invalidate a chosen session.

5.2  Stolen Tickets

   An eavesdropper or man-in-the-middle may obtain the ticket and
   attempt to use the ticket to establish a session with the server,
   however since the ticket is encrypted and the attacker does not know
   the secret key a stolen key does not help an attacker resume a
   session.  A TLS server MUST use strong encryption and integrity
   protection for the ticket to prevent an attacker from using a brute
   force mechanism to obtain the tickets contents.

5.3  Forged Tickets

   A malicious user could forge or alter a ticket in order to resume a
   session, to extend its lifetime, to impersonate as another user or
   gain additional privileges.  This attack is not possible if the
   ticket is protected using a strong integrity protection algorithm
   such as a keyed HMAC.

5.4  Denial of Service Attacks

   An adversary could store or forge a large number of tickets to send
   to the TLS server for verification.  To minimize the possibility of a
   denial of service the verification of the ticket should be
   lightweight (e.g., using efficient symmetric key cryptographic

5.5  Ticket Protection Key Lifetime

   The management of the keys used to protect the ticket is beyond the
   scope of this document.  It is advisable to limit the lifetime of
   these keys to ensure they are not overused.

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5.6  Alternate Ticket Formats and Distribution Schemes

   If a different ticket format or distribution scheme than the ones
   defined in this document is used then great care must be taken in
   analyzing the security of the solution.  In particular if a secret is
   transferred to the client it MUST be done using secure communication
   so as to prevent attackers from obtaining or modifying the key.  Also
   the ticket MUST have its integrity and privacy protected with strong
   cryptographic techniques to prevent a breach in the security of the

6.  Acknowledgments

   The authors would like to thank the following people for their help
   with preparing and reviewing this document: Eric Rescorla, Mohamad
   Badra, Tim Dierks, Nelson Bolyard, Nancy Cam-Winget, David McGrew,
   Rob Dugal and members of the TLS working group.

   [CSSC] describes a solution that is very similar to the one described
   in this document and gives a detailed analysis of the security
   considerations involved.  [RFC2712] describes a mechanism for using
   Kerberos ([RFC1510]) in TLS ciphersuites, which helped inspire the
   use of tickets to avoid server state.  []
   makes use of a similar mechanism to avoid maintaining server state
   for the cryptographic tunnel.  [SC97] also investigates the concept
   of stateless sessions.

7.  IANA considerations

   IANA has assigned a TLS extension number of TBD1 (the value 35 is
   suggested) to the SessionTicket TLS extension from the TLS registry
   of ExtensionType values defined in [I-D.ietf-tls-rfc3546bis].

   IANA has assigned a TLS HandshakeType number TBD2 to the
   SessionTicket handshake type from the TLS registry of HandshakeType
   values defined in [I-D.ietf-tls-rfc2246-bis].

8.  References

8.1  Normative References

   [AES]      National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", Federal Information
              Processing Standards (FIPS) Publication 197,
              November 2001.

   [CBC]      National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation -

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              Methods and Techniques", NIST Special Publication 800-38A,
              December 2001.

              Dierks, T. and E. Rescorla, "The TLS Protocol Version
              1.1", draft-ietf-tls-rfc2246-bis-13 (work in progress),
              June 2005.

              Blake-Wilson, S., "Transport Layer Security (TLS)
              Extensions", draft-ietf-tls-rfc3546bis-02 (work in
              progress), October 2005.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

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

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [SHA1]     National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", Federal Information Processing
              Standards    (FIPS) Publication  180-2, August 2002.

8.2  Informative References

   [CSSC]     Shacham, H., Boneh, D., and E. Rescorla, "Client-side
              caching for TLS", Transactions on Information and
              System Security (TISSEC) , Volume 7, Issue 4,
              November 2004.

              Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "EAP
              Flexible Authentication via Secure Tunneling (EAP-FAST)",
              draft-cam-winget-eap-fast-02 (work in progress),
              April 2005.

              Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", draft-ietf-tls-psk-09
              (work in progress), June 2005.

   [RFC1510]  Kohl, J. and B. Neuman, "The Kerberos Network
              Authentication Service (V5)", RFC 1510, September 1993.

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   [RFC2712]  Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              October 1999.

   [SC97]     Aura, T. and P. Nikander, "Stateless Connections",
              Proceedings of the First International Conference on
              Information and Communication Security (ICICS '97) , 1997.

Authors' Addresses

   Joseph Salowey
   Cisco Systems
   2901 3rd Ave
   Seattle, WA  98121


   Hao Zhou
   Cisco Systems
   4125 Highlander Parkway
   Richfield, OH  44286


   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group


   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bayern  81739


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