Network Working Group J. Salowey Internet-Draft H. Zhou Expires: April 21, 2006 Cisco Systems P. Eronen Nokia H. Tschofenig Siemens October 18, 2005 Transport Layer Security Session Resumption without Server-Side State draft-salowey-tls-ticket-05.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 21, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract 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 Salowey, et al. Expires April 21, 2006 [Page 1] Internet-Draft Stateless TLS Session Resumption October 2005 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 Salowey, et al. Expires April 21, 2006 [Page 2] Internet-Draft Stateless TLS Session Resumption October 2005 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 Salowey, et al. Expires April 21, 2006 [Page 3] Internet-Draft Stateless TLS Session Resumption October 2005 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) ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> SessionTicket [ChangeCipherSpec] <-------- 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 ServerHello. ClientHello (SessionTicket extension) --------> ServerHello SessionTicket [ChangeCipherSpec] <-------- Finished [ChangeCipherSpec] Finished --------> Application Data <-------> Application Data Salowey, et al. Expires April 21, 2006 [Page 4] Internet-Draft Stateless TLS Session Resumption October 2005 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 Salowey, et al. Expires April 21, 2006 [Page 5] Internet-Draft Stateless TLS Session Resumption October 2005 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 below. 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 Salowey, et al. Expires April 21, 2006 [Page 6] Internet-Draft Stateless TLS Session Resumption October 2005 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). Salowey, et al. Expires April 21, 2006 [Page 7] Internet-Draft Stateless TLS Session Resumption October 2005 struct { ProtocolVersion protocol_version; CipherSuite cipher_suite; CompressionMethod compression_method; opaque master_secret[48]; ExampleClientIdentity client_identity; uint32 timestamp; } StatePlaintext; enum { anonymous(0), certificate_based(1), psk(2) } 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 done. Salowey, et al. Expires April 21, 2006 [Page 8] Internet-Draft Stateless TLS Session Resumption October 2005 Implementations should take care to ensure that the processing of tickets does not increase the chance of denial of serve as described below. 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 algorithms). 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. Salowey, et al. Expires April 21, 2006 [Page 9] Internet-Draft Stateless TLS Session Resumption October 2005 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 system. 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. [I-D.cam-winget-eap-fast] 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 - Salowey, et al. Expires April 21, 2006 [Page 10] Internet-Draft Stateless TLS Session Resumption October 2005 Methods and Techniques", NIST Special Publication 800-38A, December 2001. [I-D.ietf-tls-rfc2246-bis] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1", draft-ietf-tls-rfc2246-bis-13 (work in progress), June 2005. [I-D.ietf-tls-rfc3546bis] 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. [I-D.cam-winget-eap-fast] 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. [I-D.ietf-tls-psk] 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. Salowey, et al. Expires April 21, 2006 [Page 11] Internet-Draft Stateless TLS Session Resumption October 2005 [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 US Email: jsalowey@cisco.com Hao Zhou Cisco Systems 4125 Highlander Parkway Richfield, OH 44286 US Email: hzhou@cisco.com Pasi Eronen Nokia Research Center P.O. Box 407 FIN-00045 Nokia Group Finland Email: pasi.eronen@nokia.com Hannes Tschofenig Siemens Otto-Hahn-Ring 6 Munich, Bayern 81739 Germany Email: Hannes.Tschofenig@siemens.com Salowey, et al. Expires April 21, 2006 [Page 12] Internet-Draft Stateless TLS Session Resumption October 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Salowey, et al. Expires April 21, 2006 [Page 13]