--- /dev/null
+
+
+
+Network Working Group J. Salowey
+Internet-Draft H. Zhou
+Expires: June 18, 2006 Cisco Systems
+ P. Eronen
+ Nokia
+ H. Tschofenig
+ Siemens
+ December 15, 2005
+
+
+ Transport Layer Security Session Resumption without Server-Side State
+ draft-salowey-tls-ticket-06.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 June 18, 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
+
+
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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 . . . . . . . . . . . . . 6
+ 3.4 Interaction with TLS session ID . . . . . . . . . . . . . 7
+ 4. Recommended Ticket Construction . . . . . . . . . . . . . . . 8
+ 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
+ 5.1 Invalidating Sessions . . . . . . . . . . . . . . . . . . 10
+ 5.2 Stolen Tickets . . . . . . . . . . . . . . . . . . . . . . 10
+ 5.3 Forged Tickets . . . . . . . . . . . . . . . . . . . . . . 10
+ 5.4 Denial of Service Attacks . . . . . . . . . . . . . . . . 10
+ 5.5 Ticket Protection Key Lifetime . . . . . . . . . . . . . . 10
+ 5.6 Alternate Ticket Formats and Distribution Schemes . . . . 11
+ 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
+ 7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 11
+ 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
+ 8.1 Normative References . . . . . . . . . . . . . . . . . . . 11
+ 8.2 Informative References . . . . . . . . . . . . . . . . . . 12
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
+ Intellectual Property and Copyright Statements . . . . . . . . 14
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+1. Introduction
+
+ This document defines a way to resume a Transport Layer Security
+ (TLS) session without requiring session-specific state at the TLS
+ server. This mechanism may be used with any TLS ciphersuite. This
+ document applies to both TLS 1.0 defined in [RFC2246] and TLS 1.1
+ defined in [I-D.ietf-tls-rfc2246-bis]. 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 a mechanism 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 mechanisms. 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
+
+
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+ If the server wants to use this mechanism, it stores its session
+ state (such as ciphersuite and master secret) to a ticket that is
+ 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
+ (empty SessionTicket extension)
+ 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
+ decrypts the received ticket, verifies that the ticket is valid and
+ has not been tampered with, 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.
+
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+ ClientHello
+ (SessionTicket extension) -------->
+ ServerHello
+ (empty SessionTicket extension)
+ SessionTicket
+ [ChangeCipherSpec]
+ <-------- Finished
+ [ChangeCipherSpec]
+ Finished -------->
+ Application Data <-------> Application Data
+
+ 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 may be sent
+ in the ClientHello and ServerHello.
+
+ If the client possesses 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 SessionTicket extension
+ unless it is sending a non-empty ticket it received through some
+ other means from the server.
+
+ The server uses an zero length SessionTicket extension to indicate to
+ the client that it will send a new session ticket using the
+ SessionTicket handshake message described in Section 3.3. The server
+ MUST send this extension in the ServerHello if the server wishes to
+ issue a new ticket to the client using the SessionTicket handshake
+ message. The server MUST NOT send this extension if the client does
+ not include a SessionTicket handshake message in the client hello.
+
+ 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.
+
+
+
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+ struct {
+ opaque ticket<0..2^16-1>;
+ } SessionTicket;
+
+
+3.3 SessionTicket handshake message
+
+ This message is sent by the server during the TLS handshake before
+ the ChangeCipherSpec message. This message MUST be sent if the
+ server included a SessionTicket extension in the ServerHello. This
+ message MUST NOT be sent if the server did not include a
+ SessionTicket extension in the ServerHello. 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 determines that it does not want to include a ticket after it
+ has included the SessionTicket extension in the ServerHello then it
+ MAY send a zero length ticket in the SessionTicket handshake 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
+ 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.
+
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+ 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 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
+ 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 determines
+ if the server is resuming a session by the subsequent handshake
+ messages.
+
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+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 [AES] in
+ CBC mode [CBC] encryption and one 128-bit key for HMAC-SHA1 [RFC2104]
+ [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 {
+ 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.
+
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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
+ 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 ticket 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-SHA1.
+
+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.
+
+
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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
+ 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 ([RFC4120]) 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
+
+ [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)
+
+
+
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+ Extensions", draft-ietf-tls-rfc3546bis-02 (work in
+ progress), October 2005.
+
+ [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.
+
+8.2 Informative 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 -
+ Methods and Techniques", NIST Special Publication 800-38A,
+ December 2001.
+
+ [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.
+
+ [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
+ Hashing for Message Authentication", RFC 2104,
+ February 1997.
+
+ [RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
+ Suites to Transport Layer Security (TLS)", RFC 2712,
+ October 1999.
+
+ [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
+ Kerberos Network Authentication Service (V5)", RFC 4120,
+ July 2005.
+
+ [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
+ for Transport Layer Security (TLS)", RFC 4279,
+ December 2005.
+
+
+
+
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+ [SC97] Aura, T. and P. Nikander, "Stateless Connections",
+ Proceedings of the First International Conference on
+ Information and Communication Security (ICICS '97) , 1997.
+
+ [SHA1] National Institute of Standards and Technology, "Secure
+ Hash Standard (SHS)", Federal Information Processing
+ Standards (FIPS) Publication 180-2, August 2002.
+
+
+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
+
+
+
+
+
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+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
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+ http://www.ietf.org/ipr.
+
+ The IETF invites any interested party to bring to its attention any
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+
+
+Disclaimer of Validity
+
+ This document and the information contained herein are provided on an
+ "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+ OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+ ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+ INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+
+Copyright Statement
+
+ Copyright (C) The Internet Society (2005). This document is subject
+ to the rights, licenses and restrictions contained in BCP 78, and
+ except as set forth therein, the authors retain all their rights.
+
+
+Acknowledgment
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+Salowey, et al. Expires June 18, 2006 [Page 14]
+\f
projects / thirdparty / gnutls.git / commitdiff
