TLS Working Group J. Salowey
Internet-Draft H. Zhou
Expires: August 23, 2005 Cisco Systems
P. Eronen
Nokia
H. Tschofenig
Siemens
February 19, 2005
TLS Session Resumption without Server-Side State
draft-salowey-tls-ticket-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a mechanism which enables the TLS server to
resume sessions and avoid keeping per-client session state. The TLS
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server encapsulates the session state into a ticket and forwards it
to the client. The client can subsequently resume a session using
the obtained ticket. This mechanism makes use of new TLS handshake
messages and TLS hello extensions.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2 Format of SessionTicket TLS extension . . . . . . . . . . 5
3.3 Format of NewSessionTicket handshake message . . . . . . . 5
4. Sample ticket construction . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1 Invalidating Sessions . . . . . . . . . . . . . . . . . . 8
5.2 Stolen Tickets . . . . . . . . . . . . . . . . . . . . . . 8
5.3 Forged Tickets . . . . . . . . . . . . . . . . . . . . . . 8
5.4 Denial of Service Attacks . . . . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1 Normative References . . . . . . . . . . . . . . . . . . . 9
8.2 Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction
This document defines a way to resume a TLS 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 [RFC3546] 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
3.1 Overview
The client indicates that it supports this mechanism by including an
empty SessionTicket TLS extension in the ClientHello message.
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 NewSessionTicket
TLS handshake message. This message is sent during the TLS handshake
before the ChangeCipherSpec message after the server has verified the
client's Finished message.
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Client Server
ClientHello -------->
(empty SessionTicket extension)
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
NewSessionTicket
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
The client caches this ticket along with the master secret, session
ID and other parameters associated with the current session. When
the client wishes to resume the session, it includes a SessionTicket
TLS extension in the SessionTicket extension within ClientHello
message. The server then verifies that the ticket has not been
tampered with, decrypts the contents, and retrieves the session state
from the contents of the ticket and uses this state to resume the
session. Since separate fields in the request are used for the
session ID and the ticket standard stateful session resume can
co-exist with the ticket based session resume described in this
specification.
ClientHello
(SessionTicket extension) -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Since the ticket is typically interpreted by the same server or group
of servers that created it, the exact format of the ticket does not
need to be the same for all implementations. A sample 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.
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It is possible that the session ticket and master session key could
be delivered through some out of band mechanism. This behavior is
beyond the scope of the document and would need to be described in a
separate specification.
3.2 Format of SessionTicket TLS extension
The format of the ticket is an opaque structure used to carry session
specific state information.
struct {
opaque ticket<0..2^16-1>;
} SessionTicket;
3.3 Format of NewSessionTicket handshake message
This message is sent during the TLS handshake before the
ChangeCipherSpec message after the server has verified the client's
Finished message.
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: NewSessionTicket; /* NEW */
} body;
} Handshake;
struct {
opaque ticket<0..2^16-1>;
} NewSessionTicket;
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4. Sample ticket construction
This section describes one possibility how the ticket could be
constructed, other implementations are possible.
The server uses two 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];
} ExampleTicket;
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 contents of the encrypted_state field (without the
length).
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struct {
ProtocolVersion protocol_version;
SessionID session_id;
CipherSuite cipher_suite;
CompressionMethod compression_method;
opaque master_secret[48];
ClientIdentity client_identity;
uint32 timestamp;
} ExampleStatePlaintext;
enum {
anonymous(0),
certificate_based(1)
} ExampleClientAuthenticationType;
struct {
ExampleClientAuthenticationType client_authentication_type;
select (ExampleClientAuthenticationType) {
case anonymous: struct {};
case certificate_based:
ASN.1Cert certificate_list<0..2^24-1>;
}
} ExampleClientIdentity;
The structure ExampleStatePlaintext stores the TLS session state
including the SessionID and 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
ExampleClientIdentity structure contains the authentication type of
the client used in the initial exchange (see
ExampleClientAuthenticationType). 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. Other authentication mechanism
such as Kerberos or pre-shared keys 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.
Implementations should take care to ensure that the processing of
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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).
6. Acknowledgments
The authors would like to thank the following people for their help
with this document: Eric Rescorla, Nancy Cam-Winget and David McGrew
[RFC2712] describes a mechanism for using Kerberos ([RFC1510]) in TLS
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ciphersuites, which helped inspire the use of tickets to avoid server
state. [EAP-FAST] makes use of a similar mechanism to avoid
maintaining server state for the cryptographic tunnel. [AURA97] also
investigates the concept of stateless sessions. [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.
7. IANA considerations
Needs a TLS extension number (for including the ticket in client
hello), and HandshakeType number (for delivering the ticket to the
client).
8. References
8.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
8.2 Informative References
[AURA97] Aura, T. and P. Nikander, "Stateless Connections",
Proceedings of the First International Conference on
Information and Communication Security (ICICS '97) , 1997.
[CSSC] Shacham, H., Boneh, D. and E. Rescorla, "Client Side
Caching for TLS",
URI http://crypto.stanford.edu/~dabo/papers/fasttrack.pdf,
2002.
[EAP-FAST]
Cam-Winget, N., McGrew, D., Salowey, J. and H. Zhou, "EAP
Flexible Authentication via Secure Tunneling (EAP-FAST)",
Internet-Draft work-in-progress, February 2004.
[RFC1510] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[RFC2712] Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
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Suites to Transport Layer Security (TLS)", RFC 2712,
October 1999.
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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