INTERNET-DRAFT S. Santesson (3xA Security)
Intended Status: Proposed Standard Q. Dang (NIST)
Expires January 2010 June 2009
Transport Layer Security (TLS) Cached Information Extension
<draft-ietf-tls-cached-info-00.txt>
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Abstract
This document defines a Transport Layer Security (TLS) extension for
cached information. This extension allows the TLS client to inform a
server of cached information from previous TLS sessions, allowing the
server to omit sending cached static information to the client during
the TLS handshake protocol exchange.
1 Introduction
TLS handshakes often include fairly static information such as server
certificate and a list of trusted Certification Authorities (CAs).
Static information such as a server certificate can be of
considerable size. This is the case in particular if the server
certificate is bundled with a complete certificate path, including
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all intermediary certificates up to the trust anchor public key.
Significant benefits can be achieved in low bandwidth and high
latency networks, in particular if the communication channel also has
a relatively high rate of transmission errors, if a known and
previously cached server certificate path can be omitted from the TLS
handshake.
This specification defines the Cached Information TLS extension,
which may be used by a client and a server to exclude transmission of
known cached parameters from the TLS handshake.
1.1 Terminology
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 RFC 2119 [RFC2119].
2 Cached Information Extension
A new extension type (cached_information(TBD)) is defined and used in
both the client hello and server hello messages. The extension type
is specified as follows.
enum {
cached_information(TBD), (65535)
} ExtensionType;
The "extension_data" field of this extension SHALL contain
"CachedInformation" according to the following structure:
enum {
certificate_chain(1), trusted_cas(2), (255)
} CachedInformationType;
struct {
HashAlgorithm hash;
opaque hash_value<1..255>;
} CachedInformationHash;
struct {
CachedInformationType type;
CachedInformationHash hashes<1..2^16-1>;
} CachedObject;
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struct {
CachedObject cached_info<1..2^24-1>;;
} CachedInformation;
Hash algorithm identifiers are provided by the RFC 5246 [RFC5246]
HashAlgorithm registry. Compliant implementations MUST support
sha1(2) as HashAlgorithm.
When CachedInformationType identifies certificate_chain, then
hash_value MUST include at least one hash value calculated over the
certificate_list element of a server side Certificate message.
When CachedInformationType identifies trusted_cas, then hash_value
MUST include at least one hash value calculated over the
certificate_authorities element of a server side CertificateRequest
message.
Other specifications MAY define more CachedInformationType types.
4 Message flow
Clients MAY include an extension of type "cached_information" in the
(extended) client hello, which SHALL contain at least one
CachedObject as specified in section 2.
Servers that receive an extended client hello containing a
"cached_information" extension, MAY indicate that they support one or
more of the cached information objects by including an extension of
type "cached_information" in the (extended) server hello, which SHALL
contain at least one CachedObject received from the client. The
CachedObject's returned by the server MUST include the types the
server supports and has accepted to replace with a hash of the cached
data.
After negotiation of the use of cached certificates has been
successfully completed (by exchanging hello messages including
"cached_certs" extensions), the server MUST replace agreed cached
information objects in its handshake messages with a corresponding
hash_value from CachedInformationHash that was included in the
cached_information extension of the server hello message.
The handshake protocol will proceed using the cached data as if it
they were provided in the handshake protocol. The finished message
will however be calculated over the actual data exchanged in the
handshake protocol. That is, the finished message will be calculated
over the hash values of cached information objects and not over the
cached objects that were omitted from transmission.
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5 Security Considerations
Hash algorithms used in this specification are required to have
reasonable random properties in order to provide reasonably unique
identifiers. Failure of a provided hash to correctly and uniquely
identify the correct set of hashed parameters may at most lead to a
failed TLS handshake followed by a new attempt without the cached
information extension. No serious security threat requires selected
hash algorithms to have strong collision resistance.
6 IANA Considerations
1) Create an entry, cached_information(TBD), in the existing registry
for ExtensionType (defined in RFC 5246 [RFC5246]).
2) Establish a registry for TLS CachedInformationType values. The
first entries in the registry are certificate_chain(1) and
trusted_cas(2). TLS CachedInformationType values in the inclusive
range 0-63 (decimal) are assigned via RFC 5226 [RFC5226] Standards
Action. Values from the inclusive range 64-223 (decimal) are
assigned via RFC 5226 Specification Required. Values from the
inclusive range 224-255 (decimal) are reserved for RFC 5226 Private
Use.
7 Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] T. Narten, H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
May 2008.
[RFC5246] T. Dierks, E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008
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Authors' Addresses
Stefan Santesson
3xA Security AB
Bjornstorp 744
247 98 Genarp
Sweden
EMail: sts@aaa-sec.com
Quynh Dang
NIST
100 Bureau Drive, Stop 8930
Gaithersburg, MD 20899-8930
USA
Email: quynh.dang@nist.gov
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Expires January 2009
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