INTERNET-DRAFT S. Santesson (3xA Security)
Intended Status: Proposed Standard
Expires: August 22, 2010 February 18, 2010
Transport Layer Security (TLS) Cached Information Extension
<draft-ietf-tls-cached-info-03.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.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Cached Information Extension . . . . . . . . . . . . . . . . . . 4
4 Message flow . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7 Normative References . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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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
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].
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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, when included in the
client hello, 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;
struct {
CachedObject cached_info<1..2^16-1>;
} 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.
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The client MUST NOT include hashes for multiple objects in the same
CachedObject structure. If more than one hash is present in the
CachedObject structure, they MUST be hashes over the same information
object using different hash algorithms.
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. Clients MAY need the ability
to cache different values depending on other information in the
Client Hello that modify what values the server uses, in particular
the Server Name Indication [RFC4366] value.
Servers that receive an extended client hello containing a
"cached_information" extension, MAY indicate that they support
caching of information objects by including an extension of type
"cached_information" with an empty extension_data field in their
(extended) server hello.
Following a successful exchange of "cached_information" extensions,
the server may replace data objects identified through the client
extension with any of the CachedInformationHash values received from
the client, which matches the replaced object.
The handshake protocol will proceed using the cached data as if it
was 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.
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.
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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
[RFC4366] S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen, T.
Wright, "Transport Layer Security (TLS) Extensions", RFC
4366, April 2006
NOTE: RFC 4366 will be updated by RFC4366bis, currently in IESG
process.
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Authors' Addresses
Stefan Santesson
3xA Security AB
Bjornstorp 744
247 98 Genarp
Sweden
EMail: sts@aaa-sec.com
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