INTERNET-DRAFT S. Santesson (3xA Security)
Intended Status: Proposed Standard
Expires: January 13, 2011 July 12, 2010
Transport Layer Security (TLS) Cached Information Extension
<draft-ietf-tls-cached-info-09.txt>
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 handshakes, allowing
the server to omit sending cached static information to the client
during the TLS handshake protocol exchange.
Status of this Memo
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Copyright and License Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Cached Information Extension . . . . . . . . . . . . . . . . . 4
3. Extension Exchange . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Cached Information . . . . . . . . . . . . . . . . . . . . 5
3.2. Reconnaissance . . . . . . . . . . . . . . . . . . . . . . 5
4. Cached Information Substitution . . . . . . . . . . . . . . . . 6
4.1. Substitution Syntax for certificate_chain . . . . . . . . 6
4.2. Substitution Syntax for trusted_cas . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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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
cached information 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 {
CachedInformationType type;
HashAlgorithm hash;
opaque hash_value<1..255>;
} CachedObject;
struct {
CachedObject cached_info<1..2^16-1>;
} CachedInformation;
When CachedInformationType identifies certificate_chain, then
hash_value MUST include a hash calculated over the certificate_list
element of a server side Certificate message, excluding the three
length bytes of the certificate_list vector.
When CachedInformationType identifies trusted_cas, then hash_value
MUST include a hash calculated over the certificate_authorities
element of a server side CertificateRequest message, excluding the
two length bytes of the certificate_authorities vector.
The hash algorithm used to calculate hash values SHALL be the hash
algorithm that was used to generate the Finished message in the
handshake exchange from which the hashed information was cached. Hash
algorithm identifiers are defined in the RFC 5246 [RFC5246]
HashAlgorithm registry.
Other specifications MAY define more CachedInformationType types.
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3. Extension Exchange
3.1. Cached Information
Clients MAY include a "cached_information" extension in the
(extended) client hello, which MAY contain zero or more cached
objects (CachedObject).
Servers that receive an extended client hello containing a
"cached_information" extension MAY indicate that they support cached
information objects by including a cached_information extension in
their (extended) server hello.
A cached_information extension provided in the server hello has the
following semantics:
o An empty cached_information extension indicates that the server
supports information caching but provides no information about
what information types it supports.
o A non-empty cached information extension indicates that the
server supports caching of each present CachedObject that matches
the specified hash value. The server MAY support other cached
objects that are not present in the extension.
Note: 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.
3.2. Reconnaissance
A client MAY include an empty cached_information extension (with
empty extension_data field) in its (extended) client hello to query
whether the server supports cached information.
Upon receiving an empty cached_information extension, a server MAY
indicate that it supports cached information in handshakes by
including a cached_information extension in its (extended) server
hello according to any of the available options in section 3.1.
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4. Cached Information Substitution
Following a successful exchange of "cached_information" extensions,
the server MAY substitute cached information in the handshake
exchange with a matching CachedObject from the client hello
"cached_information" extension.
A substitution syntax that defines how the CachedObject structure is
carried in the handshake message MUST be defined for each
CachedInformationType in a way that does not violate the syntax of
the handshake message. The substitution syntax for
certificate_chain(1) and trusted_cas(2) is provided below.
The handshake protocol SHALL proceed using the cached information as
if it was provided in the handshake protocol. The Finished message
SHALL 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
information that were omitted from transmission.
The server MUST NOT include more than one CachedObject as
substitution for the cached information.
4.1. Substitution Syntax for certificate_chain
When an object of type certificate_chain is provided in the client
hello, the server MAY substitute the cached information with a
matching hash value received from the client by expanding the
Certificate handshake message as follows.
Original handshake message syntax defined in RFC 5246 [RFC5246]:
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
Substitution syntax is defined by expanding the syntax of the opaque
ASN.1Cert structure:
CachedObject ASN.1Cert<1..2^24-1>;
4.2. Substitution Syntax for trusted_cas
When a hash for an object of type trusted_cas is provided in the
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client hello, the server MAY substitute the cached information with a
matching hash value received from the client by expanding the
CertificateRequest handshake message as follows.
Original handshake message syntax defined in RFC 5246 [RFC5246]:
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest
The substitution syntax is defined by expanding the syntax of the
opaque DistinguishedName structure:
CachedObject DistinguishedName<1..2^16-1>;
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5. Security Considerations
The hash algorithm used in this specification is required to have
reasonable random properties in order to provide reasonably unique
identifiers. There is no clearly identified requirement that this
hash algorithm must have strong collision resistance. However since
the hash algorithm is used to represent data in the finished
calculation, the security properties of the finished calculation will
change if a weaker hash algorithm is used to represent cached
information compared with the hash algorithm used to calculate the
finished message.
Caching information in an encrypted handshake (such as a renegotiated
handshake) and sending a hash of that cached information in an
unencrypted handshake might introduce integrity or data disclosure
issues as it enables an attacker to identify if a known object (such
as a known server certificate) has been used in previous encrypted
handshakes. Information object types defined in this specification,
such as server certificates, are public objects and usually not
sensitive in this regard, but implementers should be aware if any
cached information are subject to such security concerns and in such
case SHOULD NOT send a hash over encrypted data in en unencrypted
handshake.
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. Acknowledgements
The author acknowledges input from many members of the TLS working
group.
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8. 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
Scheelev. 17
223 70 Sweden
EMail: sts@aaa-sec.com
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