TLS S. Santesson
Internet-Draft 3xA Security AB
Intended status: Standards Track H. Tschofenig
Nokia Siemens Networks

Transport Layer Security (TLS) Cached Information Extension


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.

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Table of Contents

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.

2. 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 [RFC2119].

3. 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.

4. Extension Exchange

4.1. 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:

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 [I-D.ietf-tls-rfc4366-bis] value.

4.2. 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 4.1.

5. 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.

5.1. 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>;

5.2. Substitution Syntax for trusted_cas

When a hash for an object of type trusted_cas 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 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>;
          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>;

6. 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.

7. 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.

8. Acknowledgments

The author acknowledges input from many members of the TLS working group.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[I-D.ietf-tls-rfc4366-bis] 3rd, D, "Transport Layer Security (TLS) Extensions: Extension Definitions", Internet-Draft draft-ietf-tls-rfc4366-bis-12, September 2010.

9.2. Informative References

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

Authors' Addresses

Stefan Santesson 3xA Security AB Scheelev. 17 Lund, 223 70 Sweden EMail:
Hannes Tschofenig Nokia Siemens Networks Linnoitustie 6 Espoo, 02600 Finland Phone: +358 (50) 4871445 EMail: URI: