Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-19.txt

Abstract

Transport Layer Security (TLS) handshakes often include fairly static information, such as the server certificate and a list of trusted certification authorities (CAs). This information can be of considerable size, particularly if the server certificate is bundled with a complete certificate chain (i.e., the certificates of intermediate CAs up to the root CA).

This document defines an extension that allows a TLS client to inform a server of cached information, allowing the server to omit already available information.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on September 24, 2015.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Cached Information Extension
• 4. Exchange Specification
• 4.1. Server Certificate Message
• 4.2. CertificateRequest Message
• 5. Example
• 6. Security Considerations
• 7. IANA Considerations
• 7.1. New Entry to the TLS ExtensionType Registry
• 7.2. New Registry for CachedInformationType
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

Reducing the amount of information exchanged during a Transport Layer Security handshake to a minimum helps to improve performance in environments where devices are connected to a network with a low bandwidth, and lossy radio technology. With Internet of Things such environments exist, for example, when devices use IEEE 802.15.4 or Bluetooth Smart. For more information about the challenges with smart object deployments please see [RFC6574].

This specification defines a TLS extension that allows a client and a server to exclude transmission information cached in an earlier TLS handshake.

A typical example exchange may therefore look as follows. First, the client and the server executes the full TLS handshake. The client then caches the certificate provided by the server. When the TLS client connects to the TLS server some time in the future, without using session resumption, it then attaches the cached_info extension defined in this document to the client hello message to indicate that it had cached the certificate, and it provides the fingerprint of it. If the server's certificate has not changed then the TLS server does not need to send its' certificate and the corresponding certificate chain again. In case information has changed, which can be seen from the fingerprint provided by the client, the certificate payload is transmitted to the client to allow the client to update the cache.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

This document refers to the TLS protocol but the description is equally applicable to DTLS as well.

3. Cached Information Extension

This document defines a new extension type (cached_info(TBD)), which is used in client hello and server hello messages. The extension type is specified as follows.

              
      enum {
           cached_info(TBD), (65535)
      } ExtensionType;

The extension_data field of this extension, when included in the client hello, MUST contain the CachedInformation structure. The client MAY send multiple CachedObjects of the same CachedInformationType. This may, for example, be the case when the client has cached multiple certificates from a server.

              
      enum {
           cert(1), cert_req(2) (255)
      } CachedInformationType;

      struct {
           select (type) {
             case client: 
               CachedInformationType type;
               opaque hash_value<1..255>;
             case server: 
               CachedInformationType type; 
           } body;
      } CachedObject;

      struct {
           CachedObject cached_info<1..2^16-1>;
      } CachedInformation;

This document defines the following types:

Omitting the Server Certificate Message:

With the type field set to 'cert', the client MUST include the message digest of the Certificate message in the hash_value field. For this type the message digest MUST be calculated using SHA-256 [RFC4634].

Omitting the CertificateRequest Message

With the type set to 'cert_req', the client MUST include the message digest of the CertificateRequest message in the hash_value field. For this type the message digest MUST be calculated using SHA-256 [RFC4634].

New types can be added following the policy described in the IANA considerations section, see Section 7. Different message digest algorithms for use with these types can also be added by registering a new type that makes use of this updated message digest algorithm.

4. Exchange Specification

Clients supporting this extension MAY include the "cached_info" extension in the (extended) client hello. If the client includes the extension then it MUST contain one or more CachedObject attributes.

A server supporting this extension MAY include the "cached_info" extension in the (extended) server hello. By returning the "cached_info" extension the server indicates that it supports the cached info types. For each indicated cached info type the server MUST alter the transmission of respective payloads, according to the rules outlined with each type. If the server includes the extension it MUST only include CachedObjects of a type also supported by the client (as expressed in the client hello). For example, if a client indicates support for 'cert' and 'cert_req' then the server cannot respond with a "cached_info" attribute containing support for 'cert_status'.

Since the client includes a fingerprint of information it cached (for each indicated type) the server is able to determine whether cached information is stale. If the server supports this specification and notices a mismatch between the data cached by the client and its own information then the server MUST include the information in full and MUST NOT list the respective type in the "cached_info" extension.

Note: If a server is part of a hosting environment then the client may have cached multiple data items for a single server. To allow the client to select the appropriate information from the cache it is RECOMMENDED that the client utilizes the Server Name Indication extension [RFC6066].

Following a successful exchange of the "cached_info" extension in the client and server hello, the server alters sending the corresponding handshake message. How information is altered from the handshake messages is defined in Section 4.1, and in Section 4.2 for the types defined in this specification.

4.1. Server Certificate Message

When a ClientHello message contains the "cached_info" extension with a type set to 'cert' then the server MAY send the Certificate message shown in Figure 2 under the following conditions:

• The server software implements the "cached_info" extension defined in this specification.
• The 'cert' cached info extension is enabled (for example, a policy allows the use of this extension).
• The server compared the value in the hash_value field of the client-provided "cached_info" extension with the fingerprint of the Certificate message it normally sends to clients. This check ensures that the information cached by the client is current.

The original Certificate handshake message syntax is defined in RFC 5246 [RFC5246] and has the structure shown in Figure 1.

              
      opaque ASN.1Cert<1..2^24-1>;

      struct {
          ASN.1Cert certificate_list<0..2^24-1>;
      } Certificate;

              
      struct {
          opaque hash_value<1..255>;
      } CertificateRequest;

              
      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;

              
      struct {
          opaque hash_value<1..255>;
      } CertificateRequest;

              
(A) Initial (full) Exchange 

ClientHello            ->
                       <-  ServerHello
                           Certificate* // [1]
                           ServerKeyExchange*
                           CertificateRequest*
                           ServerHelloDone 

Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished               ->

                       <- [ChangeCipherSpec]
                          Finished

Application Data <-------> Application Data


(B) TLS Cached Extension Usage

ClientHello 
cached_info=(cert)     -> // [2]
                       <-  ServerHello
                           cached_info=(cert) [3] 
                           Certificate [4]
                           ServerKeyExchange*
                           ServerHelloDone 

ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished               ->

                       <- [ChangeCipherSpec]
                          Finished

Application Data <-------> Application Data