Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-23.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/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on November 12, 2016.

Copyright Notice

Copyright (c) 2016 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. Fingerprint Calculation
• 6. Example
• 7. Security Considerations
• 8. IANA Considerations
• 8.1. New Entry to the TLS ExtensionType Registry
• 8.2. New Registry for CachedInformationType
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. Example
• 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 two types:

'cert' Type for not sending the complete Server Certificate Message:

With the type field set to 'cert', the client MUST include the fingerprint of the Certificate message in the hash_value field. For this type the fingerprint MUST be calculated using the procedure described in Section 5 with the Certificate message as input data.

'cert_req' Type for not sending the complete CertificateRequest Message:

With the type set to 'cert_req', the client MUST include the fingerprint of the CertificateRequest message in the hash_value field. For this type the fingerprint MUST be calculated using the procedure described in Section 5 with the CertificateRequest message as input data.

New cached info types can be added following the policy described in the IANA considerations section, see Section 8. New message digest algorithms for use with these types can also be added by registering a new type that makes use of the updated message digest algorithm. For practical reasons we recommend to re-use hash algorithms already available with TLS ciphersuites to avoid additional code and to keep the collision probably low new hash algorithms MUST NOT have a collision resistance worse than SHA-256.

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 ('foo-bar').

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.

Appendix A shows an example hash calculation and Section 6 shows an example protocol exchange.

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 1 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 procedure for calculating the fingerprint is described in Section 5.

The original Certificate handshake message syntax is defined in [RFC5246] and has been extended with [RFC7250]. RFC 7250 allows the certificate payload to contain only the SubjectPublicKeyInfo instead of the full information typically found in a certificate. Hence, when this specification is used in combination with [RFC7250] and the negotiated certificate type is a raw public key then the TLS server omits sending a Certificate payload that contains an ASN.1 Certificate structure with the included SubjectPublicKeyInfo rather than the full certificate chain. As such, this extension is compatible with the raw public key extension defined in RFC 7250. Note: We assume that the server implementation is able to select the appropriate certificate or SubjectPublicKeyInfo from the received hash value. If the SNI extension is used by the client then the server has additional information to guide the selection of the appropriate cached info.

When the cached info specification is used then a modified version of the Certificate message is exchanged. The modified structure is shown in Figure 1.

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

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

              
ClientHello            ->
                       <-  ServerHello
                           Certificate* // (1)
                           ServerKeyExchange*
                           CertificateRequest*
                           ServerHelloDone 

Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished               ->

                       <- [ChangeCipherSpec]
                          Finished

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

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

ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished               ->

                       <- [ChangeCipherSpec]
                          Finished

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

              
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  4 434:   SEQUENCE {
  8   3:     [0] {
 10   1:       INTEGER 2
       :       }
 13   1:     INTEGER 13
 16  10:     SEQUENCE {
 18   8:      OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2)
       :       }
 28  62:     SEQUENCE {
 30  11:       SET {
 32   9:         SEQUENCE {
 34   3:           OBJECT IDENTIFIER countryName (2 5 4 6)
 39   2:           PrintableString 'NL'
       :           }
       :         }
 43  17:       SET {
 45  15:         SEQUENCE {
 47   3:           OBJECT IDENTIFIER organizationName (2 5 4 10)
 52   8:           PrintableString 'PolarSSL'
       :           }
       :         }
 62  28:       SET {
 64  26:         SEQUENCE {
 66   3:           OBJECT IDENTIFIER commonName (2 5 4 3)
 71  19:           PrintableString 'Polarssl Test EC CA'
       :           }
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       :       }
 92  30:     SEQUENCE {
 94  13:       UTCTime 24/09/2013 15:52:04 GMT
109  13:       UTCTime 22/09/2023 15:52:04 GMT
       :       }
124  65:     SEQUENCE {
126  11:       SET {
128   9:         SEQUENCE {
130   3:           OBJECT IDENTIFIER countryName (2 5 4 6)
135   2:           PrintableString 'NL'
       :           }
       :         }
139  17:       SET {
141  15:         SEQUENCE {
143   3:           OBJECT IDENTIFIER organizationName (2 5 4 10)
148   8:           PrintableString 'PolarSSL'
       :           }
       :         }
158  31:       SET {
160  29:         SEQUENCE {
162   3:           OBJECT IDENTIFIER commonName (2 5 4 3)
167  22:           PrintableString 'PolarSSL Test Client 2'
       :           }
       :         }
       :       }
191  89:     SEQUENCE {
193  19:       SEQUENCE {
195   7:         OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
204   8:         OBJECT IDENTIFIER prime256v1 (1 2 840 10045 3 1 7)
       :         }
214  66:       BIT STRING
       :         04 57 E5 AE B1 73 DF D3 AC BB 93 B8 81 FF 12 AE
       :         EE E6 53 AC CE 55 53 F6 34 0E CC 2E E3 63 25 0B
       :         DF 98 E2 F3 5C 60 36 96 C0 D5 18 14 70 E5 7F 9F
       :         D5 4B 45 18 E5 B0 6C D5 5C F8 96 8F 87 70 A3 E4
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282 157:     [3] {
285 154:       SEQUENCE {
288   9:         SEQUENCE {
290   3:           OBJECT IDENTIFIER basicConstraints (2 5 29 19)
295   2:           OCTET STRING, encapsulates {
297   0:             SEQUENCE {}
       :             }
       :           }
299  29:         SEQUENCE {
301   3:           OBJECT IDENTIFIER subjectKeyIdentifier (2 5 29 14)
306  22:           OCTET STRING, encapsulates {
308  20:             OCTET STRING
       :              7A 00 5F 86 64 FC E0 5D E5 11 10 3B B2 E6 3B C4
       :              26 3F CF E2
       :             }
       :           }
330 110:         SEQUENCE {
332   3:          OBJECT IDENTIFIER authorityKeyIdentifier (2 5 29 35)
337 103:          OCTET STRING, encapsulates {
339 101:             SEQUENCE {
341  20:               [0]
       :               9D 6D 20 24 49 01 3F 2B CB 78 B5 19 BC 7E 24 
       :               C9 DB FB 36 7C
363  66:               [1] {
365  64:                 [4] {
367  62:                   SEQUENCE {
369  11:                     SET {
371   9:                      SEQUENCE {
373   3:                       OBJECT IDENTIFIER countryName (2 5 4 6)
378   2:                       PrintableString 'NL'
       :                       }
       :                      }
382  17:                     SET {
384  15:                      SEQUENCE {
386   3:                        OBJECT IDENTIFIER organizationName 
       :                               (2 5 4 10)
391   8:                        PrintableString 'PolarSSL'
       :                        }
       :                      }
401  28:                     SET {
403  26:                      SEQUENCE {
405   3:                       OBJECT IDENTIFIER commonName (2 5 4 3)
410  19:                       PrintableString 'Polarssl Test EC CA'
       :                        }
       :                      }
       :                     }
       :                   }
       :                 }
431   9:               [2] 00 C1 43 E2 7E 62 43 CC E8
       :               }
       :             }
       :           }
       :         }
       :       }
       :     }
442  10:   SEQUENCE {
444   8:     OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2)
       :     }
454 104:   BIT STRING, encapsulates {
457 101:     SEQUENCE {
459  48:       INTEGER
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       :         BB 70 4C 96 03 AC 8E 78 70 DD F2 0E A0 B2 16 CB
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509  49:       INTEGER
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