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rfc:rfc7924

Internet Engineering Task Force (IETF) S. Santesson Request for Comments: 7924 3xA Security AB Category: Standards Track H. Tschofenig ISSN: 2070-1721 ARM Ltd.

                                                             July 2016
    Transport Layer Security (TLS) Cached Information Extension

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, thereby enabling the server to omit
 already available information.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7924.

Santesson & Tschofenig Standards Track [Page 1] RFC 7924 TLS Cached Information Extension July 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Cached Information Extension  . . . . . . . . . . . . . . . .   3
 4.  Exchange Specification  . . . . . . . . . . . . . . . . . . .   5
   4.1.  Server Certificate Message  . . . . . . . . . . . . . . .   6
   4.2.  CertificateRequest Message  . . . . . . . . . . . . . . .   7
 5.  Fingerprint Calculation . . . . . . . . . . . . . . . . . . .   7
 6.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
 7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.1.  New Entry to the TLS ExtensionType Registry . . . . . . .  10
   8.2.  New Registry for CachedInformationType  . . . . . . . . .  11
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
 Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  13
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

Santesson & Tschofenig Standards Track [Page 2] RFC 7924 TLS Cached Information Extension July 2016

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 the Internet of Things,
 such environments exist, for example, when devices use IEEE 802.15.4,
 Bluetooth Low Energy, or low power wide area networks.  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 execute 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 ClientHello message to
 indicate that it has 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 Datagram Transport Layer Security (DTLS) as
 well.

3. Cached Information Extension

 This document defines a new extension type (cached_info(25)), which
 is used in ClientHello and ServerHello messages.  The extension type
 is specified as follows.
       enum {
            cached_info(25), (65535)
       } ExtensionType;

Santesson & Tschofenig Standards Track [Page 3] RFC 7924 TLS Cached Information Extension July 2016

 The extension_data field of this extension, when included in the
 ClientHello, 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 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 reusing hash algorithms already

Santesson & Tschofenig Standards Track [Page 4] RFC 7924 TLS Cached Information Extension July 2016

 available with TLS ciphersuites.  To avoid additional code and to
 keep the collision probability 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) ClientHello.  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) ServerHello.  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 ClientHello).  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
 (SNI) extension [RFC6066].
 Following a successful exchange of the "cached_info" extension in the
 ClientHello and ServerHello, the server alters sending the
 corresponding handshake message.  How information is altered from the
 handshake messages and for the types defined in this specification is
 defined in Sections 4.1 and 4.2, respectively.
 Appendix A shows an example hash calculation, and Section 6
 illustrates an example protocol exchange.

Santesson & Tschofenig Standards Track [Page 5] RFC 7924 TLS Cached Information Extension July 2016

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:
 o  The server software implements the "cached_info" extension defined
    in this specification.
 o  The 'cert' "cached_info" extension is enabled (for example, a
    policy allows the use of this extension).
 o  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;
               Figure 1: Cached Info Certificate Message

Santesson & Tschofenig Standards Track [Page 6] RFC 7924 TLS Cached Information Extension July 2016

4.2. CertificateRequest Message

 When a fingerprint for an object of type 'cert_req' is provided in
 the ClientHello, the server MAY send the CertificateRequest message
 shown in Figure 2 under the following conditions:
 o  The server software implements the "cached_info" extension defined
    in this specification.
 o  The 'cert_req' "cached_info" extension is enabled (for example, a
    policy allows the use of this extension).
 o  The server compared the value in the hash_value field of the
    client-provided "cached_info" extension with the fingerprint of
    the CertificateRequest 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.
 o  The server wants to request a certificate from the client.
 The original CertificateRequest handshake message syntax is defined
 in [RFC5246].  The modified structure of the CertificateRequest
 message is shown in Figure 2.
       struct {
           opaque hash_value<1..255>;
       } CertificateRequest;
           Figure 2: Cached Info CertificateRequest Message
 The CertificateRequest payload is the input parameter to the
 fingerprint calculation described in Section 5.

5. Fingerprint Calculation

 The fingerprint for the two cached info objects defined in this
 document MUST be computed as follows:
 1.  Compute the SHA-256 [RFC6234] hash of the input data.  The input
     data depends on the cached info type.  This document defines two
     cached info types, described in Sections 4.1 and in 4.2.  Note
     that the computed hash only covers the input data structure (and
     not any type and length information of the record layer).
     Appendix A shows an example.
 2.  Use the output of the SHA-256 hash.

Santesson & Tschofenig Standards Track [Page 7] RFC 7924 TLS Cached Information Extension July 2016

 The purpose of the fingerprint provided by the client is to help the
 server select the correct information.  For example, in case of a
 Certificate message, the fingerprint identifies the server
 certificate (and the corresponding private key) for use with the rest
 of the handshake.  Servers may have more than one certificate, and
 therefore a hash needs to be long enough to keep the probably of hash
 collisions low.  On the other hand, the cached info design aims to
 reduce the amount of data being exchanged.  The security of the
 handshake depends on the private key and not on the size of the
 fingerprint.  Hence, the fingerprint is a way to prevent the server
 from accidentally selecting the wrong information.  If an attacker
 injects an incorrect fingerprint, then two outcomes are possible: (1)
 the fingerprint does not relate to any cached state and the server
 has to fall back to a full exchange, and (2) if the attacker manages
 to inject a fingerprint that refers to data the client has not
 cached, then the exchange will fail later when the client continues
 with the handshake and aims to verify the digital signature.  The
 signature verification will fail since the public key cached by the
 client will not correspond to the private key that was used by the
 server to sign the message.

6. Example

 In the regular, full TLS handshake exchange, shown in Figure 3, the
 TLS server provides its certificate in the certificate payload to the
 client; see step (1).  This allows the client to store the
 certificate for future use.  After some time, the TLS client again
 interacts with the same TLS server and makes use of the TLS
 "cached_info" extension, as shown in Figure 4.  The TLS client
 indicates support for this specification via the "cached_info"
 extension, see step (2), and indicates that it has stored the
 certificate from the earlier exchange (by indicating the 'cert'
 type).  With step (3), the TLS server acknowledges the support of the
 'cert' type and by including the value in the ServerHello, it informs
 the client that the content of the certificate payload contains the
 fingerprint of the certificate instead of the payload, defined in RFC
 5246, of the Certificate message; see step (4).

Santesson & Tschofenig Standards Track [Page 8] RFC 7924 TLS Cached Information Extension July 2016

 ClientHello            ->
                        <-  ServerHello
                            Certificate* // (1)
                            ServerKeyExchange*
                            CertificateRequest*
                            ServerHelloDone
 Certificate*
 ClientKeyExchange
 CertificateVerify*
 [ChangeCipherSpec]
 Finished               ->
                        <- [ChangeCipherSpec]
                           Finished
 Application Data <-------> Application Data
      Figure 3: Example Message Exchange: Initial (Full) Exchange
 ClientHello
 cached_info=(cert)     -> // (2)
                        <-  ServerHello
                            cached_info=(cert) (3)
                            Certificate (4)
                            ServerKeyExchange*
                            ServerHelloDone
 ClientKeyExchange
 CertificateVerify*
 [ChangeCipherSpec]
 Finished               ->
                        <- [ChangeCipherSpec]
                           Finished
 Application Data <-------> Application Data
    Figure 4: Example Message Exchange: TLS Cached Extension Usage

Santesson & Tschofenig Standards Track [Page 9] RFC 7924 TLS Cached Information Extension July 2016

7. Security Considerations

 This specification defines a mechanism to reference stored state
 using a fingerprint.  Sending a fingerprint of cached information in
 an unencrypted handshake, as the ClientHello and ServerHello does,
 may allow an attacker or observer to correlate independent TLS
 exchanges.  While some information elements used in this
 specification, such as server certificates, are public objects and
 usually do not contain sensitive information, other types that are
 not yet defined may.  Those who implement and deploy this
 specification should therefore make an informed decision whether the
 cached information is in line with their security and privacy goals.
 In case of concerns, it is advised to avoid sending the fingerprint
 of the data objects in clear.
 The use of the "cached_info" extension allows the server to send
 significantly smaller TLS messages.  Consequently, these omitted
 parts of the messages are not included in the transcript of the
 handshake in the TLS Finish message.  However, since the client and
 the server communicate the hash values of the cached data in the
 initial handshake messages, the fingerprints are included in the TLS
 Finish message.
 Clients MUST ensure that they only cache information from legitimate
 sources.  For example, when the client populates the cache from a TLS
 exchange, then it must only cache information after the successful
 completion of a TLS exchange to ensure that an attacker does not
 inject incorrect information into the cache.  Failure to do so allows
 for man-in-the-middle attacks.
 Security considerations for the fingerprint calculation are discussed
 in Section 5.

8. IANA Considerations

8.1. New Entry to the TLS ExtensionType Registry

 IANA has added an entry to the existing TLS "ExtensionType Values"
 registry, defined in [RFC5246], for cached_info(25) defined in this
 document.

Santesson & Tschofenig Standards Track [Page 10] RFC 7924 TLS Cached Information Extension July 2016

8.2. New Registry for CachedInformationType

 IANA has established a registry titled "TLS CachedInformationType
 Values".  The entries in the registry are:
 Value    Description
 -----    -----------
   0      Reserved
   1      cert
   2      cert_req
 224-255  Reserved for Private Use
 The policy for adding new values to this registry, following the
 terminology defined in [RFC5226], is as follows:
 o  0-63 (decimal): Standards Action
 o  64-223 (decimal): Specification Required

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <http://www.rfc-editor.org/info/rfc6066>.
 [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and SHA-based HMAC and HKDF)", RFC 6234,
            DOI 10.17487/RFC6234, May 2011,
            <http://www.rfc-editor.org/info/rfc6234>.

Santesson & Tschofenig Standards Track [Page 11] RFC 7924 TLS Cached Information Extension July 2016

9.2. Informative References

 [ASN.1-Dump]
            Gutmann, P., "ASN.1 Object Dump Program", November 2010,
            <http://manpages.ubuntu.com/manpages/precise/man1/
            dumpasn1.1.html>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC6574]  Tschofenig, H. and J. Arkko, "Report from the Smart Object
            Workshop", RFC 6574, DOI 10.17487/RFC6574, April 2012,
            <http://www.rfc-editor.org/info/rfc6574>.
 [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
            Weiler, S., and T. Kivinen, "Using Raw Public Keys in
            Transport Layer Security (TLS) and Datagram Transport
            Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
            June 2014, <http://www.rfc-editor.org/info/rfc7250>.

Santesson & Tschofenig Standards Track [Page 12] RFC 7924 TLS Cached Information Extension July 2016

Appendix A. Example

 Consider a certificate containing a NIST P256 elliptic curve public
 key displayed using Peter Gutmann's ASN.1 decoder [ASN.1-Dump] in
 Figure 5.
  0 556: SEQUENCE {
  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'
       :           }
       :         }
       :       }
 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'
       :           }
       :         }

Santesson & Tschofenig Standards Track [Page 13] RFC 7924 TLS Cached Information Extension July 2016

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
       :         C7
       :       }
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 {

Santesson & Tschofenig Standards Track [Page 14] RFC 7924 TLS Cached Information Extension July 2016

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
       :         4A 65 0D 7B 20 83 A2 99 B9 A8 0F FC 8D EE 8F 3D
       :         BB 70 4C 96 03 AC 8E 78 70 DD F2 0E A0 B2 16 CB
       :         65 8E 1A C9 3F 2C 61 7E F8 3C EF AD 1C EE 36 20

Santesson & Tschofenig Standards Track [Page 15] RFC 7924 TLS Cached Information Extension July 2016

509  49:       INTEGER
       :         00 9D F2 27 A6 D5 74 B8 24 AE E1 6A 3F 31 A1 CA
       :         54 2F 08 D0 8D EE 4F 0C 61 DF 77 78 7D B4 FD FC
       :         42 49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57
       :         45
       :       }
       :     }
       :   }
              Figure 5: ASN.1-Based Certificate: Example
 To include the certificate shown in Figure 5 in a TLS/DTLS
 Certificate message, it is prepended with a message header.  This
 Certificate message header in our example is 0b 00 02 36 00 02 33 00
 02 00 02 30, which indicates:
 Message Type:  0b -- 1-byte type field indicating a Certificate
    message
 Length:  00 02 36 -- 3-byte length field indicating a 566-byte
    payload
 Certificates Length:  00 02 33 -- 3-byte length field indicating 563
    bytes for the entire certificates_list structure, which may
    contain multiple certificates.  In our example, only one
    certificate is included.
 Certificate Length:  00 02 30 -- 3-byte length field indicating 560
    bytes of the actual certificate following immediately afterwards.
    In our example, this is the certificate content with 30 82 02 ....
    9E 57 45 shown in Figure 6.

Santesson & Tschofenig Standards Track [Page 16] RFC 7924 TLS Cached Information Extension July 2016

 The hex encoding of the ASN.1-encoded certificate payload shown in
 Figure 5 leads to the following encoding.
           30 82 02 2C 30 82 01 B2  A0 03 02 01 02 02 01 0D
           30 0A 06 08 2A 86 48 CE  3D 04 03 02 30 3E 31 0B
           30 09 06 03 55 04 06 13  02 4E 4C 31 11 30 0F 06
           03 55 04 0A 13 08 50 6F  6C 61 72 53 53 4C 31 1C
           30 1A 06 03 55 04 03 13  13 50 6F 6C 61 72 73 73
           6C 20 54 65 73 74 20 45  43 20 43 41 30 1E 17 0D
           31 33 30 39 32 34 31 35  35 32 30 34 5A 17 0D 32
           33 30 39 32 32 31 35 35  32 30 34 5A 30 41 31 0B
           30 09 06 03 55 04 06 13  02 4E 4C 31 11 30 0F 06
           03 55 04 0A 13 08 50 6F  6C 61 72 53 53 4C 31 1F
           30 1D 06 03 55 04 03 13  16 50 6F 6C 61 72 53 53
           4C 20 54 65 73 74 20 43  6C 69 65 6E 74 20 32 30
           59 30 13 06 07 2A 86 48  CE 3D 02 01 06 08 2A 86
           48 CE 3D 03 01 07 03 42  00 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 C7 A3 81 9D 30 81 9A
           30 09 06 03 55 1D 13 04  02 30 00 30 1D 06 03 55
           1D 0E 04 16 04 14 7A 00  5F 86 64 FC E0 5D E5 11
           10 3B B2 E6 3B C4 26 3F  CF E2 30 6E 06 03 55 1D
           23 04 67 30 65 80 14 9D  6D 20 24 49 01 3F 2B CB
           78 B5 19 BC 7E 24 C9 DB  FB 36 7C A1 42 A4 40 30
           3E 31 0B 30 09 06 03 55  04 06 13 02 4E 4C 31 11
           30 0F 06 03 55 04 0A 13  08 50 6F 6C 61 72 53 53
           4C 31 1C 30 1A 06 03 55  04 03 13 13 50 6F 6C 61
           72 73 73 6C 20 54 65 73  74 20 45 43 20 43 41 82
           09 00 C1 43 E2 7E 62 43  CC E8 30 0A 06 08 2A 86
           48 CE 3D 04 03 02 03 68  00 30 65 02 30 4A 65 0D
           7B 20 83 A2 99 B9 A8 0F  FC 8D EE 8F 3D BB 70 4C
           96 03 AC 8E 78 70 DD F2  0E A0 B2 16 CB 65 8E 1A
           C9 3F 2C 61 7E F8 3C EF  AD 1C EE 36 20 02 31 00
           9D F2 27 A6 D5 74 B8 24  AE E1 6A 3F 31 A1 CA 54
           2F 08 D0 8D EE 4F 0C 61  DF 77 78 7D B4 FD FC 42
           49 EE E5 B2 6A C2 CD 26  77 62 8E 28 7C 9E 57 45
           Figure 6: Hex Encoding of the Example Certificate
 Applying the SHA-256 hash function to the Certificate message, which
 starts with 0b 00 02 and ends with 9E 57 45, produces
 0x086eefb4859adfe977defac494fff6b73033b4ce1f86b8f2a9fc0c6bf98605af.

Santesson & Tschofenig Standards Track [Page 17] RFC 7924 TLS Cached Information Extension July 2016

Acknowledgments

 We would like to thank the following persons for your detailed
 document reviews:
 o  Paul Wouters and Nikos Mavrogiannopoulos (December 2011)
 o  Rob Stradling (February 2012)
 o  Ondrej Mikle (March 2012)
 o  Ilari Liusvaara, Adam Langley, and Eric Rescorla (July 2014)
 o  Sean Turner (August 2014)
 o  Martin Thomson (August 2015)
 o  Jouni Korhonen (November 2015)
 o  Dave Garrett (December 2015)
 o  Matt Miller (December 2015)
 o  Anirudh Ramachandran (March 2016)
 We would also to thank Martin Thomson, Karthikeyan Bhargavan, Sankalp
 Bagaria, and Eric Rescorla for their feedback regarding the
 fingerprint calculation.
 Finally, we would like to thank the TLS working group chairs, Sean
 Turner and Joe Salowey, as well as the responsible Security Area
 Director, Stephen Farrell, for their support and their reviews.

Santesson & Tschofenig Standards Track [Page 18] RFC 7924 TLS Cached Information Extension July 2016

Authors' Addresses

 Stefan Santesson
 3xA Security AB
 Forskningsbyn Ideon
 Lund  223 70
 Sweden
 Email: sts@aaa-sec.com
 Hannes Tschofenig
 ARM Ltd.
 Hall in Tirol  6060
 Austria
 Email: Hannes.tschofenig@gmx.net
 URI:   http://www.tschofenig.priv.at

Santesson & Tschofenig Standards Track [Page 19]

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