TLS Cached Info: Transport Layer Security (TLS) Cached Information Extension

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.

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

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;

   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
   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.

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

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.

   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).

   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

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.

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>.

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>.

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'
         :           }
         :         }

  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 {
  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

  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.

   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.

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.

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