TLS                                                         S. Santesson
Internet-Draft                                           3xA Security AB
Intended status: Standards Track                           H. Tschofenig
Expires: May 17, September 9, 2015                                      ARM Ltd.
                                                       November 13, 2014
                                                           March 8, 2015

      Transport Layer Security (TLS) Cached Information Extension
                   draft-ietf-tls-cached-info-17.txt
                   draft-ietf-tls-cached-info-18.txt

Abstract

   Transport Layer Security (TLS) handshakes often include fairly static
   information, such as the server certificate and a list of trusted
   Certification Authorities
   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 May 17, September 9, 2015.

Copyright Notice

   Copyright (c) 2014 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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cached Information Extension  . . . . . . . . . . . . . . . .   3
     3.1.  Certificate_list Fingerprint  . . . . . . . . . . . . . .   4
     3.2.  Certificate_authorities Fingerprint . . . . . . . .
   4.  Exchange Specification  . . .   4
     3.3.  Fingerprint Hash Algorithm . . . . . . . . . . . . . . .   4
   4.  Exchange Specification .   5
     4.1.  Omitting the Server Certificate Message . . . . . . . . .   5
     4.2.  Omitting the CertificateRequest Message . . . . . . . . .   5
     4.1.   6
     4.3.  Omitting the Certificate List . . . . . . . . Status Information (OCSP
           Stapling and Multi OCSP Stapling) . . . . . .   5
     4.2.  Omitting the Trusted Certificate Authorities . . . . . .   6   7
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   6   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9  10
     7.1.  New Entry to the TLS ExtensionType Registry . . . . . . .   9  10
     7.2.  New Registry for CachedInformationType  . . . . . . . . .   9  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10  12
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10  18

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 smart objects are connected
   using a low power devices use IEEE 802.15.4 radio or via
   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 usual TLS handshake.  The client
   may, for example, decide to cache 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 list 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 MUST NOT send multiple CachedObjects with of the same
   CachedInformationType.

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

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

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

   This document establishes a registry for CachedInformationType types;
   additional values can be added following defines the policy described in
   Section 7.

3.1.  Certificate_list Fingerprint

   When following types:

   Omitting the CachedInformationType identifies a certificate_list, then Server Certificate Message:

      With the hash_value type field set to 'cert', the client MUST include the hash calculated over the
   certificate_list element
      message digest of the Certificate payload provided by the
   TLS server message in an earlier exchange, excluding the three length bytes
   of hash_value field.
      For this type the certificate_list vector.

3.2.  Certificate_authorities Fingerprint

   When message digest MUST be calculated using SHA-256
      [RFC4634].

   Omitting the CachedInformationType identifies a certificate_authorities,
   then CertificateRequest Message

      With the hash_value type set to 'cert_req', the client MUST include a hash calculated over
   CertificateRequest payload provided by the TLS server
      message digest of the CertificateRequest message in an earlier
   exchange, excluding the msg_type and length hash_value
      field.

3.3.  Fingerprint Hash Algorithm

   The hash algorithm used  For this type the message digest MUST be calculated using
      SHA-256 [RFC4634].

   Omitting the Certificate Status Information (OCSP Stapling and
   Multiple OCSP Stapling) Message

      With the type set to calculate hash values is conveyed in 'cert_status', the
   'hash' field of client MUST include the CachedObject element.  The list
      message digest of registered
   hash algorithms can be found the CertificateStatus message in the TLS HashAlgorithm Registry, which
   was created by RFC 5246 [RFC5246].  The value zero (0) for 'none' and
   one (1) for 'md5' is not an allowed choice for a hash algorithm and hash_value
      field.  For this type the message digest MUST NOT be used. 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.
   Clients and servers MUST NOT include more than one CachedObject
   attribute per info type.

   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 alters alter the transmission of respective payloads, as specified for 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).

   Note that  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 the cached information
   to give it cached (for
   each indicated type) the server enough information 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 item type in the "cached_info" extension.

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

   Following a successful exchange of the "cached_info" extensions 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 per cached info type. in Section 4.1 and 4.1, Section 4.2 defines and Section 4.3 for
   the syntax of types defined in this specification.

4.1.  Omitting the fingerprinted information.

   The handshake protocol MUST proceed using Server Certificate Message

   When a ClientHello message contains the information as if it
   was provided in "cached_info" extension with
   a type set to 'cert' then the handshake protocol.  Since server MAY omit the Finished Certificate message
   is calculated over the exchanged data it will also include
   under the hash
   of following conditions:

      The server software implements the "cached_info" extension defined
      in this specification.

      The 'cert' cached data.

4.1.  Omitting the Certificate List

   When an object of type 'certificate_list' info extension is provided in the client
   hello, enabled (for example, a policy
      allows the use of this extension).

      The server MAY replace compared the list value in the hash_value field of certificates with an empty
   sequence the
      client-provided "cached_info" extension with an actual length field the fingerprint of zero (=empty vector).

   The original handshake
      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 following structure:

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

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

                Certificate Message as defined in RFC 5246.

   The fingerprint MUST be computed as follows: hash_value:=SHA-
   256(Certificate)

   Note that RFC 7250 [RFC7250] 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.1Cert ASN.1 Certificate structure of with
   the included SubjectPublicKeyInfo rather than the full certificate.
   As such, this extension is compatible with the
   SubjectPublicKeyInfo. raw public key
   extension defined in RFC 7250.

4.2.  Omitting the Trusted Certificate Authorities CertificateRequest Message

   When a fingerprint for an object of type 'certificate_authorities' 'cert_req' is provided in
   the client hello, the server MAY replace omit the CertificateRequest message with an empty sequence with an actual
   length
   under the following conditions:

      The server software implements the "cached_info" extension defined
      in this specification.

      The 'cert_req' 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 zero. 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 server wants to request a certificate from the client.

   The original CertificateRequest handshake message syntax is defined
   in RFC 5246 [RFC5246] and has the following structure:

         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;

   The fingerprint MUST be computed as follows: hash_value:=SHA-
   256(CertificateRequest)

4.3.  Omitting the Certificate Status Information (OCSP Stapling and
      Multi OCSP Stapling)

   When a fingerprint for an object of type 'cert_status' is provided in
   the client hello, the server MAY omit the CertificateStatus message
   under the following conditions:

      The server software implements the "cert_status" extension defined
      in this specification.

      The 'cert_status' 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 CertificateStatus message it normally sends to clients.  This
      check ensures that the information cached by the client is
      current.

      Both client and server support the use of OCSP Stapling and/or
      Multiple OCSP Stapling, as defined in RFC 6066 [RFC6066]  and in
      [RFC6961].

   The CertificateStatus message syntax, defined in [RFC6961], has the
   following structure:

        struct {
          CertificateStatusType status_type;
          select (status_type) {
            case ocsp: OCSPResponse;
            case ocsp_multi: OCSPResponseList;
          } response;
        } CertificateStatus;

        opaque OCSPResponse<0..2^24-1>;

        struct {
          OCSPResponse ocsp_response_list<1..2^24-1>;
        } OCSPResponseList;

   The fingerprint MUST be computed as follows: hash_value:=SHA-
   256(CertificateStatus)

5.  Example

   Figure 1 illustrates an example exchange using the TLS cached info
   extension.  In the normal TLS handshake exchange shown in flow (A)
   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 flow (B).  The TLS client indicates
   support for this specification via the "cached_info" extension, see
   [2], and indicates that it has stored the 'certificate_list' certificate from the
   earlier exchange. exchange (by indicating the 'cert' type).  With [3] the TLS
   server acknowledges the supports of this specification the 'cert' type and by including
   the value in the server hello informs the client that it alterned the
   content of the certificate
   payload (see [4], as described in
   Section 4.1). has been omitted.

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

   Certificate*

   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   Finished               ->

                          <- [ChangeCipherSpec]
                             Finished

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

                    Figure 1: Example Message Exchange

6.  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 client and server hello is, 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 in
   this regard, others may be. information, other (not yet defined cached info types) may.
   Those who implement and deploy this specification should therefore
   make an informed decision whether the cached information is inline
   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 obmit
   sending certain TLS messages.  Consequently, these omitted messages
   are not included in the transcript of the handshake in the TLS Finish
   message per value.  However, since the client communicates the hash algorithm used
   values of the cached values in this specification is required the initial handshake message 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 have
   have strong collision resistance. ensure that an attacker does not
   inject incorrect information into the cache.  Failure to do so allows
   for man-in-the-middle attacks.

7.  IANA Considerations

7.1.  New Entry to the TLS ExtensionType Registry

   IANA is requested to add an entry to the existing TLS ExtensionType
   registry, defined in RFC 5246 [RFC5246], for cached_info(TBD) defined
   in this document.

7.2.  New Registry for CachedInformationType

   IANA is requested to establish a registry for TLS
   CachedInformationType values.  The first entries in the registry are

   o  certificate_list(1)  cert(1)

   o  cert_req(2)

   o  certificate_authorities(2)  cert_status(3)

   The policy for adding new values to this registry, following the
   terminology defined in RFC 5226 [RFC5226], is as follows:

   o  0-63 (decimal): Standards Action

   o  64-223 (decimal): Specification Required
   o  224-255 (decimal): reserved for Private Use

8.  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 (in March 2012)

   o  Ilari Liusvaara, Adam Langley, and Eric Rescorla (in July 2014)

   o  Sean Turner (in August 2014)

   Additionally, 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 his their support.

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.

   [RFC3874]  Housley, R., "A 224-bit One-way Hash Function: SHA-224",
              RFC 3874, September 2004.

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              June 2013.

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.

   [RFC6574]  Tschofenig, H. and J. Arkko, "Report from the Smart Object
              Workshop", RFC 6574, April 2012.

   [RFC7250]  Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
              T. Kivinen, "Using Raw Public Keys in Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7250, June 2014.

Appendix A.  Example

   The Wireshark trace of an example TLS exchange shown in Figure 2
   illustrates the use of an ECC-based ciphersuite with a 256 bit key.
   ECC allows for a small certificate size compared to RSA with
   equivalent security strength.  The Certificate message provided by
   the server is with 557 bytes (including the record layer header) one
   of the largest message even though it only contains a single
   certificate (i.e., no intermediate CA certificates).  The client-
   provided Certificate message has a length of 570 bytes (also
   including the record layer header).

 Client --> Server:

     TLSv1.2 Record Layer: Handshake Protocol: Client Hello
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 121
         Handshake Protocol: Client Hello
             Handshake Type: Client Hello (1)
             Length: 117
             Version: TLS 1.2 (0x0303)
             Random
                 gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
                 random_bytes: c61b966bba2781c50b07c3278c43f5892b3d...
             Session ID Length: 0
             Cipher Suites Length: 10
             Cipher Suites (5 suites)
             Compression Methods Length: 1
             Compression Methods (1 method)
             Extensions Length: 66
             Extension: server_name
             Extension: signature_algorithms
             Extension: elliptic_curves
             Extension: ec_point_formats

 Client <-- Server:

     TLSv1.2 Record Layer: Handshake Protocol: Server Hello
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 87
         Handshake Protocol: Server Hello
             Handshake Type: Server Hello (2)
             Length: 83
             Version: TLS 1.2 (0x0303)
             Random
                 gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
                 random_bytes: 82d3d09b44149d738b7002da4ff5a986fe11...
             Session ID Length: 32
             Session ID: d069a74661088676b98db8346070278a7475b617a0...
             Cipher Suite: Unknown (0xc0ad)
             Compression Method: null (0)
             Extensions Length: 11
             Extension: renegotiation_info
             Extension: ec_point_formats

     TLSv1.2 Record Layer: Handshake Protocol: Certificate
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 557
         Handshake Protocol: Certificate
             Handshake Type: Certificate (11)
             Length: 553
             Certificates Length: 550
             Certificates (550 bytes)
                 Certificate Length: 547
                 Certificate (id-at-commonName=localhost,
                 id-at-organizationName=PolarSSL,id-at-countryName=NL)
                     signedCertificate
                     algorithmIdentifier (iso.2.840.10045.4.3.2)
                     Padding: 0
                     encrypted: 30650231009a2c5cd7a6dba2e5640df0b94ed...

     TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 215
         Handshake Protocol: Server Key Exchange
             Handshake Type: Server Key Exchange (12)
             Length: 211

     TLSv1.2 Record Layer: Handshake Protocol: Certificate Request
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 78
         Handshake Protocol: Certificate Request
             Handshake Type: Certificate Request (13)
             Length: 74
             Certificate types count: 1
             Certificate types (1 type)
             Signature Hash Algorithms Length: 2
             Signature Hash Algorithms (1 algorithm)
             Distinguished Names Length: 66
             Distinguished Names (66 bytes)

     TLSv1.2 Record Layer: Handshake Protocol: Server Hello Done
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 4
         Handshake Protocol: Server Hello Done
             Handshake Type: Server Hello Done (14)
             Length: 0

 Client --> Server:

     TLSv1.2 Record Layer: Handshake Protocol: Certificate
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 570
         Handshake Protocol: Certificate
             Handshake Type: Certificate (11)
             Length: 566
             Certificates Length: 563
             Certificates (563 bytes)
                 Certificate Length: 560
                 Certificate (id-at-commonName=PolarSSL Test Client 2,
                 id-at-organizationName=PolarSSL,id-at-countryName=NL)
                     signedCertificate
                     algorithmIdentifier (iso.2.840.10045.4.3.2)
                     Padding: 0
                     encrypted: 306502304a650d7b2083a299b9a80ffc8dee8...

     TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 138
         Handshake Protocol: Client Key Exchange
             Handshake Type: Client Key Exchange (16)
             Length: 134

     TLSv1.2 Record Layer: Handshake Protocol: Certificate Verify
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 80
         Handshake Protocol: Certificate Verify
             Handshake Type: Certificate Verify (15)
             Length: 76

     TLSv1.2 Record Layer: Change Cipher Spec Protocol
         Content Type: Change Cipher Spec (20)
         Version: TLS 1.2 (0x0303)
         Length: 1
         Change Cipher Spec Message

     TLSv1.2 Record Layer: Handshake Protocol:
         Encrypted Handshake Message (TLS Finished)
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 40
         Handshake Protocol: Encrypted Handshake Message

 Client <-- Server:

     TLSv1.2 Record Layer: Change Cipher Spec Protocol
         Content Type: Change Cipher Spec (20)
         Version: TLS 1.2 (0x0303)
         Length: 1
         Change Cipher Spec Message

     TLSv1.2 Record Layer: Handshake Protocol
         Encrypted Handshake Message (TLS Finished)
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 40
         Handshake Protocol: Encrypted Handshake Message

      Figure 2: Example TLS Exchange (without Cached Info Extension).

   The total size of the TLS exchange shown in Figure 2 is 1932 bytes
   whereas the exchange shown in Figure 3 reduces the size to 1323 bytes
   by omitting the Certificate and the CertificateRequest messages.  As
   it can be seen, the use of the cached info extension leads to an on-
   the-wire improvement of more than 600 bytes.

 Client --> Server:

     TLSv1.2 Record Layer: Handshake Protocol: Client Hello
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 121 + 21
         Handshake Protocol: Client Hello
             Handshake Type: Client Hello (1)
             Length: 117 + 21
             Version: TLS 1.2 (0x0303)
             Random
                 gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
                 random_bytes: c61b966bba2781c50b07c3278c43f5892b3d...
             Session ID Length: 0
             Cipher Suites Length: 10
             Cipher Suites (5 suites)
             Compression Methods Length: 1
             Compression Methods (1 method)
             Extensions Length: 66
             Extension: server_name
             Extension: signature_algorithms
             Extension: elliptic_curves
             Extension: ec_point_formats
             Extension: cached_info

 Client <-- Server:

     TLSv1.2 Record Layer: Handshake Protocol: Server Hello
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 87 + 5
         Handshake Protocol: Server Hello
             Handshake Type: Server Hello (2)
             Length: 83 + 5
             Version: TLS 1.2 (0x0303)
             Random
                 gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
                 random_bytes: 82d3d09b44149d738b7002da4ff5a986fe11...
             Session ID Length: 32
             Session ID: d069a74661088676b98db8346070278a7475b617a0...
             Cipher Suite: Unknown (0xc0ad)
             Compression Method: null (0)
             Extensions Length: 11 + 5
             Extension: renegotiation_info
             Extension: ec_point_formats
             Extension: cached_info

     TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 215
         Handshake Protocol: Server Key Exchange
             Handshake Type: Server Key Exchange (12)
             Length: 211

     TLSv1.2 Record Layer: Handshake Protocol: Server Hello Done
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 4
         Handshake Protocol: Server Hello Done
             Handshake Type: Server Hello Done (14)
             Length: 0

 Client --> Server:

     TLSv1.2 Record Layer: Handshake Protocol: Certificate
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 570
         Handshake Protocol: Certificate
             Handshake Type: Certificate (11)
             Length: 566
             Certificates Length: 563
             Certificates (563 bytes)
                 Certificate Length: 560
                 Certificate (id-at-commonName=PolarSSL Test Client 2,
                 id-at-organizationName=PolarSSL,id-at-countryName=NL)
                     signedCertificate
                     algorithmIdentifier (iso.2.840.10045.4.3.2)
                     Padding: 0
                     encrypted: 306502304a650d7b2083a299b9a80ffc8dee8...

     TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 138
         Handshake Protocol: Client Key Exchange
             Handshake Type: Client Key Exchange (16)
             Length: 134

     TLSv1.2 Record Layer: Handshake Protocol: Certificate Verify
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 80
         Handshake Protocol: Certificate Verify
             Handshake Type: Certificate Verify (15)
             Length: 76

     TLSv1.2 Record Layer: Change Cipher Spec Protocol
         Content Type: Change Cipher Spec (20)
         Version: TLS 1.2 (0x0303)
         Length: 1
         Change Cipher Spec Message

     TLSv1.2 Record Layer: Handshake Protocol:
         Encrypted Handshake Message (TLS Finished)
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 40
         Handshake Protocol: Encrypted Handshake Message

 Client <-- Server:

     TLSv1.2 Record Layer: Change Cipher Spec Protocol
         Content Type: Change Cipher Spec (20)
         Version: TLS 1.2 (0x0303)
         Length: 1
         Change Cipher Spec Message

     TLSv1.2 Record Layer: Handshake Protocol
         Encrypted Handshake Message (TLS Finished)
         Content Type: Handshake (22)
         Version: TLS 1.2 (0x0303)
         Length: 40
         Handshake Protocol: Encrypted Handshake Message

       Figure 3: Example TLS Exchange (with Cached Info Extension).

   Note: To accomplish further on-the-wire handshake size message
   reductions the Certificate message sent by the client can be reduced
   in size by using the Client Certificate URL extension.

Authors' Addresses

   Stefan Santesson
   3xA Security AB
   Scheelev. 17
   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