TLS                                                         S. Santesson
Internet-Draft                                           3xA Security AB
Intended status: Standards Track                           H. Tschofenig
Expires: August 18, 2014 May 17, 2015                                           ARM Ltd.
                                                       February 14,
                                                       November 13, 2014

      Transport Layer Security (TLS) Cached Information Extension
                   draft-ietf-tls-cached-info-16.txt
                   draft-ietf-tls-cached-info-17.txt

Abstract

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

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 18, 2014. May 17, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cached Information Extension  . . . . . . . . . . . . . . . .   3
   4.  Exchange Specification
     3.1.  Certificate_list Fingerprint  . . . . . . . . . . . . . .   4
     3.2.  Certificate_authorities Fingerprint . . . . . . . . . . .   4
     4.1.  Omitting the Certificate Chain
     3.3.  Fingerprint Hash Algorithm  . . . . . . . . . . . . . . .   4
   4.  Exchange Specification  . . . . . . . . . . . . . . . . . . .   5
     4.2.
     4.1.  Omitting the Trusted CAs Certificate List . . . . . . . . . . . . . .   5
     4.2.  Omitting the Trusted Certificate Authorities  . . . . . .   5   6
   5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8   9
     7.1.  New Entry to the TLS ExtensionType Registry . . . . . . .   8   9
     7.2.  New Registry for CachedInformationType  . . . . . . . . .   8   9
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9  10

1.  Introduction

   Transport Layer Security (TLS) handshakes often include fairly static
   information, such as

   Reducing the server certificate and a list amount of trusted
   Certification Authorities (CAs).  This information can be of
   considerable size, particularly if the server certificate is bundled
   with exchanged during a complete certificate chain (i.e., the certificates of
   intermediate CAs up to the root CA).

   Optimizing the exchange of information 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.  These types  With Internet of Things such
   environments exist, for example, when smart objects are connected
   using a low power IEEE 802.15.4 radio or via Bluetooth Low Energy. 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 of cached information from the 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_information
   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 had has not
   changed then the TLS server does not need to send the complete its' certificate chain to
   and the client corresponding certificate list again.  In case the information had
   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 its state information. 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_information(TBD)), (cached_info(TBD)), which
   is used in client hello and server hello messages.  The extension
   type is specified as follows.

         enum {
              cached_information(TBD),
              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 the same
   CachedInformationType.

         enum {
              certificate_chain(1), trusted_cas(2)
              certificate_list(1), certificate_authorities(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 the policy described in
   Section 7.

3.1.  Certificate_list Fingerprint

   When the CachedInformationType identifies a certificate_chain, certificate_list, then
   the hash_value field MUST include the hash calculated over the
   certificate_list element of the Certificate payload provided by the
   TLS server in an earlier exchange, excluding the three length bytes
   of the certificate_list vector.

3.2.  Certificate_authorities Fingerprint

   When the CachedInformationType identifies a trusted_cas, certificate_authorities,
   then the hash_value MUST include a hash calculated over the
   certificate_authorities element of the
   CertificateRequest payload provided by the TLS server in an earlier
   exchange, excluding the two msg_type and length bytes of the certificate_authorities vector. field.

3.3.  Fingerprint Hash Algorithm

   The hash algorithm used to calculate hash values is conveyed in the
   'hash' field of the CachedObject element.  The list of registered
   hash algorithms can be found 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
   MUST NOT be used.

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

4.  Exchange Specification

   Clients supporting this extension MAY include the
   "cached_information" "cached_info"
   extension in the (extended) client hello, which
   MAY contain zero or more CachedObject 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_information" "cached_info"
   extension in the (extended) server hello, which
   MAY contain one or more CachedObject attributes it supports. hello.  By returning the "cached_information"
   "cached_info" extension the server indicates that it supports caching the
   cached info types.  For each indicated cached info type the server
   MUST alters the transmission of respective payloads, as specified for
   each present CachedObject 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 matches the specified hash value.  The client includes a fingerprint of the cached information
   to give the server MAY support other enough information to determine whether cached
   objects that are not present
   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 in the "cached_info" extension.

   Note: If clients make use of the Server Name Indication [RFC6066]
   then clients Clients may need to cache multiple data items for a single server since servers may host multiple 'virtual' if
   those servers at are part of a single
   underlying network address. hosting environment.  To allow the client
   to select the appropriate information from the cached it is
   RECOMMENDED that the client uses information from the Server Name
   Indication [RFC6066].

   Following a successful exchange of the "cached_information" "cached_info" extensions in
   the client and server hello, the server omits alters sending the
   corresponding handshake message.  How information is omitted altered from the
   handshake message messages is defined per cached info type.  Section 4.1 and
   Section 4.2 defines the syntax of the fingerprinted information.

   The handshake protocol MUST proceed using the information as if it
   was provided in the handshake protocol.  The  Since the Finished message MUST be
   is calculated over the actual data exchanged in the handshake protocol.
   That is, the Finished message data it will be calculated over also include the information
   that was omitted from transmission by means of its present hash in
   the client hello and not through its presence in
   of the handshake
   exchange.

   The server MUST NOT include more than one fingerprint for a single
   information element, i.e., at maximum only one CachedObject structure
   per replaced information is provided. cached data.

4.1.  Omitting the Certificate Chain List

   When an object of type 'certificate_chain' 'certificate_list' is provided in the client
   hello, the server MAY replace the sequence list of certificates with an empty
   sequence with an actual length field of zero (=empty vector).

   The original 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;

   Note that [I-D.ietf-tls-oob-pubkey] [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 [I-D.ietf-tls-oob-pubkey] [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 structure of the
   SubjectPublicKeyInfo.

4.2.  Omitting the Trusted CAs Certificate Authorities

   When a fingerprint for an object of type 'trusted_cas' 'certificate_authorities' is
   provided in the client hello, the server MAY send a DistinguishedName in replace the
   Certificate Request
   CertificateRequest message with an empty sequence with an actual
   length field of zero
   (=empty vector). zero.

   The original 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;

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_information "cached_info" extension, see

   [2], and indicates that it has stored the certificate_chain 'certificate_list' from the
   earlier exchange.  With [3] the TLS server indicates that it also acknowledges the supports
   of this specification and informs the client that it also
   supports caching of other objects beyond the 'certificate_chain',
   namely 'trusted_cas' (also defined in this document), and the 'foo-
   bar' extension (i.e., an imaginary extension that yet needs to be
   defined).  With [4] alterned the TLS server omits sending
   content of the certificate
   chain, payload (see [4], as described in
   Section 4.1. 4.1).

   (A) Initial (full) Exchange

   client_hello

   ClientHello  ->
                          <-  server_hello,
                              certificate,  ServerHello
                              Certificate* // [1]
                              server_key_exchange,
                              server_hello_done

   client_key_exchange,
   change_cipher_spec,
   finished
                              ServerKeyExchange*
                              CertificateRequest*
                              ServerHelloDone

   Certificate*
   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   Finished                  ->

                          <- change_cipher_spec,
                             finished [ChangeCipherSpec]
                             Finished

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

   (B) TLS Cached Extension Usage

   client_hello,
   cached_information=(certificate_chain)

   ClientHello
   cached_info=(certificate_list)   -> // [2]
                          <-  server_hello,
                              cached_information=  ServerHello
                              cached_info=
                              (certificate_list) // [3]
                              (certificate_chain, trusted_cas, foo-bar)
                              certificate,
                              Certificate* // [4]
                              server_key_exchange,
                              server_hello_done

   client_key_exchange,
   change_cipher_spec,
   finished
                              ServerKeyExchange*
                              CertificateRequest*
                              ServerHelloDone

   Certificate*
   ClientKeyExchange
   CertificateVerify*
   [ChangeCipherSpec]
   Finished                  ->

                          <- change_cipher_spec,
                             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 not sensitive in
   this regard, others may be.  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 hash algorithm used in this specification is required to have
   reasonable random properties in order to provide reasonably unique
   identifiers.  There is no requirement that this hash algorithm must
   have strong collision resistance.

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_information(TBD) 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_chain(1)  certificate_list(1)

   o  trusted_cas(2)  certificate_authorities(2)

   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)

   Additionally, we would like to thank the TLS working group chairs,
   Eric Rescorla
   Sean Turner and Joe Salowey, as well as the responsible security area
   directors, Sean Turner and
   director, Stephen Farrell, for their feedback and his 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.

   [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.

9.2.  Informative References

   [I-D.ietf-tls-oob-pubkey]
              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)", draft-ietf-tls-oob-pubkey-11 (work in progress),
              January 2014.

   [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.

Authors' Addresses
   Stefan Santesson
   3xA Security AB
   Scheelev. 17
   Lund  223 70
   Sweden

   Email: sts@aaa-sec.com

   Hannes Tschofenig
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   Great Britain
   Hall in Tirol  6060
   Austria

   Email: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at