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TLS S. Santesson
Internet-Draft 3xA Security AB
Intended status: Standards Track H. Tschofenig
Expires: January 16, 2013 Nokia Siemens Networks
July 15, 2012
Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-12.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 path (including all intermediary
certificates up to the trust anchor public key).
This document defines an extension that omits the exchange of already
available information. The TLS client informs a server of cached
information, for example from a previous TLS handshake, allowing the
server to omit the 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 January 16, 2013.
Copyright Notice
Copyright (c) 2012 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
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Cached Information Extension . . . . . . . . . . . . . . . . . 5
4. Exchange Specification . . . . . . . . . . . . . . . . . . . . 7
4.1. Fingerprint of the Certificate Chain . . . . . . . . . . . 7
4.2. Fingerprint for Trusted CAs . . . . . . . . . . . . . . . 8
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 13
7.2. New Registry for CachedInformationType . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
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 path (including all intermediary
certificates up to the trust anchor public key).
Optimizing the exchange of information to a minimum helps to improve
performance in environments where devices are connected to a network
with characteristics like low bandwidth, high latency and high loss
rate. These types of networks exist, for example, when smart objects
are connected using a low power IEEE 802.15.4 radio. 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 TLS
handshake.
A typical example exchange may therefore look as follows. First, the
TLS exchange executes the usual TLS handshake. It may decide to
store the certificate provided by the server for a future exchange.
When the TLS client then connects to the TLS server some time in the
future, without using session resumption, it then attaches the
cached_information 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 not
changed then the TLS server does not need to send the full
certificate to the client again. In case the information had
changed, the certificate payload is transmitted to the client to
allow the client to update it's state information.
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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].
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3. Cached Information Extension
This document defines a new extension type (cached_information(TBD)),
which is used in client hello and server hello messages. The
extension type is specified as follows.
enum {
cached_information(TBD), (65535)
} ExtensionType;
The extension_data field of this extension, when included in the
client hello, MUST contain the CachedInformation structure.
enum {
certificate_chain(1), trusted_cas(2) (255)
} CachedInformationType;
struct {
CachedInformationType type;
HashAlgorithm hash;
opaque hash_value<1..255>;
} CachedObject;
struct {
CachedObject cached_info<1..2^16-1>;
} CachedInformation;
When the CachedInformationType identifies a certificate_chain, 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.
When the CachedInformationType identifies a trusted_cas, 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
length bytes of the certificate_authorities vector.
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' is
not an allowed choice for a hash algorithm and MUST NOT be used.
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This document establishes a registry for CachedInformationType types
and additional values can be added following the policy described in
Section 7.
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4. Exchange Specification
Clients supporting this extension MAY include the
"cached_information" extension in the (extended) client hello, which
MAY contain zero or more CachedObject attributes.
Server supporting this extension MAY include the "cached_information"
extension in the (extended) server hello, which MAY contain one or
more CachedObject attributes. By returning the "cached_information"
extension the server indicates that it supports caching of each
present CachedObject that matches the specified hash value. The
server MAY support other cached objects that are not present in the
extension.
Note: Clients may need the ability to cache different values
depending on other information in the Client Hello that modify what
values the server uses, in particular the Server Name Indication
[RFC6066] value.
Following a successful exchange of "cached_information" extensions,
the server MAY send fingerprints of the cached information in the
handshake exchange as a replacement for the exchange of the full
data. 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 Finished message MUST be
calculated over the actual data exchanged in the handshake protocol.
That is, the Finished message will be calculated over the hash values
of cached information objects and not over the cached information
that were omitted from transmission.
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.
4.1. Fingerprint of the Certificate Chain
When an object of type 'certificate_chain' is provided in the client
hello, the server MAY send a fingerprint instead of the complete
certificate chain as shown below.
The original handshake message syntax is defined in RFC 5246
[RFC5246] and has the following structure:
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opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
By using the extension defined in this document the following
information is sent:
struct {
CachedObject ASN.1Cert<1..2^24-1>;
} Certificate;
The opaque ASN.1Cert structure is replaced with the CachedObject
structure defined in this document.
Note: [I-D.ietf-tls-oob-pubkey] allows a PKIX certificate containing
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] and the negotiated
certificate type is a raw public key then the TLS server sends the
hashed Certificate payload that contains a ASN.1Cert structure of the
SubjectPublicKeyInfo.
4.2. Fingerprint for Trusted CAs
When a hash for an object of type 'trusted_cas' is provided in the
client hello, the server MAY send a fingerprint instead of the
complete certificate authorities information as shown below.
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;
By using the extension defined in this document the following
information is sent:
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struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
CachedObject DistinguishedName<1..2^16-1>;
} CertificateRequest;
The opaque DistinguishedName structure is replaced with the
CachedObject structure defined in this document.
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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 extension,
see [2], and indicates that it has stored the certificate_chain from
the earlier exchange. With [3] the TLS server indicates that it also
supports 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] the TLS server provides the fingerprint of the
certificate chain as described in Section 4.1.
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(A) Initial (full) Exchange
client_hello ->
<- server_hello,
certificate, // [1]
server_key_exchange,
server_hello_done
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
(B) TLS Cached Extension Usage
client_hello,
cached_information=(certificate_chain) -> // [2]
<- server_hello,
cached_information= // [3]
(certificate_chain, trusted_cas, foo-bar)
certificate, // [4]
server_key_exchange,
server_hello_done
client_key_exchange,
change_cipher_spec,
finished ->
<- change_cipher_spec,
finished
Application Data <-------> Application Data
Figure 1: Example Message Exchange
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6. Security Considerations
This specification defines a mechanism to reference stored state
using a fingerprint. 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.
Caching information in an encrypted handshake (such as a renegotiated
handshake) and sending a hash of that cached information in an
unencrypted handshake might introduce integrity or data disclosure
issues as it enables an attacker to identify if a known object (such
as a known server certificate) has been used in previous encrypted
handshakes. Information object types defined in this specification,
such as server certificates, are public objects and usually not
sensitive in this regard, but implementers should be aware if any
cached information are subject to such security concerns and in such
case SHOULD NOT send a hash over encrypted data in unencrypted
handshake.
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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)
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)
o trusted_cas(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
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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)
Additionally, we would like to thank the TLS working group chairs,
Eric Rescorla and Joe Salowey, as well as the security area
directors, Sean Turner and Stephen Farrell, for their feedback and
support.
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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., Gilmore, J., Weiler, S., Kivinen, T., and H.
Tschofenig, "TLS Out-of-Band Public Key Validation",
draft-ietf-tls-oob-pubkey-03 (work in progress),
April 2012.
[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.
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Authors' Addresses
Stefan Santesson
3xA Security AB
Scheelev. 17
Lund 223 70
Sweden
Email: sts@aaa-sec.com
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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