Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-17
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (tls WG) | |
|---|---|---|---|
| Authors | Stefan Santesson , Hannes Tschofenig | ||
| Last updated | 2014-11-13 | ||
| Replaces | draft-santesson-tls-certcache | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
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| Stream | WG state | WG Document | |
| Document shepherd | Joseph A. Salowey | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-tls-cached-info-17
TLS S. Santesson
Internet-Draft 3xA Security AB
Intended status: Standards Track H. Tschofenig
Expires: May 17, 2015 ARM Ltd.
November 13, 2014
Transport Layer Security (TLS) Cached Information Extension
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
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, 2015.
Copyright Notice
Copyright (c) 2014 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
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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
3.3. Fingerprint Hash Algorithm . . . . . . . . . . . . . . . 4
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5
4.1. Omitting the Certificate List . . . . . . . . . . . . . . 5
4.2. Omitting the Trusted Certificate Authorities . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 9
7.2. New Registry for CachedInformationType . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 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
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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 the same
CachedInformationType.
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enum {
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_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 certificate_authorities,
then the hash_value MUST include a hash calculated over
CertificateRequest payload provided by the TLS server in an earlier
exchange, excluding the msg_type and length 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.
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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 the transmission of respective payloads, as specified for
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 the client includes a fingerprint of the cached information
to give the server enough information 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 in the "cached_info" extension.
Note: Clients may cache multiple data items for a single server if
those servers are part of a 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_info" extensions 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. 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. Since the Finished message
is calculated over the exchanged data it will also include the hash
of the cached data.
4.1. Omitting the Certificate List
When an object of type 'certificate_list' is provided in the client
hello, the server MAY replace the list of certificates with an empty
sequence with an actual length field of zero (=empty vector).
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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 [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 structure of the
SubjectPublicKeyInfo.
4.2. Omitting the Trusted Certificate Authorities
When a fingerprint for an object of type 'certificate_authorities' is
provided in the client hello, the server MAY replace the
CertificateRequest message with an empty sequence with an actual
length field of 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_info" extension, see
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[2], and indicates that it has stored the 'certificate_list' from the
earlier exchange. With [3] the TLS server acknowledges the supports
of this specification and informs the client that it alterned the
content of the certificate payload (see [4], as described in
Section 4.1).
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(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) -> // [2]
<- ServerHello
cached_info=
(certificate_list) // [3]
Certificate* // [4]
ServerKeyExchange*
CertificateRequest*
ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ->
<- [ChangeCipherSpec]
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. 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
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_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)
o 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:
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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,
Sean Turner and Joe Salowey, as well as the responsible security area
director, Stephen Farrell, for 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
[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
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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
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