One document matched: draft-ietf-tls-cached-info-19.txt
Differences from draft-ietf-tls-cached-info-18.txt
TLS S. Santesson
Internet-Draft 3xA Security AB
Intended status: Standards Track H. Tschofenig
Expires: September 24, 2015 ARM Ltd.
March 23, 2015
Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-19.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 September 24, 2015.
Copyright Notice
Copyright (c) 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cached Information Extension . . . . . . . . . . . . . . . . 3
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 4
4.1. Server Certificate Message . . . . . . . . . . . . . . . 5
4.2. CertificateRequest Message . . . . . . . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 9
7.2. New Registry for CachedInformationType . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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 devices use IEEE 802.15.4 or
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 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 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
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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 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 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.
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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 types:
Omitting the Server Certificate Message:
With the type field set to 'cert', the client MUST include the
message digest of the Certificate message in the hash_value field.
For this type the message digest MUST be calculated using SHA-256
[RFC4634].
Omitting the CertificateRequest Message
With the type set to 'cert_req', the client MUST include the
message digest of the CertificateRequest message in the hash_value
field. For this type the message digest MUST be 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.
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
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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 client hello). 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 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
extension [RFC6066].
Following a successful exchange of the "cached_info" 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 in Section 4.1, and in Section 4.2 for the types
defined in this specification.
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 2 under the following conditions:
The server software implements the "cached_info" extension defined
in this specification.
The 'cert' 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 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 structure shown in Figure 1.
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opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
Figure 1: Certificate Message as defined in RFC 5246.
The new structure of the CertificateRequest message is shown in
Figure 2.
struct {
opaque hash_value<1..255>;
} CertificateRequest;
Figure 2: Cached Info Certificate Message.
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.1 Certificate structure with
the included SubjectPublicKeyInfo rather than the full certificate.
As such, this extension is compatible with the raw public key
extension defined in RFC 7250.
4.2. CertificateRequest Message
When a fingerprint for an object of type 'cert_req' is provided in
the client hello, the server MAY omit the CertificateRequest message
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 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.
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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;
Figure 3: CertificateRequest Message as defined in RFC 5246.
The new structure of the CertificateRequest message is shown in
Figure 4.
struct {
opaque hash_value<1..255>;
} CertificateRequest;
Figure 4: Cached Info CertificateRequest Message.
The fingerprint MUST be computed as follows: hash_value:=SHA-
256(CertificateRequest)
5. Example
Figure 5 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 from the
earlier exchange (by indicating the 'cert' type). With [3] the TLS
server acknowledges the supports of the 'cert' type and by including
the value in the server hello informs the client that the content of
the certificate payload contains the fingerprint of the certificate
instead of the RFC 5246-defined payload of the certificate message in
message [4].
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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=(cert) -> // [2]
<- ServerHello
cached_info=(cert) [3]
Certificate [4]
ServerKeyExchange*
ServerHelloDone
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ->
<- [ChangeCipherSpec]
Finished
Application Data <-------> Application Data
Figure 5: 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
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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 (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
values of the cached values in 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 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 cert(1)
o cert_req(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
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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 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.
[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.
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.
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[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.
Hall in Tirol 6060
Austria
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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