Transport Layer Security (TLS) Cached Information Extension
RFC 7924
| Document | Type | RFC - Proposed Standard (July 2016; No errata) | |
|---|---|---|---|
| Authors | Stefan Santesson , Hannes Tschofenig | ||
| Last updated | 2018-12-20 | ||
| Replaces | draft-santesson-tls-certcache | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Joseph Salowey | ||
| Shepherd write-up | Show (last changed 2015-11-02) | ||
| IESG | IESG state | RFC 7924 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Stephen Farrell | ||
| Send notices to | (None) | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack | ||
Internet Engineering Task Force (IETF) S. Santesson
Request for Comments: 7924 3xA Security AB
Category: Standards Track H. Tschofenig
ISSN: 2070-1721 ARM Ltd.
July 2016
Transport Layer Security (TLS) Cached Information Extension
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, thereby enabling the server to omit
already available information.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7924.
Santesson & Tschofenig Standards Track [Page 1]
RFC 7924 TLS Cached Information Extension July 2016
Copyright Notice
Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cached Information Extension . . . . . . . . . . . . . . . . 3
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5
4.1. Server Certificate Message . . . . . . . . . . . . . . . 6
4.2. CertificateRequest Message . . . . . . . . . . . . . . . 7
5. Fingerprint Calculation . . . . . . . . . . . . . . . . . . . 7
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10
8.2. New Registry for CachedInformationType . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
Santesson & Tschofenig Standards Track [Page 2]
RFC 7924 TLS Cached Information Extension July 2016
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 the Internet of Things,
such environments exist, for example, when devices use IEEE 802.15.4,
Bluetooth Low Energy, or low power wide area networks. 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 execute 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 ClientHello message to
indicate that it has 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 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 Datagram Transport Layer Security (DTLS) as
well.
3. Cached Information Extension
This document defines a new extension type (cached_info(25)), which
is used in ClientHello and ServerHello messages. The extension type
is specified as follows.
enum {
cached_info(25), (65535)
} ExtensionType;
Santesson & Tschofenig Standards Track [Page 3]
RFC 7924 TLS Cached Information Extension July 2016
The extension_data field of this extension, when included in the
ClientHello, 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.
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 two types:
'cert' type for not sending the complete server certificate message:
With the type field set to 'cert', the client MUST include the
fingerprint of the Certificate message in the hash_value field.
For this type, the fingerprint MUST be calculated using the
procedure described in Section 5 with the Certificate message as
input data.
'cert_req' Type for not sending the complete CertificateRequest
Message:
With the type set to 'cert_req', the client MUST include the
fingerprint of the CertificateRequest message in the hash_value
field. For this type, the fingerprint MUST be calculated using
the procedure described in Section 5 with the CertificateRequest
message as input data.
New cached info types can be added following the policy described in
the IANA Considerations (Section 8). New message digest algorithms
for use with these types can also be added by registering a new type
that makes use of the updated message digest algorithm. For
practical reasons, we recommend reusing hash algorithms already
Santesson & Tschofenig Standards Track [Page 4]
RFC 7924 TLS Cached Information Extension July 2016
available with TLS ciphersuites. To avoid additional code and to
keep the collision probability low, new hash algorithms MUST NOT have
a collision resistance worse than SHA-256.
4. Exchange Specification
Clients supporting this extension MAY include the "cached_info"
extension in the (extended) ClientHello. 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) ServerHello. 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 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 ClientHello). 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
('foo-bar').
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
(SNI) extension [RFC6066].
Following a successful exchange of the "cached_info" extension in the
ClientHello and ServerHello, the server alters sending the
corresponding handshake message. How information is altered from the
handshake messages and for the types defined in this specification is
defined in Sections 4.1 and 4.2, respectively.
Appendix A shows an example hash calculation, and Section 6
illustrates an example protocol exchange.
Santesson & Tschofenig Standards Track [Page 5]
RFC 7924 TLS Cached Information Extension July 2016
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 1 under the following conditions:
o The server software implements the "cached_info" extension defined
in this specification.
o The 'cert' "cached_info" extension is enabled (for example, a
policy allows the use of this extension).
o 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
procedure for calculating the fingerprint is described in
Section 5.
The original certificate handshake message syntax is defined in
[RFC5246] and has been extended with [RFC7250]. RFC 7250 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 chain. As such, this extension is
compatible with the raw public key extension defined in RFC 7250.
Note: We assume that the server implementation is able to select the
appropriate certificate or SubjectPublicKeyInfo from the received
hash value. If the SNI extension is used by the client, then the
server has additional information to guide the selection of the
appropriate cached info.
When the cached info specification is used, then a modified version
of the Certificate message is exchanged. The modified structure is
shown in Figure 1.
struct {
opaque hash_value<1..255>;
} Certificate;
Figure 1: Cached Info Certificate Message
Santesson & Tschofenig Standards Track [Page 6]
RFC 7924 TLS Cached Information Extension July 2016
4.2. CertificateRequest Message
When a fingerprint for an object of type 'cert_req' is provided in
the ClientHello, the server MAY send the CertificateRequest message
shown in Figure 2 under the following conditions:
o The server software implements the "cached_info" extension defined
in this specification.
o The 'cert_req' "cached_info" extension is enabled (for example, a
policy allows the use of this extension).
o 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. The procedure for calculating the fingerprint is
described in Section 5.
o The server wants to request a certificate from the client.
The original CertificateRequest handshake message syntax is defined
in [RFC5246]. The modified structure of the CertificateRequest
message is shown in Figure 2.
struct {
opaque hash_value<1..255>;
} CertificateRequest;
Figure 2: Cached Info CertificateRequest Message
The CertificateRequest payload is the input parameter to the
fingerprint calculation described in Section 5.
5. Fingerprint Calculation
The fingerprint for the two cached info objects defined in this
document MUST be computed as follows:
1. Compute the SHA-256 [RFC6234] hash of the input data. The input
data depends on the cached info type. This document defines two
cached info types, described in Sections 4.1 and in 4.2. Note
that the computed hash only covers the input data structure (and
not any type and length information of the record layer).
Appendix A shows an example.
2. Use the output of the SHA-256 hash.
Santesson & Tschofenig Standards Track [Page 7]
RFC 7924 TLS Cached Information Extension July 2016
The purpose of the fingerprint provided by the client is to help the
server select the correct information. For example, in case of a
Certificate message, the fingerprint identifies the server
certificate (and the corresponding private key) for use with the rest
of the handshake. Servers may have more than one certificate, and
therefore a hash needs to be long enough to keep the probably of hash
collisions low. On the other hand, the cached info design aims to
reduce the amount of data being exchanged. The security of the
handshake depends on the private key and not on the size of the
fingerprint. Hence, the fingerprint is a way to prevent the server
from accidentally selecting the wrong information. If an attacker
injects an incorrect fingerprint, then two outcomes are possible: (1)
the fingerprint does not relate to any cached state and the server
has to fall back to a full exchange, and (2) if the attacker manages
to inject a fingerprint that refers to data the client has not
cached, then the exchange will fail later when the client continues
with the handshake and aims to verify the digital signature. The
signature verification will fail since the public key cached by the
client will not correspond to the private key that was used by the
server to sign the message.
6. Example
In the regular, full TLS handshake exchange, shown in Figure 3, 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 Figure 4. The TLS client
indicates support for this specification via the "cached_info"
extension, see step (2), and indicates that it has stored the
certificate from the earlier exchange (by indicating the 'cert'
type). With step (3), the TLS server acknowledges the support of the
'cert' type and by including the value in the ServerHello, it informs
the client that the content of the certificate payload contains the
fingerprint of the certificate instead of the payload, defined in RFC
5246, of the Certificate message; see step (4).
Santesson & Tschofenig Standards Track [Page 8]
RFC 7924 TLS Cached Information Extension July 2016
ClientHello ->
<- ServerHello
Certificate* // (1)
ServerKeyExchange*
CertificateRequest*
ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ->
<- [ChangeCipherSpec]
Finished
Application Data <-------> Application Data
Figure 3: Example Message Exchange: Initial (Full) Exchange
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 4: Example Message Exchange: TLS Cached Extension Usage
Santesson & Tschofenig Standards Track [Page 9]
RFC 7924 TLS Cached Information Extension July 2016
7. 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 ClientHello and ServerHello does,
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 information, other types that are
not yet defined may. Those who implement and deploy this
specification should therefore make an informed decision whether the
cached information is in line 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 send
significantly smaller TLS messages. Consequently, these omitted
parts of the messages are not included in the transcript of the
handshake in the TLS Finish message. However, since the client and
the server communicate the hash values of the cached data in the
initial handshake messages, 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.
Security considerations for the fingerprint calculation are discussed
in Section 5.
8. IANA Considerations
8.1. New Entry to the TLS ExtensionType Registry
IANA has added an entry to the existing TLS "ExtensionType Values"
registry, defined in [RFC5246], for cached_info(25) defined in this
document.
Santesson & Tschofenig Standards Track [Page 10]
RFC 7924 TLS Cached Information Extension July 2016
8.2. New Registry for CachedInformationType
IANA has established a registry titled "TLS CachedInformationType
Values". The entries in the registry are:
Value Description
----- -----------
0 Reserved
1 cert
2 cert_req
224-255 Reserved for Private Use
The policy for adding new values to this registry, following the
terminology defined in [RFC5226], is as follows:
o 0-63 (decimal): Standards Action
o 64-223 (decimal): Specification Required
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
Santesson & Tschofenig Standards Track [Page 11]
RFC 7924 TLS Cached Information Extension July 2016
9.2. Informative References
[ASN.1-Dump]
Gutmann, P., "ASN.1 Object Dump Program", November 2010,
<http://manpages.ubuntu.com/manpages/precise/man1/
dumpasn1.1.html>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6574] Tschofenig, H. and J. Arkko, "Report from the Smart Object
Workshop", RFC 6574, DOI 10.17487/RFC6574, April 2012,
<http://www.rfc-editor.org/info/rfc6574>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
Santesson & Tschofenig Standards Track [Page 12]
RFC 7924 TLS Cached Information Extension July 2016
Appendix A. Example
Consider a certificate containing a NIST P256 elliptic curve public
key displayed using Peter Gutmann's ASN.1 decoder [ASN.1-Dump] in
Figure 5.
0 556: SEQUENCE {
4 434: SEQUENCE {
8 3: [0] {
10 1: INTEGER 2
: }
13 1: INTEGER 13
16 10: SEQUENCE {
18 8: OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2)
: }
28 62: SEQUENCE {
30 11: SET {
32 9: SEQUENCE {
34 3: OBJECT IDENTIFIER countryName (2 5 4 6)
39 2: PrintableString 'NL'
: }
: }
43 17: SET {
45 15: SEQUENCE {
47 3: OBJECT IDENTIFIER organizationName (2 5 4 10)
52 8: PrintableString 'PolarSSL'
: }
: }
62 28: SET {
64 26: SEQUENCE {
66 3: OBJECT IDENTIFIER commonName (2 5 4 3)
71 19: PrintableString 'Polarssl Test EC CA'
: }
: }
: }
92 30: SEQUENCE {
94 13: UTCTime 24/09/2013 15:52:04 GMT
109 13: UTCTime 22/09/2023 15:52:04 GMT
: }
124 65: SEQUENCE {
126 11: SET {
128 9: SEQUENCE {
130 3: OBJECT IDENTIFIER countryName (2 5 4 6)
135 2: PrintableString 'NL'
: }
: }
Santesson & Tschofenig Standards Track [Page 13]
RFC 7924 TLS Cached Information Extension July 2016
139 17: SET {
141 15: SEQUENCE {
143 3: OBJECT IDENTIFIER organizationName (2 5 4 10)
148 8: PrintableString 'PolarSSL'
: }
: }
158 31: SET {
160 29: SEQUENCE {
162 3: OBJECT IDENTIFIER commonName (2 5 4 3)
167 22: PrintableString 'PolarSSL Test Client 2'
: }
: }
: }
191 89: SEQUENCE {
193 19: SEQUENCE {
195 7: OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
204 8: OBJECT IDENTIFIER prime256v1 (1 2 840 10045 3 1 7)
: }
214 66: BIT STRING
: 04 57 E5 AE B1 73 DF D3 AC BB 93 B8 81 FF 12 AE
: EE E6 53 AC CE 55 53 F6 34 0E CC 2E E3 63 25 0B
: DF 98 E2 F3 5C 60 36 96 C0 D5 18 14 70 E5 7F 9F
: D5 4B 45 18 E5 B0 6C D5 5C F8 96 8F 87 70 A3 E4
: C7
: }
282 157: [3] {
285 154: SEQUENCE {
288 9: SEQUENCE {
290 3: OBJECT IDENTIFIER basicConstraints (2 5 29 19)
295 2: OCTET STRING, encapsulates {
297 0: SEQUENCE {}
: }
: }
299 29: SEQUENCE {
301 3: OBJECT IDENTIFIER subjectKeyIdentifier (2 5 29 14)
306 22: OCTET STRING, encapsulates {
308 20: OCTET STRING
: 7A 00 5F 86 64 FC E0 5D E5 11 10 3B B2 E6 3B C4
: 26 3F CF E2
: }
: }
330 110: SEQUENCE {
332 3: OBJECT IDENTIFIER authorityKeyIdentifier (2 5 29 35)
337 103: OCTET STRING, encapsulates {
339 101: SEQUENCE {
Santesson & Tschofenig Standards Track [Page 14]
RFC 7924 TLS Cached Information Extension July 2016
341 20: [0]
: 9D 6D 20 24 49 01 3F 2B CB 78 B5 19 BC 7E 24
: C9 DB FB 36 7C
363 66: [1] {
365 64: [4] {
367 62: SEQUENCE {
369 11: SET {
371 9: SEQUENCE {
373 3: OBJECT IDENTIFIER countryName (2 5 4 6)
378 2: PrintableString 'NL'
: }
: }
382 17: SET {
384 15: SEQUENCE {
386 3: OBJECT IDENTIFIER organizationName
: (2 5 4 10)
391 8: PrintableString 'PolarSSL'
: }
: }
401 28: SET {
403 26: SEQUENCE {
405 3: OBJECT IDENTIFIER commonName (2 5 4 3)
410 19: PrintableString 'Polarssl Test EC CA'
: }
: }
: }
: }
: }
431 9: [2] 00 C1 43 E2 7E 62 43 CC E8
: }
: }
: }
: }
: }
: }
442 10: SEQUENCE {
444 8: OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2)
: }
454 104: BIT STRING, encapsulates {
457 101: SEQUENCE {
459 48: INTEGER
: 4A 65 0D 7B 20 83 A2 99 B9 A8 0F FC 8D EE 8F 3D
: BB 70 4C 96 03 AC 8E 78 70 DD F2 0E A0 B2 16 CB
: 65 8E 1A C9 3F 2C 61 7E F8 3C EF AD 1C EE 36 20
Santesson & Tschofenig Standards Track [Page 15]
RFC 7924 TLS Cached Information Extension July 2016
509 49: INTEGER
: 00 9D F2 27 A6 D5 74 B8 24 AE E1 6A 3F 31 A1 CA
: 54 2F 08 D0 8D EE 4F 0C 61 DF 77 78 7D B4 FD FC
: 42 49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57
: 45
: }
: }
: }
Figure 5: ASN.1-Based Certificate: Example
To include the certificate shown in Figure 5 in a TLS/DTLS
Certificate message, it is prepended with a message header. This
Certificate message header in our example is 0b 00 02 36 00 02 33 00
02 00 02 30, which indicates:
Message Type: 0b -- 1-byte type field indicating a Certificate
message
Length: 00 02 36 -- 3-byte length field indicating a 566-byte
payload
Certificates Length: 00 02 33 -- 3-byte length field indicating 563
bytes for the entire certificates_list structure, which may
contain multiple certificates. In our example, only one
certificate is included.
Certificate Length: 00 02 30 -- 3-byte length field indicating 560
bytes of the actual certificate following immediately afterwards.
In our example, this is the certificate content with 30 82 02 ....
9E 57 45 shown in Figure 6.
Santesson & Tschofenig Standards Track [Page 16]
RFC 7924 TLS Cached Information Extension July 2016
The hex encoding of the ASN.1-encoded certificate payload shown in
Figure 5 leads to the following encoding.
30 82 02 2C 30 82 01 B2 A0 03 02 01 02 02 01 0D
30 0A 06 08 2A 86 48 CE 3D 04 03 02 30 3E 31 0B
30 09 06 03 55 04 06 13 02 4E 4C 31 11 30 0F 06
03 55 04 0A 13 08 50 6F 6C 61 72 53 53 4C 31 1C
30 1A 06 03 55 04 03 13 13 50 6F 6C 61 72 73 73
6C 20 54 65 73 74 20 45 43 20 43 41 30 1E 17 0D
31 33 30 39 32 34 31 35 35 32 30 34 5A 17 0D 32
33 30 39 32 32 31 35 35 32 30 34 5A 30 41 31 0B
30 09 06 03 55 04 06 13 02 4E 4C 31 11 30 0F 06
03 55 04 0A 13 08 50 6F 6C 61 72 53 53 4C 31 1F
30 1D 06 03 55 04 03 13 16 50 6F 6C 61 72 53 53
4C 20 54 65 73 74 20 43 6C 69 65 6E 74 20 32 30
59 30 13 06 07 2A 86 48 CE 3D 02 01 06 08 2A 86
48 CE 3D 03 01 07 03 42 00 04 57 E5 AE B1 73 DF
D3 AC BB 93 B8 81 FF 12 AE EE E6 53 AC CE 55 53
F6 34 0E CC 2E E3 63 25 0B DF 98 E2 F3 5C 60 36
96 C0 D5 18 14 70 E5 7F 9F D5 4B 45 18 E5 B0 6C
D5 5C F8 96 8F 87 70 A3 E4 C7 A3 81 9D 30 81 9A
30 09 06 03 55 1D 13 04 02 30 00 30 1D 06 03 55
1D 0E 04 16 04 14 7A 00 5F 86 64 FC E0 5D E5 11
10 3B B2 E6 3B C4 26 3F CF E2 30 6E 06 03 55 1D
23 04 67 30 65 80 14 9D 6D 20 24 49 01 3F 2B CB
78 B5 19 BC 7E 24 C9 DB FB 36 7C A1 42 A4 40 30
3E 31 0B 30 09 06 03 55 04 06 13 02 4E 4C 31 11
30 0F 06 03 55 04 0A 13 08 50 6F 6C 61 72 53 53
4C 31 1C 30 1A 06 03 55 04 03 13 13 50 6F 6C 61
72 73 73 6C 20 54 65 73 74 20 45 43 20 43 41 82
09 00 C1 43 E2 7E 62 43 CC E8 30 0A 06 08 2A 86
48 CE 3D 04 03 02 03 68 00 30 65 02 30 4A 65 0D
7B 20 83 A2 99 B9 A8 0F FC 8D EE 8F 3D BB 70 4C
96 03 AC 8E 78 70 DD F2 0E A0 B2 16 CB 65 8E 1A
C9 3F 2C 61 7E F8 3C EF AD 1C EE 36 20 02 31 00
9D F2 27 A6 D5 74 B8 24 AE E1 6A 3F 31 A1 CA 54
2F 08 D0 8D EE 4F 0C 61 DF 77 78 7D B4 FD FC 42
49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57 45
Figure 6: Hex Encoding of the Example Certificate
Applying the SHA-256 hash function to the Certificate message, which
starts with 0b 00 02 and ends with 9E 57 45, produces
0x086eefb4859adfe977defac494fff6b73033b4ce1f86b8f2a9fc0c6bf98605af.
Santesson & Tschofenig Standards Track [Page 17]
RFC 7924 TLS Cached Information Extension July 2016
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 (March 2012)
o Ilari Liusvaara, Adam Langley, and Eric Rescorla (July 2014)
o Sean Turner (August 2014)
o Martin Thomson (August 2015)
o Jouni Korhonen (November 2015)
o Dave Garrett (December 2015)
o Matt Miller (December 2015)
o Anirudh Ramachandran (March 2016)
We would also to thank Martin Thomson, Karthikeyan Bhargavan, Sankalp
Bagaria, and Eric Rescorla for their feedback regarding the
fingerprint calculation.
Finally, 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 and their reviews.
Santesson & Tschofenig Standards Track [Page 18]
RFC 7924 TLS Cached Information Extension July 2016
Authors' Addresses
Stefan Santesson
3xA Security AB
Forskningsbyn Ideon
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
Santesson & Tschofenig Standards Track [Page 19]