TLS Cached Info: Transport Layer Security (TLS) Cached Information Extension
A TLS extension that lets a client say it already has the server's certificate, so the server sends a fingerprint instead of the certificate chain.
Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", RFC 7924, DOI 10.17487/RFC7924, July 2016, <https://www.rfc-editor.org/info/rfc7924>.
@misc{rfc7924,
series = {Request for Comments},
number = 7924,
howpublished = {RFC 7924},
publisher = {RFC Editor},
doi = {10.17487/RFC7924},
url = {https://www.rfc-editor.org/info/rfc7924},
author = {S. Santesson and H. Tschofenig},
title = {{Transport Layer Security (TLS) Cached Information Extension}},
pagetotal = 19,
year = 2016,
month = jul,
} RFC text
Contents
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.
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
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;
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
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.
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
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.
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).
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
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.
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>.
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>.
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'
: }
: }
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 {
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
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.
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.
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.
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
ISO Plain Language
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§1Why Shrink the Handshake
Reducing the amount of information exchanged during a Transport Layer Security handshake helps performance where devices are on a network with 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.
The RFC defines a TLS extension that allows a client and a server to exclude transmission of information cached in an earlier TLS handshake. The typical exchange works like this. The client and the server first perform a full handshake, and the client caches the certificate the server provided. Later, when the client connects again without using session resumption, it attaches the cached_info extension to the ClientHello, saying that it has the certificate and giving its fingerprint. If the server's certificate has not changed, the server does not need to send the certificate and its chain again. If the information has changed, which the fingerprint reveals, the certificate payload is transmitted so the client can update its cache.
Small devices on radio links pay for every byte of the handshake, and much of it is the server's certificate, which rarely changes. This extension lets a returning client say 'I already have that certificate' and prove it with a fingerprint, so the server can send a short value instead of the whole chain.
§3The Cached Information Extension
The RFC defines a new extension type, cached_info(25), used in the ClientHello and ServerHello messages. The extension data in a ClientHello must contain the CachedInformation structure. The client may send multiple CachedObjects of the same CachedInformationType, for example when it has cached several certificates from one server.
The structures are:
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;Two types are defined. With the type set to 'cert', the client must include the fingerprint of the Certificate message in the hash_value field, computed by the procedure in Section 5 with the Certificate message as the input data. With the type set to 'cert_req', the client must include the fingerprint of the CertificateRequest message, computed the same way with that message as the input.
New cached information types can be added, and new message digest algorithms can be added by registering a new type that uses them. The RFC recommends reusing hash algorithms already available with TLS ciphersuites, and requires that new hash algorithms must not have collision resistance worse than SHA-256.
The extension is a list of entries. Each entry names a type of information and, when the client sends it, the fingerprint of that information. Two types are defined now: the server's certificate and the server's certificate request. A client may list several entries of the same type when it has more than one certificate cached.
enum {
cached_info(25), (65535)
} ExtensionType;
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;§4How the Exchange Works
A client that supports the extension may include it in the ClientHello; if it does, it must contain one or more CachedObject attributes. A server that supports the extension may include it in the ServerHello, and by returning the extension it indicates that it supports the cached information types it lists. For each indicated type the server must alter the transmission of the corresponding payload, and it must only list types the client also indicated in its ClientHello.
Because the client sends a fingerprint of what it cached, the server can tell whether that information is stale. If the server notices a mismatch between the client's cached data and its own, it must include the information in full and must not list that type in the cached_info extension. In a hosting environment a client may have cached several items for one server, so the RFC recommends that the client uses the Server Name Indication extension to help the server select the right cached information.
For the 'cert' type the server may send a modified Certificate message, a structure holding only a hash_value of at most 255 octets, under three conditions: the server software implements the extension, the 'cert' type is enabled by policy, and the server has compared the client's hash_value with the fingerprint of the Certificate message it would normally send. That comparison is what makes sure the cached information is current.
The extension also interacts with raw public keys as defined in RFC 7250, which lets the certificate payload carry only the SubjectPublicKeyInfo instead of a full certificate. When the negotiated certificate type is a raw public key, the server omits a certificate payload that contains an ASN.1 certificate structure with the included SubjectPublicKeyInfo rather than the full certificate chain.
For the 'cert_req' type the server may send a modified CertificateRequest message, again just a hash_value, under the same three conditions plus one more: the server wants to request a certificate from the client.
The client lists what it has cached, the server replies with the list it agrees to use, and from then on the server sends short fingerprints instead of the full messages for those types. If the fingerprints do not match what the server would send now, the server sends the full information and leaves that type out of its reply. The client tells the server which cached item to use through the Server Name Indication extension.
| Handshake message | Full exchange | With cached_info |
|---|---|---|
| ClientHello | No extension | cached_info=(cert) with the fingerprint |
| ServerHello | No extension | cached_info=(cert) |
| Certificate | Full certificate chain | Fingerprint only |
| CertificateRequest | Full request | Fingerprint only |
§5Calculating the Fingerprint
The fingerprint for the two cached information objects must be computed in two steps. First, compute the SHA-256 hash of the input data, where the input data depends on the cached information type: the Certificate message for 'cert' and the CertificateRequest message for 'cert_req'. Second, use the output of that hash. The RFC notes that the computed hash covers only the input data structure and not any type and length information of the record layer.
The purpose of the fingerprint is to help the server select the correct information. For a Certificate message it identifies the server certificate, and the corresponding private key, for use with the rest of the handshake. A server may hold more than one certificate, so the hash has to be long enough to keep the probability of collisions low. The design also aims to reduce the amount of data exchanged, and the security of the handshake depends on the private key rather than on the size of the fingerprint. The fingerprint is therefore a way to stop the server accidentally selecting the wrong information.
If an attacker injects an incorrect fingerprint there are two outcomes. Either the fingerprint relates to no cached state, in which case the server falls back to a full exchange, or the attacker manages to inject a fingerprint that refers to data the client has not cached, in which case the exchange fails later, when the client verifies the digital signature. The signature verification fails because the public key the client cached does not correspond to the private key the server used to sign the message.
The fingerprint is a SHA-256 hash of the exact message the server would have sent. It is not a secret and it does not protect the handshake; it only tells the server which cached item to pick. A wrong fingerprint either causes a full handshake or makes the signature check fail later, so it cannot be used to slip a false certificate past the client.
§6The Example Exchanges
In a regular full TLS handshake, the server provides its certificate in the certificate payload, which lets the client store it for later. When the client returns to the same server and uses the cached_info extension, it indicates support with the extension in the ClientHello and states that it stored the certificate from the earlier exchange by indicating the 'cert' type. The server acknowledges support of the 'cert' type in the ServerHello, and by including the value there it tells the client that the certificate payload now contains the fingerprint of the certificate instead of the certificate itself as defined in RFC 5246.
Appendix A works the calculation through with a certificate holding a NIST P256 elliptic curve public key, shown with an ASN.1 decoder. When that certificate is put into a TLS or DTLS Certificate message it is preceded by a message header, in the example 0b 00 02 36 00 02 33 00 02 00 02 30: a one byte type field indicating a Certificate message, a three byte length field of 566 bytes for the payload, a three byte length of 563 bytes for the whole certificates list, and a three byte length of 560 bytes for the certificate itself. Applying SHA-256 to the Certificate message, which starts with 0b 00 02 and ends with 9E 57 45, produces the hash 0x086eefb4859adfe977defac494fff6b73033b4ce1f86b8f2a9fc0c6bf98605af.
The RFC walks through a complete example with a real certificate, showing the bytes that go into the message and the resulting fingerprint. The point of the example is that the fingerprint is computed over the message exactly as it would have been sent, so the server and client agree on the same bytes.
- CLIENTHELLO cached_info=(cert)The client says it has the certificate cached and sends its fingerprint.
- SERVERHELLO cached_info=(cert)The server confirms that it supports the 'cert' type.
- CERTIFICATE (fingerprint only)The certificate payload carries the fingerprint instead of the certificate.
§7Security and Privacy
The mechanism references stored state using a fingerprint. Sending a fingerprint of cached information in an unencrypted handshake, as the ClientHello and ServerHello do, may allow an attacker or observer to correlate independent TLS exchanges. Some information elements used by the extension, such as server certificates, are public objects and usually do not carry sensitive information, but other types that are not yet defined may. Implementers and deployers should therefore decide whether the cached information fits their security and privacy goals, and where there are concerns it is advised to avoid sending the fingerprint of the data objects in clear.
Because the omitted parts are no longer sent, they 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. When a client populates the cache from a TLS exchange, it must only cache information after the successful completion of that exchange, so that an attacker cannot inject incorrect information into the cache. Failing to do so allows man-in-the-middle attacks.
A fingerprint sent in the clear can let an observer link two separate TLS connections that share a cached item. The RFC also insists that the client only stores what a completed handshake gave it, because a cache seeded by an attacker is a way to mount a man-in-the-middle attack later.
§8Registry Entries
The RFC adds an entry to the existing TLS ExtensionType registry defined in RFC 5246, for cached_info(25).
It also establishes a new registry, the TLS CachedInformationType Values registry, holding the reserved value 0, cert at 1, cert_req at 2, and the range 224 to 255 reserved for private use. Values from 0 to 63 are added by Standards Action, and values from 64 to 223 are added by Specification Required, following the terminology of RFC 5226.
Two registry changes are needed. The extension gets the number 25 in the TLS extension registry, and a new registry lists the kinds of information that can be cached: 1 for the certificate and 2 for the certificate request, with room for private values at the top of the range.
- 0Reserved
- 1cert
- 2cert_req
- 224-255Reserved for private use