draft-ietf-tls-cached-info-19.txt   draft-ietf-tls-cached-info-20.txt 
TLS S. Santesson TLS S. Santesson
Internet-Draft 3xA Security AB Internet-Draft 3xA Security AB
Intended status: Standards Track H. Tschofenig Intended status: Standards Track H. Tschofenig
Expires: September 24, 2015 ARM Ltd. Expires: April 21, 2016 ARM Ltd.
March 23, 2015 October 19, 2015
Transport Layer Security (TLS) Cached Information Extension Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-19.txt draft-ietf-tls-cached-info-20.txt
Abstract Abstract
Transport Layer Security (TLS) handshakes often include fairly static Transport Layer Security (TLS) handshakes often include fairly static
information, such as the server certificate and a list of trusted information, such as the server certificate and a list of trusted
certification authorities (CAs). This information can be of certification authorities (CAs). This information can be of
considerable size, particularly if the server certificate is bundled considerable size, particularly if the server certificate is bundled
with a complete certificate chain (i.e., the certificates of with a complete certificate chain (i.e., the certificates of
intermediate CAs up to the root CA). intermediate CAs up to the root CA).
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 24, 2015. This Internet-Draft will expire on April 21, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cached Information Extension . . . . . . . . . . . . . . . . 3 3. Cached Information Extension . . . . . . . . . . . . . . . . 3
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 4 4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5
4.1. Server Certificate Message . . . . . . . . . . . . . . . 5 4.1. Server Certificate Message . . . . . . . . . . . . . . . 5
4.2. CertificateRequest Message . . . . . . . . . . . . . . . 6 4.2. CertificateRequest Message . . . . . . . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. Fingerprint Calculation . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.2. New Registry for CachedInformationType . . . . . . . . . 9 8.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 8.2. New Registry for CachedInformationType . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 10 10.1. Normative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
Reducing the amount of information exchanged during a Transport Layer Reducing the amount of information exchanged during a Transport Layer
Security handshake to a minimum helps to improve performance in Security handshake to a minimum helps to improve performance in
environments where devices are connected to a network with a low environments where devices are connected to a network with a low
bandwidth, and lossy radio technology. With Internet of Things such bandwidth, and lossy radio technology. With Internet of Things such
environments exist, for example, when devices use IEEE 802.15.4 or environments exist, for example, when devices use IEEE 802.15.4 or
Bluetooth Smart. For more information about the challenges with Bluetooth Smart. For more information about the challenges with
smart object deployments please see [RFC6574]. smart object deployments please see [RFC6574].
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opaque hash_value<1..255>; opaque hash_value<1..255>;
case server: case server:
CachedInformationType type; CachedInformationType type;
} body; } body;
} CachedObject; } CachedObject;
struct { struct {
CachedObject cached_info<1..2^16-1>; CachedObject cached_info<1..2^16-1>;
} CachedInformation; } CachedInformation;
This document defines the following types: This document defines the following two types:
Omitting the Server Certificate Message: 'cert' Type for not sending the complete Server Certificate Message:
With the type field set to 'cert', the client MUST include the With the type field set to 'cert', the client MUST include the
message digest of the Certificate message in the hash_value field. fingerprint of the Certificate message in the hash_value field.
For this type the message digest MUST be calculated using SHA-256 For this type the fingerprint MUST be calculated using the
[RFC4634]. procedure described in Section 5 with the Certificate message as
input data.
Omitting the CertificateRequest Message 'cert_req' Type for not sending the complete CertificateRequest
Message:
With the type set to 'cert_req', the client MUST include the With the type set to 'cert_req', the client MUST include the
message digest of the CertificateRequest message in the hash_value fingerprint of the CertificateRequest message in the hash_value
field. For this type the message digest MUST be calculated using field. For this type the fingerprint MUST be calculated using the
SHA-256 [RFC4634]. procedure described in Section 5 with the CertificateRequest
message as input data..
New types can be added following the policy described in the IANA New cached info types can be added following the policy described in
considerations section, see Section 7. Different message digest the IANA considerations section, see Section 8. New message digest
algorithms for use with these types can also be added by registering algorithms for use with these types can also be added by registering
a new type that makes use of this updated message digest algorithm. a new type that makes use of the updated message digest algorithm.
There are no specific requirements for the use of specific hash
algorithms but for practical reason it is useful to re-use algorithms
already available with TLS ciphersuites to avoid additional code and
to keep the collision probably low.
4. Exchange Specification 4. Exchange Specification
Clients supporting this extension MAY include the "cached_info" Clients supporting this extension MAY include the "cached_info"
extension in the (extended) client hello. If the client includes the extension in the (extended) client hello. If the client includes the
extension then it MUST contain one or more CachedObject attributes. extension then it MUST contain one or more CachedObject attributes.
A server supporting this extension MAY include the "cached_info" A server supporting this extension MAY include the "cached_info"
extension in the (extended) server hello. By returning the extension in the (extended) server hello. By returning the
"cached_info" extension the server indicates that it supports the "cached_info" extension the server indicates that it supports the
cached info types. For each indicated cached info type the server cached info types. For each indicated cached info type the server
MUST alter the transmission of respective payloads, according to the MUST alter the transmission of respective payloads, according to the
rules outlined with each type. If the server includes the extension rules outlined with each type. If the server includes the extension
it MUST only include CachedObjects of a type also supported by the it MUST only include CachedObjects of a type also supported by the
client (as expressed in the client hello). For example, if a client client (as expressed in the client hello). For example, if a client
indicates support for 'cert' and 'cert_req' then the server cannot indicates support for 'cert' and 'cert_req' then the server cannot
respond with a "cached_info" attribute containing support for respond with a "cached_info" attribute containing support for ('foo-
'cert_status'. bar'.
Since the client includes a fingerprint of information it cached (for Since the client includes a fingerprint of information it cached (for
each indicated type) the server is able to determine whether cached each indicated type) the server is able to determine whether cached
information is stale. If the server supports this specification and information is stale. If the server supports this specification and
notices a mismatch between the data cached by the client and its own notices a mismatch between the data cached by the client and its own
information then the server MUST include the information in full and information then the server MUST include the information in full and
MUST NOT list the respective type in the "cached_info" extension. MUST NOT list the respective type in the "cached_info" extension.
Note: If a server is part of a hosting environment then the client 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 may have cached multiple data items for a single server. To allow
the client to select the appropriate information from the cache it is the client to select the appropriate information from the cache it is
RECOMMENDED that the client utilizes the Server Name Indication RECOMMENDED that the client utilizes the Server Name Indication
extension [RFC6066]. extension [RFC6066].
Following a successful exchange of the "cached_info" extension in the Following a successful exchange of the "cached_info" extension in the
client and server hello, the server alters sending the corresponding client and server hello, the server alters sending the corresponding
handshake message. How information is altered from the handshake handshake message. How information is altered from the handshake
messages is defined in Section 4.1, and in Section 4.2 for the types messages is defined in Section 4.1, and in Section 4.2 for the types
defined in this specification. defined in this specification.
Appendix A shows an example hash calculation and Section 6 shows an
example protocol exchange.
4.1. Server Certificate Message 4.1. Server Certificate Message
When a ClientHello message contains the "cached_info" extension with When a ClientHello message contains the "cached_info" extension with
a type set to 'cert' then the server MAY send the Certificate message a type set to 'cert' then the server MAY send the Certificate message
shown in Figure 2 under the following conditions: shown in Figure 1 under the following conditions:
The server software implements the "cached_info" extension defined o The server software implements the "cached_info" extension defined
in this specification. in this specification.
The 'cert' cached info extension is enabled (for example, a policy o The 'cert' cached info extension is enabled (for example, a policy
allows the use of this extension). allows the use of this extension).
The server compared the value in the hash_value field of the o The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of client-provided "cached_info" extension with the fingerprint of
the Certificate message it normally sends to clients. This check the Certificate message it normally sends to clients. This check
ensures that the information cached by the client is current. 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 RFC The original Certificate handshake message syntax is defined in RFC
5246 [RFC5246] and has the structure shown in Figure 1. 5246 [RFC5246] and has been extended with RFC 7250 [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.
opaque ASN.1Cert<1..2^24-1>; 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 { struct {
ASN.1Cert certificate_list<0..2^24-1>; opaque hash_value[1..255];
} Certificate; } Certificate;
Figure 1: Certificate Message as defined in RFC 5246. Figure 1: Cached Info Certificate Message.
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 4.2. CertificateRequest Message
When a fingerprint for an object of type 'cert_req' is provided in When a fingerprint for an object of type 'cert_req' is provided in
the client hello, the server MAY omit the CertificateRequest message the client hello, the server MAY send the CertificateRequest message
under the following conditions: shown in Figure 2 message under the following conditions:
The server software implements the "cached_info" extension defined o The server software implements the "cached_info" extension defined
in this specification. in this specification.
The 'cert_req' cached info extension is enabled (for example, a o The 'cert_req' cached info extension is enabled (for example, a
policy allows the use of this extension). policy allows the use of this extension).
The server compared the value in the hash_value field of the o The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of client-provided "cached_info" extension with the fingerprint of
the CertificateRequest message it normally sends to clients. This the CertificateRequest message it normally sends to clients. This
check ensures that the information cached by the client is check ensures that the information cached by the client is
current. current. The procedure for calculating the fingerprint is
described in Section 5.
The server wants to request a certificate from the client. o The server wants to request a certificate from the client.
The original CertificateRequest handshake message syntax is defined The original CertificateRequest handshake message syntax is defined
in RFC 5246 [RFC5246] and has the following structure: in RFC 5246 [RFC5246]. The modified structure of the
CertificateRequest message is shown in Figure 2.
opaque DistinguishedName<1..2^16-1>;
struct { struct {
ClientCertificateType certificate_types<1..2^8-1>; opaque hash_value<1..255>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest; } CertificateRequest;
Figure 3: CertificateRequest Message as defined in RFC 5246. Figure 2: Cached Info CertificateRequest Message.
The new structure of the CertificateRequest message is shown in The CertificateRequest payload is the input parameter to the
Figure 4. fingerprint calculation described in Section 5.
struct { 5. Fingerprint Calculation
opaque hash_value<1..255>;
} CertificateRequest;
Figure 4: Cached Info CertificateRequest Message. The fingerprint MUST be computed as follows:
The fingerprint MUST be computed as follows: hash_value:=SHA- 1. Compute the SHA-256 [RFC4634] hash of the input data. The input
256(CertificateRequest) data depends on the cached info type. This document defines two
cached info types, described in Section 4.1 and in Section 4.2.
Note that the computed hash only covers the input data structure
(and not any type and length information of the record layer).
5. Example 2. Truncate the output of the SHA-256 hash. When a hash value is
truncated to 32 bits, the leftmost 32 bits (that is, the most
significant 32 bits in network byte order) from the binary
representation of the hash value MUST be used as the truncated
value. An example of a 256-bit hash output truncated to 32 bits
is shown in Figure 3.
Figure 5 illustrates an example exchange using the TLS cached info 256-bit hash:
0x265357902fe1b7e2a04b897c6025d7a2265357902fe1b7e2a04b897c6025d7a2
32-bit truncated hash:
0x26535790
Figure 3: Truncated Hash Example.
The purpose of the fingerprint provided by the client is to help the
server select the correct information. For example, in case of the
certificate message the fingerprint identifies the server certificate
(and the corresponding private key) for use for 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. (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 server to sign the
message.
6. Example
Figure 4 illustrates an example exchange using the TLS cached info
extension. In the normal TLS handshake exchange shown in flow (A) extension. In the normal TLS handshake exchange shown in flow (A)
the TLS server provides its certificate in the Certificate payload to the TLS server provides its certificate in the Certificate payload to
the client, see step [1]. This allows the client to store the the client, see step [1]. This allows the client to store the
certificate for future use. After some time the TLS client again certificate for future use. After some time the TLS client again
interacts with the same TLS server and makes use of the TLS cached interacts with the same TLS server and makes use of the TLS cached
info extension, as shown in flow (B). The TLS client indicates info extension, as shown in flow (B). The TLS client indicates
support for this specification via the "cached_info" extension, see support for this specification via the "cached_info" extension, see
[2], and indicates that it has stored the certificate from the [2], and indicates that it has stored the certificate from the
earlier exchange (by indicating the 'cert' type). With [3] the TLS earlier exchange (by indicating the 'cert' type). With [3] the TLS
server acknowledges the supports of the 'cert' type and by including server acknowledges the supports of the 'cert' type and by including
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ClientKeyExchange ClientKeyExchange
CertificateVerify* CertificateVerify*
[ChangeCipherSpec] [ChangeCipherSpec]
Finished -> Finished ->
<- [ChangeCipherSpec] <- [ChangeCipherSpec]
Finished Finished
Application Data <-------> Application Data Application Data <-------> Application Data
Figure 5: Example Message Exchange Figure 4: Example Message Exchange
6. Security Considerations 7. Security Considerations
This specification defines a mechanism to reference stored state This specification defines a mechanism to reference stored state
using a fingerprint. Sending a fingerprint of cached information in using a fingerprint. Sending a fingerprint of cached information in
an unencrypted handshake, as the client and server hello is, may an unencrypted handshake, as the client and server hello is, may
allow an attacker or observer to correlate independent TLS exchanges. allow an attacker or observer to correlate independent TLS exchanges.
While some information elements used in this specification, such as While some information elements used in this specification, such as
server certificates, are public objects and usually do not contain server certificates, are public objects and usually do not contain
sensitive information, other (not yet defined cached info types) may. sensitive information, other not yet defined types may. Those who
Those who implement and deploy this specification should therefore implement and deploy this specification should therefore make an
make an informed decision whether the cached information is inline informed decision whether the cached information is inline with their
with their security and privacy goals. In case of concerns, it is security and privacy goals. In case of concerns, it is advised to
advised to avoid sending the fingerprint of the data objects in avoid sending the fingerprint of the data objects in clear.
clear.
The use of the cached info extension allows the server to obmit The use of the cached info extension allows the server to send
sending certain TLS messages. Consequently, these omitted messages significantly smaller TLS messages. Consequently, these omitted
are not included in the transcript of the handshake in the TLS Finish parts of the messages are not included in the transcript of the
message per value. However, since the client communicates the hash handshake in the TLS Finish message. However, since the client and
values of the cached values in the initial handshake message the the server communicate the hash values of the cached data in the
fingerprints are included in the TLS Finish message. initial handshake messages the fingerprints are included in the TLS
Finish message.
Clients MUST ensure that they only cache information from legitimate Clients MUST ensure that they only cache information from legitimate
sources. For example, when the client populates the cache from a TLS sources. For example, when the client populates the cache from a TLS
exchange then it must only cache information after the successful exchange then it must only cache information after the successful
completion of a TLS exchange to ensure that an attacker does not completion of a TLS exchange to ensure that an attacker does not
inject incorrect information into the cache. Failure to do so allows inject incorrect information into the cache. Failure to do so allows
for man-in-the-middle attacks. for man-in-the-middle attacks.
7. IANA Considerations Security consideratios for the fingerprint calculation are discussed
in Section 5.
7.1. New Entry to the TLS ExtensionType Registry 8. IANA Considerations
8.1. New Entry to the TLS ExtensionType Registry
IANA is requested to add an entry to the existing TLS ExtensionType IANA is requested to add an entry to the existing TLS ExtensionType
registry, defined in RFC 5246 [RFC5246], for cached_info(TBD) defined registry, defined in RFC 5246 [RFC5246], for cached_info(TBD) defined
in this document. in this document.
7.2. New Registry for CachedInformationType 8.2. New Registry for CachedInformationType
IANA is requested to establish a registry for TLS IANA is requested to establish a registry for TLS
CachedInformationType values. The first entries in the registry are CachedInformationType values. The first entries in the registry are
o cert(1) o cert(1)
o cert_req(2) o cert_req(2)
The policy for adding new values to this registry, following the The policy for adding new values to this registry, following the
terminology defined in RFC 5226 [RFC5226], is as follows: terminology defined in RFC 5226 [RFC5226], is as follows:
skipping to change at page 9, line 49 skipping to change at page 11, line 4
CachedInformationType values. The first entries in the registry are CachedInformationType values. The first entries in the registry are
o cert(1) o cert(1)
o cert_req(2) o cert_req(2)
The policy for adding new values to this registry, following the The policy for adding new values to this registry, following the
terminology defined in RFC 5226 [RFC5226], is as follows: terminology defined in RFC 5226 [RFC5226], is as follows:
o 0-63 (decimal): Standards Action o 0-63 (decimal): Standards Action
o 64-223 (decimal): Specification Required o 64-223 (decimal): Specification Required
o 224-255 (decimal): reserved for Private Use o 224-255 (decimal): reserved for Private Use
8. Acknowledgments 9. Acknowledgments
We would like to thank the following persons for your detailed We would like to thank the following persons for your detailed
document reviews: document reviews:
o Paul Wouters and Nikos Mavrogiannopoulos (December 2011) o Paul Wouters and Nikos Mavrogiannopoulos (December 2011)
o Rob Stradling (February 2012) o Rob Stradling (February 2012)
o Ondrej Mikle (in March 2012) o Ondrej Mikle (in March 2012)
o Ilari Liusvaara, Adam Langley, and Eric Rescorla (in July 2014) o Ilari Liusvaara, Adam Langley, and Eric Rescorla (in July 2014)
o Sean Turner (in August 2014) o Sean Turner (in August 2014)
Additionally, we would like to thank the TLS working group chairs, o Martin Thomson (in August 2015)
Sean Turner and Joe Salowey, as well as the responsible security area
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. director, Stephen Farrell, for their support.
9. References 10. References
9.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms [RFC4634] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMAC-SHA)", RFC 4634, July 2006. (SHA and HMAC-SHA)", RFC 4634, DOI 10.17487/RFC4634, July
2006, <http://www.rfc-editor.org/info/rfc4634>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extension Definitions", RFC 6066, January 2011. Extensions: Extension Definitions", RFC 6066, DOI
10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
9.2. Informative References 10.2. Informative References
[ASN.1-Dump]
Gutmann, P., "ASN.1 Object Dump Program", February 2013,
<http://www.cs.auckland.ac.nz/~pgut001/>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6574] Tschofenig, H. and J. Arkko, "Report from the Smart Object [RFC6574] Tschofenig, H. and J. Arkko, "Report from the Smart Object
Workshop", RFC 6574, April 2012. Workshop", RFC 6574, DOI 10.17487/RFC6574, April 2012,
<http://www.rfc-editor.org/info/rfc6574>.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
T. Kivinen, "Using Raw Public Keys in Transport Layer Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Security (TLS) and Datagram Transport Layer Security Transport Layer Security (TLS) and Datagram Transport
(DTLS)", RFC 7250, June 2014. 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 an 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 bytes
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
is starts with 0b 00 02 and ends with 9E 57 45, produces
0x086eefb4859adfe977defac494fff6b73033b4ce1f86b8f2a9fc0c6bf98605af.
Subsequently, this output is truncated to 32 bits, which leads to a
fingerpint of 0x086eefb4.
Authors' Addresses Authors' Addresses
Stefan Santesson Stefan Santesson
3xA Security AB 3xA Security AB
Scheelev. 17 Scheelev. 17
Lund 223 70 Lund 223 70
Sweden Sweden
Email: sts@aaa-sec.com Email: sts@aaa-sec.com
 End of changes. 59 change blocks. 
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