draft-ietf-tls-cached-info-17.txt   draft-ietf-tls-cached-info-18.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: May 17, 2015 ARM Ltd. Expires: September 9, 2015 ARM Ltd.
November 13, 2014 March 8, 2015
Transport Layer Security (TLS) Cached Information Extension Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-17.txt draft-ietf-tls-cached-info-18.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).
This document defines an extension that allows a TLS client to inform This document defines an extension that allows a TLS client to inform
a server of cached information, allowing the server to omit already a server of cached information, allowing the server to omit already
available information. available information.
Status of This Memo Status of This Memo
skipping to change at page 1, line 40 skipping to change at page 1, line 40
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 May 17, 2015. This Internet-Draft will expire on September 9, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 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.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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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
3.1. Certificate_list Fingerprint . . . . . . . . . . . . . . 4
3.2. Certificate_authorities Fingerprint . . . . . . . . . . . 4
3.3. Fingerprint Hash Algorithm . . . . . . . . . . . . . . . 4
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5 4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5
4.1. Omitting the Certificate List . . . . . . . . . . . . . . 5 4.1. Omitting the Server Certificate Message . . . . . . . . . 5
4.2. Omitting the Trusted Certificate Authorities . . . . . . 6 4.2. Omitting the CertificateRequest Message . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.3. Omitting the Certificate Status Information (OCSP
Stapling and Multi OCSP Stapling) . . . . . . . . . . . . 7
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 9 7.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10
7.2. New Registry for CachedInformationType . . . . . . . . . 9 7.2. New Registry for CachedInformationType . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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 smart objects are connected environments exist, for example, when devices use IEEE 802.15.4 or
using a low power IEEE 802.15.4 radio or via Bluetooth Smart. For Bluetooth Smart. For more information about the challenges with
more information about the challenges with smart object deployments smart object deployments please see [RFC6574].
please see [RFC6574].
This specification defines a TLS extension that allows a client and a This specification defines a TLS extension that allows a client and a
server to exclude transmission information cached in an earlier TLS server to exclude transmission information cached in an earlier TLS
handshake. handshake.
A typical example exchange may therefore look as follows. First, the A typical example exchange may therefore look as follows. First, the
client and the server executes the usual TLS handshake. The client client and the server executes the usual TLS handshake. The client
may, for example, decide to cache the certificate provided by the may, for example, decide to cache the certificate provided by the
server. When the TLS client connects to the TLS server some time in server. When the TLS client connects to the TLS server some time in
the future, without using session resumption, it then attaches the the future, without using session resumption, it then attaches the
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This document defines a new extension type (cached_info(TBD)), which This document defines a new extension type (cached_info(TBD)), which
is used in client hello and server hello messages. The extension is used in client hello and server hello messages. The extension
type is specified as follows. type is specified as follows.
enum { enum {
cached_info(TBD), (65535) cached_info(TBD), (65535)
} ExtensionType; } ExtensionType;
The extension_data field of this extension, when included in the The extension_data field of this extension, when included in the
client hello, MUST contain the CachedInformation structure. The client hello, MUST contain the CachedInformation structure. The
client MUST NOT send multiple CachedObjects with the same client MUST NOT send multiple CachedObjects of the same
CachedInformationType. CachedInformationType.
enum { enum {
certificate_list(1), certificate_authorities(2) (255) cert(1), cert_req(2) (255)
} CachedInformationType; } CachedInformationType;
struct { struct {
select (type) { select (type) {
case client: case client:
CachedInformationType type; CachedInformationType type;
HashAlgorithm hash;
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 establishes a registry for CachedInformationType types; This document defines the following types:
additional values can be added following the policy described in
Section 7.
3.1. Certificate_list Fingerprint Omitting the Server Certificate Message:
When the CachedInformationType identifies a certificate_list, then With the type field set to 'cert', the client MUST include the
the hash_value field MUST include the hash calculated over the message digest of the Certificate message in the hash_value field.
certificate_list element of the Certificate payload provided by the For this type the message digest MUST be calculated using SHA-256
TLS server in an earlier exchange, excluding the three length bytes [RFC4634].
of the certificate_list vector.
3.2. Certificate_authorities Fingerprint Omitting the CertificateRequest Message
When the CachedInformationType identifies a certificate_authorities, With the type set to 'cert_req', the client MUST include the
then the hash_value MUST include a hash calculated over message digest of the CertificateRequest message in the hash_value
CertificateRequest payload provided by the TLS server in an earlier field. For this type the message digest MUST be calculated using
exchange, excluding the msg_type and length field. SHA-256 [RFC4634].
3.3. Fingerprint Hash Algorithm Omitting the Certificate Status Information (OCSP Stapling and
Multiple OCSP Stapling) Message
The hash algorithm used to calculate hash values is conveyed in the With the type set to 'cert_status', the client MUST include the
'hash' field of the CachedObject element. The list of registered message digest of the CertificateStatus message in the hash_value
hash algorithms can be found in the TLS HashAlgorithm Registry, which field. For this type the message digest MUST be calculated using
was created by RFC 5246 [RFC5246]. The value zero (0) for 'none' and SHA-256 [RFC4634].
one (1) for 'md5' is not an allowed choice for a hash algorithm and
MUST NOT be used. New types can be added following the policy described in the IANA
considerations section, see Section 7. Different message digest
algorithms for use with these types can also be added by registering
a new type that makes use of this updated message digest algorithm.
4. Exchange Specification 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.
Clients and servers MUST NOT include more than one CachedObject
attribute per info type.
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 alters the transmission of respective payloads, as specified for MUST alter the transmission of respective payloads, according to the
each type. If the server includes the extension it MUST only include rules outlined with each type. If the server includes the extension
CachedObjects of a type also supported by the client (as expressed in it MUST only include CachedObjects of a type also supported by the
the client hello). client (as expressed in the client hello). For example, if a client
indicates support for 'cert' and 'cert_req' then the server cannot
respond with a "cached_info" attribute containing support for
'cert_status'.
Note that the client includes a fingerprint of the cached information Since the client includes a fingerprint of information it cached (for
to give the server enough information 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 item in the "cached_info" extension. MUST NOT list the respective type in the "cached_info" extension.
Note: Clients may cache multiple data items for a single server if Note: If a server is part of a hosting environment then the client
those servers are part of a hosting environment. To allow the client may have cached multiple data items for a single server. To allow
to select the appropriate information from the cached it is the client to select the appropriate information from the cache it is
RECOMMENDED that the client uses information from the Server Name RECOMMENDED that the client utilizes the Server Name Indication
Indication [RFC6066]. extension [RFC6066].
Following a successful exchange of the "cached_info" extensions in Following a successful exchange of the "cached_info" extension in the
the client and server hello, the server alters sending the client and server hello, the server alters sending the corresponding
corresponding handshake message. How information is altered from the handshake message. How information is altered from the handshake
handshake messages is defined per cached info type. Section 4.1 and messages is defined in Section 4.1, Section 4.2 and Section 4.3 for
Section 4.2 defines the syntax of the fingerprinted information. the types defined in this specification.
The handshake protocol MUST proceed using the information as if it 4.1. Omitting the Server Certificate Message
was provided in the handshake protocol. Since the Finished message
is calculated over the exchanged data it will also include the hash
of the cached data.
4.1. Omitting the Certificate List When a ClientHello message contains the "cached_info" extension with
a type set to 'cert' then the server MAY omit the Certificate message
under the following conditions:
When an object of type 'certificate_list' is provided in the client The server software implements the "cached_info" extension defined
hello, the server MAY replace the list of certificates with an empty in this specification.
sequence with an actual length field of zero (=empty vector).
The original handshake message syntax is defined in RFC 5246 The 'cert' cached info extension is enabled (for example, a policy
[RFC5246] and has the following structure: allows the use of this extension).
The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of
the Certificate message it normally sends to clients. This check
ensures that the information cached by the client is current.
The original Certificate handshake message syntax is defined in RFC
5246 [RFC5246] and has the following structure:
opaque ASN.1Cert<1..2^24-1>; opaque ASN.1Cert<1..2^24-1>;
struct { struct {
ASN.1Cert certificate_list<0..2^24-1>; ASN.1Cert certificate_list<0..2^24-1>;
} Certificate; } Certificate;
Note that [RFC7250] allows the certificate payload to contain only Certificate Message as defined in RFC 5246.
the SubjectPublicKeyInfo instead of the full information typically
found in a certificate. Hence, when this specification is used in
combination with [RFC7250] and the negotiated certificate type is a
raw public key then the TLS server omits sending a Certificate
payload that contains an ASN.1Cert structure of the
SubjectPublicKeyInfo.
4.2. Omitting the Trusted Certificate Authorities The fingerprint MUST be computed as follows: hash_value:=SHA-
256(Certificate)
When a fingerprint for an object of type 'certificate_authorities' is Note that RFC 7250 [RFC7250] allows the certificate payload to
provided in the client hello, the server MAY replace the contain only the SubjectPublicKeyInfo instead of the full information
CertificateRequest message with an empty sequence with an actual typically found in a certificate. Hence, when this specification is
length field of zero. 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.
The original handshake message syntax is defined in RFC 5246 4.2. Omitting the CertificateRequest Message
[RFC5246] and has the following structure:
When a fingerprint for an object of type 'cert_req' is provided in
the client hello, the server MAY omit the CertificateRequest message
under the following conditions:
The server software implements the "cached_info" extension defined
in this specification.
The 'cert_req' cached info extension is enabled (for example, a
policy allows the use of this extension).
The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of
the CertificateRequest message it normally sends to clients. This
check ensures that the information cached by the client is
current.
The server wants to request a certificate from the client.
The original CertificateRequest handshake message syntax is defined
in RFC 5246 [RFC5246] and has the following structure:
opaque DistinguishedName<1..2^16-1>; opaque DistinguishedName<1..2^16-1>;
struct { struct {
ClientCertificateType certificate_types<1..2^8-1>; ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>; supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>; DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest; } CertificateRequest;
The fingerprint MUST be computed as follows: hash_value:=SHA-
256(CertificateRequest)
4.3. Omitting the Certificate Status Information (OCSP Stapling and
Multi OCSP Stapling)
When a fingerprint for an object of type 'cert_status' is provided in
the client hello, the server MAY omit the CertificateStatus message
under the following conditions:
The server software implements the "cert_status" extension defined
in this specification.
The 'cert_status' cached info extension is enabled (for example, a
policy allows the use of this extension).
The server compared the value in the hash_value field of the
client-provided "cached_info" extension with the fingerprint of
the CertificateStatus message it normally sends to clients. This
check ensures that the information cached by the client is
current.
Both client and server support the use of OCSP Stapling and/or
Multiple OCSP Stapling, as defined in RFC 6066 [RFC6066] and in
[RFC6961].
The CertificateStatus message syntax, defined in [RFC6961], has the
following structure:
struct {
CertificateStatusType status_type;
select (status_type) {
case ocsp: OCSPResponse;
case ocsp_multi: OCSPResponseList;
} response;
} CertificateStatus;
opaque OCSPResponse<0..2^24-1>;
struct {
OCSPResponse ocsp_response_list<1..2^24-1>;
} OCSPResponseList;
The fingerprint MUST be computed as follows: hash_value:=SHA-
256(CertificateStatus)
5. Example 5. Example
Figure 1 illustrates an example exchange using the TLS cached info Figure 1 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_list' from the earlier exchange (by indicating the 'cert' type). With [3] the TLS
earlier exchange. With [3] the TLS server acknowledges the supports server acknowledges the supports of the 'cert' type and by including
of this specification and informs the client that it alterned the the value in the server hello informs the client that the certificate
content of the certificate payload (see [4], as described in payload has been omitted.
Section 4.1).
(A) Initial (full) Exchange (A) Initial (full) Exchange
ClientHello -> ClientHello ->
<- ServerHello <- ServerHello
Certificate* // [1] Certificate* // [1]
ServerKeyExchange* ServerKeyExchange*
CertificateRequest* CertificateRequest*
ServerHelloDone ServerHelloDone
Certificate* Certificate*
ClientKeyExchange ClientKeyExchange
CertificateVerify* CertificateVerify*
[ChangeCipherSpec] [ChangeCipherSpec]
Finished -> Finished ->
<- [ChangeCipherSpec] <- [ChangeCipherSpec]
Finished Finished
Application Data <-------> Application Data Application Data <-------> Application Data
(B) TLS Cached Extension Usage (B) TLS Cached Extension Usage
ClientHello ClientHello
cached_info=(certificate_list) -> // [2] cached_info=(cert) -> // [2]
<- ServerHello <- ServerHello
cached_info= cached_info=(cert) [3]
(certificate_list) // [3]
Certificate* // [4]
ServerKeyExchange* ServerKeyExchange*
CertificateRequest*
ServerHelloDone ServerHelloDone
Certificate*
ClientKeyExchange ClientKeyExchange
CertificateVerify* CertificateVerify*
[ChangeCipherSpec] [ChangeCipherSpec]
Finished -> Finished ->
<- [ChangeCipherSpec] <- [ChangeCipherSpec]
Finished Finished
Application Data <-------> Application Data Application Data <-------> Application Data
Figure 1: Example Message Exchange Figure 1: Example Message Exchange
6. Security Considerations 6. 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 not sensitive in server certificates, are public objects and usually do not contain
this regard, others may be. Those who implement and deploy this sensitive information, other (not yet defined cached info types) may.
specification should therefore make an informed decision whether the Those who implement and deploy this specification should therefore
cached information is inline with their security and privacy goals. make an informed decision whether the cached information is inline
In case of concerns, it is advised to avoid sending the fingerprint with their security and privacy goals. In case of concerns, it is
of the data objects in clear. advised to avoid sending the fingerprint of the data objects in
clear.
The hash algorithm used in this specification is required to have The use of the cached info extension allows the server to obmit
have strong collision resistance. sending certain TLS messages. Consequently, these omitted messages
are not included in the transcript of the handshake in the TLS Finish
message per value. However, since the client communicates the hash
values of the cached values in the initial handshake message the
fingerprints are included in the TLS Finish message.
Clients MUST ensure that they only cache information from legitimate
sources. For example, when the client populates the cache from a TLS
exchange then it must only cache information after the successful
completion of a TLS exchange to ensure that an attacker does not
inject incorrect information into the cache. Failure to do so allows
for man-in-the-middle attacks.
7. IANA Considerations 7. IANA Considerations
7.1. New Entry to the TLS ExtensionType Registry 7.1. 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 7.2. 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 certificate_list(1) o cert(1)
o certificate_authorities(2) o cert_req(2)
o cert_status(3)
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 8. 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)
skipping to change at page 10, line 13 skipping to change at page 11, line 19
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)
Additionally, we would like to thank the TLS working group chairs, Additionally, we would like to thank the TLS working group chairs,
Sean Turner and Joe Salowey, as well as the responsible security area Sean Turner and Joe Salowey, as well as the responsible security area
director, Stephen Farrell, for his support. director, Stephen Farrell, for their support.
9. References 9. References
9.1. Normative References 9.1. 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, March 1997.
[RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224", [RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224",
RFC 3874, September 2004. RFC 3874, September 2004.
[RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMAC-SHA)", RFC 4634, July 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [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, August 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011. Extension Definitions", RFC 6066, January 2011.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013.
9.2. Informative References 9.2. Informative References
[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. May 2008.
[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, April 2012.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and [RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7250, June 2014. (DTLS)", RFC 7250, June 2014.
Appendix A. Example
The Wireshark trace of an example TLS exchange shown in Figure 2
illustrates the use of an ECC-based ciphersuite with a 256 bit key.
ECC allows for a small certificate size compared to RSA with
equivalent security strength. The Certificate message provided by
the server is with 557 bytes (including the record layer header) one
of the largest message even though it only contains a single
certificate (i.e., no intermediate CA certificates). The client-
provided Certificate message has a length of 570 bytes (also
including the record layer header).
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 121
Handshake Protocol: Client Hello
Handshake Type: Client Hello (1)
Length: 117
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: c61b966bba2781c50b07c3278c43f5892b3d...
Session ID Length: 0
Cipher Suites Length: 10
Cipher Suites (5 suites)
Compression Methods Length: 1
Compression Methods (1 method)
Extensions Length: 66
Extension: server_name
Extension: signature_algorithms
Extension: elliptic_curves
Extension: ec_point_formats
Client <-- Server:
TLSv1.2 Record Layer: Handshake Protocol: Server Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 87
Handshake Protocol: Server Hello
Handshake Type: Server Hello (2)
Length: 83
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: 82d3d09b44149d738b7002da4ff5a986fe11...
Session ID Length: 32
Session ID: d069a74661088676b98db8346070278a7475b617a0...
Cipher Suite: Unknown (0xc0ad)
Compression Method: null (0)
Extensions Length: 11
Extension: renegotiation_info
Extension: ec_point_formats
TLSv1.2 Record Layer: Handshake Protocol: Certificate
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 557
Handshake Protocol: Certificate
Handshake Type: Certificate (11)
Length: 553
Certificates Length: 550
Certificates (550 bytes)
Certificate Length: 547
Certificate (id-at-commonName=localhost,
id-at-organizationName=PolarSSL,id-at-countryName=NL)
signedCertificate
algorithmIdentifier (iso.2.840.10045.4.3.2)
Padding: 0
encrypted: 30650231009a2c5cd7a6dba2e5640df0b94ed...
TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 215
Handshake Protocol: Server Key Exchange
Handshake Type: Server Key Exchange (12)
Length: 211
TLSv1.2 Record Layer: Handshake Protocol: Certificate Request
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 78
Handshake Protocol: Certificate Request
Handshake Type: Certificate Request (13)
Length: 74
Certificate types count: 1
Certificate types (1 type)
Signature Hash Algorithms Length: 2
Signature Hash Algorithms (1 algorithm)
Distinguished Names Length: 66
Distinguished Names (66 bytes)
TLSv1.2 Record Layer: Handshake Protocol: Server Hello Done
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 4
Handshake Protocol: Server Hello Done
Handshake Type: Server Hello Done (14)
Length: 0
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Certificate
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 570
Handshake Protocol: Certificate
Handshake Type: Certificate (11)
Length: 566
Certificates Length: 563
Certificates (563 bytes)
Certificate Length: 560
Certificate (id-at-commonName=PolarSSL Test Client 2,
id-at-organizationName=PolarSSL,id-at-countryName=NL)
signedCertificate
algorithmIdentifier (iso.2.840.10045.4.3.2)
Padding: 0
encrypted: 306502304a650d7b2083a299b9a80ffc8dee8...
TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 138
Handshake Protocol: Client Key Exchange
Handshake Type: Client Key Exchange (16)
Length: 134
TLSv1.2 Record Layer: Handshake Protocol: Certificate Verify
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 80
Handshake Protocol: Certificate Verify
Handshake Type: Certificate Verify (15)
Length: 76
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol:
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Client <-- Server:
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Figure 2: Example TLS Exchange (without Cached Info Extension).
The total size of the TLS exchange shown in Figure 2 is 1932 bytes
whereas the exchange shown in Figure 3 reduces the size to 1323 bytes
by omitting the Certificate and the CertificateRequest messages. As
it can be seen, the use of the cached info extension leads to an on-
the-wire improvement of more than 600 bytes.
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Client Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 121 + 21
Handshake Protocol: Client Hello
Handshake Type: Client Hello (1)
Length: 117 + 21
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: c61b966bba2781c50b07c3278c43f5892b3d...
Session ID Length: 0
Cipher Suites Length: 10
Cipher Suites (5 suites)
Compression Methods Length: 1
Compression Methods (1 method)
Extensions Length: 66
Extension: server_name
Extension: signature_algorithms
Extension: elliptic_curves
Extension: ec_point_formats
Extension: cached_info
Client <-- Server:
TLSv1.2 Record Layer: Handshake Protocol: Server Hello
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 87 + 5
Handshake Protocol: Server Hello
Handshake Type: Server Hello (2)
Length: 83 + 5
Version: TLS 1.2 (0x0303)
Random
gmt_unix_time: Jan 14, 2015 12:43:58.000000000 CET
random_bytes: 82d3d09b44149d738b7002da4ff5a986fe11...
Session ID Length: 32
Session ID: d069a74661088676b98db8346070278a7475b617a0...
Cipher Suite: Unknown (0xc0ad)
Compression Method: null (0)
Extensions Length: 11 + 5
Extension: renegotiation_info
Extension: ec_point_formats
Extension: cached_info
TLSv1.2 Record Layer: Handshake Protocol: Server Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 215
Handshake Protocol: Server Key Exchange
Handshake Type: Server Key Exchange (12)
Length: 211
TLSv1.2 Record Layer: Handshake Protocol: Server Hello Done
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 4
Handshake Protocol: Server Hello Done
Handshake Type: Server Hello Done (14)
Length: 0
Client --> Server:
TLSv1.2 Record Layer: Handshake Protocol: Certificate
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 570
Handshake Protocol: Certificate
Handshake Type: Certificate (11)
Length: 566
Certificates Length: 563
Certificates (563 bytes)
Certificate Length: 560
Certificate (id-at-commonName=PolarSSL Test Client 2,
id-at-organizationName=PolarSSL,id-at-countryName=NL)
signedCertificate
algorithmIdentifier (iso.2.840.10045.4.3.2)
Padding: 0
encrypted: 306502304a650d7b2083a299b9a80ffc8dee8...
TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 138
Handshake Protocol: Client Key Exchange
Handshake Type: Client Key Exchange (16)
Length: 134
TLSv1.2 Record Layer: Handshake Protocol: Certificate Verify
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 80
Handshake Protocol: Certificate Verify
Handshake Type: Certificate Verify (15)
Length: 76
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol:
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Client <-- Server:
TLSv1.2 Record Layer: Change Cipher Spec Protocol
Content Type: Change Cipher Spec (20)
Version: TLS 1.2 (0x0303)
Length: 1
Change Cipher Spec Message
TLSv1.2 Record Layer: Handshake Protocol
Encrypted Handshake Message (TLS Finished)
Content Type: Handshake (22)
Version: TLS 1.2 (0x0303)
Length: 40
Handshake Protocol: Encrypted Handshake Message
Figure 3: Example TLS Exchange (with Cached Info Extension).
Note: To accomplish further on-the-wire handshake size message
reductions the Certificate message sent by the client can be reduced
in size by using the Client Certificate URL extension.
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
Hannes Tschofenig Hannes Tschofenig
ARM Ltd. ARM Ltd.
Hall in Tirol 6060 Hall in Tirol 6060
Austria Austria
Email: Hannes.tschofenig@gmx.net Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at URI: http://www.tschofenig.priv.at
 End of changes. 57 change blocks. 
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