draft-ietf-tls-cached-info-23.txt   rfc7924.txt 
TLS S. Santesson Internet Engineering Task Force (IETF) S. Santesson
Internet-Draft 3xA Security AB Request for Comments: 7924 3xA Security AB
Intended status: Standards Track H. Tschofenig Category: Standards Track H. Tschofenig
Expires: November 12, 2016 ARM Ltd. ISSN: 2070-1721 ARM Ltd.
May 11, 2016 July 2016
Transport Layer Security (TLS) Cached Information Extension Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-23.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, thereby enabling the server to omit
available information. already available information.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on November 12, 2016. 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 Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cached Information Extension . . . . . . . . . . . . . . . . 3 3. Cached Information Extension . . . . . . . . . . . . . . . . 3
4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5 4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5
4.1. Server Certificate Message . . . . . . . . . . . . . . . 5 4.1. Server Certificate Message . . . . . . . . . . . . . . . 6
4.2. CertificateRequest Message . . . . . . . . . . . . . . . 6 4.2. CertificateRequest Message . . . . . . . . . . . . . . . 7
5. Fingerprint Calculation . . . . . . . . . . . . . . . . . . . 7 5. Fingerprint Calculation . . . . . . . . . . . . . . . . . . . 7
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10 8.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10
8.2. New Registry for CachedInformationType . . . . . . . . . 10 8.2. New Registry for CachedInformationType . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 9.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11 9.2. Informative References . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 11 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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 the Internet of Things,
environments exist, for example, when devices use IEEE 802.15.4 or such environments exist, for example, when devices use IEEE 802.15.4,
Bluetooth Smart. For more information about the challenges with Bluetooth Low Energy, or low power wide area networks. For more
smart object deployments please see [RFC6574]. information about the challenges with smart object deployments,
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 full TLS handshake. The client client and the server execute the full TLS handshake. The client
then caches the certificate provided by the server. When the TLS then caches the certificate provided by the server. When the TLS
client connects to the TLS server some time in the future, without client connects to the TLS server some time in the future, without
using session resumption, it then attaches the cached_info extension using session resumption, it then attaches the "cached_info"
defined in this document to the client hello message to indicate that extension defined in this document to the ClientHello message to
it had cached the certificate, and it provides the fingerprint of it. indicate that it has cached the certificate, and it provides the
If the server's certificate has not changed then the TLS server does fingerprint of it. If the server's certificate has not changed, then
not need to send its certificate and the corresponding certificate the TLS server does not need to send its certificate and the
chain again. In case information has changed, which can be seen from corresponding certificate chain again. In case information has
the fingerprint provided by the client, the certificate payload is changed, which can be seen from the fingerprint provided by the
transmitted to the client to allow the client to update the cache. client, the certificate payload is transmitted to the client to allow
the client to update the cache.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This document refers to the TLS protocol but the description is This document refers to the TLS protocol, but the description is
equally applicable to DTLS as well. equally applicable to Datagram Transport Layer Security (DTLS) as
well.
3. Cached Information Extension 3. Cached Information Extension
This document defines a new extension type (cached_info(TBD)), which This document defines a new extension type (cached_info(25)), which
is used in client hello and server hello messages. The extension is used in ClientHello and ServerHello messages. The extension type
type is specified as follows. is specified as follows.
enum { enum {
cached_info(TBD), (65535) cached_info(25), (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 ClientHello, MUST contain the CachedInformation structure. The
client MAY send multiple CachedObjects of the same client MAY send multiple CachedObjects of the same
CachedInformationType. This may, for example, be the case when the CachedInformationType. This may, for example, be the case when the
client has cached multiple certificates from a server. client has cached multiple certificates from a server.
enum { enum {
cert(1), cert_req(2) (255) cert(1), cert_req(2) (255)
} CachedInformationType; } CachedInformationType;
struct { struct {
select (type) { select (type) {
skipping to change at page 4, line 25 skipping to change at page 4, line 31
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 two types: This document defines the following two types:
'cert' Type for not sending the complete 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
fingerprint of the Certificate message in the hash_value field. fingerprint of the Certificate message in the hash_value field.
For this type the fingerprint MUST be calculated using the For this type, the fingerprint MUST be calculated using the
procedure described in Section 5 with the Certificate message as procedure described in Section 5 with the Certificate message as
input data. input data.
'cert_req' Type for not sending the complete CertificateRequest 'cert_req' Type for not sending the complete CertificateRequest
Message: 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
fingerprint of the CertificateRequest message in the hash_value fingerprint of the CertificateRequest message in the hash_value
field. For this type the fingerprint MUST be calculated using the field. For this type, the fingerprint MUST be calculated using
procedure described in Section 5 with the CertificateRequest the procedure described in Section 5 with the CertificateRequest
message as input data. message as input data.
New cached info types can be added following the policy described in New cached info types can be added following the policy described in
the IANA considerations section, see Section 8. New message digest the IANA Considerations (Section 8). New message digest algorithms
algorithms for use with these types can also be added by registering for use with these types can also be added by registering a new type
a new type that makes use of the updated message digest algorithm. that makes use of the updated message digest algorithm. For
For practical reasons we recommend to re-use hash algorithms already practical reasons, we recommend reusing hash algorithms already
available with TLS ciphersuites to avoid additional code and to keep available with TLS ciphersuites. To avoid additional code and to
the collision probably low new hash algorithms MUST NOT have a keep the collision probability low, new hash algorithms MUST NOT have
collision resistance worse than SHA-256. a collision resistance worse than SHA-256.
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) ClientHello. 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) ServerHello. 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 ClientHello). 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 ('foo- respond with a "cached_info" attribute containing support for
bar'). ('foo-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
RECOMMENDED that the client utilizes the Server Name Indication is RECOMMENDED that the client utilizes the Server Name Indication
extension [RFC6066]. (SNI) 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 ClientHello and ServerHello, the server alters sending the
handshake message. How information is altered from the handshake corresponding handshake message. How information is altered from the
messages is defined in Section 4.1, and in Section 4.2 for the types handshake messages and for the types defined in this specification is
defined in this specification. defined in Sections 4.1 and 4.2, respectively.
Appendix A shows an example hash calculation and Section 6 shows an Appendix A shows an example hash calculation, and Section 6
example protocol exchange. illustrates 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
shown in Figure 1 under the following conditions: message shown in Figure 1 under the following conditions:
o 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.
o The 'cert' cached info extension is enabled (for example, a policy o The 'cert' "cached_info" extension is enabled (for example, a
allows the use of this extension). policy allows the use of this extension).
o 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. The ensures that the information cached by the client is current. The
procedure for calculating the fingerprint is described in procedure for calculating the fingerprint is described in
Section 5. Section 5.
The original Certificate handshake message syntax is defined in The original certificate handshake message syntax is defined in
[RFC5246] and has been extended with [RFC7250]. RFC 7250 allows the [RFC5246] and has been extended with [RFC7250]. RFC 7250 allows the
certificate payload to contain only the SubjectPublicKeyInfo instead certificate payload to contain only the SubjectPublicKeyInfo instead
of the full information typically found in a certificate. Hence, of the full information typically found in a certificate. Hence,
when this specification is used in combination with [RFC7250] and the when this specification is used in combination with [RFC7250] and the
negotiated certificate type is a raw public key then the TLS server negotiated certificate type is a raw public key, then the TLS server
omits sending a Certificate payload that contains an ASN.1 omits sending a certificate payload that contains an ASN.1
Certificate structure with the included SubjectPublicKeyInfo rather certificate structure with the included SubjectPublicKeyInfo rather
than the full certificate chain. As such, this extension is than the full certificate chain. As such, this extension is
compatible with the raw public key extension defined in RFC 7250. compatible with the raw public key extension defined in RFC 7250.
Note: We assume that the server implementation is able to select the Note: We assume that the server implementation is able to select the
appropriate certificate or SubjectPublicKeyInfo from the received appropriate certificate or SubjectPublicKeyInfo from the received
hash value. If the SNI extension is used by the client then the hash value. If the SNI extension is used by the client, then the
server has additional information to guide the selection of the server has additional information to guide the selection of the
appropriate cached info. appropriate cached info.
When the cached info specification is used then a modified version of When the cached info specification is used, then a modified version
the Certificate message is exchanged. The modified structure is of the Certificate message is exchanged. The modified structure is
shown in Figure 1. shown in Figure 1.
struct { struct {
opaque hash_value<1..255>; opaque hash_value<1..255>;
} Certificate; } Certificate;
Figure 1: Cached Info Certificate Message. Figure 1: Cached Info Certificate Message
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 send the CertificateRequest message the ClientHello, the server MAY send the CertificateRequest message
shown in Figure 2 message under the following conditions: shown in Figure 2 under the following conditions:
o 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.
o 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).
o 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. The procedure for calculating the fingerprint is current. The procedure for calculating the fingerprint is
described in Section 5. described in Section 5.
o 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 [RFC5246]. The modified structure of the CertificateRequest in [RFC5246]. The modified structure of the CertificateRequest
message is shown in Figure 2. message is shown in Figure 2.
struct { struct {
opaque hash_value<1..255>; opaque hash_value<1..255>;
} CertificateRequest; } CertificateRequest;
Figure 2: Cached Info CertificateRequest Message. Figure 2: Cached Info CertificateRequest Message
The CertificateRequest payload is the input parameter to the The CertificateRequest payload is the input parameter to the
fingerprint calculation described in Section 5. fingerprint calculation described in Section 5.
5. Fingerprint Calculation 5. Fingerprint Calculation
The fingerprint for the two cached info objects defined in this The fingerprint for the two cached info objects defined in this
document MUST be computed as follows: document MUST be computed as follows:
1. Compute the SHA-256 [RFC6234] hash of the input data. The input 1. Compute the SHA-256 [RFC6234] hash of the input data. The input
data depends on the cached info type. This document defines two data depends on the cached info type. This document defines two
cached info types, described in Section 4.1 and in Section 4.2. cached info types, described in Sections 4.1 and in 4.2. Note
Note that the computed hash only covers the input data structure that the computed hash only covers the input data structure (and
(and not any type and length information of the record layer). not any type and length information of the record layer).
Appendix A shows an example. Appendix A shows an example.
2. Use the output of the SHA-256 hash. 2. Use the output of the SHA-256 hash.
The purpose of the fingerprint provided by the client is to help the The purpose of the fingerprint provided by the client is to help the
server select the correct information. For example, in case of the server select the correct information. For example, in case of a
certificate message the fingerprint identifies the server certificate Certificate message, the fingerprint identifies the server
(and the corresponding private key) for use for with the rest of the certificate (and the corresponding private key) for use with the rest
handshake. Servers may have more than one certificate and therefore of the handshake. Servers may have more than one certificate, and
a hash needs to be long enough to keep the probably of hash 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 collisions low. On the other hand, the cached info design aims to
reduce the amount of data being exchanged. The security of the reduce the amount of data being exchanged. The security of the
handshake depends on the private key and not on the size 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 fingerprint. Hence, the fingerprint is a way to prevent the server
from accidentally selecting the wrong information. If an attacker from accidentally selecting the wrong information. If an attacker
injects an incorrect fingerprint then two outcomes are possible: (1) injects an incorrect fingerprint, then two outcomes are possible: (1)
The fingerprint does not relate to any cached state and the server 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 has to fall back to a full exchange, and (2) if the attacker manages
inject a fingerprint that refers to data the client has not cached to inject a fingerprint that refers to data the client has not
then the exchange will fail later when the client continues with the cached, then the exchange will fail later when the client continues
handshake and aims to verify the digital signature. The signature with the handshake and aims to verify the digital signature. The
verification will fail since the public key cached by the client will signature verification will fail since the public key cached by the
not correspond to the private key that was used by server to sign the client will not correspond to the private key that was used by the
message. server to sign the message.
6. Example 6. Example
In the regular, full TLS handshake exchange, shown in Figure 3, the In the regular, full TLS handshake exchange, shown in Figure 3, 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 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
info extension, as shown in Figure 4. The TLS client indicates "cached_info" extension, as shown in Figure 4. The TLS client
support for this specification via the "cached_info" extension, see indicates support for this specification via the "cached_info"
step (2), and indicates that it has stored the certificate from the extension, see step (2), and indicates that it has stored the
earlier exchange (by indicating the 'cert' type). With step (3) the certificate from the earlier exchange (by indicating the 'cert'
TLS server acknowledges the supports of the 'cert' type and by type). With step (3), the TLS server acknowledges the support of the
including the value in the server hello informs the client that the 'cert' type and by including the value in the ServerHello, it informs
content of the certificate payload contains the fingerprint of the the client that the content of the certificate payload contains the
certificate instead of the RFC 5246-defined payload of the fingerprint of the certificate instead of the payload, defined in RFC
certificate message in step (4). 5246, of the Certificate message; see step (4).
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
Figure 3: Example Message Exchange: Initial (full) Exchange. Figure 3: Example Message Exchange: Initial (Full) Exchange
ClientHello ClientHello
cached_info=(cert) -> // (2) cached_info=(cert) -> // (2)
<- ServerHello <- ServerHello
cached_info=(cert) (3) cached_info=(cert) (3)
Certificate (4) Certificate (4)
ServerKeyExchange* ServerKeyExchange*
ServerHelloDone ServerHelloDone
ClientKeyExchange ClientKeyExchange
CertificateVerify* CertificateVerify*
[ChangeCipherSpec] [ChangeCipherSpec]
Finished -> Finished ->
<- [ChangeCipherSpec] <- [ChangeCipherSpec]
Finished Finished
Application Data <-------> Application Data Application Data <-------> Application Data
Figure 4: Example Message Exchange: TLS Cached Extension Usage. Figure 4: Example Message Exchange: TLS Cached Extension Usage
7. 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 ClientHello and ServerHello does,
allow an attacker or observer to correlate independent TLS exchanges. may allow an attacker or observer to correlate independent TLS
While some information elements used in this specification, such as exchanges. While some information elements used in this
server certificates, are public objects and usually do not contain specification, such as server certificates, are public objects and
sensitive information, other not yet defined types may. Those who usually do not contain sensitive information, other types that are
implement and deploy this specification should therefore make an not yet defined may. Those who implement and deploy this
informed decision whether the cached information is inline with their specification should therefore make an informed decision whether the
security and privacy goals. In case of concerns, it is advised to cached information is in line with their security and privacy goals.
avoid sending the fingerprint of the data objects in clear. 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 The use of the "cached_info" extension allows the server to send
significantly smaller TLS messages. Consequently, these omitted significantly smaller TLS messages. Consequently, these omitted
parts of the messages are not included in the transcript of the parts of the messages are not included in the transcript of the
handshake in the TLS Finish message. However, since the client and handshake in the TLS Finish message. However, since the client and
the server communicate the hash values of the cached data in the the server communicate the hash values of the cached data in the
initial handshake messages the fingerprints are included in the TLS initial handshake messages, the fingerprints are included in the TLS
Finish message. 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.
Security considerations for the fingerprint calculation are discussed Security considerations for the fingerprint calculation are discussed
in Section 5. in Section 5.
8. IANA Considerations 8. IANA Considerations
8.1. New Entry to the TLS ExtensionType Registry 8.1. New Entry to the TLS ExtensionType Registry
IANA is requested to add an entry to the existing TLS ExtensionType IANA has added an entry to the existing TLS "ExtensionType Values"
registry, defined in [RFC5246], for cached_info(TBD) defined in this registry, defined in [RFC5246], for cached_info(25) defined in this
document. document.
8.2. New Registry for CachedInformationType 8.2. New Registry for CachedInformationType
IANA is requested to establish a registry for TLS IANA has established a registry titled "TLS CachedInformationType
CachedInformationType values. The first entries in the registry are Values". The entries in the registry are:
o cert(1)
o cert_req(2) 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 The policy for adding new values to this registry, following the
terminology defined in [RFC5226], is as follows: terminology defined in [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 9. References
9. 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 Matt Miller (December 2015)
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.
10. References
10.1. 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, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[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, DOI 10.17487/ (TLS) Protocol Version 1.2", RFC 5246,
RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <http://www.rfc-editor.org/info/rfc5246>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, DOI Extensions: Extension Definitions", RFC 6066,
10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>. <http://www.rfc-editor.org/info/rfc6066>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI (SHA and SHA-based HMAC and HKDF)", RFC 6234,
10.17487/RFC6234, May 2011, DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>. <http://www.rfc-editor.org/info/rfc6234>.
10.2. Informative References 9.2. Informative References
[ASN.1-Dump] [ASN.1-Dump]
Gutmann, P., "ASN.1 Object Dump Program", February 2013, Gutmann, P., "ASN.1 Object Dump Program", November 2010,
<http://www.cs.auckland.ac.nz/~pgut001/>. <http://manpages.ubuntu.com/manpages/precise/man1/
dumpasn1.1.html>.
[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,
DOI 10.17487/RFC5226, May 2008, DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>. <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, DOI 10.17487/RFC6574, April 2012, Workshop", RFC 6574, DOI 10.17487/RFC6574, April 2012,
<http://www.rfc-editor.org/info/rfc6574>. <http://www.rfc-editor.org/info/rfc6574>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>. June 2014, <http://www.rfc-editor.org/info/rfc7250>.
Appendix A. Example Appendix A. Example
Consider a certificate containing an NIST P256 elliptic curve public Consider a certificate containing a NIST P256 elliptic curve public
key displayed using Peter Gutmann's ASN.1 decoder [ASN.1-Dump] in key displayed using Peter Gutmann's ASN.1 decoder [ASN.1-Dump] in
Figure 5. Figure 5.
0 556: SEQUENCE { 0 556: SEQUENCE {
4 434: SEQUENCE { 4 434: SEQUENCE {
8 3: [0] { 8 3: [0] {
10 1: INTEGER 2 10 1: INTEGER 2
: } : }
13 1: INTEGER 13 13 1: INTEGER 13
16 10: SEQUENCE { 16 10: SEQUENCE {
skipping to change at page 14, line 50 skipping to change at page 16, line 4
: } : }
442 10: SEQUENCE { 442 10: SEQUENCE {
444 8: OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2) 444 8: OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2)
: } : }
454 104: BIT STRING, encapsulates { 454 104: BIT STRING, encapsulates {
457 101: SEQUENCE { 457 101: SEQUENCE {
459 48: INTEGER 459 48: INTEGER
: 4A 65 0D 7B 20 83 A2 99 B9 A8 0F FC 8D EE 8F 3D : 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 : 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 : 65 8E 1A C9 3F 2C 61 7E F8 3C EF AD 1C EE 36 20
509 49: INTEGER 509 49: INTEGER
: 00 9D F2 27 A6 D5 74 B8 24 AE E1 6A 3F 31 A1 CA : 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 : 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 : 42 49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57
: 45 : 45
: } : }
: } : }
: } : }
Figure 5: ASN.1-based Certificate: Example. Figure 5: ASN.1-Based Certificate: Example
To include the certificate shown in Figure 5 in a TLS/DTLS To include the certificate shown in Figure 5 in a TLS/DTLS
Certificate message it is prepended with a message header. This 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 Certificate message header in our example is 0b 00 02 36 00 02 33 00
02 00 02 30, which indicates: 02 00 02 30, which indicates:
Message Type: 0b -- 1 byte type field indicating a Certificate Message Type: 0b -- 1-byte type field indicating a Certificate
message message
Length: 00 02 36 -- 3 byte length field indicating a 566 bytes Length: 00 02 36 -- 3-byte length field indicating a 566-byte
payload payload
Certificates Length: 00 02 33 -- 3 byte length field indicating 563 Certificates Length: 00 02 33 -- 3-byte length field indicating 563
bytes for the entire certificates_list structure, which may bytes for the entire certificates_list structure, which may
contain multiple certificates. In our example only one contain multiple certificates. In our example, only one
certificate is included. certificate is included.
Certificate Length: 00 02 30 -- 3 byte length field indicating 560 Certificate Length: 00 02 30 -- 3-byte length field indicating 560
bytes of the actual certificate following immediately afterwards. bytes of the actual certificate following immediately afterwards.
In our example, this is the certificate content with 30 82 02 .... In our example, this is the certificate content with 30 82 02 ....
9E 57 45 shown in Figure 6. 9E 57 45 shown in Figure 6.
The hex encoding of the ASN.1 encoded certificate payload shown in The hex encoding of the ASN.1-encoded certificate payload shown in
Figure 5 leads to the following encoding. Figure 5 leads to the following encoding.
30 82 02 2C 30 82 01 B2 A0 03 02 01 02 02 01 0D 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 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 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 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 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 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 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 33 30 39 32 32 31 35 35 32 30 34 5A 30 41 31 0B
skipping to change at page 16, line 41 skipping to change at page 17, line 44
72 73 73 6C 20 54 65 73 74 20 45 43 20 43 41 82 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 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 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 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 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 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 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 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 49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57 45
Figure 6: Hex Encoding of the Example Certificate. Figure 6: Hex Encoding of the Example Certificate
Applying the SHA-256 hash function to the Certificate message, which Applying the SHA-256 hash function to the Certificate message, which
is starts with 0b 00 02 and ends with 9E 57 45, produces starts with 0b 00 02 and ends with 9E 57 45, produces
0x086eefb4859adfe977defac494fff6b73033b4ce1f86b8f2a9fc0c6bf98605af. 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 Authors' Addresses
Stefan Santesson Stefan Santesson
3xA Security AB 3xA Security AB
Scheelev. 17 Forskningsbyn Ideon
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
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