| TLS | S. Santesson |
| Internet-Draft | 3xA Security AB |
| Intended status: Standards Track | H. Tschofenig |
| Expires: September 9, 2015 | ARM Ltd. |
| March 8, 2015 |
Transport Layer Security (TLS) Cached Information Extension
draft-ietf-tls-cached-info-18.txt
Transport Layer Security (TLS) handshakes often include fairly static information, such as the server certificate and a list of trusted certification authorities (CAs). This information can be of considerable size, particularly if the server certificate is bundled with a complete certificate chain (i.e., the certificates of intermediate CAs up to the root CA).
This document defines an extension that allows a TLS client to inform a server of cached information, allowing the server to omit already available information.
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This Internet-Draft will expire on September 9, 2015.
Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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Reducing the amount of information exchanged during a Transport Layer Security handshake to a minimum helps to improve performance in environments where devices are connected to a network with a low bandwidth, and lossy radio technology. With Internet of Things such environments exist, for example, when devices use IEEE 802.15.4 or Bluetooth Smart. For more information about the challenges with smart object deployments please see [RFC6574].
This specification defines a TLS extension that allows a client and a server to exclude transmission information cached in an earlier TLS handshake.
A typical example exchange may therefore look as follows. First, the client and the server executes the usual TLS handshake. The client may, for example, decide to cache the certificate provided by the server. When the TLS client connects to the TLS server some time in the future, without using session resumption, it then attaches the cached_info extension defined in this document to the client hello message to indicate that it had cached the certificate, and it provides the fingerprint of it. If the server's certificate has not changed then the TLS server does not need to send its' certificate and the corresponding certificate list again. In case information has changed, which can be seen from the fingerprint provided by the client, the certificate payload is transmitted to the client to allow the client to update the cache.
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document refers to the TLS protocol but the description is equally applicable to DTLS as well.
This document defines a new extension type (cached_info(TBD)), which is used in client hello and server hello messages. The extension type is specified as follows.
enum {
cached_info(TBD), (65535)
} ExtensionType;
The extension_data field of this extension, when included in the client hello, MUST contain the CachedInformation structure. The client MUST NOT send multiple CachedObjects of the same CachedInformationType.
enum {
cert(1), cert_req(2) (255)
} CachedInformationType;
struct {
select (type) {
case client:
CachedInformationType type;
opaque hash_value<1..255>;
case server:
CachedInformationType type;
} body;
} CachedObject;
struct {
CachedObject cached_info<1..2^16-1>;
} CachedInformation;
This document defines the following types:
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.
Clients supporting this extension MAY include the "cached_info" extension in the (extended) client hello. If the client includes the extension then it MUST contain one or more CachedObject attributes.
A server supporting this extension MAY include the "cached_info" extension in the (extended) server hello. By returning the "cached_info" extension the server indicates that it supports the cached info types. For each indicated cached info type the server MUST alter the transmission of respective payloads, according to the rules outlined with each type. If the server includes the extension it MUST only include CachedObjects of a type also supported by the client (as expressed in the 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'.
Since the client includes a fingerprint of information it cached (for each indicated type) the server is able to determine whether cached information is stale. If the server supports this specification and notices a mismatch between the data cached by the client and its own information then the server MUST include the information in full and MUST NOT list the respective type in the "cached_info" extension.
Note: If a server is part of a hosting environment then the client may have cached multiple data items for a single server. To allow the client to select the appropriate information from the cache it is RECOMMENDED that the client utilizes the Server Name Indication extension [RFC6066].
Following a successful exchange of the "cached_info" extension in the client and server hello, the server alters sending the corresponding handshake message. How information is altered from the handshake messages is defined in Section 4.1, Section 4.2 and Section 4.3 for the types defined in this specification.
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:
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
Certificate Message as defined in RFC 5246.
The original Certificate handshake message syntax is defined in RFC 5246 [RFC5246] and has the following structure:
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.
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 original CertificateRequest handshake message syntax is defined in RFC 5246 [RFC5246] and has the following structure:
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest;
The fingerprint MUST be computed as follows: hash_value:=SHA-256(CertificateRequest)
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 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)
Figure 1 illustrates an example exchange using the TLS cached info extension. In the normal TLS handshake exchange shown in flow (A) the TLS server provides its certificate in the Certificate payload to the client, see step [1]. This allows the client to store the certificate for future use. After some time the TLS client again interacts with the same TLS server and makes use of the TLS cached info extension, as shown in flow (B). The TLS client indicates support for this specification via the "cached_info" extension, see [2], and indicates that it has stored the certificate from the earlier exchange (by indicating the 'cert' type). With [3] the TLS server acknowledges the supports of the 'cert' type and by including the value in the server hello informs the client that the certificate payload has been omitted.
(A) Initial (full) Exchange
ClientHello ->
<- ServerHello
Certificate* // [1]
ServerKeyExchange*
CertificateRequest*
ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ->
<- [ChangeCipherSpec]
Finished
Application Data <-------> Application Data
(B) TLS Cached Extension Usage
ClientHello
cached_info=(cert) -> // [2]
<- ServerHello
cached_info=(cert) [3]
ServerKeyExchange*
ServerHelloDone
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished ->
<- [ChangeCipherSpec]
Finished
Application Data <-------> Application Data
Figure 1: Example Message Exchange
This specification defines a mechanism to reference stored state using a fingerprint. Sending a fingerprint of cached information in an unencrypted handshake, as the client and server hello is, may allow an attacker or observer to correlate independent TLS exchanges. While some information elements used in this specification, such as server certificates, are public objects and usually do not contain sensitive information, other (not yet defined cached info types) may. Those who implement and deploy this specification should therefore make an informed decision whether the cached information is inline with their security and privacy goals. In case of concerns, it is advised to avoid sending the fingerprint of the data objects in clear.
The use of the cached info extension allows the server to obmit 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.
IANA is requested to add an entry to the existing TLS ExtensionType registry, defined in RFC 5246 [RFC5246], for cached_info(TBD) defined in this document.
IANA is requested to establish a registry for TLS CachedInformationType values. The first entries in the registry are
The policy for adding new values to this registry, following the terminology defined in RFC 5226 [RFC5226], is as follows:
We would like to thank the following persons for your detailed document reviews:
Additionally, 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.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC3874] | Housley, R., "A 224-bit One-way Hash Function: SHA-224", 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 (TLS) Protocol Version 1.2", RFC 5246, August 2008. |
| [RFC6066] | Eastlake, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011. |
| [RFC6961] | Pettersen, Y., "The Transport Layer Security (TLS) Multiple Certificate Status Request Extension", RFC 6961, June 2013. |
| [RFC5226] | Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. |
| [RFC6574] | Tschofenig, H. and J. Arkko, "Report from the Smart Object Workshop", RFC 6574, April 2012. |
| [RFC7250] | Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S. and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, June 2014. |
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.