--- 1/draft-ietf-tls-cached-info-20.txt 2015-12-23 02:15:15.598634477 -0800 +++ 2/draft-ietf-tls-cached-info-21.txt 2015-12-23 02:15:15.634635343 -0800 @@ -1,19 +1,19 @@ TLS S. Santesson Internet-Draft 3xA Security AB Intended status: Standards Track H. Tschofenig -Expires: April 21, 2016 ARM Ltd. - October 19, 2015 +Expires: June 23, 2016 ARM Ltd. + December 21, 2015 Transport Layer Security (TLS) Cached Information Extension - draft-ietf-tls-cached-info-20.txt + draft-ietf-tls-cached-info-21.txt Abstract Transport Layer Security (TLS) handshakes often include fairly static information, such as the server certificate and a list of trusted certification authorities (CAs). This information can be of considerable size, particularly if the server certificate is bundled with a complete certificate chain (i.e., the certificates of intermediate CAs up to the root CA). @@ -29,21 +29,21 @@ 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on April 21, 2016. + This Internet-Draft will expire on June 23, 2016. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -65,21 +65,21 @@ 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8.1. New Entry to the TLS ExtensionType Registry . . . . . . . 10 8.2. New Registry for CachedInformationType . . . . . . . . . 10 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 12 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 1. Introduction Reducing the amount of information exchanged during a Transport Layer Security handshake to a minimum helps to improve performance in environments where devices are connected to a network with a low bandwidth, and lossy radio technology. With 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]. @@ -89,21 +89,21 @@ handshake. A typical example exchange may therefore look as follows. First, the client and the server executes the full TLS handshake. The client then caches the certificate provided by the server. When the TLS client connects to the TLS server some time in the future, without using session resumption, it then attaches the cached_info extension defined in this document to the 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 + not need to send its certificate and the corresponding certificate chain again. In case information has changed, which can be seen from the fingerprint provided by the client, the certificate payload is transmitted to the client to allow the client to update the cache. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. @@ -134,41 +134,41 @@ 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>; + CachedObject cached_info<1..2^8-1>; } CachedInformation; This document defines the following two types: 'cert' Type for not sending the complete Server Certificate Message: With the type field set to 'cert', the client MUST include the fingerprint of the Certificate message in the hash_value field. For this type the fingerprint MUST be calculated using the procedure described in Section 5 with the Certificate message as input data. 'cert_req' Type for not sending the complete CertificateRequest Message: With the type set to 'cert_req', the client MUST include the fingerprint of the CertificateRequest message in the hash_value field. For this type the fingerprint MUST be calculated using the procedure described in Section 5 with the CertificateRequest - message as input data.. + message as input data. New cached info types can be added following the policy described in the IANA considerations section, see Section 8. New message digest algorithms for use with these types can also be added by registering a new type that makes use of the updated message digest algorithm. There are no specific requirements for the use of specific hash algorithms but for practical reason it is useful to re-use algorithms already available with TLS ciphersuites to avoid additional code and to keep the collision probably low. @@ -181,21 +181,21 @@ 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 ('foo- - bar'. + bar'). Since the client includes a fingerprint of information it cached (for each indicated type) the server is able to determine whether cached information is stale. If the server supports this specification and notices a mismatch between the data cached by the client and its own information then the server MUST include the information in full and MUST NOT list the respective type in the "cached_info" extension. Note: If a server is part of a hosting environment then the client may have cached multiple data items for a single server. To allow @@ -224,31 +224,35 @@ o The 'cert' cached info extension is enabled (for example, a policy allows the use of this extension). o The server compared the value in the hash_value field of the client-provided "cached_info" extension with the fingerprint of the Certificate message it normally sends to clients. This check ensures that the information cached by the client is current. The procedure for calculating the fingerprint is described in Section 5. - The original Certificate handshake message syntax is defined in RFC - 5246 [RFC5246] and has been extended with RFC 7250 [RFC7250]. RFC - 7250 allows the certificate payload to contain only the - SubjectPublicKeyInfo instead of the full information typically found - in a certificate. Hence, when this specification is used in - combination with [RFC7250] and the negotiated certificate type is a - raw public key then the TLS server omits sending a Certificate - payload that contains an ASN.1 Certificate structure with the - included SubjectPublicKeyInfo rather than the full certificate chain. - As such, this extension is compatible with the raw public key - extension defined in RFC 7250. + The original Certificate handshake message syntax is defined in + [RFC5246] and has been extended with [RFC7250]. RFC 7250 allows the + certificate payload to contain only the SubjectPublicKeyInfo instead + of the full information typically found in a certificate. Hence, + when this specification is used in combination with [RFC7250] and the + negotiated certificate type is a raw public key then the TLS server + omits sending a Certificate payload that contains an ASN.1 + Certificate structure with the included SubjectPublicKeyInfo rather + than the full certificate chain. As such, this extension is + compatible with the raw public key extension defined in RFC 7250. + Note: We assume that the server implementation is able to select the + appropriate certificate or SubjectPublicKeyInfo from the received + hash value. If the SNI extension is used by the client then the + server has additional information to guide the selection of the + appropriate cached info. When the cached info specification is used then a modified version of the Certificate message is exchanged. The modified structure is shown in Figure 1. struct { opaque hash_value[1..255]; } Certificate; Figure 1: Cached Info Certificate Message. @@ -268,41 +272,42 @@ o The server compared the value in the hash_value field of the client-provided "cached_info" extension with the fingerprint of the CertificateRequest message it normally sends to clients. This check ensures that the information cached by the client is current. The procedure for calculating the fingerprint is described in Section 5. o The server wants to request a certificate from the client. The original CertificateRequest handshake message syntax is defined - in RFC 5246 [RFC5246]. The modified structure of the - CertificateRequest message is shown in Figure 2. + in [RFC5246]. The modified structure of the CertificateRequest + message is shown in Figure 2. struct { opaque hash_value<1..255>; } CertificateRequest; Figure 2: Cached Info CertificateRequest Message. The CertificateRequest payload is the input parameter to the fingerprint calculation described in Section 5. 5. Fingerprint Calculation The fingerprint MUST be computed as follows: - 1. Compute the SHA-256 [RFC4634] 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 cached info types, described in Section 4.1 and in Section 4.2. Note that the computed hash only covers the input data structure (and not any type and length information of the record layer). + Appendix A shows an example. 2. Truncate the output of the SHA-256 hash. When a hash value is truncated to 32 bits, the leftmost 32 bits (that is, the most significant 32 bits in network byte order) from the binary representation of the hash value MUST be used as the truncated value. An example of a 256-bit hash output truncated to 32 bits is shown in Figure 3. 256-bit hash: 0x265357902fe1b7e2a04b897c6025d7a2265357902fe1b7e2a04b897c6025d7a2 @@ -328,77 +333,74 @@ has to fall back to a full exchange. (2) If the attacker manages to inject a fingerprint that refers to data the client has not cached then the exchange will fail later when the client continues with the handshake and aims to verify the digital signature. The signature verification will fail since the public key cached by the client will not correspond to the private key that was used by server to sign the message. 6. Example - Figure 4 illustrates an example exchange using the TLS cached info - extension. In the normal TLS handshake exchange shown in flow (A) - the TLS server provides its certificate in the Certificate payload to - the client, see step [1]. This allows the client to store the + In the regular, full TLS handshake exchange, shown in Figure 4, 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 + info extension, as shown in Figure 5. 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 content of - the certificate payload contains the fingerprint of the certificate - instead of the RFC 5246-defined payload of the certificate message in - message [4]. - - (A) Initial (full) Exchange + step (2), and indicates that it has stored the certificate from the + earlier exchange (by indicating the 'cert' type). With step (3) the + TLS server acknowledges the supports of the 'cert' type and by + including the value in the server hello informs the client that the + content of the certificate payload contains the fingerprint of the + certificate instead of the RFC 5246-defined payload of the + certificate message in step (4). ClientHello -> <- ServerHello - Certificate* // [1] + Certificate* // (1) ServerKeyExchange* CertificateRequest* ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished -> <- [ChangeCipherSpec] Finished Application Data <-------> Application Data - (B) TLS Cached Extension Usage + Figure 4: Example Message Exchange: Initial (full) Exchange. ClientHello - cached_info=(cert) -> // [2] + cached_info=(cert) -> // (2) <- ServerHello - cached_info=(cert) [3] - Certificate [4] + cached_info=(cert) (3) + Certificate (4) ServerKeyExchange* ServerHelloDone ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished -> <- [ChangeCipherSpec] Finished Application Data <-------> Application Data - Figure 4: Example Message Exchange + Figure 5: Example Message Exchange: TLS Cached Extension Usage. 7. Security Considerations This specification defines a mechanism to reference stored state using a fingerprint. Sending a fingerprint of cached information in an unencrypted handshake, as the 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 types may. Those who @@ -415,96 +417,101 @@ initial handshake messages the fingerprints are included in the TLS Finish message. Clients MUST ensure that they only cache information from legitimate sources. For example, when the client populates the cache from a TLS exchange then it must only cache information after the successful completion of a TLS exchange to ensure that an attacker does not inject incorrect information into the cache. Failure to do so allows for man-in-the-middle attacks. - Security consideratios for the fingerprint calculation are discussed + Security considerations for the fingerprint calculation are discussed in Section 5. 8. IANA Considerations 8.1. New Entry to the TLS ExtensionType Registry 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. + registry, defined in [RFC5246], for cached_info(TBD) defined in this + document. 8.2. New Registry for CachedInformationType IANA is requested to establish a registry for TLS CachedInformationType values. The first entries in the registry are o cert(1) o cert_req(2) The policy for adding new values to this registry, following the - terminology defined in RFC 5226 [RFC5226], is as follows: + terminology defined in [RFC5226], is as follows: 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. 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 (in March 2012) + o Ondrej Mikle (March 2012) - o Ilari Liusvaara, Adam Langley, and Eric Rescorla (in July 2014) + o Ilari Liusvaara, Adam Langley, and Eric Rescorla (July 2014) - o Sean Turner (in August 2014) + o Sean Turner (August 2014) - o Martin Thomson (in August 2015) + 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. + director, Stephen Farrell, for their support and their reviews. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . - [RFC4634] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms - (SHA and HMAC-SHA)", RFC 4634, DOI 10.17487/RFC4634, July - 2006, . - [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/ RFC5246, August 2008, . [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, . + [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms + (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI + 10.17487/RFC6234, May 2011, + . + 10.2. Informative References [ASN.1-Dump] Gutmann, P., "ASN.1 Object Dump Program", February 2013, . [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, . @@ -516,21 +523,21 @@ [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., Weiler, S., and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014, . Appendix A. Example Consider a certificate containing an NIST P256 elliptic curve public key displayed using Peter Gutmann's ASN.1 decoder [ASN.1-Dump] in - Figure 5. + Figure 6. 0 556: SEQUENCE { 4 434: SEQUENCE { 8 3: [0] { 10 1: INTEGER 2 : } 13 1: INTEGER 13 16 10: SEQUENCE { 18 8: OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2) : } @@ -656,23 +664,23 @@ : 65 8E 1A C9 3F 2C 61 7E F8 3C EF AD 1C EE 36 20 509 49: INTEGER : 00 9D F2 27 A6 D5 74 B8 24 AE E1 6A 3F 31 A1 CA : 54 2F 08 D0 8D EE 4F 0C 61 DF 77 78 7D B4 FD FC : 42 49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57 : 45 : } : } : } - Figure 5: ASN.1-based Certificate: Example. + Figure 6: ASN.1-based Certificate: Example. - To include the certificate shown in Figure 5 in a TLS/DTLS + To include the certificate shown in Figure 6 in a TLS/DTLS Certificate message it is prepended with a message header. This Certificate message header in our example is 0b 00 02 36 00 02 33 00 02 00 02 30, which indicates: Message Type: 0b -- 1 byte type field indicating a Certificate message Length: 00 02 36 -- 3 byte length field indicating a 566 bytes payload @@ -676,26 +684,25 @@ Length: 00 02 36 -- 3 byte length field indicating a 566 bytes payload Certificates Length: 00 02 33 -- 3 byte length field indicating 563 bytes for the entire certificates_list structure, which may contain multiple certificates. In our example only one certificate is included. Certificate Length: 00 02 30 -- 3 byte length field indicating 560 bytes of the actual certificate following immediately afterwards. - In our example, this is the certificate content with 30 82 02 .... - 9E 57 45 shown in Figure 6. + 9E 57 45 shown in Figure 7. The hex encoding of the ASN.1 encoded certificate payload shown in - Figure 5 leads to the following encoding. + Figure 6 leads to the following encoding. 30 82 02 2C 30 82 01 B2 A0 03 02 01 02 02 01 0D 30 0A 06 08 2A 86 48 CE 3D 04 03 02 30 3E 31 0B 30 09 06 03 55 04 06 13 02 4E 4C 31 11 30 0F 06 03 55 04 0A 13 08 50 6F 6C 61 72 53 53 4C 31 1C 30 1A 06 03 55 04 03 13 13 50 6F 6C 61 72 73 73 6C 20 54 65 73 74 20 45 43 20 43 41 30 1E 17 0D 31 33 30 39 32 34 31 35 35 32 30 34 5A 17 0D 32 33 30 39 32 32 31 35 35 32 30 34 5A 30 41 31 0B 30 09 06 03 55 04 06 13 02 4E 4C 31 11 30 0F 06 @@ -719,26 +726,25 @@ 72 73 73 6C 20 54 65 73 74 20 45 43 20 43 41 82 09 00 C1 43 E2 7E 62 43 CC E8 30 0A 06 08 2A 86 48 CE 3D 04 03 02 03 68 00 30 65 02 30 4A 65 0D 7B 20 83 A2 99 B9 A8 0F FC 8D EE 8F 3D BB 70 4C 96 03 AC 8E 78 70 DD F2 0E A0 B2 16 CB 65 8E 1A C9 3F 2C 61 7E F8 3C EF AD 1C EE 36 20 02 31 00 9D F2 27 A6 D5 74 B8 24 AE E1 6A 3F 31 A1 CA 54 2F 08 D0 8D EE 4F 0C 61 DF 77 78 7D B4 FD FC 42 49 EE E5 B2 6A C2 CD 26 77 62 8E 28 7C 9E 57 45 - Figure 6: Hex Encoding of the Example Certificate. + Figure 7: Hex Encoding of the Example Certificate. Applying the SHA-256 hash function to the Certificate message, which is starts with 0b 00 02 and ends with 9E 57 45, produces 0x086eefb4859adfe977defac494fff6b73033b4ce1f86b8f2a9fc0c6bf98605af. - Subsequently, this output is truncated to 32 bits, which leads to a fingerpint of 0x086eefb4. Authors' Addresses Stefan Santesson 3xA Security AB Scheelev. 17 Lund 223 70 Sweden