--- 1/draft-ietf-tls-cached-info-22.txt 2016-05-11 12:15:58.494968281 -0700 +++ 2/draft-ietf-tls-cached-info-23.txt 2016-05-11 12:15:58.530969186 -0700 @@ -1,19 +1,19 @@ TLS S. Santesson Internet-Draft 3xA Security AB Intended status: Standards Track H. Tschofenig -Expires: July 29, 2016 ARM Ltd. - January 26, 2016 +Expires: November 12, 2016 ARM Ltd. + May 11, 2016 Transport Layer Security (TLS) Cached Information Extension - draft-ietf-tls-cached-info-22.txt + draft-ietf-tls-cached-info-23.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 July 29, 2016. + This Internet-Draft will expire on November 12, 2016. Copyright Notice Copyright (c) 2016 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 @@ -60,26 +60,26 @@ 3. Cached Information Extension . . . . . . . . . . . . . . . . 3 4. Exchange Specification . . . . . . . . . . . . . . . . . . . 5 4.1. Server Certificate Message . . . . . . . . . . . . . . . 5 4.2. CertificateRequest Message . . . . . . . . . . . . . . . 6 5. Fingerprint Calculation . . . . . . . . . . . . . . . . . . . 7 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 + 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 - 10.2. Informative References . . . . . . . . . . . . . . . . . 12 + 10.2. Informative References . . . . . . . . . . . . . . . . . 11 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 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]. @@ -127,28 +127,28 @@ client has cached multiple certificates from a server. enum { cert(1), cert_req(2) (255) } CachedInformationType; struct { select (type) { case client: CachedInformationType type; - opaque hash_value[4]; + opaque hash_value<1..255>; case server: CachedInformationType type; - }; + } body; } CachedObject; struct { - CachedObject cached_info<1..255>; + CachedObject cached_info<1..2^16-1>; } 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 @@ -157,26 +157,27 @@ '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. New cached info types can be added following the policy described in - the IANA considerations section, see Section 8. New hash algorithms - can also be added by registering a new type. For practical reasons - we recommend to re-use hash algorithms already available with TLS - ciphersuites to avoid additional code and to keep the collision - probably low new hash algorithms MUST NOT have a collision resistance - worse than SHA-256 when truncated to 4 bytes. + 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. + For practical reasons we recommend to re-use hash algorithms already + available with TLS ciphersuites to avoid additional code and to keep + the collision probably low new hash algorithms MUST NOT have a + collision resistance worse than SHA-256. 4. Exchange Specification 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 @@ -244,21 +245,21 @@ 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[4]; + opaque hash_value<1..255>; } Certificate; Figure 1: Cached Info Certificate Message. 4.2. CertificateRequest Message When a fingerprint for an object of type 'cert_req' is provided in the client hello, the server MAY send the CertificateRequest message shown in Figure 2 message under the following conditions: @@ -275,53 +276,41 @@ 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 [RFC5246]. The modified structure of the CertificateRequest message is shown in Figure 2. struct { - opaque hash_value[4]; + 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: + The fingerprint for the two cached info objects defined in this + document MUST be computed as follows: 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 - - 32-bit truncated hash: - 0x26535790 - - Figure 3: Truncated Hash Example. + 2. Use the output of the SHA-256 hash. The purpose of the fingerprint provided by the client is to help the server select the correct information. For example, in case of the certificate message the fingerprint identifies the server certificate (and the corresponding private key) for use for with the rest of the handshake. Servers may have more than one certificate and therefore a hash needs to be long enough to keep the probably of hash collisions low. On the other hand, the cached info design aims to reduce the amount of data being exchanged. The security of the handshake depends on the private key and not on the size of the @@ -332,26 +321,26 @@ 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 - In the regular, full TLS handshake exchange, shown in Figure 4, the + In the regular, full TLS handshake exchange, shown in Figure 3, 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 Figure 5. The TLS client indicates + info extension, as shown in Figure 4. The TLS client indicates support for this specification via the "cached_info" extension, see 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 -> @@ -365,41 +354,41 @@ ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished -> <- [ChangeCipherSpec] Finished Application Data <-------> Application Data - Figure 4: Example Message Exchange: Initial (full) Exchange. + Figure 3: Example Message Exchange: Initial (full) Exchange. ClientHello cached_info=(cert) -> // (2) <- ServerHello cached_info=(cert) (3) Certificate (4) ServerKeyExchange* ServerHelloDone ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished -> <- [ChangeCipherSpec] Finished Application Data <-------> Application Data - Figure 5: Example Message Exchange: TLS Cached Extension Usage. + Figure 4: 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 @@ -462,25 +452,22 @@ 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) - o Dave Garrett (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 @@ -524,21 +511,21 @@ [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 6. + Figure 5. 0 556: SEQUENCE { 4 434: SEQUENCE { 8 3: [0] { 10 1: INTEGER 2 : } 13 1: INTEGER 13 16 10: SEQUENCE { 18 8: OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2) : } @@ -613,22 +599,22 @@ 308 20: OCTET STRING : 7A 00 5F 86 64 FC E0 5D E5 11 10 3B B2 E6 3B C4 : 26 3F CF E2 : } : } 330 110: SEQUENCE { 332 3: OBJECT IDENTIFIER authorityKeyIdentifier (2 5 29 35) 337 103: OCTET STRING, encapsulates { 339 101: SEQUENCE { 341 20: [0] - : 9D 6D 20 24 49 01 3F 2B CB 78 B5 19 BC 7E 24 C9 - : DB FB 36 7C + : 9D 6D 20 24 49 01 3F 2B CB 78 B5 19 BC 7E 24 + : C9 DB FB 36 7C 363 66: [1] { 365 64: [4] { 367 62: SEQUENCE { 369 11: SET { 371 9: SEQUENCE { 373 3: OBJECT IDENTIFIER countryName (2 5 4 6) 378 2: PrintableString 'NL' : } : } 382 17: SET { @@ -665,45 +651,45 @@ : 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 6: ASN.1-based Certificate: Example. + Figure 5: ASN.1-based Certificate: Example. - To include the certificate shown in Figure 6 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 header in our example is 0b 00 02 36 00 02 33 00 02 00 02 30, which indicates: Message Type: 0b -- 1 byte type field indicating a Certificate message Length: 00 02 36 -- 3 byte length field indicating a 566 bytes payload Certificates Length: 00 02 33 -- 3 byte length field indicating 563 bytes for the entire certificates_list structure, which may contain multiple certificates. In our example only one certificate is included. Certificate Length: 00 02 30 -- 3 byte length field indicating 560 bytes of the actual certificate following immediately afterwards. In our example, this is the certificate content with 30 82 02 .... - 9E 57 45 shown in Figure 7. + 9E 57 45 shown in Figure 6. The hex encoding of the ASN.1 encoded certificate payload shown in - Figure 6 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 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 @@ -727,30 +713,27 @@ 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 7: 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 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 - fingerprint of 0x086eefb4. Authors' Addresses - Stefan Santesson 3xA Security AB Scheelev. 17 Lund 223 70 Sweden Email: sts@aaa-sec.com Hannes Tschofenig ARM Ltd.