Network Working Group J. Benet Internet-Draft Protocol Labs Intended status: Standards Track M. Sporny Expires: February 13, 2019 Digital Bazaar August 12, 2018 The Multihash Data Format draft-multiformats-multihash-00 Abstract Cryptographic hash functions often have multiple output sizes and encodings. This variability makes it difficult for applications to examine a series of bytes and determine which hash function produced them. Multihash is a universal data format for encoding outputs from hash functions. It is useful to write applications that can simultaneously support different hash function outputs as well as upgrade their use of hashes over time; Multihash is intended to address these needs. Feedback This specification is a joint work product of Protocol Labs [1], the W3C Digital Verification Community Group [2], and the W3C Credentials Community Group [3]. Feedback related to this specification should logged in the issue tracker [4] or be sent to public- credentials@w3.org [5]. . Status of This Memo This Internet-Draft is submitted in full conformance with the 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 https://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 February 13, 2019. Benet & Sporny Expires February 13, 2019 [Page 1] Internet-Draft The Multihash Data Format August 2018 Copyright Notice Copyright (c) 2018 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. The Multihash Fields . . . . . . . . . . . . . . . . . . . . 3 2.1. Multihash Core Data Types . . . . . . . . . . . . . . . . 3 2.1.1. unsigned variable integer . . . . . . . . . . . . . . 3 2.2. Multihash Fields . . . . . . . . . . . . . . . . . . . . 4 2.2.1. Hash Function Type . . . . . . . . . . . . . . . . . 4 2.2.2. Digest Length . . . . . . . . . . . . . . . . . . . . 4 2.2.3. Digest Value . . . . . . . . . . . . . . . . . . . . 4 2.3. A Multihash Example . . . . . . . . . . . . . . . . . . . 5 3. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Normative References . . . . . . . . . . . . . . . . . . 5 3.2. Informative References . . . . . . . . . . . . . . . . . 5 3.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Appendix A. Security Considerations . . . . . . . . . . . . . . 6 Appendix B. Test Values . . . . . . . . . . . . . . . . . . . . 6 B.1. SHA-1 . . . . . . . . . . . . . . . . . . . . . . . . . . 6 B.2. SHA-256 . . . . . . . . . . . . . . . . . . . . . . . . . 6 B.3. SHA-512/256 . . . . . . . . . . . . . . . . . . . . . . . 6 B.4. SHA-512 . . . . . . . . . . . . . . . . . . . . . . . . . 7 B.5. blake2b512 . . . . . . . . . . . . . . . . . . . . . . . 7 B.6. blake2b256 . . . . . . . . . . . . . . . . . . . . . . . 7 B.7. blake2s256 . . . . . . . . . . . . . . . . . . . . . . . 7 B.8. blake2s128 . . . . . . . . . . . . . . . . . . . . . . . 7 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 7 Appendix D. IANA Considerations . . . . . . . . . . . . . . . . 7 D.1. The Multihash Algorithms Registry . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction Multihash is particularly important in systems which depend on cryptographically secure hash functions. Attacks may break the cryptographic properties of secure hash functions. These cryptographic breaks are particularly painful in large tool Benet & Sporny Expires February 13, 2019 [Page 2] Internet-Draft The Multihash Data Format August 2018 ecosystems, where tools may have made assumptions about hash values, such as function and digest size. Upgrading becomes a nightmare, as all tools which make those assumptions would have to be upgraded to use the new hash function and new hash digest length. Tools may face serious interoperability problems or error-prone special casing. How many programs out there assume a git hash is a SHA-1 hash? How many scripts assume the hash value digest is exactly 160 bits? How many tools will break when these values change? How many programs will fail silently when these values change? This is precisely why Multihash was created. It was designed for seamlessly upgrading systems that depend on cryptographic hashes. When using Multihash, a system warns the consumers of its hash values that these may have to be upgraded in case of a break. Even though the system may still only use a single hash function at a time, the use of multihash makes it clear to applications that hash values may use different hash functions or be longer in the future. Tooling, applications, and scripts can avoid making assumptions about the length, and read it from the multihash value instead. This way, the vast majority of tooling - which may not do any checking of hashes - would not have to be upgraded at all. This vastly simplifies the upgrade process, avoiding the waste of hundreds or thousands of software engineering hours, deep frustrations, and high blood pressure. 2. The Multihash Fields A multihash follows the TLV (type-length-value) pattern and consists of several fields composed of a combination of unsigned variable length integers and byte information. 2.1. Multihash Core Data Types The following section details the core data types used by the Multihash data format. 2.1.1. unsigned variable integer A data type that enables one to express an unsigned integer of variable length. When encoding an unsigned variable integer, the unsigned integer is serialized seven bits at a time, starting with the least significant Benet & Sporny Expires February 13, 2019 [Page 3] Internet-Draft The Multihash Data Format August 2018 bits. The most significant bit in each output byte intecates if there is a continuation byte. It is not possible to express a signed integer with this data type. +-------+----------------------------+ | Value | Encoding (bits) | +-------+----------------------------+ | 1 | 00000001 | | 127 | 01111111 | | 128 | 10000000 00000001 | | 255 | 11111111 00000001 | | 300 | 10101100 00000010 | | 16384 | 10000000 10000000 00000001 | +-------+----------------------------+ Table 1: Examples of Unsigned Variable Integers Implementations MUST restrict the size of the varint to a max of nine bytes (63 bits). In order to avoid memory attacks on the encoding, the aforementioned practical maximum length of of nine bytes is used. There is no theoretical limit, and future specs can grow this number if it is truly necessary to have code or length values larger than 2^31. 2.2. Multihash Fields A multihash follows the TLV (type-length-value) pattern. 2.2.1. Hash Function Type The hash function type is an unsigned variable integer [6] identifying the hash function. The possible values for this field are provided in The Multihash Algorithms Registry [7]. 2.2.2. Digest Length The digest length is an unsigned variable integer [8] counting the length of the digest in bytes. 2.2.3. Digest Value The digest value is the hash function digest with a length of exactly what is specified in the digest length, which is specified in bytes. Benet & Sporny Expires February 13, 2019 [Page 4] Internet-Draft The Multihash Data Format August 2018 2.3. A Multihash Example For example, the following is an expression of a SHA2-256 hash in hexadecimal notation (spaces added for readability purposes): 12 20 41dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8 The first byte (0x12) specifies the SHA2-256 hash function. The second byte (0x20) specifies the length of the hash, which is 32 bytes. The rest of the data specifies the value of the output of the hash function. 3. References 3.1. 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, . [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, . [RFC7693] Saarinen, M-J., Ed. and J-P. Aumasson, "The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC)", RFC 7693, DOI 10.17487/RFC7693, November 2015, . 3.2. Informative References [RFC6150] Turner, S. and L. Chen, "MD4 to Historic Status", RFC 6150, DOI 10.17487/RFC6150, March 2011, . [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, March 2011, . 3.3. URIs [1] https://protocol.ai/ [2] https://w3c-dvcg.github.io/ Benet & Sporny Expires February 13, 2019 [Page 5] Internet-Draft The Multihash Data Format August 2018 [3] https://w3c-ccg.github.io/ [4] https://github.com/w3c-dvcg/multihash/issues [5] mailto:public-credentials@w3.org [6] #cdt-uvi [7] #mh-registry [8] #cdt-uvi [9] http://www.iana.org/assignments/multihash-algorithms [10] https://github.com/multiformats/multihash/blob/master/ hashtable.csv Appendix A. Security Considerations There are a number of security considerations to take into account when implementing or utilizing this specification. TBD Appendix B. Test Values The multihash examples are chosen to show different hash functions and different hash digest lengths at play. The input test data for all of the examples in this section is: B.1. SHA-1 The fields for this multihash are - hashing function: sha1 (0x11), length: 20 (0x14), digest: 8a173fd3e32c0fa78b90fe42d305f202244e2739 B.2. SHA-256 The fields for this multihash are - hashing function: sha2-256 (0x12), length: 32 (0x20), digest: 41dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8 B.3. SHA-512/256 The fields for this multihash are - hashing function: sha2-512 (0x13), length: 32 (0x20), digest: 52eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4 Benet & Sporny Expires February 13, 2019 [Page 6] Internet-Draft The Multihash Data Format August 2018 B.4. SHA-512 The fields for this multihash are - hashing function: sha2-512 (0x13), length: 64 (0x40), digest: 52eb4dd19f1ec522859e12d89706156570 f8fbab1824870bc6f8c7d235eef5f4c2cbbafd365f96fb12b1d98a0334870c2ce9035 5da25e6a1108a6e17c4aaebb0 B.5. blake2b512 The fields for this multihash are - hashing function: blake2b-512 (0xb240), length: 64 (0x40), digest: d91ae0cb0e48022053ab0f8f0dc78d28 593d0f1c13ae39c9b169c136a779f21a0496337b6f776a73c1742805c1cc15e792ddb 3c92ee1fe300389456ef3dc97e2 B.6. blake2b256 The fields for this multihash are - hashing function: blake2b-256 (0xb220), length: 32 (0x20), digest: 7d0a1371550f3306532ff44520b649f8be05b72674e46fc24468ff74323ab030 B.7. blake2s256 The fields for this multihash are - hashing function: blake2s-256 (0xb260), length: 32 (0x20), digest: a96953281f3fd944a3206219fad61a40b992611b7580f1fa091935db3f7ca13d B.8. blake2s128 The fields for this multihash are - hashing function: blake2s-128 (0xb250), length: 16 (0x10), digest: 0a4ec6f1629e49262d7093e2f82a3278 Appendix C. Acknowledgements The editors would like to thank the following individuals for feedback on and implementations of the specification (in alphabetical order). Appendix D. IANA Considerations D.1. The Multihash Algorithms Registry The following initial entries should be added to the Multihash Algorithms Registry to be created and maintained at (the suggested URI) http://www.iana.org/assignments/multihash-algorithms [9]: +--------------+------------------+------------+--------------------+ | Codec | Identifier (hex) | Status | Specification | +--------------+------------------+------------+--------------------+ Benet & Sporny Expires February 13, 2019 [Page 7] Internet-Draft The Multihash Data Format August 2018 | identity | 0x00 | active | Unknown | | md4 | 0xd4 | deprecated | RFC 6150 [RFC6150] | | md5 | 0xd5 | deprecated | RFC 6151 [RFC6151] | | sha1 | 0x11 | active | RFC 6234 [RFC6234] | | sha2-256 | 0x12 | active | RFC 6234 [RFC6234] | | sha2-512 | 0x13 | active | RFC 6234 [RFC6234] | | dbl-sha2-256 | 0x56 | active | Unknown | | sha3-224 | 0x17 | active | Unknown | | sha3-256 | 0x16 | active | Unknown | | sha3-384 | 0x15 | active | Unknown | | sha3-512 | 0x14 | active | Unknown | | shake-128 | 0x18 | active | Unknown | | shake-256 | 0x19 | active | Unknown | | keccak-224 | 0x1A | active | Unknown | | keccak-256 | 0x1B | active | Unknown | | keccak-384 | 0x1C | active | Unknown | | keccak-512 | 0x1D | active | Unknown | | murmur3-128 | 0x22 | active | Unknown | | murmur3-32 | 0x23 | active | Unknown | | blake2b-8 | 0xb201 | active | RFC 7693 [RFC7693] | | blake2b-16 | 0xb202 | active | RFC 7693 [RFC7693] | | blake2b-24 | 0xb203 | active | RFC 7693 [RFC7693] | | blake2b-32 | 0xb204 | active | RFC 7693 [RFC7693] | | blake2b-40 | 0xb205 | active | RFC 7693 [RFC7693] | | blake2b-48 | 0xb206 | active | RFC 7693 [RFC7693] | | blake2b-56 | 0xb207 | active | RFC 7693 [RFC7693] | | blake2b-64 | 0xb208 | active | RFC 7693 [RFC7693] | | blake2b-72 | 0xb209 | active | RFC 7693 [RFC7693] | | blake2b-80 | 0xb20a | active | RFC 7693 [RFC7693] | | blake2b-88 | 0xb20b | active | RFC 7693 [RFC7693] | | blake2b-96 | 0xb20c | active | RFC 7693 [RFC7693] | | blake2b-104 | 0xb20d | active | RFC 7693 [RFC7693] | | blake2b-112 | 0xb20e | active | RFC 7693 [RFC7693] | | blake2b-120 | 0xb20f | active | RFC 7693 [RFC7693] | | blake2b-128 | 0xb210 | active | RFC 7693 [RFC7693] | | blake2b-136 | 0xb211 | active | RFC 7693 [RFC7693] | | blake2b-144 | 0xb212 | active | RFC 7693 [RFC7693] | | blake2b-152 | 0xb213 | active | RFC 7693 [RFC7693] | | blake2b-160 | 0xb214 | active | RFC 7693 [RFC7693] | | blake2b-168 | 0xb215 | active | RFC 7693 [RFC7693] | | blake2b-176 | 0xb216 | active | RFC 7693 [RFC7693] | | blake2b-184 | 0xb217 | active | RFC 7693 [RFC7693] | | blake2b-192 | 0xb218 | active | RFC 7693 [RFC7693] | | blake2b-200 | 0xb219 | active | RFC 7693 [RFC7693] | | blake2b-208 | 0xb21a | active | RFC 7693 [RFC7693] | | blake2b-216 | 0xb21b | active | RFC 7693 [RFC7693] | | blake2b-224 | 0xb21c | active | RFC 7693 [RFC7693] | | blake2b-232 | 0xb21d | active | RFC 7693 [RFC7693] | Benet & Sporny Expires February 13, 2019 [Page 8] Internet-Draft The Multihash Data Format August 2018 | blake2b-240 | 0xb21e | active | RFC 7693 [RFC7693] | | blake2b-248 | 0xb21f | active | RFC 7693 [RFC7693] | | blake2b-256 | 0xb220 | active | RFC 7693 [RFC7693] | | blake2b-264 | 0xb221 | active | RFC 7693 [RFC7693] | | blake2b-272 | 0xb222 | active | RFC 7693 [RFC7693] | | blake2b-280 | 0xb223 | active | RFC 7693 [RFC7693] | | blake2b-288 | 0xb224 | active | RFC 7693 [RFC7693] | | blake2b-296 | 0xb225 | active | RFC 7693 [RFC7693] | | blake2b-304 | 0xb226 | active | RFC 7693 [RFC7693] | | blake2b-312 | 0xb227 | active | RFC 7693 [RFC7693] | | blake2b-320 | 0xb228 | active | RFC 7693 [RFC7693] | | blake2b-328 | 0xb229 | active | RFC 7693 [RFC7693] | | blake2b-336 | 0xb22a | active | RFC 7693 [RFC7693] | | blake2b-344 | 0xb22b | active | RFC 7693 [RFC7693] | | blake2b-352 | 0xb22c | active | RFC 7693 [RFC7693] | | blake2b-360 | 0xb22d | active | RFC 7693 [RFC7693] | | blake2b-368 | 0xb22e | active | RFC 7693 [RFC7693] | | blake2b-376 | 0xb22f | active | RFC 7693 [RFC7693] | | blake2b-384 | 0xb230 | active | RFC 7693 [RFC7693] | | blake2b-392 | 0xb231 | active | RFC 7693 [RFC7693] | | blake2b-400 | 0xb232 | active | RFC 7693 [RFC7693] | | blake2b-408 | 0xb233 | active | RFC 7693 [RFC7693] | | blake2b-416 | 0xb234 | active | RFC 7693 [RFC7693] | | blake2b-424 | 0xb235 | active | RFC 7693 [RFC7693] | | blake2b-432 | 0xb236 | active | RFC 7693 [RFC7693] | | blake2b-440 | 0xb237 | active | RFC 7693 [RFC7693] | | blake2b-448 | 0xb238 | active | RFC 7693 [RFC7693] | | blake2b-456 | 0xb239 | active | RFC 7693 [RFC7693] | | blake2b-464 | 0xb23a | active | RFC 7693 [RFC7693] | | blake2b-472 | 0xb23b | active | RFC 7693 [RFC7693] | | blake2b-480 | 0xb23c | active | RFC 7693 [RFC7693] | | blake2b-488 | 0xb23d | active | RFC 7693 [RFC7693] | | blake2b-496 | 0xb23e | active | RFC 7693 [RFC7693] | | blake2b-504 | 0xb23f | active | RFC 7693 [RFC7693] | | blake2b-512 | 0xb240 | active | RFC 7693 [RFC7693] | | blake2s-8 | 0xb241 | active | RFC 7693 [RFC7693] | | blake2s-16 | 0xb242 | active | RFC 7693 [RFC7693] | | blake2s-24 | 0xb243 | active | RFC 7693 [RFC7693] | | blake2s-32 | 0xb244 | active | RFC 7693 [RFC7693] | | blake2s-40 | 0xb245 | active | RFC 7693 [RFC7693] | | blake2s-48 | 0xb246 | active | RFC 7693 [RFC7693] | | blake2s-56 | 0xb247 | active | RFC 7693 [RFC7693] | | blake2s-64 | 0xb248 | active | RFC 7693 [RFC7693] | | blake2s-72 | 0xb249 | active | RFC 7693 [RFC7693] | | blake2s-80 | 0xb24a | active | RFC 7693 [RFC7693] | | blake2s-88 | 0xb24b | active | RFC 7693 [RFC7693] | | blake2s-96 | 0xb24c | active | RFC 7693 [RFC7693] | | blake2s-104 | 0xb24d | active | RFC 7693 [RFC7693] | Benet & Sporny Expires February 13, 2019 [Page 9] Internet-Draft The Multihash Data Format August 2018 | blake2s-112 | 0xb24e | active | RFC 7693 [RFC7693] | | blake2s-120 | 0xb24f | active | RFC 7693 [RFC7693] | | blake2s-128 | 0xb250 | active | RFC 7693 [RFC7693] | | blake2s-136 | 0xb251 | active | RFC 7693 [RFC7693] | | blake2s-144 | 0xb252 | active | RFC 7693 [RFC7693] | | blake2s-152 | 0xb253 | active | RFC 7693 [RFC7693] | | blake2s-160 | 0xb254 | active | RFC 7693 [RFC7693] | | blake2s-168 | 0xb255 | active | RFC 7693 [RFC7693] | | blake2s-176 | 0xb256 | active | RFC 7693 [RFC7693] | | blake2s-184 | 0xb257 | active | RFC 7693 [RFC7693] | | blake2s-192 | 0xb258 | active | RFC 7693 [RFC7693] | | blake2s-200 | 0xb259 | active | RFC 7693 [RFC7693] | | blake2s-208 | 0xb25a | active | RFC 7693 [RFC7693] | | blake2s-216 | 0xb25b | active | RFC 7693 [RFC7693] | | blake2s-224 | 0xb25c | active | RFC 7693 [RFC7693] | | blake2s-232 | 0xb25d | active | RFC 7693 [RFC7693] | | blake2s-240 | 0xb25e | active | RFC 7693 [RFC7693] | | blake2s-248 | 0xb25f | active | RFC 7693 [RFC7693] | | blake2s-256 | 0xb260 | active | RFC 7693 [RFC7693] | +--------------+------------------+------------+--------------------+ Table 2: Multihash Algorithms Registry NOTE: The most up to date place for developers to find the table above is https://github.com/multiformats/multihash/blob/master/ hashtable.csv [10]. Authors' Addresses Juan Benet Protocol Labs 548 Market Street, #51207 San Francisco, CA 94104 US Phone: +1 619 957 7606 Email: juan@protocol.ai URI: http://juan.benet.ai/ Benet & Sporny Expires February 13, 2019 [Page 10] Internet-Draft The Multihash Data Format August 2018 Manu Sporny Digital Bazaar 203 Roanoke Street W. Blacksburg, VA 24060 US Phone: +1 540 961 4469 Email: msporny@digitalbazaar.com URI: http://manu.sporny.org/ Benet & Sporny Expires February 13, 2019 [Page 11]