Internet DRAFT - draft-multiformats-multihash
draft-multiformats-multihash
Network Working Group J. Benet
Internet-Draft Protocol Labs
Intended status: Informational M. Sporny
Expires: 21 February 2024 Digital Bazaar
20 August 2023
The Multihash Data Format
draft-multiformats-multihash-07
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
(https://protocol.ai/) and the W3C Credentials Community Group
(https://w3c-ccg.github.io/). Feedback related to this specification
should logged in the issue tracker (https://github.com/w3c-
ccg/multihash/issues) or be sent to public-credentials@w3.org
(mailto:public-credentials@w3.org).
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 21 February 2024.
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Copyright Notice
Copyright (c) 2023 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. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Multihash Fields . . . . . . . . . . . . . . . . . . . . 3
2.1. Multihash Core Data Types . . . . . . . . . . . . . . . . 3
2.1.1. unsigned variable integer . . . . . . . . . . . . . . 4
2.2. Multihash Fields . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Hash Function Identifier . . . . . . . . . . . . . . 5
2.2.2. Digest Length . . . . . . . . . . . . . . . . . . . . 5
2.2.3. Digest Value . . . . . . . . . . . . . . . . . . . . 5
2.3. A Multihash Example . . . . . . . . . . . . . . . . . . . 5
3. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Normative References . . . . . . . . . . . . . . . . . . 5
3.2. Informative References . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . . . . . 7
B.4. SHA-512 . . . . . . . . . . . . . . . . . . . . . . . . . 7
B.5. blake2b512 . . . . . . . . . . . . . . . . . . . . . . . 7
B.6. blake2b256 . . . . . . . . . . . . . . . . . . . . . . . 7
B.7. blake2s256 . . . . . . . . . . . . . . . . . . . . . . . 7
B.8. blake2s128 . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Appendix D. IANA Considerations . . . . . . . . . . . . . . . . 8
D.1. The Multihash Identifier Registry . . . . . . . . . . . . 8
D.2. The 'mh' Digest Algorithm . . . . . . . . . . . . . . . . 9
D.3. The 'mh' Named Information Hash Algorithm . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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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
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.
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2.1.1. unsigned variable integer
A data type that enables one to express an unsigned integer of
variable length. The format uses the Little Endian Base 128 (LEB128)
encoding that is defined in Appendix C of the DWARF Debugging
Information Format [DWARF] standard, initially released in 1993.
As suggested by the name, this variable length encoding is only
capable of representing unsigned integers. Further, while there is
no theoretical maximum integer value that can be represented by the
format, implementations MUST NOT encode more than nine (9) bytes
giving a practical limit of integers in a range between 0 and 2^63 -
1.
When encoding an unsigned variable integer, the unsigned integer is
serialized seven bits at a time, starting with the least significant
bits. The most significant bit in each output byte indicates if
there is a continuation byte. It is not possible to express a signed
integer with this data type.
+=======+============================+======================+
| Value | Encoding (bits) | hexadecimal notation |
+=======+============================+======================+
| 1 | 00000001 | 0x01 |
+-------+----------------------------+----------------------+
| 127 | 01111111 | 0x7F |
+-------+----------------------------+----------------------+
| 128 | 10000000 00000001 | 0x8001 |
+-------+----------------------------+----------------------+
| 255 | 11111111 00000001 | 0xFF01 |
+-------+----------------------------+----------------------+
| 300 | 10101100 00000010 | 0xAC02 |
+-------+----------------------------+----------------------+
| 16384 | 10000000 10000000 00000001 | 0x808001 |
+-------+----------------------------+----------------------+
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 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.
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2.2. Multihash Fields
A multihash follows the TLV (type-length-value) pattern.
2.2.1. Hash Function Identifier
The hash function identifier is an unsigned variable integer (#cdt-
uvi) identifying the hash function. The possible values for this
field are provided in The Multihash Identifier Registry (#mh-
registry).
2.2.2. Digest Length
The digest length is an unsigned variable integer (#cdt-uvi) 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.
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):
0x12 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
[DWARF] Workgroup, D. D. I. F., Ed., "DWARF Debugging Information
Format, Version 3", December 2005,
<http://dwarfstd.org/doc/Dwarf3.pdf>.
[FIPS202] Technology, I. T. L. N. I. O. S. A., Ed., "SHA-3 Standard:
Permutation-Based Hash and Extendable-Output Functions",
FIPS 202, DOI 10.6028/NIST.FIPS.202, August 2015,
<https://doi.org/10.6028/NIST.FIPS.202>.
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[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,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC7693] Saarinen, M., Ed. and J. Aumasson, "The BLAKE2
Cryptographic Hash and Message Authentication Code (MAC)",
RFC 7693, DOI 10.17487/RFC7693, November 2015,
<https://www.rfc-editor.org/info/rfc7693>.
3.2. Informative References
[RFC6150] Turner, S. and L. Chen, "MD4 to Historic Status",
RFC 6150, DOI 10.17487/RFC6150, March 2011,
<https://www.rfc-editor.org/info/rfc6150>.
[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,
<https://www.rfc-editor.org/info/rfc6151>.
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:
Merkle–Damgård
B.1. SHA-1
0x11148a173fd3e32c0fa78b90fe42d305f202244e2739
The fields for this multihash are - hashing function: sha1 (0x11),
length: 20 (0x14), digest: 0x8a173fd3e32c0fa78b90fe42d305f202244e2739
B.2. SHA-256
0x122041dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8
The fields for this multihash are - hashing function: sha2-256
(0x12), length: 32 (0x20), digest:
0x41dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8
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B.3. SHA-512/256
0x132052eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4
The fields for this multihash are - hashing function: sha2-512
(0x13), length: 32 (0x20), digest:
0x52eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4
B.4. SHA-512
0x134052eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4c2cbbafd365f96fb12b1d98a0334870c2ce90355da25e6a1108a6e17c4aaebb0
The fields for this multihash are - hashing function: sha2-512
(0x13), length: 64 (0x40), digest: 0x52eb4dd19f1ec522859e12d897061565
70f8fbab1824870bc6f8c7d235eef5f4c2cbbafd365f96fb12b1d98a0334870c2ce90
355da25e6a1108a6e17c4aaebb0
B.5. blake2b512
0xb24040d91ae0cb0e48022053ab0f8f0dc78d28593d0f1c13ae39c9b169c136a779f21a0496337b6f776a73c1742805c1cc15e792ddb3c92ee1fe300389456ef3dc97e2
The fields for this multihash are - hashing function: blake2b-512
(0xb240), length: 64 (0x40), digest: 0xd91ae0cb0e48022053ab0f8f0dc78d
28593d0f1c13ae39c9b169c136a779f21a0496337b6f776a73c1742805c1cc15e792d
db3c92ee1fe300389456ef3dc97e2
B.6. blake2b256
0xb220207d0a1371550f3306532ff44520b649f8be05b72674e46fc24468ff74323ab030
The fields for this multihash are - hashing function: blake2b-256
(0xb220), length: 32 (0x20), digest:
0x7d0a1371550f3306532ff44520b649f8be05b72674e46fc24468ff74323ab030
B.7. blake2s256
0xb26020a96953281f3fd944a3206219fad61a40b992611b7580f1fa091935db3f7ca13d
The fields for this multihash are - hashing function: blake2s-256
(0xb260), length: 32 (0x20), digest:
0xa96953281f3fd944a3206219fad61a40b992611b7580f1fa091935db3f7ca13d
B.8. blake2s128
0xb250100a4ec6f1629e49262d7093e2f82a3278
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The fields for this multihash are - hashing function: blake2s-128
(0xb250), length: 16 (0x10), digest:
0x0a4ec6f1629e49262d7093e2f82a3278
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 Identifier Registry
The Multihash Identifier Registry contains hash functions supported
by Multihash each with its canonical name, its value in hexadecimal
notation, and its status. The following initial entries should be
added to the registry to be created and maintained at (the suggested
URI) http://www.iana.org/assignments/multihash-identifiers
(http://www.iana.org/assignments/multihash-identifiers):
+===========================+============+========+===============+
| Name | Identifier | Status | Specification |
+===========================+============+========+===============+
| identity | 0x00 | active | Unknown |
+---------------------------+------------+--------+---------------+
| sha1 | 0x11 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha2-256 | 0x12 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha2-512 | 0x13 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha3-512 | 0x14 | active | FIPS 202 |
| | | | [FIPS202] |
+---------------------------+------------+--------+---------------+
| sha3-384 | 0x15 | active | FIPS 202 |
| | | | [FIPS202] |
+---------------------------+------------+--------+---------------+
| sha3-256 | 0x16 | active | FIPS 202 |
| | | | [FIPS202] |
+---------------------------+------------+--------+---------------+
| sha3-224 | 0x17 | active | FIPS 202 |
| | | | [FIPS202] |
+---------------------------+------------+--------+---------------+
| sha2-384 | 0x20 | active | RFC 6234 |
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| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha2-256-trunc254-padded | 0x1012 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha2-224 | 0x1013 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha2-512-224 | 0x1014 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| sha2-512-256 | 0x1015 | active | RFC 6234 |
| | | | [RFC6234] |
+---------------------------+------------+--------+---------------+
| blake2b-256 | 0xb220 | active | RFC 7693 |
| | | | [RFC7693] |
+---------------------------+------------+--------+---------------+
| poseidon-bls12_381-a2-fc1 | 0xb401 | active | Unknown |
+---------------------------+------------+--------+---------------+
Table 2: Multihash Identifier Registry
NOTE: The most up to date place for developers to find the table
above, plus all multihash headers in "draft" status, is
https://github.com/multiformats/multicodec/blob/master/table.csv
(https://github.com/multiformats/multicodec/blob/master/table.csv).
D.2. The 'mh' Digest Algorithm
This memo registers the "mh" digest-algorithm in the HTTP Digest
Algorithm Values (https://www.iana.org/assignments/http-dig-alg/http-
dig-alg.xhtml) registry with the following values:
Digest Algorithm: mh
Description: The multibase-serialized value of a multihash-supported
algorithm.
References: this document
Status: standard
D.3. The 'mh' Named Information Hash Algorithm
This memo registers the "mh" hash algorithm in the Named Information
Hash Algorithm (https://www.iana.org/assignments/named-information/
named-information.xhtml#hash-alg) registry with the following values:
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ID: 49
Hash Name String: mh
Value Length: variable
Reference: this document
Status: current
Authors' Addresses
Juan Benet
Protocol Labs
548 Market Street, #51207
San Francisco, CA 94104
United States of America
Phone: +1 619 957 7606
Email: juan@protocol.ai
URI: http://juan.benet.ai/
Manu Sporny
Digital Bazaar
203 Roanoke Street W.
Blacksburg, VA 24060
United States of America
Phone: +1 540 961 4469
Email: msporny@digitalbazaar.com
URI: http://manu.sporny.org/
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