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.

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Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. The Multihash Fields
• 2.1. Multihash Core Data Types
• 2.1.1. unsigned variable integer
• 2.2. Multihash Fields
• 2.2.1. Hash Function Type
• 2.2.2. Digest Length
• 2.2.3. Digest Value
• 2.3. A Multihash Example
• 3. References
• 3.1. Normative References
• 3.2. Informative References
• Appendix A. Security Considerations
• Appendix B. Test Values
• B.1. SHA-1
• B.2. SHA-256
• B.3. SHA-512/256
• B.4. SHA-512
• B.5. blake2b512
• B.6. blake2b256
• B.7. blake2s256
• B.8. blake2s128
• Appendix C. Acknowledgements
• Appendix D. IANA Considerations
• D.1. The Multihash Algorithms Registry
• Authors' Addresses

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.

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 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.

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 identifying the hash function. The possible values for this field are provided in The Multihash Algorithms Registry.

2.2.2. Digest Length

The digest length is an unsigned variable integer 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

12 20 41dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8

For example, the following is an expression of a SHA2-256 hash in hexadecimal notation (spaces added for readability purposes):

3. References

3.1. Normative References

3.2. Informative References

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

B.4. SHA-512

The fields for this multihash are - hashing function: sha2-512 (0x13), length: 64 (0x40), digest: 52eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4c2cbbafd365f96fb12b1d98a0334870c2ce90355da25e6a1108a6e17c4aaebb0

B.5. blake2b512

The fields for this multihash are - hashing function: blake2b-512 (0xb240), length: 64 (0x40), digest: d91ae0cb0e48022053ab0f8f0dc78d28593d0f1c13ae39c9b169c136a779f21a0496337b6f776a73c1742805c1cc15e792ddb3c92ee1fe300389456ef3dc97e2

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:

NOTE: The most up to date place for developers to find the table above is https://github.com/multiformats/multihash/blob/master/hashtable.csv.

Authors' Addresses