The Multihash Data Format

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Table of Contents

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.¶

0x12 20 41dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8

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

0x11148a173fd3e32c0fa78b90fe42d305f202244e2739

0x122041dd7b6443542e75701aa98a0c235951a28a0d851b11564d20022ab11d2589a8

0x132052eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4

0x134052eb4dd19f1ec522859e12d89706156570f8fbab1824870bc6f8c7d235eef5f4c2cbbafd365f96fb12b1d98a0334870c2ce90355da25e6a1108a6e17c4aaebb0

0xb24040d91ae0cb0e48022053ab0f8f0dc78d28593d0f1c13ae39c9b169c136a779f21a0496337b6f776a73c1742805c1cc15e792ddb3c92ee1fe300389456ef3dc97e2

0xb220207d0a1371550f3306532ff44520b649f8be05b72674e46fc24468ff74323ab030

0xb26020a96953281f3fd944a3206219fad61a40b992611b7580f1fa091935db3f7ca13d

        0xb250100a4ec6f1629e49262d7093e2f82a3278

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

Authors' Addresses