Network Working Group J. Benet
Internet-Draft Protocol Labs
Intended status: Standards Track M. Sporny
Expires: July 4, 2019 Digital Bazaar
December 31, 2018

The Multibase Data Format


Raw binary data is often encoded using a mechanism that enables the data to be included in human-readable text-based formats. This mechanism is often referred to as "base-encoding the data". Base-encoding is often used when expressing binary data in hyperlinks, cryptographic keys in web pages, or security tokens in application software. There are a variety of base-encodings, such as base32, base58, and base64. It is not always possible to differentiate one base-encoding from another. The purpose of this specification is to provide a mechanism to be able to deterministically identify the base-encoding for a particular string of data.


This specification is a joint work product of Protocol Labs, the W3C Digital Verification Community Group, and the W3C Credentials Community Group. Feedback related to this specification should logged in the issue tracker or be sent to .

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This Internet-Draft will expire on July 4, 2019.

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Table of Contents

1. Introduction

This specification describes a forward-compatible data model for expressing raw binary data in a variety of base-encoding formats such as base32, base58. and base64.

When text is encoded as bytes, we can usually use a one-size-fits-all encoding (UTF-8) because we're always encoding to the same set of 256 bytes. When that doesn't work, usually for historical or performance reasons, we can usually infer the encoding from the context.

However, when bytes are encoded as text (using a base encoding), the choice of base encoding is often restricted by the context. Worse, these restrictions can change based on where the data appears in the text. In some cases, we can only use [a-z0-9]. In others, we can use a larger set of characters but need a compact encoding. This has lead to a large set of "base encodings", one for every use-case. Unlike when encoding text to bytes, we can't just standardize around a single base encoding because there is no optimal encoding for all cases.

Unfortunately, it's not always clear what base encoding is used; that's where this specification comes in. It answers the question:

Given data 'd' encoded into text 's', what base is it encoded with?

2. The Multibase Format

base-encoding-character base-encoded-data

A multibase-encoded value follows a simple format:

The encoding algorithm is a single character value that is always the first byte of the data. The possible values for this field are provided in The Multibase Algorithm Registry.

2.1. A Multibase Example


The following is an encoding of "Hello World!" using the version of base-58 that utilizes the Bitcoin encoding character set:

3. 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.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.

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 multibase examples are chosen to show different encoding algorithms and different output lengths at play. The input test data for all of the examples in this section is:

Multibase is awesome! \o/

B.1. Hexadecimal upper-case encoding


B.2. Base-32 upper-case encoding, no padding


B.3. Base-58 Bitcoin encoding


B.4. Base-64 with padding and MIME-encoding


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 Multibase Algorithms Registry

The following initial entries should be added to the Multibase Algorithms Registry to be created and maintained at (the suggested URI)

Multihash Algorithms Registry
Algorithm Identifier (character) Status Specification
identity 0x00 active 8-bit binary (encoder and decoder keeps data unmodified)
base1 1 active unary (11111)
base2 0 active binary (01010101)
base8 7 active octal
base10 9 active decimal
base16 f active hexadecimal
base16upper F active hexadecimal
base32hex v active RFC 4648 no padding - highest char
base32hexupper V active RFC 4648 no padding - highest char
base32hexpad t active RFC 4648 with padding
base32hexpadupper T active RFC 4648 with padding
base32 b active RFC 4648 no padding
base32upper B active RFC 4648 no padding
base32pad c active RFC 4648 with padding
base32padupper C active RFC 4648 with padding
base32z h active z-base-32 (used by Tahoe-LAFS)
base58flickr Z active base58 flicker
base58btc z active base58 bitcoin
base64 m active RFC 4648 no padding
base64pad M active RFC 4648 with padding - MIME encoding
base64url u active RFC 4648 no padding
base64urlpad U active RFC 4648 with padding

NOTE: The most up to date place for developers to find the table above is

Authors' Addresses

Juan Benet Protocol Labs 548 Market Street, #51207 San Francisco, CA 94104 US Phone: +1 619 957 7606 EMail: URI:
Manu Sporny Digital Bazaar 203 Roanoke Street W. Blacksburg, VA 24060 US Phone: +1 540 961 4469 EMail: URI: