This document registers a tag for serializing IPLD content identifiers (CIDs) in Concise Binary Object Representation (CBOR).

Tag: 42
Data item: byte string
Semantics: IPLD content identifier
Created: 2019-08-20

CID is a format for referencing content in distributed information systems, like IPFS. It leverages content addressing, cryptographic hashing, and self-describing formats. It is the core identifier used by IPFS and IPLD.

An IPLD content identifier in CBOR has tag 42 and is encoded as a byte string (major type 2). The CID is encoded in its binary representation (base256) and MUST be prepended by the identity Multibase prefix (a null-byte).

This CID bafyreie74tgmnxqwojhtumgh5dzfj46gi4mynlfr7dmm7duwzyvnpw7h7m is encoded as base32. When encoded as CBOR it will be converted to its (base256) binary representation. In this example it also has the identify Multibase prefix: 00017112209fe4ccc6de16724f3a30c7e8f254f3c6471986acb1f8d8cf8e96ce2ad7dbe7fb. The binary text is then stored with tag 42:

D8 2A                                   # tag(42)
   58 25                                # bytes(37)
      00017112209FE4CCC6DE16724F3A30C7E8F254F3C6471986ACB1F8D8CF8E96CE2AD7DBE7FB

Volker Mische <volker@protocol.ai>