Cid Type
A Content Identifier (CID) used to reference content-addressed data in the AT Protocol.
CIDs are self-describing content hashes that uniquely identify a piece of data.
Only CIDv1 is supported; CIDv0 (starting with Qmb) is rejected.
CIDs in the AT Protocol use CIDv1 with DAG-CBOR codec and SHA-256 hash. This type performs syntactic validation only (base-encoded alphanumeric string of 8-256 characters). See https://github.com/multiformats/cid for the CID specification.