libp2p-peer-ids.md

Libp2p Peer IDs

Peer ID Spec Multihash spec Multicodec table

Similar exploration to the one in this document

Decoding a Peer ID

According to the Peer ID spec:

If it starts with 1 or Qm, it's a bare base58btc encoded multihash. Decode it according to the base58btc algorithm.

For example, QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa is a base58btc encoded multihash.

The multihash has the following format:

<varint hash function code><varint digest size in bytes><hash function output>

QmQCU2EcMqAqQPR2i9bChDtGNJchTbq5TbXJJ16u19uLTa decodes to code 18 (0x12 in hex) which is sha2-256, size 32 (bytes) of the hash

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Another example, 12D3KooWRBy97UB99e3J6hiPesre1MZeuNQvfan4gBziswrRJsNK is a base58btc encoded multihash.

Somewhat confusing is that this is an identity multihash, i.e. it's not hashed.

12D3KooWRBy97UB99e3J6hiPesre1MZeuNQvfan4gBziswrRJsNK decodes to code 0 (0x00 in hex) which is identity and has the size 36 bytes

Open questions

• How do we actually determine that it's an Ed25519 key?
  • The js-libp2p codebase determines based on the first three characters: 12D.
  • The base58btc characters 12D map to 2 bytes, 0, 70
• Why is the Ed25519 key length 36 bytes and not 32 bytes? The peerID spec says it follows the Ed25519 standard which says the length is 32 bytes.