Serialization of unsigned or partially signed transactions

Electrum 2.0 uses an extended serialization format for transactions. The purpose of this format is to send unsigned and partially signed transactions to cosigners or to cold storage.

This is achieved by extending the ‘pubkey’ field of a transaction input.

Extended public keys

The first byte of the pubkey indicates if it is an extended pubkey:

0x02, 0x03, 0x04: legal Bitcoin public key (compressed or not).
0xFF, 0xFE, 0xFD: extended public key.

Extended public keys are of 3 types:

0xFF: bip32 xpub and derivation
0xFE: legacy electrum derivation: master public key + derivation
0xFD: unknown pubkey, but we know the Bitcoin address.

Public key

This is the legit Bitcoin serialization of public keys.

0x02 or 0x03	compressed public key (32 bytes)
0x04	uncompressed public key (64 bytes)

BIP32 derivation

0xFF

xpub (78 bytes)

bip32 derivation (2*k bytes)

Legacy Electrum Derivation

0xFE

mpk (64 bytes)

derivation (4 bytes)

Bitcoin address

Used if we don’t know the public key, but we know the address (or the hash 160 of the output script). The cosigner should know the public key.

0xFD	hash_160_of_script (20 bytes)