Bitcoin Cash Specification

Request: Filter Load (“filterload”)

Inserts a transaction and merkle block filter into the receiving peer, overwriting any existing filter the peer has stored for the sender.

Effect on Transactions

This message installs a bloom filter into the peer. Subsequent inv and merkleblock messages only provide transactions that in match this bloom filter in some manner. The following items in a transaction are checked against the bloom filter:

  • The transaction hash
  • Each data field in every locking script in the transaction
    • Most importantly, this allows public keys and public key hashes (essentially Bitcoin addresses) to be added to the bloom filter, allowing a wallet to detect an incoming transfer.
  • Each previous output for the inputs of the transaction (see Outpoints)
    • This allows a wallet to detect that a different wallet has spent funds that are co-controlled
  • Each data field in every unlocking script in the transaction.

Effect on Merkle Blocks

If a filtered block is requested via an inventory message, the installed bloom filter is used to choose which transactions are included in the response using the same matching algorithm as described above for transaction inventories.

Message Format

| Field | Length | Format | Description |
|—|—|—|—| | bloom filter size | variable | variable length integer | The size of the bloom filter to be used. | | bloom filter data | variable | bloom_filter_size bytes | Raw bloom filter data (up to 36,000 bytes). | | number of hash functions | 4 bytes | unsigned integer(LE) | The number of hash functions used when generating this filter (max 50). | | tweak | 4 bytes | unsigned integer(LE) | Arbitrary number (nonce) that helps randomize this bloom filter. | | flags | 1 byte | bit field | See bloom filter flags. |

Bloom Filter Flags

The lowest two bits of flags control how the installed bloom filter is automatically updated. The remaining bits are reserved and should be set to 0.

Value Name Description
0 BLOOM_UPDATE_NONE Do not update the bloom filter
1 BLOOM_UPDATE_ALL Add outpoints to the bloom filter when any of the locking script data elements match the bloom filter
2 BLOOM_UPDATE_P2PUBKEY_ONLY Add outpoints to the bloom filter when any of the locking script data elements match the bloom filter, only when the locking script is a pay-to-public-key or multi-sig script