p2pool/docs/MERGE_MINING.MD

Merge mining tx_extra tag format

• TX_EXTRA_MERGE_MINING_TAG (1 byte)
  • A single byte of value 3
• Size (in bytes) of the following fields (1 byte)
  • Technically a Varint, but it's always a single byte in practice
• Merkle tree parameters (1 or multiple bytes - 64-bit value encoded as varint)
• Merkle root hash (32 bytes)

Merkle tree parameters

A 64-bit value encoding the total number of aux chains hashed into the Merkle root hash, and an aux nonce that defines the order of aux chains when building the Merkle tree.

Bit	Description
0...2	N-1 - number of bits used to store n_aux_chains (value 0 means 1 bit, ..., value 7 means 8 bits)
3...N+2	n_aux_chains-1 (value 0 means 1 chain, and so on)
N+3...N+34	aux_nonce (32-bit value)
N+35...63	reserved (current implementations must set these bits to all 0 when writing and ignore these bits when reading)

N must be the smallest number of bits enough to represent n_aux_chains-1

Reference code: PoolBlock::encode_merkle_tree_data and PoolBlock::decode_merkle_tree_data in pool_block.h

Merkle tree construction's first step (Monero-specific)

If n_aux_chains is not a power of 2, their hashes are combined starting from the end of the list of hashes until there are a power of 2 hashes left. Then they're processed normally.

Example 1: 6 hashes H0, H1, H2, H3, H4, H5 will be transformed into 4 hashes H0, H1, H(H2|H3), H(H4|H5) where H is the hash function and | is concatenation.

Example 2: 5 hashes H0, H1, H2, H3, H4 will be transformed into 4 hashes H0, H1, H2, H(H3|H4)

The order of hashes in the Merkle tree

Each of the aux chains must provide a 32-byte value unique_id that singles it out from any other chain. Genesis block's hash is a good choice for this value, but it can be anything else really.

unique_id is used to enforce the order in which aux hashes are added to the Merkle tree. This prevents an attack where miners can mine two versions of the same chain (for example when double spending) without any additional cost.

Forked chains can choose whether to keep their unique_id or change it. If they keep it, it will be impossible to merge mine with the chain they forked from beause they will always be assigned the same slot in the Merkle tree.

A deterministic pseudo-random function SHA256(unique_id|nonce|"m") % N is applied to determine which slot is used by which chain. nonce is brute-forced until all N chains are assigned different slots. This limits N to no more than 15-16 in practice.

Reference code: get_aux_slot and find_aux_nonce in merkle.cpp

Proposed RPC API

P2Pool must be able to get mining jobs from merge mined chains and submit PoW solutions to them.

merge_mining_get_id

Request: an empty JSON

Response: a JSON containing these fields:

Field	Description
result	OK or an error message
id	A unique 32-byte hex-encoded value that identifies this merge mined chain.

merge_mining_get_job

Request: a JSON containing these fields:

Field	Description
height	Monero height
prev_id	Hash of the previous Monero block
address	A wallet address on the merge mined chain
aux_hash	Merge mining job that is currently being used

Response: a JSON containing these fields:

Field	Description
result	OK or an error message
aux_blob	A hex-encoded blob of data. Merge mined chain defines the contents of this blob. It's opaque to P2Pool and will not be changed by it.
aux_hash	A 32-byte hex-encoded hash of the aux_blob. Merge mined chain defines how exactly this hash is calculated. It's opaque to P2Pool.
aux_diff	Mining difficulty (decimal number).

If aux_hash is the same as in the request, all other fields will be ignored by P2Pool, so they don't have to be included in the response. Moreover, {"result":"OK"} response will be interpreted as a response having the same aux_hash as in the request. This enables an efficient polling.

merge_mining_submit_solution

Request: a JSON containing these fields:

Field	Description
aux_blob	Blob of data returned by merge_mining_get_job.
aux_hash	A 32-byte hex-encoded hash of the aux_blob - the same value that was returned by merge_mining_get_job.
blob	Monero block template that has enough PoW to satisfy difficulty returned by merge_mining_get_job. It also must have a merge mining tag in tx_extra of the coinbase transaction.
merkle_proof	A proof that aux_hash was included when calculating Merkle root hash from the merge mining tag

Note that merkle_proof only contains a vector of 32-byte hashes for aux_hash to be combined with. It can be verified by running this pseudo-code and functions from merkle.cpp (adapt it to your codebase):

verify_merkle_proof(aux_hash, merkle_proof, get_aux_slot(unique_id, aux_nonce, n_aux_chains), n_aux_chains, merkle_root_hash)

aux_nonce and n_aux_chains can be extracted from the Merkle tree parameters (see above). merkle_root_hash can be extracted from the merge mining tag (see above).