57 lines
2.4 KiB
Python
57 lines
2.4 KiB
Python
 
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"""Merkle proofs (B-83, TX-22, ELC-26).
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`blockchain.transaction.get_merkle` answers with the branch that links a transaction to
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its block's merkle root. Checking it against a header **we hold** is what turns "a server
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says this tx is at height h" into something a hostile server cannot fake without mining.
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"""
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import hashlib
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_HEADER_LENGTH_BYTES = 80
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_MERKLE_ROOT_OFFSET = 36
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_MERKLE_ROOT_LENGTH = 32
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_MAX_BRANCH_LENGTH = 40 # 2^40 transactions in a block is already absurd
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def _sha256d(data: bytes) -> bytes:
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return hashlib.sha256(hashlib.sha256(data).digest()).digest()
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def header_merkle_root(header_hex: str) -> bytes:
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"""The merkle root committed to by a raw 80-byte header, in internal byte order."""
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header = bytes.fromhex(header_hex)
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if len(header) != _HEADER_LENGTH_BYTES:
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raise ValueError(f"a block header is {_HEADER_LENGTH_BYTES} bytes, got {len(header)}")
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return header[_MERKLE_ROOT_OFFSET : _MERKLE_ROOT_OFFSET + _MERKLE_ROOT_LENGTH]
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def merkle_root_from_branch(txid: str, branch: list[str], position: int) -> bytes:
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"""The merkle root (internal byte order) a transaction `txid` at index `position` of
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its block reaches through `branch` — the sibling hashes, bottom-up, in the display
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order Electrum uses for txids."""
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if position < 0 or len(branch) > _MAX_BRANCH_LENGTH or position >> len(branch):
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raise ValueError("merkle position does not fit the branch")
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current = bytes.fromhex(txid)[::-1]
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index = position
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for sibling_hex in branch:
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sibling = bytes.fromhex(sibling_hex)[::-1]
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if len(sibling) != 32:
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raise ValueError("merkle branch element is not 32 bytes")
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current = _sha256d(sibling + current) if index & 1 else _sha256d(current + sibling)
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index >>= 1
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return current
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def proof_matches_header(txid: str, height: int, proof: object, header_hex: str) -> bool:
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"""Does `proof` — a `get_merkle` reply — put `txid` in the block at `height` whose
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header is `header_hex`? Any malformed reply is simply "no"."""
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try:
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if not isinstance(proof, dict) or int(proof["block_height"]) != height:
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return False
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branch = proof["merkle"]
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if not isinstance(branch, list):
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return False
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root = merkle_root_from_branch(txid, [str(item) for item in branch], int(proof["pos"]))
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return root == header_merkle_root(header_hex)
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except (KeyError, TypeError, ValueError):
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return False
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