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Merge pull request #1027 from kdmukai/psbt_parser_docstrings

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Clarify psbt_parser.py docstrings and comments; minor cleanup
This commit is contained in:
Nick Klockenga
2026-09-04 15:18:33 -04:00
committed by GitHub
parent e4af6a7e38 8574c13f39
commit 85cd9a0211
1 changed files with 30 additions and 39 deletions
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src/seedsigner/models/psbt_parser.py
+30 -39
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@@ -32,33 +32,21 @@ class PSBTVerificationError(Exception):
class PSBTOutputOwnershipClaimError(PSBTVerificationError):
"""
An output scope claims this seed's fingerprint on a key the seed does not derive.
An output claims this seed's fingerprint on a key the seed does not derive.
This is not a psbt that merely fails to be ours. A fingerprint is
coordinator-supplied metadata, so this is a psbt asserting that a key belongs to this
seed when it does not. On an output that assertion is how a fake change output is
dressed up as the user's own, so it should be treated as an attack.
This is asserting that an output belongs to this seed when it does not. We treat this
deception as an attack.
"""
pass
class PSBTInputOwnershipClaimError(PSBTVerificationError):
"""
An input scope claims this seed's fingerprint on a key the seed does not derive.
An input claims this seed's fingerprint on a key the seed does not derive.
The same false claim as PSBTOutputOwnershipClaimError, but on an input the threat
picture inverts. A forged input claim has no path to losing funds: it cannot produce a
signature (embit re-derives the real key and refuses on a mismatch), and it cannot
alter the amounts, the fee, or how outputs are classified. The likely causes are
instead a psbt assembled for a different wallet, a corrupted entry, or a collaborative
spend that happens to include a key whose 4-byte fingerprint collides with ours (1 in
2^32 chance).
The psbt still fails, deliberately, following embit's lead: sign_with raises on this
same condition and abandons the entire signing pass, so tolerating the entry here
would only defer the failure to a worse spot. Changing this behavior, if desired,
should happen in embit first. Until then the trade-off is accepted: a
collaborative-spend counterparty could grief such a transaction into unsignability.
The same type of false claim as the output case above, but a forged input claim simply
renders the psbt unsignable (embit re-derives the real key and will refuse to sign on
a mismatch) so there's no point in continuing.
"""
pass
@@ -133,9 +121,9 @@ class PSBTParser():
self.destination_amounts = []
self.op_return_data: bytes = None
# Indexed alongside psbt.inputs / psbt.outputs. Each entry is the derivation path
# the seed genuinely owns in each scope or None where it owns nothing. Determined
# in _verify_claimed_derivation_paths.
# Contains one entry per input in psbt.inputs and per output in psbt.outputs. Each
# entry is either the derivation path the seed genuinely owns there, or it is set
# to `None`.
self.verified_input_derivation_paths: List[List[int] | None] = []
self.verified_output_derivation_paths: List[List[int] | None] = []
@@ -176,12 +164,12 @@ class PSBTParser():
"""
Establishes, in order:
1. _fill_missing_fingerprints: backfills all-zero fingerprints, but only for
scopes the seed provably derives.
1. _fill_missing_fingerprints: backfills all-zero fingerprints, but only where
the seed provably derives the key.
2. _verify_claimed_derivation_paths: each input and output scope that claims to
be controlled by the seed is verified. Raises an Input/Output
OwnershipClaimError if a claimed scope fails verification.
2. _verify_claimed_derivation_paths: each input and output that claims to be
controlled by the seed declares a derivation path that must be verified.
Raises an Input/Output OwnershipClaimError if a claim fails verification.
3. _reject_if_seed_cannot_sign: raises PSBTSeedCannotSignError if none of the
inputs can be signed by the seed. A mismatch rather than an attack, caught
@@ -257,6 +245,10 @@ class PSBTParser():
def _parse_inputs(self, child_key_derivation_cache: dict):
"""
Totals the input amounts and determines the wallet policy. Every input must
resolve to the same policy, otherwise a RuntimeError is raised.
"""
self.input_amount = 0
self.num_inputs = len(self.psbt.inputs)
for inp in self.psbt.inputs:
@@ -314,7 +306,8 @@ class PSBTParser():
elif self.policy["type"] == "p2sh":
sc = script.p2sh(out.redeem_script)
# single-sig
# single-sig: p2pkh, p2sh-p2wpkh, and p2wpkh; taproot handled separately
# below.
elif "pkh" in self.policy["type"]:
my_pubkey = None
@@ -431,6 +424,8 @@ class PSBTParser():
policy = {"type": script_type}
# expected multisig
# TODO: rename this local. It shadows the embit `script` module for the rest of
# this function, so script.p2wsh() and the other constructors are unreachable.
script = None
if script_type:
if "p2wsh" in script_type and scope.witness_script is not None:
@@ -459,19 +454,19 @@ class PSBTParser():
@staticmethod
def _parse_multisig(sc):
def _parse_multisig(multisig_script):
"""Takes a script and extracts m,n and pubkeys from it"""
# OP_m <len:pubkey> ... <len:pubkey> OP_n OP_CHECKMULTISIG
# check min size
if len(sc.data) < 37 or sc.data[-1] != 0xAE:
if len(multisig_script.data) < 37 or multisig_script.data[-1] != 0xAE:
raise ValueError("Not a multisig script")
m = sc.data[0] - 0x50
m = multisig_script.data[0] - 0x50
if m < 1 or m > 16:
raise ValueError("Invalid multisig script")
n = sc.data[-2] - 0x50
n = multisig_script.data[-2] - 0x50
if n < m or n > 16:
raise ValueError("Invalid multisig script")
s = BytesIO(sc.data)
s = BytesIO(multisig_script.data)
# drop first byte
s.read(1)
# read pubkeys
@@ -482,7 +477,7 @@ class PSBTParser():
raise ValueError("Invlid pubkey")
pubkeys.append(ec.PublicKey.parse(s.read(33)))
# check that nothing left
if s.read() != sc.data[-2:]:
if s.read() != multisig_script.data[-2:]:
raise ValueError("Invalid multisig script")
return m, n, pubkeys
@@ -670,11 +665,7 @@ class PSBTParser():
match). Returns the verified derivation path (as a list of ints) or None.
Every key in the scope that claims this seed's fingerprint is re-derived and
checked. A claim that does not hold up raises
PSBT[Output|Input]OwnershipClaimError. This includes fingerprint collisions (two
different keys with the same 4-byte fingerprint):
* On the output side, a collision is considered an attack.
* On the input side it is merely disallowed because it is unsignable by embit.
checked. A false claim raises PSBT[Output|Input]OwnershipClaimError.
One edge case:
* A multisig could use this seed in more than one cosigner slot, each