def _recover_secret(threshold: int, shares: List[Tuple[int, bytes]]) -> bytes:
# If the threshold is 1, then the digest of the shared secret is not used.
if threshold == 1:
return shares[0][1]
shared_secret = shamir.interpolate(shares, _SECRET_INDEX)
digest_share = shamir.interpolate(shares, _DIGEST_INDEX)
digest = digest_share[:_DIGEST_LENGTH_BYTES]
random_part = digest_share[_DIGEST_LENGTH_BYTES:]
if digest != _create_digest(random_part, shared_secret):
raise MnemonicError("Invalid digest of the shared secret.")
return shared_secret
def _split_secret(
threshold: int, share_count: int, shared_secret: bytes
) -> List[Tuple[int, bytes]]:
if threshold < 1:
raise ValueError(
"The requested threshold ({}) must be a positive integer.".format(threshold)
)
if threshold > share_count:
raise ValueError(
"The requested threshold ({}) must not exceed the number of shares ({}).".format(
threshold, share_count
)
)
if share_count > MAX_SHARE_COUNT:
raise ValueError(
"The requested number of shares ({}) must not exceed {}.".format(
share_count, MAX_SHARE_COUNT
)
)
# If the threshold is 1, then the digest of the shared secret is not used.
if threshold == 1:
return [(i, shared_secret) for i in range(share_count)]
random_share_count = threshold - 2
shares = [(i, random.bytes(len(shared_secret))) for i in range(random_share_count)]
random_part = random.bytes(len(shared_secret) - _DIGEST_LENGTH_BYTES)
digest = _create_digest(random_part, shared_secret)
base_shares = shares + [
(_DIGEST_INDEX, digest + random_part),
(_SECRET_INDEX, shared_secret),
]
for i in range(random_share_count, share_count):
shares.append((i, shamir.interpolate(base_shares, i)))
return shares
