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# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.
from __future__ import absolute_import, division, print_function
import six
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric.utils import Prehashed
def _truncate_digest(digest, order_bits):
digest_len = len(digest)
if 8 * digest_len > order_bits:
digest_len = (order_bits + 7) // 8
digest = digest[:digest_len]
if 8 * digest_len > order_bits:
rshift = 8 - (order_bits & 0x7)
assert 0 < rshift < 8
mask = 0xFF >> rshift << rshift
# Set the bottom rshift bits to 0
digest = digest[:-1] + six.int2byte(six.indexbytes(digest, -1) & mask)
return digest
def _calculate_digest_and_algorithm(backend, data, algorithm):
if not isinstance(algorithm, Prehashed):
hash_ctx = hashes.Hash(algorithm, backend)
hash_ctx.update(data)
data = hash_ctx.finalize()
else:
algorithm = algorithm._algorithm
if len(data) != algorithm.digest_size:
raise ValueError(
"The provided data must be the same length as the hash "
"algorithm's digest size."
)
return (data, algorithm)