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fix truncation of hash input with order bit size not multiple of 8 #215

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Merged
merged 1 commit into from
Nov 11, 2020
Merged

fix truncation of hash input with order bit size not multiple of 8 #215

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22 changes: 18 additions & 4 deletions src/ecdsa/keys.py
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Original file line number Diff line number Diff line change
Expand Up @@ -77,7 +77,7 @@
from .numbertheory import square_root_mod_prime, SquareRootError
from .ecdsa import RSZeroError
from .util import string_to_number, number_to_string, randrange
from .util import sigencode_string, sigdecode_string
from .util import sigencode_string, sigdecode_string, bit_length
from .util import (
oid_ecPublicKey,
encoded_oid_ecPublicKey,
Expand Down Expand Up @@ -694,14 +694,28 @@ def verify_digest(
# signature doesn't have to be a bytes-like-object so don't normalise
# it, the decoders will do that
digest = normalise_bytes(digest)
if allow_truncate:
digest = digest[: self.curve.baselen]
if len(digest) > self.curve.baselen:
if not allow_truncate and len(digest) > self.curve.baselen:
raise BadDigestError(
"this curve (%s) is too short "
"for your digest (%d)" % (self.curve.name, 8 * len(digest))
)
number = string_to_number(digest)
if allow_truncate:
max_length = bit_length(self.curve.order)
# we don't use bit_length(number) as that truncates leading zeros
length = len(digest) * 8

# See NIST FIPS 186-4:
#
# When the length of the output of the hash function is greater
# than N (i.e., the bit length of q), then the leftmost N bits of
# the hash function output block shall be used in any calculation
# using the hash function output during the generation or
# verification of a digital signature.
#
# as such, we need to shift-out the low-order bits:
number >>= max(0, length - max_length)

try:
r, s = sigdecode(signature, self.pubkey.order)
except (der.UnexpectedDER, MalformedSignature) as e:
Expand Down