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gh-155428: align hmac.digest and hmac.HMAC.digest outputs when using small block sizes #155429

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gh-155428: align hmac.digest and hmac.HMAC.digest outputs when using small block sizes #155429

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39 changes: 23 additions & 16 deletions Lib/hmac.py
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Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,27 @@ def digest_wrapper(d=b''):
return digest_wrapper


def _select_blocksize(obj, default, stacklevel=3):
if hasattr(obj, 'block_size'):
blocksize = obj.block_size
if blocksize < 16:
import warnings

warnings.warn(f"block_size of {blocksize} seems too small; "
f"using our default of {default}.",
RuntimeWarning, stacklevel=stacklevel)
blocksize = default
else:
import warnings

warnings.warn("No block_size attribute on given digest object; "
f"Assuming {default}.",
RuntimeWarning, stacklevel=stacklevel)
blocksize = default
return blocksize



class HMAC:
"""RFC 2104 HMAC class. Also complies with RFC 4231.

Expand Down Expand Up @@ -116,8 +137,6 @@ def _init_builtin_hmac(self, key, msg, digestmod):
self.block_size = self._hmac.block_size

def _init_old(self, key, msg, digestmod):
import warnings

digest_cons = _get_digest_constructor(digestmod)
if _is_shake_constructor(digest_cons):
raise ValueError(f"unsupported hash algorithm {digestmod}")
Expand All @@ -127,19 +146,7 @@ def _init_old(self, key, msg, digestmod):
self._inner = digest_cons()
self.digest_size = self._inner.digest_size

if hasattr(self._inner, 'block_size'):
blocksize = self._inner.block_size
if blocksize < 16:
warnings.warn(f"block_size of {blocksize} seems too small; "
f"using our default of {self.blocksize}.",
RuntimeWarning, 2)
blocksize = self.blocksize # pragma: no cover
else:
warnings.warn("No block_size attribute on given digest object; "
f"Assuming {self.blocksize}.",
RuntimeWarning, 2)
blocksize = self.blocksize # pragma: no cover

blocksize = _select_blocksize(self._inner, self.blocksize)
if len(key) > blocksize:
key = digest_cons(key).digest()

Expand Down Expand Up @@ -272,7 +279,7 @@ def _compute_digest_fallback(key, msg, digest):
raise ValueError(f"unsupported hash algorithm {digest}")
inner = digest_cons()
outer = digest_cons()
blocksize = getattr(inner, 'block_size', 64)
blocksize = _select_blocksize(inner, HMAC.blocksize)
if len(key) > blocksize:
key = digest_cons(key).digest()
key = key.ljust(blocksize, b'\0')
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-09-14-26-48.gh-issue-155428.wi0_fI.rst
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@@ -0,0 +1,3 @@
Ensure that :func:`hmac.digest` and :meth:`hmac.HMAC.digest` output
identical values when using a digest's block size smaller than 16. Patch by
Bénédikt Tran.
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