`hmac.digest()` and `HMAC(...).digest()` disagree when blocksize is smaller than 16

Bug report

Bug description:

hmac.digest() and HMAC(...).digest() return DIFFERENT MACs for any digest whose block_size is below 16, because hmac._compute_digest_fallback reads block_size raw while hmac.HMAC._init_old clamps it to 64.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-155429