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> This is not time independent. Is it an issue?
You're correct, the length check does leak the length of the expected digest as a performance enhancement (otherwise, your comparison runtime is bounded by the length of the attackers input).
Generally, exposing the length and thereby potentially the underlying cryptographic hash function (eg. 20 bytes -> hmac-sha1) is not considered a security risk for this type of scenario, whereas leaking key material certainly is. I considered including this nuance in the documentation and probably should.
> It's better to write isinstance(a, bytes). You should raise a
> TypeError if a is not a bytes or str.
Ack, thanks.