Lemma hmac256drbgabs_reseed_Zlength_key:
forall a s additional_data,
Zlength (hmac256drbgabs_key a) = Z.of_nat SHA256.DigestLength ->
Zlength (hmac256drbgabs_key (hmac256drbgabs_reseed a s additional_data)) = Z.of_nat SHA256.DigestLength.