Use of wrong group constant for calculating e in ECDSA_SIG_recover_key_GFp

[smlu]

There is a bug in public_key_impl::ECDSA_SIG_recover_key_GFp where the bit size of the group prime p is retrieved via call to EC_GROUP_get_degree. However, according to SEC1-v2 "4.1.4 Verifying Operation", part 3.2 , the bit size of the prime order n of the base point G should be used to compute e from M.
While this issue is pedantic, and in practice, the bit size of prime p and group order n for EC secp256r1 (P-256) is the same, it would be advisable to use EC_GROUP_order_bits instead. This becomes especially important for future cases if other ECs are supported where bitsize(n) != bitsize(p).