Use EVP_aes_128_ctr() on OpenSSL >= 1.0.1
Starting with OpenSSL 1.0.1, there is an EVP value to provide counter-mode AES. We definitely want to be using this one for our counter mode implementation in aes.c, since it appears to automatically use the best available AES implementation, including tricky ones that use AESNI/bitsliced/vectorized implementations.
Bitsliced and vectorized implementations of counter mode are not only faster than the straightforward assembly versions, but also run in constant-time.
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