openssl library: aes-256-cbc vs aes-256-cbc-hmac-sha256

Question

The openssl_get_cipher_methods() function returns, within the list of available algorithms, both aes-256-cbc and aes-256-cbc-hmac-sha256 so I was trying to understand how to use them with the openssl_encrypt/decrypt() functions but I see no difference.

php > echo openssl_encrypt('hello world!', 'aes-256-cbc', '5823r4s5b36f45f333c7be91e6819b91', 0, base64_decode('7urdW9TS74FKFgHFSb2sqA=='), $tag);
JmeNeIeK2yU3pVUMGX5VsQ==
php > echo $tag;
php > echo openssl_encrypt('hello world!', 'aes-256-cbc-hmac-sha256', '5823r4s5b36f45f333c7be91e6819b91', 0, base64_decode('7urdW9TS74FKFgHFSb2sqA=='), $tag);
JmeNeIeK2yU3pVUMGX5VsQ==
php > echo $tag;
php > 

So, what's going on? Am I missing something?

Answer 1

aes-256-cbc-hmac-sha256 is a specialist cipher exposed by libcrypto for use in TLS by libssl. It is not meant for general purpose use. Don't use it unless you really know what you are doing.

The man page says this about it:

Authenticated encryption with AES in CBC mode using SHA256 (SHA-2, 256-bits) as HMAC, with keys of 128 and 256 bits length respectively. The authentication tag is 256 bits long.

WARNING: this is not intended for usage outside of TLS and requires calling of some undocumented ctrl functions. These ciphers do not conform to the EVP AEAD interface.

https://www.openssl.org/docs/man3.0/man3/EVP_aes_256_cbc_hmac_sha256.html