Imagine that an attacker knows:
- A correct ciphertext.
- The algorithm: AES-256-CBC.
- The implementation: openssl CLI.
- The IV.
- The HMAC-SHA-256 of the ciphertext.
But does not know the key (let's assume the key was chosen in a sane way and is not derived, but full 256-bit). Also, this is the only ciphertext associated with this key.
There is no server. Is this enough to be vulnerable to the padding oracle attack? Meaning: can an attacker get the plaintext?