Symmetric Encryption
====================

**Functions**:

.. function:: SymEnc(key []byte, iv []byte, plaintext []byte) ([]byte)

    Encrypt the plaintext using AES-CBC mode with the provided key and IV.

    Returns the ciphertext, which will contain the IV (you do not need to store
    the IV separately).
    
    This function will panic if the plaintext is not a multiple of the AES block
    size, which is 16 bytes. You can use the constant
    ``userlib.AESBlockSizeBytes`` in your code.

    :param key: 16-byte symmetric key for encryption
    :type key: []byte
    :param iv: 16-byte initialization vector
    :type iv: []byte
    :param plaintext: Message to encrypt, length a multiple of 16 bytes
    :type plaintext: []byte

    :returns: Ciphertext, length a multiple of 16 bytes
    :rtype: []byte

.. function:: SymDec(key []byte, ciphertext []byte) ([]byte)

    Decrypt the ciphertext using the key.
    
    This function will panic if the ciphertext is not a multiple of the AES
    block size, which is 16 bytes. You can use the constant
    ``userlib.AESBlockSizeBytes`` in your code.

    :param key: 16-byte symmetric key for decryption
    :type key: []byte
    :param ciphertext: Message to decrypt, length a multiple of 16 bytes
    :type ciphertext: []byte

    :returns: Plaintext, length a multiple of 16 bytes
    :rtype: []byte

.. warning::
    Remember: one key, one purpose. If we use a key for HKDF or HMAC, we should
    not use the same key for symmetric encryption.