Convert base64 string to byte string

Questions & Help Questions
8

My Code:
message = "A very important message."
{:ok, aes_256_key} = ExCrypto.generate_aes_key(:aes_256, :base64)
{:ok, {init_vec, cipher_text}} = ExCrypto.encrypt(aes_256_key, message)
{:ok, val} = ExCrypto.decrypt(aes_256_key, init_vec, cipher_text)

Get error blew:
(exit) an exception was raised:
** (MatchError) no match of right hand side value: {:error, "IV must be exactly 128 bits and key must be exactly 128, 192 or 256 bits"}