You can turn it into a list:

iex(11)> "Like this 😀" |> Base.encode64
"TGlrZSB0aGlzIPCfmIA="
iex(12)> "TGlrZSB0aGlzIPCfmIA=" |> Base.decode64!() |> :binary.bin_to_list()
[76, 105, 107, 101, 32, 116, 104, 105, 115, 32, 240, 159, 152, 128]

I already told you a few times, the documentation of that library is far from par.

Also @chrismcg told you to try Base.url_decode64!/1, which you obviously didn’t, as it works for me:

iex(1)> message = "Foo"
"Foo"
iex(2)> {:ok, key64} = ExCrypto.generate_aes_key(:aes_256, :base64)
{:ok, "YBFgCMzhnxIOVeirWf5hgzbFmUcvZcHY0yKRJJ4Ocks="}
iex(3)> key = Base.url_decode64!(key64)
<<96, 17, 96, 8, 204, 225, 159, 18, 14, 85, 232, 171, 89, 254, 97, 131, 54, 197,
  153, 71, 47, 101, 193, 216, 211, 34, 145, 36, 158, 14, 114, 75>>
iex(4)> {:ok, {iv, ciphered}} = ExCrypto.encrypt(key, message)
{:ok,
 {<<195, 207, 172, 38, 128, 248, 158, 10, 14, 239, 231, 195, 48, 154, 244, 177>>,
  <<4, 238, 11, 86, 7, 6, 5, 86, 13, 182, 189, 113, 193, 85, 194, 141>>}}
iex(5)> {:ok, clear} = ExCrypto.decrypt(key, iv, ciphered)
{:ok, "Foo"}

It seems as if you really need to actively read and understand the sources of this library to be able to use it.

My suggestion is, that you use :bytes with the key generator and then convert the resulting binary to exactly the format you really need. No surprises anymore because of bad documentation, that claims to return base64 encoded string but actually does return an Base64-URL-encoded string…

As there is no native kind of array in elixir, you can’t.

But at least there is the Base module, which allows you to decode base64 encoded stuff to a binary.