crypto.ex

defmodule Crypto do
  def encrypt(data, key) when is_binary(data) and is_binary(key) do
    iv = :crypto.strong_rand_bytes(16)
    cipher = :crypto.block_encrypt(:aes_cbc256, hash_key(key), iv, pad(data))

    Base.encode64("#{iv}#{cipher}")
  end

  def decrypt(digest, key) when is_binary(digest) and is_binary(key) do
    text = Base.decode64!(digest)

    iv = Kernel.binary_part(text, 0, 16)
    cypher = Kernel.binary_part(text, 16, 16)

    :aes_cbc256
    |> :crypto.block_decrypt(hash_key(key), iv, cypher)
    |> unpad()
  end

  defp hash_key(key) do
    :crypto.hash(:sha256, key)
  end

  # pad data using PKCS#5
  defp pad(msg) do
    bytes_remaining = rem(byte_size(msg), 16)
    padding_size = 16 - bytes_remaining
    msg <> :binary.copy(<<padding_size>>, padding_size)
  end

  defp unpad(msg) do
    padding_size = :binary.last(msg)
    if padding_size <= 16 do
      msg_size = byte_size(msg)
      if binary_part(msg, msg_size, -padding_size) == :binary.copy(<<padding_size>>, padding_size) do
        {:ok, binary_part(msg, 0, msg_size - padding_size)}
      else
        :error
      end
    else
      :error
    end
  end
end

data = "yes"
key = "secret"

data
|> Crypto.encrypt(key)
|> IO.inspect()
|> Crypto.decrypt(key)
|> IO.inspect()