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Wechat API wrapper in Elixir.

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wechat lib plug crypto wechat_message_encryptor.ex
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lib/plug/crypto/wechat_message_encryptor.ex 

defmodule Plug.Crypto.WechatMessageEncryptor do

  @moduledoc """

  Encrypt and decrypt wechat messages.

  """



  def decrypt(encrypted, encoding_aes_key) do

    plain_text =

      encrypted

      |> Base.decode64!

      |> decrypt_aes(encoding_aes_key)

      |> decode_padding



    # random(16B) + msg_len(4B) + msg + appid

    <<_ :: binary-size(16),

      msg_len :: integer-size(32),

      msg :: binary-size(msg_len),

      appid :: binary>> = plain_text



    {appid, msg}

  end



  defp decrypt_aes(encrypted, encoding_aes_key) do

    aes_key = Base.decode64!(encoding_aes_key <> "=")

    iv = binary_part(aes_key, 0, 16)

    :crypto.block_decrypt(:aes_cbc, aes_key, iv, encrypted)

  end



  defp decode_padding(padded_text) do

    len = byte_size(padded_text)

    <<pad :: utf8>> = binary_part(padded_text, len, -1)

    case pad < 1 or pad > 32 do

      true -> binary_part(padded_text, 0, len)

      false -> binary_part(padded_text, 0, len - pad)

    end

  end

end