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Wechat API wrapper in Elixir.

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wechat lib wechat utils cipher.ex
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lib/wechat/utils/cipher.ex 

defmodule Wechat.Utils.Cipher do

  @moduledoc """

  Decrypt wechat msg.

  """



  def decrypt(msg_encrypt, aes_key) do

    plain_text =

      msg_encrypt

      |> Base.decode64!

      |> decrypt_aes(aes_key)

      |> decode_padding



    # http://mp.weixin.qq.com/wiki/2/3478f69c0d0bbe8deb48d66a3111ff6e.html

    # 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(aes_encrypt, aes_key) do

    iv = binary_part(aes_key, 0, 16)

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

  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