我用的是官方给的C#示例代码Cryptography.cs中的AES_decrypt解密方法，传进来的Input="D7yzvUNAL930rd28wf21s4hvXhz0L6Uit/p2Di6C5DHyYGpEgdBRnKjBec34JwoQXicwaZC7fOVihW80F4VtsdvE//1vr7oAbqjDv8KenVp+ajKYpJnyQ4zMRhIC+a31fCVOMC03FfzV/QuC94kBP55a+Za3sJgvAn+ZbsNqZI5DkyuzkhQN8OBqCzFhizGmy0xpM0MEA4agpvE+RuNO1rhHTtuJB5yltw1FiYzecSXJ+y/D2r81VkRn2eYjh2ltsoyfbDR7Is6ookXIFxTfyeNyeHxMeT4KN5WpCDmSbTcUYrbBUlkGLJ9n/rU8YOywma6G7aTb7KZKOqCxgfoUlYEZPk4FUL/TK7ShriFDMVCLyjQJ15Ob++agDWtcxhfUfe6HIoIpRW8mKNBQiY/Jd1svvuskA1wLef1RTtzKfMpagSCX/laZINmdnX4zrF6kIaR9P4xQrWXbsTFHTYe0Mg=="

这个是官方文档中的消息推送介绍里的示例包体

EncodingAESKey="1223334444555556666667777777888888887777777"

public static string AES_decrypt(String Input, string EncodingAESKey, ref string appid)

{

    byte[] Key;

    Key = Convert.FromBase64String(EncodingAESKey + "=");

    byte[] Iv = new byte[16];

    Array.Copy(Key, Iv, 16);

    byte[] btmpMsg = AES_decrypt(Input, Iv, Key);

    int len = BitConverter.ToInt32(btmpMsg, 16);

    len = IPAddress.NetworkToHostOrder(len);

    byte[] bMsg = new byte[len];

    byte[] bAppid = new byte[btmpMsg.Length - 20 - len];

    Array.Copy(btmpMsg, 20, bMsg, 0, len);

    Array.Copy(btmpMsg, 20+len , bAppid, 0, btmpMsg.Length - 20 - len);

    string oriMsg = Encoding.UTF8.GetString(bMsg);

    appid = Encoding.UTF8.GetString(bAppid);

    return oriMsg;

}

代码走到byte[] bAppid= new byte[btmpMsg.Length - 20 - len]时btmpMsg.Length=352，len=1273545308。

然后捕获异常：System.OverflowException:“Arithmetic operation resulted in an overflow.”

此异常最初是在此调用堆栈中引发的: 

    Tencent.Cryptography.AES_decrypt(string, string, ref string) - 位于 Cryptography.cs

    Tencent.WXBizMsgCrypt.DecryptMsg(string, string, string, string, ref string) - 位于 WXBizMsgCrypt.cs