public static string Encrypt(string plainText, string AESKey)
{
    RijndaelManaged rijndaelCipher = new RijndaelManaged();
    byte[] inputByteArray = Encoding.UTF8.GetBytes(plainText);//得到需要加密的字节数组 
    rijndaelCipher.Key = Convert.FromBase64String(AESKey);//加解密双方约定好密钥：AESKey
    rijndaelCipher.GenerateIV();
    byte[] keyIv = rijndaelCipher.IV;
    byte[] cipherBytes = null;
    using (MemoryStream ms = new MemoryStream())
    {
        using (CryptoStream cs = new CryptoStream(ms, rijndaelCipher.CreateEncryptor(), CryptoStreamMode.Write))
        {
            cs.Write(inputByteArray, 0, inputByteArray.Length);
            cs.FlushFinalBlock();
            cipherBytes = ms.ToArray();//得到加密后的字节数组
            cs.Close();
            ms.Close();
        }
    }
    var allEncrypt = new byte[keyIv.Length + cipherBytes.Length];
    Buffer.BlockCopy(keyIv, 0, allEncrypt, 0, keyIv.Length);
    Buffer.BlockCopy(cipherBytes, 0, allEncrypt, keyIv.Length * sizeof(byte), cipherBytes.Length);
    return Convert.ToBase64String(allEncrypt);
}

public static string Decrypt(string showText, string AESKey)
{
    string result = string.Empty;
    try
    {
        byte[] cipherText = Convert.FromBase64String(showText);
        int length = cipherText.Length;
        SymmetricAlgorithm rijndaelCipher = Rijndael.Create();
        rijndaelCipher.Key = Convert.FromBase64String(AESKey);//加解密双方约定好的密钥
        byte[] iv = new byte[16];
        Buffer.BlockCopy(cipherText, 0, iv, 0, 16);
        rijndaelCipher.IV = iv;
        byte[] decryptBytes = new byte[length - 16];
        byte[] passwdText = new byte[length - 16];
        Buffer.BlockCopy(cipherText, 16, passwdText, 0, length - 16);
        using (MemoryStream ms = new MemoryStream(passwdText))
        {
            using (CryptoStream cs = new CryptoStream(ms, rijndaelCipher.CreateDecryptor(), CryptoStreamMode.Read))
            {
                cs.Read(decryptBytes, 0, decryptBytes.Length);
                cs.Close();
                ms.Close();
            }
        }
        result = Encoding.UTF8.GetString(decryptBytes).Replace("\0", "");   ///将字符串后尾的'\0'去掉
    }
    catch { }
    return result;
}

string jiaMi = MyAESTools.Encrypt(textBox1.Text, "abcdefgh12345678abcdefgh12345678");

string jieMi = MyAESTools.Decrypt(textBox3.Text, "abcdefgh12345678abcdefgh12345678");