System.Convert.FromBase64String C# (CSharp) Method

        public static extern byte[] FromBase64String(String s);
        

        public static string Decrypt(byte[] cipherText, byte[] IV)
        {
            // Check arguments.
            if (cipherText == null || cipherText.Length <= 0)
            {
                throw new ArgumentNullException("cipherText");
            }
            if (IV == null || IV.Length <= 0)
            {
                throw new ArgumentNullException("IV");
            }

            // Declare the string used to hold the decrypted text.
            string plaintext = null;

            // Create an Aes object with the specified key and IV.
            using (Aes aesAlg = Aes.Create())
            {
                aesAlg.Key = Convert.FromBase64String(_key);
                aesAlg.IV  = IV;

                // Create a decryptor to perform the stream transform.
                ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);

                // Create the streams used for decryption.
                using (MemoryStream msDecrypt = new MemoryStream(cipherText))
                    using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                        using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                        {
                            // Read the decrypted bytes from the decrypting stream and place them in a string.
                            plaintext = srDecrypt.ReadToEnd();
                        }
            }
            return(plaintext);
        }