获取 Base-64 字符串中的无效字符

我有一个 16 字节的十六进制密钥"F81AFDEA26D680BF"和一个 16 字节的十六进制加密文本，作为"3508D26A7064CF68"。我需要使用 DES 来解密上面的文本。我收到错误"Base-64 字符串中的字符无效"。我使用的代码是

static byte[] bytes = Encoding.ASCII.GetBytes(KeyHexAscii("F81AFDEA26D680BF"));
public static string Decrypt(string cryptedString)
    {
        if (String.IsNullOrEmpty(cryptedString))
        {
            throw new ArgumentNullException("The string which needs to be decrypted can not be null.");
        }
        DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
        MemoryStream memoryStream = new MemoryStream(Convert.FromBase64String(cryptedString));
        CryptoStream cryptoStream = new CryptoStream(memoryStream,  cryptoProvider.CreateDecryptor(bytes, bytes), CryptoStreamMode.Read);
        StreamReader reader = new StreamReader(cryptoStream);
        return reader.ReadToEnd();
    }
  public static string Encrypt(string originalString)
    {
        if (String.IsNullOrEmpty(originalString))
        {
            throw new ArgumentNullException("The string which needs to be encrypted can not be null.");
        }
        DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
        MemoryStream memoryStream = new MemoryStream();
        CryptoStream cryptoStream = new CryptoStream(memoryStream, cryptoProvider.CreateEncryptor(bytes, bytes), CryptoStreamMode.Write);
        StreamWriter writer = new StreamWriter(cryptoStream);
        writer.Write(originalString);
        writer.Flush();
        cryptoStream.FlushFinalBlock();
        writer.Flush();
        return Convert.ToBase64String(memoryStream.GetBuffer(), 0, (int)memoryStream.Length);
    }