Code
public class ProSecurity
{
    private SymmetricAlgorithm mobjCryptoService;
    /// <summary>
    /// 。
    /// </summary>
    public ProSecurity()
    {
        mobjCryptoService = new RijndaelManaged();
    }
    private byte[] GetLegalKey(string Key)
    {
        string sTemp = Key;
        mobjCryptoService.GenerateKey();
        byte[] bytTemp = mobjCryptoService.Key;
        int KeyLength = bytTemp.Length;
        if (sTemp.Length > KeyLength)
            sTemp = sTemp.Substring(0, KeyLength);
        else if (sTemp.Length < KeyLength)
            sTemp = sTemp.PadRight(KeyLength, ' ');
        return ASCIIEncoding.ASCII.GetBytes(sTemp);
    }
    private byte[] GetLegalIV(string Key)
    {
        string sTemp = Key;
        mobjCryptoService.GenerateIV();
        byte[] bytTemp = mobjCryptoService.IV;
        int IVLength = bytTemp.Length;
        if (sTemp.Length > IVLength)
            sTemp = sTemp.Substring(0, IVLength);
        else if (sTemp.Length < IVLength)
            sTemp = sTemp.PadRight(IVLength, ' ');
        return ASCIIEncoding.ASCII.GetBytes(sTemp);
    }
    /// <summary>
    /// 對稱加密方法。
    /// <para></para>
    /// <para>例外狀況:</para>
    /// 沒有指定加密的字符以及偏移量。
    /// </summary>
    /// <param name="Source"></param>
    /// <returns></returns>
    public string Encrypto(string Source, string vKey)
    {
        byte[] bytIn = UTF8Encoding.UTF8.GetBytes(Source);
        MemoryStream ms = new MemoryStream();
        mobjCryptoService.Key = GetLegalKey(vKey);
        mobjCryptoService.IV = GetLegalIV(vKey);
        ICryptoTransform encrypto = mobjCryptoService.CreateEncryptor();
        CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Write);
        cs.Write(bytIn, 0, bytIn.Length);
        cs.FlushFinalBlock();
        ms.Close();
        byte[] bytOut = ms.ToArray();
        return Convert.ToBase64String(bytOut);
    }
    /// <summary>
    /// 對稱解密方法。
    /// <para></para>
    /// <para>例外狀況:</para>
    /// 要解密的字符不滿足8位元不帶正負號的整數陣列。
    /// </summary>
    /// <param name="Source"></param>
    /// <returns></returns>
    public string Decrypto(string Source, string vKey)
    {
        byte[] bytIn = Convert.FromBase64String(Source);
        MemoryStream ms = new MemoryStream(bytIn, 0, bytIn.Length);
        mobjCryptoService.Key = GetLegalKey(vKey);
        mobjCryptoService.IV = GetLegalIV(vKey);
        ICryptoTransform encrypto = mobjCryptoService.CreateDecryptor();
        CryptoStream cs = new CryptoStream(ms, encrypto, CryptoStreamMode.Read);
        StreamReader sr = new StreamReader(cs);
        return sr.ReadToEnd();
    }
}