如何使用特定的密钥解密加密的字符串

Question

First of all this is my code so far.

public Form1()
{
    InitializeComponent();
    desObj = Rijndael.Create();
}

string cData;
byte[] cBytes;
byte[] pBytes;
byte[] pBytes2;
byte[] pKey;
SymmetricAlgorithm desObj;

public static string InputBox(string title, string promptText)
{
    Form form = new Form();
    Label label = new Label();
    TextBox textBox = new TextBox();
    Button buttonOk = new Button();
    Button buttonCancel = new Button();

    form.Text = title;
    label.Text = promptText;

    buttonOk.Text = "OK";
    buttonCancel.Text = "Cancel";
    buttonOk.DialogResult = DialogResult.OK;
    buttonCancel.DialogResult = DialogResult.Cancel;

    label.SetBounds(9, 20, 372, 13);
    textBox.SetBounds(12, 36, 372, 20);
    buttonOk.SetBounds(228, 72, 75, 23);
    buttonCancel.SetBounds(309, 72, 75, 23);

    label.AutoSize = true;

    textBox.MaxLength = 16;
    textBox.Anchor = textBox.Anchor | AnchorStyles.Right;
    buttonOk.Anchor = AnchorStyles.Bottom | AnchorStyles.Right;
    buttonCancel.Anchor = AnchorStyles.Bottom | AnchorStyles.Right;

    form.ClientSize = new Size(396, 107);
    form.Controls.AddRange(new Control[] { label, textBox, buttonOk, buttonCancel });
    form.ClientSize = new Size(Math.Max(300, label.Right + 10), form.ClientSize.Height);
    form.FormBorderStyle = FormBorderStyle.FixedDialog;
    form.StartPosition = FormStartPosition.CenterScreen;
    form.MinimizeBox = false;
    form.MaximizeBox = false;
    form.AcceptButton = buttonOk;
    form.CancelButton = buttonCancel;
    DialogResult dR = form.ShowDialog();
    string test = textBox.Text;
    MessageBox.Show("test: "+test);
    if (test.Length != 16)
    {
        int pKey2NeededLength = 0;
        for (int i = 0; i < (test.Length + 16); i++)
        {
            if ((i + test.Length) == 16)
            {
                pKey2NeededLength = i;
                MessageBox.Show("pKey2NeededLength: "+pKey2NeededLength);
                break;
            }
        }
        StringBuilder sB = new StringBuilder();
        string[] pArray = { "1", "12", "123", "1234", "12345", "123456", "1234567", "12345678", "123456789", "1234567891", "12345678912", "123456789123", "1234567891234", "12345678912345", "123456789123456", "1234567891234567" };
        sB.Append(test + pArray[pKey2NeededLength - 1]);
        test = sB.ToString();
        MessageBox.Show("test(after sB): "+test);
    }
    return test;
}

private void button1_Click(object sender, EventArgs e)//ENCRYPT
{
    cData = richTextBox1.Text;
    pBytes = Encoding.ASCII.GetBytes(cData);
    string pKey2 = InputBox("Encryption-Key", "Enter a Encryption-Key:");
    pKey = Encoding.ASCII.GetBytes(pKey2);
    desObj.Key = pKey;
    desObj.Mode = CipherMode.CBC;
    desObj.Padding = PaddingMode.PKCS7;
    System.IO.MemoryStream mS = new System.IO.MemoryStream();
    CryptoStream cS = new CryptoStream(mS, desObj.CreateEncryptor(), CryptoStreamMode.Write);
    cS.Write(pBytes, 0, pBytes.Length);
    cS.Close();
    cBytes = mS.ToArray();
    mS.Close();
    richTextBox1.Text = Encoding.ASCII.GetString(cBytes);
}

private void button2_Click(object sender, EventArgs e)//DECRYPT
{
    string pKey3 = InputBox("Decryption-Key", "Enter a Decryption-Key:");
    MessageBox.Show("pKey3: "+pKey3);
    byte[] pBytes3 = Encoding.ASCII.GetBytes(pKey3);
    MessageBox.Show("pBytes3: "+pBytes3);
    System.IO.MemoryStream mS1 = new System.IO.MemoryStream(pBytes3/*cBytes*/);
    CryptoStream cS1 = new CryptoStream(mS1, desObj.CreateDecryptor(), CryptoStreamMode.Read);
    cS1.Read(pBytes3/*cBytes*/, 0, pBytes3/*cBytes*/.Length);
    pBytes2 = mS1.ToArray();
    cS1.Close();
    mS1.Close();
    richTextBox1.Text = Encoding.ASCII.GetString(pBytes2);
}

Now what i'm trying to do: My programm has 1 richTextBox and 2 buttons one button is for encrypting the text of the textbox and the other one for decrypting it. So if you press the encrypt-button you are asked for a Key(maximum characters: 16), if your entered key has a lower number of characters than 16, it gets filled up, fe "test" becomes "test123456789123". The encrypting part workes so far, but i'm having troubles with decrypting it. if you press the decrypting-button you are asked to enter a key too, and it gets also filled up, so test becomes the same 16character-key like above. But after i have entered "test" it gives me the following error:

An unhandled exception of type 'System.Security.Cryptography.CryptographicException' occurred in mscorlib.dll

Additional information: Character spacing are invalid and can not be removed .

Hopefully somebody knows what to do! Thanks in advance :)

EDIT: So this works decrypting the encrypted text:

private void button2_Click(object sender, EventArgs e)//DECRYPT
{
    //string pKey3 = InputBox("Decryption-Key", "Enter a Decryption-Key:");
    System.IO.MemoryStream mS1 = new System.IO.MemoryStream(cBytes);
    CryptoStream cS1 = new CryptoStream(mS1, desObj.CreateDecryptor(), CryptoStreamMode.Read);
    cS1.Read(cBytes, 0, cBytes.Length);
    pBytes2 = mS1.ToArray();
    cS1.Close();
    mS1.Close();
    string garbage = Encoding.ASCII.GetString(pBytes2);
    string decrypt = garbage.Substring(0, pBytes.Length);
    richTextBox1.Text = decrypt;
}

but this only works directly after formating and not when i close the programm and open it again, so i want to get the text in the textbox and decrypt it after the string that is entered as decryption key in this case pKey3 !

Answer 1

The code above doesn't store and then restore the IV value. The IV value will be randomized upon first use of the desObj . So if the ciphertext is directly decrypted with the same object instance the IV will still be set correctly. If you re-instantiate the object it won't be. Usually the IV is stored together with the ciphertext.

Furthermore, the above code seems to store the cihpertext as ASCII. Ciphertext bytes may have any value, including the bottom 32 unprintable ones and value 127 which is reserved. Instead the ciphertext should be encoded as base 64. Information may be lost when decoding the bytes to ASCII.

Notes:

• you should use a PBKDF such as PBKDF2 to derive a key from a password. Directly using a password as key is not secure
• single DES or double key triple DES is not considered secure anymore, better upgrade to AES and think about adding an authentication tag (MAC) over your IV and ciphertext

Answer 2

I noticed on first look that you get password during decryption but you use it as content in MemoryStream. Because you have password already set in encryption method, it looks like that you tried decrypt entered password with encryption password.