RC2 Decryption problem (lost of data)

Question

1.00/5 (1 vote)

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private void button1_Click_1(object sender, EventArgs e)
{
    string enc = rc("some string", "some key", true);
    MessageBox.Show(enc);
    string dec = rc(enc, "some key", false);
    MessageBox.Show(dec);
}

private string rc(string text, string key, bool enc)
{

    string plainText = text;
    byte[] buffer = ASCIIEncoding.ASCII.GetBytes(plainText);
    RC2CryptoServiceProvider rc2 = new RC2CryptoServiceProvider();
    rc2.Key = ASCIIEncoding.ASCII.GetBytes(key);
    rc2.GenerateIV();
    if (enc)
    {
        string encryptedString = Convert.ToBase64String(
        rc2.CreateEncryptor().TransformFinalBlock(
        buffer, 0, buffer.Length));
        return encryptedString;
    }
    else
    {
        buffer = Convert.FromBase64String(text);
        byte[] b_dec = rc2.CreateDecryptor().TransformFinalBlock(buffer, 0, buffer.Length);
        string decryptedString = ASCIIEncoding.ASCII.GetString(b_dec);
        return decryptedString;
    }
}

Why:
- encrypted string is different each time
- decrypted string is wrong(all characters before 'ing' are unreadable)
?
Please, help.

Posted 20-Nov-10 23:20pm

halabella

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3 solutions

Solution 3

I don't know much about RC2, but are you sure that it is not supposed to make a new encrypted string everytime? Probably not, but I've heard of it before.

Posted 21-Nov-10 0:54am

LloydA111

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halabella · Accepted Answer · 2010-11-21T00:24:00

Solution 1

rc2.GenerateIV() was needed to replace with something fixed as this:
rc2.IV = new byte[] { 23, 10, 99, 73, 44, 69, 35, 80 };

Posted 21-Nov-10 0:24am

halabella

CPallini · Accepted Answer · 2010-11-21T00:53:00

The problem is that the following line

halabella wrote:
rc2.GenerateIV();

every time is called generates a new random vector.

Quick fix:

Add the following class member variable:

byte [] iv =null;

then modify the function this way:

private string rc(string text, string key, bool enc)
{
    string plainText = text;
    byte[] buffer = ASCIIEncoding.ASCII.GetBytes(plainText);
    RC2CryptoServiceProvider rc2 = new RC2CryptoServiceProvider();
    rc2.Key = ASCIIEncoding.ASCII.GetBytes(key);
    if (iv == null)
    {
        rc2.GenerateIV();
        iv = rc2.IV;
    }
    else
    {
        rc2.IV = iv;
    }
    if (enc)
    {
        string encryptedString = Convert.ToBase64String(
        rc2.CreateEncryptor().TransformFinalBlock(
        buffer, 0, buffer.Length));
        return encryptedString;
    }
    else
    {
        buffer = Convert.FromBase64String(text);
        byte[] b_dec = rc2.CreateDecryptor().TransformFinalBlock(buffer, 0, buffer.Length);
        string decryptedString = ASCIIEncoding.ASCII.GetString(b_dec);
        return decryptedString;
    }
}

Of course you may find a more elegant soultion depending on your needs