power operation

[csss]

    private void decrypt_Click(object sender, EventArgs e)
    {
        v = Convert.ToDouble (txtencrypt.Text);            
        t = Math.Pow(v, d);   
        msg = Convert.ToInt16(Math.Pow(v,d) % Convert.ToDouble(n));
        txtdecrypt.Text = Convert.ToString(msg);
    }

 This is the code im doing for decryption in rsa algorithm but i got a problem in power operation.
 For a small power value,the power operation performed well but when im using highest value as a power i got exponential value as a output. 

[Mike Askew]

Can we have some example values for v, n and d?

[mshauny]

Please explain what exactly are using v,n and d for. are they IV's and Keys ?
Did you know you can use .NET building Encryption and Decryption algorithms ?

.NET RSA security

Hope you find it useful.

[tinstaafl]

try this, (long)Math.Pow(v, d)

[csss]

the value is
v=11
d=23
n=187

[Mike Askew]

When running those figures I recieve:
t = 8.95430243255237E+23
msg = 149

I presume the issue is coming from the result of t?

@tinstaafl using your suggestion just results in the two results above being flipped to negatives and values changed, i.e:
t = -9.22337203685478E+18
msg = -162

Tested using the code:

            double v = 11;
            double d = 23;
            double n = 187;
            double t = (long)Math.Pow(v, d);
            Console.WriteLine(t);
            Int16 msg = Convert.ToInt16(t % Convert.ToDouble(n));
            Console.WriteLine(Convert.ToString(msg));

Note the line Convert.ToInt16(t % Convert.ToDouble(n)); where I have removed the second power function and used the result of your variable t as your just getting the same value twice when it isn't needed.

[tinstaafl]

Using your original code and the numbers you gave, by changing t to decimal and casting the power function to decimal I get 895430243255237000000000 for t and 58 for msg

            double v = 11;
            decimal t = (decimal)Math.Pow(v, 23);
            double msg = Convert.ToInt16(t % Convert.ToDecimal(187));
            Console.WriteLine(Convert.ToString(msg)+" "+t.ToString());

[csss]

plain text is 88
cipher text is 11(i.e. "v" value)
d=23,n=187
but while doing the power operation with long data type,im getting the plain text as -162

[csss]

Thanks tinstaafl for your suggestion
im also getting the value 895430243255237000000000 but i need an exact values instead of 000000000. But the normal calculation the answer is 895430243255237372246531

[ddanbe]

Perhaps you could consider this.

[tinstaafl]

Good idea ddanbe, this produces the results you want:

First you have to be targeting atleast Framework 4, which means VS 2010 or later, then you have to add a reference to System.Numerics.dll, then you have include it using System.Numerics;

            BigInteger v = 11;
            BigInteger t = BigInteger.Pow(v,23);
            BigInteger msg = (BigInteger.Remainder(t,187));
            Console.WriteLine(Convert.ToString(msg)+" "+t.ToString());

I think the problem with using the Math.Pow method is, it only returns a double, which limits it's accuracy on really large number like this.

[csss]

Please give some other suggestion.

[ddanbe]

You could implement your own biginteger class if you don't have one. Google.
Examples:
http://www.codeproject.com/Articles/2728/C-BigInteger-Class
http://www.boyet.com/Articles/PiCalculator.html

[TnTinMN]

If you can live with the 28 digit accuracy of the decimal type for the result of the exponentiation, then you can implement your own power function.

    private void test()
    {
        Int32 v = 11;
        Int32 d = 23;
        Int32 n = 187;

        decimal exp = DecPower(v, d);

        decimal m = decimal.Remainder(exp, n);
        string msg = m.ToString();

    }

    private decimal DecPower(decimal Num, Int32 power)
    {
        decimal ret = 1;
        for (Int32 i = 1; i <= power; i++)
        {
            ret = decimal.Multiply(ret, Num);
        }
        return ret;
    }