I am getting this error: System.OverflowException: 'The value is not a number.' I was under the impression that big integer could store any sized value (500 ^ 500 in this case), so I don't understand why this is happening.
public int decode(int code)
{
int totient = (p - 1) * (q - 1);
int d = modInverse(e, totient);
int pq = p * q;
BigInteger decodedMessage = new BigInteger(Math.Pow(code, d) % pq);
return (int)decodedMessage;
}
BigInteger decodedMessage = new BigInteger(Math.Pow(code, d) % pq);
Well, Math.Pow(code, d) % pq is not a BigInteger, it's an expression of type double. Converting the result to a BigInteger won't have an effect until the computation is complete (and has overflowed).
Math.Pow can easily overflow to Double.PositiveInfinity with large numbers, and Double.PositiveInfinity % someNumber yields Double.NaN. Calling new BigInteger(Double.NaN) yields the error you have described.
You need to do the computation in BigInteger. Fortunately, there's a method for exactly that purpose (BigInteger.ModPow):
BigInteger decodedMessage = BigInteger.ModPow(code, d, pq);
(BigInteger.ModPow requires BigInteger parameters, but there are implicit conversions from int to BigInteger.)
It seems that pq is 0, next throws the same exception:
new BigInteger(0.0 % 0);
Division of double by zero results in Double.NaN which is not a valid value to instantiate a BigInteger according to docs:
Exceptions
OverflowException
valueis NaN, NegativeInfinity, or PositiveInfinity.
Or, as @Heinzi correctly mentioned in comments Math.Pow results in Infinity.