/* Creates an odd BigInt with the specified number of digits.
* Returns it by reference in the "number" parameter. */
void PrimeGenerator::makePrimeCandidate(BigInt &number,
unsigned long int digitCount)
{
PrimeGenerator::MakeRandom(number, digitCount);
//make the number odd
if (!number.IsOdd())
number.SetDigit(0, number.GetDigit(0) + 1);
//make sure the leading digit is not a zero
if (number.GetDigit(number.Length() - 1) == 0)
number.SetDigit(number.Length() - 1, (std::rand() % 9) + 1);
}
/* Transforms a BigInt cyphertext into a std::string cyphertext. */
string RSA::decode(const BigInt &message)
{
string decoded;
// The special symbol '1' we added to the beginning of the encoded message
// will now be positioned at message[message.Length() - 1], and
// message.Length() -1 must be divisible by 3 without remainder. Thus we
// can ignore the special symbol by only using digits in the range
// from message[0] to message[message.Length() - 2].
for (unsigned long int i(0); i < message.Length() / 3; i++)
{
// Decode the characters using the ASCII values in the BigInt digits.
char ASCII = 100 * char(message.GetDigit(i * 3));
ASCII += 10 * char(message.GetDigit(i * 3 + 1));
decoded.push_back(ASCII + char(message.GetDigit(i * 3 + 2)));
}
return decoded;
}
