encryption : RSA algorithm

Question

I am implementing the RSA algorithm for encryption and decryption as given here:

http://www.di-mgt.com.au/rsa_alg.html

But could not understand the random pr

Answer 1

Following the practical notes at the end of the linked page you would arrive at something like this for the prime generation:

unsigned int get_prime(int e)
{
    while (true)
    {
        unsigned int suspect = 1 + (unsigned int)(65535.0 * rand() / (RAND_MAX + 1.0));
        suspect &= 0x0000FFFF; // make sure only the lower 16bit are set
        suspect |= 0xC001; // set the two highest and the lowest bit
        while (!test_prime(suspect))
        {
            suspect += 2;
        }
        if (suspect < 65536 && gcd(e, suspect - 1) == 1)
            return suspect;
    }
}

test_prime is supposed to be an implementation of the Miller-Rabin test. The function above makes certain assumptions and has some drawbacks:

• int is 32 bit
• RAND_MAX is larger than 65536
• rand() is usually not a good random number generator to use for serious encryption
• The generated primes are 16bit so obviously not large enough for serious encryption anyway

Don't use this in any production code.

According to the article it seems ok to choose e fixed.