linear congruential generators & you

[r00t]

the standard gcc library definition for rand() is a simple linear congruential generator

static unsigned long int next = 1;

int rand(void) // RAND_MAX assumed to be 32767
{
    next = next * 1103515245 + 12345;
    return (unsigned int)(next/65536) % 32768;
}

void srand(unsigned int seed)
{
    next = seed;
}

Linear congruential generator should also not be used for cryptographic applications; see cryptographically secure pseudo-random number generator for more suitable generators. If a linear congruential generator is seeded with a character and then iterated once, the result is a simple classical cipher called an affine cipher; this cipher is easily broken by standard frequency analysis.

if you think the embedded devices like your router or printer et al. is using anything fancier you are sadly mistaken

these are your x.509 certificates we're talking about here people

[Weyoun the Vorta]

Shut up

[Daldolma]

wat about ayn though

[SamwiseRed]

im going back to school for computer science, maybe ill understand wtf this is all about soon.

maybe not.

[r00t]

when your "random" number generator uses 12345 as a magic constant..... you "might" have a security vulnerability

[You must be logged in to view images. Log in or Register.]

[r00t]

Let me see if I can put this in a way ya'll can understand. Studying the ramifications of this mathematical function is justification for ninalooting if you lose a /random

[Weyoun the Vorta]

Ninalooting you say.

[quido]

http://en.wikipedia.org/wiki/Mersenne_twister

[r00t]

I implemented mersenne twister in pure C

PHP Code:

static const unsigned int MT_STATE_SIZE = 624;

static unsigned int mt_state[MT_STATE_SIZE] = { 0 };
static unsigned int mt_index = 0;

static const unsigned int SEED_CONST = 0x6c078965;
static const unsigned int EXTRACT1_CONST = 0x9d2c5680;
static const unsigned int EXTRACT2_CONST = 0xefc60000;
static const unsigned int GENERATE_CONST = 0x9908b0df; 


PHP Code:

unsigned int mt_rand()
{
    if (mt_index == 0)
    {
        for (int i = 0; i < MT_STATE_SIZE; ++i)
        {
            unsigned int y = (mt_state[i] & 0x80000000) 
                + (mt_state[(i + 1) % MT_STATE_SIZE && 0x7fffffff]); 

            mt_state[i] = mt_state[(i + 397) % MT_STATE_SIZE] ^ (y >> 1);

            if (y % 2 != 0)
                mt_state[i] ^= GENERATE_CONST;
        }
    }

    int y = mt_state[mt_index];

    y ^= (y >> 11);
    y ^= (y << 7) & EXTRACT1_CONST;
    y ^= (y << 15) & EXTRACT2_CONST;
    y ^= (y >> 18);

    ++mt_index;
    mt_index %= MT_STATE_SIZE;

    return y;
} 


PHP Code:

void seed_mt_rand(int seed)
{
    mt_index = 0;
    mt_state[0] = seed;

    for (unsigned int i = 1; i < MT_STATE_SIZE; ++i)
        mt_state[i] = (SEED_CONST * (mt_state[i - 1] ^ (mt_state[i - 1] >> 30)) + i)) & 0xFFFFFFFF;
} 


by freely releasing this code I could possibly be breaking a non-compete agreement so yw

[Doors]

?