[lwip-users] [lwip] Module for secure TCP sequence number generation

[Paul Sheer]

Here is some code that uses a Galois Field as a hash
function. These envolve calculation of ((g**n) mod p),
and are hence very slow. but the code is really simple
and small. The ISN algorithm is very close to Linux's.

comments welcome

--

Summary:

extern u32_t MHz_clock_counter;

static u32_t qe2 (u32_t x, u32_t y, u32_t n)
{
    long long s = 1, t = x, u = y;
    while (u) {
        if (u & 1) s = (s * t) % n;
        u >>= 1;
        t = (t * t) % n; }
    return (u32_t) s;
}

#define hash1(x)        qe2 (MY_G1, (x), MY_PRIME1)
#define hash2(x)        qe2 (MY_G2, (x), MY_PRIME2)
#define hash3(x)        qe2 (MY_G3, (x), MY_PRIME3)

/* local_addr ignored for now */
u32_t tcp_next_iss (u16_t port1, u16_t port2, u32_t local_addr,
                    u32_t remote_addr)
{
    static u32_t last_clock = 0, local_clock = 0, spurt_clock =
        0, random_increment = 0;
    u32_t clock = MHz_clock_counter;
    local_clock += clock - last_clock;
    spurt_clock += clock - last_clock;
    last_clock = clock;
    if (spurt_clock > (random_increment >> 7)) {
        local_clock += random_increment & 0xFFFFFF;
        spurt_clock = 0;
        random_increment = hash1 (clock); }
    return local_clock + (hash2 ((u32_t) ((u32_t) port1 << 16) | port2)
                          ^ hash3 (remote_addr) ^ PER_HOST_RANDOM_KEY);
}

-paul

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`
end


Paul Sheer Consulting IT Services . . Tel . . . +27 (0)21 6869634
Email . . . address@hidden . . . . . . Pager . . . 088 0057245
Linux development, cryptography, recruitment,  support,  training
http://www.icon.co.za/~psheer . . . . http://rute.sourceforge.net
L I N U X . . . . . . . . . . . .  The Choice of a GNU Generation

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