Diffie-Hellman Key Exchange

dh.c

// The biggest 64bit prime
#define P 0xffffffffffffffc5ull
#define G 5

#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <stdlib.h>

// calc a * b % p , avoid 64bit overflow
static inline uint64_t
mul_mod_p(uint64_t a, uint64_t b) {
	uint64_t m = 0;
	while(b) {
		if(b&1) {
			uint64_t t = P-a;
			if ( m >= t) {
				m -= t;
			} else {
				m += a;
			}
		}
		if (a >= P - a) {
			a = a * 2 - P;
		} else {
			a = a * 2;
		}
		b>>=1;
	}
	return m;
}

static inline uint64_t
pow_mod_p(uint64_t a, uint64_t b) {
	if (b==1) {
		return a;
	}
	uint64_t t = pow_mod_p(a, b>>1);
	t = mul_mod_p(t,t);
	if (b % 2) {
		t = mul_mod_p(t, a);
	}
	return t;
}

// calc a^b % p
uint64_t
powmodp(uint64_t a, uint64_t b) {
	if (a > P)
		a%=P;
	return pow_mod_p(a,b);
}

uint64_t
randomint64() {
	uint64_t a = rand();
	uint64_t b = rand();
	uint64_t c = rand();
	uint64_t d = rand();
	return a << 48 | b << 32 | c << 16 | d;
}

static void
test() {
	uint64_t a = randomint64();
	uint64_t b = randomint64();
	uint64_t A = powmodp(G, a);
	uint64_t B = powmodp(G, b);
	uint64_t secret1 = powmodp(B,a);
	uint64_t secret2 = powmodp(A,b);
	assert(secret1 == secret2);
	printf("a=%I64x b=%I64x s=%I64x\n", a,b,secret1);
}

int
main() {
	int i;
	for (i=0;i<100;i++) {
		test();
	}

	return 0;
}

Just asking, I am including this code in our production servers. Did you find any obvious problems in it? (any inconsistencies with different architectures or something?)

BTW we don't need very high encryption, just the basic one.