Support multiple cyclic groups for smaller subnets

We will choose the first one in the list that is large enough to support
the number of hosts chosen. This speeds up scanning small networks considerably:
e.g. /16s can be scanned in <200ms (+ cooldown)

src/cyclic.c

@@ -57,13 +57,48 @@
 #include "aesrand.h"
 
 #define LSRC "cyclic"
-#define PRIME 4294967311 // 2^32 + 15
-#define KNOWN_PRIMROOT 3
 
-// distinct prime factors of 2^32 + 15
-static const uint64_t psub1_f[] = { 2, 3, 5, 131, 364289 }; 
+typedef struct cyclic_group {
+	uint64_t prime;
+	uint64_t known_primroot;
+	size_t num_prime_factors;	// number of unique prime factors of (prime-1)
+	uint64_t prime_factors[10];	// unique prime factors of (prime-1)
+} cyclic_group_t;
 
-// selected primitive root that we'll use as the generator
+// We will pick the first cyclic group from this list that is
+// larger than the number of IPs in our whitelist. E.g. for an
+// entire Internet scan, this would be cyclic32
+// Note: this list should remain ordered by size (primes) ascending.
+static cyclic_group_t groups[] = {
+{ // 2^16 + 1
+	.prime = 65537,
+	.known_primroot = 3,
+	.prime_factors = {2},
+	.num_prime_factors = 1
+},
+{ // 2^24 + 43
+	.prime = 16777259,
+	.known_primroot = 2,
+	.prime_factors = {2, 23, 103, 3541},
+	.num_prime_factors = 4
+},
+{ // 2^28 + 3
+	.prime = 268435459,
+	.known_primroot = 2,
+	.prime_factors = {2, 3, 19, 87211},
+	.num_prime_factors = 4
+},
+{ // 2^32 + 15
+	.prime = 4294967311,
+	.known_primroot = 3,
+	.prime_factors = {2, 3, 5, 131, 364289},
+	.num_prime_factors = 5
+}
+};
+
+
+// selected prime/primitive root that we'll use as the generator
+static uint64_t prime = 0;
 static uint64_t primroot = 0;
 static uint64_t current = 0;
 
@@ -73,14 +108,14 @@ static uint64_t num_addrs = 0;
 #define NOT_COPRIME 0
 
 // check whether two integers are coprime
-static int check_coprime(uint64_t check)
+static int check_coprime(uint64_t check, const cyclic_group_t *group)
 {
-	for (unsigned i=0; i < sizeof(psub1_f)/sizeof(psub1_f[0]); i++) {
-		if (psub1_f[i] > check && !(psub1_f[i] % check)) {
+	for (unsigned i=0; i < group->num_prime_factors; i++) {
+		if (group->prime_factors[i] > check && !(group->prime_factors[i] % check)) {
 			return NOT_COPRIME;
-		} else if (psub1_f[i] < check && !(check % psub1_f[i])) {
+		} else if (group->prime_factors[i] < check && !(check % group->prime_factors[i])) {
 			return NOT_COPRIME;
-		} else if (psub1_f[i] == check) {
+		} else if (group->prime_factors[i] == check) {
 			return NOT_COPRIME;
 		}
 	}
@@ -88,18 +123,18 @@ static int check_coprime(uint64_t check)
 }
 
 // find gen of cyclic group Z modulo PRIME
-static uint64_t find_primroot(void)
+static uint64_t find_primroot(const cyclic_group_t *group)
 {
 	// what luck, rand() returns a uint32_t!
 	uint32_t candidate = (uint32_t) aesrand_getword() & 0xFFFF;
-	while(check_coprime(candidate) != COPRIME) {
+	while(check_coprime(candidate, group) != COPRIME) {
 		++candidate;
 	}
 	// pre-modded result is gigantic so use GMP
 	mpz_t base, power, prime, primroot;
-	mpz_init_set_d(base, (double) KNOWN_PRIMROOT);
+	mpz_init_set_d(base, (double) group->known_primroot);
 	mpz_init_set_d(power, (double) candidate);
-	mpz_init_set_d(prime, (double) PRIME);
+	mpz_init_set_d(prime, (double) group->prime);
 	mpz_init(primroot);
 	mpz_powm(primroot, base, power, prime);
 	uint64_t retv = (uint64_t) mpz_get_ui(primroot);
@@ -114,6 +149,24 @@ int cyclic_init(uint32_t primroot_, uint32_t current_)
 {
 	assert(!(!primroot_ && current_));
 
+	// Initialize blacklist
+	if (blacklist_init_from_files(zconf.whitelist_filename,
+							zconf.blacklist_filename)) {
+		return -1;
+	}
+	num_addrs = blacklist_count_allowed();
+
+	uint32_t i;
+	const cyclic_group_t *cur_group = NULL;
+	for (i=0; i<sizeof(groups)/sizeof(groups[0]); i++) {
+		if (groups[i].prime > num_addrs) {
+			cur_group = &groups[i];
+			log_info("cyclic", "using prime %lu, known_primroot %lu", cur_group->prime, cur_group->known_primroot);
+			prime = groups[i].prime;
+			break;
+		}
+	}
+
 	if (zconf.use_seed) {
 		aesrand_init(zconf.seed+1);
 	} else {
@@ -121,7 +174,7 @@ int cyclic_init(uint32_t primroot_, uint32_t current_)
 	}
 	if (!primroot_) {
 		do {
-			primroot = find_primroot();
+			primroot = find_primroot(cur_group);
 		} while (primroot >= (1LL << 32));
 		log_debug(LSRC, "primitive root: %lld", primroot);
 		current = (uint32_t) aesrand_getword() & 0xFFFF;
@@ -142,12 +195,6 @@ int cyclic_init(uint32_t primroot_, uint32_t current_)
 		}
 	}
 	zconf.generator = primroot;
-	if (blacklist_init_from_files(zconf.whitelist_filename,
-							zconf.blacklist_filename)) {
-	  return -1;
-	}
-
-	num_addrs = blacklist_count_allowed();
 	// make sure current is an allowed ip
 	cyclic_get_next_ip();
 
@@ -156,7 +203,7 @@ int cyclic_init(uint32_t primroot_, uint32_t current_)
 
 uint32_t cyclic_get_curr_ip(void)
 {
-	return (uint32_t) blacklist_lookup_index(current);
+	return (uint32_t) blacklist_lookup_index(current-1);
 }
 
 uint32_t cyclic_get_primroot(void)
@@ -168,7 +215,7 @@ static inline uint32_t cyclic_get_next_elem(void)
 {
 	do {
 		current *= primroot;
-		current %= PRIME;
+		current %= prime;
 	} while (current >= (1LL << 32));
 	return (uint32_t) current;
 }
@@ -177,8 +224,8 @@ uint32_t cyclic_get_next_ip(void)
 {
 	while (1) {
 		uint32_t candidate = cyclic_get_next_elem();
-		if (candidate < num_addrs) {
-			return blacklist_lookup_index(candidate);
+		if (candidate-1 < num_addrs) {
+			return blacklist_lookup_index(candidate-1);
 		}
 		zsend.blacklisted++;
 	}