Additional comments for the testing PRNG and a seeding fix.

gmaxwell · gmaxwell · commit 34b898dc84e8 · 2015-02-07T00:22:49.000Z
Rw has additional short-cycle inputs because 2^32/0x464fffff &gt;= 2.
diff --git a/src/testrand.h b/src/testrand.h
@@ -11,7 +11,9 @@
 #include "libsecp256k1-config.h"
 #endif
 
-/** Seed the pseudorandom number generator. */
+/* A non-cryptographic RNG used only for test infrastructure. */
+
+/** Seed the pseudorandom number generator for testing. */
 SECP256K1_INLINE static void secp256k1_rand_seed(uint64_t v);
 
 /** Generate a pseudorandom 32-bit number. */
diff --git a/src/testrand_impl.h b/src/testrand_impl.h
@@ -18,15 +18,19 @@ SECP256K1_INLINE static void secp256k1_rand_seed(uint64_t v) {
     secp256k1_Rz = v >> 32;
     secp256k1_Rw = v;
 
+    /* There are two seeds with short (length 1) cycles for the Rz PRNG. */
     if (secp256k1_Rz == 0 || secp256k1_Rz == 0x9068ffffU) {
         secp256k1_Rz = 111;
     }
-    if (secp256k1_Rw == 0 || secp256k1_Rw == 0x464fffffU) {
+    /* There are four seeds with short (length 1) cycles for the Rw PRNG. */
+    if (secp256k1_Rw == 0 || secp256k1_Rw == 0x464fffffU ||
+         secp256k1_Rw == 0x8c9ffffeU || secp256k1_Rw == 0xd2effffdU) {
         secp256k1_Rw = 111;
     }
 }
 
 SECP256K1_INLINE static uint32_t secp256k1_rand32(void) {
+    /* MWC PRNG for tests. */
     secp256k1_Rz = 36969 * (secp256k1_Rz & 0xFFFF) + (secp256k1_Rz >> 16);
     secp256k1_Rw = 18000 * (secp256k1_Rw & 0xFFFF) + (secp256k1_Rw >> 16);
     return (secp256k1_Rw << 16) + (secp256k1_Rw >> 16) + secp256k1_Rz;

Original file line number	Diff line number	Diff line change
`@@ -18,15 +18,19 @@ SECP256K1_INLINE static void secp256k1_rand_seed(uint64_t v) {`
`18`	`18`	`secp256k1_Rz = v >> 32;`
`19`	`19`	`secp256k1_Rw = v;`
`20`	`20`
	`21`	`+ /* There are two seeds with short (length 1) cycles for the Rz PRNG. */`
`21`	`22`	`if (secp256k1_Rz == 0 \|\| secp256k1_Rz == 0x9068ffffU) {`
`22`	`23`	`secp256k1_Rz = 111;`
`23`	`24`	`}`
`24`		`- if (secp256k1_Rw == 0 \|\| secp256k1_Rw == 0x464fffffU) {`
	`25`	`+ /* There are four seeds with short (length 1) cycles for the Rw PRNG. */`
	`26`	`+ if (secp256k1_Rw == 0 \|\| secp256k1_Rw == 0x464fffffU \|\|`
	`27`	`+ secp256k1_Rw == 0x8c9ffffeU \|\| secp256k1_Rw == 0xd2effffdU) {`
`25`	`28`	`secp256k1_Rw = 111;`
`26`	`29`	`}`
`27`	`30`	`}`
`28`	`31`
`29`	`32`	`SECP256K1_INLINE static uint32_t secp256k1_rand32(void) {`
	`33`	`+ /* MWC PRNG for tests. */`
`30`	`34`	`secp256k1_Rz = 36969 * (secp256k1_Rz & 0xFFFF) + (secp256k1_Rz >> 16);`
`31`	`35`	`secp256k1_Rw = 18000 * (secp256k1_Rw & 0xFFFF) + (secp256k1_Rw >> 16);`
`32`	`36`	`return (secp256k1_Rw << 16) + (secp256k1_Rw >> 16) + secp256k1_Rz;`