Hi,
the attached patch improves the scrambling of the seed given by the
RANDOM_SEED intrinsic. Previously we shuffled the bytes in the seed
back and forth, the new implementation replaces this with a "XOR
cipher" (https://en.wikipedia.org/wiki/XOR_cipher). Also, now the
scrambling is also done for the random_seed_i8 variant, previously
only the i4 variant was scrambled.
Similarly to the previous implementation, this is of course not a
defence against somebody who is determined to generate a poor seed.
But it's a protection from naive users setting all seed values to 0 or
to some small constant value, or similar low entropy seeds.
Since this is bordering on obvious, I'm planning to commit this in a
few days unless somebody objects. Regtested on x86_64-pc-linux-gnu.
libgfortran:
2016-08-16 Janne Blomqvist <j...@gcc.gnu.org>
* intrinsics/random.c (xor_keys): New array with "secret" keys.
(scramble_seed): XOR given seed with xor_keys array rather than
shuffling bytes.
(unscramble_seed): Remove function.
(random_seed_i4): Use new scramble_seed.
(random_seed_i8): Likewise.
frontend:
2016-08-16 Janne Blomqvist <j...@gcc.gnu.org>
* intrinsics.texi (RANDOM_NUMBER): Remove reference to
init_random_seed in example.
(RANDOM_SEED): Remove warning to not set all seed values to 0.
--
Janne Blomqvist
diff --git a/gcc/fortran/intrinsic.texi b/gcc/fortran/intrinsic.texi
index 67b0231..8cca9b1 100644
--- a/gcc/fortran/intrinsic.texi
+++ b/gcc/fortran/intrinsic.texi
@@ -11155,7 +11155,6 @@ Subroutine
@smallexample
program test_random_number
REAL :: r(5,5)
- CALL init_random_seed() ! see example of RANDOM_SEED
CALL RANDOM_NUMBER(r)
end program
@end smallexample
@@ -11192,9 +11191,6 @@ alias any other stream in the system, where @var{N} is
the number of
threads that have used @code{RANDOM_NUMBER} so far during the program
execution.
-Note that setting any of the seed values to zero should be avoided as
-it can result in poor quality random numbers being generated.
-
@item @emph{Standard}:
Fortran 95 and later
diff --git a/libgfortran/intrinsics/random.c b/libgfortran/intrinsics/random.c
index 3b91389..ddfbbd4 100644
--- a/libgfortran/intrinsics/random.c
+++ b/libgfortran/intrinsics/random.c
@@ -715,28 +715,33 @@ arandom_r16 (gfc_array_r16 *x)
#endif
+/* Number of elements in master_state array. */
+#define SZU64 (sizeof (master_state) / sizeof (uint64_t))
-static void
-scramble_seed (unsigned char *dest, unsigned char *src, int size)
-{
- int i;
- for (i = 0; i < size; i++)
- dest[(i % 2) * (size / 2) + i / 2] = src[i];
-}
+/* Keys for scrambling the seed in order to avoid poor seeds. */
+static const uint64_t xor_keys[] = {
+ 0xbd0c5b6e50c2df49ULL, 0xd46061cd46e1df38ULL, 0xbb4f4d4ed6103544ULL,
+ 0x114a583d0756ad39ULL, 0x4b5ad8623d0aaab6ULL, 0x3f2ed7afbe0c0f21ULL,
+ 0xdec83fd65f113445ULL, 0x3824f8fbc4f10d24ULL, 0x5d9025af05878911ULL,
+ 0x500bc46b540340e9ULL, 0x8bd53298e0d00530ULL, 0x57886e40a952e06aULL,
+ 0x926e76c88e31cdb6ULL, 0xbd0724dac0a3a5f9ULL, 0xc5c8981b858ab796ULL,
+ 0xbb12ab2694c2b32cULL
+};
+
+
+/* Since a XOR cipher is symmetric, we need only one routine, and we
+ can use it both for encryption and decryption. */
static void
-unscramble_seed (unsigned char *dest, unsigned char *src, int size)
+scramble_seed (uint64_t *dest, const uint64_t *src)
{
- int i;
-
- for (i = 0; i < size; i++)
- dest[i] = src[(i % 2) * (size / 2) + i / 2];
+ for (int i = 0; i < (int) SZU64; i++)
+ dest[i] = src[i] ^ xor_keys[i];
}
-
/* random_seed is used to seed the PRNG with either a default
set of seeds or user specified set of seeds. random_seed
must be called with no argument or exactly one argument. */
@@ -744,7 +749,7 @@ unscramble_seed (unsigned char *dest, unsigned char *src,
int size)
void
random_seed_i4 (GFC_INTEGER_4 *size, gfc_array_i4 *put, gfc_array_i4 *get)
{
- unsigned char seed[sizeof (master_state)];
+ uint64_t seed[SZU64];
#define SZ (sizeof (master_state) / sizeof (GFC_INTEGER_4))
/* Check that we only have one argument present. */
@@ -771,12 +776,12 @@ random_seed_i4 (GFC_INTEGER_4 *size, gfc_array_i4 *put,
gfc_array_i4 *get)
init_rand_state (rs, false);
/* Unscramble the seed. */
- unscramble_seed (seed, (unsigned char *) rs->s, sizeof seed);
+ scramble_seed (seed, rs->s);
/* Then copy it back to the user variable. */
for (size_t i = 0; i < SZ ; i++)
memcpy (&(get->base_addr[(SZ - 1 - i) * GFC_DESCRIPTOR_STRIDE(get,0)]),
- seed + i * sizeof(GFC_UINTEGER_4),
+ (unsigned char*) seed + i * sizeof(GFC_UINTEGER_4),
sizeof(GFC_UINTEGER_4));
/* Finally copy the value of p after the seed. */
@@ -808,21 +813,19 @@ random_seed_i4 (GFC_INTEGER_4 *size, gfc_array_i4 *put,
gfc_array_i4 *get)
/* We copy the seed given by the user. */
for (size_t i = 0; i < SZ; i++)
- memcpy (seed + i * sizeof(GFC_UINTEGER_4),
+ memcpy ((unsigned char*) seed + i * sizeof(GFC_UINTEGER_4),
&(put->base_addr[(SZ - 1 - i) * GFC_DESCRIPTOR_STRIDE(put,0)]),
sizeof(GFC_UINTEGER_4));
/* We put it after scrambling the bytes, to paper around users who
provide seeds with quality only in the lower or upper part. */
- scramble_seed ((unsigned char *) master_state, seed, sizeof seed);
+ scramble_seed (master_state, seed);
njumps = 0;
init_rand_state (rs, true);
rs->p = put->base_addr[SZ * GFC_DESCRIPTOR_STRIDE(put, 0)] & 15;
}
-
-
__gthread_mutex_unlock (&random_lock);
}
#undef SZ
@@ -833,6 +836,8 @@ iexport(random_seed_i4);
void
random_seed_i8 (GFC_INTEGER_8 *size, gfc_array_i8 *put, gfc_array_i8 *get)
{
+ uint64_t seed[SZU64];
+
/* Check that we only have one argument present. */
if ((size ? 1 : 0) + (put ? 1 : 0) + (get ? 1 : 0) > 1)
runtime_error ("RANDOM_SEED should have at most one argument present.");
@@ -857,9 +862,12 @@ random_seed_i8 (GFC_INTEGER_8 *size, gfc_array_i8 *put,
gfc_array_i8 *get)
if (!rs->init)
init_rand_state (rs, false);
+ /* Unscramble the seed. */
+ scramble_seed (seed, rs->s);
+
/* This code now should do correct strides. */
for (size_t i = 0; i < SZ; i++)
- memcpy (&(get->base_addr[i * GFC_DESCRIPTOR_STRIDE(get,0)]), &rs->s[i],
+ memcpy (&(get->base_addr[i * GFC_DESCRIPTOR_STRIDE(get,0)]), &seed[i],
sizeof (GFC_UINTEGER_8));
get->base_addr[SZ * GFC_DESCRIPTOR_STRIDE(get, 0)] = rs->p;
@@ -890,9 +898,10 @@ random_seed_i8 (GFC_INTEGER_8 *size, gfc_array_i8 *put,
gfc_array_i8 *get)
/* This code now should do correct strides. */
for (size_t i = 0; i < SZ; i++)
- memcpy (&master_state[i], &(put->base_addr[i *
GFC_DESCRIPTOR_STRIDE(put,0)]),
+ memcpy (&seed[i], &(put->base_addr[i * GFC_DESCRIPTOR_STRIDE(put,0)]),
sizeof (GFC_UINTEGER_8));
+ scramble_seed (master_state, seed);
njumps = 0;
init_rand_state (rs, true);
rs->p = put->base_addr[SZ * GFC_DESCRIPTOR_STRIDE(put, 0)] & 15;
