Use a XOR cipher instead of byte shuffling to protect against bad seeds.

libgfortran:
2016-08-19  Janne Blomqvist  <jb@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:
2016-08-19  Janne Blomqvist  <jb@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.

From-SVN: r239613

2 files changed, 41 insertions, 23 deletions

diff --git a/libgfortran/ChangeLog b/libgfortran/ChangeLog
index 6fd1222..10dc07a 100644
--- a/libgfortran/ChangeLog
+++ b/libgfortran/ChangeLog
@@ -1,4 +1,13 @@
-2016-08-16  Janne Blomqvist  <jb@gcc.gnu.org>
+2016-08-19  Janne Blomqvist  <jb@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.
+
+2016-08-19  Janne Blomqvist  <jb@gcc.gnu.org>
 
         * intrinsics/random.c (master_init): New variable.
         (init_rand_state): Move below getosrandom (), maybe initialize
diff --git a/libgfortran/intrinsics/random.c b/libgfortran/intrinsics/random.c
index 35c7611..739dbeb 100644
--- a/libgfortran/intrinsics/random.c
+++ b/libgfortran/intrinsics/random.c
@@ -722,28 +722,33 @@ 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.  */
@@ -751,7 +756,7 @@ 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.  */
@@ -778,12 +783,12 @@ 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.  */
@@ -814,13 +819,13 @@ 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;
       master_init = true;
       init_rand_state (rs, true);
@@ -828,8 +833,6 @@ random_seed_i4 (GFC_INTEGER_4 *size, gfc_array_i4 *put, gfc_array_i4 *get)
       rs->p = put->base_addr[SZ * GFC_DESCRIPTOR_STRIDE(put, 0)] & 15;
     }
 
-
-
   __gthread_mutex_unlock (&random_lock);
     }
 #undef SZ
@@ -840,6 +843,8 @@ 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.");
@@ -864,9 +869,12 @@ 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;
@@ -896,9 +904,10 @@ 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;
       master_init = true;
       init_rand_state (rs, true);