Oops! A trivial typo in obfuscate_bitmap() made the obfuscation

function rather less uniform-looking than I'd intended. I _thought_
it looked a bit fishy, but had assumed it was just the human
tendency to see patterns where none exist. Now fixed, and some real
test vectors confirm that this time the obfuscation function is
actually what I intended it to be.

This means that all masked game IDs generated before this revision
are now invalid. That's a shame, but the game is only a day old and
I think I can reasonably justify it as teething trouble.

[originally from svn r5883]

diff --git a/mines.c b/mines.c
index 725fdcf477f1bf38f6bdcbbc1d675cae5c895957..5023faadd84579463c3766bc030b43463bea464b 100644 (file)
--- a/mines.c
+++ b/mines.c
@@ -1792,7 +1792,7 @@ static void obfuscate_bitmap(unsigned char *bmp, int bits, int decode)
                SHA_Final(&final, digest);
                digestpos = 0;
            }
-           steps[i].targetstart[j] ^= digest[digestpos]++;
+           steps[i].targetstart[j] ^= digest[digestpos++];
        }
 
        /*
@@ -1810,6 +1810,69 @@ static char *new_mine_layout(int w, int h, int n, int x, int y, int unique,
     unsigned char *bmp;
     int i, area;
 
+#ifdef TEST_OBFUSCATION
+    static int tested_obfuscation = FALSE;
+    if (!tested_obfuscation) {
+       /*
+        * A few simple test vectors for the obfuscator.
+        * 
+        * First test: the 28-bit stream 1234567. This divides up
+        * into 1234 and 567[0]. The SHA of 56 70 30 (appending
+        * "0") is 15ce8ab946640340bbb99f3f48fd2c45d1a31d30. Thus,
+        * we XOR the 16-bit string 15CE into the input 1234 to get
+        * 07FA. Next, we SHA that with "0": the SHA of 07 FA 30 is
+        * 3370135c5e3da4fed937adc004a79533962b6391. So we XOR the
+        * 12-bit string 337 into the input 567 to get 650. Thus
+        * our output is 07FA650.
+        */
+       {
+           unsigned char bmp1[] = "\x12\x34\x56\x70";
+           obfuscate_bitmap(bmp1, 28, FALSE);
+           printf("test 1 encode: %s\n",
+                  memcmp(bmp1, "\x07\xfa\x65\x00", 4) ? "failed" : "passed");
+           obfuscate_bitmap(bmp1, 28, TRUE);
+           printf("test 1 decode: %s\n",
+                  memcmp(bmp1, "\x12\x34\x56\x70", 4) ? "failed" : "passed");
+       }
+       /*
+        * Second test: a long string to make sure we switch from
+        * one SHA to the next correctly. My input string this time
+        * is simply fifty bytes of zeroes.
+        */
+       {
+           unsigned char bmp2[50];
+           unsigned char bmp2a[50];
+           memset(bmp2, 0, 50);
+           memset(bmp2a, 0, 50);
+           obfuscate_bitmap(bmp2, 50 * 8, FALSE);
+           /*
+            * SHA of twenty-five zero bytes plus "0" is
+            * b202c07b990c01f6ff2d544707f60e506019b671. SHA of
+            * twenty-five zero bytes plus "1" is
+            * fcb1d8b5a2f6b592fe6780b36aa9d65dd7aa6db9. Thus our
+            * first half becomes
+            * b202c07b990c01f6ff2d544707f60e506019b671fcb1d8b5a2.
+            * 
+            * SHA of that lot plus "0" is
+            * 10b0af913db85d37ca27f52a9f78bba3a80030db. SHA of the
+            * same string plus "1" is
+            * 3d01d8df78e76d382b8106f480135a1bc751d725. So the
+            * second half becomes
+            * 10b0af913db85d37ca27f52a9f78bba3a80030db3d01d8df78.
+            */
+           printf("test 2 encode: %s\n",
+                  memcmp(bmp2, "\xb2\x02\xc0\x7b\x99\x0c\x01\xf6\xff\x2d\x54"
+                         "\x47\x07\xf6\x0e\x50\x60\x19\xb6\x71\xfc\xb1\xd8"
+                         "\xb5\xa2\x10\xb0\xaf\x91\x3d\xb8\x5d\x37\xca\x27"
+                         "\xf5\x2a\x9f\x78\xbb\xa3\xa8\x00\x30\xdb\x3d\x01"
+                         "\xd8\xdf\x78", 50) ? "failed" : "passed");
+           obfuscate_bitmap(bmp2, 50 * 8, TRUE);
+           printf("test 2 decode: %s\n",
+                  memcmp(bmp2, bmp2a, 50) ? "failed" : "passed");
+       }
+    }
+#endif
+
     grid = minegen(w, h, n, x, y, unique, rs);
 
     if (game_desc) {