CDVDMAN: revised algorithm for unscrambling the PS2 logo. Fixes failu…

…res to correctly unscramble the logo if the first byte of subsequent passes does not represent a black pixel.
ps2homebrew · Feb 17, 2019 · ea01c62 · ea01c62
1 parent f16c52d
commit ea01c62
Showing 1 changed file with 9 additions and 3 deletions.
diff --git a/modules/iopcore/cdvdman/cdvdman.c b/modules/iopcore/cdvdman/cdvdman.c
@@ -385,12 +385,18 @@ static int cdvdman_read_sectors(u32 lsn, unsigned int sectors, void *buf)
             break;
         }
 
-        // PS2LOGO Decryptor algorithm; based on misfire's code (https://github.com/mlafeldt/ps2logo)
+        /* PS2LOGO Decryptor algorithm; based on misfire's code (https://github.com/mlafeldt/ps2logo)
+           The PS2 logo is stored within the first 12 sectors, scrambled.
+           This algorithm exploits the characteristic that the value used for scrambling will be recorded,
+           when it is XOR'ed against a black pixel. The first pixel is black, hence the value of the first byte
+           was the value used for scrambling. */
         if (lsn < 13) {
             u32 j;
             u8 *logo = (u8 *)ptr;
-            u8 key = logo[0];
-            if (logo[0] != 0) {
+            static u8 key = 0;
+            if (lsn == 0) //First sector? Copy the first byte as the value for unscrambling the logo.
+                key = logo[0];
+            if (key != 0) {
                 for (j = 0; j < (SectorsToRead * 2048); j++) {
                     logo[j] ^= key;
                     logo[j] = (logo[j] << 3) | (logo[j] >> 5);