Decode FORS indices similarly to WOTS

As proposed by NIST's David Cooper: https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/88tuvtb7nN4/m/DA1QCoJWBAAJ
sphincs · May 4, 2023 · c8116bc · c8116bc
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diff --git a/SHA256SUMS b/SHA256SUMS
@@ -1,36 +1,36 @@
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-a838509fa6ec49ade2638efc35d9e29fdb56bd9b843d5e1f48210a2cab218332 sphincs-haraka-256f-robust
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 b8e617db2099e617dfc372ff732eead88872aea791e2fe82628568d75dd03c78 sphincs-sha2-192f-robust
 84b1a342683bcad658efb6c65f7367c6b30623e74e3a24c2238d19eaf74722ab sphincs-sha2-192f-simple
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diff --git a/ref/fors.c b/ref/fors.c
@@ -56,7 +56,7 @@ static void message_to_indices(uint32_t *indices, const unsigned char *m)
     for (i = 0; i < SPX_FORS_TREES; i++) {
         indices[i] = 0;
         for (j = 0; j < SPX_FORS_HEIGHT; j++) {
-            indices[i] ^= ((m[offset >> 3] >> (offset & 0x7)) & 0x1) << j;
+            indices[i] ^= ((m[offset >> 3] >> (~offset & 0x7)) & 0x1) << (SPX_FORS_HEIGHT-1-j);
             offset++;
         }
     }

diff --git a/sha2-avx2/fors.c b/sha2-avx2/fors.c
@@ -126,7 +126,7 @@ static void message_to_indices(uint32_t *indices, const unsigned char *m)
     for (i = 0; i < SPX_FORS_TREES; i++) {
         indices[i] = 0;
         for (j = 0; j < SPX_FORS_HEIGHT; j++) {
-            indices[i] ^= ((m[offset >> 3] >> (offset & 0x7)) & 0x1) << j;
+            indices[i] ^= ((m[offset >> 3] >> (~offset & 0x7)) & 0x1) << (SPX_FORS_HEIGHT-1-j);
             offset++;
         }
     }

diff --git a/shake-a64/fors.c b/shake-a64/fors.c
@@ -91,7 +91,7 @@ static void message_to_indices(uint32_t *indices, const unsigned char *m)
     for (i = 0; i < SPX_FORS_TREES; i++) {
         indices[i] = 0;
         for (j = 0; j < SPX_FORS_HEIGHT; j++) {
-            indices[i] ^= ((m[offset >> 3] >> (offset & 0x7)) & 0x1) << j;
+            indices[i] ^= ((m[offset >> 3] >> (~offset & 0x7)) & 0x1) << (SPX_FORS_HEIGHT-1-j);
             offset++;
         }
     }

diff --git a/shake-avx2/fors.c b/shake-avx2/fors.c
@@ -102,7 +102,7 @@ static void message_to_indices(uint32_t *indices, const unsigned char *m)
     for (i = 0; i < SPX_FORS_TREES; i++) {
         indices[i] = 0;
         for (j = 0; j < SPX_FORS_HEIGHT; j++) {
-            indices[i] ^= ((m[offset >> 3] >> (offset & 0x7)) & 0x1) << j;
+            indices[i] ^= ((m[offset >> 3] >> (~offset & 0x7)) & 0x1) << (SPX_FORS_HEIGHT-1-j);
             offset++;
         }
     }