Fix handling of integer wraparound in megolm.c

src/megolm.c

@@ -108,8 +108,12 @@ void megolm_advance_to(Megolm *megolm, uint32_t advance_to) {
         uint32_t mask = (~(uint32_t)0) << shift;
         int k;
 
-        /* how many times to we need to rehash this part? */
-        int steps = (advance_to >> shift) - (megolm->counter >> shift);
+        /* how many times do we need to rehash this part?
+         *
+         * '& 0xff' ensures we handle integer wraparound correctly
+         */
+        unsigned int steps =
+            ((advance_to >> shift) - (megolm->counter >> shift)) & 0xff;
 
         if (steps == 0) {
             continue;

tests/test_megolm.cpp

@@ -82,4 +82,20 @@ std::uint8_t random_bytes[] =
     assert_equals(expected3, megolm_get_data(&mr), MEGOLM_RATCHET_LENGTH);
 }
 
+{
+    TestCase test_case("Megolm::advance wraparound");
+
+    Megolm mr1, mr2;
+
+    megolm_init(&mr1, random_bytes, 0xffffffffUL);
+    megolm_advance_to(&mr1, 0x1000000);
+    assert_equals(0x1000000U, mr1.counter);
+
+    megolm_init(&mr2, random_bytes, 0);
+    megolm_advance_to(&mr2, 0x2000000);
+    assert_equals(0x2000000U, mr2.counter);
+
+    assert_equals(megolm_get_data(&mr2), megolm_get_data(&mr1), MEGOLM_RATCHET_LENGTH);
+}
+
 }