diff --git a/um_crypto/qmc/src/v2_rc4/cipher.rs b/um_crypto/qmc/src/v2_rc4/cipher.rs
index ad63fba..602d410 100644
--- a/um_crypto/qmc/src/v2_rc4/cipher.rs
+++ b/um_crypto/qmc/src/v2_rc4/cipher.rs
@@ -31,8 +31,9 @@ impl QMC2RC4 {
         let n = self.key.len();
 
         for (datum, offset) in data.iter_mut().zip(offset..) {
-            let idx = get_segment_key(offset as u64, self.key[offset % n], self.hash) as usize;
-            *datum ^= self.key[idx % n];
+            let idx = get_segment_key(offset as u64, self.key[offset % n], self.hash);
+            let idx = idx % (n as u64);
+            *datum ^= self.key[idx as usize];
         }
     }
 
@@ -43,10 +44,11 @@ impl QMC2RC4 {
         let block_offset = offset % OTHER_SEGMENT_SIZE;
 
         let seed = self.key[id % n];
-        let skip = get_segment_key(id as u64, seed, self.hash) % 512;
+        let skip = get_segment_key(id as u64, seed, self.hash);
+        let skip = (skip & 0x1FF) as usize;
 
         debug_assert!(data.len() <= OTHER_SEGMENT_SIZE - block_offset);
-        let key_stream = self.key_stream.iter().skip(skip as usize + block_offset);
+        let key_stream = self.key_stream.iter().skip(skip + block_offset);
         for (datum, &key) in data.iter_mut().zip(key_stream) {
             *datum ^= key;
         }