diff --git a/src/gsm/a5.c b/src/gsm/a5.c
index 36978c4..e330c75 100644
--- a/src/gsm/a5.c
+++ b/src/gsm/a5.c
@@ -104,9 +104,8 @@
 	x ^= x >> 16;
 	x ^= x >> 8;
 	x ^= x >> 4;
-	x ^= x >> 2;
-	x ^= x >> 1;
-	return x & 1;
+	x &= 0xf;
+	return (0x6996 >> x) & 1;
 }
 
 /*! \brief Compute majority bit from 3 taps