Really add consttime_memequal

diff --git a/compat/consttime_memequal.h b/compat/consttime_memequal.h
new file mode 100644 (file)
index 0000000..9830648
--- /dev/null
+++ b/compat/consttime_memequal.h
@@ -0,0 +1,28 @@
+/*
+ * Written by Matthias Drochner <drochner@NetBSD.org>.
+ * Public domain.
+ */
+
+#ifndef CONSTTIME_MEMEQUAL_H
+#define CONSTTIME_MEMEQUAL_H
+inline static int
+consttime_memequal(const void *b1, const void *b2, size_t len)
+{
+       const unsigned char *c1 = b1, *c2 = b2;
+       unsigned int res = 0;
+
+       while (len--)
+               res |= *c1++ ^ *c2++;
+
+       /*
+        * Map 0 to 1 and [1, 256) to 0 using only constant-time
+        * arithmetic.
+        *
+        * This is not simply `!res' because although many CPUs support
+        * branchless conditional moves and many compilers will take
+        * advantage of them, certain compilers generate branches on
+        * certain CPUs for `!res'.
+        */
+       return (1 & ((res - 1) >> 8));
+}
+#endif /* CONSTTIME_MEMEQUAL_H */