summaryrefslogtreecommitdiffstats
diff options
context:
space:
mode:
authorRoy Marples <roy@marples.name>2019-04-19 21:47:37 +0100
committerRoy Marples <roy@marples.name>2019-04-19 21:47:37 +0100
commitaee631aadeef4283c8a749c1caf77823304acf5e (patch)
tree624c956154dc53b415fcb2889ebb0e5434b54ce3
parentcfde89ab66cb4e5957b1c4b68ad6a9449e2784da (diff)
downloaddhcpcd-aee631aadeef4283c8a749c1caf77823304acf5e.tar.xz
Really add consttime_memequal
-rw-r--r--compat/consttime_memequal.h28
1 files changed, 28 insertions, 0 deletions
diff --git a/compat/consttime_memequal.h b/compat/consttime_memequal.h
new file mode 100644
index 00000000..98306484
--- /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 */