Add a function to maybe memcpy() a value, in constant time.

2 files changed, 29 insertions, 0 deletions

diff --git a/src/lib/ctime/di_ops.c b/src/lib/ctime/di_ops.c
index 7448a9973e..0859dd8bee 100644
--- a/src/lib/ctime/di_ops.c
+++ b/src/lib/ctime/di_ops.c
@@ -279,3 +279,30 @@ select_array_member_cumulative_timei(const uint64_t *entries, int n_entries,
 
   return i_chosen;
 }
+
+/**
+ * If <b>s</b> is true, then copy <b>n</b> bytes from <b>src</d> to
+ * <b>dest</b>.  Otherwise leave <b>dest</b> alone.
+ *
+ * This function behaves the same as
+ *
+ *      if (s)
+ *         memcpy(dest, src, n);
+ *
+ * except that it tries to run in the same amount of time whether <b>s</b> is
+ * true or not.
+ **/
+void
+memcpy_if_true_timei(bool s, void *dest, const void *src, size_t n)
+{
+  // If s is true, mask will be ~0.  If s is false, mask will be 0.
+  const char mask = (char) -(signed char)s;
+
+  char *destp = dest;
+  const char *srcp = src;
+  for (size_t i = 0; i < n; ++i) {
+    *destp = (*destp & ~mask) | (*srcp & mask);
+    ++destp;
+    ++srcp;
+  }
+}
diff --git a/src/lib/ctime/di_ops.h b/src/lib/ctime/di_ops.h
index 4ff8f03165..9fe2884ecc 100644
--- a/src/lib/ctime/di_ops.h
+++ b/src/lib/ctime/di_ops.h
@@ -73,4 +73,6 @@ int select_array_member_cumulative_timei(const uint64_t *entries,
                                          int n_entries,
                                          uint64_t total, uint64_t rand_val);
 
+void memcpy_if_true_timei(bool s, void *dest, const void *src, size_t n);
+
 #endif /* !defined(TOR_DI_OPS_H) */