1 files changed, 34 insertions, 2 deletions

diff --git a/crypto/constant_time_locl.h b/crypto/constant_time_locl.h
index 0d3acd58b3..ccf7b62f5f 100644
--- a/crypto/constant_time_locl.h
+++ b/crypto/constant_time_locl.h
@@ -54,7 +54,7 @@ extern "C" {
 #endif
 
 /*
- * The following methods return a bitmask of all ones (0xff...f) for true
+ * The boolean methods return a bitmask of all ones (0xff...f) for true
  * and 0 for false. This is useful for choosing a value based on the result
  * of a conditional in constant time. For example,
  *
@@ -67,7 +67,7 @@ extern "C" {
  * can be written as
  *
  * unsigned int lt = constant_time_lt(a, b);
- * c = a & lt | b & ~lt;
+ * c = constant_time_select(lt, a, b);
  */
 
 /*
@@ -107,6 +107,21 @@ static inline unsigned int constant_time_eq(unsigned int a, unsigned int b);
 /* Convenience method for getting an 8-bit mask. */
 static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b);
 
+/*
+ * Returns (mask & a) | (~mask & b).
+ *
+ * When |mask| is all 1s or all 0s (as returned by the methods above),
+ * the select methods return either |a| (if |mask| is nonzero) or |b|
+ * (if |mask| is zero).
+ */
+static inline unsigned int constant_time_select(unsigned int mask,
+	unsigned int a, unsigned int b);
+/* Convenience method for unsigned chars. */
+static inline unsigned char constant_time_select_8(unsigned char mask,
+	unsigned char a, unsigned char b);
+/* Convenience method for signed integers. */
+static inline int constant_time_select_int(unsigned int mask, int a, int b);
+
 static inline unsigned int constant_time_msb(unsigned int a)
 	{
 	return (unsigned int)((int)(a) >> (sizeof(int) * 8 - 1));
@@ -162,6 +177,23 @@ static inline unsigned char constant_time_eq_8(unsigned int a, unsigned int b)
 	return (unsigned char)(constant_time_eq(a, b));
 	}
 
+static inline unsigned int constant_time_select(unsigned int mask,
+	unsigned int a, unsigned int b)
+	{
+	return (mask & a) | (~mask & b);
+	}
+
+static inline unsigned char constant_time_select_8(unsigned char mask,
+	unsigned char a, unsigned char b)
+	{
+	return (unsigned char)(constant_time_select(mask, a, b));
+	}
+
+inline int constant_time_select_int(unsigned int mask, int a, int b)
+	{
+	return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
+	}
+
 #ifdef __cplusplus
 }
 #endif