From: Samuel Neves <sneves@dei.uc.pt>
Date: Fri, 3 Oct 2014 23:13:36 +0000 (+0100)
Subject: Use only unsigned arithmetic in constant-time operations
X-Git-Tag: master-post-reformat~317
X-Git-Url: https://git.openssl.org/?p=openssl.git;a=commitdiff_plain;h=d2fa182988afa33d9e950358de406cc9fb36d000

Use only unsigned arithmetic in constant-time operations

Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
---

diff --git a/crypto/constant_time_locl.h b/crypto/constant_time_locl.h
index 7b339a7c31..ccb64082fd 100644
--- a/crypto/constant_time_locl.h
+++ b/crypto/constant_time_locl.h
@@ -129,17 +129,12 @@ 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));
+	return -(a >> (sizeof(unsigned int) * 8 - 1));
 	}
 
 static inline unsigned int constant_time_lt(unsigned int a, unsigned int b)
 	{
-	unsigned int lt;
-	/* Case 1: msb(a) == msb(b). a < b iff the MSB of a - b is set.*/
-	lt = ~(a ^ b) & (a - b);
-	/* Case 2: msb(a) != msb(b). a < b iff the MSB of b is set. */
-	lt |= ~a & b;
-	return constant_time_msb(lt);
+	return constant_time_msb(a^((a^b)|((a-b)^b)));
 	}
 
 static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
@@ -149,12 +144,7 @@ static inline unsigned char constant_time_lt_8(unsigned int a, unsigned int b)
 
 static inline unsigned int constant_time_ge(unsigned int a, unsigned int b)
 	{
-	unsigned int ge;
-	/* Case 1: msb(a) == msb(b). a >= b iff the MSB of a - b is not set.*/
-	ge = ~((a ^ b) | (a - b));
-	/* Case 2: msb(a) != msb(b). a >= b iff the MSB of a is set. */
-	ge |= a & ~b;
-	return constant_time_msb(ge);
+	return ~constant_time_lt(a, b);
 	}
 
 static inline unsigned char constant_time_ge_8(unsigned int a, unsigned int b)