libmp: initial attempt at constant time code, faster reductions for special primes (for ecc)

introduce MPtimesafe flag to request time invariant computation
disables normalization so significant digits are not leaked.

1 files changed, 12 insertions, 10 deletions

diff --git a/sys/src/libmp/port/mpcmp.c b/sys/src/libmp/port/mpcmp.c
index a2e3cf724..7ab5a16b6 100644
--- a/sys/src/libmp/port/mpcmp.c
+++ b/sys/src/libmp/port/mpcmp.c
@@ -8,10 +8,14 @@ mpmagcmp(mpint *b1, mpint *b2)
 {
 	int i;
 
-	i = b1->top - b2->top;
-	if(i)
-		return i;
-
+	i = b1->flags | b2->flags;
+	if(i & MPtimesafe)
+		return mpvectscmp(b1->p, b1->top, b2->p, b2->top);
+	if(i & MPnorm){
+		i = b1->top - b2->top;
+		if(i)
+			return i;
+	}
 	return mpveccmp(b1->p, b1->top, b2->p, b2->top);
 }
 
@@ -19,10 +23,8 @@ mpmagcmp(mpint *b1, mpint *b2)
 int
 mpcmp(mpint *b1, mpint *b2)
 {
-	if(b1->sign != b2->sign)
-		return b1->sign - b2->sign;
-	if(b1->sign < 0)
-		return mpmagcmp(b2, b1);
-	else
-		return mpmagcmp(b1, b2);
+	int sign;
+
+	sign = (b1->sign - b2->sign) >> 1;	// -1, 0, 1
+	return sign | (sign&1)-1 & mpmagcmp(b1, b2)*b1->sign;
 }