scalar: use unsigned radix 32

Improves performance of all scalar operations; in particular adding,
subtraction, negating and unpacking.

Closes #11
See #13

ristretto.go

@@ -112,13 +112,17 @@ func (p *Point) SetElligator(buf *[32]byte) *Point {
 // Sets p to s * q, where q is the point for which the table t was
 // computed. Returns p.
 func (p *Point) ScalarMultTable(t *ScalarMultTable, s *Scalar) *Point {
-	t.t().ScalarMult(p.e(), (*[32]uint8)(s))
+	var buf [32]byte
+	s.SetBytes(&buf)
+	t.t().ScalarMult(p.e(), &buf)
 	return p
 }
 
 // Sets p to s * q.  Returns p.
 func (p *Point) ScalarMult(q *Point, s *Scalar) *Point {
-	p.e().ScalarMult(q.e(), (*[32]uint8)(s))
+	var buf [32]byte
+	s.SetBytes(&buf)
+	p.e().ScalarMult(q.e(), &buf)
 	return p
 }
 
@@ -127,13 +131,17 @@ func (p *Point) ScalarMult(q *Point, s *Scalar) *Point {
 // Warning: this method uses a non-constant time inmplementation and thus leaks
 // information about s.  Use this function only if s is public knowledge.
 func (p *Point) PublicScalarMult(q *Point, s *Scalar) *Point {
-	p.e().VarTimeScalarMult(q.e(), (*[32]uint8)(s))
+	var buf [32]byte
+	s.SetBytes(&buf)
+	p.e().VarTimeScalarMult(q.e(), &buf)
 	return p
 }
 
 // Sets p to s * B, where B is the edwards25519 basepoint. Returns p.
 func (p *Point) ScalarMultBase(s *Scalar) *Point {
-	edwards25519.BaseScalarMultTable.ScalarMult(p.e(), (*[32]uint8)(s))
+	var buf [32]byte
+	s.SetBytes(&buf)
+	edwards25519.BaseScalarMultTable.ScalarMult(p.e(), &buf)
 	return p
 }