ecc: Add helper for multiplying a scalar with a curve generator

The SPAKE2+ key exchange protocol requires the ability to multiply a
random number with the curve generator. Instead of exposing the curve
generator expose a helper function that provides the multiplication.

3 files changed, 15 insertions, 0 deletions

diff --git a/ell/ecc.c b/ell/ecc.c
index 59bff7bd..ff147b84 100644
--- a/ell/ecc.c
+++ b/ell/ecc.c
@@ -940,6 +940,18 @@ LIB_EXPORT bool l_ecc_point_multiply(struct l_ecc_point *ret,
 	return true;
 }
 
+LIB_EXPORT bool l_ecc_point_multiply_g(struct l_ecc_point *ret,
+					const struct l_ecc_scalar *scalar)
+{
+	if (unlikely(!ret || !scalar))
+		return false;
+
+	_ecc_point_mult(ret, &scalar->curve->g, scalar->c, NULL,
+						scalar->curve->p);
+
+	return true;
+}
+
 LIB_EXPORT bool l_ecc_point_add(struct l_ecc_point *ret,
 					const struct l_ecc_point *a,
 					const struct l_ecc_point *b)
diff --git a/ell/ecc.h b/ell/ecc.h
index 2f6faa4e..de116f01 100644
--- a/ell/ecc.h
+++ b/ell/ecc.h
@@ -83,6 +83,8 @@ bool l_ecc_scalar_add(struct l_ecc_scalar *ret, const struct l_ecc_scalar *a,
 bool l_ecc_point_multiply(struct l_ecc_point *ret,
 				const struct l_ecc_scalar *scalar,
 				const struct l_ecc_point *point);
+bool l_ecc_point_multiply_g(struct l_ecc_point *ret,
+				const struct l_ecc_scalar *scalar);
 bool l_ecc_point_add(struct l_ecc_point *ret, const struct l_ecc_point *a,
 				const struct l_ecc_point *b);
 bool l_ecc_point_inverse(struct l_ecc_point *p);
diff --git a/ell/ell.sym b/ell/ell.sym
index c457a762..0f593e1f 100644
--- a/ell/ell.sym
+++ b/ell/ell.sym
@@ -593,6 +593,7 @@ global:
 	l_ecc_point_get_data;
 	l_ecc_point_inverse;
 	l_ecc_point_multiply;
+	l_ecc_point_multiply_g;
 	l_ecc_point_new;
 	l_ecc_point_from_sswu;
 	l_ecc_point_clone;