Add functions for point negation and subtraction

2 files changed, 32 insertions, 0 deletions

diff --git a/include/libuecc/ecc.h b/include/libuecc/ecc.h
index 982f7c9..5d75150 100644
--- a/include/libuecc/ecc.h
+++ b/include/libuecc/ecc.h
@@ -87,6 +87,13 @@ void ecc_25519_store_packed(ecc_int256_t *out, const ecc_25519_work_t *in);
 int ecc_25519_is_identity(const ecc_25519_work_t *in);
 
 /**
+ * Negates a point of the Elliptic Curve
+ *
+ * The same pointer may be given for input and output
+ */
+void ecc_25519_negate(ecc_25519_work_t *out, const ecc_25519_work_t *in);
+
+/**
  * Doubles a point of the Elliptic Curve
  *
  * ecc_25519_double(out, in) is equivalent to ecc_25519_add(out, in, in), but faster.
@@ -102,6 +109,12 @@ void ecc_25519_double(ecc_25519_work_t *out, const ecc_25519_work_t *in);
  */
 void ecc_25519_add(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc_25519_work_t *in2);
 
+/**
+ * Subtracts two points of the Elliptic Curve
+ *
+ * The same pointers may be given for input and output.
+ */
+void ecc_25519_sub(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc_25519_work_t *in2);
 
 /**
  * Does a scalar multiplication of a point of the Elliptic Curve with an integer of a given bit length
diff --git a/src/ec25519.c b/src/ec25519.c
index a9d519c..d673aee 100644
--- a/src/ec25519.c
+++ b/src/ec25519.c
@@ -498,6 +498,18 @@ int ecc_25519_is_identity(const ecc_25519_work_t *in) {
 	return (check_zero(in->X)&check_zero(Y_Z));
 }
 
+void ecc_25519_negate(ecc_25519_work_t *out, const ecc_25519_work_t *in) {
+	int i;
+
+	for (i = 0; i < 32; i++) {
+		out->Y[i] = in->Y[i];
+		out->Z[i] = in->Z[i];
+	}
+
+	sub(out->X, zero, in->X);
+	sub(out->T, zero, in->T);
+}
+
 void ecc_25519_double(ecc_25519_work_t *out, const ecc_25519_work_t *in) {
 	unsigned int A[32], B[32], C[32], D[32], E[32], F[32], G[32], H[32], t0[32], t1[32], t2[32], t3[32];
 
@@ -542,6 +554,13 @@ void ecc_25519_add(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc
 	mult(out->Z, F, G);
 }
 
+void ecc_25519_sub(ecc_25519_work_t *out, const ecc_25519_work_t *in1, const ecc_25519_work_t *in2) {
+	ecc_25519_work_t in2_neg;
+
+	ecc_25519_negate(&in2_neg, in2);
+	ecc_25519_add(out, in1, &in2_neg);
+}
+
 void ecc_25519_scalarmult_bits(ecc_25519_work_t *out, const ecc_int256_t *n, const ecc_25519_work_t *base, unsigned bits) {
 	ecc_25519_work_t Q2, Q2p;
 	ecc_25519_work_t cur = ecc_25519_work_identity;