Add x-only ecmult_const version for x=n/d

src/ecmult_const.h

@@ -18,4 +18,23 @@
  */
 static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *q, int bits);
 
+/**
+ * Same as secp256k1_ecmult_const, but takes in an x coordinate of the base point
+ * only, specified as fraction n/d. Only the x coordinate of the result is returned.
+ *
+ * If known_on_curve is 0, a verification is performed that n/d is a valid X
+ * coordinate, and 0 is returned if not. Otherwise, 1 is returned.
+ *
+ * d being NULL is interpreted as d=1.
+ *
+ * Constant time in the value of q, but not any other inputs.
+ */
+static int secp256k1_ecmult_const_xonly(
+    secp256k1_fe* r,
+    const secp256k1_fe *n,
+    const secp256k1_fe *d,
+    const secp256k1_scalar *q,
+    int bits,
+    int known_on_curve);
+
 #endif /* SECP256K1_ECMULT_CONST_H */

src/ecmult_const_impl.h

@@ -228,4 +228,58 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
     secp256k1_fe_mul(&r->z, &r->z, &Z);
 }
 
+static int secp256k1_ecmult_const_xonly(secp256k1_fe* r, const secp256k1_fe *n, const secp256k1_fe *d, const secp256k1_scalar *q, int bits, int known_on_curve) {
+
+    /* This algorithm is a generalization of Peter Dettman's technique for
+     * avoiding the square root in a random-basepoint x-only multiplication
+     * on a Weierstrass curve:
+     * https://mailarchive.ietf.org/arch/msg/cfrg/7DyYY6gg32wDgHAhgSb6XxMDlJA/
+     */
+    secp256k1_fe g, i;
+    secp256k1_ge p;
+    secp256k1_gej rj;
+
+    /* Compute g = (n^3 + B*d^3). */
+    secp256k1_fe_sqr(&g, n);
+    secp256k1_fe_mul(&g, &g, n);
+    if (d) {
+        secp256k1_fe b;
+        secp256k1_fe_sqr(&b, d);
+        secp256k1_fe_mul(&b, &b, d);
+        secp256k1_fe_mul(&b, &b, &secp256k1_fe_const_b);
+        secp256k1_fe_add(&g, &b);
+        if (!known_on_curve) {
+            secp256k1_fe c;
+            secp256k1_fe_mul(&c, &g, d);
+            if (secp256k1_fe_jacobi_var(&c) < 0) return 0;
+        }
+    } else {
+        secp256k1_fe_add(&g, &secp256k1_fe_const_b);
+        if (!known_on_curve) {
+            if (secp256k1_fe_jacobi_var(&g) < 0) return 0;
+        }
+    }
+
+    /* Compute base point P = (n*g, g^2), the effective affine version of
+     * (n*g, g^2, sqrt(d*g)), which has corresponding affine X coordinate
+     * n/d. */
+    secp256k1_fe_mul(&p.x, &g, n);
+    secp256k1_fe_sqr(&p.y, &g);
+    p.infinity = 0;
+
+    /* Perform x-only EC multiplication of P with q. */
+    secp256k1_ecmult_const(&rj, &p, q, bits);
+
+    /* The resulting (X, Y, Z) point on the effective-affine isomorphic curve
+     * corresponds to (X, Y, Z*sqrt(d*g)) on the secp256k1 curve. The affine
+     * version of that has X coordinate (X / (Z^2*d*g)). */
+    secp256k1_fe_sqr(&i, &rj.z);
+    secp256k1_fe_mul(&i, &i, &g);
+    if (d) secp256k1_fe_mul(&i, &i, d);
+    secp256k1_fe_inv(&i, &i);
+    secp256k1_fe_mul(r, &rj.x, &i);
+
+    return 1;
+}
+
 #endif /* SECP256K1_ECMULT_CONST_IMPL_H */

src/tests.c

@@ -4028,6 +4028,68 @@ void ecmult_const_mult_zero_one(void) {
     ge_equals_ge(&res2, &point);
 }
 
+void ecmult_const_mult_xonly(void) {
+    int i;
+
+    /* Test correspondence between secp256k1_ecmult_const and secp256k1_ecmult_const_xonly. */
+    for (i = 0; i < 2*count; ++i) {
+        secp256k1_ge base;
+        secp256k1_gej basej, resj;
+        secp256k1_fe n, d, resx, v;
+        secp256k1_scalar q;
+        int res;
+        /* Random base point. */
+        random_group_element_test(&base);
+        /* Random scalar to multiply it with. */
+        random_scalar_order_test(&q);
+        /* If i is odd, n=d*base.x for random non-zero d */
+        if (i & 1) {
+            do {
+                random_field_element_test(&d);
+            } while (secp256k1_fe_normalizes_to_zero_var(&d));
+            secp256k1_fe_mul(&n, &base.x, &d);
+        } else {
+            n = base.x;
+        }
+        /* Perform x-only multiplication. */
+        res = secp256k1_ecmult_const_xonly(&resx, &n, (i & 1) ? &d : NULL, &q, 256, i & 2);
+        CHECK(res);
+        /* Perform normal multiplication. */
+        secp256k1_gej_set_ge(&basej, &base);
+        secp256k1_ecmult(&resj, &basej, &q, NULL);
+        /* Check that resj's X coordinate corresponds with resx. */
+        secp256k1_fe_sqr(&v, &resj.z);
+        secp256k1_fe_mul(&v, &v, &resx);
+        CHECK(check_fe_equal(&v, &resj.x));
+    }
+
+    /* Test that secp256k1_ecmult_const_xonly correctly rejects X coordinates not on curve. */
+    for (i = 0; i < 2*count; ++i) {
+        secp256k1_fe x, n, d, c, r;
+        int res;
+        secp256k1_scalar q;
+        random_scalar_order_test(&q);
+        /* Generate random X coordinate not on the curve. */
+        do {
+            random_field_element_test(&x);
+            secp256k1_fe_sqr(&c, &x);
+            secp256k1_fe_mul(&c, &c, &x);
+            secp256k1_fe_add(&c, &secp256k1_fe_const_b);
+        } while (secp256k1_fe_jacobi_var(&c) >= 0);
+        /* If i is odd, n=d*x for random non-zero d. */
+        if (i & 1) {
+            do {
+                random_field_element_test(&d);
+            } while (secp256k1_fe_normalizes_to_zero_var(&d));
+            secp256k1_fe_mul(&n, &x, &d);
+        } else {
+            n = x;
+        }
+        res = secp256k1_ecmult_const_xonly(&r, &n, (i & 1) ? &d : NULL, &q, 256, 0);
+        CHECK(res == 0);
+    }
+}
+
 void ecmult_const_chain_multiply(void) {
     /* Check known result (randomly generated test problem from sage) */
     const secp256k1_scalar scalar = SECP256K1_SCALAR_CONST(
@@ -4059,6 +4121,7 @@ void run_ecmult_const_tests(void) {
     ecmult_const_random_mult();
     ecmult_const_commutativity();
     ecmult_const_chain_multiply();
+    ecmult_const_mult_xonly();
 }
 
 typedef struct {