silentpayments: add routine for creating labelled spend pubkeys (for …

…addresses)

include/secp256k1_silentpayments.h

@@ -169,6 +169,29 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_create_l
     unsigned int m
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
+/** Create Silent Payment labelled spend public key.
+ *
+ *  Given a recipient's spend public key B_spend and a label_tweak, calculate
+ *  the corresponding serialized labelled spend public key:
+ *
+ *  B_m = B_spend + label_tweak * G
+ *
+ *  The result is used by the receiver to create a Silent Payment address, consisting
+ *  of the serialized and concatenated scan public key and (labelled) spend public key each.
+ *
+ *  Returns: 1 if labelled spend public key creation was successful. 0 if an error occured.
+ *  Args:                  ctx: pointer to a context object
+ *  Out: l_addr_spend_pubkey33: pointer to the resulting labelled spend public key
+ *   In: receiver_spend_pubkey: pointer to the receiver's scan pubkey
+ *                 label_tweak: pointer to the the receiver's spend
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_create_address_spend_pubkey(
+    const secp256k1_context *ctx,
+    unsigned char *l_addr_spend_pubkey33,
+    const secp256k1_pubkey *receiver_spend_pubkey,
+    const unsigned char *label_tweak32
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
 #ifdef __cplusplus
 }
 #endif

src/modules/silentpayments/main_impl.h

@@ -238,4 +238,29 @@ int secp256k1_silentpayments_create_label_tweak(const secp256k1_context *ctx, un
     return 1;
 }
 
+int secp256k1_silentpayments_create_address_spend_pubkey(const secp256k1_context *ctx, unsigned char *l_addr_spend_pubkey33, const secp256k1_pubkey *receiver_spend_pubkey, const unsigned char *label_tweak32) {
+    secp256k1_ge B_m;
+    size_t ser_size;
+    int ser_ret;
+
+    /* Sanity check inputs. */
+    VERIFY_CHECK(ctx != NULL);
+    VERIFY_CHECK(l_addr_spend_pubkey33 != NULL);
+    VERIFY_CHECK(receiver_spend_pubkey != NULL);
+    VERIFY_CHECK(label_tweak32 != NULL);
+
+    /* Calculate B_m = B_spend + label_tweak * G */
+    secp256k1_pubkey_load(ctx, &B_m, receiver_spend_pubkey);
+    if (!secp256k1_ec_pubkey_tweak_add_helper(&B_m, label_tweak32)) {
+        return 0;
+    }
+
+    /* Serialize B_m */
+    ser_ret = secp256k1_eckey_pubkey_serialize(&B_m, l_addr_spend_pubkey33, &ser_size, 1);
+    VERIFY_CHECK(ser_ret && ser_size == 33);
+    (void)ser_ret;
+
+    return 1;
+}
+
 #endif