silentpayments: add recipient light client support

Expose a shared secret routine in the public API and a output_pubkey
creation routine. These are simple wrappers around the same functions
the module is using under the hood. They are exposed for the light
client as the minimal set of operations they need to do scanning without
access to the transaction outputs.

This means the light client will need to manage their own scanning
state, so wherever possible it is preferrable to use the
`_recipient_scan_ouputs` function.

Author: josibake

include/secp256k1_silentpayments.h

@@ -273,6 +273,58 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_recipien
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4)
     SECP256K1_ARG_NONNULL(6) SECP256K1_ARG_NONNULL(7) SECP256K1_ARG_NONNULL(8);
 
+/** Create Silent Payment shared secret.
+ *
+ * Given the public input data (A_tweaked = input_hash * A_sum), calculate the shared secret using ECDH:
+ *
+ * shared_secret = b_scan * A_tweaked   [Recipient, Light client scenario]
+ *
+ * The resulting shared secret is needed as input for creating silent payments
+ * outputs belonging to the same recipient scan public key.
+ *
+ *  Returns: 1 if shared secret creation was successful. 0 if an error occured.
+ *  Args:                  ctx: pointer to a context object
+ *  Out:       shared_secret33: pointer to the resulting 33-byte shared secret
+ *  In:     recipient_scan_key: pointer to the recipient's scan key
+ *                 public_data: pointer to the input public key sum, tweaked with the input_hash
+ *                              (see `_recipient_compute_public_data`)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_recipient_create_shared_secret(
+    const secp256k1_context *ctx,
+    unsigned char *shared_secret33,
+    const unsigned char *recipient_scan_key,
+    const secp256k1_silentpayments_public_data *public_data
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+/** Create Silent Payment output public key.
+ *
+ *  Given a shared_secret, a public key B_spend, and an output counter k,
+ *  calculate the corresponding output public key:
+ *
+ *  P_output_xonly = B_spend + hash(shared_secret || ser_32(k)) * G
+ *
+ *  This function is used by the recipient when scanning for outputs without access to the
+ *  transaction outputs (e.g. using BIP158 block filters).
+ *  When scanning with this function, it is the scanners responsibility to determine if the generated
+ *  output exists in a block before proceeding to the next value of `k`.
+ *
+ *  Returns: 1 if output creation was successful. 0 if an error occured.
+ *  Args:               ctx: pointer to a context object
+ *  Out:     P_output_xonly: pointer to the resulting output x-only pubkey
+ *  In:     shared_secret33: shared secret, derived from either sender's
+ *                           or receiver's perspective with routines from above
+ *    receiver_spend_pubkey: pointer to the receiver's spend pubkey (labelled or unlabelled)
+ *                        k: output counter (initially set to 0, must be incremented for each
+ *                           additional output created or after each output found when scanning)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_recipient_create_output_pubkey(
+    const secp256k1_context *ctx,
+    secp256k1_xonly_pubkey *P_output_xonly,
+    const unsigned char *shared_secret33,
+    const secp256k1_pubkey *receiver_spend_pubkey,
+    unsigned int k
+) 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

@@ -566,4 +566,23 @@ int secp256k1_silentpayments_recipient_scan_outputs(
     return 1;
 }
 
+int secp256k1_silentpayments_recipient_create_shared_secret(const secp256k1_context *ctx, unsigned char *shared_secret33, const unsigned char *recipient_scan_key, const secp256k1_silentpayments_public_data *public_data) {
+    secp256k1_pubkey A_tweaked;
+    /* Sanity check inputs */
+    ARG_CHECK(shared_secret33 != NULL);
+    ARG_CHECK(recipient_scan_key != NULL);
+    ARG_CHECK(public_data != NULL);
+    ARG_CHECK(public_data->data[0] == 1);
+    if (!secp256k1_silentpayments_recipient_public_data_load(ctx, &A_tweaked, NULL, public_data)) {
+        return 0;
+    }
+    return secp256k1_silentpayments_create_shared_secret(ctx, shared_secret33, recipient_scan_key, &A_tweaked, NULL);
+}
+
+int secp256k1_silentpayments_recipient_create_output_pubkey(const secp256k1_context *ctx, secp256k1_xonly_pubkey *P_output_xonly, const unsigned char *shared_secret33, const secp256k1_pubkey *receiver_spend_pubkey, unsigned int k)
+{
+    return secp256k1_silentpayments_create_output_pubkey(ctx, P_output_xonly, shared_secret33, receiver_spend_pubkey, k);
+}
+
+
 #endif