From 9039d6b673fee6d5f98b1cd79a92bd6241b19a6a Mon Sep 17 00:00:00 2001
From: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>
Date: Wed, 20 Dec 2023 23:01:58 +0100
Subject: [PATCH] silentpayments: implement output spending privkey creation
 (for receiver)

---
 include/secp256k1_silentpayments.h     | 29 ++++++++++++++++++++++++
 src/modules/silentpayments/main_impl.h | 31 ++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/include/secp256k1_silentpayments.h b/include/secp256k1_silentpayments.h
index 7f02ec111e..8b3c0c6a35 100644
--- a/include/secp256k1_silentpayments.h
+++ b/include/secp256k1_silentpayments.h
@@ -221,6 +221,35 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_create_o
     const unsigned char *label_tweak32
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
+/** Create Silent Payment output private key (for spending receiver's funds).
+ *
+ *  Given a shared_secret, a recipient's spend private key b_spend, an output
+ *  counter k, and an optional label_tweak, calculate the corresponding
+ *  output private key d:
+ *
+ *  b_m = b_spend + label_tweak
+ *  (if no label tweak is used, them b_m = b_spend)
+ *  d = (b_m + hash(shared_secret || ser_32(k))) mod n
+ *
+ *  Returns: 1 if private key creation was successful. 0 if an error occured.
+ *  Args:                  ctx: pointer to a context object
+ *  Out:         output_seckey: pointer to the resulting spending private key
+ *  In:        shared_secret33: shared secret, derived from either sender's
+ *                              or receiver's perspective with routines from above
+ *       receiver_spend_seckey: pointer to the receiver's spend private key
+ *                           k: output counter (usually set to 0, should be increased for
+ *                              every additional output to the same recipient)
+ *               label_tweak32: an optional 32-byte label tweak (NULL if no label is used)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_create_output_seckey(
+    const secp256k1_context *ctx,
+    unsigned char *output_seckey,
+    const unsigned char *shared_secret33,
+    const unsigned char *receiver_spend_seckey,
+    unsigned int k,
+    const unsigned char *label_tweak32
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/modules/silentpayments/main_impl.h b/src/modules/silentpayments/main_impl.h
index 2859ed36ef..abbc94c203 100644
--- a/src/modules/silentpayments/main_impl.h
+++ b/src/modules/silentpayments/main_impl.h
@@ -320,4 +320,35 @@ int secp256k1_silentpayments_create_output_pubkey(const secp256k1_context *ctx,
     return 1;
 }
 
+int secp256k1_silentpayments_create_output_seckey(const secp256k1_context *ctx, unsigned char *output_seckey, const unsigned char *shared_secret33, const unsigned char *receiver_spend_seckey, unsigned int k, const unsigned char *label_tweak32) {
+    secp256k1_scalar t_k_scalar;
+    secp256k1_scalar final_seckey;
+    int ret;
+
+    /* Sanity check inputs */
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(output_seckey != NULL);
+    memset(output_seckey, 0, 32);
+    ARG_CHECK(shared_secret33 != NULL);
+    ARG_CHECK(receiver_spend_seckey != NULL);
+
+    /* Apply label tweak if provided */
+    ret = secp256k1_scalar_set_b32_seckey(&final_seckey, receiver_spend_seckey);
+    VERIFY_CHECK(ret);
+    (void)ret;
+    if (label_tweak32 != NULL) {
+        secp256k1_scalar tweak_scalar;
+        secp256k1_scalar_set_b32(&tweak_scalar, label_tweak32, NULL);
+        secp256k1_eckey_privkey_tweak_add(&final_seckey, &tweak_scalar);
+    }
+
+    /* Compute and return d = (b_m + t_k) mod n */
+    secp256k1_silentpayments_create_t_k(&t_k_scalar, shared_secret33, k);
+    secp256k1_eckey_privkey_tweak_add(&final_seckey, &t_k_scalar);
+    secp256k1_scalar_get_b32(output_seckey, &final_seckey);
+    secp256k1_scalar_clear(&final_seckey);
+
+    return 1;
+}
+
 #endif