silentpayments: add private tweak data creation routine

include/secp256k1_silentpayments.h

@@ -28,7 +28,41 @@ extern "C" {
  * operations.
  */
 
-/* TODO: add function API for sender side. */
+/** Create Silent Payment tweak data from input private keys.
+ *
+ * Given a list of n private keys a_1...a_n (one for each silent payment
+ * eligible input to spend) and a serialized outpoint_smallest, compute
+ * the corresponding input private keys tweak data:
+ *
+ * a_tweaked = (a_1 + a_2 + ... + a_n) * hash(outpoint_smallest || A)
+ *
+ * (where A = A_1 + A_2 + ... + A_n)
+ *
+ * If necessary, the private keys are negated to enforce the right y-parity.
+ * For that reason, the private keys have to be passed in via two different parameter
+ * pairs, depending on whether they were used for creating taproot outputs or not.
+ * The resulting data is needed to create a shared secret for the sender side.
+ *
+ *  Returns: 1 if shared secret creation was successful. 0 if an error occured.
+ *  Args:                  ctx: pointer to a context object
+ *  Out:  private_tweak_data32: pointer to the resulting 32-byte private tweak data
+ *  In:          plain_seckeys: pointer to an array of 32-byte private keys of non-taproot inputs
+ *                              (can be NULL if no private keys of non-taproot inputs are used)
+ *             n_plain_seckeys: the number of sender's non-taproot input private keys
+ *             taproot_seckeys: pointer to an array of 32-byte private keys of taproot inputs
+ *                              (can be NULL if no private keys of taproot inputs are used)
+ *           n_taproot_seckeys: the number of sender's taproot input private keys
+ *         outpoint_smallest36: serialized smallest outpoint
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_create_private_tweak_data(
+    const secp256k1_context *ctx,
+    unsigned char *private_tweak_data32,
+    const unsigned char *plain_seckeys,
+    size_t n_plain_seckeys,
+    const unsigned char *taproot_seckeys,
+    size_t n_taproot_seckeys,
+    const unsigned char *outpoint_smallest36
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(7);
 
 /* TODO: add function API for receiver side. */
 

src/modules/silentpayments/main_impl.h

@@ -9,7 +9,98 @@
 #include "../../../include/secp256k1.h"
 #include "../../../include/secp256k1_silentpayments.h"
 
-/* TODO: implement functions for sender side. */
+/** Set hash state to the BIP340 tagged hash midstate for "BIP0352/Inputs". */
+static void secp256k1_silentpayments_sha256_init_inputs(secp256k1_sha256* hash) {
+    secp256k1_sha256_initialize(hash);
+    hash->s[0] = 0xd4143ffcul;
+    hash->s[1] = 0x012ea4b5ul;
+    hash->s[2] = 0x36e21c8ful;
+    hash->s[3] = 0xf7ec7b54ul;
+    hash->s[4] = 0x4dd4e2acul;
+    hash->s[5] = 0x9bcaa0a4ul;
+    hash->s[6] = 0xe244899bul;
+    hash->s[7] = 0xcd06903eul;
+
+    hash->bytes = 64;
+}
+
+static void secp256k1_silentpayments_calculate_input_hash(secp256k1_scalar *input_hash_scalar, const unsigned char *outpoint_smallest36, secp256k1_ge *pubkey_sum) {
+    secp256k1_sha256 hash;
+    unsigned char hash_ser[32];
+    unsigned char pubkey_sum_ser[33];
+    size_t ser_size;
+    int ser_ret;
+
+    secp256k1_silentpayments_sha256_init_inputs(&hash);
+    secp256k1_sha256_write(&hash, outpoint_smallest36, 36);
+    ser_ret = secp256k1_eckey_pubkey_serialize(pubkey_sum, pubkey_sum_ser, &ser_size, 1);
+    VERIFY_CHECK(ser_ret && ser_size == sizeof(pubkey_sum_ser));
+    (void)ser_ret;
+    secp256k1_sha256_write(&hash, pubkey_sum_ser, sizeof(pubkey_sum_ser));
+    secp256k1_sha256_finalize(&hash, hash_ser);
+    secp256k1_scalar_set_b32(input_hash_scalar, hash_ser, NULL);
+}
+
+int secp256k1_silentpayments_create_private_tweak_data(const secp256k1_context *ctx, unsigned char *private_tweak_data32, const unsigned char *plain_seckeys, size_t n_plain_seckeys, const unsigned char *taproot_seckeys, size_t n_taproot_seckeys, const unsigned char *outpoint_smallest36) {
+    size_t i;
+    secp256k1_scalar a_sum, addend;
+    secp256k1_ge A_sum_ge;
+    secp256k1_gej A_sum_gej;
+    secp256k1_scalar input_hash_scalar;
+
+    /* Sanity check inputs. */
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(private_tweak_data32 != NULL);
+    memset(private_tweak_data32, 0, 32);
+    ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
+    ARG_CHECK(plain_seckeys == NULL || n_plain_seckeys >= 1);
+    ARG_CHECK(taproot_seckeys == NULL || n_taproot_seckeys >= 1);
+    ARG_CHECK((plain_seckeys != NULL) || (taproot_seckeys != NULL));
+    ARG_CHECK((n_plain_seckeys + n_taproot_seckeys) >= 1);
+    ARG_CHECK(outpoint_smallest36 != NULL);
+
+    /* Compute input private keys sum: a_sum = a_1 + a_2 + ... + a_n */
+    a_sum = secp256k1_scalar_zero;
+    for (i = 0; i < n_plain_seckeys; i++) {
+        int ret = secp256k1_scalar_set_b32_seckey(&addend, &plain_seckeys[i*32]);
+        VERIFY_CHECK(ret);
+        (void)ret;
+
+        secp256k1_scalar_add(&a_sum, &a_sum, &addend);
+        VERIFY_CHECK(!secp256k1_scalar_is_zero(&a_sum));
+    }
+    /* private keys used for taproot outputs have to be negated if they resulted in an odd point */
+    for (i = 0; i < n_taproot_seckeys; i++) {
+        secp256k1_ge addend_point;
+        int ret = secp256k1_ec_pubkey_create_helper(&ctx->ecmult_gen_ctx, &addend, &addend_point, &taproot_seckeys[i*32]);
+        VERIFY_CHECK(ret);
+        (void)ret;
+        /* declassify addend_point to allow using it as a branch point (this is fine because addend_point is not a secret) */
+        secp256k1_declassify(ctx, &addend_point, sizeof(addend_point));
+        secp256k1_fe_normalize_var(&addend_point.y);
+        if (secp256k1_fe_is_odd(&addend_point.y)) {
+            secp256k1_scalar_negate(&addend, &addend);
+        }
+
+        secp256k1_scalar_add(&a_sum, &a_sum, &addend);
+        VERIFY_CHECK(!secp256k1_scalar_is_zero(&a_sum));
+    }
+    if (secp256k1_scalar_is_zero(&a_sum)) {
+        /* TODO: do we need a special error return code for this case? */
+        return 0;
+    }
+
+    /* Compute input_hash = hash(outpoint_L || A_sum) */
+    secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &A_sum_gej, &a_sum);
+    secp256k1_ge_set_gej(&A_sum_ge, &A_sum_gej);
+    secp256k1_silentpayments_calculate_input_hash(&input_hash_scalar, outpoint_smallest36, &A_sum_ge);
+
+    /* Compute a_tweaked = a_sum * input_hash */
+    secp256k1_scalar_mul(&a_sum, &a_sum, &input_hash_scalar);
+    secp256k1_scalar_get_b32(private_tweak_data32, &a_sum);
+
+    return 1;
+}
 
 /* TODO: implement functions for receiver side. */