silentpayments: implement shared secret creation

Add function for calculating either a*B or A*b. This function is used by both
the sender and the recipient (i.e. a*B == A*b).

Add functions for generating the tagged input hash, as this is used when
creating the shared secret.

In a later commit, `create_shared_secret` will be exposed in the API for
the receiver to use when scanning as a light client.

Author: theStack

src/modules/silentpayments/main_impl.h

@@ -7,6 +7,8 @@
 #define SECP256K1_MODULE_SILENTPAYMENTS_MAIN_H
 
 #include "../../../include/secp256k1.h"
+#include "../../../include/secp256k1_ecdh.h"
+#include "../../../include/secp256k1_extrakeys.h"
 #include "../../../include/secp256k1_silentpayments.h"
 
 /** Sort an array of silent payment recipients. This is used to group recipients by scan pubkey to
@@ -41,4 +43,84 @@ int secp256k1_silentpayments_recipient_sort(const secp256k1_context* ctx, const
     return 1;
 }
 
+/** 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(unsigned char *input_hash, const unsigned char *outpoint_smallest36, secp256k1_ge *pubkey_sum) {
+    secp256k1_sha256 hash;
+    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, input_hash);
+}
+
+/* secp256k1_ecdh expects a hash function to be passed in or uses its default
+ * hashing function. We don't want to hash the ECDH result yet (it will be
+ * hashed later with a counter `k`), so we define a custom function which simply
+ * returns the pubkey without hashing.
+ */
+static int secp256k1_silentpayments_ecdh_return_pubkey(unsigned char *output, const unsigned char *x32, const unsigned char *y32, void *data) {
+    secp256k1_ge point;
+    secp256k1_fe x, y;
+    size_t ser_size;
+    int ser_ret;
+
+    (void)data;
+    /* Parse point as group element */
+    if (!secp256k1_fe_set_b32_limit(&x, x32) || !secp256k1_fe_set_b32_limit(&y, y32)) {
+        return 0;
+    }
+    secp256k1_ge_set_xy(&point, &x, &y);
+
+    /* Serialize as compressed pubkey */
+    ser_ret = secp256k1_eckey_pubkey_serialize(&point, output, &ser_size, 1);
+    VERIFY_CHECK(ser_ret && ser_size == 33);
+    (void)ser_ret;
+
+    return 1;
+}
+
+int secp256k1_silentpayments_create_shared_secret(const secp256k1_context *ctx, unsigned char *shared_secret33, const unsigned char *secret_component, const secp256k1_pubkey *public_component, unsigned char *input_hash) {
+    unsigned char tweaked_secret_component[32];
+    /* Sanity check inputs */
+    ARG_CHECK(shared_secret33 != NULL);
+    memset(shared_secret33, 0, 33);
+    ARG_CHECK(public_component != NULL);
+    ARG_CHECK(secret_component != NULL);
+
+    /* Tweak secret component with input hash, if available */
+    memcpy(tweaked_secret_component, secret_component, 32);
+    if (input_hash != NULL) {
+        if (!secp256k1_ec_seckey_tweak_mul(ctx, tweaked_secret_component, input_hash)) {
+            return 0;
+        }
+    }
+
+    /* Compute shared_secret = tweaked_secret_component * Public_component */
+    if (!secp256k1_ecdh(ctx, shared_secret33, public_component, tweaked_secret_component, secp256k1_silentpayments_ecdh_return_pubkey, NULL)) {
+        return 0;
+    }
+
+    return 1;
+}
+
 #endif