introduce struct to hold DLEQ proof related data for silent payments

- structure contains 33-byte shared secret point + 64-byte DLEQ proof
  + index of recipient in original unsorted array of silent payment
    recipients
- add functions to serialise and parse the structure to/from bytes

Author: stratospher

include/secp256k1_silentpayments.h

@@ -43,6 +43,19 @@ typedef struct {
     size_t index;
 } secp256k1_silentpayments_recipient;
 
+/* This struct contains details of the DLEQ proof
+ *
+ * Fields:
+ * - shared_secret : 33-byte shared secret point
+ * - proof: 64-byte serialized DLEQ proof
+ * - index: Indicates which recipient the proof pertains to based on the original (not sorted) ordering of the addresses
+ */
+typedef struct {
+    unsigned char shared_secret[33];
+    unsigned char proof[64];
+    size_t index;
+} secp256k1_silentpayments_dleq_data;
+
 /** Create Silent Payment outputs for recipient(s).
  *
  *  Given a list of n private keys a_1...a_n (one for each silent payment
@@ -399,6 +412,30 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_silentpayments_recipien
     unsigned int k
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
+/** Serialize a secp256k1_silentpayments_dleq_data object into a 101-byte sequence.
+ *  101-byte sequence = 33 bytes shared secret + 64 bytes proof + 4 bytes index
+ *                      where index is position in an array of pointers to silent payment recipients
+ *
+ *  Out:       output: pointer to a 101-byte array to place the serialized `secp256k1_silentpayments_dleq_data` in
+ *  In:    dleq_data: pointer to an initialized secp256k1_silentpayments_dleq_data object
+ */
+SECP256K1_API void secp256k1_silentpayments_dleq_data_serialize(
+    unsigned char *output33,
+    const secp256k1_silentpayments_dleq_data *dleq_data
+)SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
+/** Parse a 101-byte sequence into a secp256k1_silentpayments_dleq_data object.
+ *  101-byte sequence = 33 bytes shared secret + 64 bytes proof + 4 bytes index
+ *                      where index is position in an array of pointers to silent payment recipients
+ *
+ *  Out:  dleq_data: pointer to a secp256k1_silentpayments_dleq_data object.
+ *  In:        input: pointer to a serialized secp256k1_silentpayments_dleq_data.
+ */
+SECP256K1_API void secp256k1_silentpayments_dleq_data_parse(
+        secp256k1_silentpayments_dleq_data *dleq_data,
+        const unsigned char *input
+)SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
 #ifdef __cplusplus
 }
 #endif

src/modules/silentpayments/main_impl.h

@@ -657,4 +657,16 @@ int secp256k1_silentpayments_recipient_create_output_pubkey(const secp256k1_cont
 }
 
 
+void secp256k1_silentpayments_dleq_data_serialize(unsigned char *output, const secp256k1_silentpayments_dleq_data *dleq_data) {
+    memcpy(output, dleq_data->shared_secret, 33);
+    memcpy(output + 33, dleq_data->proof, 64);
+    secp256k1_write_be32(output + 33 + 64, dleq_data->index);
+}
+
+void secp256k1_silentpayments_dleq_data_parse(secp256k1_silentpayments_dleq_data *dleq_data, const unsigned char *input) {
+    memcpy(dleq_data->shared_secret, input, 33);
+    memcpy(dleq_data->proof, input + 33, 64);
+    dleq_data->index = secp256k1_read_be32(input + 33 + 64);
+}
+
 #endif