json: Add helper to extract a secret from JSON

common/json.c

@@ -143,6 +143,12 @@ bool json_to_bool(const char *buffer, const jsmntok_t *tok, bool *b)
 	return false;
 }
 
+bool json_to_secret(const char *buffer, const jsmntok_t *tok, struct secret *dest)
+{
+	return hex_decode(buffer + tok->start, tok->end - tok->start,
+			  dest->data, sizeof(struct secret));
+}
+
 u8 *json_tok_bin_from_hex(const tal_t *ctx, const char *buffer, const jsmntok_t *tok)
 {
 	u8 *result;

common/json.h

@@ -2,6 +2,7 @@
 #define LIGHTNING_COMMON_JSON_H
 #include "config.h"
 #include <bitcoin/preimage.h>
+#include <bitcoin/privkey.h>
 #include <ccan/tal/tal.h>
 #include <stdbool.h>
 #include <stdint.h>
@@ -49,6 +50,9 @@ bool json_to_int(const char *buffer, const jsmntok_t *tok, int *num);
 /* Extract boolean from this */
 bool json_to_bool(const char *buffer, const jsmntok_t *tok, bool *b);
 
+/* Extract a secret from this. */
+bool json_to_secret(const char *buffer, const jsmntok_t *tok, struct secret *dest);
+
 /* Is this a number? [0..9]+ */
 bool json_tok_is_num(const char *buffer, const jsmntok_t *tok);