[dnssec] add hex_to_string() API

dnssec/Makefile.am

@@ -11,6 +11,8 @@ libdnssec_la_SOURCES =	\
 	src/binary.c \
 	src/binary.h \
 	src/error.h \
+	src/hex.c \
+	src/hex.h \
 	src/key.c \
 	src/key.h \
 	src/keys/legacy.c \

dnssec/src/hex.c

+#include "binary.h"
+
+static const char BIN_TO_HEX[] = { '0', '1', '2', '3', '4', '5', '6', '7',
+				   '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
+
+char *hex_to_string(const dnssec_binary_t *data)
+{
+	char *str = malloc(data->size * 2 + 1);
+	if (!str) {
+		return NULL;
+	}
+
+	for (size_t i = 0; i < data->size; i++) {
+		str[2*i]   = BIN_TO_HEX[data->data[i] >> 4];
+		str[2*i+1] = BIN_TO_HEX[data->data[i] & 0x0f];
+	}
+	str[2 * data->size] = '\0';
+
+	return str;
+}

dnssec/src/hex.h

+#pragma once
+
+#include "binary.h"
+
+char *hex_to_string(const dnssec_binary_t *data);