gluon-mesh-vpn-wireguard: add gluon-hex-to-b64

gluon-hex-to-b64 takes base64 content such as a fastd private key
in legacy form via stdin and emits it in base64 encoded (WireGuard) form.

Provides basic return codes.

package/gluon-mesh-vpn-wireguard/Makefile

@@ -6,7 +6,13 @@ include ../gluon.mk
 
 define Package/gluon-mesh-vpn-wireguard
   TITLE:=Support for connecting meshes via wireguard
-  DEPENDS:=+gluon-core +libgluonutil +gluon-mesh-vpn-core +wireguard-tools +wgpeerselector +libubus
+  DEPENDS:=+gluon-core +libgluonutil +gluon-mesh-vpn-core +wireguard-tools +wgpeerselector +libubox +libubus
+endef
+
+define Package/gluon-mesh-vpn-wireguard/install
+	$(Gluon/Build/Install)
+	$(INSTALL_DIR) $(1)/usr/sbin
+	$(INSTALL_BIN) $(PKG_BUILD_DIR)/gluon-hex-to-b64 $(1)/usr/sbin/
 endef
 
 $(eval $(call BuildPackageGluon,gluon-mesh-vpn-wireguard))

package/gluon-mesh-vpn-wireguard/src/Makefile

-all: respondd.so
+all: respondd.so gluon-hex-to-b64
 
 CFLAGS += -Wall -Werror-implicit-function-declaration
 
+gluon-hex-to-b64: gluon-hex-to-b64.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -Wall -o $@ $^ $(LDLIBS) -lubox
+
 respondd.so: respondd.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -shared -fPIC -D_GNU_SOURCE -o $@ $^ $(LDLIBS) -lgluonutil -lubus

package/gluon-mesh-vpn-wireguard/src/gluon-hex-to-b64.c

+// SPDX-FileCopyrightText: 2022 Jan-Niklas Burfeind <gluon@aiyionpri.me>
+// SPDX-License-Identifier: BSD-2-Clause
+// SPDX-FileContributor: read_hex() by Matthias Schiffer <mschiffer@universe-factory.net>
+
+#include <libubox/utils.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/**
+ * how many blocks should be encoded at once - can be configured
+ */
+#define BLOCK_AMOUNT 32
+
+/**
+ * smallest possible block size to encode in b64 without further contex
+ * is three bytes - do not change
+ */
+#define CHUNK_SIZE (3*BLOCK_AMOUNT)
+
+/** print usage info and exit as failed */
+static void usage(void) {
+	fprintf(stderr, "Usage: gluon-hex-to-b64\n");
+	exit(1);
+}
+
+/**
+ * read a string of hexadecimal characters and return them as bytes
+ * return false in case any non-hexadecimal characters are provided
+ * return true on success
+ */
+static bool read_hex(uint8_t key[CHUNK_SIZE], const char *hexstr) {
+	if (strspn(hexstr, "0123456789abcdefABCDEF") != strlen(hexstr))
+		return false;
+
+	size_t i;
+	for (i = 0; i < CHUNK_SIZE; i++)
+		sscanf(&hexstr[2 * i], "%02hhx", &key[i]);
+
+	return true;
+}
+
+int main(int argc, char *argv[]) {
+	if (argc != 1)
+		usage();
+
+	unsigned char hex_input[CHUNK_SIZE * 2 + 1];
+	uint8_t as_bytes[CHUNK_SIZE];
+	int byte_count;
+	int b64_buflen = B64_ENCODE_LEN(CHUNK_SIZE);
+	int b64_return;
+	size_t ret;
+
+	char str[b64_buflen];
+
+	do {
+		ret = fread(hex_input, 1, sizeof(hex_input) - 1, stdin);
+		hex_input[ret] = '\0';
+
+		/* in case fread did not fill six characters */
+		if (ret != sizeof(hex_input)-1) {
+			/* drop newline by replacing it with a null character */
+			hex_input[strcspn(hex_input, "\n")] = 0;
+
+			/*
+			 * count length of resulting string and make sure it's even,
+			 * as bytes are represented using pairs of hex characters
+			 */
+			ret = strlen(hex_input);
+			if (ret % 2 == 1) {
+				fprintf(stderr, "Error: Incomplete hex representation of a byte.\n");
+				exit(EXIT_FAILURE);
+			}
+		}
+
+		byte_count = ret / 2;
+		b64_buflen = B64_ENCODE_LEN(byte_count);
+
+		/* in case read_hex fails due to invalid characters */
+		if (!read_hex(as_bytes, hex_input)) {
+			fprintf(stderr, "Error: Invalid hexadecimal input.\n");
+			exit(EXIT_FAILURE);
+		}
+
+		b64_return = b64_encode(as_bytes, byte_count, str, b64_buflen);
+
+		/* trailing '\0' is not counted by b64_encode(), so we subtract one character */
+		if (b64_buflen - 1 != b64_return) {
+			fprintf(stderr, "Error: Encoding bytes as b64 failed.\n");
+			exit(EXIT_FAILURE);
+		}
+
+		printf("%s", str);
+	/* repeat until a non full block is read */
+	} while (ret == sizeof(hex_input)-1);
+	printf("\n");
+
+	exit(EXIT_SUCCESS);
+}