Move decode_hex into its own file

- Error detection against strotul(3) has been improved, as done in other
places.
- New function encode_hex has been implemented, which will be used
by future commits.

CMakeLists.txt

@@ -5,6 +5,7 @@ add_executable(${PROJECT_NAME}
     base64.c
     cftw.c
     handler.c
+    hex.c
     html.c
     http.c
     jwt.c

Makefile

@@ -14,6 +14,7 @@ OBJECTS = \
 	base64.o \
 	cftw.o \
 	handler.o \
+	hex.o \
 	html.o \
 	http.o \
 	jwt.o \

auth.c

@@ -1,4 +1,5 @@
 #include "auth.h"
+#include "hex.h"
 #include "http.h"
 #include "jwt.h"
 #include <cjson/cJSON.h>
@@ -70,22 +71,6 @@ end:
     return ret;
 }
 
-static int decode_hex(const char *const hex, unsigned char *buf, size_t n)
-{
-    for (const char *s = hex; *s; s += 2)
-    {
-        const char nibble[sizeof "00"] = {*s, *(s + 1)};
-
-        if (!n)
-            return -1;
-
-        *buf++ = strtoul(nibble, NULL, 16);
-        n--;
-    }
-
-    return n ? -1 : 0;
-}
-
 static int find_cookie(const cJSON *const users, const char *const cookie)
 {
     const cJSON *u;
@@ -107,9 +92,9 @@ static int find_cookie(const cJSON *const users, const char *const cookie)
             fprintf(stderr, "%s: missing key\n", __func__);
             return -1;
         }
-        else if (decode_hex(key, dkey, sizeof dkey))
+        else if (hex_decode(key, dkey, sizeof dkey))
         {
-            fprintf(stderr, "%s: decode_hex failed\n", __func__);
+            fprintf(stderr, "%s: hex_decode failed\n", __func__);
             return -1;
         }
 
@@ -177,9 +162,9 @@ static int generate_cookie(const cJSON *const json, const char *const path,
     int ret = -1;
     char *jwt = NULL;
 
-    if (decode_hex(key, dkey, sizeof dkey))
+    if (hex_decode(key, dkey, sizeof dkey))
     {
-        fprintf(stderr, "%s: decode_hex failed\n", __func__);
+        fprintf(stderr, "%s: hex_decode failed\n", __func__);
         goto end;
     }
     else if (!(jwt = jwt_encode(name, dkey, sizeof dkey)))
@@ -220,9 +205,9 @@ static int compare_pwd(const char *const salt, const char *const password,
         fprintf(stderr, "%s: malloc(3): %s\n", __func__, strerror(errno));
         goto end;
     }
-    else if (decode_hex(salt, dec_salt, sizeof dec_salt))
+    else if (hex_decode(salt, dec_salt, sizeof dec_salt))
     {
-        fprintf(stderr, "%s: decode_hex failed\n", __func__);
+        fprintf(stderr, "%s: hex_decode failed\n", __func__);
         goto end;
     }
 

hex.c

@@ -0,0 +1,49 @@
+#include "hex.h"
+#include <errno.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int hex_encode(const void *const b, char *hex, const size_t buflen,
+    size_t hexlen)
+{
+    const char *buf = b;
+
+    for (size_t i = 0; i < buflen; i++)
+    {
+        const int r = snprintf(hex, hexlen, "%02hhx", *(const char *)buf++);
+
+        if (r < 0 || r >= hexlen)
+            return -1;
+
+        hexlen -= r;
+        hex += 2;
+    }
+
+    return 0;
+}
+
+int hex_decode(const char *const hex, void *const b, size_t n)
+{
+    unsigned char *buf = b;
+
+    for (const char *s = hex; *s; s += 2)
+    {
+        const char nibble[sizeof "00"] = {*s, *(s + 1)};
+
+        if (!n)
+            return -1;
+
+        char *end;
+
+        errno = 0;
+        *buf++ = strtoul(nibble, &end, 16);
+
+        if (errno || *end)
+            return -1;
+
+        n--;
+    }
+
+    return n ? -1 : 0;
+}

hex.h

@@ -0,0 +1,9 @@
+#ifndef HEX_H
+#define HEX_H
+
+#include <stddef.h>
+
+int hex_encode(const void *buf, char *hex, size_t buflen, size_t hexlen);
+int hex_decode(const char *hex, void *buf, size_t n);
+
+#endif /* HEX_H */