diff --git a/common/utils/utils.c b/common/utils/utils.c
index cf5673fa56ec63a73d3e0193359bee0694811553..a807a27096f661002d87ab5023291aa62b34fe89 100644
--- a/common/utils/utils.c
+++ b/common/utils/utils.c
@@ -32,12 +32,17 @@ void *malloc_or_fail(size_t size) {
  **              Others:        None                                       **
  **                                                                        **
  ** Outputs:     None                                                      **
- **              Return:        Converted value                            **
+ **              Return:        Converted value (-1 on error)              **
  **              Others:        None                                       **
  **                                                                        **
  ***************************************************************************/
-uint8_t hex_char_to_hex_value (char c)
+int hex_char_to_hex_value (char c)
 {
+  if (!((c >= 'a' && c <= 'f') ||
+        (c >= 'A' && c <= 'F') ||
+        (c >= '0' && c <= '9')))
+    return -1;
+
   if (c >= 'A') {
     /* Remove case bit */
     c &= ~('a' ^ 'A');
@@ -60,17 +65,32 @@ uint8_t hex_char_to_hex_value (char c)
  **              Others:        None                                       **
  **                                                                        **
  ** Outputs:     hex_value:     Converted value                            **
- **              Return:        None                                       **
+ **              Return:        0 on success, -1 on error                  **
  **              Others:        None                                       **
  **                                                                        **
  ***************************************************************************/
-void hex_string_to_hex_value (uint8_t *hex_value, const char *hex_string, int size)
+int hex_string_to_hex_value (uint8_t *hex_value, const char *hex_string, int size)
 {
   int i;
 
+  if (strlen(hex_string) != size*2) {
+    fprintf(stderr, "the string '%s' should be of length %d\n", hex_string, size*2);
+    return -1;
+  }
+
   for (i=0; i < size; i++) {
-    hex_value[i] = (hex_char_to_hex_value(hex_string[2 * i]) << 4) | hex_char_to_hex_value(hex_string[2 * i + 1]);
+    int a = hex_char_to_hex_value(hex_string[2 * i]);
+    int b = hex_char_to_hex_value(hex_string[2 * i + 1]);
+    if (a == -1 || b == -1) goto error;
+    hex_value[i] = (a << 4) | b;
   }
+  return 0;
+
+error:
+  fprintf(stderr, "the string '%s' is not a valid hexadecimal string\n", hex_string);
+  for (i=0; i < size; i++)
+    hex_value[i] = 0;
+  return -1;
 }
 
 char *itoa(int i) {
diff --git a/common/utils/utils.h b/common/utils/utils.h
index a4444a7793b0180925322b573145a43e970f4e93..cfe3df34b9356e02fe33e050198af154b415a472 100644
--- a/common/utils/utils.h
+++ b/common/utils/utils.h
@@ -8,9 +8,9 @@ void *calloc_or_fail(size_t size);
 void *malloc_or_fail(size_t size);
 
 // Converts an hexadecimal ASCII coded digit into its value. **
-uint8_t hex_char_to_hex_value (char c);
+int hex_char_to_hex_value (char c);
 // Converts an hexadecimal ASCII coded string into its value.**
-void hex_string_to_hex_value (uint8_t *hex_value, const char *hex_string, int size);
+int hex_string_to_hex_value (uint8_t *hex_value, const char *hex_string, int size);
 
 char *itoa(int i);
 
diff --git a/openair3/NAS/TOOLS/conf_usim.c b/openair3/NAS/TOOLS/conf_usim.c
index 50043632759cc9afda4b52cf30bb05f2484a32fc..3a5c6dbf6d146285fd0e3dedc37b5b8bf6d07ecf 100644
--- a/openair3/NAS/TOOLS/conf_usim.c
+++ b/openair3/NAS/TOOLS/conf_usim.c
@@ -287,11 +287,14 @@ void gen_usim_data(usim_data_conf_t *u, usim_data_t *usim_data,
 	usim_data->nasconfig.Timer_T3245_Behaviour.length = 1;
 	usim_data->nasconfig.Timer_T3245_Behaviour.value[0] = 0x00;
 
-	/* initialize the subscriber authentication security key */
-	hex_string_to_hex_value(usim_data->keys.usim_api_k,
-			u->usim_api_k, USIM_API_K_SIZE);
-	hex_string_to_hex_value(usim_data->keys.opc, u->opc,
-	OPC_SIZE);
+        /* initialize the subscriber authentication security key */
+        if (hex_string_to_hex_value(usim_data->keys.usim_api_k,
+                                    u->usim_api_k, USIM_API_K_SIZE) == -1 ||
+            hex_string_to_hex_value(usim_data->keys.opc,
+                                    u->opc, OPC_SIZE) == -1) {
+          fprintf(stderr, "fix your configuration file\n");
+          exit(1);
+        }
 }
 
 bool write_usim_data(const char *directory, int user_id, usim_data_t *usim_data){