compare strings in constant time

[syndrome2]

I understand that on a well-tuned system, the success of a timing attack is doubtful, but why not increase the security for free? compare_in_constant_time.patch.txt

diff '--color=auto' -ru a/shadow.c b/shadow.c
--- a/shadow.c  2022-01-26 18:01:11.000000000 +0200
+++ b/shadow.c  2023-07-19 08:27:01.806569020 +0300
@@ -41,6 +41,19 @@
 #define HOST_NAME_MAX _POSIX_HOST_NAME_MAX
 #endif

+int compare_in_constant_time(const char * str1, const char * str2)
+{
+   char result = 0;
+   while (1)
+   {
+       char a = *str1 ^ *str2;
+       result = result | a;
+       if ((*str1 == 0) || (*str2 == 0)) {break;}
+       str1++; str2++;
+   }
+   return (int)result;
+}
+
 void
 shadowauth(const char *myname, int persist)
 {
@@ -94,7 +107,7 @@
        errx(1, "Authentication failed");
    }
    explicit_bzero(rbuf, sizeof(rbuf));
-   if (strcmp(encrypted, hash) != 0) {
+   if (compare_in_constant_time(encrypted, hash) != 0) {
        syslog(LOG_AUTHPRIV | LOG_NOTICE, "failed auth for %s", myname);
        errx(1, "Authentication failed");
    }