+/* Return zero if strings are the same, non-zero if they are not.
+ * The comparison is performed in a way that prevents an attacker to obtain
+ * information about the nature of the strings just monitoring the execution
+ * time of the function.
+ *
+ * Note that limiting the comparison length to strings up to 512 bytes we
+ * can avoid leaking any information about the password length and any
+ * possible branch misprediction related leak.
+ */
+int time_independent_strcmp(char *a, char *b) {
+ char bufa[REDIS_AUTHPASS_MAX_LEN], bufb[REDIS_AUTHPASS_MAX_LEN];
+ /* The above two strlen perform len(a) + len(b) operations where either
+ * a or b are fixed (our password) length, and the difference is only
+ * relative to the length of the user provided string, so no information
+ * leak is possible in the following two lines of code. */
+ int alen = strlen(a);
+ int blen = strlen(b);
+ int j;
+ int diff = 0;
+
+ /* We can't compare strings longer than our static buffers.
+ * Note that this will never pass the first test in practical circumstances
+ * so there is no info leak. */
+ if (alen > sizeof(bufa) || blen > sizeof(bufb)) return 1;
+
+ memset(bufa,0,sizeof(bufa)); /* Constant time. */
+ memset(bufb,0,sizeof(bufb)); /* Constant time. */
+ /* Again the time of the following two copies is proportional to
+ * len(a) + len(b) so no info is leaked. */
+ memcpy(bufa,a,alen);
+ memcpy(bufb,b,blen);
+
+ /* Always compare all the chars in the two buffers without
+ * conditional expressions. */
+ for (j = 0; j < sizeof(bufa); j++) {
+ diff |= (bufa[j] ^ bufb[j]);
+ }
+ /* Length must be equal as well. */
+ diff |= alen ^ blen;
+ return diff; /* If zero strings are the same. */
+}
+
projects / redis.git / blobdiff