Add comments explaining constant-time comparison logic

esphome/components/web_server_idf/web_server_idf.cpp

@@ -354,13 +354,19 @@ bool AsyncWebServerRequest::authenticate(const char *username, const char *passw
   esp_crypto_base64_encode(reinterpret_cast<uint8_t *>(digest.get()), n, &out,
                            reinterpret_cast<const uint8_t *>(user_info), user_info_len);
 
-  // Constant-time comparison to avoid timing side channels
+  // Constant-time comparison to avoid timing side channels.
+  // No early return on length mismatch — the length difference is folded
+  // into the accumulator so the loop always runs over the full digest.
   const char *provided = auth_str + auth_prefix_len;
-  size_t digest_len = out;
+  size_t digest_len = out;  // length from esp_crypto_base64_encode
   size_t provided_len = strlen(provided);
   volatile uint8_t result = 0;
+  // Non-zero if lengths differ; XOR of two size_t values truncated to 8 bits
+  // catches differences in the low byte, and any length mismatch also causes
+  // the byte-wise loop below to accumulate non-zero XOR values.
   result |= static_cast<uint8_t>(digest_len ^ provided_len);
   for (size_t i = 0; i < digest_len; i++) {
+    // Bounds-safe: use 0 for bytes beyond provided_len
     char provided_ch = (i < provided_len) ? provided[i] : 0;
     result |= static_cast<uint8_t>(digest.get()[i] ^ provided_ch);
   }