Derive provided_len from string size and bound loop to digest_len

- Use auth.value().size() instead of strlen() to avoid rescanning
  attacker-controlled header content.
- Iterate over digest_len (expected length) instead of max_len so a
  long Authorization header cannot force extra work. The full-width
  length XOR already rejects any length mismatch.

esphome/components/web_server_idf/web_server_idf.cpp

@@ -356,21 +356,22 @@ bool AsyncWebServerRequest::authenticate(const char *username, const char *passw
 
   // Constant-time comparison to avoid timing side channels.
   // No early return on length mismatch — the length difference is folded
-  // into the accumulator and the loop runs over the longer of the two
-  // strings so extra trailing bytes are also compared.
+  // into the accumulator so any mismatch is rejected.
   const char *provided = auth_str + auth_prefix_len;
   size_t digest_len = out;  // length from esp_crypto_base64_encode
-  size_t provided_len = strlen(provided);
+  // Derive provided_len from the already-sized std::string rather than
+  // rescanning with strlen (avoids attacker-controlled scan length).
+  size_t provided_len = auth.value().size() - auth_prefix_len;
   // Use full-width XOR so any bit difference in the lengths is preserved
   // (uint8_t truncation would miss differences in higher bytes, e.g.
   // digest_len vs digest_len + 256).
   volatile size_t result = digest_len ^ provided_len;
-  size_t max_len = (digest_len > provided_len) ? digest_len : provided_len;
-  for (size_t i = 0; i < max_len; i++) {
-    // Bounds-safe: substitute 0 for bytes beyond each string's length
-    char digest_ch = (i < digest_len) ? digest.get()[i] : 0;
+  // Iterate over the expected digest length only — the full-width length
+  // XOR above already rejects any length mismatch, and bounding the loop
+  // prevents a long Authorization header from forcing extra work.
+  for (size_t i = 0; i < digest_len; i++) {
     char provided_ch = (i < provided_len) ? provided[i] : 0;
-    result |= static_cast<uint8_t>(digest_ch ^ provided_ch);
+    result |= static_cast<uint8_t>(digest.get()[i] ^ provided_ch);
   }
   return result == 0;
 }