Mask a whole file before you attach it to an AI chat

Why a file, and why before the upload

Support desks and AI assistants increasingly ask for the whole file: a 40 MB application log, a .env to explain a startup error, or a HAR capture of a failing login. An AI chat cannot mask a file for you, because by the time it could read it, the file has already been sent. A HAR capture is the worst case: it records every request header, so it holds your live session cookies and Authorization: Bearer tokens, which are enough to sign in as you until they expire.

How a big file is masked without splitting a secret

The file is read in 4 MB slices and masked by a background worker, so the page stays responsive and the progress bar moves. A first quick pass only looks for text that already has the shape of a placeholder, such as [EMAIL_1], so the masker never issues one that is already in the file and un-masking can never change a line you did not mask. The second pass masks.

• Text is masked in chunks of about 256 KB that end only at a line break. The partial last line is carried into the next chunk. A single line longer than a chunk, such as minified JSON, has to be cut inside the line, but never inside a value a rule has found, and the next chunk still sees the text before the cut.
• If a -----BEGIN … PRIVATE KEY----- block has started but its END line has not arrived yet, the whole block is carried forward until it has, so a key is always hidden as one [PRIVATE_KEY_1].
• Each chunk also sees the previous 2 KB of lines as context. A rule that needs a nearby word, such as a US Social Security number that is only masked when SSN stands within 40 characters before it, works across the chunk edge too.
• One placeholder list covers the whole file: an address that appears on line 3 and line 900,000 is [EMAIL_1] both times, so the AI can still see it is the same person.

The same 229 detectors as the paste box run on every line: cloud and API keys (including 196 provider key formats ported from gitleaks), wallet recovery phrases, JWTs, private keys, bearer tokens, cookies, passwords in config lines and connection strings, e-mail addresses, phone numbers, IBANs, card numbers, eleven national ID numbers and IP addresses. Each rule, with what it deliberately leaves alone, is listed on the secret types page. On a sparse application log, the engine masks roughly 10 MB a second on one core of a typical server CPU; a phone is slower, and a log with a secret on every line is slower too.

What happens to a HAR file

A .har file, or a .json file that starts with {"log": the way browsers export a HAR, is masked by field as well as by pattern, because a session cookie called sid with a random value has no pattern to find:

• request and response header values for Cookie (every value, names kept), Set-Cookie (the value, attributes kept), Authorization and Proxy-Authorization (the credential, the scheme kept), and any header whose name contains token, api-key, apikey, secret or session;
• every request.cookies[].value and response.cookies[].value;
• queryString[] and postData.params[] values whose name contains one of those words or password, credential or signature, and the same parameters inside request.url, redirectURL, Location and Referer headers, and a form-encoded postData.text;
• every other string, including response bodies, through the same detectors as a text file.

"url": "…/v1/me?access_token=q8Zt…&lang=en"
  → "url": "…/v1/me?access_token=[SECRET_1]&lang=en"
{"name": "cookie", "value": "sid=8f3a9c…; theme=dark"}
  → {"name": "cookie", "value": "sid=[COOKIE_1]; theme=[COOKIE_2]"}
{"name": "authorization", "value": "Bearer eyJhbGciOi…"}
  → {"name": "authorization", "value": "Bearer [BEARER_1]"}

The same value keeps the same placeholder everywhere, so the token in the URL and in queryString are both [SECRET_1]. Only the text inside strings changes: the masked HAR is still valid JSON with the same structure, and it opens in a HAR viewer. A HAR file is read into memory whole, so a very large capture needs a browser with memory to spare; plain-text files are streamed.

The download and the mapping file

The masked copy is named after the original with .masked before the extension: app.log becomes app.masked.log, capture.har becomes capture.masked.har. The mapping is a separate JSON file listing each placeholder and the exact text it replaced. You only need it if the tab is closed before the AI's answer comes back: load it in step 2 and the answer restores exactly as it would have in this tab. For a UTF-8 file, un-masking the whole masked copy with its mapping gives back the original byte for byte; that is how this page is tested, on a 50 MB log and on a HAR capture.