Guides: what is in your files, and who can see it

Answers, not adverts

Every one of these was written by checking it. The field names, the byte offsets and the arithmetic in them came from reading the formats and from building files and looking at what was actually in them, and anything that could not be checked was left out and said so. Each ends at the tool on this site that does the thing, and every one of those runs in your browser with nothing uploaded.

What is in my file that I did not put there

Does a Photo Show Where It Was Taken? Usually yes, to about a third of a metre. A photograph from a phone normally carries the latitude and longitude of the spot it was taken, the time… What Is Hidden in a Word Document A .docx is a zip archive full of XML, and the words on the page are only one part of it. Data Still in a Spreadsheet You Deleted An .xlsx is a zip archive of XML parts, and deleting something from the grid changes one part and leaves the rest. Can Text Under a PDF Redaction Be Recovered? Usually, yes, and it takes no skill: select the blacked-out area and copy it, or open the file with anything that extracts text.

Can I trust what somebody sent me

How to Tell If an Email Is Really From Who It Says Ignore the From line: it is text the sender typed, and nothing in delivery checks it. What a QR Code Can Hide, and How to Check One A QR code is text and nothing more, and the danger is not in the square but in the fact that a person cannot read it before acting on it. How to Check a File Has Not Been Changed Take the SHA-256 of the file you have and compare it, character for character, with the value the publisher gave you.

What am I giving away

What a Website Learns About You Before You Click Before you click anything, a site has your IP address and the handful of headers every request carries. What utm_source and fbclid on a Link Actually Do Campaign tags and click identifiers are two different things wearing the same clothes. Is It Safe to Upload a File to a Free Online Tool? It depends much less on the tool's promises than on one fact: the moment you upload, the file is a copy on somebody else's computer, and every…

Doing it safely

How to Make a Password You Can Actually Remember Pick the words at random from a published list instead of inventing a password yourself. How to Send a File Without It Sitting on a Server Encrypt it to the recipient's public key, or hand it over directly between two browsers.

What this is, and what it is not

Everything here was checked before it was written. The names of fields, the byte offsets, the parameter lists and the arithmetic were taken from reading the file formats and from building files and looking inside them, not from memory. Where something could not be verified it was left out, and several of these pages say so where the gap would otherwise be invisible.

They are about the problem, not about this site. Each one explains a thing that is true whatever you use to deal with it, and says plainly what it does not cover and what checking cannot tell you. The tools at the end are the ones here because they are the ones to hand, and every one of them works in your browser without sending anything anywhere.

Nothing on this page tracks you. No analytics, no cookies set by this page, and no request to any other company. That is the same on every page of this site, and there is a page that lets you check it rather than take it on trust.

Questions people ask

Who are these written for?

Somebody who has run into the problem and wants to understand it, not somebody looking for a product. They assume you are capable and that you have not happened to look at this particular thing before.

Are they kept up to date?

They are written against the file formats and the specifications, which change slowly. Where something is a judgement about how a tool behaves today rather than a property of the format, the page says which it is.

Can I use them at work?

Read them anywhere. The tools they lead to are free for personal use, and using them in the course of a business needs a commercial licence.

Something here is wrong. What then?

Say so and it gets corrected. The whole argument of this site is that it tells you the truth about your own files, and a page here that is wrong damages that more than it helps anything.

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