Build guides

How the tools on this site were actually made: the engine choices, the code, and the mistakes. Written for people who want to build something similar, or who want to check that the privacy claims hold up before trusting them.

These are English only. They are long technical writing whose value is precision, and a machine-translated approximation of a precise claim is just a wrong claim.

• Building a PDF password tool that never uploads your file

  This tool encrypts a PDF with AES-256 in the browser tab, with no server involved. The interesting part is not the encryption, which we did not write. It is everything around it: picking an engine, discovering that the engine drags a hard hosting requirement behind it, and three separate failure modes that all present as a spinner that never stops.

  Uses the tool: Password-protect a PDF

• Building a compressor you give a file size, not a quality slider

  Every image compressor has a quality slider from 1 to 100. Nobody wants a quality of 63. They want a photo under 100 KB because a form refuses anything larger. This tool inverts the control: you state the outcome, and the engine searches for the parameters. That search is the single most reused piece of code on the site, and it knows nothing about images.

  Uses the tool: Compress an image to an exact size

• Building a metadata remover that does not touch your pixels

  Photographs carry more than they look like they do: the camera, the lens, the timestamp and very often the exact coordinates where the shutter opened. Removing that is easy to do badly. The naive approach is to decode the image and re-encode it, which does strip the metadata and also quietly degrades every pixel in the photo. This tool does not decode anything.

  Uses the tool: Remove metadata from a photo

• Building a CSV converter that survives contact with Excel

  Converting a CSV to a spreadsheet is one library call. That is exactly why there are a thousand converters and why most of them hand back a file that is subtly wrong. The call is the easy part. Everything that decides whether the output is usable happens on a machine the author never sees, when somebody else opens it in Excel.

  Uses the tool: CSV to Excel, and back

• Building a video compressor that fits an exact size limit

  Fitting a video under a size limit is the same problem as fitting an image under one, except that a single attempt can take a minute. The search strategy that works for images is unusable here, and the engine is large enough that shipping it at all turned into its own problem.

  Uses the tool: Compress a video to fit a limit

• Building redaction that destroys the text instead of hiding it

  Black rectangles over a PDF are how confidential documents get published with their secrets intact. It has happened to newspapers, law firms and governments, and it keeps happening because the failure is invisible: the page looks correct, and the text is still in the file, selectable by anyone who copies it out. This tool cannot fail that way, and the reason is that it throws away the document.

  Uses the tool: Redact a document or photo

For the shorter version that covers the whole site rather than one tool, see how YappyKit works.