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Split a File Into Parts in the Browser: The SHA-256 Manifest and a Verified Rejoin

#files #javascript #webdev #tools

I work on OnePassApps, so I am writing about a tool we ship.

Splitting a big file usually means uploading it: to a converter site, to a drive, to yourself. The File Studio at app.onepassapps.com splits a file into parts and hands you back a bundle with a manifest, in the tab.

What the split panel actually says

The panel is three stages, and the copy is specific:

  • Part size (MiB) - "A whole number from 1 to 64."
  • Result - "Ready", then "Created 1 part and a verified manifest for 3.1 MiB."
  • Bundle - the ZIP is named after the file: split-quarterly-notes.txt-parts.zip
  • Original SHA-256 - the full 64-character hash of the file you started with
  • Each part row carries its own name, its size and its own hash, shown truncated as "SHA-256 ffb369fd..."
  • "Created in 0.7 s" and "Parts and manifest ready to download."

A real run

I dropped a 3.1 MiB text file (42,000 repeated lines) on the stage and ran the split in the browser. What the panel gave back:

Created 1 part and a verified manifest for 3.1 MiB.
Bundle        split-quarterly-notes.txt-parts.zip
Original SHA-256  ffb369fd0962f7111f6bd2b0f6a9ef3328b3b94da5a4ef501ea47f5f2fac1690
Created in 0.7 s

quarterly-notes-part1-of-1.txt   3.1 MiB   SHA-256 ffb369fd...
Enter fullscreen mode Exit fullscreen mode

Two details worth keeping: the part is named quarterly-notes-part1-of-1.txt, so the name carries the run's own part count, and the bundle is a ZIP that holds the parts and the manifest, not just the parts.

The join side is the interesting half

The Join parts tab is where the manifest earns its keep. Its own copy:

  • Verification - "SHA-256, part by part"
  • Result - "The restored file is only offered when its checksum matches the original."
  • The drop hint - "manifest.json plus every part from one split bundle - each part is checked by SHA-256"
  • The action - "Verify and join parts"

So the manifest is not a list of instructions, it is the thing the parts get checked against: each part is verified by SHA-256, and the restored file is offered only when the checksum matches the original. Nothing about the file names is trusted, and "Nothing leaves this tab." holds on this side too - the manifest and the parts are read locally, like everything else here.

What it costs

The panel shows the plan next to the tool: "Standard is free right now", "$0 / month", "100,000 credits every 30 days - No end date set - No card required", and "No automatic paid subscription. Existing paid subscriptions keep their terms."

If you want the full walkthrough of the other File Studio tools, the guides list has the rest of them.

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