Is it possible to compute a file's SHA1 ID usi

Question

If this were possible to do prior to posting a form, it may save me having to upload the file to my server...

Answer 1

Not natively, no, and this is a bad idea anyway. Every byte in the file will have to be loaded into memory by Javascript, and you'd need a way to get it there.

If you must do this and you've got a way to put the file's binary information into your script, then there's plenty of third-party scripts you can use. Here's one, for example.

Answer 2

Actually you can read the contents of a client-side file now, as long as it's chosen in a file upload field and you are using Firefox. See the input.files array. You can then indeed hash it, although it might be rather slow.

See How would I get a Hash value of a users file with Javascript or Flash? for an example and a compact SHA-1 implementation.

Answer 3

You could do this with a Java applet. I've never used any of them, but there are quite a few Java upload applets out there. The hash algorithm itself is available with Java and can be accessed through java.security.MessageDigest. If the client doesn't have the Java plug-in available you could just fail back to a regular upload and hash on the server.

A side note: depending upon why you're hashing the file you'll probably want to re-hash it on the server after the upload rather than trust the client.

Answer 4

To do that you would have to load the file's binary information into JavaScript. Which is not possible.

But here's an implementation of SHA1 in JavaScript.

Answer 5

One can read their local file using the HTML5 File interface: https://developer.mozilla.org/en-US/docs/Web/API/File

And then you can use a library for like Crypto.js https://code.google.com/p/crypto-js/ to finish the hash over the read text.

Answer 6

It is possible to use SHA1, though performance isn't going to be the best...

For anything over a few hundred KB's you will have to run some benchmarks and determine if indeed its a viable solution.

See this link for a good implementation (passpack and quite a few OS projects use it)

Edit: As other have already replied, actually getting the file contents may be a whole different matter - so unless you use something like Google Gears or Adobe AIR it should be virtually impossible.