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I need to generate a RSA key pair with Javascript. This key pair will be used for encrypting a one time blob of data (20-100kb), then discarded. This data can be reduced to a couple of hundred kb or less if I only encrypt data that needs to be secure and not other resources such as image files for the application.

Should I write my own implementation or are there any libraries that I can trust to be both stable and secure?

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Definitely do not write your own implementation. Just don't. Better to redesign your whole application, than to write your own implementation... – AviD♦ Jul 26 '11 at 5:40

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You could take a look at the GWT Crypto package. Its a port of some of the BouncyCastle stuff into GWT, so you should be able to take the compiled output and massage it a bit so you can call it from your current Javascript. It currently supports RSA keypair generation and encryption/decryption, as well as AES and DES. I think that's about it though.

I personally have not used this library, but since it was ported from BC it should be safer than writing your own implementation. That said, I dont know where they sourced their SecureRandom implementation from. You may need to look to see if its actually a cryptographically secure implementation or not. I took a quick look and they are just doing something with SHA and the seed bytes, but I dont know if that qualifies or not.

One last thing. I have found using RSA to encrypt large amounts of data to be quite slow. Perhaps you may want to use a hybrid solution of some sort where you encrypt an AES256 key using the RSA keypair and then use AES to encrypt the data? Could be faster...

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I can change my code to just encrypt the parts of the package that differ between clients (ie. the secure stuff). How do I know BouncyCastle is secure? – Casebash Jul 26 '11 at 5:00
Aside from paying someone you trust to inspect it? I suppose you dont. However, BouncyCastle is the crypto provider in Google's Android. Perhaps if its good enough for them its good enough for you? That's really a tough question and I dont know what right answer is. As far as I know it doesnt have any government certifications or anything... – senecaso Jul 26 '11 at 5:13
It appears that the Google Web Toolkit is not really designed to support this use case: -groups.google.com/forum/#!topic/google-web-toolkit/2a6HnMJJ0ss. I could use GWT Exporter, but I think its best just to find a javascript library – Casebash Jul 27 '11 at 5:38
Ya, like I said, you would have to "massage" the output. Still, its almost certainly safer than implementing your own! :) You could try jscryptolib, but I dont know how trusted it is, not to mention there hasnt been any activity on the project for ~2yrs. The other implementations that come up with a google search seem to be personal/student projects, so I dont know how much faith should be put into them being correct. – senecaso Jul 27 '11 at 6:26

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