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I'm trying to set up a web service so that it will download zip files periodically, based on a file that tells it if more are ready.

That's simple enough (I'm sure), but what I'd like to do is to sign these zip files with a digital signature (I have a public/private certificate prepared), and to get the WP7 app to verify it before using it.

How can I do this? It doesn't even seem like WP7 supports asymmetric encryption. What's my best option?

EDIT: Speaking of which, ARE there any asymmetric encryption methods? If so, it would be good if I could use them to further secure my data.

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2 Answers 2

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Dustin Horne as created an Open Source Cryptography library for Silverlight and made it available on http://scrypt.codeplex.com/

It doesn't claim support for WP7 but would probably be a very good starting point for further investigation.

However, what are you trying to achieve by signing the zip?

If it's just coming from your own server and you want to confirm data intergrity, would a hash of the contents not be sufficient to confirm it's as you created it on your server?

If you're trying to protect the contents of the zip from prying eyes would transmission only over HTTPS and a protected access to the resource not be sufficient?

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Mm, a hash might work, but I want to make sure it can't be modified later on, hence the signature. Also, asymmetric encryption isn't important, I just realised. I can just password protect the zip. –  Liam Dawson Nov 26 '10 at 11:41
Are you trying to prevent modification? (If so, by whom and does it really matter?) Or do you just want to make sure that you're using something that hasn't been modified? - If the second then you just need the server to provide what the hash of the valid file should be. You then hash it once downloaded and compare to check that you've not got a modified file. –  Matt Lacey Nov 26 '10 at 12:27

Yes it supports WP7. I'll be making a couple mods soon to fix some VB .NET compatibility issues (not having explicit public constructors in some places such as the Signature Providers).

Update: @dawmail - Yes this is the right question. Someone mentioned my RSA library above and I was clarifying that it supports WP7. :) Also the above mentioned compatibility issues have been resolved as well as support added for CspBlob formatted key data.

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right question? –  Liam Dawson Jan 29 '11 at 14:57

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