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What I want to do is use asymmetric crypto to guarantee that a file comes from a source. I have software that runs on lots of wireless devices, and each of those has a license file with an encrypted date that they expire. I'd like to be able to encrypt it in the office and have the devices decrypt/read the date. The purpose of using asymmetric crypto is so someone can't pull the key out of the software and encrypt their own license file. We want to be the only ones who can generate a license file.

I'm mildly familiar with RSA asymmetric cryptography. My understanding is you encrypt with the public key and decrypt with the private key. I read on http://stackoverflow.com/questions/6352023/rsa-can-you-create-a-public-key-from-a-private-key that the private key comes with the public key by RSA Specification, so I can't just reverse it and keep the private key on the devices and the public key at the office, or is there a way for me to get around this?

What sort of encryption should I use for what i'm trying to accomplish?

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You should use digital signatures instead of encryption. $\;$ –  Ricky Demer Dec 7 '13 at 0:19
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up vote 3 down vote accepted

You should resort to digital signature to solve your problem. If RSA is used to generate digital signature, private key is first used to "decrypt" the file that you want to sign. The software can verify the "decrypted" file comes from you since it was "encrypted" using your public key.

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