I'm making a simple licensing system for my apps.
I don't know about cryptography, but I know that I need a algorithm that consists of 2 keys: private and public.
I need to encrypt some data (expiration date and customer email) using my private key, and then my app will decrypt the data using the public key to compare expiration date.
Is there a known algorithm that does what I need?
EDIT:Problem solved. Helper class to use dsa avaliable Here
Check out RSA. Most modern platforms will have implementations of RSA in their standard library.
This isn't how private key encryption is supposed to work at all. You encrypt the data using the public key, it is only able to be decrypted using the private key.
edit: please ignore this I was completely wrong. I'll leave it in for other people who thunk like me to learn from
Sounds like you need a library!
I recommend checking out LibTomCrypt
What you want to do is actually called "signing" in the crypto world. You encrypt something with your private key, but since the public key is public, anyone can decrypt it. The algorithms that do this are called "asymmetric ciphers" (since the encryption key is different than the decryption key).
To be concrete, the RSA algorithm will do what you want in a secure way.
Do yourself a favour and do not try to implement it yourself; rather, take an existing implementation like the one from the OpenSSL library. It has an Apache-style license so you're probably allowed to use it in your application.
(However, note that such a licensing system is never completely secure: somebody can still modify your executable and remove the check. But clearly that is more effort than, say, simply modifying a registry value.)