Passing Values from one page to another

I am passing ID of login user as Session variable and i hv to pass id of one more item and that i m passing in query string.But this is not safe, so how can i encrypt and decrypt that id so if user make some changes in id in addressbar of explorer, it didn't get the data.You can use this link to encrypt the data before putting it in a session variable, then decrypt the data again when you want to read it:

Encryption/Decryption with .NET

regards
But i m not using session variable in 2nd case. Ihv to pass the 2nd id in query string. But i want encryption and decruption for that or is that possible that quesry string not visile to end user.