Using Cryptography

The proper use of cryptography is way beyond the scope of this tutorial. However, there are a couple of rules that apply unless you're wearing a pointed hat and known the all DES S-boxes by heart:

Do not try to invent your own crypto.

This means: if you want to use cryptography in any way, use standard algorithms. Do not try to roll your own. Creating cryptographic algorithms that withstand an attack is one of the last black arts on this planet. Do not meddle with wizards...

Use standard libraries.

Do not try to implement cryptographic algorithms yourself. Use well-tested libraries such as OpenSSL.

Known what you're doing.

A while ago, I looked into an application that used public key certificates as installation key. Which looks fine and dandy and very modern. However the way the application verified the key was sort of interesting. The developers had compiled the private key into the application, and verified the key by recomputing the signature and comparing that signature to the one that was stored in the file.