Encrypt topic title and content?

Hi there!

I need to encrypt/decrypt the post/topic content and topic title in the db. I need to use a user provided string to do this.

Can anybody give me an overview of how to get going on this?

I would need the user to provide this text every time he logs in. I should store this in the session in order for IP to use it in every create/update of any topic/ post.

I would need for this text to be used for every encode/decode of posts/topic.

I dont want anyone to be able to see the contents of a topic post without the specific post/topic text.