I agree with Bainternet on the recommendation of Yoast's SEO plug-in. There's absolutely nothing better, and I've tried a lot of them.
That said, your goal of auto-generating the description and keywords is a bit ... misguided. It's not the direction you want to go. If you're going to do SEO at all, take the time to do it right and draft a different description and set of keywords for each page and post. Auto-generation will get you nowhere. Yoast even has an article explaining the negative impact of such auto-generation when your links and pages start getting used on other sites.
If you're still intent on auto-generation of descriptions, most SEO systems (Yoast's included) will allow you to do that. Auto-generating keywords is different.
If you're open to a paid solution (not a free plug-in), I recently came across ClickBump SEO. You can auto-generate your focus keyword based on the post/page title, and automatically generate a list of highly-rated similar keywords. If you're already using them in the post's content, it tells you. If not, you can work them in to maximize your SEO score.