don't need dictionary's keys - hash table?

[kdotsky at gmail.com]

> depending on your application, a bloom filter might be a good enough:
>
>      http://en.wikipedia.org/wiki/Bloom_filter
>

Thanks (everyone) for the comments.  I like the idea of the bloom
filter or using an md5 hash, since a rare collision will not be a
show-stopper in my case.