As the topic says: how to search for posts containing two or more specific tags?
If you click on one tag (like hash), you see in the right a list of "related tags". Click one one of them, e.g. passwords, and you'll arrive at a page which lists questions having both tags. You can continue this with more tags.
Also, the search field is then pre-filled with these tags in the right syntax (
[hash] [passwords] in my example), where you then can add more search terms (in the non-tag form, too, like
[hash] [passwords] MD5).