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You can do some really nifty things with a hash. I've used them as a key to encrypt a specific volume. It works like this: you have a document you want to encrypt with a symmetric cypher like AES or 3DES. You want to have a really strong password for it, but dont have to remember the long string of characters of the password itself. So you get the hash of a phrase. For example, you take a sha512 hash of the phrase 'I was born in texas.' This produces a string of gibberish that will be impossible to guess using all the computing power that exists on the planet untilnthe heat death of the universe. Howver, and this is important ... you don't have to remember the huge strig of characters. Instead, when you need the key, you just generatenthe hash ofnthe phrase, remembering not to capitalize the state name (grin). You now have the pass, and can decrypt the file. There are many, many variations of the same thing that you can use. For example, there are many mathematical oddities out there that follow a special form. Mersene numbers are a good example.

Let's say that Alice had an encrypted file of Bobs. She doesnt know the password. So Bob sends Alice a messagenwith a subjectmline of '412'. Well 412 could mean anything. However, in this case, Alice knowa that Bob means that he is really talkimg about the number represented by 2^412-1. Thw password could either be the number itself, or an agreed upon hash of same. The hash is actually better because it is always the same number of characters, whereas the mersene number (look it up) of 412 is 125 digits long and these Mersene number for 912 would be 2748 digits.

The key here, if you'll pardon the pun is that the two parties have to agree on the form to be used ahead of time in order for it to work. Unfortunately, a system like this is very much succeptable to being beaten out of you with rubber hoses. That is a big drawback, but there are an infinite number of ways to make dead drops on the internet. If people are even somewhat creatice, it doesnt take much to come up with a system like this.