Let's make sure we're talking about cryptographic hashing functions.

Let's say I make a prediction that Barack Obama is a werewolf. I don't want anyone to know I've made this prediction, because otherwise the Secret Werewolf Police will come and eat me, but I do want to lord it over everyone after the fact.

I take the string "Barack Obama is a werewolf" and I put it through a hashing function, (for purposes of this example, the hashing function MD5). This produces the output 37ecb0a3164e6422bedc0f8db82e45ec. The original string about Barack Obama is not recoverable from this output, because MD5 is a lossy function, but anyone else putting that string through the MD5 hashing function will get the same output. The hash output is like a fingerprint for the original string.

So if I put 37ecb0a3164e6422bedc0f8db82e45ec in a public place a year before Barack Obama transforms into a werewolf on live television, after the fact I can give people the original string and they can verify it for themselves against the hash output.

I use a Chrome plugin for most tasks that involve basic encoding of text values. (If I'm honest, I mostly use it for rot13, and more exotic uses aren't that common). There are also online hash generators for different hashing functions. Some popular hashing functions for this purpose are MD5 and SHA1.

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