For BitBacker, we use 128-bit AES encryption, which means our keys are really long and annoying – 32 characters long when printed in hex. And not only do the users sometimes have to type them in, but they have to write them down on paper. (We can't store the key on our servers because then we'd be able to read the user's files; and we obviously can't trust it to their hard drive because that's what we're backing up.)
Somehow, we have to present these random 128-bit keys to the user, and I think I've found a pretty good way. We use RFC 1751, which defines a "Convention for Human-Readable 128-bit Keys" – basically just a mapping of blocks of bits to strings of English words. Here's an example in Python using the RFC 1751 module in PyCrypto:
>>> key = os.urandom(16) # Generate 16 random bytes (128 bits) >>> bin_to_hex(key) # Show the key in hex (32 characters) '61aa60e43a5e7fdb4b86a4897b52a0dc' >>> y = RFC1751.key_to_english(key) >>> y # Show the pass phrase version of the key 'BUSY BARN RUB DOLE TAUT TOOK ALTO PRY KIT WALL MUG CURT' >>> # The transformation is always reversible >>> bin_to_hex(RFC1751.english_to_key(y)) '61aa60e43a5e7fdb4b86a4897b52a0dc'
The keys are still very long, of course, and this is unavoidable for our application. But when translated to words, I think it's easier to write them down or type them in without making a mistake. The image below shows BitBacker giving me a pass phrase. (This feature hasn't even gone into beta yet – it's little more than a mockup. So please don't judge it too harshly!)
When the user clicks "Continue" here, BitBacker actually makes him re-enter the generated pass phrase he wrote down. To be honest, BitBacker's pass phrase handling is quite annoying. But that's a heck of a lot better than losing your pass phrase, which would make your backups inaccessible! This is the one place in all of BitBacker that isn't optimized for "least user annoyance". Encryption keys are just way too important to mess around with, and I think that most existing software is far too lax with them (including BitBacker's competitors).
(This was derived from a comment I left on Jeff Atwood's "Software Registration Keys" post.)