All keys are sequences of bits
The notion of a cryptographic key was created by analogy to physical locks and keys, but it looks like the cryptographers may have had the deeper insight: the authorizing function of any "key" (a PGP key, a door key, a voting machine key) is really a relatively tiny amount of information (usually digital information, even in the case of pin-and-tumbler locks, since there are typically explicit digital descriptions of any key: note that "[a]ll lock makers assign [integer] values to each pin depth so that keys can be replicated by number rather than requiring the physical key", per Marc Tobias's explanation of bump keys, which includes a nice diagram of the fact that door keys are really physical analog approximations of what their manufacturers conceive of as small digital secrets).
The neat new example of this: a photograph of a pin-and-tumbler lock key is equivalent to the key itself. Not only can you make photographs from keys, you can make keys from photographs.
