relative sizes of URLs and hashes

Graham wrote

> (a) in the scheme of things, I think a 20-byte hash is quite small --
> smaller than most URLs.  If a given hash is used more than once in a
> message, and associated just once with a URL, the result might actually be
> shorter.   (I am assuming a different approach here to that in some of my
> recent postings.)

I consider the two to be roughly the same size, because a
cryptographically secure hash is not compressible, while a URL
is. There's considerable uncertainty in the "roughly", since URLs vary
in size, and since there's some possibility of specifically choosing a
short name for a heavily-used URL (e.g. the description of
capabilities for a standard mobile device) in order to conserve
bandwidth.

I don't think that length is a real problem either for
URL-as-description or hash-as-description schemes. The factor-of-2
cost of sending both a URL and a hash on every single request would be
a drawback of any scheme that required that, though.

  Bill Newman
  [email protected]