I felt dumb back in 2001 when I suggested that the solution to IMC’s ip address privacy concerns was to use a one-way hash (e.g. MD5), and a few basic facts where pointed out to me.

  • IPv4 provides a total possible search space for ~4 billion highly predictable records, a dictionary style attack against this is well within reason.
  • the effective address space is actually *much* less then 4 billion due to the way that IP addresses are distributed.
  • it is likely that the search space will actually be *drastically* smaller (a few hundred) based on the amount of information the attacker has already been able to gather.

I’m not a cypherpunk, but the numbers kind of speak for themselves. Hashing of IP addresses is useless. Which is why Indymedia doesn’t log IP addresses.

So its a little odd to see that bastion of geeky wisdom Slashdot, talking about how they’ve “voluntarily taken on the privacy burden of MD5’ing incoming IP addresses”.

I guess that prevents against accidental discovery, and maybe thats their threat model. (that must be nice)

This has been a public service announcement.