Dick Hardt’s OSCON 2005 keynote talked about how to identify yourself online. Here’s an IRC conversation the ensued after I accidently issued a sudo command on a friend’s system, one that is monitored very closely by the sysadmin.

[22:35] <paulv>    matthew immediately declared that we were already cracked. :)
[22:35] <matthew>  yeah, it's all good.  Although, can you prove you're Jim?
[22:35] <jweirich> yes
[22:35] <jweirich> ask me a Ruby question
[22:35] <paulv>    implement the Y-combinator!
[22:35] <matthew>  heh
[22:35] <paulv>    then I'll believe.
[22:36] <jweirich> p proc { |le|
[22:36] <jweirich>   proc { |f| f.call(f) } \
[22:36] <jweirich>   .call ( proc { |f|
[22:36] <jweirich>            le.call (proc { |x| f.call(f).call(x) }) })
[22:36] <jweirich> }.call(proc { |recurse|
[22:36] <jweirich>   proc { |n|
[22:36] <jweirich>     if n == 0 then
[22:36] <jweirich>       1
[22:36] <jweirich>     else
[22:36] <jweirich>       n * recurse.call (n-1)
[22:36] <jweirich>     end
[22:36] <jweirich>   }
[22:36] <jweirich> }).call(5)  
[22:37] <matthew>  wow.  This is better than gpg keys.
[22:37] <matthew>  I hope you had that laying around 
[22:37] <jweirich> nah, did it from memory ;)
[22:37] <paulv>    this *is* better than GPG keys.

(and for those that missed the winking smiley, no, I did not do this from memory)

Update

I see that Dick Hardt’s OSCON 2005 presentation on Identity 2.0 is available at http://www.identity20.com/media/OSCON2005. This is worth seeing.