Crypto Idea; Multi-Part Sigs

[nobody at alumni.cco.caltech.edu]

I've been thinking of an idea I've heard kicking around the list somewhat;
that of digital pseudonyms and groups of people going by one name; likewise
single people going by names that sound like groups.

Let's say that three people decide to create a digital identity for
themselves,
maybe they plan on dealing with Blacknet as a group. So one of the first
things they might do after getting together might be to create a pgp keypair
with the name of the group on it. But who gets to hold the secret key?
Remember
they wish to be thought of and treated like a single entity. However they all
need to be able to decrypt documents addressed to them, and none of them
should be able to sign a document without the consent of the others.

Is it possible to produce a set of keys (for example, 3 private 1 public)
such that       -all three private keys are needed to sign a message from that
                I.D.  and
                -any of the private keys can decrypt anything encrypted with
                the single pubkey.

The way I see it, three (or more) people could have a joint keyID, and if
they wanted to post a message from that ID, the first person would write
the message and sign it with his/her segment of the key. She/he would then
encrypt with the second member's pubkey and annonymously remail it to #2, who
would sign with the second key segment, re-encrypt with #3's pubkey, and send
it on to #3, who would sign with the final key segment and anonymously
post to
the 'Pool'. If a response were posted, all three could read it without
depending on the others.

Is this possible?  In whole or in part? I don't know the math well enough to
figure it out myself, sorry if this is an obvious one.

Happy Hunting, -Chris
<cdodhner at indirect.com>
______________________________________________________________________________
Christian Douglas Odhner     | "The NSA can have my secret key when they pry
cdodhner at indirect.com	     | it from my cold, dead, hands... But they shall
pgp 2.3 public key by finger | NEVER have the password it's encrypted with!"
  "If guns are outlawed, only the government will have guns." -E. Abbey
My opinions are shareware. For a registered copy, send me 15$ in DigiCash.
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