[nycbug-talk] [ccc related] MD5 considered harmful today

Miles Nordin carton at Ivy.NET
Wed Dec 31 02:45:46 EST 2008

>>>>> "jc" == Jesse Callaway <bonsaime at gmail.com> writes:

    jc> man that sucks... so even if this issue in the paper is
    jc> addressed, it won't matter until the browsers fix the
    jc> revocation mechanism.

i don't think revocation is part of the fix.  The revocation problem
is just funny because it adds to the feeling of powerlessness the
incompetently-run CA, and makes me happy because they've been milking
us all for cash all along.  Their first thoughut must have been
``Rogue cert?!  oh no.  quick, revoke it!''  nope.  PWNTx2!

This attack would be foiled if the CA's would simply stop using MD5
tomorrow.  They can't invalidate any fake signing certificates out
there already, but there probably aren't any unless this same group
made a working cert secretly.  Because the collision blocks are in the
keys, AIUI an attacker needs to get a new key signed to pull it off.
If the CA switches to SHA from now on, someone holding an honest MD5
cert from before the switch can't use the same trick because there are
no collision blocks hidden in his honest RSA key.

The revocation mechanism isn't broken or underimplemented in FF3/IE7
according to the authors---rather it's badly designed in X.509, so
there is no improvement to that hilarious revocpwnage planned, not
even an unimplemented one.

The GnuPG revocation mechanism has neither problem they mention.  All
revocations are signatures, but GnuPG revocations are signatures on
whole keys, not on serial numbers like X.509, so you cannot manipulate
me into revoking a key I don't want to by setting your serial number
the same as the target.  And the revokee doesn't have to consent to
the mechanism by publishing a URL in their key---it is possible in
GnuPG to revoke your signature on someone else's key without their
consent by simply uploading it:

pub   1024D/6E9400D6 2004-07-11
uid                  amber fechko (xi) <xi at telekinetic.net>
rev          DA5BFE1D 2004-12-20  Miles Nordin <carton at ivy.net>

It's not very nice, but I think she deserved it.

Amber disappeared and set her MX record for telekinetic.net to (that is, to the domain name, in text.  not the IP
address  foolio.).  In GnuPG signatures and revocations
represent the binding or lack thereof, of key material to a uid, so
when she shut down her email address clearly on purpose I decided she
might like me to warn people that the binding wasn't valid any more.
I guess no one else agreed with me, but I stand by my revocation.  not
that I have a choice---i can't unrevoke.

I think it would be funny if these guys made a real CA cert with their
exploit and started selling certs signed by their fake key for $2 each
or something.  not illegitimate certs, like, email-contact-verified
certs, the regular legitimate kind, just cheaper.  Why not?  It's
probably even legal in some jurisdiction if not in most.  and most
webmasters just want to turn the browser bar green.  It works now, so
for $2 why not?  I'd buy one.  If it starts turning browser bars red
some day, buy a more expensive cert _some day_, not now. The whole
cert thing was such a racket to begin with, i wish they'd start
selling fake ones.
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