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Re: Minor PGP vulnerability
From: warlord () MIT EDU (Derek Atkins)
Date: Wed, 16 Jul 1997 12:05:01 -0400


This is old news.  Paul Leyland (pcl () ox ac uk) has posted about this
at least two years ago.  Also, there are a few incorrect facts in your
original mail.

As you might know, PGP uses a 32-Bit number, called key-ID, as
an internal index for storing and recognizing keys. Although
the key-ID's are quite randomly distributed within 31 of the
32 bits (the key-ID is always odd), the scheme how this key id
is derived from the (public) key is not cryptographically secure.

Actually, PGP uses 64 bits internally; although it only displays 32
bits to the user.  However, these 64 bits are, as you say, insecure in
a cryptographic sense.  The PGP 5.0 DSS/DH keys are not subject to
this attack, since the keyID is a cryptographic derivation from the
key.  Only the old style RSA keys are succeptible, since the keyID is
just the low bits of the public key modulus.

As a consequence, when obtaining PGP keys from insecure sources,
you should always check for the existance of a key with the same
key-ID in your own public keyring. To verify a key, always use
the fingerprint and never the key-ID.

Actually, there is a problem in PGP's RSA fingerprinting algorithm,
too.  You can create a key with the same fingerprint as another key,
however the size and keyid cannot match as well.  This means you
should *ALWAYS* check the fingerprint, keyid, AND key size in order to
verify a key; an attacker can only forge at most two of the three
checks.

-derek

--
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/      PP-ASEL      N1NWH
       warlord () MIT EDU                        PGP key available



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