Just to clarify (and not to start algorithm wars):
LUC's claimed advantage over "raw" RSA is that encryption and
multiplication do not commute. In "raw" RSA, they do. Let RSA(m) be
the encryption of a message m with "raw" RSA, i.e., RSA(m) = m^e mod
n. Then
E(m1 * m2) = E(m1) * E(m2)
This is quite useful property for constructing blind signatures in
digital cash systems. It is also opens the door to chosen ciphertext
attacks. If I want to get the decryption of some ciphertext c, I can
ask you to decrypt c1 and c2 where c = c1 * c2; the resulting m1 and
m2, multiplied together, will give the decryption of c.
Similarly, if I want the signature of some message m, I can ask you to
sign some m1 and m2 where m = m1 * m2, then multiply the signatures.
Or, if I already have the signatures of m1 and m2, then I can compute
the signatures of any product of any product of powers of m1 and m2.
In LUC, the only time this property holds is when m1 = m2, so I can
get signatures of a power of some message, but not as rich as set as
in RSA.
Now, for the clarification:
PEM and other standard applications of RSA do *not* have this
multiplicative property, because a message is "padded" with a certain
prefix before it is encrypted or signed. The padding is intended
specifically to address concerns such as this. (See PKCS #1 for
details.) In PEM the message is also hashed before it is signed. "Raw"
mathematical relationships do not apply, since the chance that the
product has the right form is so small.
-- Burt Kaliski
______________________________ Reply Separator _________________________________
Subject: RFC1423 asymmetric algorithms & LUC
Author: "David M. Balenson" <balenson(_at_)tis(_dot_)com> at INTERNET
Date: 09/19/94 12:55 PM
Message was resent -- Original recipients were:
To: balenson(_at_)tis(_dot_)com
-------------------------------------------------------------------------------
I would like to propose the addition of LUC to the PEM
Asymmetric Algorithms as in RFC1423
Note that LUC is now on the ISO algorithm list
LUC is now ISO standard ISO 9979-0003 (July 1994)
LUC Digital Signature
LUC is a new Public Key Cryptography system. Used as a Digital Signature it
is a secure alternative to the RSA system.
Cryptographic advantage:
LUC has a cryptographic advantage over RSA when used as a Digital Signature=