🚀 go-pugleaf

In article <1eg516INNrrq@uniwa.uwa.edu.au>, markd@uniwa.uwa.edu.au (Mark
Diamond) writes:
| The question arose in a previous reply to this posting of whether
| triple, double or any other repeated DES encryption was stronger than a
| single encryption.  The previous reply indicated that the question was
| still open.  It is not open ... it has been settled and the results
| presented at CRYPTO 92.
| 
| Essentially the question about the strength of double encryption is the
| same as the question about whether DES is or is not a group.
| 
| If DES were a group then for two keys K1 and K2
| 
|        DES( DES(Plaintext, K1), K2) == DES(Plaintext, K3) for some
| 
| K3 which presumably the eavesdropper is trying to discover ... the point
| being that they would not have to find K1 and K2, but only a kind of
| "combination" K3.
| 
| The recently demonstrated fact that DES is *not* a group indicates that
| a double encryption *is* stronger than single encryption.

Sorry, no.  Repeating a falsehood does not make it true.  All we know is that
it MIGHT be stronger.

Consider TES, the Trivial Encryption Standard - it's just like DES except that
TES(P,k) = k || DES(P,k) (|| is concatenation).  That is, TES "gives away the
store" at the beginning of the ciphertext.  It's clear that if DES isn't a
group, neither is TES.  It's also clear that double TES is exactly as strong
as single TES.

If there were a generic unknown-plaintext attack against DES, it would work
just as well against double DES, requiring double the time.  (Yes, this would
make double DES just a bit stronger - but multiplicative factors - especially
such small ones! - are ignored when we discuss the strength of cryptosystems.

A pure chosen-plaintext attack against DES would not move directly to double
DES.  But that only says something about ONE PARTICULAR ATTACK; it says
nothing about the inherent strength of the cryptosystem.

							-- Jerry