Cryptography Reference
In-Depth Information
contained in the software/hardware, which means that it is basically accessible.
The hacker mentioned above could sit in another country and work for that
country's national intelligence agency. He would be listening in on everything
and nobody would know!
The manufacturer would appropriately use an asymmetric method to hide his
malicious intentions, and encrypt the user password with his own public key.
That would add yet another eavesdropping party — the manufacturer himself. If
hybrid methods were used, one could additionally send along the user's private
key rather than the session key every time, for example, piece by piece together
with the position in the key.
This is an extraordinarily enticing method! It mightn't be used yet; the more
important it is to know it now. It is similar to steganography, discussed in
Section 1.3, in some respect; it is
steganography versus cryptography
,soto
speak. But the implementation is simpler in the case discussed here: while
the steganographer has to mix in complete ciphertexts, we could do it with a
128-bit key, for example.
There are countermeasures, as the term 'steganography' suggests: you have to
take capacity in the data stream away from the manufacturer, preventing him
from stealthily sending a key along. What it takes are cleanly defined network
protocols, fully described headers, disclosed algorithms, access to the user's
session key (unless it is predefined), and finally a defined padding in block
algorithms (see Section 5.1.2).
Of course, as a practitioner you know that things will never get to this point;
such requirements remain pious hopes. The best countermeasure I can think of
might be an external crypto-interface that would allow the user to procure the
entire ciphering and key management from a different manufacturer, or perhaps
program it himself. But even this proposal might hit resistance. The concept
is possible for software (though not in all cases). For hardware in general and
chip cards in particular, however, this separation might not be possible. The
Crypto AG story above shows how sad the real world can be.
The Perfect Fraud: Cleptography
Not meaning to be entirely serious, Young and Yung [Young] used this name for
a further perfection of the method described here. Nothing is hidden by stegano-
graphic methods any more;
cryptography versus cryptography
is used instead.
Cleptographic software or hardware uses asymmetric methods. By cleverly
choosing
free
parameters — during
random
generation,
for
example,
in
