Cryptography Reference
In-Depth Information
The Innards of Deep Crack
What do the parameters of the machine look like? Its core is a 40-MHz special
chip with 24 independent search units, each of which manages one DES decryp-
tion within 16 clock pulses. Sixty-four such chips are housed on one board.
There are 12 such boards, plugged into scrapped Sun 4/470 computer boxes,
and two such computers work in parallel. This results in a search speed of
approximately 90 billion keys per second, i.e., roughly 2.5 times faster than the
entire free capacity on the Internet that had been deployed for the RSA Chal-
lenge! Each chip tests concurrently for certain criteria to find whether or not
the plaintext created is meaningful, and stops when hitting a success. A control
computer running Linux or Windows95 is responsible for further evaluation,
and continues the search, if need be. The average search time is 4.5 days.
Of particular interest is the test on plaintext, i.e., whether or not a tested key is
the correct one. The hardware doesn't find the correct key itself; it rather tests,
for example, to see whether the plaintext created consists only of characters
from a certain subset, for instance ASCII characters. If this is true, then a
second ciphertext block (consisting of 8 bytes) is decrypted instantly, and the
test is repeated. If this test is positive, too, then the search unit stops and
informs the controlling PC which, in turn, takes over and runs further analyses.
This means that the hardware is not responsible for finding the correct key, but
for singling out as many 'bad' keys as possible! As a sideline, the machine
also works with texts encrypted in CBC mode (see Section 5.1.1). Remember
this 'gradual filtering' of a data stream. National intelligence organizations also
work by this principle. We will get back to this issue in Chapter 8.
Meanwhile, there is at least one official successor: Copacobana (
www.
copacobana.org
), which stands for 'Cost-Optimized Parallel Code Breaker',
is a machine with an FPGA basis developed jointly by the universities at Kiel
and Cologne, Germany. You can buy it for 8980 euros (probably not in the
supermarket though). It can handle about 48 billion DES ciphers per second,
while consuming only 600 watts. This means that searching the full key space
would take about nine days. It can also be used to break other block ciphers
programmed in FPGA.
Other Considerations
It may be reasonably expected that DES cryptanalysis will be offered as a
service unofficially. So you'll have to strike out the sentence that can still be
