Cryptography Reference
In-Depth Information
The design of the
pkzip
cipher is not bad at all. But its single steps (a) through
(e) are not secure enough. Each step can be cracked in itself without knowing
the previous step. This is an important difference to product algorithms like the
ones discussed in this topic. The single steps in
pkzip
differ a lot, but without
doing any good.
5.7.2 Classified Stuff in Air: The D-Networks and the A5
Algorithm
Cellular telephones are great as long as Mr Mallory can't listen in on them.
Some owner of a C-network phone was obviously not aware of this problem. In
the C-network, phones worked analogously, like radio senders and receivers.
Though voice was scrambled, listening in on them wasn't a major problem
for techno freaks more or less familiar with the matter. In short, if you made
confidential phone calls within the C-network you might just as well have
written the contents on postcards and given them to the next passerby to put
them in a letterbox. That wouldn't be as fast as a phone call, but more secure
if the passerby looked fairly trustworthy.
Things changed with the advent of the GSM standard. In Germany, these are
the networks D1, D2, and E-Plus. These networks digitize, compress, encrypt,
and broadcast voice in single 114-bit data packets to the next base station.
Up to eight subscribers can concurrently use a frequency in one time slot
based on timesharing — every one of them is 'on' for about half a millisecond.
Moreover, the transmit frequency can be set anew over and again during the
transmission. This is called
frequency hopping
. Theoretically, 124 frequencies
are available in each cell; the practical number of frequencies used per cell
seems to still be in the single-digit figure range. Frequency hopping enables
broadcasting behind coarse grids like steel bridges, while making eavesdropping
more difficult for attackers.
The base station receives the packets individually, decrypts them and forwards
them via radio relay or fiberglass cables. If the receiver happens to be using
a GSM handset, then his base station encrypts the data packets received, and
the receiver's handset decrypts them, puts them together again, and converts
them back into voice. Otherwise, this process happens upon the transition into
a different telephone network. All this shows the following clearly.
Eavesdropping on D-network conversations is not meant for freaks with tuned
broadband receivers. You need very expensive special hardware. If the infor-
mation on frequency hopping itself is also encrypted (which I don't know),
then the cost would multiply once more considerably.
