Cryptography Reference
In-Depth Information
have no chance, unless you use a quotation from a topic that has been
stored electronically (there are even bible CDs and movie dialogs on the
Internet!). The crazier the better; typos are allowed and even desired.
If you write the ciphering program yourself, you should allow for passphrases.
Passwords would be possible anyway as a special case, for conservative users.
If your algorithm wants a 128-bit key, then use the passphrase to build a suitable
checksum. This is called
key crunching
.
The only problem with a passphrase is entering it. You normally type a pass-
word blindly. Unfortunately, blind typing of entire sentences is pretty hard for
some people. PGP allows you to make your input visible in exceptional cases.
It doesn't mean that somebody wants to look over your shoulder (there are
telescopes!). It might be helpful to always display the passphrase in a small
window pane (e.g., 5 characters) that scrolls horizontally. Should somebody
watch you they won't be able to quickly read it as they pass by, and it helps
people who have to look at what they type.
5.1.5 Bottom Line
This section has certainly showed you that the implementation of security
software requires all kinds of tricks. Apart from purely cryptological criteria
(such as modes or padding), you also have to bear in mind the entire security
environment: password entry, managing secret data, key space size, and so on.
In general, you will probably not want to implement encryption programs your-
self, but rather understand and evaluate their security problems. Well, now you
have learned a few problems you should be aware of when buying such soft-
ware. A software vendor should know how to deal with your questions. If they
don't, or if they even keep the algorithm used hidden, then be as suspicious as
you can afford in view of the market situation.
5.2 DES Modifications
Returning to our discussion of algorithms, we still remain in the DES envi-
ronment. Based on current knowledge, brute force is still the only practicable
attack against this method, i.e., its only exploitable vulnerability is its too short
a key length. The slight suspicion whether or not the NSA built in a back-
door in DES remains, of course. There has not been a shortage of attempts to
