Cryptography Reference
In-Depth Information
The result of either of the two equations depends on three quantities:
K,T
,
and
V
or
S
. This means that, if we choose
T
for
K
and
V
'very randomly',
an attacker has no chance, even if he were to find
K
and
V
.
No question, the method was carefully designed and studied. But also no ques-
tion excessive security can be effectively annihilated if the implementation is
bad. For example, a programmer might take the term 'timestamp' too literally
and
make the set of possible secret keys
K
too small, thus producing a strongly
reduced key space. It would be theoretically sufficient if the timestamp
or
the
secret key were taken from a sufficiently large set of values.
The problem of finding a 'secret corner' on the computer is not a cryptological
problem, but one that belongs to system security.
K
is normally created with
a 'sufficient amount of randomness' when the program is started. More about
this issue in the next section.
'Computer Randomness'
Randomness in a computer is usually not desirable, for example, when a Win-
dows computer crashes purely by chance. On the other hand, you will find that
creating randomness in a
targeted
way is not as simple as it may sound. This is
not a matter of the statistical properties of randomness. We are only interested
in preventing an attacker from anticipating or guessing the values created.
There are many ways to utilize pseudo-random events. However, this depends a
lot on the operating system and the computer type used. We cannot discuss such
programming techniques in generalized form. You will find some suggestions
in Figure 5.7.
Password Selection—the Trick with Wait Times
If you program a password entry yourself, you should absolutely prevent users
from entering 'bad passwords' in the program! Think of the experience peo-
ple had with the Enigma: only the establishment of appropriate work rules
made the radio operators think of keys better than 'aaa', 'asd', 'sdf', etc. Only
an alarmingly small number of users I watched as they logged into a UNIX
system typed long passwords. Older systems allowed users to use very short
passwords and, well, many chose two-letter passwords. One single letter may
have appeared to them to be too risky after all.
By the way, there is a trick that let's you live well with a one-letter password,
but only in one special case. I wrote myself a program that locks the screen
