Novell LINUX ENTERPRISE DESKTOP 10 SP2 - DEPLOYMENT GUIDE 08-05-2008 Deployment Manual page 791

Serial terminals connected to serial ports are still used in many places. Unlike network
interfaces, they do not rely on a network protocol to communicate with the host. A
simple cable or an infrared port is used to send plain characters back and forth between
the devices. The cable itself is the weakest point of such a system: with an older printer
connected to it, it is easy to record anything that runs over the wires. What can be
achieved with a printer can also be accomplished in other ways, depending on the effort
that goes into the attack.
Reading a file locally on a host requires other access rules than opening a network
connection with a server on a different host. There is a distinction between local secu-
rity and network security. The line is drawn where data must be put into packets to be
sent somewhere else.
44.1.1 Local Security
Local security starts with the physical environment in the location where the computer
is running. Set up your machine in a place where security is in line with your expectations
and needs. The main goal of local security is to keep users separate from each other,
so no user can assume the permissions or the identity of another. This is a general rule
to be observed, but it is especially true for the user root, who holds the supreme
power on the system. root can take on the identity of any other local user without
being prompted for the password and read any locally stored file.
44.1.2 Passwords
On a Linux system, passwords are not stored as plain text and the text string entered is
not simply matched with the saved pattern. If this were the case, all accounts on your
system would be compromised as soon as someone got access to the corresponding
file. Instead, the stored password is encrypted and, each time it is entered, is encrypted
again and the two encrypted strings are compared. This only provides more security if
the encrypted password cannot be reverse-computed into the original text string.
This is actually achieved by a special kind of algorithm, also called trapdoor algorithm,
because it only works in one direction. An attacker who has obtained the encrypted
string is not able to get your password by simply applying the same algorithm again.
Instead, it would be necessary to test all the possible character combinations until a
combination is found that looks like your password when encrypted. With passwords
eight characters long, there are quite a number of possible combinations to calculate.
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