Security - Telenor

these approaches can be used to design practical
systems that provide provable security in a strong
sense. Most algorithms are designed using a system-theoretic
approach combining well-established
principles with partial security proofs.
All cryptographic algorithms can be attacked by
a brute force attack on the key space. Assuming
an enemy has knowledge of the algorithm involved,
he can try to decrypt the ciphertext with
all possible keys. Meaningful plaintext will appear
when the proper key is found. If the length of the
secret key is n bits, there are 2 n different keys to
try. This means that the key length is a simple
parameter used to indicate the strength provided
by a cryptographic algorithm. However, it is
important to recognise that a sufficient key
length is a necessary but not a sufficient requirement
for a strong algorithm. An algorithm could
have a long key, but still be vulnerable to other
attacks more efficient than exhaustive key
search. A report from 1996 [9] gives guidelines
for selection of key lengths as shown in Table 1.
It is important to stress that this table is only relevant
for symmetric algorithms. The situation
for asymmetric ciphers is much more complex
and adequate key lengths for such systems are
completely different from those in Table 1. An
excellent guide is [10].
4 Encryption Policy
The implementation of encryption solutions is
not only a technical matter, but also sensitive
political issues are involved. For a long time
serious use of cryptography was restricted to
diplomatic and military use and encryption technology
was considered to be of strategic importance
to national security. National and international
regulations were developed to monitor the
use and dissemination of the technology. Most
countries still enforce some kind of export control,
but the rules have gradually become more
liberal.
The introduction of confidentiality as a standard
service in telecommunication systems will not
only protect the involved link against fraud and
malicious eavesdropping, but it will also prohibit
police and security agencies involved in legal
interception as part of their combat against terrorism
and organised crime. In many countries
such interception has been an important tool
against serious criminal activity, but normally
a special court order must be available before
the interception can take place.
In mobile systems like GSM and DECT, the
radio link is considered to be an exposed and
vulnerable channel and the standards specify
encryption of this link. The solutions involved
Telektronikk 3.2000
Type of attacker Key length needed
Pedestrian hacker 45-50
Small business 55
Corporate department 60
Big company 70
Intelligence agency 75
have been a carefully chosen design balancing
the specific threats involved and the need for
general exportability. For such systems there are
no export controls on the terminals, but a license
is normally needed for network equipment.
5 Open or Secret Algorithms
As stated above, the strength of a cryptographic
system should not rely on the secrecy of the
system description. In order to provide optimal
analysis and confidence in a cryptographic algorithm,
it will be important to have public available
specifications that have been studied by
independent experts. By standing such public
scrutiny over a long period of time, an algorithm
can achieve the necessary trust and assurance.
This is a strong argument in favour of open algorithms
and it seems clear that this is the only
model for wide acceptance of general-purpose
cryptographic algorithms.
Open algorithms may be subject to a variety of
research analysis and many results are published
as “breaks”, even if the announced attack cannot
be conducted within the operational environment
of the system. Normally any attack with complexity
lower than exhaustive search is reported
as “cracking the algorithm”, often resulting in
much publicity and worried users. In many cases
such results are mostly of academic interest and
have minimal impact on the security of the
actual system.
However, in the same way as openness is no
guarantee for strong algorithms, a secret algorithm
does not implicitly mean that the algorithm
is weak. Cryptographic algorithms used
in military systems for protection of classified
information are seldom or never revealed. These
organisations have a strong internal competence
in design and analysis of algorithms and do not
have the same need for an open process. From
history they know the difference between attacking
a known versus an unknown crypto system.
In order not to provide an enemy cryptanalyst
with any advantage, they prefer the use of secret
algorithms.
Table 1 Guidelines for
selection of key lengths
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