Security - Telenor

16
6 Use of Encryption in
Telecommunications Area
Traditionally the use of encryption technology in
the telecommunications area has been seen along
the following lines:
1. Protection of network management information.
Operator initiated encryption of dedicated
connections in order to secure important
connections vital to the operation and maintenance
of the network itself. This approach may
work well within one domain controlled by one
operator, but may face problems in multiple
domain solutions implementing the Open Networks
Provision policy.
2. End-to-end encryption by dedicated users
with high security requirements. Users that
exchange confidential information over public
networks using secure phones or crypto-faxes.
Such solutions normally depend on expensive
and proprietary equipment. Due to the complexity
of key management, the solutions do not
always scale well and there are no standards for
such connections.
3. Encryption of vulnerable links. Modern
mobile networks like GSM, DECT, UMTS and
satellite-based networks involve a radio link
especially vulnerable to eavesdropping (as well
as other security threats). In order to protect user
data and signalling information in such systems,
encryption techniques have been introduced on
the radio link between the mobile terminal and
access points to the fixed network. 1)
4. Entity authentication. Mobile communications
lacks the conventional access point to the
network and there is a strong need for authentication
of users involved in a call. It may also
be necessary for the user to authenticate the network.
In most systems such authentication is
based on a “challenge and response protocol” in
which an entity authenticates itself by proving
knowledge or possession of a unique secret key.
The challenge and key are then input to some
cryptographic algorithm which outputs the correct
response. 2)
7 The Data Encryption
Standard (DES)
For many years the US Data Encryption Standard
(DES) [3] has been the de-facto standard
for commercial encryption. DES was proposed
in 1975 and approved in 1977 as a Federal Standard
for protection of sensitive, but unclassified
information. DES was designed and proposed by
IBM, endorsed by the National Bureau of Standards
(NBS, now NIST) and later approved by
ANSI as US standard. Around 1985 there was
work within ISO to establish DES as an international
crypto standard, but this project was
halted due to political problems. DES is widely
used by banks for protection of electronic funds
transfer.
DES is a symmetric block cipher, which encrypts
a 64-bits data block under the control of
a 56-bits key. From the very first moment there
was strong criticism against the key length of
DES and it was argued that the key space could
be searched by strong opponents using dedicated
hardware devices. In July 1998, using custom
designed chips and a personal computer, the
Electronic Foundation built “DES Cracker” [11].
Costing less than USD 250,000 and taking less
than a year to build, DES Cracker broke a DESencoded
message in fifty-six hours. There was
nothing terribly novel about the decryption
machine except that it was built. From Table 1 in
Section 3 we see that this result fits nicely with
the predictions from 1996.
The immediate response to the shortcomings
of the DES key length has been to implement
Triple-DES systems, in which the DES algorithm
is used in three consecutive steps using
two or three different keys. The long-term solution
will be to develop a replacement algorithm(s),
see section on AES below.
Even if DES is approaching the end of its life
cycle, it has been an example of a carefully designed
algorithm, which have resisted open analysis
over many years. Today we know that it
was designed to withstand attacks that were not
publicly known back in the seventies [12]. Even
new attacks have appeared over the years, but
they had little impact on the practical strength
provided by DES.
1) Due to interoperability requirements, this encryption has to be based on standardised algorithms
known both to the mobile station and the network.
2) This authentication is often a protocol between the subscriber’s SIM module and the operator and
may be based on an operator-controlled algorithm. In this case it is not necessary to have one standardised
algorithm.
Telektronikk 3.2000