IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Hardware & Software Remarks
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User, control and management information
A user is primarily interested in the transmission of user information to remote
users. Depending on the actual reference model being used, however (e.g.
ISDN), control, signalling and management information is also transferred
between the systems (terminal devices, coupling elements) for the purpose of
setting up and clearing down connections, negotiating quality of service
parameters, and configuring and monitoring the network by network providers
etc.
The network concerned has the task of transmitting user information without
changing it and without interpreting it; i.e. only the terminal devices must be
capable of interpreting user information. In this way the information can be
secured irrespective of the rest of the network infrastructure, if necessary even
using proprietary security functions (closed user group). It must be possible
for control, signalling and management information on the transport layers to
be evaluated, modified or generated by network elements belonging to the
network provider. As a result, this information largely avoids any protection
provided independently of the network provider (e.g. encryption). The
safeguarding of this information calls for trusting co-operation with the
network provider, as well as application of the relevant standards. Threats may
arise from the fact that the security functions of certain products are
incorrectly assessed. When cryptographic devices are selected, it is essential to
examine precisely which components of the information are secured or
filtered. Likewise, looking at it the other way, it is necessary to check which
information remains unsecured despite the use of crypto devices, and to what
extent this can be tolerated.
Example: With ISDN, the user information is generally carried via the B
channels. However, the D channel, which is primarily used for signalling, can
also be used for the transmission of packetised data. If the objective is to
protect all user data, it is plain that safeguarding the B channels is not
sufficient in cases where packetised data is transmitted via the D channel.
Security in circuit-switched networks
In circuit-switched networks, the establishment of a connection sets up
channels of a defined bandwidth, which are exclusively available to the
communicating parties. After the connection has been established, the user
data is transmitted, then the connection is cleared down. The network provider
can set up fixed connections, in which case there is no need for the connection
to be established and cleared down – usually performed by the subscriber. One
example of a circuit-switched network is ISDN.
When a connection is established, user data channels are set up between the
communication partners on OSI layer 1; in ISDN these are referred to as B
channels. In order to ensure the confidentiality of the transferred user data, the
channel can be encrypted. If it is also intended to secure the signalling
channel, in the case of N-ISDN therefore the D channel (layers 1-3), it must be
borne in mind that both the communication partner’s terminal and the network
provider’s exchanges can appear as distant stations for a terminal transmitting
data. The D channel is not normally encrypted, because this would mean
imposing particular requirements on the network provider. In this case
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IT-Baseline Protection Manual: Oktober 2000