IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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server have a corresponding implementation. With regard to negotiation of
the procedure, care should be taken to ensure that client and server are both
configured so that only the procedures specified in the IT security
guidelines are accepted. Steps must also be taken to ensure that
unencrypted communication is not chosen as callback variant if no
compatible procedure could be negotiated between client and server.
Explicit negotiation of the unencrypted communication must also be
prohibited.
Tunnelling at Layer 3: IPsec
Whereas the Layer 2 protocols make use of the security mechanisms of the
underlying PPP protocol, with the IPsec Layer 3 specification separate
security procedures and mechanisms are specified. One limitation of IPsec is
the fact that only IP-based communication is supported. However, in most
cases this is not a serious disadvantage as today most operating systems and
applications are able to utilise IP-based communication.
As regards security requirements, the situation regarding IPsec is as follows:
- User authentication
Tunnel protocols at Layer 3 assume that authentication of the tunnel end
points has been completed already before the tunnel is established and do
not offer any separate mechanisms. The only exception here is the IPsec
IKE procedure (formerly ISAKMP/Oakley), which allows mutual
authentication of the tunnel end points at application level. However,
authentication at user level is not possible with this. But since a Layer 3
protocol is actually transmitted over a Layer 2 protocol, in principle use
can be made here of the security mechanisms of both protocol layers.
When RAS is being used, the mechanisms for user authentication of the
underlying Layer 2 protocol actually have to be used as otherwise an
unauthorised third person could circumvent the security mechanism, for
example, through a physical attack on the client.
- Data encryption
The standard IPsec prescribes that IPsec-compliant implementations must
as a minimum make available the DES and Triple-DES encryption
procedures as well as the MD5 and SHA-1 hash functions. However there
is no reason why another procedure should not be used here, although in
this case the same procedure must also be available to the communication
partner. In general, only generally recognised and established procedures
should be used. The keys used for symmetric encryption procedures should
be at least 80 bits long.
- Key management
IPsec defines several methods for the generation, exchange and
management of keys. With the "Manual IPsec" procedure, there is no
automatic key management. In general keys are exchanged by the
communication partners over a secure channel (e.g. courier, encrypted email).
The interval for the regular exchange of keys here is much larger
than with the automatic procedures, such as the above-mentioned IKE
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IT-Baseline Protection Manual: Oktober 2000