IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Personnel Remarks
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- Resistance to forging: for anyone who is not in possession of the key it
must be "practically impossible" to calculate the MAC value of a new
message, even if he has come into possession of a number of old messages
with the associated MAC values.
If Alice and Bob have a MAC and a common, secret MAC key, Alice
authenticates her message simply by calculating the MAC value of the
message and sending it to Bob together with the message. In turn, Bob
calculates the MAC value of the received message with the MAC key, which
is also known to him. If this tallies with Alice’s value, he can assume that the
message is authentic (i.e. that it has not been altered and that it really
originates from Alice). Alice has therefore authenticated her message to Bob
by using the key that is known only to Bob and herself.
MACs are often designed on the basis of symmetric encryption methods. The
best known variant is the encryption of a message with DES or another block
cipher algorithm in CBC or CFB mode. This involves appending the last
encrypted block to the message as the MAC. Apart from this, however, there
are also MACs that are not based on encryption methods. The MAC value of a
message can be seen as the non-forgeable, key-dependent, cryptographic
checksum of the message. The use of MACs for the purpose of authentication
requires that both parties reliably protect the secret authentication key.
As a side-effect of integrity protection, the procedures outlined above can be
used at the same time by the recipient of the message to check that the
message, which has been verified as being unmanipulated, could only have
been sent by the sender who is actually known to the recipient. This
conclusion can be drawn because only this sender has the necessary keys for
encrypting and determining the check information.
III. Proof of authenticity
Certain criteria must be met regarding the authentication of users with respect
to communication partners/IT systems or of clients with respect to servers:
- Illegitimate accesses must be detected and warded off
- Legitimate accesses must be permitted
- Sensitive data must remain protected even when transmitted across
networks
To ensure this, procedures are required which allow all participants to
establish the identity of their communication partners unequivocally. This
includes a time aspect: Alice wants to convince Bob in real time that it is
indeed she who is communicating with him. The main techniques for
authentications of this nature are cryptographic challenge-response protocols.
In these, Bob sends data to Alice and requests (challenges) her to prove to him
that she possesses a secret (i.e. an item of key information); Alice
demonstrates to him that she has this possession without divulging the secret
itself by sending him a response that is dependent on the secret and on his
challenge. Bob in turn uses the response that she has sent to check that the
correct secret really was used to calculate the response.
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IT-Baseline Protection Manual: Oktober 2000