IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Personnel Remarks
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- Key generation is generally complex and time-consuming, because care
must be taken not to generate "weak" key pairs.
Hybrid procedures attempt to combine the advantages of both types of
encryption: they use asymmetric encryption to transfer a session key for a
symmetric procedure, and encrypt the bulk data with the symmetric procedure.
The session key is usually used for only one session (transmission) and is then
destroyed. The asymmetric key pair may be used for a long period, depending
on the circumstances.
II. Protection of integrity
The objective of integrity protection is to enable the recipient of a message to
establish whether he has received the message without it being corrupted. The
basic principle of integrity protection is to transmit the message unencrypted
and unchanged, but at the same time to send certain checking information with
the message, which enables the actual message to be checked to ensure that it
is uncorrupted. The prerequisite for this, however, is that the recipient must
receive the check data in an unmanipulated state. The check data therefore has
to satisfy the following conditions:
- The amount of check information must be kept as small a possible so as to
minimise the amount of additional information to be transmitted.
- It must be possible to detect virtually any manipulation, even of only a
single bit of the message, on the basis of the check information.
- It must be possible to transmit the check information in an unmanipulable
form, or it must be possible to detect manipulations.
Typically there are two methods that are used for the calculation of check
information: hash functions and message authentication codes.
A (one-way) hash function is a data transformation with the following
characteristics:
- Compression characteristic: bit sequences of any length are mapped to bit
sequences of a fixed, generally shorter length (typically 128 - 160 bits).
- "One-way" characteristic: for a specified hash value it must be "practically
impossible" to find a message whose hash value is the specified hash value.
- Collision resistance: it must be "practically impossible" to find two
messages which lead to the same hash value.
A and B can check the integrity of a message with the aid of a hash function
known to both communication partners: Alice hashes her message, and
transmits this and the hash value to Bob in such a way that the accuracy of the
hash value is guaranteed. Similarly, Bob hashes the message he receives and
compares his result with the hash value supplied by Alice. If the two values
tally, he can assume that no bit of the message has been changed.
A message authentication code (MAC) is a cryptographic checksum for
message security, in other words a data transformation in which a secret key is
additionally included in the calculation, with the following characteristics:
- Compression characteristic: bit sequences of any length are mapped to bit
sequences of a fixed, generally shorter length.
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IT-Baseline Protection Manual: Oktober 2000