IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Organisation Remarks
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In the case of asymmetrical procedures, the chosen mechanism strength
should be such that solving the underlying mathematical problems requires
an unjustifiably high or practically impossible amount of computation (the
mechanism strength that should be chosen is therefore dependent on the
present state of algorithm development and of computing technology).
Currently it can be assumed that you will be "on the safe side" with:
- module lengths of 768 bits with RSA or
- subgroup orders of the order of magnitude of 160 bits in the case of
ElGamal procedures on a suitable elliptical curve
.
No "unknown" algorithms should be used, i.e. the algorithms that are used
should be ones which have been published, have been intensively
investigated by a broad spectrum of experts and which are not known to
have any security weaknesses. Vendors frequently offer security products
with new algorithms which are supposedly "even more secure and even
faster" than other algorithms. However, great caution must be exercised
when using unknown algorithms from sources whose cryptographic
competence is not sufficiently proven.
- Symmetrical or hybrid procedures?
For performance reasons, no implementations based solely on public key
techniques are used for encryption purposes. All common implementations
of public key cryptography use hybrid procedures (see S 3.23).
In applications with large or open user groups it is usually advisable to use
a hybrid procedure (because of the advantages for key management).
Where user groups are small and closed (and in particular of course in the
case of a single user), it is possible to keep to symmetrical procedures. If
hybrid procedures are used, it makes sense to tailor the symmetrical and
asymmetrical parts to work together. With the asymmetrical procedure it is
generally the case that before a key change many keys for the symmetrical
procedure are encrypted, so therefore the asymmetrical algorithm should
normally be somewhat stronger.
- Feasibility of technical requirements
The enciphering algorithms must be designed such that the technical
requirements, in particular the required performance, can be fulfilled if a
suitable implementation is put in place. These include requirements
relating to error propagation (for example if data is sent via very noisy
channels), but also requirements as to synchronisation overhead and time
delay (for example if "real-time" encryption of large quantities of data is
required).
Example: Voice encryption with ISDN
When a communication network is being planned, a range of parameters
have to be taken into account which have an influence on the expected
speech quality and which become noticeable in the form of noise, clicking,
crosstalk or singing. Such influencing factors include the encryption
procedures, for example. In order to be able to achieve satisfactory speech
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IT-Baseline Protection Manual: Oktober 2000