IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Personnel Remarks
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When A and B want to establish a confidential connection, they proceed as
follows:
1. They agree on an encryption procedure.
2. They agree on a key or a pair of keys.
3. A encrypts a message and sends this to B.
4. B decrypts the ciphertext sent from A.
There are two major classes of encryption procedures:
Symmetrical encryption procedures use the same key both for encryption and
decryption. Symmetrical techniques are therefore also occasionally referred to
as "one-key" techniques, because knowledge of one key is sufficient to be able
to encrypt and decrypt texts.
Well-known symmetrical encryption procedures include DES, Triple-DES,
IDEA and RC5, for example.
Symmetrical procedures are further differentiated, distinguishing between
stream ciphers and block ciphers.
In the case of stream ciphers, a key is used to generate a bit sequence (or bit
stream) with as random an appearance as possible, which is added to the plain
bit sequence (modulo 2). The plain bit sequence is therefore encrypted bit by
bit (by the addition of key stream bits). It is essential for the security of stream
ciphers that two (different) messages are never encrypted with the same key
stream – this must be ensured with the aid of special measures
(synchronisation information in the form of a message key). Examples of
stream ciphers are RC4 and SEAL.
Stromchiffre: Blockchiffre:
Bitstromgenerator
0
1
Schlüsselstrombit
Klarbit XOR Geheimbit
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IT-Baseline Protection Manual: Oktober 2000
0
1
1
0
Klartextblock
Schlüssel
Verschlüsselungsfunktion
1
1
1
1
0
0
Geheimtextblock
In the case of block ciphers, on the other hand, an entire block of bits is
encrypted in one encryption cycle; nowadays this typically comprises 64 bits.
Most symmetrical encryption methods are block ciphers; these also include
DES, IDEA and RC5. A range of operating modes have been defined (and
standardised) for block ciphers. These are:
- ECB (Electronic Code Book) mode, where every block is encrypted
separately – independently of the other blocks
- CBC (Cipher Block Chaining) mode and CFB (Cipher Feed Back) mode;
in these modes a dependence is established between the ciphertext blocks