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7.7
DES algorithm.
DES uses a 56 bit key. (8 bits of the 16 hexadecimal digits are used
as parity bits.). If anyone should know parts of the plain text and try
to decrypt this using different keys, at worst about 7.2 x 10°°
attempts must be tried (72 million billion attempts).
Tne DES algorithm is used in the "Chipher Biock Chaining" (CBC)
mode. The encrypted block will depend on the previous block. The
first block is dependent of one of the blocks exchanged in the key
negotiation.
During this negotiation, before a user key has been agreed upon,
data is encrypted using key no. 0. By using this key negotiation,
identical messages
transmitted several times, are not encrypted in
the same way. (Only the message length will repeat itself).
7-22
LZTR104006 Rt.00
Zat 14.4/14.4-7 and
19.2/19.2-7
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