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ryptior
7.5.6
Description of the data channel and encryption mode.
In encrypted synchronous data mode, the DTE will have a
transparent full duplex data channel. The delay through both
modems at 9600 bps with trellis coding is about 70ms. This
delay will typically be 25ms in normal synchronous mode.
The connection sequence in synch. encryption mode is as
follows:
First the modems are connected.
Then the V.42/V.42 bis protocol is set up.
Then the answering modem requests password in plain text.
The originating modem sends this password in plain text.
The answering modem decides which key index to use and
sends this information to the remote encrypted with key 0.
The originating modem decrypts the information and activates
the correct local key.
Then both modems starts a bit synching procedure. The
synching of the receive and transmit channel is independent.
When the bit synching process is nearly finished, the modem
turns 106 on and starts reading data from DTE.
When the bit synching process has finished, the modems set
109 on and enters data state.
In data state, the modems receives bytes synchronously from
both line and DTE. The bytes are stored in buffers. When the
buffers has accumulated 8 bytes, these 8 bytes are encrypted
or decrypted as a block. After encryption/decryption the block
is put in transmit buffers waiting to be sent. The number of
characters in the buffers are kept at a constant size because
both receiver and transmitter is clocked from the same
source.
The encryption/decryption uses DES-Cipher Block Chaining
(CBC) mode of operation with a random initial vector. (The
same as in asynchronous encryption mode) This means that
all encrypted sessions will send different bit patterns on the
line, even if the plain text data is the same.
The DES-CBC mode is self synchronising. This means that
any bit errors in an encrypted block will produce bit errors in
the decrypted block. But the next error free block will be
decrypted into an error free block.
If the modems
loose synch. the CBC mode will not
synchronise again. This case is prevented in the modem
by
automatically starting retrains or entering a new handshake
when serious line distortions are detected.
7-20
LZTR104006 R1.00
Zat 14.4/14.4-7 and 19.2/19.2-7
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