Cryptography Reference
In-Depth Information
1
⊕
4
3
P
2
K
D
C
Receiver
Figure 4.12.
Decryption using CBC mode
4. Replace the contents of the top register with
C
1
.
5. Now repeat from step 2. In other words, decrypt the next block of ciphertext
C
2
and place the result in the bottom register (the bottom register now contains
exactly the same value as the sender had in their bottom register prior to the
second encryption); add the contents of the bottom register to the contents
of the top register, the result of which is the next plaintext block
P
2
; replace
the contents of the top register with
C
2
. Continue in this manner until the last
ciphertext block has been processed.
Put more concisely, in CBC mode the current plaintext block is added to
the previous ciphertext block, and then the result is encrypted with the key.
Decryption is thus the reverse process, which involves decrypting the current
ciphertext and then adding the previous ciphertext block to the result.
IMPACT OF TRANSMISSION ERRORS IN CBC MODE
Just as we did for CFBmode, we now look at the effect of a 1-bit transmission error
when CBC mode is used. Suppose that we are using CBC mode as depicted in
Figures 4.11 and 4.12, using a block cipher with a block size of 128, and that a 1-bit
transmission error occurs in ciphertext block
C
i
, but that all previous ciphertext
blocks were received correctly. Clearly all the previous plaintext blocks
P
1
up
to
P
i
−
1
are unaffected since they were decrypted prior to
C
i
arriving. We now
consider what happens when the erroneous block
C
i
is processed. This situation
is depicted in Figure 4.13.
1. When
C
i
is received, it is decrypted, resulting in an incorrect value being placed
in the bottom register. The current content of the top register consists of
C
i
−
1
