Cryptography Reference
In-Depth Information
PROPERTIES OF CBC MODE
Although CBC mode is a different design to CFB mode (behaving much less
like a stream cipher) the properties of CBC mode with respect to message
dependency, error propagation and synchronicity are almost identical to those
of CFB mode. However,
it does not have the implementation benefits of
CFB mode.
It is worth noting that CBC mode forms the basis for a well-known data
origin authentication mechanism that we will discuss in Section 6.3.3. This brings
implementation benefits for applications that require both symmetric encryption
and data origin authentication and is perhaps why CBC mode has historically
been the more popular of these two modes of operation.
4.6.4
Counter mode
The last of the modes of operation that we will describe in full is
Counter
(CTR) mode.
ENCRYPTION AND DECRYPTION USING CTR MODE
Counter mode can be thought of as a counter-based version of CFBmode without
the feedback. The main difference is that we assume that both the sender and
receiver have access to a reliable
counter
, which computes a new shared value
each time a ciphertext block is exchanged. This shared counter is not necessarily a
secret value, but both sides must keep the counter synchronised. Both encryption
and decryption in CTR mode are depicted in Figure 4.14.
1
5
8
Counter
Counter
Increment
counter
E
2
6
E
K
K
3
7
⊕
⊕
P
P
4
C
C
Sender
Receiver
Figure 4.14.
Encryption and decryption using CTR mode
