Problem - Kevin Tafuro

5.9 Using a Generic CTR Mode Implementation
Problem
You want to use counter (CTR) mode and your library doesn’t provide an interface,
or you want to use a more high-level interface than your library provides. Alternatively,
you would like a portable CTR interface, or you have only a block cipher
implementation and you would like to use CTR mode.
Solution
CTR mode encrypts by generating keystream, then combining the keystream with
the plaintext via XOR. This mode generates keystream one block at a time by
encrypting plaintexts that are the same, except for an ever-changing counter, as
shown in Figure 5-4. Generally, the counter value starts at zero and is incremented
sequentially.
Few libraries provide a CTR implementation, because it has only recently come into
favor, despite the fact that it is a very old mode with great properties. We provide
code implementing this mode in the following “Discussion” section.
Discussion
1st counter block
P 1
Figure 5-4. Counter (CTR) mode
Start
E K
C 1
You should probably use a higher-level abstraction, such as the one
discussed in Recipe 5.16. Use a raw mode only when absolutely necessary,
because there is a huge potential for introducing asecurity vulnerability
by accident. If you still want to use CTR mode, be sure to use a
message authentication code with it.
CTR mode is a stream-based mode. Encryption occurs by XOR’ing the keystream
bytes with the plaintext bytes. The keystream is generated one block at a time by
Using a Generic CTR Mode Implementation | 197
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P 11
Start +10
E K
. . . . . .
C 11
10th counter block
= XOR