Introduction to Cryptography with Open-Source Software

Block ciphers and the data encryption st andard 177

c

0

c

1

c

2

c

n

p

1

p

2

p

n

E

k

(c

0

) E

k

(c

1

) E

k

(c

n−1

)

FIGURE 8.7: Cipher feedback mode.

Counter mode (CTR)

This mode is less well known than the others, as it wasn’t accepted as a

standard mode until more r e c e ntly. However, it is bo th very easy to implement

and very secure. To start, a nonce value such as was needed for the other

modes is needed. The nonce needs to be expanded to a full block. This is

done by concatenating with the nonce a counter for the message blocks. For

a 128 block size, standard sizes are 48 bits for the message number, 16 bits

for the nonce data, and the remaining 6 4 bits for the counter ...