Cryptography Reference
In-Depth Information
15
. For each of the modes of operation discussed in Section 4.6, what is the effect
of a 2-bit transmission error in the ciphertext if the two erroneous bits occur:
(a) on the same ciphertext block?
(b) on two consecutive ciphertext blocks?
(c) on two ciphertext blocks that are separated by at least one error-free
ciphertext block?
16
. CFB mode is normally used in reduced feedback mode.
(a) Sketch a diagram that illustrates how 8-bit CFB mode works when used
with a block cipher operating on block sizes of 32 bits.
(b) Explain what happens if there is a 1-bit transmission error in a ciphertext
sent using this cryptosystem.
17
. Modes of operation such as CFB and CBC require the use of an initial variable
(IV), which must be known by both the sender and receiver. Discuss the pros
and cons of the following suggestions for selection of IVs:
(a) randomly generate each IV;
(b) use a counter that is incremented after each message as the IV;
(c) use the last ciphertext block of the previous message as the IV.
18
. For each of the modes of operation that we discussed in Section 4.6:
(a) What is the impact of the transmission loss of an entire block of ciphertext
(in other words the ciphertext block is 'dropped')?
(b) What is the impact of a 1-bit error as a result of a computational error during
the encryption process itself (in other words, the error occurs at Alice's end
when she computes ciphertext block
C
i
)?
19
. There are several modes of operation proposed for block ciphers that we have
not discussed in any detail. Choose an example of one suchmode of operation.
(a) What applications has your chosen mode of operation been designed for?
(b) What properties does your mode of operation offer?
(c) What is the impact of a 1-bit transmission error in your chosen mode of
operation?
