CS402 Cryptography: Worksheet
1. The letter “N” was found to be the most frequent letter in a large ciphertext produced using
a Caeser cipher over the alphabet A= 0, B = 1, ..., Z = 25. Decipher the following portion
of the ciphertext:
CQRB RBJ F N JT LRY QNA .
2. (a) Describe the Vigen`ere cipher.
(b) A Vigen`ere cipher over the 27-letter alphabet A= 0, B = 1, ..., Z = 25,= 26 was used
to produce the ciphertext
EK W W EK KW W E KK DQDI KLOF DT K KS RKC A P W K ZW QP X J W V K
SW N DDS IK DX E GKE W HK DW RQLV MRQP K DM W P HS R
OKQR OY P D RMQK QRLLOH DBK N P SRN P HK LLDH DN ZB CH KX
IB IK DP F W P KX W S WM RH LY K W DLY LRQLIJ W DDS MP H KM M R
LL X P DQZZ REK GP P P LIS .
The corresponding plaintext begins
A LON G LO NG TI M E AGO .
Determine the first 17 words of plaintext.
3. What basic property of the Vigen`ere cipher makes it significantly more secure than the
Caeser cipher for sending very short messages (such as a bank PIN or credit card details)?
4. What basic property of the Enigma cipher makes it significantly more secure than the Vi-
gen`ere cipher against a ciphertext only attack.
5. The ciphertext
T M Y
was produced by applying the affine enciphering function fE:Z26 →Z26, x 7→ 9x+ 16 to
single letter message units over the alphabet A= 0, B = 1, ..., Z = 25. Determine the
plaintext.
6. The ciphertext
ES DCW N M H
was produced by applying a Hill cipher to 2-letter message units over the alphabet A=
0, B = 1, ..., Z = 25, with enciphering finction
fE:x
y7→ Ax
ywhere A=2 3
7 5 .
Calculate A−1mod 26 and hence determine the first FOUR letters of plaintext.
7. Carefully explain each of the following italicized terms:
(a) Kerckhoff’s principle
(b) two factor authentication
(c) symmetric cipher
(d) public key cipher
(e) block cipher
(f) stream cipher
(g) key space
(h) ciphertext only attack
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