Cryptography Reference
In-Depth Information
2
. The Caesar Cipher can be implemented by two 'wheels', where each wheel has
the letters of the alphabet written on it and the inner wheel can be rotated
within the outer wheel. Explain how to encrypt and decrypt with the Caesar
Cipher using such an implementation.
3
. The Simple Substitution Cipher is a major improvement on the Caesar Cipher.
(a) In what different ways is it an improvement?
(b) If a Simple Substitution Cipher is used (with an unknown key) and we
intercept the ciphertext OXAO then which of the following four-letter words
could be the plaintext: JOHN, SKID, SPAS, LOOT, PLOP, OSLO?
(c) If a Simple Substitution Cipher is used (with an unknown key) and we
intercept the ciphertext BRKKLK thenwhat dowe knowabout the plaintext?
(d) Suppose that in the case of intercepting BRKKLK we also know that the
plaintext is the name of a country. What is the plaintext?
(e) What important lesson does the BRKKLK example illustrate?
4
. The
Atbash Cipher
consists of replacing 'plaintext' letters A, B, C to Z by the
'ciphertext' letters Z, Y, X to A, respectively.
(a) 'Decrypt' XZKVIXZROORV.
(b) How would you describe the relationship between the Atbash Cipher and
the Simple Substitution Cipher?
5
. Explain how you might adjust the normal English letter frequencies depicted
in Table 2.1 in order to conduct single letter frequency analysis, given that the
context of a plaintext is:
(a) a personal letter;
(b) part of the technical manual for a SUN SOLARIS operating system;
(c) a document written in Polish.
6
. Decrypt the following ciphertext, which was obtained from English plaintext
using a Simple Substitution Cipher (to make things slightly easier we have
included the characters [space] and [.] in the ciphertext in the same positions
that they appear in the plaintext):
UGVPQFG OQ OLG PQCWNG. QDG EZF ZN SQW OLG NOCBGDON
OQ VGEWD EHQCO ZDSQWFEOZQD NGPCWZOJ. OLZN FGEDN LEWB
UQWY. LQUGRGW UG EVNQ UEDO OLGF OQ GDMQJ OLGFNGVRGN.
OLG PQCWNG EDB OLG VGPOCWGN NLQCVB HG SCD XVGENG VGO CN
YDQU ZS OLGWG EWG EDJ XWQHVGFN NQ OLEO UG PED EOOGFXO OQ
PQWWGPO OLGF.
7
. The ciphertext MBR OJFGA SWNTE CNK QJDIL NURW MBR XHMR was
obtained by encrypting the plaintext THE QUICK BROWN FOX JUMPS OVER
THE GATE using a Simple Substitution Cipher.
(a) How much of the key is determined if this plaintext/ciphertext pair are
known?
