CS 167  Cryptography Homework 1, 50 Points
Due: Friday, January 18, 2008

What is the ciphertext for ATTACKATDAWN using a shift cipher with shift key
k=5?

ROT13 is a simple shift cipher
for English text such that when it is applied twice
it returns the original plaintext.
Argue why there is no single
shift cipher for English text such that when it is applied three times
it returns the original plaintext.
Give an example of a simple shift cipher for English other than
ROT13 such that
when it is applied some number
m < 26 times it returns the original plaintext.
What is the value of m in this case?

The 4th century Indian text, Kama Sutra,
proposes, among other things,
a method for secret writing where each letter is paired with
another and each occurrence of a letter in
a document is replaced by its mate.
For example, we could pair (A,V), (B,N), (C,E), (D,F), (E,C), (F,D),
and so on, so that
every "A" is replaced by "V" and every "V" is replaced by "A",
every "B" is replaced by "N" and every "N" is replaced by "B",
etc.

Give an example encryption using a Kama Sutra cipher.

How many possible keys are there for the Kama Sutra cipher?

What is the security of the Kama Sutra cipher with respect to the four
kinds of attacks that can be done on a cryptosystem?

Suppose Eve has learned that Bob is using an Affine cipher,
and she has been able to apply
a knownplaintext attack to learn that the plaintext
``ADOG'' has ``FLHR'' as its ciphertext. What is Bob's key?

Suppose Eve has learned that Bob is using a Vigenere cipher
with keyword length m < 12, and she has been able to apply
a chosenplaintext attack to learn that the plaintext
``AAAAAAAA'' has ``BCDEBCDE'' as its
ciphertext. What are the possible values for m?