Core 139, Fall, 2004
Exam 2
Name_________________________
1. Match each of the following people or groups with the event(s) with which they are
associated. Note, more than one person or group may be matched with an event. Each person or
group is used at least once, possibly more. Every event should have at least one association.
a. Marian Rejewski
b. Horst Feistel
c. Arthur Scherbius
d. Diffie, Hellman, Merkle
e. Alan Turing
f. Rivest, Shamir, Adelman
g. Tommy Flowers
h. Alice Kober
i. Phil Zimmerman
j. Michael Ventris
k.Ellis and Cocks
________ published "On Computable Numbers" with the
theoretical idea of a universal computer
________ developed Colossus
________ first published public key encryption method
________ major contribution to understanding Linear B
________ first published idea of public key encryption
________ first discovered public key encryption method
________ widely distributed public key encryption for
anyone to use
________ invented Enigma machine
________ developed idea of "bombes" to automatically
find possible rotor settings for enigma
using a "crib"
________ developed first method for breaking Enigma
based on repeated message keys
________ invented "Lucifer" encryption, the basis for DES
________ first published method secure key-exchange
Core 139, Fall, 2004
Exam 2
Name_________________________
2. Describe how the following parts of the Enigma machine worked.
(a) rotors (or scramblers)
(b) reflector
(c) plugboard
Core 139, Fall, 2004
Exam 2
Name_________________________
3. Describe the method developed by Turing using "cribs" for breaking the Enigma encryption.
How did the Turing Bombes assist this method? What aspect of the procedures used by the
Germans made this approach possible?
4. Describe two other errors (other than the one in question 3) that the Germans made in their use
of the Enigma that helped Allied cryptanalysts.
Core 139, Fall, 2004
Exam 2
Name_________________________
5. Give three reasons why the Navajo language was well-suited for being the basis of a code.
Explain why each reason was important.
6. The DES code is a block cipher that encrypts block of binary values.
(a) How many bits are in one block for DES?
(b) How many bits are in the key for DES?
(c) How many subkeys are used for the "rounds" of DES (this is the same as the number of
rounds) and how many bits in these subkeys?
(d) What are two modes of use for DES?
Core 139, Fall, 2004
Exam 2
Name_________________________
7. In public key encryption, each person (Alice, Bob, Eve, ...) has both a public key and a private
key. The public keys for everyone are made available to anyone else. The private keys are kept
secret by their owner.
(a) How can Alice send a message to Bob so that Bob can be sure that it came from Alice – that
is how can Alice "sign" her message? (Note: the message is not hidden from others for this part,
see part (b)) How does Bob verify that the message actually came from Alice?
(b) Since encryption and decryption is much slower for public key codes (like RSA) how can
Alice adapt the public key method to encrypt and send a long message to Bob almost as fast as
with symmetric key codes (like DES), but without exchanging keys in advance.
Core 139, Fall, 2004
Exam 2
Name_________________________
8. (a) Suppose one algorithm for solving problem P1 is shown to have complexity O(N2), where
N is the "size" of the data. If one version of the problem has N = 10,000 and takes one minute to
compute, then how long would you expect a problem with size N = 100,000 to take to compute?
(b) Suppose one algorithm for solving problem P2 is shown to have complexity O(2N), where N
is the "size" of the data. If one version of the problem has N = 10,000 and takes one minute to
compute, then how long would you expect a problem with size N = 10,010 to take to compute?
(Hint: 210 = 1024)
9. Use the ideas of computational complexity (the ideas of big-O notation used in problem 8) to
explain why the RSA encryption method will be at least as secure, perhaps more so, when
computers are 1000 times as powerful as they are today. (Note: the method is secure, but the use
of the method may change by using longer keys.)