This homework is due November 18 at 10.15. It can be
delivered to Stefan personally at his office or put in his
mail slot at the department. Do not use the depart mail box.
It can also be given to Stefan in connection with one of the lectures.
Solutions handed in late are not accepted and will not be graded.
Some forms of collaboration are allowed and required. The size of a
group of collaboration should be two people.
The group should hand in only one solution and for each problem it
should be clearly marked which of the members have contributed.
There will be a list outside of Stefan's office where you can book
a time. All members of the group must be present when the homework
I do not consider the set of problems below easy. Thus a
performance of getting half the total score on this set
is at least equivalent to the grade 4 on this subset of
the course. Credit may be given for partially solved
1. Multiplication (60p)
Your task is to implement multiplication algorithms for
large integers. You should implement both the schoolbook
algorithm and Karatsuba's algorithm.
The multiplication algorithms should be implemented from scratch
using either C or Java.
However, you may use standard libraries, such as Java's BigInteger,
for reading and converting numbers from and to decimal form.
Compare the running times of the algorithms experimentally.
(Optionally, compare the running time of your algorithms with
the algorithm based on FFT in the textbook by Goodrich and Tamassia.)
The code should follow
Java Code Conventions
(or similar C conventions).
IndexTreeList.java is intended to be a good
Your code should be well documented and easy to use.
The documentation could be generated using
The API should be easy to understand and use. Only include necessary
methods and carefully design your data types.
The code should be robust. Erroneous parameters and function
calls should be handled gracefully.
The code should be efficient.
The code must, of course, be correct. Include code for testing
all functions. I highly recommend the following
article. You don't have to use the software tool described in
the article but the ideas are still very useful.
2. Fill in the gaps (10p+10p+10p+10p)
Give detailed proofs of the following statements.
page 14: If this condition is true for all a between 1 and p-1
then N must be prime since gcd(a, N) = 1 for
1 <= a <= N-1 which clearly implies that N is prime.
page 15: It is not hard to see that
x = a1U1 + a2U2 mod p1p2
fulfills the first set of equations ...
page 15: It is not difficult to extend this to larger r
(we leave the details as an exercise) and ...
page 16: But if N is prime, then the equation
x2 = 1 mod N has only the solutions
x = ±1 and ...
3. Prime numbers (20p)
Let p be a prime such that p = 1 mod 8.
Find, and prove, a formula for the number of solutions to
xd = ±1 mod p where
d = (p - 1)/8, (p - 1)/4, and (p - 1)/2.
4.Lemma 4.2 (20p)
Prove Lemma 4.2 in the lecture notes.
5. Cracking RSA (30p+30p)
a) Using any free software, write a program to crack RSA.
You may use at most 5 minutes of user time on a work station and
you must be able to convincingly demonstrate that your program
is working. The bigger the keys you manage to crack, the more
points are awarded.
b) Is it possible to crack RSA without factoring?
What if you have no extra information except the public key
and the encrypted message? What if you have some extra
You may go about this problem in many different ways. You
will be rewarded for presenting published results
(don't forget do give proper references) and creative
thinking of your own is also very much appreciated.