Cryptography Reference
In-Depth Information
4. Draw a circuit diagram and verify the output sequence of the LFSR.
2.11.
We want to perform an attack on another LFSR-based stream cipher. In order
to process letters, each of the 26 uppercase letters and the numbers 0, 1, 2, 3, 4, 5
are represented by a 5-bit vector according to the following mapping:
A
↔
0 = 00000
2
.
Z
↔
25 = 11001
2
0
↔
26 = 11010
2
.
5
↔
31 = 11111
2
We happen to know the following facts about the system:
The degree of the LFSR is
m
= 6.
Every message starts with the header
WPI
.
We observe now on the channel the following message (the fourth letter is a
zero):
j5a0edj2b
1. What is the initialization vector?
2. What are the feedback coefficients of the LFSR?
3. Write a program in your favorite programming language which generates the
whole sequence, and find the whole plaintext.
4. Where does the thing after
WPI
live?
5. What type of attack did we perform?
2.12.
Assume the
IV
and the key of Trivium each consist of 80 all-zero bits. Com-
pute the first 70 bits
s
1
,...,
s
70
during the warm-up phase of Trivium. Note that
these are only internal bits which are not used for encryption since the warm-up
phase lasts for 1152 clock cycles.
