Need to work cryptography task

Learning Goal: I’m working on a computer science question and need an explanation and answer to help me learn.


  1. A stream cipher uses a LFSR with three units. An attacker captures a ciphertext “0101001010011”. The attacker knows that the first 6 bits of the message are “110001.” Using this information, the attacker defeats this encryption, i.e., he is able to determine the initial seed values (S1, S2 and S3) and Keys (K1, K2 and K3). What are those values? (Points 25)

Calculation Hints:


S 1 through S 6 can be computed by XORing 6 bit message with the first 6 bits of ciphertext.Then formulate the following equations for S 4 through S 6 :

S m+1= S m × K m + S m-1 × K m-1 + …… S 2 × K 2 + S 1 × K 1 mod 2

S m+2= S m+1 × K m + S m × K m-1 + …… S 3 × K 2 + S 2 × K 1 mod 2

S m+3= S m+2 × K m + S m+1 × K m-1 + …… S 4 × K 2 + S 3 × K 1 mod 2

Number of units in the shift register determines the value of m.

Many of you may not feel comfortable solving systems of equation. You can apply the following brute force method:

Check, all possible sets of values of K 1, K 2 and K 3 listed in the following table to find out which set satisfies all the above equations (i.e., makes both sides of the equations equal) :

K1 K2 K3
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
  1. An organization uses same stream cipher key multiple times. An attacker who knows that, captures two ciphertexts C1 and C2 and finds that,

C1⊕C2= 0000101000011101000000110001001100010000000101000000011000000011000010100000100100011011

The attacker also knows that the plaintexts are composed of only English capital letters without any punctuations or space, and that part of the plaintexts contains the word: “PRESIDENT”. He was able to recover both the plaintexts. Show, how. (Points 25)

Please see the attached screenshot

Calculate your order
Pages (275 words)
Standard price: $0.00
Client Reviews
Our Guarantees
100% Confidentiality
Information about customers is confidential and never disclosed to third parties.
Original Writing
We complete all papers from scratch. You can get a plagiarism report.
Timely Delivery
No missed deadlines – 97% of assignments are completed in time.
Money Back
If you're confident that a writer didn't follow your order details, ask for a refund.

Calculate the price of your order

You will get a personal manager and a discount.
We'll send you the first draft for approval by at
Total price:
Power up Your Academic Success with the
Team of Professionals. We’ve Got Your Back.
Power up Your Study Success with Experts We’ve Got Your Back.