# COMP90043 Cryptography and Security Assignment 1 solved

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## Description

Objectives
This assignment is designed to improve your understanding of the Euclid’s algorithm, classical ciphers and basics of probability. It’s also aimed at improving your problem-solving
and written communication skills.
Questions
1. Classical Ciphers [20 marks]
Consider the following version of a classical cipher where plaintext and ciphertext
elements are the integers from 0 to 35. Note that this alphabet may be used when
plaintexts are 26 English characters and 10 numeric characters. The encryption function, which maps any plaintext p to a ciphertext c, is given by
c = E(a,b)(p) = (ap + b) mod 36,
where a and b are integers less than 36.
(a) What is the decryption function for the scheme?
(b) A key is called trivial if c = p for all input p. How many non-trivial keys are
possible for this scheme?
(c) Would this cipher be considered as mono-alphabetic cipher or poly-alphabetic
cipher? Why?
(d) You are given a large amount of ciphertext characters encrypted using this
scheme. Assuming its plaintext was written in English, show how an attacker
can retrieve the key.
(e) An oracle is available to you which can output the encrypted ciphertext for
arbitrary plaintext you give. Briefly describe an efficient way to retrieve the key
using the oracle.
2. General Security [8 marks]
Which of the following factors might be the most concern by the public in regards to
using the COVIDSafe app1
1https://www.health.gov.au/resources/apps-and-tools/covidsafe-app
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(a) Confidentiality (b) Integrity (c) Availability
3. Euclid’s algorithm [15 marks]
Perform the following implementation tasks in a language of your choice. You are at
free to employ any underlying integer arithmetic library. In order to get full marks,
your algorithm has to be able to work in realistic cryptographic environments (consider
101000 as input).
(a) Implement the extended GCD algorithm as discussed in lectures and print the
code here.
(b) Implement a function which takes two positive integers a, n as inputs, and returns
the inverse of (a mod n) based on your extended GCD algorithm (that you just
implemented above). Print the code for this function.
(c) Use the above function to find the inverse of (X mod 16777259), where X is your
student number. You don’t need to show steps for the calculation.
4. Poly-alphabetic Cipher [21 marks]
For this question, we consider the Hill cipher given in the textbook on an alphabet
A consisting of 26 English characters (A-Z), 10 numeric characters (0-9) and space,
which corresponds to integers 0 to 36. Here the plaintext is processed successively in
blocks of size m. The encryption algorithm takes a block with m plaintext digits and
transforms into a cipher block of size m using a key matrix of size m×m by the linear
transformation, which is given by:
c1 = (k1,1p1 + k1,2p2 + · · · + k1,mpm) mod 37
c2 = (k2,1p1 + k2,2p2 + · · · + k2,mpm) mod 37
· · ·
cm = (km,1p1 + km,2p2 + · · · + km,mpm) mod 37
Note: For this question, correspondence between plaintext and number modulo 37 are
as follows “A” ↔ 0, “B” ↔ 1, “C” ↔ 2, . . . , “Z” ↔ 25, “0” ↔ 26, “1” ↔ 27,
“2” ↔ 28, . . . , “9” ↔ 35 and “ ” (space) ↔ 36
(a) How many different keys are possible in this system?
(b) This cipher is easily broken with a known plaintext attack. An adversary discovers the following ciphertext is encrypted using this cipher with m = 5 (55
characters in total, no spaces):
A8VS3XRDEON6JEVXGJID13C07L4C1R4Q965XWRA5DQGYWTNHYO4ND8Z
If the following combination of plaintext and ciphertext is given (please replace
both “?????” by the last five digits of your student number), decrypt the cipher
by giving the plaintext as well as both encryption and decryption keys.
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Plaintext X9B6T6JAW3UEY7FHIW?????5Z
Ciphertext 2Q59ZZ1Z?????UMDNY2JHINTS
You need to show step-by-step details of your working. Make sure to include
the details of any package, functions used, and/or programs developed. Simply
showing the final result and/or a program would not receive marks.
5. Probability [11 marks]
Let x be the fourth digit of your student ID (without leading zero), y be the sixth
digit of your student ID. The value N used in this task is given by 5x + 6y + 15.
For the below tasks, you need to show your working by providing formula used, and/or
short explanation. Also give the numerical final answer (e.g. 1024 instead of 2
10).
(a) What is your value of N based on your student ID? You may simply show N,
but please make sure that your calculation of N is correct, as you will need this
value for the rest of tasks.
(b) Assuming that we have 230 students enrolled in this subject, and all student
numbers are randomly generated. What’s the probability that at least one of
your classmate shares the same N with you? Your result should be rounded to
three digits after the decimal point.
(c) How many ways to place N different balls into five different bins?
(d) How many ways to place N identical balls into five different bins, so that all bins
are non-empty?
(e) How many ways to place N identical balls into five different bins?
(f) How many ways to place N identical balls into five identical bins, so that at most
two bins are non-empty?
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Submission and Evaluation
• You must submit a PDF document via the COMP90043 Assignment 1 submission
entry on the LMS by the due date. Handwritten, scanned images, and/or Microsoft
Word submissions are not acceptable — if you use Word, create a PDF version for
submission.
• Late submission will be possible, but a late submission will attract a penalty of 10%
per day (or part thereof). Requests for extensions on medical grounds will need to be
supported by a medical certificate. Any request received less than 48 hours before the
assessment date (or after the date) will generally not be accepted except in the most
extreme circumstances.
• This assignment will be marked out of 75 marks, and will contribute to 7.5% of your
total marks in this subject. Marks are primarily allocated for correctness of your
thinking and clarity of your communication, rather than (only) the correct result
without justification.
• We expect your work to be neat — parts of your submission that are difficult to read
or decipher will be deemed incorrect. Make sure that you have enough time towards
the end of the assignment to present your solutions carefully. Time you put in early
will usually turn out to be more productive than a last-minute effort.
• You are reminded that your submission for this assignment is to be your own individual
work. For many students, discussions with friends will form a natural part of the
undertaking of the assignment work. However, it is still an individual task. You
are welcome to discuss strategies to answer the questions, but not to share the work
(even draft solutions) on social media or discussion board. It is University policy
that cheating by students in any form is not permitted, and that work submitted for
assessment purposes must be the independent work of the student concerned.