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# Software Systems Mini Assignment #3 Cryptography in C

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COMP 206 School of Computer Science

Software Systems
Mini Assignment #3

Cryptography in C
In cryptography, a Caesar cipher, also known as Caesar’s cipher, the shift cipher, Caesar’s code
or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type
of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed
number of positions down the alphabet. For example, with a left shift of 3, D would be replaced
by A, E would become B, and so on.
Example: The transformation can be represented by aligning two alphabets; the cipher alphabet
is the plain alphabet rotated left or right by some number of positions. For instance, here is a
Caesar cipher using a left rotation of three places, equivalent to a right shift of 23 (the shift
parameter is used as the key):
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW
When encrypting, a person looks up each letter of the message in the “plain” line and
writes down the corresponding letter in the “cipher” line.
Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
Deciphering is done in reverse, with a right shift of 3.
Notice that modulo is helpful, assuming that, A=0, B=1, C=3, …, Z=25. Then to shift
characters one needs to do the following:
Int letter = ‘A’, key = 3, newLetter;
newLetter = (letter + key) % 26; // since 26 letters in the alphabet
Note that this does not work in the opposite direction since subtracting 3 could
lead to a negative number.
Notice further, we can convert ASCII to A=0, B=1, C=3, etc. by performing this
subtraction: int letter; letter = letter – ‘A’; for upper case characters. You would need to
subtract by ‘a’ for lower case characters. This would work for the entire alphabet.
McGill University COMP 206 School of Computer Science
Vybihal Assignment #3 Page 2 of 3
This mini assignment is asking you to write a C language program to implement this
cipher.
The program takes two inputs:
1) A sentence typed in by the user at a prompt.
2) The key the user plans to use (For the assignment, we will restrict it to a left shift
between 1 and 25. Ex, a 3 means shift left by 3 places), typed in at a prompt.
The program displays:
1) the original sentence and
2) the encrypted sentence.
3) It then decrypts the encrypted message and displays the decrypted message.
The decrypted message and the original message should agree.
The input sentence will only be text using alphabets with spaces as in the above
example. We will not test it for numbers or special characters.
Expected format:
Bash-prompt \$ ./a.out
Sentence: Mary
Key: 2
Original message: Mary
Encrypted message: Kypw
Decrypted message: Mary
Bash-prompt \$
McGill University COMP 206 School of Computer Science
Vybihal Assignment #3 Page 3 of 3
HOW IT WILL BE GRADED
Points removed for bad practices:
• -1 for not following instructions
• -1 for not indenting, spacing, and/or commenting
• -1 for not using good variable names
This assignment is worth 20 points:
• +2 Input sentence stored in an array and key in variable
• +2 Output as described above
• +4 Arrays, proper use of
• +2 Modulo, proper use of
• +5 Encryption, proper functioning of, for all letters in sentence
• +5 Decryption, proper functioning of, for all letters in sentence

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