Assignment 3
The science of writing secret codes is called cryptography. For thousands of years cryptography has made secret messages that only the sender and recipient could read, even if someone captured the messenger and read the coded message. A secret code system is called a cipher. In cryptography, we call the message that we want to be secret the plaintext. The plaintext could look like this:
Hello there! The keys to the house are hidden under the flower pot.
Converting the plaintext into the encoded message is called encrypting the plaintext. The plaintext is encrypted into the ciphertext. The ciphertext looks like random letters, and we cannot understand what the original plaintext was just by looking at the ciphertext.
Here is the previous example encrypted into ciphertext:
Yvccf kyviv! Kyv bvpj kf kyv yfljv riv yzuuve leuvi kyv wcfnvi gfk.
But if you know about the cipher used to encrypt the message, you can decrypt the ciphertext back to the plaintext. (Decryption is the opposite of encryption.)
CS 4350 – Unix Systems Programming
Spring 2020 – Husain Gholoom – Senior Lecturer in Computer Science Page 2
Many ciphers also use keys. Keys are secret values that let you decrypt ciphertext that was encrypted using a specific cipher.
Think of the cipher as being like a door lock. You can only unlock it with a particular key.
encrypt a message by taking each letter in the message (in cryptography, these letters are called symbols because they can be letters ( A-Z) ( a-z ), numbers( 0-9 ) , all symbols and replacing it with a “shifted” letter.
Using letters are , If you shift the letter A by one space, you get the letter B. If you shift the letter A by two spaces, you get the letter C. For example , this is a picture of some letters shifted over by three spaces.
The number of spaces you shift is the key in the Cipher. The example above shows the letter translations for the key 3. If you encrypt the plaintext “Howdy” with a key of 3, then:
• The “H” becomes “K”. • The letter “o” becomes “r”. • The letter “w” becomes “z”. • The letter “d” becomes “g”. • The letter “y” becomes “b”.
The ciphertext of “Howdy” with key 3 becomes “Krzgb”.
We will keep any non-letter characters the same. To decrypt “Krzgb” with the key 3, we go from the bottom boxes back to the top:
• The letter “K” becomes “H”. • The letter “r” becomes “o”. • The letter “z” becomes “w”. • The letter “g” becomes “d”. • The letter “b” becomes “y”.
You will write a C program that accept a message and the word encrypt , decrypt , or exit ( not case sensitive ) . The program then will do the following :
– Encrypt a message – Enter a key and :
a. Count and display number of capital letters , Small letters , digits and symbols in the message. b. Count number of characters in the message. c. Convert all small characters to capital letters and vice versa the display the message. d. Encrypt the original message e. Concatenate the encrypted message with the original message
– Decrypt a message – Enter a key and :
f. Count and display number of capital letters , Small letters , digits and symbols in the message. g. Count number of characters in the message. h. Convert all small characters to capital letters and vice versa then Display the message. i. Decrypt the original message. j. Concatenate the decrypted message with the original message
– Exit the program. Terminate the program.
Guideline: 1) Must use 2 dimensional array
2) key number must be between 1 and 93. Anything else should be rejected.
3) Must have one function for populating the 2d array and encrypting the message and all other functions.
4) Must have one function for populating the 2d array and perform the decrypting the message and all other functions.
5) Must pass parameters ( message and the key ) to encrypt or decrypt functions .
6) Must have one function to perform the validation ( encrypt , decrypt , exit and the key number ).
7) You may use the functions that are provided by C libraries when writing this program.
8) Your encryption / decryption arrays will contain the following characters and numbers in the that sequence. A – Z followed by
a - z followed by
0 - 9 followed by the following symbols in
! (exclamation mark)
” (Quotation mark)
# (Number sign)
$ (Dollar sign)
% (Percent sign)
& (Ampersand)
‘ (Apostrophe)
( (round brackets or parentheses)
) (round brackets or parentheses)
* (Asterisk)
+ (Plus sign)
, (Comma)
-‐ (Hyphen
. (Full stop, dot)
/ (Slash)
: (Colon)
; (Semicolon)
< (Less-than sign)
= (Equals sign)
> (Greater-than sign; Inequality)
? (Question mark)
@ (At sign)
[ (square brackets or box brackets)
\ (Backslash)
] (square brackets or box brackets)
^ (Caret or circumflex accent)
_ (underscore, understrike, underbar or lowline)
` (Grave accent)
{ (curly brackets or braces)
- (vertical bar, vbar, vertical line or vertical slash)
} (curly brackets or braces)
~ (Tilde; swung dash)
9 ) Must place the following information on top left of the first page of your assignment as comments
