Description

Assignment 03A: Encryption Algorithms
Cryptography or cryptology; from Greek κρυπτός kryptós, “hidden, secret”; and γράφειν graphein, “writing”, or ­λογία­logia, “study”, respectively is the practice and study of techniques for secure communication in the presence of third parties.
Wikipedia
In this assignment, you are going to be working with a set of encryption algorithms. The problems are independent, but they need to be solve in the order indicated.
You are going to be working with three different types of encryption algorithms, I’m presenting the first one in this document, once you have written the first one, you will use that to decrypt a second document that contains the information about the second encryption algorithm. With the second algorithm and a new giving key, you will decrypt the third and last problem.
Marking scheme:
Encryption Algorithm Marks Required
Basic Encryption 3% YES
Second algorithm 3% NO
Third algorithm 2% NO

This assignment is worth 6% of your final mark. If you decide to write just the first part, you will get 50% in the assignment, but I will consider it as a successful submission. If you decide to go ahead and write the second part, you will be marked with a 100%, and if you decide to go ahead and do the third part, that will be extra marks.
Basic Encryption Algonquin
In this encryption algorithm, each byte of plaintext (unencrypted) data has been adjusted arithmetically by adding an “encryption key” value to the byte. An 8-bit byte of data can have values that range from 0-255 (expressing the 256 8-bit patterns from 00000000 to 11111111).
To produce the ciphertext, the encryption key value is added to each plaintext byte. Where the resulting sum exceeds the maximum value allowed in a byte (the sum is greater than 255), the newly calculated number will be “wrapped around” to the start of the range of available numbers.
The following chart shows a sample of several plaintext data byte values going through the conversion to ciphertext using an example encryption key of 5. For example, the plaintext letter ‘A’ (byte value 65) has the encryption key 5 added to it to become the ciphertext letter ‘F’ (byte value 70).

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You are to write a small C program cipher, to encrypt / decrypt a file. The program should have the following functionality:
cipher [ OPTIONS ] SOURCE DESTINATION
OPTIONS:
­d KEY
decrypt the file SOURCE using KEY and writes back into DESTINATION
­e KEY
encrypt the file SOURCE using KEY and writes back into DESTINATION ­h: help in using the command
Option d and e are exclusive, if d is on, e can not be on, and both of them require an argument, the encryption KEY
In case that the user does not give all the required command arguments, you program should print a usage message and exit with EXIT_FAILURE.
Assignment Progression
Now that you have finished your first algorithm, use your cipher and the KEY 8234, to decrypt the file mystery01 file that you’ll find in BB.

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