Assignment 2: a file encrypt or
Contents
Subset 0: File and directory commands
Subset 1: XOR encryption
Subset 2: Directory traversal
Subset 3: ECB encryption
Subset 4: Cipher block chaining encryption
Assumptions and Clarifications
Change Log
Aims
to improve your understanding of filesystem objects
to give you experience writing C code to manipulate binary files
to further experience practical uses of bitwise operations
to give you experience writing a relevant low-level data manipulation program in C
Introduction
Your task in this assignment is to write tide, a terribly insecure single-file encryption/decryption tool. Throughout this assignment, you will explore some basic filesystem operations, as well as implement several rudimentary encryption algorithms.
Encryption is the process of converting information into an obscured format, which can (in theory), only be converted back into useful
information by an authorized party who knows the encryption process and key. Encryption is an incredibly useful tool, and is the reason why the internet can function in the way it does, with sensitive information freely transmitted across it.
File encryption is particularly useful to safeguard data in the case that it is stolen. Encrypting your files could prevent someone from being able to access your photos in the event that your laptop gets stolen.
In this assignment, you will implement three different algorithms for file encryption: XOR (eXclusive OR), ECB (Electronic Code Book) and CBC (Cipher Block Chaining). Each of these algorithms function slightly differently, but all work towards the same purpose of obscuring information, that can only be correctly interpreted by an authorised party.
XOR encryption works by employing the bitwise XOR operation on every bit of some given data. A key, which when broken up into its constituent bits, is expanded to match the length of the data being encrypted. The XOR operation is then employed between these two
bitstreams to yield the encrypted data. This encrypted data can be decrypted only by re-running the same XOR operation with the same key. In tide, standalone XOR encryption will only employ the the single-byte key 0xA9 .
ECB encryption works by bit-shifting data by the amount specified by some key (a password). Each character in a 'block' of the input data is
shifted by the value of the character in the corresponding position within the password. The encrypted data can be decrypted only by shifting it back by the value of the corresponding position within the password. In tide, passwords will be a fixed length of 16 characters.
CBC encryption is different from the above two algorithms as each block of the encrypted data contributes to the encryption of the next block.
We will combine both XOR encryption and ECB encryption to develop an encryption algorithm where it is significantly harder for an unauthorised party to read our encrypted data by guessing our password.
However, before all of this, tide needs to be able to function as a basic standalone program. As such, we will implement several filesystem
manipulation operations. You will also implement two different methods of searching for files, which will make the user's life easier in finding what they might need to encrypt.
Getting Started
Create a new directory for this assignment called tide , change to this directory, and fetch the provided code by running these commands:
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$ mkdir -m 700 tide $ cd tide $ 1521 fetch tide |
If you're not working at CSE, you can download the provided files as a zip file or a tar file.
This will get you tide.c , which contains code to start the assignment. As provided, it will compile and run, but lacks any real functionality:
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$ make dcc -Wall -Werror main.c tide.c -o tide $ ./tide Welcome to tide! To see what commands are available, type help.
search-from-file source-file (sf) Searches for a file by its content for the provided bytes, supplied from a file encrypt-cbc filename (ec) Encrypts a file with CBC decrypt-cbc filename (dc) Decrypts a file with CBC quit (q) Quits the program tide> q Thanks for using tide. Have a nice day! |
However, tide.c also contains some provided functions to make your task easier. For example, the sort_strings function will sort an array of strings into alphabetical order in-place. You should read through the provided code in this file before you begin work on this assignment.
You may also find the provided constants, data types and function signatures in tide.h to be useful.
Reference implementation
We've written a solution to the assignment, compiled it and made it available to you as a reference implementation
A reference implementation is a common, efficient, and effective method to provide or define an operational specification; and it's something you will likely work with after you leave UNSW.
You can run the reference implementation as: 1521 tide , and see the correct output and behaviour for any input: