Homework 2 – Symmetric Key Encryption


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CSCE 465 Computer and Network Security 1
Homework 2 – Symmetric Key Encryption
1 Overview
The learning objective of this homework is for students to get familiar with the concepts and principles in
the secret-key encryption. After finishing the homework, students should be able to understand better and
gain a first-hand experience on encryption algorithms and encryption modes. Moreover, students will be
able to use tools and write programs to encrypt/decrypt messages.
2 Lab and Programming Tasks [100 pts]
There are a total of 5 lab/programming tasks.
Lab Environment
Please note most mentioned software has already been installed in the provided pre-built Ubuntu VM.
Installing OpenSSL. In this lab, we will use openssl commands and libraries.
If your VM does not have it, you should first install openssl package using the following command:
% sudo apt-get install openssl
It should be noted that the above command only install the openssl binaries. If you want to use
openssl libraries in your programs, you need to install several other things for the programming environment, including the header files, libraries, manuals, etc. You can use the following command to do this:
% apt-get source openssl
Untar the tar ball, and run the following commands.
You should read the INSTALL file first:
% ./config
% make
% make test
% sudo make install
Installing GHex. In this lab, we need to be able to view and modify files of binary format.GHex is a hex
editor for GNOME, it allows the user to load data from any file, view and edit it in either hex or ascii.
You can use the following command to install it (it should be noted that the name of the command is
called /usr/bin/ghex2 at the time of writing). You can call ghex2 to use it.
% sudo apt-get install ghex.
CSCE 465 Computer and Network Security 2
Fixing Apt-get You may encounter problems with apt-get, which is because the resource repository is
outdated. You can fix it as follows:
1. % sudo vi /etc/apt/sources.list
(and please replace all “” links to
2. % sudo apt-get update
2.1 Task 1: Frequency Analysis [10 pts]
It is well-known that monoalphabetic substitution cipher (also known as monoalphabetic cipher) is not
secure, because it can be subjected to frequency analysis. In this lab, you are given a cipher-text that is
encrypted using a monoalphabetic cipher; namely, each letter in the original text is replaced by another
letter, where the replacement does not vary (i.e., a letter is always replaced by the same letter during the
encryption). Your job is to find out original text of file ciphertext.txt using frequency analysis. It is
known that the original text is an English article.
Guidelines. Using the frequency analysis, you can find out the plaintext for some of the characters quite
easily. For those characters, you may want to change them back to its plaintext, as you may be able to get
more clues. It is better to use capital letters for plaintext, so for the same letter, we know which is plaintext
and which is ciphertext. You can use the tr command to do this. For example, in the following, we replace
letters a, e, and t in in.txt with letters X, G, E, respectively; the results are saved in out.txt.
$ tr ’aet’ ’XGE’ < in.txt > out.txt
There are many online resources that you can use. We list four useful links in the following:
• : This Wikipedia page provides frequencies for a typical English plaintext.
• : Bigram frequency.
• : Trigram frequency.
Please submit the key and the plaintext recovered, describe the method you used and submit the code.
2.2 Task 2: Encryption using different ciphers and modes [10 pts]
In this task, we will play with various encryption algorithms and modes. You can use the following
openssl enc command to encrypt/decrypt a file. To see the manuals, you can type man openssl
and man enc.
% openssl enc ciphertype -e -in plain.txt -out cipher.bin \
-K 00112233445566778889aabbccddeeff \
-iv 0102030405060708
Please replace the ciphertype with a specific cipher type, such as -aes-128-cbc, -aes-128-cfb,
-des, etc. In this task, you should try at least 3 different ciphers and three different modes. You can find
the meaning of the command-line options and all the supported cipher types by typing “man enc”. We
include some common options for the openssl enc command in the following:
CSCE 465 Computer and Network Security 3
-in <file> input file
-out <file> output file
-e encrypt
-d decrypt
-K/-iv key/iv in hex is the next argument
-[pP] print the iv/key (then exit if -P)
Please submit the message, ciphertext and the command used in each mode of encryption.
2.3 Task 3: Encryption Mode – ECB vs. CBC [20 pts]
The file pic original.bmp contains a simple picture. We would like to encrypt this picture, so people
without the encryption keys cannot know what is in the picture. Please encrypt the file using the ECB
(Electronic Code Book) and CBC (Cipher Block Chaining) modes, and then do the following:
1. Let us treat the encrypted picture as a picture, and use a picture viewing software to display it. However, For the .bmp file, the first 0x36 bytes contain the header information about the picture, we have
to set it correctly, so the encrypted file can be treated as a legitimate .bmp file. We will replace the
header of the encrypted picture with that of the original picture. You can use the ghex tool to directly
modify binary files.
2. Display the encrypted picture using any picture viewing software. Can you derive any useful information about the original picture from the encrypted picture?
Select a picture of your choice and repeat the experiment above.
Please explain your steps, observations and the encrypted images.
2.4 Task 4: Encryption Mode – Corrupted Cipher Text [20 pts]
To understand the properties of various encryption modes, we would like to do the following exercise:
1. Create a text file that is at least 64 bytes long.
2. Encrypt the file using the AES-128 cipher.
3. Unfortunately, a single bit of the 30th byte in the encrypted file got corrupted. You can achieve this
corruption using ghex.
4. Decrypt the corrupted file (encrypted) using the correct key and IV.
Please answer the following questions: (1) How much information can you recover by decrypting the
corrupted file, if the encryption mode is ECB, CBC, CFB, or OFB, respectively? Please answer this question
before you conduct this task, and then find out whether your answer is correct or wrong after you finish this
task. (2) Please explain why. (3) What are the implications of these differences?
CSCE 465 Computer and Network Security 4
2.5 Task 5: Programming using the Crypto Library [20 pts]
So far, we have learned how to use the tools provided by openssl to encrypt and decrypt messages. In
this task, we will learn how to use openssl’s crypto library to encrypt/descrypt messages in programs.
OpenSSL provides an API called EVP, which is a high-level interface to cryptographic functions. Although OpenSSL also has direct interfaces for each individual encryption algorithm, the EVP library provides a common interface for various encryption algorithms. To ask EVP to use a specific algorithm, we
simply need to pass our choice to the EVP interface. A sample code is given in http://www.openssl.
org/docs/crypto/EVP_EncryptInit.html. Please get yourself familiar with this program, and
then do the following exercise.
You are given a plaintext and a ciphertext, and you know that aes-128-cbc is used to generate the
ciphertext from the plaintext, and you also know that the numbers in the IV are all zeros (not the ASCII
character ‘0’). Another clue that you have learned is that the key used to encrypt this plaintext is an English
word shorter than 16 characters; the word that can be found from a typical English dictionary. Since the
word has less than 16 characters (i.e. 128 bits), space characters (hexadecimal value 0x20) are appended to
the end of the word to form a key of 128 bits. Your goal is to write a program to find out this key. You can
download a English word list from the Internet. We have also provided one in words.txt. The plaintext
and ciphertext is in the following:
Plaintext (total 21 characters): This is a top secret.
Ciphertext (in hex format): 8d20e5056a8d24d0462ce74e4904c1b5
Note 1: If you choose to store the plaintex message in a file, and feed the file to your program, you need
to check whether the file length is 21. Some editors may add a special character to the end of the file. If that
happens, you can use the ghex tool to remove the special character.
Note 2: In this task, you are supposed to write your own program to invoke the crypto library. No credit
will be given if you simply use the openssl commands to do this task.
Note 3: To compile your code, you may need to include the header files in openssl, and link to
openssl libraries. To do that, you need to tell your compiler where those files are. In your Makefile,
you may want to specify the following:
gcc -I$(INC) -L$(LIB) -o enc yourcode.c -lcrypto
Please submit the key and your code to recover it.
3 Submission
You need to submit a detailed lab report on Gradescope, with
screenshots, to describe what you have done and what you have observed. You also need to provide explanation to the observations that are interesting or surprising. Please also list the important code snippets
followed by explanation. Simply attaching code without any explanation will not receive credits.
CSCE 465 Computer and Network Security 5
Copyright c 2006 – 2009 Wenliang Du, Syracuse University.
The development of this document is funded by the National Science Foundation’s Course, Curriculum, and
Laboratory Improvement (CCLI) program under Award No. 0618680 and 0231122. Permission is granted
to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License,
Version 1.2 or any later version published by the Free Software Foundation. A copy of the license can be
found at

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