Lab 2 - Secret-Key Encryption

Introduction to Information Security - CS 458 
Lab 2 - Secret-Key Encryption

1 Overview
The learning objective of this lab is for students to get familiar with the concepts in the secret-key encryption.
After finishing the lab, students should be able to gain a first-hand experience on encryption algorithms,
encryption modes, and initial vector (IV). Moreover, students will be able to use tools and write programs
to encrypt/decrypt messages. This lab covers the following topics:
• Secret-key encryption
• Substitution cipher and frequency analysis
• Encryption modes, IV, and paddings
• Common mistakes in using encryption algorithms
• Programming using the crypto library
2 Lab Tasks
2.1 Task 1: Frequency Analysis
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 the original text using frequency analysis. It is known that the original
text is an English article.
Your job is to use the frequency analysis to figure out the encryption key and the original plaintext given a
ciphertext ( ciphertext.txt ). The file ciphertext.txt can be download from the course Blackboard .
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:
• http://www.richkni.co.uk/php/crypta/freq.php : This website can produce the statistics from a ciphertext, including the single-letter frequencies, bigram frequencies (2-letter sequence), and trigram
frequencies (3-letter sequence), etc.
• https://en.wikipedia.org/wiki/Frequency analysis: This Wikipedia page provides frequencies for a typical English plaintext.
• https://en.wikipedia.org/wiki/Bigram: Bigram frequency.
• https://en.wikipedia.org/wiki/Trigram: Trigram frequency.
1Credit: Wenliang Du, Syracuse University
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2.2 Task 2: Encryption using Different Ciphers and Modes
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 00010203040506070809aabbccddeeff \
-iv 0a0b0c0d0e0f010203040506070809
Please replace the ciphertype with a specific cipher type, such as -aes-128-cbc , -aes-128-cfb , -bf-cbc ,
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:
-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)
• To create a plaintext file, e.g., plain.txt , you can the following commands:
$ touch plain.txt
$ gedit plain.txt
• Since we did not cover the Cipher Feedback (CFB) and the Output Feedback (OFB) in class, you need
to read Block cipher mode of operation.
• To observe the encrypted contents of a file, e.g., cipher.bin , in hexadecimal format, use the command
line hex viewing tool xxd .
$ xxd cipher.bin
2.3 Task 3: Encryption Mode – ECB vs. CBC
The file pic original.bmp can be downloaded from course Blackboard, and it 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 54 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 Bless a hex
editor tool (already installed on our VM) to directly modify binary files. We can also use the following
commands to get the header from p1.bmp , the data from p2.bmp (from offset 55 to the end of the
file), and then combine the header and data together into a new file.
$ head -c 54 p1.bmp > header
$ tail -c +55 p2.bmp > body
$ cat header body > new.bmp
2. Display the encrypted picture using a picture viewing program (we have installed an image viewer
program called eog on our VM). Can you derive any useful information about the original picture from
the encrypted picture? Please explain your observations.
2.3.1 Select a picture of your choice, repeat the experiment above, and report your
observations.
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2.4 Task 4 : Padding
For block ciphers, when the size of a plaintext is not a multiple of the block size, padding may be required.
All the block ciphers normally use PKCS#5 padding, which is known as standard block padding. We will
conduct the following experiments to understand how this type of padding works:
1. Use ECB, CBC, CFB, and OFB modes to encrypt a file (you can pick any cipher). Please report which
modes have paddings and which ones do not. For those that do not need paddings, please explain why.
2. Let us create three files, which contain 5 bytes, 10 bytes, and 16 bytes, respectively. We can use the
following echo -n command to create such files. The following example creates a file f1.txt with
length 5 (without the -n option, the length will be 6, because a newline character will be added by
echo ):
$ echo -n "12345" > f1.txt
We then use openssl enc -aes-128-cbc -e to encrypt these three files using 128-bit AES with
CBC mode. Please describe the size of the encrypted files.
We would like to see what is added to the padding during the encryption. To achieve this goal, we
will decrypt these files using openssl enc -aes-128-cbc -d . Unfortunately, decryption by default
will automatically remove the padding, making it impossible for us to see the padding. However, the
command does have an option called -nopad , which disables the padding, i.e., during the decryption,
the command will not remove the padded data. Therefore, by looking at the decrypted data, we can
see what data are used in the padding. Please use this technique to figure out what paddings are added
to the three files.
It should be noted that padding data may not be printable, so you need to use a hex tool to display
the content. The following example shows how to display a file in the hex format:
$ hexdump -C p1.txt
00000000 31 32 33 34 35 36 37 38 39 49 4a 4b 4c 0a. |123456789IJKL.|
$ xxd p1.txt
00000000: 3132 3334 3536 3738 3949 4a4b 4c0a. 123456789IJKL.
2.5 Task 5: Error Propagation – Corrupted Cipher Text
To understand the error propagation property of various encryption modes, we would like to do the following
exercise:
1. Create a text file that is at least 1000 bytes long.
2. Encrypt the file using the AES-128 cipher.
3. Unfortunately, a single bit of the 55th byte in the encrypted file got corrupted. You can achieve this
corruption using the bless hex editor.
4. Decrypt the corrupted ciphertext file using the correct key and IV.
Please answer the following question: 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.
Please provide justification.
2.6 Task 6: Initial Vector (IV)
Most of the encryption modes require an initial vector (IV). Properties of an IV depend on the cryptographic
scheme used. If we are not careful in selecting IVs, the data encrypted by us may not be secure at all, even
though we are using a secure encryption algorithm and mode. The objective of this task is to help students
understand the problems if an IV is not selected properly. Please do the following experiments:
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2.6.1 Task 6.1. Uniqueness of the IV
A basic requirement for IV is uniqueness, which means that no IV may be reused under the same key. To
understand why, please encrypt the same plaintext using (1) two different IVs, and (2) the same IV. Please
describe your observation, based on which, explain why IV needs to be unique.
2.6.2 Task 6.2. Common Mistake: Use the Same IV
One may argue that if the plaintext does not repeat, using the same IV is safe. Let us look at the Output
Feedback (OFB) mode. Assume that the attacker gets hold of a plaintext ( P1 ) and a ciphertext ( C1 ),
can he/she decrypt other encrypted messages if the IV is always the same? You are given the following
information, please try to figure out the actual content of P2 based on C2 , P1 , and C1 .
Plaintext (P1): This is a known message!
Ciphertext (C1): a469b1c502c1cab966965e50425438e1bb1b5f9037a4c15913
Plaintext (P2): (unknown to you)
Ciphertext (C2): bf73bcd3509299d566c35b5d450337e1bb175f903fafc15913
If we replace OFB in this experiment with CFB (Cipher Feedback), how much of P2 can be revealed? You
only need to answer the question; there is no need to demonstrate that.
The attack used in this experiment is called the known-plaintext attack , which is an attack model for
cryptanalysis where the attacker has access to both the plaintext and its encrypted version (ciphertext). If
this can lead to the revealing of further secret information, the encryption scheme is not considered as secure.
2.6.3 Task 6.3. Common Mistake: Use a Predictable IV
From the previous tasks, we now know that IVs cannot repeat. Another important requirement on IV is
that IVs need to be unpredictable for many schemes, i.e., IVs need to be randomly generated. In this task,
we will see what is going to happen if IVs are predictable.
Assume that Bob just sent out an encrypted message, and Eve knows that its content is either Yes or No ;
Eve can see the ciphertext and the IV used to encrypt the message, but since the encryption algorithm AES
is quite strong, Eve has no idea what the actual content is. However, since Bob uses predictable IVs, Eve
knows exactly what IV Bob is going to use next. The following summarizes what Bob and Eve know:
Encryption method: 128-bit AES with CBC mode.
Key (in hex): 00112233445566778899aabbccddeeff (known only to Bob)
Ciphertext (C1): bef65565572ccee2a9f9553154ed9498 (known to both)
IV used on P1 (known to both)
(in ascii): 1234567890123456
(in hex) : 31323334353637383930313233343536
Next IV (known to both)
(in ascii): 1234567890123457
(in hex) : 31323334353637383930313233343537
A good cipher should not only tolerate the known-plaintext attack described previously, it should also
tolerate the chosen-plaintext attack , which is an attack model for cryptanalysis where the attacker can
obtain the ciphertext for an arbitrary plaintext. Since AES is a strong cipher that can tolerate the chosenplaintext attack, Bob does not mind encrypting any plaintext given by Eve; he does use a different IV for
each plaintext, but unfortunately, the IVs he generates are not random, and they can always be predictable.
Your job is to construct a message P2 and ask Bob to encrypt it and give you the ciphertext. Your objective
is to use this opportunity to figure out whether the actual content of P1 is Yes or No .
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2.7 Additional Readings
There are more advanced cryptanalysis on IV that is beyond the scope of this lab. Students can read the
article posted in this URL: https://defuse.ca/cbcmodeiv.htm. Because the requirements on IV really depend
on cryptographic schemes, it is hard to remember what properties should be maintained when we select an IV.
However, we will be safe if we always use a new IV for each encryption, and the new IV needs to be generated
using a good pseudo random number generator, so it is unpredictable by adversaries. Students can read this
Wikipedia page for ideas: Initialization vector (https://en.wikipedia.org/wiki/Initialization vector).
2.8 Task 7: Programming using the Crypto Library - Extra Credit 5%
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/decrypt messages in programs.
In this task, you are given a plaintext and a ciphertext, and your job is to find the key that is used for the
encryption. You do know the following facts:
• The aes-128-cbc cipher is used for the encryption.
• The key used to encrypt this plaintext is an English word shorter than 16 characters; the word can
be found from a typical English dictionary. Since the word has less than 16 characters (i.e. 128 bits),
pound signs (#: hexadecimal value is 0x23 ) 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 the encryption key. You can download an English word list from
the Internet or form course Blackboard ( words.txt ). The plaintext, ciphertext, and IV are listed in the
following:
Plaintext (total 21 characters): This is a secret tool
Ciphertext (in hex format): ece6753e938f8f903cabbbe12d395bf5f7eae38ad918a2d3e1c3a832476d5c7a
IV (in hex format): 010203040506070809000a0b0c0d0e0f
You need to pay attention to the following issues:
• If you choose to store the plaintext message in a file, and feed the file to your program, you need to
check whether the file length is 21. If you type the message in a text editor, you need to be aware that
some editors may add a special character to the end of the file. The easiest way to store the message
in a file is to use the following command (the -n flag tells echo not to add a trailing newline):
$ echo -n "This is a secret tool" > file
• 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. Sample code can be found from
the following URL: OpenSSL: EVP Encrypt Int
• When you compile your code using gcc , do not forget to include the -lcrypto flag, because your
code needs the crypto library. See the following example:
$ gcc -o myenc myenc.c -lcrypto
3 DES/AES explained as a Flash animation
• DES: https://www.youtube.com/watch?v=Vcld7CMAnNs
• AES: https://www.youtube.com/watch?v=gP4PqVGudtg
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4 Submission
You need to submit a detailed lab report, 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.
Your report must be a .pdf file that is uploaded to Blackboard for Lab 2 .
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