Lab Exercise 6 – Cryptography Solution

Name:

1. Overview

This lab exercise will provide some hands-on experience with symmetric and asymmetric encryption using command-line tools in Linux.

2. Resources required

This exercise requires Kali Linux VM running in the Virginia Cyber Range.  Please log in at https://console.virginiacyberrange.net/.

3. Initial Setup

From your Virginia Cyber Range course, select the Cyber Basics environment.  Click “start” to start your environment and “join” to get to your Linux desktop.

4. Tasks

Task 1: Symmetric Encryption with mcrypt

Mcrypt is a symmetric file and stream encryption utility for Linux and Unix that replaces the weaker crypt utility.  Mcrypt can be used to encrypt files using several different symmetric encryption algorithms.  By default it uses the Rijndael cipher, which is the algorithm on which the Advanced Encryption Standard (AES) is based. 

Mcrypt is not installed by default on your virtual machine.  Open a terminal and use the Linux package manager to install this software at the command line as follows (the second command may take a few minutes):

    $ sudo apt-get update

$ sudo apt-get install mcrypt

Although we will be using mcrypt in default mode, it is very powerful and full-featured.  To see all of the command-line options available to mcrypt, use the following command:

$ mcrypt --help

Mcrypt provides a variety of symmetric encryption techniques (you would use the -m option at the command line to access these).  For a list of the various symmetric encryption modes available to mcrypt, use the following command:

$ mcrypt --list

Next we need a file to encrypt.  You can download a text file from the Virginia Cyber Range using the command below, or you can create a text file using a text editor (mousepad) on your Linux virtual machine and save it in your home directory. 

    $ wget artifacts.virginiacyberrange.net/gencyber/textfile1.txt

You can examine the contents of the file using the Linux cat command.

Question 1:  CUT AND PASTE THE CONTENTS OF THE FILE HERE: (.5 point)




Use mcrypt to encrypt your textfile.  Mcrypt will ask for an encryption key – you can simply type a passphrase at the command line (you will use the same passphrase to decrypt the file so make sure to remember it).  Be sure that you are in the same directory location as your text file and encrypt it as follows.

    $ mcrypt textfile1.txt

If you list your directory you should see textfile1.txt.nc – the encrypted version of the file replaced the plaintext version.  Use the cat command to view the file.  It should be unintelligible.


Question 2:  CUT AND PASTE THE CONTENTS OF THE FILE HERE: (.5 point)




You could now send this file to someone else and as long as they have the passphrase, they can decrypt and read it.  Now you can safely delete textfile1.txt (as long as you remember your passphrase so you can decrypt textfile1.txt.nc)!

    $ rm textfile1.txt

Use mcrypt with the –d switch to decrypt your file.  Be sure to use the same passphrase as in step 3, above.

    $ mcrypt –d textfile1.txt.nc

Your unencrypted file should be restored to textfile1.txt (use cat to be sure).





Task 2: Asymmetric Encryption using Gnu Privacy Guard (gpg)

Asymmetric encryption using Gnu Privacy Guard (gpg), an open-source implementation of Pretty-Good Privacy (pgp).  Gpg is included in your Kali Linux VM so we don’t need to install anything.  Below we will take basic steps to create a public/private key pair, then encrypt a file using our own public key and decrypt it using our own private key.  There are lots more features and options, however.  Review the man page for the gpg utility for more details.

First we have to create an encryption key

    $ gpg –-gen-key

You should be prompted for:
    • Your name
    • Your email address (and remember what you entered!).

If everything looks ok you can select O for Okay when prompted.

You will next be prompted for a password to protect the key.  Remember this password!

Now you must generate entropy by using the keyboard, moving the mouse, etc. until sufficient entropy is available to create your key.  This entropy is needed in the generation of random numbers as part of the key creation process.  This can take several minutes in a virtual machine.

Once complete, you should get output listing a public key fingerprint and some other data.

Question 3:  CUT AND PASTE THE OUTPUT HERE: (.5 point)




Download (or create) a second textfile. 

    $ wget artifacts.virginiacyberrange.net/gencyber/textfile2.txt

Use cat to examine the file.

Question 4:  CUT AND PASTE THE CONTENTS OF THE FILE HERE: (.5 point)




Now we’ll encrypt the file using our public key.

    $ gpg –e –r your-email-address textfile2.txt

A new file will be added called textfile2.txt.gpg.  Use cat to examine the file.  It should be unreadable.

Question 5:  CUT AND PASTE THE CONTENTS OF THE FILE HERE: (.5 point)




Next, delete the old file and use gpg to decrypt the file using your private key.

    $ rm textfile2.txt
    $ gpg -d textfile2.txt.gpg

Enter the password that you created back in step 1.  Your unencrypted file should be displayed!

Now that you know that your key works for encryption and decryption, you can share your public key with others so that they can encrypt files to be decrypted with your private key.  Use the following syntax to export your key to a text file.

    $ gpg --export -a your-email-address > public.key

Examine the key using cat.  The -a flag has the key encoded in ascii (text).  Some people append a text version of their public key to their email signatures, making is easy for others to use to encrypt files and send to them. 

Question 6:  CUT AND PASTE THE CONTENTS OF THE FILE HERE: (.5 point)




From here, you could share your public key with others at a key-signing party, upload it to a key server, or otherwise make it available for others to use to encrypt documents that only you can decrypt. 


Task 3: RSA Encryption/Decryption*

Let’s take another look at asymmetric encryption to perform RSA and generate keys to encrypt and decrypt a message.  Pay particular attention to the output of the variables of the algorithm because you will be implementing them in your next programming assignment!

Let’s use the openssl library to generate a public/private key pair and encrypt a file.  To begin, generate a pair of public and private keys by running the following command:
openssl genrsa -out pub_priv_pair.key 1024 
The genrsa flag lets openssl know that you want to generate an RSA key, the -out flag specifies the name of the output file, and the value 1024 repre­sents the length of the key.  Longer keys are more secure.  Remember: don’t share your private key with anyone.  You can view the key pair you generated by running the following command: 
openssl rsa -text -in pub_priv_pair.key 
The rsa flag tells openssl to interpret the key as an RSA key and the -text flag displays the key in human­readable format.
Question 7:  CUT AND PASTE YOUR OUTPUT HERE.  Label the areas of the output that correspond to the RSA algorithm components (p, q, n, integer e, d, PR) (1 point) **Note: if you plan to use your public/private key pair in real life, please obfuscate your private key in the cut and paste.



You can extract the public key from this file by running the following command: 
openssl rsa -in pub_priv_pair.key -pubout -out public_key.key 
The -pubout flag tells openssl to extract the public key from the file. You can view the public key by running the following command, in which the -pubin flag instructs openssl to treat the input as a public key: 
openssl rsa -text -pubin -in public_key.key 

Question 8:  CUT AND PASTE YOUR OUTPUT HERE. Label the areas of the output that correspond to the RSA algorithm components (n, integer e, PU) (1 point)




Next, let’s create a text file to encrypt:
echo "Cryptography is fun!" > plain.txt 
Next, use the RSA utility rsautl to create an encrypt plain.txt to and encrypted binary file cipher.bin using your public key:
openssl rsautl -encrypt -pubin -inkey public_key.key -in plain.txt -out cipher.bin -oaep 
Notice that we included the -oaep flag. Secure implementations of RSA must also use the OAEP algorithm. Whenever you’re encrypting and decrypting files using openssl, be sure to apply this flag to make the operations secure.
Next, decrypt the binary using the following command: 
openssl rsautl -decrypt -inkey pub_priv_pair.key -in cipher.bin -out plainD.txt -oaep 
Lastly, you can view the decrypted message plainD.txt using the cat command and you should see your original message.

Task 4: Other Encryption/Decryption

Question 9:  Decrypt the following Caesar Cipher:  psvclaolcpynpuphjfilyyhunl (1 point)




Question 10:  Generate the MD5 hash of the following sentence: I love hash browns for breakfast. (Do not include the period when generating the MD5). (1 point)




By submitting this assignment you are digitally signing the honor code, “I pledge that I have neither given nor received help on this assignment”.


END OF EXERCISE



References

Mcrypt: http://mcrypt.sourceforge.net/
Gpg: https://gnupg.org/
Openssl: https://www.openssl.org/

*Openssl task credit to Ethical Hacking by Daniel Graham