Lab05. Practical cryptography

Important read to be graded!

Use OpenStack CDCI template to start a new VM. To access the VM, login to using your UPB credentials, and from there ssh into the private IP from OpenStack using “ubuntu” as a username and your ssh key.

Due to the fact that we have to work remote, please make sure that you record your screen while working. Here is how.

# start the recording after you ssh into the machine
ubuntu@cdci-v2:~/cdci/labs/lab03$ asciinema rec lab03_mihai.cast
# !!!IMPORTANT before you start working echo your name in the terminal!!!
ubuntu@cdci-v2:~/cdci/labs/lab03$ echo "Mihai Chiroiu's terminal!"
# ============ IMPORTANT ============
# do the tasks: enter docker
ubuntu@cdci-v2:~/cdci/labs/lab03$ ./ 
root@attacker:/# pwd
root@attacker:/# exit
# exited docker
# stop recording 
ubuntu@cdci-v2:~/cdci/labs/lab03$ exit
asciinema: recording finished
asciinema: press <enter> to upload to, <ctrl-c> to save locally

View the recording at:

# the IP of server has changed, sorry
ubuntu@cdci-v2:~/cdci/labs/lab03$ sudo echo "" > /etc/hosts
# upload the recording
ubuntu@cdci-v2:~/cdci/labs/lab03$ asciinema upload lab03_mihai.cast

When you finish your work, submit the details on the form . Double check to see if all is good (


  • Hashing data to provide integrity
  • Use openssl for symmetric and asymmetric encryption
  • Convert to base64 an encrypted message



01. [5p] Virtual machine setup

First, make sure that your virtual machine is updated (run the provided script, or create one).

root@cdci:/# cat
# (c) Mihai Chiroiu - CDCI

git clone

Next, in one terminal start the provided Mininet topology.

root@cdci:/# cd cdci/lab05
root@cdci:/# /usr/bin/python3

If there are any problems with starting the topology (if all is good you should see the Mininet prompt ”>”) use the given cleanup script and try to restart the topology.

02. [10p] Internet connectivity

Before you begin, make sure that you have Internet connectivity on all two nodes. R1 should be the gateway for the all of them. Write down the IP addresses of all the nodes (including the gateway). Use the provided scripts to access the nodes.

Download the following archive and extract it both nodes (Hint: wget is installed). . Download the archive also on your local computer and inspect the images.

root@ip-172-30-0-165:/# ./ 

root@ip-172-30-0-165:/# ./ 
root@h2:~# wget
root@h2:~# unzip 
root@h2:~# pwd  

03. [10p] Hashing

  1. Use the MD5 hashing function and compare the files from the archive extracted previously. How do the hashes differ? What about the files, are they the same? Now, use other hashing functions and compare the result.
  2. Download the latest version of the Firefox browser for Windows and compare the hash value to the official one.
root@h1:~/upb-vs-harvard# md5sum upb.bmp 
2cbfabbbfb2adf0fea6ff8fbbaeaabae  upb.bmp
root@h1:~/upb-vs-harvard# md5sum harvard.bmp 
2cbfabbbfb2adf0fea6ff8fbbaeaabae  harvard.bmp  

root@h1:~/#  wget
root@h1:~/#  sha256sum Firefox\ Setup\ 65.0b9.exe

04. [10p] Symmetric encryption

The openssl tool provides different mechanisms to encrypt data using symmetric cyphers. List all the available cyphers and modes of operation. Encrypt the ‘This is a cool lab’ text using aes-256-cbc mode and the password ‘thisisasupersecretpassword’. The output of the previous command is a not human-readable and cannot be easily used, add the ‘-base64’ parameter and verify the output.

  • Copy and decrypt the message on the other node.
root@h1:~# openssl list -cipher-commands
aes-128-cbc       aes-128-ecb       aes-192-cbc       aes-192-ecb       
root@h1:~# echo "This is a cool lab" | openssl aes-256-cbc -k thisisasupersecretpassword
root@h1:~# echo "This is a cool lab" | openssl aes-256-cbc -k thisisasupersecretpassword -md sha512 -p -base64 -pbkdf2
iv =9B51ACC36B704923244B65190C3A1A0B

root@h2:~# echo "U2FsdGVkX18fZ4BRp9AMb3QeOQYXFoR0lFCO7tXYAXiQ/tJmwAsbOM8yYjmWD6ku" | openssl aes-256-cbc -k thisisasupersecretpassword -md sha512 -base64 -d -pbkdf2
This is a cool lab

05. [15p] ECB-mode encryption

In this exercise we will show how using a wrong encryption mode can break the encryption scheme. Let us try to encrypt the upb.bmp image using the AES-256-ECB mode.

  • Encrypt the image file using AES-256-ECB.
  • Recover the image-related metadata / header . This is required if we want the picture to be displayed properly. Note that the header is 54 bytes ( You can use the “dd” tool for this purpose.
  • Last, transfer the encrypted image to your file (from the node to the virtual machine, and from the virtual machine to your computer).

You can transfer file from the nodes (docker containers) to the host (virtual machine) using scp.

root@h1:~/# openssl aes-256-ecb -k thisisasupersecretpassword -in upb.bmp -out upb_logo_enc.bmp -nosalt -pbkdf2
root@h1:~/# dd if=upb.bmp of=upb_logo_enc.bmp bs=6 count=9 conv=notrunc
9+0 records in
9+0 records out
54 bytes copied, 5.6056e-05 s, 963 kB/s
root@h1:~/# scp upb_logo_enc.bmp ubuntu@
ubuntu@'s password: 
upb_logo_enc.bmp                                                     100%  733KB 381.8KB/s   00:01  

06. [10p] Digital signing

Symmetric encryption is typically used for encrypting data, hashing is used for data integrity and asymmetric encryption is used for authentication. We can use ‘openssl dgst’ tool for signing files using DSA.

  • Create a DSA key pair and use it to sign the image file encrypted before. Use SHA256 for hashing the file.
  • Send the signed data to the other node and verify the signature. How could you distribute the public part of the DSA key?

For file transfer between the two nodes you can use scp via the virtual machine. You can also start the ssh daemon on the nodes if you want.

root@h1:~/# openssl genpkey -genparam -algorithm DSA -pkeyopt dsa_paramgen_bits:2048 -pkeyopt dsa_paramgen_q_bits:256 -pkeyopt dsa_paramgen_md:sha256 -out dsaparams.pem
root@h1:~/# openssl genpkey -paramfile dsaparams.pem -out dsaprivkey.pem
root@h1:~/# openssl dsa -in dsaprivkey.pem -pubout > dsapubkey.pem
root@h1:~/# openssl dgst -sha256 -sign dsaprivkey.pem upb_logo_enc.bmp > upb.sig
root@h1:~/# scp upb_logo_enc.bmp upb.sig dsapubkey.pem ubuntu@

root@h2:~# scp ubuntu@ ubuntu@ ubuntu@ .
root@h2:~# ls
dsapubkey.pem  upb.sig  upb_logo_enc.bmp
root@h2:~# openssl dgst -sha256 -verify dsapubkey.pem -signature upb.sig upb_logo_enc.bmp
Verified OK

07. [10p] Certificate Signing Request

Asymmetric encryption schemes are used in certificates to authenticate and encrypt data in transit. In this exercise we are going to create a CSR (Certificate Signing Request), which includes the public key of your server. Note that this CSR must be signed by a Certificate Authority before being used.

  • Generate a 2048 bits private-public RSA key. Note, that while in practice the private key should be protected using a symmetric key, in this lab we assume that you don’t. Display the public key part of the generated pair. How about the private one?
  • Use the previously generated key and create a new CSR that should be saved to the ‘ServerCertificateRequest.csr’ file.
  • Print the public key stored in the CSR certificate and compare it with the one generated in the first step.
root@h1:~# openssl genrsa -out RSAKEYPAIR.pem 2048
root@h1:~# openssl rsa -in RSAKEYPAIR.pem -pubout
root@h1:~# openssl rsa -in RSAKEYPAIR.pem -text
root@h1:~# openssl req -out ServerCertificateRequest.csr -new -key RSAKEYPAIR.pem
root@h1:~# openssl req -in ServerCertificateRequest.csr -pubkey

08. [10p] Digital signing

In this exercise you will be required to analyze an already signed certificate from the website.

  • Use the s_client suite from openssl and download the top chain certificate locally (Hint: signed by GlobalSign Root CA). Note that there might be a chain of certificates, save each one in a different file.
  • What is the public key of the certificate? Compare it to the one viewed in browser (use Firefox for a simplified view).
  • Use ‘openssl verify’ to test the correctness of the certificate. Does the verification of the certificate work?
root@h1:/# openssl s_client -host -port 443 -prexit -showcerts
root@h1:/# cat 

root@h1:/# openssl x509 -in -pubkey
-----END PUBLIC KEY-----

root@h1:/# openssl verify -verbose         OK

# if we use the first certificate
root@h1:/# openssl verify -verbose 
C = US, ST = California, L = Mountain View, O = Google LLC, CN =
error 20 at 0 depth lookup: unable to get local issuer certificate
error verification failed
cdci/labs/5.txt · Last modified: 2020/05/15 17:07 by mihai.chiroiu
CC Attribution-Share Alike 3.0 Unported Valid CSS Driven by DokuWiki do yourself a favour and use a real browser - get firefox!! Recent changes RSS feed Valid XHTML 1.0