Secure Text Transfer Using Diffie-Hellman Key Exchange Based On Cloud

Security on cloud has been a hot topic. Even the tech giants like google and amazon spend hefty capital to strengthen their security. We, here have implemented a secure text transfer using diffie-hellman key exchange algorithm.

Diffie-Hellman Key Exchange

One of the brute approaches for secure transfer can be that user A can encrypt the using a key and later the key could be shared with user B. This approach can work up to an extent but it always has a risk of third-party eavesdroppping. We needed a system in which two users, without knowing the secret key, can independently generate same key at both ends. This key could later be used to encrypt and decrypt the text. The diffie-hellman key exchange algorithm comes into picture.

Diffie–Hellman key exchange (DH) is a method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols named after Whitfield Diffie and Martin Hellman. DH is one of the earliest practical examples of public key exchange implemented within the field of cryptography.
In public key cryptosystem, enciphering and deciphering are governed by distinct keys, E and D, such that computing D from E is computationally infeasible (e.g., requiring more than 10^100 instructions). The enciphering key E can thus be publicly disclosed without compromising the deciphering key D. This was the main ideology behind Diffie-Hellman Key Exchange Protocol. Each user of the network can, therefore, place his enciphering key in a public directory. This enables any user of the system to send a message to any other user enciphered in such a way that only the intended receiver can decipher it. As such, a public key cryptosystem is a multiple access cipher. A private conversation can therefore be held between any two individuals regardless of whether they have ever communicated before. Each one sends messages to the other enciphered in the receiver’s public enciphering key and deciphers the messages he receives using his own secret deciphering key.

Working example of Diffie-Hellman

p is a prime number
g is a primitive root modulo of p

• Alice and Bob agree to use a modulus p = 23 and base g = 5
• Alice gets her private key (key which she should not share with anyone) generated as 4.
• Thus, public key generated for Alice shall be 5^4 %23 = 625%23 = 4
• Bob gets his private key (key which he should not share with anyone) generated as 3.
• Thus, public key generated for Bob shall be 5^3 %23 = 125%23 = 10
• Now, Alice gets the public key of Bob and generates a secret key. i.e. (public key of Bob ^ Private Key of Alice) mod p => (10^4 ) % 23 => 10000 % 23 => 18
• On the other side, Bob also uses a similar method to generate a secret key i.e. (public key of Alice ^ Private Key of Bob) mod p => (4^3 ) % 23 => 64 % 23 => 18

Dependencies

python-flask
hashlib
pycrypto
secretsharing
tkinter
webbrowser

Code Explanation

This implementation can be explained in two parts

• stand-alone-application
• web-application

stand-alone-application

• This portion deals with encryption and decryption of file
• The file is encrypted using AES algorithm
• Menu option also helps to toggle the menu to upload and download files
  
  

src/stand-alone-application/DH.py: This file deals with generating keys using diffie-hellman. It generates three keys: Private Key, Public Key, Secret Key (used for encryption and decryption)
src/stand-alone-application/ENCDEC.py: This file is used for encoding and decoding using AES algorithm.
src/stand-alone-application/thrain.py: This file acts as a mediator and connect the main program with other code files.
src/stand-alone-application/main.py: This file deals with the GUI. It is the main file [yeah, trust me!].

web-application

Once file is encrypted it has to be uploaded on an online directory. Another directory is needed where public-key of all the users is stored. Thus, we built an online directory and hosted it on cloud. The unique thing about hosting is that dynamic files are being generated while adding a new user or uploading a text file. Thus, we needed a cloud service which could run the program and incorporate the dynamic files. We tried free services like pivotal and heroku but then amazon AWS came to our rescue.
src/web-application/app.py Contains the website in python-flask which acts like a directory.

Hosting on AWS

• fork this repository
• create an amazon EC2 instance.
• Select and create Amazon Linux AMI 2018.03.0 (HVM), SSD Volume Type
• While creating the machine, toggle to menu Configure Security Group menu.
• Here, enable port SSH, HTTP, RDP and in port, change Source to Anywhere
• Download and keep the publicKeyPair.pem file
• Install putty and puttygen
• Open puttygen, select the publicKeyPair.pem and generate a private key. Now, save it as Save private key
• Open the AWS machine dashboard ad copy the IPv4 Public IP of the instance created
• Open putty
• In putty, copy the IP in Host Name(or IP address)
• In putty, toggle the SSH>Auth and here, select the private key you generated.
• Click open and voila! The terminal of instance opens

• ---

• Enter "login as:" ec2-user
• Enter following command
• $sudo bash
• $yum install python-pip
• $yum install git
• $pip install flask
• Now, clone the forked repository on local machine
• In src/web-application/app.py, comment on line 169 and uncomment line 168
• Update the github repository
• On the terminal of instance, clone the repository and enter command
• $python app.py

NOTE: For instance we have uploaded the screenshots of the process in dump/images.
NOTE 2: This hosting technique is newbie. The project can be accessed by the public IP of the instance. If you have a better approach, please let us know :')

FOr any further queries please fell free to connect to us at hardikgaur@geu.ac.in or parthendo@geu.ac.in