Weak RSA

Weak RSA writeup by Thamizhiniyan C S

Overview

Hello everyone, In this writeup we are going to solve Weak RSA from HackTheBox.

Link for the machine : https://app.hackthebox.com/challenges/6

Lets Start 🙌

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Initial Setup

First download the given file.

The give file is a zip file. Extract the zip file using the following command and the given password:

Command: unzip <zip_file>

password: hackthebox

The zip file contains two files: flag.enc and key.pub

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Enumeration

From the challenge name, we can assume that the encryption used to encrypt the flag.enc

file is RSA.

We need the private key to decrypt the flag.enc file, since the public key is given which shows that asymmetric encryption is used to encrypt the file flag.enc.

The public key in the key.pub is smaller, i.e., of less length. Its possible to retrieve private keys from short or smaller public keys.

We can do this by using the tool RsaCtfTool.

GitHub - RsaCtfTool/RsaCtfTool: RSA attack tool (mainly for ctf) - retreive private key from weak public key and/or uncipher dataGitHub

To install and run this tool execute the commands given below:

// Installing the necessary dependencies
sudo apt-get install libgmp3-dev libmpc-dev

// Downloading the repository
git clone https://github.com/RsaCtfTool/RsaCtfTool.git

// Creating a virtual environment to download the requirements for the tool to run
python3 -m venv env

// Activating the virtual environment
source ./env/bin/activate

// NOTE: To deactivate the virtual environment after job is done use the following command:
deactivate

// Installing the requirements
pip3 install -r ./RsaCtfTool/requirements.txt

Now we have successfully installed the tool. Now we have to retrieve the private key by using the following command:

python3 ./RsaCtfTool/RsaCtfTool.py --publickey ./key.pub --private

// Here 
// --publickey - mention the path to public key file.
// --private - Display private key if recovered.

We have successfully recovered the private key. Now copy and store the private key in a private.key file.

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Getting the Flag

Now we can use this private key to decrypt the contents of the flag.enc file. We can do this by using the openssl utility.

Use the following command to decrypt the flag.enc file and store the output in flag.txt file.

openssl pkeyutl -in flag.enc -out flag.txt -decrypt -inkey ./private.key

// Here
// pkeyutl - Public key algorithm cryptographic operation command.
// -in - input file.
// -inkey - input private key file.
// -decrypt - Decrypt input data with private key.	
// -out - Decrypted output file name and location.

We have successfully decrypted the file and got the flag!!!!!

Thankyou !!!!!