RSACrack is a toolbox for deriving private key files from a given public key file, modulus or base64 encoded key.

At the time of writing it supports cracking RSA keys of 128, 256 bits in minutes. If you need to perform factorization in less time, feel free to spin up an EC2 instance that is compute optimised. My tests so far have shown 256bits can be done in less than 50 seconds using this toolbox.

I developed this toolbox so I could better understand how RSA works and gain an understanding of the various factorization libraries. Putting this together makes it simple for non-cryptographers to develop proof of concept key reconstruction without having to worry about the details.

The simplest way to get started is to:

docker pull b4den/rsacrack

docker run -it b4den/rsacrack "$(cat location_of_your_public_key_file.pub)"

For key bit lengths of less than or equal to 256 bits, after a couple of minutes the output should look something like:

[*] results are: ['<snip>', '<snip>', <snip>]
[*] Key extraction done.
-----BEGIN RSA PRIVATE KEY-----
		<snip>
-----END RSA PRIVATE KEY-----

I'd strongly suggest using the quick method above, but if you want to build the docker image locally, that's supported too.

Please note this may take some time due to cado-nfs and openssl library compile times.

git clone https://github.com/b4den/rsacrack.git

docker build . -t rsacrack

docker run -it b4den/rsacrack:latest "$(cat location_of_your_public_key_file.pub)"

The notes for generating RSA key pairs are given below. If you're on a macbook then you should be able to generate keypairs using small key lengths. Otherwise, an earlier version of openssl is provided in the toolbox for this purpose.

openssl genrsa -out private_key.pem 256

openssl rsa -in private_key.pem -pubout > public_key.pub

The application takes one and only one argument. From there the modulus, exponent and eventually primes are deduced from that key.

Valid inputs

• public key strings docker run -it b4den/rsacrack "$(cat public_key.pem)"
• public key value docker run -it b4den/rsacrack "base64encoded-pubkey"
• public key modulus docker run -it b4den/rsacrack modulusint

Some newer versions of openssl won't let you generate rsa keys of 128/256 bit sizes. As a workaround the image comes pre-compiled with openssl 1.0.0: docker run --entrypoint openssl -it b4den/rsacrack genrsa 256

For Macbook's they come with LibreSSL so you should be able to use your builtin openssl library for key generation using small key lengths.

Do check out the other repositories that this code relies on. factordb is a lookup site where n is small (<=128 bits). For key lengths greater than 128, our toolbox relies on the cado-nfs library.

• cado-nfs
• factordb
• 0day.work