goRsaTool is an RSA tool for CTF challenges, it attempts multiple attacks against a public key and/or an RSA encrypted ciphertext binary in an effort to recover either the private key, the plain text of the message or both.

Inspired by my time spent contributing to RsaCtfTool and originally for the purpose of learning golang. Now I maintain this tool as a side project with the goal of support as wide a range of factorization methods and RSA attacks as possible.

• factordb attack (i.e. is the modulus already fully factored on factordb.com)
• small q attack (smallq)
• small e attack / low public exponent attack (hastads)
• novelty primes attack - 31337 and 1337 primes. (notableprimes)
• mersenne primes - factor n when p is a mersenne prime (notableprimes)
• lucas primes - factor n when p is a lucas prime (notableprimes)
• past CTF primes attack (pastctfprimes)
• fermat factorization for close p & q (fermat)
• londahl factorization for close p & q (londahl)
• wiener's attack for large public exponents (3 variants) (wiener)
• wiener's attack on multiprime RSA (wiener)
• pollards p-1 attack (pollardsp1)
• williams p+1 attack (williamsp1)
• pollards rho factorization - original Pollard's Monte Carlo factorization method (pollardsrho)
• pollard rho brent factorization - Richard Brents improved version of Pollard's monte carlo factorization (pollardsrhobrent)
• Qi Cheng factorization from "A New Class of Unsafe Primes" (qicheng)
• solve for plaintext with CRT components provided (Dp, Dq, p, q, c)
• ecm (Lenstra elliptic curve method) using GMP-ECM library (ecm)
• Franklin Reiter related message attack - Requires 1 key, 2 ciphertexts which are related with some minor different suffix. See the example keys in the examples/ subdirectory. (franklinreiter)
• small fraction factorization - finding factors of n when p and q are close to a small fraction (e.g. 37/32). (smallfractions)
• faulty rsa implementation where c = me mod n instead of ct = m^e mod n (brokenrsa module)
• Public key consisting of many small primes (corCTF 2021 4096 challenge) (manysmallprimes)
• Private key recovery when 50+% of the LSB of D are known. (partiald)
• Sexy primes - primes seperated by 6. (fermat)
• Known prime - not really an attack but a helpful shortcut (knownprime)
• Recovering plaintext when phi(n) are not coprime provided we have at least 1 prime and partial KPT (defectivee)
• Recover private key and plaintext when n is a square. (squaren)

• hastads broadcast attack (hastadsbroadcast)
• common factors attack (share p among multiple moduli) (commonfactors)
• common modulus attack (2 keys share n but have different e) (commonmodulus)

• recover RSA modulus given signatures and plaintexts (use -siglist flag)
• recover RSA modulus given an encryption oracle (oraclemodulus, see examples/recover_modulus_from_oracle.txt)
• recover RSA modulus given two RS256 JWT tokens (jwtmodulus).

• Requires go 1.16 +
• Get dependencies, you will need:
  • Golang
  • Git
  • FLINT2 (Fast Library for Number Theory)
  • GMP-ECM (GMP Elliptic Curve Method factorization)

This works on Ubuntu (tested 20.04 LTS) and Windows 10 WSL 2.0 (tested in Ubuntu22.04.3 WSL) as well as Kali Linux 2024.1.

sudo apt update && sudo apt install git golang libflint-dev libecm-dev make
git clone https://github.com/sourcekris/goRsaTool
cd goRsaTool
sudo make install

• Note: a breaking change was published in Kali Linux in a package called libflint-2.8.5 which was later fixed in libflint17 which is based on FLINT 2.9.0. If you get goflint package build errors, upgrade to libflint17 or build FLINT2 2.9.0+ from source. On my system sudo apt update && sudo apt install libflint-dev was enough to correct the build errors.

For Arch Linux including Athena OS:

sudo pacman -S git go flint gmp-ecm make
git clone https://github.com/sourcekris/goRsaTool
cd goRsaTool
sudo make install

Note: This will install goRsaTool binary into /usr/local/bin.

For Mac OSX (tested on Mojave) you need Golang installed. I used the official .pkg distrubution from the Golang homepage. To install dependencies I used Homebrew to install FLINT and built GMP-ECM library from source. This guide assumes Xcode is already setup / working.

Install FLINT first, it comes with GMP which is a dependancy of GMP-ECM.

$ brew install flint

There's no prepackaged version of GMP-ECM on OSX. There is a MacPorts gmp-ecm package but it is only the ecm binary and does not include the headers needed. To install GMP-ECM from source follow these steps.

Tested on Mac OS 12.1 (Monterey, on arm64)

$ cd ~
$ git clone https://gitlab.inria.fr/zimmerma/ecm.git
$ cd ecm
$ brew install autoconf automake libtool
$ glibtoolize
$ autoreconf -i
$ ./configure --with-gmp=/opt/homebrew/
$ make
$ sudo make install

Finally, get the goRsaTool package

$ go get github.com/sourcekris/goRsaTool

$ ./gorsatool -createkey -n 115367564564210182766242534110944507919869313713243756429 -e 3
-----BEGIN RSA PUBLIC KEY-----
MB0CGAS0flryFxnpDN8t2jlPVnTt6YdoEyEXjQIBAw==
-----END RSA PUBLIC KEY-----

$ ./gorsatool -dumpkey -key ./key.pub
key.pub:
n = 115367564564210182766242534110944507919869313713243756429
e = 3

$ ./gorsatool -key ./key.pub -attack all -verbose
rsatool: rsatool.go:72: starting up...
2020/05/13 21:48:28 fermat factorization attempt beginning with timeout 5m0s
2020/05/13 21:53:28 small q attempt beginning with timeout 3m0s
rsatool: rsatool.go:45: key factored with attack: smallq
-----BEGIN RSA PRIVATE KEY-----
MEoCAQACGAS0flryFxnpDN8t2jlPVnTt6YdoEyEXjQIBAwIBAAIDM7EfAhUXTSkV
Ec7+LmR48UUVuXtlKE1fhdMCAQACAQACAxw+pw==
-----END RSA PRIVATE KEY-----

$ cat numbers.txt
n = 115367564564210182766242534110944507919869313713243756429
e = 3
$ ./gorsatool -key numbers.txt -attack ecm -verbose
rsatool: rsatool.go:72: starting up...
2020/05/13 21:50:31 ecm factorization attempt beginning with timeout 5m0s
-----BEGIN RSA PRIVATE KEY-----
MEoCAQACGAS0flryFxnpDN8t2jlPVnTt6YdoEyEXjQIBAwIBAAIDM7EfAhUXTSkV
Ec7+LmR48UUVuXtlKE1fhdMCAQACAQACAxw+pw==
-----END RSA PRIVATE KEY-----

$ ./gorsatool -list
crtsolver
factordb
hastads
hastadsbroadcast
commonfactors
commonmodulus
partiald
knownprime
brokenrsa
notableprimes
pastctf
smallq
wiener
wienermultiprime
qicheng
fermat
londahl
smallfractions
manysmallprimes
ecm
franklinreiter
pollardsp1
pollardsrho
pollardrhobrent
williamsp1
defectivee
oraclemodulus
mersenne
lucas
novelty
pastprimes
pastctfprimes
sexyprimes
squaren

./gorsatool -key ./key.pub -attack wiener

./gorsatool -key examples/pollardsp1.pub -attack pollardsp1

./gorsatool -keylist examples/hastadsbroadcast1.key,examples/hastadsbroadcast2.key,examples/hastadsbroadcast3.key -attack hastadsbroadcast

./rsatool -ptlist message1.txt,message2.txt -siglist sig1,sig2

Note: See the test.sh file for more examples using almost every attack in the list.

• Kris Hunt (@ctfkris)