CTFlearn-Writeups/Cryptography/RSA Beginner

RSA Beginner

The main idea finding the flag using RSA function and its operations.

Step-1:

After we download rsa (1).txt from the cloud, we try to understand what is the content.

If anyone is unaware of RSA Encryption, they can checkout here:

https://en.wikipedia.org/wiki/RSA_(cryptosystem)

Step-2:

The contents of rsa (1).txt are as follows:

e: 3
c: 219878849218803628752496734037301843801487889344508611639028
n: 245841236512478852752909734912575581815967630033049838269083

Step-3:

We run a simple Flag.py script to get the flag.

import gmpy2
from gmpy2 import mpz

e=mpz(3)
c=mpz(219878849218803628752496734037301843801487889344508611639028)
n=mpz(245841236512478852752909734912575581815967630033049838269083)

#use factordb
q=mpz(416064700201658306196320137931)
p=mpz(590872612825179551336102196593)

phi=gmpy2.mul(p-1,q-1)
d=gmpy2.invert(e,phi)
f=gmpy2.powmod(c,d,n)
g=bytes.fromhex(hex(f)[2:])

print("[+] Flag is : ",g)

Step-4:

The output of python3 Flag.py is as follows:

[+] Flag is : b'abctf{rs4_is_aw3s0m3}'

Step-5:

Finally the flag becomes: abctf{rs4_is_aw3s0m3}