CTFlearn-Writeups/Cryptography/RSA Noob

RSA Noob

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

Step-1:

After we download rsanoob(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: 1
c: 9327565722767258308650643213344542404592011161659991421
n: 245841236512478852752909734912575581815967630033049838269083

Step-3:

'e' and 'n' - Public key. 'c' - Cipher text.

Given n to us, we try to find p & q online from http://factordb.com/index.php

Step-4:

We get the p & q online as follows:

Now we can feed inputs manually to yield flag.

Step-5:

Running this Flag.py script:

from Crypto.Util.number import inverse
import binascii

e = 1
c = 9327565722767258308650643213344542404592011161659991421
n = 245841236512478852752909734912575581815967630033049838269083

# From factordb

p = 416064700201658306196320137931
q = 590872612825179551336102196593

phi = (p-1) * (q-1)

d = inverse(e,phi)
m = pow(c,d,n)

hex_str = hex(m)[2:] # Removing '0x'
print(binascii.unhexlify(hex_str))

Step-6:

We get the flag by python3 Flag.py

We get this output: b'abctf{b3tter_up_y0ur_e}'

Step-7:

Finally the flag becomes: abctf{b3tter_up_y0ur_e}