Table of contents
Understanding the Challenge
This challenge gives us 2 files:
adversarial-crypto-infant.py: Python script, ASCII text executable
output.txt: ASCII text, with very long lines (621)
output.txt seems to contain an RSA private key and encrypted message
Solution
I open up adversarial-crypto-infant.py.
I instantly see n is not set to be the product of 2 primes, but directly to a large number
I create my own python file to decrypt the message
Since n is not the product of primes theres no p, q for generating d, the private key, therefore I decrypt using p, q = n
Running the script:
from Crypto.Util.number import getStrongPrime, bytes_to_long, long_to_bytes
d = pow(e, -1, (n-1)*(n-1))
flag = pow(c, d, n)
flag = long_to_bytes(flag)
print(flag)
We get the flag:
FCSC{d0bf88291bcd488f28a809c9ae79d53da9caefc85b3790f57615e61c70a45f3c}