RACTF 2020 Crypto - Really Speedy Algirithm


#Really Speedy Algorithm

Connect to the network service to get the flag. The included template script may be of use.

Warning: The challenge server is currently under heavy load, so 
is miss-judging how fast solutions are being sent. If your script 
is "too slow", this may be the issue. We're working to resolve it. 
If your script is timing out a lot, DM @Bottersnike#3605 on 
Discord with your script and I can run it for you.

This challenge is all about scripting your RSA knowledge, you are given a duration of about a second, to reply with the desired response 100 times in a row to obtain the flag.

Quickly netcatting the server one could see

[*] Welcome to my little RSA game.
[*] You will be presented with a number of questions.
[*] You have at most 1000ms to solve any given question.
[*] That should be plenty for any human good at crypography.
[*] Good luck.
[c] Challenge 1:
[:] p: 11259532070749083557537863095082912058641704056954116353307644260341925427893606142834585810360891048369681609478993096256214633920167758208604293863143279
[:] phi: 131064587799066203944005827312562617566669586196802373713166674946117857476631324909300704603924316375524117034979774573457951330106727138654981406704336485209955642518891120054845088878754681912501769601803479805113260316058019238267654072352658937053161061649042885639007433350438630146424838391623764024288
[:] e: 65537
[:] ct: 128888444188911299316060627894674165443895799030567545881798184573905727932049715322993601264858919464128364177444155939775493201453351612767170937300333987078025128111979600272358211481975171316360005673613236601924964071568574643794655671843495059544037652057137373486252046141981703380542284670110596876893
[?] pt:
[!] A good cryptologist should be faster than that!

It gives some parameters, and we have to provide asked parameter based on the given parameters.

All the challenges can be summarized in one of the following forms:-

  • Calculate pt, given e, phi, p
    • d = inverse of e under phi
    • q is phi/(p-1) + 1 as phi = (p-1)*(q-1)
    • n = p*q
    • pt = (ct^d) % n
  • Calculate ct, given pt, p, q and e
    • n = p*q
    • ct = (pt^e) % n
  • Calculate q, given n and p
    • q = n//p
  • Calculate p, given n and q
    • p = n//q
  • Calculate n, given p and q
    • n = p*q
  • Calculate d, given p, q and e
    • phi = (p-1) * (q-1)
    • d = inverse of e under phi

Input format begins with specific tags

  • Provided parameter begins with the tag [:]
  • Asked parameter begins with the tag [?]
  • Challenge number begins with tag [c]
  • Crucial information begins with tag [!]
  • General statements/information begins with tag [*]

Putting all the above information in a really hacky script

from pwn import remote
from gmpy2 import invert

#HOST, PORT = "" ,62467
HOST, PORT = "", 31337
REM = remote(HOST, PORT)

def recieve():
    n = None  # only the recieved parameters returned
    p = None
    q = None
    ct = None
    pt = None
    e = None
    d = None
    phi = None
    param = None  # The parameter specified in a challenge to find
    while True:
        data = REM.recvn(3).decode()
        print(data, end="")
        if data.startswith('[!]'): # line of critical information
        elif data.startswith('[:]'):  # line of parameter specification type
            param_name = REM.recvuntil(b': ').decode().strip()[:-1]
            val = int(REM.recvline().decode().strip())
            if param_name == 'n':
                n = val
            elif param_name == 'p':
                p = val
            elif param_name == 'q':
                q = val
            elif param_name == 'ct':
                ct = val
            elif param_name == 'pt':
                pt = val
            elif param_name == 'phi':
                phi = val
            elif param_name == 'e':
                e = val
            elif param_name == 'd':
                d = val
        elif data.startswith('[?]'): # line specifying which parameter to find
            param = REM.recvuntil(b': ').decode().strip()[:-1]
            return (n, p, q, ct, pt, e, d, phi, param)
        elif data.startswith('[*]') or data.startswith('[c]'): # random informative line
    return (n, p, q, ct, pt, e, d, phi, param)

def solve(values):
    Takes the parameters and calculates the desired parameter
    # print(values)
    n, p, q, ct, pt, e, d, phi, param = values
    if param == 'pt':
        d = invert(e, phi)
        q = phi // (p - 1) + 1
        n = p * q
        pt = pow(ct, d, n)
        return pt
    elif param == 'ct':
        ct = pow(pt, e, p * q)
        return ct
    elif param == 'q':
        return n // p
    elif param == 'p':
        return n // q
    elif param == 'n':
        return p * q
    elif param == 'd':
        phi = (p - 1) * (q - 1)
        return int(invert(e, phi))
        print("NOT IMPLEMENTED", param)

def send(val):

while True:
    rec = solve(recieve())

Having bad connection with high latency, I requested one of the admins to run the script and got flag in return :smile: