PicoCTF_2017: Encrypted Shell

Category: Cryptography Points: 190 Description:

This service gives a shell, but it's password protected! We were able intercept this encrypted traffic which may contain a successful password authentication. Can you get shell access and read the contents of flag.txt? The service is running at shell2017.picoctf.com:38314.

Hint:

Are any of the parameters used in the key exchange weaker than they should be?

Write-up

This challenge revolves on breaking the Diffie-Hellman key exchange. The hint given tells us that something is weak and upon observing the server code closely, we find that a = random.randint(1, 2**46), which is surprisingly, a very small number.

So, we are then able to use the "Baby Step Giant Step" algorithm to try and reverse a from the given A. Additionally, since we know the range of a, we can run a sqrt(a) to limit the amount of small steps we have to take.

Code

Therefore, the flag is 467de743e8f82e09b555426e322adba5.

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