Really Small Algorithm

[Cryptography – 150 pts ]

This problem is titled Really Small Algorithm. Since the title has the initials R-S-A, I assumed it was an RSA challenge.

Lets open the address in netcat:

It opens up to a very familiar set of numbers, N, E, and C (or in this case, CT. C or CT stands for Cipher or CipherText).

 So, since we have everything, this shouldn’t be that hard to decipher.

After a few minutes of online searching, I came across a stack overflow post where one guy suggested using a tool called RsaCtfTool.

Here is the link to it:
https://github.com/Ganapati/RsaCtfTool

After I git cloned it to my desktop on kali linux, I opened up a terminal. I cd into the directory of the tool.

 Lets go back to the post to see how to format our command

Which explains to do it as python RSACtfTool – n {n} -e {e} – -uncipher {c}

I replicate the command using the numbers we got from the challenge (However, I substituted python3 for python, as that was the version of python I had installed on my machine.

Lets try it:

 It worked! Even though we had some errors, the tool ‘unciphered’ the cipher. (Down there at the bottom)

Flag: ractf{S0m3t1mesS1zeDoesM4773r}

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