# [MagTF] Crypto1sFun

## Challenge by WelloWorld

So, this is a medium-hard crypto challenge with a little bit of forensic. The solver gets a server and port and a description says:

“Today you will learn to work with two important mechanisms: The first one is very known today as a good mechanism, and the second one is old and not used today as normal mechanism.”

The mechanisms are: Deffie-Hellman key exchange and the WEP for wifi authentication (wifi is not really here). If they try to connect through sockets to the server, they will find out it sends to them the string:

“Wello Bob!\ng=\np=\n”

With replacing with real random numbers so now they have g and p values. They will try some things but at the end when Alice sends g and p to Bob she expects to get her b(actually B but it gets it too) value. So when the user sends b= the server will send him the A of him and each one of them need to calculate the key by the values they got.

After that the server will send to the socket a little clue about WEP (and ciphertext and IV to be decipher) that says:

“HAHAHA IEEE WOH”

(The clue is IEEE by the way), So they get from the packet two things: the ciphertext and the IV for the decryption. With the previous key (DH) they calculated as string of key , they need calculate the plaintext by WEP mechanism (KSA,PRGA, XORing) and they get the string “MagTF{DeffieHelman+_WEP=FUN}” and a little bit of crc32 because of the WEP mechanism.

Example:
Connect to the host:port then:

Server:

g=101
p=181


Client:

b= 736 #private number
B=39 #(101**736)%181=39


Server:

a= 465 #private number
A=133 #(B**a)%181


The key is 48 (either (133736)%181 or (39465)%181)
Then the client recieves the message:
> Hehehe ieee woh, decrypt it:
> {Encrypted message}
> {Init vector}

end of connection

The solution after getting the key of DH is(after the server sent the data):