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beVX Conference Challenge – OffensiveCon

During the event of OffensiveCon, we launched a reverse engineering and encryption challenge and gave the attendees the change to win great prizes.
The challenge was divided into two parts, a file – can be downloaded from here: https://www.beyondsecurity.com/bevxcon/bevx-challenge-1 – that you had to download and reverse engineer and server that you had to access to have a running version of this file.
The challenge could not have been resolved without access to the server as the encryption key that you were supposed to extract was only available in the running version on the server.
We had some great solutions sent to us, some of them were posted below – some arrived after the deadline, and some were not eligible as their solution was incomplete, but in the end we had three winners.
First place winner got an all paid, flight and hotel, and entry to our security conference beVX in September, second place prize winner got flight and entry to our security conference and the third place winner got a free entry to our event.

Challenge Source Code
If you don’t want to get a solution or hints to how to solve it – don’t continue reading – you have been warned 🙂

Solution (Tim)
The “encryption” routine operates on 1 bit of the key at a time, modifying an internal ongoing value each step. If the bit is a zero then this value is squared. If the bit is a one then the value is squared and then further multiplied by the value of the “message” that is selected to encrypt.
Because you can stop the encryption at any point, you can encrypt progressively more of the message, allowed each bit to be extracted by comparing the result with the result for the previous number of bits.
Script output:

Solution (mongo)
Key: “beVX Sep 20!”

Solution (Dmitry)
Secret key is “beVX Sep 20!”
Solution:
It is easy to detect that valid row numbers are 0..15 (thanks to error messages).
“Private Key Encryption” handling routine sets bit 3 (& 8) of number of rows thus allowing access to rows 16..23. Key is stored in rows 18..20. Each row represents 32-bit value.
Encryption is just calculation of pow(msgRow, keyRow, 1<<32)
Fastest method (using timing attack) allows recovering of row value in single pass. Each non-zero bit in exponent requires additional call to decrypt(), that causes sensitive delay.
But due to difficulties in automation of SSH interactive communication I derives each row in 3 steps:
1. Find number of bits if exponent (by counting “Continue Encryption? (y/n)” prompts)
2. Find highest 16 bits of exponent (by stopping encryption 16 bits before its end and brute-forcing 16 bit exponent value)
3. Find complete exponent (by brute-forcing lowest 16 bits)

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