Can COViD steal Bob’s idea?

Points: 968 [1000]

Description

Bob wants Alice to help him design the stream cipher’s keystream generator base on his rough idea. Can COViD steal Bob’s “protected” idea?

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This challenge:

• Unlocks other challenge(s)
• Is eligible for Awesome Write-ups Award

Solution

1. We are given a PCAP file that contains all the required information needed for the challenge. So the first thing I did was to filter everything to TCP only.
2. Analyzing the first few packets will reveal that Alice sent the following messages message through TCP

   p = 298161833288328455288826827978944092433
   g = 216590906870332474191827756801961881648
   g^a = 181553548982634226931709548695881171814
   g^b = 64889049934231151703132324484506000958

   Hi Alice, could you please help me to design a keystream generator according to the file I share in the file server so that I can use it to encrypt my 500-bytes secret message? Please make sure it run with maximum period without repeating the keystream. The password to protect the file is our shared Diffie-Hellman key in digits. Thanks.

3. The subsequent few TCP packets shows pyftpd protocol sending a ZIP file over.
4. If we were to export the ZIP file, we will realise that the ZIP file is corrupted.
5. Then further analysis shows that I have forgotten to append an additional packet worth of ZIP data. So fixing that will yield a nice ZIP file that is password protected.
6. So googling for Diffie Hellman crackers, I stumbled upon a post on CTFTime by Tudor_OG: https://ctftime.org/writeup/12240
7. Clicking on the calculator he gave, I was able to crack the original a and b values (but honestly you only need one…)
8. Throwing into python using the formula s = g^b^a (mod p), we can derive the secret key.

   a = 211631375588570729261040810141700746731
   gb = 64889049934231151703132324484506000958
   p = 298161833288328455288826827978944092433
   s = pow(gb, a, p)
   print(s)
   # Output: 246544130863363089867058587807471986686
   

9. We can then test the secret with the zip, which will give us the cryptography design:
10. The flag is then the secret value that we derived!

Flag

govtech-csg{246544130863363089867058587807471986686}

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