From: hochspezialisiert@sec.in.tum.de
To: itsec_examdraft@sec.in.tum.de
Subject: [Draft Question — Cryptography]

Confidential (ONLY FOR AUTHORIZED PERSONNEL)

Question:
Consider a toy "QR-encryption" scheme where a message is encoded into
a QR code and then randomized by permuting its tiles with a secret seed.

(a) Under ideal assumptions, explain why this scheme could satisfy
Kerckhoffs' principle.  
(b) Now assume an attacker can test permutations against the QR format
(find patterns, error correction, alignment markers). Why does this
reduce the effective security of the scheme?  

Solution (DO NOT DISTRIBUTE):
(a) If the algorithm is known and the seed remains secret, only the
seed determines security → aligns with Kerckhoffs’ principle.  
(b) QR codes have strong structural redundancy (finder patterns,
error correction). This gives the attacker an oracle to prune wrong
seeds quickly → brute force feasible.  

Verification Token: