problemgraphy

Temper resistant

5 points | exact answer | by klmn | | edited | no votes (0 votes; 0 up, 0 down)

One uses a cryptosystem with encryption function $E_{a, b, c}\colon \Z_p \to (\Z_p)^2$ defined as $$E_{a, b, c}(x) = (a + x \bmod p, b x + c \bmod p),$$ where $p=101$.
Alice encrypts each plaintext $x \in \Z_p$ with a random key and sends cipher text $(c_i)_{i=0}^3$ to Bob. Tomas wants to mess with Bob, so he intercepts the messages and changes some of them. Bob has the following keys that Alices used

For which of the following received ciphertexts will Bob detect tempering?

  1. $c_1 = (77, 56)$
  2. $c_2 = (60, 34)$
  3. $c_3 = (23, 77)$

What is the probability of undetectable tampering? Round to 4 decimal places.

Submit the answer as a sequence of numbers, eg. 2 3 0.1234.

#modular-arithmetic

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