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INT-CTXT game adversary

sa flag

I am trying to answer this question. But I think I got it wrong. I am not sure why and how I can improve it.

enter image description here

This is the adversary I came up with. Is this correct?

Adversary A:
M <- 0^256; M' <- 0^256
C <- Enc(M)
C[0]C[1]C[2] <- C
h' = SHA256(M')
C'[0] <- C[0]
C'[1] <- C[1] XOR M'
C'[2] <- C[2] XOR h'
return C'[0]C'[1]C'[2]

The idea was I needed to forge a new ciphertext C' where I only had to change a bit from C to count as a new C'. So I kept C[0] (the random thing). Then C2 XOR M lets us recover F(K, C[0] + 1) and C2 XOR h lets us recover F(K, C[0] + 2).

Should M' be something else? Thank you for any input!

Daniel S avatar
ru flag
HINT: $F(K,C'[0])\oplus C'[1]$ does not equal $M'$.
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