in the case where the arbitary constant p (the one we use for the encrypt function i.e. ek(m) ≡ k1 ⋅m + k2 (mod p) and decrypt function i.e. dk(c) ≡ k1′ ⋅ (c − k2) (mod p)) is not publicly known, is the affine cipher vulnerable to a chosen plaintext attack? If so, how many pairs of plaintext - ciphertext is needed for the attacker to find the key?