A block cipher with encryption function e when used in OFB mode produces a keystream of pseudo- random blocks s1,s2.s3.... where s1=e(IV) and si+1-e(si). These blocks are then used as a stream cipher to yield ciphertexts yi-sixi where xi is the ith block of plaintext. What would happen if, by coincidence, s3=s20. Would this threaten the security of the system? How?