Question

9. Describe three attack types that a message authentication code can prevent. [6 marks] 10.Explain why a checksum or CRC, by itself, does not provide security. [5 Marks)

Answer 1

A MAC function must resist existential forgery under chosen-plaintext attacks. This means that even if an attacker has access to an oracle which possesses the secret key and generates MACs for messages of the attacker's choosing, the attacker cannot guess the MAC for other messages (which were not used to query the oracle) without performing infeasible amounts of computation.In a chosen-plaintext attack the enemy can (adaptively) request the ciphertexts of discretionary plaintext messages. This is formalized by permitting the foe to collaborate with an encryption prophet, saw as a black box. The's assailant will likely uncover all or part of the mystery encryption key.

A replay attack (otherwise called playback attack ) is a type of system assault in which a substantial information transmission is noxiously or deceitfully rehashed or postponed. This is completed either by the originator or by an enemy who blocks the information and re-transmits it, potentially as a major aspect of a disguise assault by IP bundle replacement.However, to allow the receiver to be able to detect replay attacks, the message itself must contain data that assures that this same message can only be sent once (e.g. time stamp, sequence number or use of a one-time MAC).

Numerically expressed, a collision attack discovers two unique messages m1 and m2, to such an extent that hash(m1) = hash(m2). In a traditional collision attack, the aggressor has no influence over the substance of either message, however they are subjectively picked by the calculation.