$(12$pts$)$ Explain the following on hash functions.

$($a$)$ When we talk about security of a hash function, what do we refer to$?$

One wayness, second preimage ressistance, collision ressistance.

$($b$)$ What is the main advantage of MD$5$ over SHA$1?$

Speed.

$($c$)$ Give two applications of hash functions.

Authentication, stream ciphers.