$2.$ Let the message space be M $=[$a$,$ b$]$ with the probability distribution Pr$[$M $=$ a$]=3/4,$ Pr$[$M $=$ b$]=1/4.$ Let key space be K $=[$k$1,$ k$2,$ k$3]$ with the probability distribution Pr$[$K $=$ k$1]=1/2,$ Pr$[$K $=$ k$2]=$ Pr$[$K $=$ k$3]=1/4.$ The ciphertext space C $=[1,2,3,4]$ was produced using an encryption scheme given in the below matrix. For example, Enc$($k$1,$ a$)=1,$ and Dec$($k$2,3)=$ b$.$

a

b

k$1$

$1$

$2$

k$2$

$2$

$3$

k$3$

$3$

$4$

What is Pr$[$M $=$ b $|$ C $=4)?$ Does it equal to Pr$[$M $=$ b$]?$ Is this encryption scheme perfectly secret?