Decrypt the ciphertext below:

$03$ a$3$ da $539783$ cf $1$e f$16318$ f$0515352$ f$6$

df $3$e $411$b $268$a d$35$b bc $143293$ cc d$48098$

The key is encrypted with Caesar cipher: utrs

$($hint $1$: decrypt the key first, then apply to the above cipher for decryption$)$

hint $2$: the correct answer shall be:

Cybersecurity is fun