Which functions can be used as Pseudo Random functions? Can block Ciphers be used for this purpose with our own encryption function? Since the cloud security algorithm I am trying to implement says that for a PRF, any secure encryption algorithm or a keyed hash function can be used. The security parameter I have chosen is 15 bits while AES and SHA use a large key size than this. So which block cipher and encyption function should I use for a PRF?
I would recommend against using ONLY 15-bits as a key for your encryption algorithm. A key size that small can be broken via brute force by any standard machine. There's a reason that AES (where the key can be 128 bits, 192 bits, or 256 bits) or SHA-256 are so widely used. The extended key lengths of those algorithms add security against brute force attacks from adversaries.