In order to provide varieties of security and efficiency levels ; RC5 has a variable block size ( 32 , 64 or 128 bits ) , variable key size ( 0 to 2040 bits ) and variable number of rounds ( 0 to 255 ) . Unlike many schemes , RC5 has a variable block size ( 32 , 64 or 128 bits ) , key size ( 0 to 2040 bits ) and number of rounds ( 0 to 255 ) .