Abstract: Several multilevel access control schemes have been proposed. However, they all have one or all of the following drawbacks: 1) the users must store large amount of common information when the number of classes of users is large; 2) the system must be rebuilt when there is a need to add/delete a user class or to change the clearance of some user classes; and 3) it is difficult to change keys for the users. With the aim of overcoming these drawbacks, the problem of efficiently implementing authorized access control in multilevel security systems using cryptographic techniques is studied in this paper. A new dynamic access control scheme based on key distribution is proposed. In the scheme, each user is assigned a security clearance. The user in a higher security class can read and store information items that belong to users in a lower security class, but the opposite direction of this operation is infeasible. Hence, authorized access control through the use of this type of key distribution schemes can be implemented. The key distribution scheme is based on Rabin public key system and Chinese remainder theorem. It has the following advantages over the previous ones: 1) the algorithms of key generation and derivation are simple; 2) there is no need to change the keys of other user classes when adding/deleting a user class; and 3) there is no need to change the whole system when some user classes change their keys for some security reasons.