ElGamal with differentiated decryption on K+1 access levels

The algorithm is an extension of the El Gamal encryption system. The difference from the classical form is the way the keys are generated and distributed. This system allows the encryption of the message m
i
… by Alice
i…
and its decryption by Bob
i…
. Moreover, there exists the possibility of decrypting a group of messages or all the messages by other beneficiaries, each using a secret special key. This key is distributed depending on the access level, meaning, on the access right. The user of the key of level 0 can decrypt all messages, the user which holds one of the keys of level 1 can decrypt a subset of the set of messages that can be decrypted with the key of level 0, the one which holds one of the keys of level 2 can decrypt a subset of the set of messages that can be decrypted with the key of level 1 and so on, and the one which holds one of the keys of level k can decrypt a message m
i..
.