A certificate authorization device, comprising: a public/private key generation unit generating public/private key pairs; a secret value negotiation unit negotiating a secret value with an entity, wherein the secret value is a key contained in a root node of a key tree; wherein the key tree consists of the root node and a plurality of sub-nodes; wherein each sub-node contains a key; a hash function unit generating a hash function set; a key set computation unit receiving the secret value from the secret value negotiation unit and receiving the hash function set from the hash function unit, wherein the key set computation unit generates a derived key set of the entity, wherein said derived key set consists of a plurality of keys derived from the secret value and the hash function set; and wherein the keys contained in the plurality of sub-nodes are respectively the plurality of keys of the derived key set; a certificate generation unit receiving the public/private key pairs from the public/private key generation unit and the derived key set from the key set computation unit, wherein the certificate generation unit generates for each leaf node of the key tree a corresponding certificate; wherein each certificate comprises the key contained in its corresponding leaf node, an index of its corresponding leaf node, a public key of the public/private key pairs, a certificate authority signature and one or more indexes of one or more hash functions of the hash function set from which the key contained in its corresponding leaf node is derived; a certificate revocation unit acquiring at least one derived secret key contained in at least one common parent node of a minimum number of sub-trees whose leaf-nodes only correspond to revoked certificates of a revoked certificate set, and updating a certificate revocation list by adding said at least one derived secret key, the index of said at least one common parent node and a height of said at least one common parent node; and a certificate revocation list broadcast unit receiving the certificate revocation list and broadcasting the certificate revocation list, wherein the derived secret key of each of the sub-nodes is generated by applying a randomly selected hash function of the hash function set to the key contained in the parent node of said sub-node; and wherein said common parent node is not the root node of the key tree. a public/private key generation unit generating public/private key pairs; a secret value negotiation unit negotiating a secret value with an entity, wherein the secret value is a key contained in a root node of a key tree; wherein the key tree consists of the root node and a plurality of sub-nodes; wherein each sub-node contains a key; a hash function unit generating a hash function set; a key set computation unit receiving the secret value from the secret value negotiation unit and receiving the hash function set from the hash function unit, wherein the key set computation unit generates a derived key set of the entity, wherein said derived key set consists of a plurality of keys derived from the secret value and the hash function set; and wherein the keys contained in the plurality of sub-nodes are respectively the plurality of keys of the derived key set; a certificate generation unit receiving the public/private key pairs from the public/private key generation unit and the derived key set from the key set computation unit, wherein the certificate generation unit generates for each leaf node of the key tree a corresponding certificate; wherein each certificate comprises the key contained in its corresponding leaf node, an index of its corresponding leaf node, a public key of the public/private key pairs, a certificate authority signature and one or more indexes of one or more hash functions of the hash function set from which the key contained in its corresponding leaf node is derived; a certificate revocation unit acquiring at least one derived secret key contained in at least one common parent node of a minimum number of sub-trees whose leaf-nodes only correspond to revoked certificates of a revoked certificate set, and updating a certificate revocation list by adding said at least one derived secret key, the index of said at least one common parent node and a height of said at least one common parent node; and a certificate revocation list broadcast unit receiving the certificate revocation list and broadcasting the certificate revocation list, wherein the derived secret key of each of the sub-nodes is generated by applying a randomly selected hash function of the hash function set to the key contained in the parent node of said sub-node; and wherein said common parent node is not the root node of the key tree.