A host device for processing confidential information, which reads an encrypted content from a target device storing encrypted confidential information that includes the encrypted content, and decrypts the encrypted content for use, said device comprising: a confidential information processing unit operable to perform operations according only to a plurality of predetermined sequences; a CPU which directs said confidential information processing unit to initiate the sequences; a first interface which inputs and outputs data including the confidential information between said confidential information processing unit and the target device; and wherein: m keys {K1,..., Km} are stored, as the encrypted confidential information, in the target device or said host device; the key Km is a content key for encrypting a content; n keys {Kb1,..., Kbn} are stored in the target device or a different target device, n being a natural number; and said device being characterized by: a second interface which inputs and outputs data including the confidential information between said confidential information processing unit and said CPU; and a key Ki is encrypted with a key K (i - 1), i being a natural number satisfying 1 ≤ i ≤ m; the sequences include a key conversion sequence which converts a Ki encrypted with a K (i - 1) into a Ki encrypted with a key different from the K (i - 1), the to-be-converted Ki being a key among the m keys, the different key is a key among the keys {Kb0,..., Kb (n - 1)}; and said first interface and said second interface output only the encrypted confidential information outside said confidential information processing unit, in the case where the key conversion sequence is initiated, wherein said host device is operable to perform a first authentication processing for the target device; a key K0 for encrypting the key K1 is an authentication key Ka0 generated by the first authentication processing; and the key Kbj is encrypted with a Kb (j - 1), j being a natural number satisfying 1 ≤ j ≤ n; and wherein: said host device is further operable to perform a second authentication processing for the target device or the different target device; the key Kb0 is an authentication key generated by the second authentication processing, and said confidential information processing unit further includes a flag storage unit operable to store a target flag which indicates whether or not the first authentication processing and the second authentication processing have been performed on the same target device; and in the case where said target flag does not indicate the first authentication processing and the second authentication processing have been performed on the same target device, the key conversion sequence is prohibited to be initiated by said host CPU. a confidential information processing unit operable to perform operations according only to a plurality of predetermined sequences; a CPU which directs said confidential information processing unit to initiate the sequences; a first interface which inputs and outputs data including the confidential information between said confidential information processing unit and the target device; and wherein: m keys {K1,..., Km} are stored, as the encrypted confidential information, in the target device or said host device; the key Km is a content key for encrypting a content; n keys {Kb1,..., Kbn} are stored in the target device or a different target device, n being a natural number; and a second interface which inputs and outputs data including the confidential information between said confidential information processing unit and said CPU; and a key Ki is encrypted with a key K (i - 1), i being a natural number satisfying 1 ≤ i ≤ m; the sequences include a key conversion sequence which converts a Ki encrypted with a K (i - 1) into a Ki encrypted with a key different from the K (i - 1), the to-be-converted Ki being a key among the m keys, the different key is a key among the keys {Kb0,..., Kb (n - 1)}; and said first interface and said second interface output only the encrypted confidential information outside said confidential information processing unit, in the case where the key conversion sequence is initiated, said host device is operable to perform a first authentication processing for the target device; a key K0 for encrypting the key K1 is an authentication key Ka0 generated by the first authentication processing; and the key Kbj is encrypted with a Kb (j - 1), j being a natural number satisfying 1 ≤ j ≤ n; and wherein: said host device is further operable to perform a second authentication processing for the target device or the different target device; the key Kb0 is an authentication key generated by the second authentication processing, and