Abstract:
Provided is a storage system including: a host interface connected via a network to a host computer; a disk interface connected to a disk drive; a memory module that stores control information of a cache memory for an access to the disk drive and the storage system; a processor that controls the storage system; a mature network that interconnects the host interface, the disk interface, the memory module, and the processor; and an encryption module that encrypts data read/written by the host computer, in which the processor reads data from a given area of the disk drive from the memory module, decrypts the read data with an encryption key corresponding to this data, encrypts the decrypted data with an encryption key different from the one that has just been used to decrypt the data, and writes the encrypted data in an area different from the given area. Accordingly, customers can be provided with a secure, highly reliable storage system with its confidentiality preserving capability enhanced.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese application P2005-211247 filed on Jul. 21, 2005, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND 
   This invention relates to a storage system. More specifically, this invention relates to a storage system comprises a storage controller such as a disk array controller, which stores data in one or more disk drives, a tape library controller, an optical disk library controller, a solid-state disk controller (e.g., semiconductor disk controller), or a storage controller that uses a non-volatile memory, typically, flash memory. 
   Companies and public offices store an increasing amount of digital data recording personal information, and now those who let such information leak have to face legal consequences. It is therefore an urgent task for any organization that keeps personal information and other digital data to make sure that the information is managed securely and is protected against the risk of leakage. 
   A common technique that is currently available for this task is to encrypt data in a storage system by using an appliance-type encryptor in conjunction with a storage controller (see “Securing Networked Storage whitepaper”, DECRU Inc., 2004 and US 2004/0153642 A). 
   With data in a storage system encrypted by this method, it is difficult for a person who obtains the storage system, or a magnetic disk drive (HDD) mounted to the storage system, through theft or other illegal measures to decode the data. 
   Also known are a volume mirror function, with which different logical volumes in a storage system share the same data, and a snapshot function (see “Data Protection with Storage Networks Part II”, pp. 25 to 45, [online], 2004, SNIA, Internet &lt;URL:http://www.snia.org/education/tutorials/fall2004/backup/data_protection_partII.pdf&gt; and “Examination of Disk-based Data Protection Technologies”, pp. 23 to 36, [online], 2005, SNIA, Internet &lt;URL:http://www.snia.org/education/tutorials/spr2005/datamanagement/ExaminationofDiskBasedDataProtection-v5.pdf&gt;). 
   There is also a write operation called write after and employed to write in a cache memory as well as in a disk drive. A specific example can be found in a scalable storage system of JP 07-20994 A. This storage system has plural host adapters, which are connected to an upstream CPU, plural disk adapters, which are connected to array disks, and a short-term cache memory, which is shared among the adapters. The adapters and the cache memory are detachably attached to a common bus, which is shared among the adapters and the cache memory. The scale of the storage system is enlarged by merely adding as many adapters and cache memories as necessary. The adapters, cache memory, and the common bus are duplicated to enable the storage system run in a degenerate mode in the event of a failure. The adapters and the cache memory can be hot plugged in and out of the common bus, thereby making it possible to perform maintenance work and replace parts without shutting down the storage system. 
   SUMMARY 
   However, prior art gives no consideration at all to balancing the trade-off between highly secure encryption that users demand, on top of the data replication function which has long been utilized by users, and the host computer performance which is lowered by the encryption and replication functions. 
   Data encrypted by prior art can be decoded by third parties in the case where encryption key information is stolen from a system having an encrypting appliance as the one described above as a result of poor running and management of the system or other man-made errors. 
   When a storage system having a storage controller and an appliance-type encryptor puts the data replication function and the snapshot taking function into use, the same encryption key is used to encrypt two or more pieces of data. This increases the system&#39;s vulnerability against theft of encryption key by allowing a person who illicitly obtains a key to decode more than one piece of data with a single key. 
   This problem will be described further with reference to a schematic diagram of  FIG. 22 . 
   In  FIG. 22 , host computers  104 , a storage system  101  and an encrypting appliance  201  are connected to one another via a network  105 . 
   The encrypting appliance  201  is in an upper layer of the storage system  101 . The host computers  104  request the storage system  101  to write data, which is encrypted by the encrypting appliance  201 . The encrypted data is written in the storage system  101 . The host computers  104  place a read request for data in the storage system  101 , and the requested data is sent to the host computers  104  after being decrypted by the encrypting appliance  201 . 
   Data replicating unit  204  in the storage system  101  creates copy pairs from logical volumes accessed by the host computers  104 , and pairs a logical volume LVOL 1 , which is denoted by  202 , with a logical volume LVOL 2 , which is denoted by  203 . In this example, data encrypted with the same encryption key, “Key One”, is copied to two logical volumes. 
   The snapshot function, which is one of functions in data replication, is executed in the storage system  101 . Here, a snapshot is taken by mirror split, and the mirroring relation between the logical volume LVOL 1   202  and the logical volume LVOL 2   203  is dissolved. After the mirroring relation is broken up, data written in the logical volume LVOL 1   202  is not mirrored to the logical volume LVOL 2   203 . 
   Once this happens, the encrypting appliance  201  has no way of knowing data replication operations in the storage system  101 , and therefore uses the same encryption key to encrypt every new data to be written in the logical volume LVOL 1   202 , with the result that data encrypted with the same encryption key is doubled in number. If the snapshot processing is repeated after that, the same encryption key is used by even more logical volumes. 
   This invention has been made in view of those problems, and it is therefore an object of this invention to provide to customers a secure, highly reliable storage system with its confidentiality preserving capability enhanced by making data encryption and the data replication function work in cooperation with each other and thus eliminating any vulnerable points in data protection that can be removed. 
   According to this invention, there is provided a storage system including: a host interface connected via a network to a host computer; a disk interface connected to a disk drive; a memory module that stores control information of a cache memory for an access to the disk drive and the storage system; a processor that controls the storage system; a mature network that interconnects the host interface, the disk interface, the memory module, and the processor; and an encryption module that encrypts data read/written by the host computer, in which the processor reads data from a given area of the disk drive, decrypts the read data with an encryption key corresponding to this data, encrypts the decrypted data with an encryption key different from the one that has just been used to decrypt the data, and writes the encrypted data in an area different from the given area. 
   This invention achieves efficient cooperation between more secure and confidential management of encrypted data and the data replication function, which is a characteristic function of a storage system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a configuration block diagram of a computer system according to a first embodiment of this invention. 
       FIG. 2  is a block diagram showing detailed configurations of a host interface unit and an MP unit according to the first embodiment of this invention. 
       FIG. 3  is a block diagram showing detailed configurations of a disk interface unit and an MP unit according to the first embodiment of this invention. 
       FIG. 4  is a block diagram showing a detailed configuration of a memory unit according to the first embodiment of this invention. 
       FIG. 5  is an explanatory diagram schematically showing processing of writing data according to the first embodiment of this invention. 
       FIG. 6  is a flow chart for data write processing according to the first embodiment of this invention. 
       FIG. 7  is a flow chart for processing initially executed in data replication according to the first embodiment of this invention. 
       FIG. 8  is an explanatory diagram of a volume management table according to the first embodiment of this invention. 
       FIG. 9  is an explanatory diagram schematically showing processing of writing data according to a second embodiment of this invention. 
       FIG. 10  is a flow chart for data write processing according to the second embodiment of this invention. 
       FIG. 11  is an explanatory diagram schematically showing processing of writing data according to a modification example of the second embodiment of this invention. 
       FIG. 12  is a block diagram of the configuration of a memory unit according to the second embodiment of this invention. 
       FIG. 13  is an explanatory diagram schematically showing processing of writing data according to a third embodiment of this invention. 
       FIG. 14  is a flow chart for data write processing according to the third embodiment of this invention. 
       FIG. 15  is an explanatory diagram of a volume management table according to the third embodiment of this invention. 
       FIG. 16  is an explanatory diagram schematically showing processing of writing data according to a fourth embodiment of this invention. 
       FIG. 17  is a flow chart for data write processing according to the fourth embodiment of this invention. 
       FIG. 18  is a flow chart for another example of data write processing according to the fourth embodiment of this invention. 
       FIG. 19  is a flow chart for data restoring processing according to the fourth embodiment of this invention. 
       FIG. 20  is an explanatory diagram schematically showing processing of accessing data according to a fifth embodiment of this invention. 
       FIG. 21  is an explanatory diagram of a volume management table according to the fifth embodiment of this invention. 
       FIG. 22  is an explanatory diagram of a conventional computer system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of this invention will be described below. 
   First Embodiment 
     FIG. 1  is a configuration block diagram of a computer system according to a first embodiment of this invention. 
   Plural hosts  104  ( 104 A,  104 B, and  104 C) are connected to a storage system  101  via a network  105 . A disk drive group  102  is connected to the storage system  101 . A disk drive group  103  is connected to the network  105 . A management terminal  107  is connected to the storage system  101  via a network  106 . 
   The hosts  104  send a request to the storage system  101  via the network  105 , and receive a result of the request via the network  105 . The storage system  101  reads data from the disk drive group  102  or  103  following a request from the hosts  104 . 
   The storage system  101  has host interface units  111 , disk interface units  113 , MP (processor) units  112 , memory units  114  and a management unit  115 , which are interconnected by a mature network  116 . The disk interface units  113  have encryption function units  117 . 
   The host interface units  111  receive a request sent over a network, and send a result of the request to the sender of the request. 
   The disk interface units  113  are connected to the disk drive group  102  to read and write data in the disk drive group  102 . The disk interface units  113  set the configuration of the disk drive group  102 . 
   The MP units  112  execute prescribed processing in the storage system  101 . The MP units  112  analyze a request received by the host interface units  111  and execute necessary processing to meet the request. 
   The memory units  114  store data temporarily. The memory units  114  function as cache memories where data to be written in the disk drive group  102  is temporarily stored. The memory units  114  also function as a shared memory which store information to be shared among the units of the storage system  101 . 
   The management unit  115  is connected to the MP units  112 , and manages the storage system  101 . 
   The storage system  101  in this embodiment has two host interface units  111 , two disk interface units  113 , and two memory units  114  for dualization. The two host interface units  111  and the two disk interface units  113  each have one of the MP units  112 , and there are four MP units  112  in total. The storage system  101  of this invention is not limited to this configuration and the number of the host interface units  111 , the number of the disk interface units  113 , and the number of the memory units  114 , may be one or more than one. 
   The disk drive groups  102  and  103  each have one or more magnetic disk drives. In this embodiment, the disk drive group  102  has sixteen magnetic disk drives. The storage system  101  is designed such that the disk interface unit  113 A accesses eight out of the sixteen magnetic disk drives in the disk drive group  102  whereas the disk interface unit  113 B accesses the remaining eight disk drives. 
   The disk drive group  103  is connected directly to the network  105 . The hosts  104  access the disk drive group  103  via the network  105  or via the storage system  101 . The disk drive group  103  is, for example, a disk array device or a virtual disk drive. 
   The disk drive groups  102  and  103  in this embodiment have magnetic disk drives, but may instead have other storage media such as a tape library, an optical disk library, a semiconductor disk drive, a flash memory array, and a DVD library. 
   The management terminal  107  is connected to the management unit  115  of the storage system  101  via the network  106 . The management terminal  107  communicates with the management unit  115  of the storage system  101  to manage various settings and other matters of the storage system  101 . 
     FIG. 2  is a block diagram showing detailed configurations of the host interface units  111  and the MP units  112 . 
   Each of the host interface units  111  has a host interface control unit  311 , a control unit  312 , and a memory  317 . The control unit  312  has an internal bus/SW function unit  313 , a DMA function unit  314 , and a mature network interface control unit  315 . 
   The host interface control unit  311  has one or more connection paths connected to the network  105  to send and receive data over the network  105 . 
   The internal bus/SW function unit  313  has a function of a bus that interconnects the units of the host interface unit  111  and a function of a switch that transfers data exchanged among the units of the host interface unit  111 . 
   The DMA function unit  314  has a function of sending and receiving data via the mature network  116 . The mature network interface control unit  315  has a connection path connected to the mature network  116  to send and receive data over the mature network  116 . 
   The memory  317  functions as a cache memory for data sent and received by the host interface unit ( 111 A,  111 B, or  111 C) to which this memory  317  belongs. 
   Each of the MP units  112  has an MP (processor)  321 , a network interface  322 , a memory  323 , and a bridge  324 . 
   The MP (processor)  321  is a processor that handles the majority of processing done by the MP units  112 . 
   The network interface  322  has a connection path connected to the management unit  115 , and exchanges data with the management unit  115 . 
   The memory  323  stores programs executed by the MP  321  and various types of information. 
   The bridge  324  has a connection path connected to the internal bus/SW function unit  313  in one of the host interface units  111  to exchange data with the one of the host interface units  111 . The bridge  324  may not directly be connected to the internal bus/SW function unit  313 . For instance, the bridge  324  may have a connection path connected to the mature network  116  to communicate with its associated host interface via the mature network  116 . Other connection methods may also be employed. 
     FIG. 3  is a block diagram showing detailed configurations of the disk interface units  113  and the MP units  112 . 
   The disk interface units  113  are built similarly to the host interface units  111  described above. To elaborate, each of the disk interface units  113  has a disk interface control unit  319 , a control unit  312 , and a memory  317 . The control unit  312  has an internal bus/SW function unit  313 , a DMA function unit  314 , and a mature network interface control unit  315 . 
   The control unit  312  in each of the disk interface units  113  also has a RAID function unit  316  and an encryption engine  318 . 
   The disk interface control unit  319  has one or more connection paths connected to the disk drive group  102 , and exchanges data with the disk drive group  102 . 
   The RAID function unit  316  implements a RAID function of magnetic disk drives provided in the disk drive group  102 . Through the RAID function, logical volumes are set in the disk drive group  102 . 
   The encryption engine  318  encrypts, with an encryption key, data that passes through the disk interface unit ( 113 A or  113 B) to which this encryption engine  318  belongs. The encryption processing by the encryption engine  318  and management of encryption keys are executed by one of the MP units  112  that is associated with this disk interface unit. In other words, the MP units  112  executing the function of the encryption engine  318  make the encryption function units  117 . 
     FIG. 4  is a block diagram showing a detailed configuration of the memory units  114 . 
   Each of the memory units  114  has a memory  411  and a control unit  416 . 
   The control unit  416  has a memory controller  412 , an internal bus/SW function unit  413 , a DMA function unit  414 , and a mature network interface control unit  415 . 
   The memory  411  is, for example, a RAM and stores data temporarily. 
   The internal bus/SW function unit  413 , the DMA function unit  414 , and the mature network interface control unit  415  respectively have the same functions as the above-described units  313 ,  314 , and  315  in the host interface units  111  or the disk interface units  113 . 
   The memory controller  412  controls reading and writing of data in the memory  411 . 
   Described next is how data is encrypted in this embodiment. 
     FIG. 5  is an explanatory diagram schematically showing processing in which the hosts  104  writes data in the storage system  101 . 
   Logical volumes  00  and  01  are set in the storage system  101 . A logical volume is a logical area that is recognizable as one disk drive to the hosts  104 . The logical volumes are set in advance upon instruction from the management terminal  107  or the like. 
   The actual, physical location of the logical volume  00  is set in plural magnetic disk drives  504 A to  504 H of the disk drive group  102 . The actual, physical location of the logical volume  01  is set in plural magnetic disk drives  505 A to  505 H of the disk drive group  102 . The disk interface unit  113 A accesses the magnetic disk drives  504  whereas the disk interface unit  113 B accesses the magnetic disk drives  505 . 
   The logical volume  00  and the logical volume  01  form a volume pair  503  which implements a mirroring function with the logical volume  00  serving as the primary volume. Data written in the logical volume  00  is also written in the logical volume  01 . As a result, data in the logical volume  01  matches data in the logical volume  00 . 
   Given below is how the hosts  104  operate when writing write data “DT 0 ” in the logical volume  00  set in the storage system  101 . 
   One of the hosts  104  makes a request for write data to the logical volume  00 , and the host interface unit  111 A receives the request. The host interface unit  111 A stores the write data DT 0  in the memory unit  114 A. Set in the memory unit  114 A are cache memory areas corresponding to the logical volumes. The host interface unit  111 A then stores, in the shared memory area set in the memory units  114 , information reporting that the data DT 0  has been written in a cache memory area of the memory unit  114 A. 
   The disk interface units  113 A and  113 B obtain the information from the shared memory, thereby detecting that the write data DT 0  is stored in the memory unit  114 A. Then the disk interface units  113 A and  113 B store the write data DT 0  held in the memory unit  114 A in areas of the disk drive group  102  that are specified in the write request. 
   In storing the write data DT 0 , the disk interface unit  113 A looks up a volume management table to obtain an encryption key for an area of the disk drive group  102  that is specified in the write request, namely, the logical volume  00 . The volume management table holds, as shown in  FIG. 8 , information about which encryption key is used to encrypt which logical volume. 
   Obtaining an encryption key, the disk interface unit  113 A uses the encryption key to encrypt the write data DT 0  in the encryption function unit  117 A. The encrypted data is stored in the area of the disk drive group  102  that is specified in the write request. 
   The disk interface unit  113 B works the similar way and looks up the volume management table to obtain an encryption key for the logical volume  01  as an area of the disk drive group  102  that is specified in the write request. Then the disk interface unit  113 B uses the obtained encryption key to encrypt the write data DT 0  in the encryption function unit  117 B, and stores the encrypted data in the disk drive group  102 . 
     FIG. 6  is a flow chart for data write processing in the storage system  101 . 
   As described with reference to  FIG. 5 , the host interface unit  111 A in the storage system  101  stores the write data DT 0  in a cache memory area of the memory unit  114 A, and stores information to that effect in the shared memory area of the memory units  114 . Data write in the logical volume  00  is thus processed as requested by the write request. The data is also copied to the logical volume  01 , which is paired with the logical volume  00  to form a mirroring pair (S 601 ). 
   Based on the information stored in the shared memory area, the disk interface unit  113 A creates a write task to write the data (S 602 ). 
   The write task makes the following processing executed in the disk interface unit  113 A. 
   First, the volume management table is searched for an entry concerning the logical volume  00 , which is specified in the write request, in order to determine whether to encrypt the logical volume  00  or not. In the case where the logical volume  00  is to be encrypted, an encryption key assigned to the logical volume  00  is obtained. The obtained encryption key is used in the encryption function unit  117 A to encrypt the write data DT 0  (S 603 ). 
   The encrypted data requested to be written is written in the area specified in the write request, whereby the write task is ended (S 604 ). 
   Similarly, the disk interface unit  113 B creates, from the information stored in the shared memory area, a write task to write data (S 605 ). 
   According to the write task, an encryption key is obtained that is assigned to the logical volume  01  specified in the write request, and the write data DT 0  is encrypted with the key (S 606 ). The encrypted data is written in the area specified in the write request, whereby the write task is ended (S 607 ). 
   Write requests made by the hosts  104  are processed as illustrated in the flow chart of  FIG. 6 . Data requested to be written is encrypted if necessary. 
   The processing of  FIG. 6  is actually executed by the MP units  112  of the host interface units  111  or of the disk interface units  113 . The following description continues to give the host interface units  111  or the disk interface units  113  as the implementer of the processing, but it is the MP units  112  that actually execute the processing. Any one of the MP units  112 A to  112 D can take the lead in executing the processing. 
   Described next is how a volume pair is set to logical volumes. 
   There are various ways to set a volume pair. Initial setting is necessary to make a volume pair from two logical volumes that are originally separate logical volumes with one of the two serving as a primary logical volume and the other serving as a secondary logical volume. More specifically, data in the primary logical volume is copied to the secondary logical volume to make the two logical volumes synchronize with each other. This processing is called initial copy processing. 
   The initial copy processing is followed by the processing described with reference to the flow chart of  FIG. 6  in which data is written in both the primary and secondary logical volumes. 
   Data in the primary logical volume is encrypted with an encryption key that is assigned to the primary logical volume. Another encryption key is assigned to the secondary logical volume. 
   Accordingly, initial copy processing is executed in which encrypted data in the primary logical volume is read and decrypted, and the decrypted data is then encrypted with an encryption key that is assigned to the secondary logical volume to be stored in the secondary logical volume. 
     FIG. 7  is a flow chart for processing initially executed in data replication in the storage system  101 . 
   First, the disk interface unit  113 A reads data from the logical volume that is set as the primary volume, and stores the read data in a work area of the memory units  114 , namely, a cache memory area (S 608 ). At this point, the read data is decrypted with an encryption key that is assigned to the primary logical volume, and the decrypted data is stored in the cache memory (S 609 ). 
   Next, the disk interface unit  113 B destages the data stored in the memory units  114  to the logical volume that is set as the secondary logical volume (S 610 ). At this point, the disk interface unit  113 B obtains an encryption key that is assigned to the secondary logical volume, and encrypts this data with the obtained encryption key (S 611 ). The disk interface unit  113 B then stores the encrypted data in an area of the disk drive group  102  that corresponds to the secondary logical volume (S 612 ). 
   Through the processing of  FIG. 7 , data in a logical volume set as the primary logical volume is stored in a logical volume set as the secondary logical volume. The disk interface units  113  look up the volume management table to obtain encryption keys assigned to the primary and secondary logical volumes, decrypt the data with an encryption key assigned to the primary logical volume, and encrypt the data with an encryption key assigned to the secondary logical volume. 
     FIG. 8  is an explanatory diagram of a volume management table. 
   The volume management table is, as mentioned above, a table showing which logical volume uses which encryption key. 
   The volume management table is set in advance by an administrator or the like and stored in the memory units  114  of the storage system  101 . The volume management table can be placed anywhere as long as it is accessible to the encryption function units  117 . For example, the memory  317  of one of the disk interface units  113  may hold the volume management table. 
   The volume management table has a volume encryptable/unencryptable table  710  and an encryption key table  720 . 
   The volume encryptable/unencryptable table  710  contains a number  711 , a logical volume ID  701 , an encryptable/unencryptable field  702 , and an owner ID  703 . 
   The number  711  indicates an identifier given to each entry. The logical volume ID  701  indicates a logical volume name serving as an identifier. The encryptable/unencryptable field  702  holds an identifier indicating whether a logical volume identified by the logical volume ID  701  is to be encrypted or not. “1” held in the encryptable/unencryptable field  702  indicates that this logical volume is to be encrypted whereas “0” held in the encryptable/unencryptable field  702  indicates that this logical volume is not to be encrypted. The owner ID  703  indicates the identifier of an owner accessing this logical volume. For example, the identifiers of the hosts  104  and the identifiers of users of the hosts  104  are stored as the owner ID  703 . 
   The encryption key table  720  contains a number  705 , a logical volume ID  706 , and an encryption key  704 . 
   The number  705  indicates an identifier given to each entry. The logical volume ID  706  indicates a logical volume name serving as an identifier. The encryption key  704  indicates an encryption key assigned to a logical volume that is identified by the logical volume ID  706 . 
   The encryption function units  117  consult the volume management table to judge whether to encrypt a logical volume or not. In the case where the logical volume is to be encrypted, the encryption function units  117  obtain, as a parameter, from among encryption keys held in the column of the encryption key  704 , one that is assigned to this logical volume. The encryption function units  117  encrypt data to be stored in the logical volume with the obtained encryption key. 
   As has been described, in a computer system according to the first embodiment of this invention, data requested by a host to be written in a logical volume is encrypted, before written in the logical volume, with an encryption key that is assigned to this logical volume. Thus different logical volumes store data encrypted with different encryption keys, and the storage system  101  is improved in data security. 
   Second Embodiment 
   A second embodiment of this invention will be described next. 
   In the first embodiment described above, the disk interface units  113  have the encryption function units  117 . The encryption units  117  in the second embodiment are attached to other units (the host interface units  111  or the memory units  114 ) than the disk interface units  113 . In the second embodiment, components identical with those in the first embodiment are denoted by the same reference symbols and descriptions thereof are omitted. 
     FIG. 9  is an explanatory diagram schematically showing processing in which the hosts  104  write data in the storage system  101  in a computer system according to the second embodiment. 
   Logical volumes  00  and  01  are set in the storage system  101 . The logical volumes are set in advance upon instruction from the management terminal  107  or the like. 
   The actual, physical location of the logical volume  00  is set in the magnetic disk drives  504  of the disk drive group  102 . The actual, physical location of the logical volume  01  is set in the magnetic disk drives  505  of the disk drive group  102 . The disk interface unit  113 A accesses the magnetic disk drives  504  whereas the disk interface unit  113 B accesses the magnetic disk drives  505 . 
   The logical volume  00  and the logical volume  01  form a volume pair which implements a mirroring function with the logical volume  00  serving as the primary volume. 
   Given below is how the hosts  104  operate when writing write data “DT 0 ” in the logical volume  00  set in the storage system  101 . 
   One of the hosts  104  makes a write request to write in the logical volume  00 , and the host interface unit  111 A receives the request. 
   The host interface unit  111 A looks up the volume management table to obtain an encryption key for the logical volume  00  specified in the write request. Obtaining an encryption key, the host interface unit  111 A uses the encryption key to encrypt the write data DT 0  in the encryption function unit  117 A. The encrypted write data DT 0  is stored in an area of the memory unit  114 A that is specified in the write request. The host interface unit  111 A then stores, in the shared memory area set in the memory unit  114 A or  114 B, information reporting that the data DT 0  is stored in the memory unit  114 A. 
   At this point, the disk interface unit  113 A obtains the information from the shared memory and detects that the encrypted write data DT 0  is stored in the memory unit  114 A. Then the disk interface unit  113 A stores the write data DT 0  held in the cache memory area in the logical volume  00  as an area of the disk drive group  102  that is specified in the write request. This data has been encrypted with an encryption key that is assigned to the logical volume  00  specified in the write request. 
   The steps of write processing to write in the secondary logical volume  01  are as follows. 
   First, the host interface unit  111 B obtains the information from the shared memory and detects that the encrypted write data DT 0  is stored in the memory unit  114 A. To process the encrypted data DT 0 , the host interface unit  111 B looks up the volume management table to obtain an encryption key for the logical volume  00  specified in the write request. Then the host interface unit  111 B uses the obtained encryption key to decrypt the write data DT 0  in the encryption function unit  117 B. Next, the host interface unit  111 B looks up the volume management table to obtain an encryption key for the logical volume  01  set as the secondary logical volume, and uses the obtained encryption key to encrypt the decrypted data. The encrypted data is stored in a cache memory area of the memory unit  114 B. The host interface unit  111 B then stores, in the shared memory, information reporting that the encrypted data is stored in the memory unit  114 B. 
   The disk interface unit  113 B obtains the information from the shared memory and detects that the encrypted write data DT 0  is stored in the memory unit  114 B. Then the disk interface unit  113 B stores the write data DT 0  held in the cache memory area in the logical volume  01  as an area of the disk drive group  102  that is specified in the write request. The data has been encrypted with an encryption key that is assigned to the logical volume  01  specified in the write request. 
     FIG. 10  is a flow chart for data write processing in the storage system  101  of this embodiment. 
   As described with reference to  FIG. 9 , the host interface unit  111 A in the storage system  101  first looks up the volume management table to obtain an encryption key for a logical volume specified in the write request. Then the host interface unit  111 A encrypts the write data DT 0  with the obtained encryption key (S 901 ). 
   The host interface unit  111 A next stores the encrypted data in a cache memory area of the memory unit  114 A (S 902 ), and stores information to that effect in the shared memory area of the memory units  114 . Data write in the logical volume  00  is thus processed as requested by the write request. 
   The host interface unit  111 B obtains the information from the shared memory and, detecting that the encrypted data is stored in the memory unit  114 B, executes the data replication processing (S 903 ). 
   The host interface unit  111 B first reads the encrypted data from the cache memory area and onto a buffer area set in the memory  317  of the host interface unit  111 B (S 904 ). 
   Next, the host interface unit  111 B looks up the volume management table to obtain an encryption key for the logical volume  00  specified in the write request. The obtained encryption key is used in the encryption function unit  117 B to decrypt the data (S 905 ). 
   The host interface unit  111 B then looks up the volume management table to obtain an encryption key that is assigned to the logical volume  01  set as the secondary logical volume. The obtained key is used to encrypt the decrypted data (S 906 ). The encrypted data is stored in a cache memory area of the memory unit  114 B (S 907 ). The host interface unit  111 B stores, in the cache memory area of the memory unit  114 , information reporting that the encrypted data is stored in the memory unit  114 B. Data write in the logical volume  01  is thus processed as requested by the write request. 
   Write requests made by the hosts  104  are processed as illustrated in the flow chart of  FIG. 10 . Data requested to be written is encrypted if necessary. 
   The encryption function units  117  can thus be attached to the host interface units  111 . With the host interface units  111  having the encryption function units  117 , data sent from the hosts  104  can be encrypted immediately instead of shuttling over the mature network  116  many times, and the load inside the storage system  101  is accordingly lessened. The load inside the storage system  101  is lessened also when encrypted data is to be stored in the external disk drive group  103 , since data is encrypted/decrypted by the host interface units  111  which directly communicate with the external disk drive group  103 . 
   Described next as a modification example of the second embodiment is a case in which the memory units  114  have the encryption function units  117 . 
     FIG. 11  is an explanatory diagram schematically showing processing in which the hosts  104  write data in the storage system  101  in a computer system according to a modification example of the second embodiment. 
   As described above, the host interface unit  111 A processes a write request from the hosts  104 , and stores write data in a cache memory area of the memory unit  114 A. Before stored in the memory unit  114 A, the write data is encrypted in the encryption function unit  117 A with an encryption key assigned to a logical volume in which the write data is to be written. 
   More specifically, prior to storing the write data in the cache memory area, the host interface unit  111 A looks up the volume management table to obtain an encryption key for the logical volume  00  specified in the write request. The obtained encryption key is used to encrypt the write data in the encryption function unit  117 A. The encrypted data is then written in the requested area of the disk drive group  102  by the disk interface unit  113 A as described above. 
   The disk interface unit  113 B looks up the volume management table to obtain an encryption key, with regard to the encrypted data stored in the cache memory area, for the logical volume  00  specified in the write request. The obtained encryption key is used in the encryption function unit  117 B to decrypt the encrypted data stored in the cache memory area of the memory unit  114 A. The decrypted data is stored in a cache memory area of the memory unit  114 B. The disk interface unit  113 B then looks up the volume management table for an encryption key that is assigned to the logical volume  01  set as the secondary logical volume. The obtained encryption key is used to encrypt the decrypted data, and the encrypted data is stored in a cache memory area of the memory unit  114 B. Thereafter, the disk interface unit  113 B stores the encrypted data held in the cache memory area in an area of the disk drive group  102  that is specified in the write request. 
   The function of the encryption function units  117  can be controlled either by the MP units  112  of the host interface units  111  or by the MP units  112  of the disk interface units  113 . 
     FIG. 12  is a block diagram showing the configuration of the memory units  114  of this embodiment. 
   The configuration of the memory units  114  in  FIG. 12  is the same as the memory unit configuration shown in  FIG. 4 , except an encryption engine  417  which is not included in any memory unit of the first embodiment. This means that the MP units  112  of the host interface units  111  or the MP units  112  of the disk interface units  113  processing the function of the encryption engine  417  make the encryption function units  117 . 
   The encryption function units  117  can thus be attached to the memory units  114  of the storage system  101 . With the memory units  114  having the encryption function units  117 , data encryption/decryption can be processed inside-cache memory areas without using the band of the mature network  116 . 
   Third Embodiment 
   A third embodiment of this invention will be described next. 
   The third embodiment deals with processing of a copy pair made up of logical volumes in a computer system according to the first or second embodiment. In the third embodiment, components identical with those in the first embodiment are denoted by the same reference symbols and descriptions thereof are omitted. 
     FIG. 13  is an explanatory diagram schematically showing processing in which the hosts  104  write data in the storage system  101  in a computer system according to the third embodiment. 
   In the storage system  101  of this embodiment, the host interface units  111  have the encryption function units  117  as in the second embodiment. The encryption function units  117  may instead be attached to the disk interface units  113  or the memory units  114 . 
   The storage system  101  of this embodiment has three logical volumes  00 ,  01  and  02 . 
   The actual, physical location of the logical volume  00  is set in the magnetic disk drives  504  of the disk drive group  102 . The actual, physical location of the logical volume  01  is set in the magnetic disk drives  505  of the disk drive group  102 . The actual, physical location of the logical volume  02  is set in magnetic disk drives  506  of the disk drive group  103 , which is external to the storage system  101 . The disk interface unit  113 A accesses the logical volume  00  and the disk interface unit  113 B accesses the logical volume  01 . The logical volume  02  is accessed by the host interface unit  111 B. 
   The logical volumes form copy pairs through a mirroring function. Specifically, the logical volume  00  is paired with the logical volume  01  to form a copy pair. The logical volume  00  is paired with the logical volume  02  to form another copy pair. 
   In the first or second embodiment, different encryption keys are prepared for two logical volumes forming a copy pair. In this embodiment, logical volumes paired as a copy pair use the same encryption key when the copy pair is in a synchronized state (called Sync, PAIR, or Mirror Active). This is based on a view that using the same encryption key for two logical volumes that are paired as a copy pair and in a synchronized state raises no security problems since the two logical volumes store the same data. 
   After a copy pair in a synchronized state is broken up (called PAIR DELETE or SIMPLEX), or after a copy pair enters a suspended state (called Mirror Split or Mirror Brake), write request data that is received subsequently is encrypted with a new encryption key. 
   The operation of the computer system according to this embodiment is described below. 
   When one of the hosts  104  makes a write request to write data in the logical volume  00 , the storage system  101  encrypts the requested data with an encryption key that is assigned to the logical volume  00  specified in the write request, and stores the data in the logical volume  00  as in the second embodiment. 
   At the same time, the requested data is encrypted with the same encryption key that is assigned to the logical volume  00 , namely, Key 0 , and the encrypted data is stored in the logical volume  01 , which is paired with the logical volume  00  as a copy pair. Similarly, the requested data is encrypted with the same encryption key Key 0  and stored in the logical volume  02 . 
   At this point, a Mirror Split command is issued to the storage system  101 , thereby setting the copy pair made up of the logical volume  00  and the logical volume  01  to a suspended state ( 1104  in  FIG. 13 ). What follows is a description on how the storage system  101  operates in this case. 
   Detecting that the copy pair has entered a suspended state ( 1104 ), the storage system  101  changes the encryption key of the logical volume  00  on the primary side to another encryption key. The logical volume  01  on the secondary side continues to store the data encrypted with the former encryption key Key 0  since the copy pair is no longer in a synchronized state. 
   The copy pair made up of the logical volume  02  and the logical volume  00  is still in a synchronized state, and the logical volume  02  changes its encryption key at the same time the logical volume  00  on the primary side changes its own. 
   In the case where a write request is made to write data in the logical volume  00  after the encryption key change, the write data is encrypted with the new, replacement encryption key, Key 1 , and the encrypted data is stored in the logical volume  00 . Similarly, the write data encrypted with the encryption key Key 1  is stored in the logical volume  02 . 
     FIG. 14  is a flow chart for data write processing in the storage system  101  of this embodiment. Here, two logical volumes are already paired as a copy pair and in a synchronized state. 
   First, it is detected whether the copy pair has moved from a synchronized state to other states. In a step S 1201 , it is judged whether or not the copy pair has been broken up (DELETE). In a step S 1202 , it is judged whether or not the copy pair is in a suspended state (SUSPEND). 
   When it is judged that the copy pair has moved from a synchronized state, a copy pair state change flag is set and the processing moves to a step S 1205 . 
   When it is judged that the copy pair has not moved from a synchronized state, the processing moves to a step S 1203 , where the volume management table is consulted to encrypt data with an encryption key that is assigned to a logical volume specified in the write request. The encrypted data is stored in a cache memory area of the memory units  114  (S 1204 ). 
   In the step S 1205 , a different encryption key is created. The created encryption key is assigned to the logical volume specified in the write request. Then the created encryption key and the ID of this logical volume are registered in the volume management table (S 1206 ). At this point, a new encryption key registration flag is set whereas the copy pair state change flag is reset. This prevents encryption key updating processing from being performed each time write processing is executed. 
   As the processing of the step S 1206  is executed, the write data is encrypted with the newly set encryption key (S 1207 ). 
   The encrypted data is stored in a cache memory area of the memory units  114  (S 1204 ). 
   After the step S 1204  is finished, a write task to write in the primary logical volume is created (S 1208 ), and a write task to write in the secondary logical volume is created (S 1212 ). 
   Thereafter, it is judged whether there have been uncoordinated encryption key changes or not (S 1209 ). In the step S 1209 , the encryption key is checked once more before the processing of transferring the encrypted data from a cache memory area to the disk drive group  102  in case a failure in one of the magnetic disk drives causes a sudden change in copy pair state. Whether there have been uncoordinated encryption key changes or not is judged from the presence or absence of a new encryption key registration flag. 
   When it is judged that the encryption key has not been changed and that there have been no uncoordinated key changes, the processing of storing the encrypted data in the disk drive group  102  is immediately executed, and then the processing is ended. 
   On the other hand, when the encryption key has been changed to another encryption key and that there have been uncoordinated key changes, the volume management table is again searched for an encryption key and the data is encrypted with the obtained encryption key. At this point, the data is decrypted before encrypted, if necessary (S 1210 ). 
   Then a write task to write in the primary logical volume is newly created (S 1211 ), whereby the processing is ended. 
   In this way, write data received after the copy pair state is changed is encrypted with an encryption key different from the one used prior to the copy pair state change. The key updating processing of the step S 1205  is executed only once when a copy pair state change is detected first. In the first step S 1212 , write processing to write in the secondary volume is kept as data write processing to be copied when the copy pair in a suspended state returns to a synchronized state in the future (differential data management). This processing is in general called Resync. 
     FIG. 15  is an explanatory diagram of a volume management table according to this embodiment. 
   The volume management table of this embodiment is similar to that of the first embodiment described with reference to  FIG. 8 , except that an area ID  1305  is added to the encryption key table  720  in this embodiment. 
   The area ID  1305  is provided for finer classification of encryption key information. The range, or size, of an “area” can be chosen suitably. Logical Block Address (LBA), for example, is employed as the area ID  1305 . This makes it possible to assign an encryption key to a specific area of a logical volume. 
   The volume management table of this embodiment has more than one encryption key table  720  per logical volume. As described above, an encryption key assigned to a logical volume is replaced with another encryption key when there is a change in copy pair state. The replaced encryption key is kept as a history in the encryption key table  720 . For instance, when a copy pair shifts from a synchronized state to a suspended state, the current encryption key is updated and the old encryption key is kept in the history section of the encryption key table. When the copy pair shifts from a suspended state to a resynchronized state, the history section is searched to obtain the pre-update encryption key. 
   The processing according to the first through third embodiments may be employed individually or in combination. For instance, the copy function of the first embodiment may be combined with the copy function of the third embodiment. In this case, when key management information is increased in amount by the use of, for example, the volume management table shown in  FIG. 15  and the increased information exceeds preset capacity limit (e.g., xx MB), the key management method is switched to the one in the first embodiment ( FIG. 8 ). An unlimited increase in amount of key management information can thus be avoided. 
   Another way to mix the embodiments is through deciding which component is to have the encryption function, and the encryption function units  117  may be attached to different types of components. This makes it possible to choose from among the encryption function units  117  in the host interface units  111 , the encryption function units  117  in the disk interface units  113 , and the encryption function units  117  in the memory units  114  in accordance with the traffic on mature network band. 
   The methods in the above embodiments can be combined in various other ways. Switching from one method to another is achieved by, for example, setting a method switch flag, which is consulted by the MP units  112  to process in accordance with the encryption key management method currently chosen. 
   Fourth Embodiment 
   A fourth embodiment of this invention will be described next. 
   The fourth embodiment deals with processing related to a snapshot function in a computer system according to the first or second embodiment. In the fourth embodiment, components identical with those in the first through third embodiments are denoted by the same reference symbols and descriptions thereof are omitted. 
   With a snapshot function, data changed as a result of write requested by the hosts  104  (differential data) is stored without making the change reflected on the original data prior to the write processing. Thereafter, when a given operation is made (snapshot command), the differential data is made reflected on the original data for data update. 
   There are two types of snapshot function, Redirect-on-Write (also referred to as RoW) and Copy-on-Write (also referred to as CoW). 
   CoW is described first. 
   According to CoW, when a snapshot command is issued, data written before the snapshot command is stored in a new, different area serving as a shelter area (e.g., a different logical volume). Data requested to be written after the snapshot command is issued is written in the former area. 
     FIG. 16  is an explanatory diagram schematically showing processing in which the hosts  104  write data in the storage system  101  in a computer system according to the fourth embodiment. 
   In the storage system  101  of this embodiment, the host interface units  111  have the encryption function units  117  as in the second embodiment. The encryption function units  117  may instead be attached to the disk interface units  113  or the memory units  114 . 
   The storage system  101  encrypts data requested by the hosts  104  to be written, and stores the encrypted data in a cache memory area of the memory units  114  as described above. The data is then stored in a given logical volume by the disk interface units  113 . 
   At this point, a snapshot command is issued from the hosts  104 . What follows is a description on how the snapshot command is processed by RoW in this case. 
   The storage system  101  detects that a snapshot command has been issued from the host  104 . When a write request is received from the hosts  104  subsequently, the storage system  101  changes an encryption key that is assigned to a logical volume specified in the write request to a new encryption key. The new encryption key is used to encrypt requested data, and the encrypted data is written in the logical volume. 
   Data DT 0 , which is stored before the snapshot command, is moved to a newly set shelter area. Here, Area One is set as a shelter in the external disk drive group  103  and the data DT 0  is stored in Area One. 
   As a result, write data received after the snapshot command is issued is encrypted with a different encryption key whereas data preceding the snapshot command is moved to a shelter area. 
     FIG. 17  is a flow chart for data write processing in the storage system  101  of this embodiment. 
   The storage system judges whether a snapshot command has been issued or not (S 1501 ). When it is judged that a snapshot command has been issued, the processing moves to a step S 1504 . When it is judged that a snapshot command has not been issued, the processing moves to a step S 1502 , where the volume management table is consulted to encrypt data with an encryption key that is assigned to a logical volume specified in the write request. The encrypted data is stored in a cache memory area of the memory units  114  (S 1503 ). 
   In the step S 1504 , a different encryption key is created. The created encryption key is assigned to the logical volume specified in the write request. Then the created encryption key and the ID of this logical volume are registered in the volume management table (S 1505 ). At this point, a new encryption key registration flag is set. Then the write data is encrypted with the newly set encryption key (S 1506 ). 
   The encrypted data is stored in a cache memory area of the memory units  114  (S 1503 ). 
   After the step S 1503  is finished, a write task to write in the primary logical volume is created (S 1507 ). 
   Thereafter, it is judged whether there have been uncoordinated encryption key changes or not (S 1508 ). In the step S 1508 , the encryption key is checked once more before the processing of transferring the encrypted data from a cache memory area to the disk drive group  102  in case a failure in one of the magnetic disk drives causes a sudden change in copy pair state. Whether there have been uncoordinated encryption key changes or not is judged from the presence or absence of a new encryption key registration flag. 
   When it is judged that the encryption key has not been changed and that there have been no uncoordinated key changes, the processing of storing the encrypted data in the disk drive group  102  is immediately executed, and then the processing is ended. 
   On the other hand, when the encryption key has been changed to another encryption key and that there have been uncoordinated key changes, the volume management table is again searched for an encryption key and the data is encrypted with the obtained encryption key. At this point, the data is decrypted before encrypted, if necessary (S 1509 ). 
   Then a write task to write in the primary logical volume is newly created (S 1510 ), whereby the processing is ended. 
     FIG. 18  is a flow chart for another example of data write processing in the storage system  101  of this embodiment. 
   As described above, write data requested after a snapshot command is issued is encrypted with a new encryption key, so that different encryption keys are used for post-snapshot data and pre-snapshot data (old data). 
   In an alternative method, old data, which has been encrypted once with an encryption key, is re-encrypted with a different encryption key when a snapshot command is issued and the re-encrypted data is stored in a shelter area whereas the former encryption key is kept used for write data requested after the snapshot command is issued. 
   The storage system  101  executes processing of this flow chart when judging that a snapshot command has been issued. 
   First, the storage system  101  obtains data prior to the snapshot command (old data) from the primary logical volume, namely, a logical volume specified in the write request of the hosts  104 , and stores the old data in a cache memory area of the memory units  114  (S 1601 ). 
   Next, the volume management table is looked up for an encryption key assigned to the logical volume where the old data has been stored. The obtained encryption key is used in the encryption function units  117  to decrypt the old data (S 1602 ). 
   Processing executed next is to store the decrypted data in a shelter area (S 1603 ). First, the volume management table is searched for an encryption key assigned to a logical volume that contains the shelter area. The obtained encryption key is used to encrypt the old data (S 1604 ). 
   The encrypted data is then written in the logical volume that contains the shelter area (S 1606 ). 
   Through this processing, data preceding the snapshot command is encrypted with a different encryption key and moved to a shelter area. Write data requested after the snapshot command is issued is encrypted with the encryption key that has been used from before the snapshot command. 
   Thus, in the fourth embodiment of this invention, old data prior to a snapshot command and data written after the snapshot command are encrypted with different encryption keys. 
   This embodiment is also applicable to RoW described above. 
   In RoW, when a snapshot command is issued, data written before the snapshot command (old data) remains stored in its original area whereas write data requested after the snapshot command is written in a new area (for example, a different logical volume). 
   RoW does not include copying old data for the evacuation purpose. Instead, new data is written in a different location from old data, and only the data location management pointer for the original logical volume is updated (Redirect) with new location information. This method is therefore applicable to a case where the focus is only on data that is newly written. In an alternative method, concurrently with new update write, old data, which has been encrypted once with an encryption key, is re-encrypted with a different encryption key and re-written in the same place. 
   In this case, the steps S 608  to S 612  of  FIG. 6  are executed but in the last step the data is written in its original place. 
   Described next as a modification example of this embodiment is a snapshot function employing a journal. 
   Here, when a write request is made, update data of old data is stored in time-series in an area called a journal (or log). The update data is made reflected on the old data at a subsequent point in time, for example, when a snapshot command is issued, thereby creating a version of old data at the time of the snapshot. Write data requested after that is stored in the journal as update data of this old data. 
   There are two major ways to process a journal. 
   One is to record old data in a journal each time update data is made reflected on the old data. This method is called before image journal. The other is to record only update data one piece at a time in a journal. This method is called after image journal. 
   Operations of storing data in a journal are managed with an area different from the primary volume set as the journal (for example, the secondary volume is set as a journal). Encryption keys therefore should be managed appropriately by the methods described in the first through third embodiments. 
   Now, a description is given on processing of restoring a version of old data at a specific point in time from old data stored in a journal. 
     FIG. 19  is a flow chart for before image journal method data restoring processing. 
   In the before image journal method, update data is encrypted with an encryption key assigned to an area where it is stored. 
   First, the disk interface units  113  read a version of old data to be restored from the journal, and stores the read data in a cache memory area of the memory units  114  (S 1901 ). The read data may be stored in the memory unit  323  of one of the MP units  112  that takes the lead in the processing in addition to the memory units  114 . 
   Next, the read old data is decrypted (S 1902 ). Specifically, the disk interface units  113  look up the volume management table for an encryption key that is assigned to the read old data. The obtained encryption key is used in the encryption function units  117  to decrypt the read old data. 
   Processing executed next is to write the decrypted old data in an area for restoration (S 1903 ). 
   First, the disk interface units  113  search the volume management table for an encryption key assigned to the area for restoration (S 1904 ). Data in the area for restoration may be destaged directly to the primary logical volume, which receives a write request, or may be destaged to the secondary logical volume, which receives a copy (snapshot) from the primary logical volume. 
   Next, the obtained encryption key is used in the encryption function units  117  to encrypt the old data (S 1905 ). 
   The encrypted data is written in the area for restoration by the disk interface units  113  (S 1906 ). 
   Through the above processing, a version of old data is restored in an area for restoration. 
   In the processing of  FIG. 19 , the processing of reading old data (S 1901  to S 1903 ) and the processing of writing restored data (S 1904  to S 1906 ) can be executed asynchronously and independently of each other. Accordingly, all pieces of old data up to the time point to be restored may be read and decrypted at once before executing the data restoring processing. 
   A case where the after image journal method is employed will be described next. 
   A snapshot taken in the after image journal method is a base image, which is a snapshot of the primary volume at a point preceding an arbitrary time point to be restored that is obtained in advance. 
   Processing for this case is basically the same as the one shown in  FIG. 19 . Specifically, old data is read from a restoration target area (base image) (S 1901 ), the read data is decrypted (S 1902 ), and the decrypted data is re-encrypted to be written again (S 1904  to S 1906 ). 
   In the after image journal method, the journal, the primary volume, and the restoration target area (a snapshot of the primary volume or the primary volume itself) are managed as different areas with different encryption keys. This way, the original primary volume, log and restoration volume are respectively encrypted with appropriate encryption keys, thereby enhancing the security. 
   Fifth Embodiment 
   A fifth embodiment of this invention will be described next. 
   The above embodiments describe processing inside the storage system  101 . This embodiment describes cooperation between the storage system  101  and an external disk controller. In this embodiment, components identical with those in the first embodiment are denoted by the same reference symbols and descriptions thereof are omitted. 
     FIG. 20  is an explanatory diagram schematically showing processing in which the hosts  104  access data in the disk drive group  103 . 
   More specifically, illustrated in  FIG. 20  is how the computer system operates when the host  104 C accesses encrypted data in a logical volume set in the disk drive group  103 , which is connected to the host  104 C via the network  105 . The secondary logical volume is set in the disk drive group  103  through the replication function. 
   The host  104 C in this case needs to know encryption key information that the storage system  101  has. Therefore, encryption key management information  1720  is stored in the memory unit  114 A of the storage system  101 . The encryption key management information  1720  contains a volume management table shown in  FIG. 21 . 
   To access data DT 0  in the disk drive group  103 , the host  104 C consults the encryption key management information  1720  to obtain an encryption key assigned to an area where the data DT 0  is stored. The host  104 C uses the obtained encryption key to encrypt or decrypt the data. 
   In the case where the disk drive group  103  has an encryption function as does the storage system  101 , it is the disk drive group  103 , instead of the host  104 C, that consults the encryption key management information  1720  when the host  104 C accesses. The disk drive group  103  obtains, from the encryption key management information  1720 , an encryption key assigned to an area where the data DT 0  is stored, uses the obtained encryption key to encrypt or decrypt the data, and hands over the encrypted or decrypted data to the host  104 C. 
   In order to enable an external device to access or receive the encryption key management information  1720 , the storage system  101  has to be equipped with a communication measure. 
   The storage system  101  therefore has a measure that gives the management terminal  107  secure access to the encryption key management information  1720  via the network  106  (communication path encryption such as SSL or IPsec). 
   Also, a communication measure for permitting access to the encryption key management information  1720  is provided between the storage system  101  and the hosts  104  or the disk drive group  103 . 
   The hosts  104  request these communication measures for permission. When permission is obtained, the hosts  104  use a special communication measure to request the storage system  101  for access to the encryption key management information  1720  via the network  105 . The storage system  101  consults, via the management unit  115 , access permission information, which is information set by the management terminal  107  to show under what conditions the hosts  104  are granted access. When the hosts  104  meet the access granting conditions, the encryption key management information  1720  is sent to the hosts  104 . The host computers  104  thus obtain encryption key information and can now access encrypted data in the disk drive group  103 . 
     FIG. 21  is an explanatory diagram of the volume management table contained in the encryption key management information  1720  of this embodiment. 
   The volume management table of this embodiment has, in addition to the items of the volume management table described with reference to  FIG. 8  or  15 , a logical volume ID  1807 , a device ID  1808  and a practical logical volume ID  1809 . 
   The logical volume ID  1807  indicates a logical volume identifier set in the storage system  101 . The device ID indicates an identifier given to each device constituting the disk drive group  103 . The practical logical volume ID  1809  indicates a logical volume identifier that is used by and within the disk drive group  103 . 
   Thus, in the fifth embodiment, equipping the storage system  101  with a measure that allows access to the encryption key management information  1702  makes it possible to execute processing of encrypting or decrypting data in a logical volume of the disk drive group  103  externally connected. The processing in this case is similar to the one described in the first through fourth embodiments. 
   While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.