To provide an encryption/decryption processing method for a shared encryption file and a program thereof which can encrypt and decrypt of a shared encryption file shared among a plurality of clients without performing troublesome processing such as update of an encryption key and re-encryption of the encrypted file in operations of creating/renaming/deleting the shared encryption file or in transition to another file server.

A key is assigned for each shared encryption folder, and all files subordinate to each folder are encrypted with the key. A marker file indicating that the folder is encrypted is created subordinately to each of all the folders, and an identifier for uniquely identifying the key used for encryption is described in the marker file. A client decides a key to be used on the basis of an identifier acquired from the marker file, and encrypts or decrypts files.

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