A mobile device comprising: a storage configured to store encrypted content which is generated by encrypting content, and to store a first partial decryption key allocated to the mobile device, wherein the stored first partial decryption key is one of a plurality of partial decryption keys which is split from a decryption key; a communicator configured to communicate with at least one external device (200, 300) and to receive a partial decryption key from each of the at least one external device (200, 300), wherein each received partial decryption key is one of the plurality of partial decryption keys which is split from a decryption key; and a controller configured: in response to determining that the mobile device is located within a proximate spacing of the at least one external device (200, 300), to perform an operation of recovering the decryption key from the stored first partial decryption key and the at least one received partial decryption key; in response to the determining that the mobile device becomes located outside the proximate spacing during a time period when the operation of recovering the decryption key proceeds, to stop the operation of recovering the decryption key; in response to determining that the mobile device is located within the proximate spacing and the decryption key has been recovered, to perform an operation of decrypting the encrypted content that is shared by the mobile device and at least one external device (200, 300), by using the recovered decryption key; and in response to determining that the mobile device becomes located outside the proximate spacing during a time period when the operation of decrypting the encrypted content proceeds, to stop the operation of decrypting the encrypted content and re-encrypt the decrypted content. a storage configured to store encrypted content which is generated by encrypting content, and to store a first partial decryption key allocated to the mobile device, wherein the stored first partial decryption key is one of a plurality of partial decryption keys which is split from a decryption key; a communicator configured to communicate with at least one external device (200, 300) and to receive a partial decryption key from each of the at least one external device (200, 300), wherein each received partial decryption key is one of the plurality of partial decryption keys which is split from a decryption key; and a controller configured: in response to determining that the mobile device is located within a proximate spacing of the at least one external device (200, 300), to perform an operation of recovering the decryption key from the stored first partial decryption key and the at least one received partial decryption key; in response to the determining that the mobile device becomes located outside the proximate spacing during a time period when the operation of recovering the decryption key proceeds, to stop the operation of recovering the decryption key; in response to determining that the mobile device is located within the proximate spacing and the decryption key has been recovered, to perform an operation of decrypting the encrypted content that is shared by the mobile device and at least one external device (200, 300), by using the recovered decryption key; and in response to determining that the mobile device becomes located outside the proximate spacing during a time period when the operation of decrypting the encrypted content proceeds, to stop the operation of decrypting the encrypted content and re-encrypt the decrypted content. in response to determining that the mobile device is located within a proximate spacing of the at least one external device (200, 300), to perform an operation of recovering the decryption key from the stored first partial decryption key and the at least one received partial decryption key; in response to the determining that the mobile device becomes located outside the proximate spacing during a time period when the operation of recovering the decryption key proceeds, to stop the operation of recovering the decryption key; in response to determining that the mobile device is located within the proximate spacing and the decryption key has been recovered, to perform an operation of decrypting the encrypted content that is shared by the mobile device and at least one external device (200, 300), by using the recovered decryption key; and in response to determining that the mobile device becomes located outside the proximate spacing during a time period when the operation of decrypting the encrypted content proceeds, to stop the operation of decrypting the encrypted content and re-encrypt the decrypted content.