Abstract:
A method of image forming comprises the steps of: converting image data into coded data with a code string using a coding key; forming a small pattern string on a sheet of printing paper by printing small patterns, each small pattern corresponding to a code constructing the coded data; recording a coding key or data corresponding to the coding key on a recording medium combined with the printing paper; advising a user to input a decoding key; reading out recorded data from the recording medium; determining whether the decoding key corresponds with the recorded data; scanning the small pattern string recorded on the printing paper when the decoding key corresponds with the recoded data; converting the small pattern string scanned from the printing paper into the code string; decoding the code string into the original image data using the decoding key; and printing an image on a sheet of printing paper based on the image data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming method and an image forming apparatus which may be applied to/as a printer, a copier, a facsimile, an MFP (Multi-Function Peripheral) and so forth, and which in particular can form a code pattern on a manuscript and decode the code pattern. 
     2. Background Information 
     One conventional example in the related art is Japanese Laid-Open Patent Application No. 2002-175 (hereinafter to be referred to as Patent Reference 1). Patent Reference 1 introduces a method in which image data is converted into a code pattern before being printed out so that contents confidentiality may be protected even when the printing media is left behind. 
     In this case, however, if wrong decoding keys are inputted in decoding the coded data, illegible contents will be printed out, and this will be a waste of printing paper. Moreover, if a weak coding system is used, it may be possible that the decoding keys get figured out by relating the inputted keys to the outputted contents, and this may lead to a case of information leakage. In addition, even when correct decoding keys are inputted, if the coded data is put through an image forming apparatus which is inapplicable for decoding that code pattern, i.e. an image forming apparatus using different coding system etc., illegible contents will be printed out as in the case of inputting wrong decoding keys. 
     As for other conventional examples in the related art, there are Japanese Laid-Open Patent Application No. 2004-088586 (hereinafter to be referred to as Patent Reference 2) and Japanese Laid-Open Patent Application No. 2002-337426 (hereinafter to be referred to as Patent Reference 3). 
     Patent Reference 2 introduces a method of imposing copy restriction by attaching an IC chip storing data of a copy inhibit area to a manuscript. If copying of this manuscript with the IC chip is attempted, the inhibit area is changed into a blank part before printing. 
     In this case, however, since the contents printed on the manuscript are not coded, there is a possibility that information leakage occurs when the manuscript is left behind. In addition, if the IC chip is detached from the manuscript, there will be no copy restriction. 
     Patent Reference 3 introduces a method in which an RFID (Radio Frequency Identification) tag with software written on is attached to an edge of a sheet of printing paper on which a manual of the software is also printed. Therefore, Patent Reference 3 introduces a technology that enables the software and its manual to be put together, but it does not show any measure against leakage of confidential information. 
     In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming method and an improved image forming apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to resolve the above-described problems and to provide an image forming method and an image forming device which can prevent leakage of confidential information written on a manuscript in case when the manuscript is left on a manuscript table of the image forming device or other place, and which can prevent unnecessary image forming on printing paper due to incorrect inputs of decoding keys. 
     It is also an object of the present invention to provide an image forming method and an image forming device which can prevent coded documents from being figured out in case when a manuscript is left on a manuscript table of the image forming device or other place. 
     It is another object of the present invention to provide an image forming method and an image forming device which can immediately determine whether coded data that a user is trying to decode is decodable with that image forming device, and thereby can save unnecessary efforts and time in acquiring legible document. 
     In accordance with a first aspect of the present invention, a method of image forming comprises the steps of: converting image data into coded data with a first code string using a coding key; forming a first small pattern string on a sheet of printing paper by printing first small patterns on the printing paper, each first small pattern corresponding to a code constructing the coded data; recording a coding key or data corresponding to the coding key on a first recording medium combined with the printing paper; advising a user to input a decoding key from an inputting means; reading out recorded data from the first recording medium; determining whether the decoding key inputted from the inputting means corresponds with the recorded data read out from the first recording medium; scanning the first small pattern string recorded on the printing paper when the decoding key inputted from an inputting means corresponds with the recoded data read out from the first recording medium; converting the first small pattern string scanned from the printing paper into the first code string; decoding the first code string into the original image data using the decoding key; and printing an image on a sheet of printing paper based on the image data. 
     In accordance with a second aspect of the present invention, a method of image forming comprises the steps of: converting image data into coded data with a first code string using a coding key; forming a first small pattern string on a sheet of printing paper by printing first small patterns on the printing paper, each first small pattern corresponding to a code constructing the coded data; converting each code included in a second code string of the coding key or data corresponding to the coding key into second small patterns; forming a second small pattern string by printing the second small patterns on a predetermined region of the printing paper; advising a user to input a decoding key from an inputting means; scanning the second small pattern string from the predetermined region of the printing paper; obtaining the coding key or the data corresponding to the coding key by converting the second small pattern string scanned from the predetermined region into a code string; determining whether the decoding key inputted from the inputting means corresponds with the coding key or the data corresponding to the coding key obtained by converting the second small pattern string scanned from the predetermined region into the code string; scanning the first small pattern string printed on the printing paper when the decoding key inputted from the inputting means corresponds with the coding key or the data corresponding to the coding key obtained by converting the second small pattern string scanned from the predetermined region into the code string; converting the first small pattern string scanned from the printing paper into the first code string; decoding the first code string into the original image data using the decoding key; and printing an image on a sheet of printing paper based on the image data. 
     In accordance with a third aspect of the present invention, an image forming apparatus comprises a manuscript table, a scanner, an electric tag reader, a paper feeding tray, a printing unit, a paper feeder, an electric tag writer, an inputting unit, and a controller. The scanner scans image on a sheet of printing paper placed on the manuscript table. The electric tag reader is arranged on the manuscript table. The paper feeding tray stocks one or more sheet of printing paper. The printing unit prints an image on a sheet of printing paper. The paper feeder feeds the printing paper to the printing unit and discharging the printing paper with the printed image to a discharge tray. The electric tag writer is arranged on a paper carrier path. The inputting unit is a structure for a user to input a decoding key. The controller switches between a coding mode and a decoding mode. In the coding mode, the controller converts image data into coded data with a first code string using a coding key, makes the printing unit form a first small pattern string on a sheet of printing paper by printing first small patterns on the printing paper, and makes the electric tag writer record the coding key or data corresponding to the coding key in a first recording medium combined with the printing paper. Each first small pattern corresponds to a code constructing the coded data. In the decoding mode, the controller makes the electric tag reader read out recorded data from the first recording medium, determines whether the decoding key inputted from the inputting unit corresponds with the recorded data read out from the first recording medium, makes the scanner scan the first small pattern string recorded on the printing paper when the decoding key inputted from the inputting unit corresponds with the recoded data read out from the first recording medium, converts the first small pattern string scanned from the printing paper into the first code string, decodes the first code string into the original image data using the decoding key, and makes the printing unit print an image on a sheet of printing paper based on the image data. 
     In accordance with a fourth aspect of the present invention, an image forming apparatus comprises a manuscript table, a scanner, a paper feeding tray, a printing unit, a paper feeder, an inputting unit, and a controller. The scanner for scans image on a sheet of printing paper placed on the manuscript table. The paper feeding tray stocks one or more sheet of printing paper. The printing unit prints an image on a sheet of printing paper. The paper feeder feeds the printing paper to the printing unit and discharging the printing paper printed with the printed image to a discharge tray. The inputting unit is a structure for a user to input a decoding key. The controller switches between a coding mode and a decoding mode. In the coding mode, the controller converts image data into coded data with a first code string using a coding key, makes the printing unit form a first small pattern string on a sheet of printing paper by printing first small patterns on the printing paper, converts each code included in a second code string of the coding key or data corresponding to the coding key into second small patterns, and makes the printing unit form a second small pattern string by printing the second small patterns on a predetermined region of the printing paper. Each first small pattern corresponding to a code constructing the coded data. In the decoding mode, the controller makes the scanner scan the second small pattern string from the predetermined region of the printing paper, obtains the coding key or the data corresponding to the coding key by converting the second small pattern string scanned from the predetermined region into a code string, determines whether the decoding key inputted from the inputting means corresponds with the coding key or the data corresponding to the coding key obtained by converting the second small pattern string scanned from the predetermined region into the code string, makes the scanner scan the first small pattern string printed on the printing paper when the decoding key inputted from the inputting means corresponds with the coding key or the data corresponding to the coding key obtained by converting the second small pattern string scanned from the predetermined region into the code string, converts the first small pattern string scanned from the printing paper into the first code string, decodes the first code string into the original image data using the decoding key, and makes the printing unit print an image on a sheet of printing paper based on the image data. 
     These and other objects, features, aspects, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG. 1  is a diagram showing a schematic structure of an image forming system according to an embodiment of the present invention; 
         FIG. 2  is an overhead view of a sheet of printing paper having an electronic tag attached to an edge of it according to the embodiment of the present invention; 
         FIG. 3  is an operation screen view for setting up the image forming apparatus according to the embodiment of the present invention; 
         FIG. 4  is an operation screen view for setting up the image forming apparatus according to the embodiment of the present invention; 
         FIG. 5  is an operation screen view for setting up the image forming apparatus according to the embodiment of the present invention; 
         FIG. 6  is a flowchart showing processes to be conducted by a system controller of the image forming apparatus according to the embodiment of the present invention; 
         FIG. 7  is a diagram representing a correspondence between 4-bit hexadecimal codes ‘0’ to ‘F’ and small patterns according to the embodiment of the present invention; 
         FIG. 8  is a pattern diagram representing an image of a manuscript which has been printed out in a coding mode according to the embodiment of the present invention; 
         FIG. 9  is an flowchart showing the processes to be conducted by the system controller of the image forming apparatus according to the embodiment of the present invention; 
         FIG. 10  is a pattern diagram representing an image of a manuscript which has been printed out in a coding mode according to a modified embodiment of the present invention; and 
         FIG. 11  is a pattern diagram representing an image of a manuscript which has been printed out in a coding mode according to other modified embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     In this embodiment, an image forming apparatus with a structure which enables images to be inputted from outside a housing or from a position as close as possible to a side edge part of the housing is realized with a minimum number of component portions and without losing the thickness of optical component portions. 
       FIG. 1  is a diagram showing a schematic structure of an image forming system applying the present invention. 
     This system has a structure in which an image forming apparatus  10  (e.g. an MFP) is connected with a host computer HC, and possesses a document protection function applying image coding and decoding. The following description will refer to a case in which the image forming apparatus  10  is used as a printer under a coding mode for producing a coded document, and as a copier under a decoding mode for decoding the coded document into a non-coded document. 
     When the image forming apparatus  10  is used as a printer, the image forming apparatus  10  forms images on the printing paper based on image data provided by the host computer HC and data set up in the image forming apparatus  10 . 
     As shown in  FIG. 1 , the image forming apparatus  10  has paper feeding trays  111 ,  112  and  113  in three stages where sheets of printing papers  121 ,  122  and  123  are set, respectively. For instance, the printing papers  121  and  122  may be sheets of plain paper, and the printing paper  123  may be sheets of printing paper with electronic tags  33  attached thereto. Corresponding to those paper feeding trays  111 ,  112  and  113 , paper feeding rollers  131 ,  132  and  133  are arranged, respectively, and corresponding to the paper feeding rollers  131 ,  132  and  133 , conveyance rollers  141 ,  142  and  143  are arranged, respectively. By this structure, the printing papers  121 ,  122  and  123  can be fed to an printing unit  20  via the paper feeding rollers  131 ,  132  and  133 , and then via the conveyance rollers  141 ,  142  and  143 , respectively. 
     In the printing unit  20  (i.e. a print engine in this embodiment), a surface of a photoconductive drum  21  is to be charged evenly by a charging roller  22 . The charges on the photoconductive drum  21  will be eliminated selectively by a laser beam LB to form an electrostatic latent image, after which toner will be transferred to the charged part of the photoconductive drum  21  by a developer  23  to develop the electrostatic latent image. 
     A registration roller  24  functions to once stop the paper feeding of the printing paper being fed from the conveyance roller  143 , and resume the paper feeding at predetermined timing to feed the printing paper between an electrostatic transferring roller  25  and the photoconductive drum  21 . The electrostatic transferring roller  25  functions to make the toner image on the photoconductive drum  21  be transferred on the printing paper being fed through electrostatic image transferring. The printing paper having the toner image transferred thereto will path through a fixing unit  30  to be heated and pressured, and thereby it will have the toner image fixed thereto. Residual toner on the photoconductive drum  21  after the transferring will be removed by a cleaner  26 . 
     After the fixing process, the printing paper will path through a paper exit roller  145  and a post-processing device  16  to be discharged on an exit tray  15 . In the vicinity of the paper exit roller  145 , a paper exit sensor  17  for detecting discharging printing paper is arranged. 
     A scanner  32  arranged under a manuscript table  31  functions to scan images on a manuscript placed on the manuscript table  31   
     Now, as shown in  FIG. 2 , a sheet of printing paper  123  has an electronic tag  33  attached to an edge of it. 
     Furthermore, as shown in  FIG. 1 , a tag reader  34  is arranged near the manuscript table  31 , and it is capable of reading the contents stored in the electronic tug  33  of the printing paper  123  without contacting it. A tag writer  35  is arranged on a paper path in the front of the printing unit  20 , and it is capable of writing a unique code  50  and a coding key correspondence code  51  (q.v.  FIG. 2 ) to the electronic tag  33  when forming a coded image on the printing paper  123 . Here, depending on a size of the image forming apparatus  10  and a effective area of radio wave for reading from or writing into the electronic tag  33 , it is also possible to arrange a tag reader/writer at one place instead of having the tag reader  34  and the tag writer  35  arranged separately at different places. 
     The unique code  50  and the coding key correspondence code  51  are used in the decoding mode. The unique code  50  is used in determining whether the image forming apparatus  10  is applicable for decoding a coded pattern formed on a manuscript  12  placed on the manuscript table  31 . The coding key correspondence code  51  is used in determining whether decoding keys inputted from an operation inputting unit  44  are appropriate or not. The coding key correspondence code  51  corresponds with the coding keys and it includes a coded data generated by coding the coding key. 
     For the purpose of controlling the internal units of the image forming apparatus  10 , the image forming apparatus  10  has a system controller  40 . The system controller  40  is connected to the scanner  32 , the tag reader  34 , the tag writer  35 , an image data memory  41  (which includes a work area), a setup data memory  42 , a unique data memory  43 , the operation inputting unit  44 , a conveyance roller controller  45  and a printing unit controller  46 . 
     In case of using the image forming apparatus  10  as a printer, the system controller  40  will arrange such that the image data provided by the host computer HC will be once stored in the image data memory  41 . This image data is originally provided by a printer driver PD during a printing routine of an application program from, and it is written in a PDL (page description language) which does not depend on a device resolution etc. 
     In case of using the image forming apparatus  10  as a copier, the system controller  40  is to transmit a read-out instruction to the scanner  32 . In response to this read-out instruction, the scanner  32  will read out the image data of the manuscript on the manuscript table  31  and transmit the read out data to the system controller  40 . As the system controller  40  receives the image data, it will arrange such that this data will be once stored in the image data memory  41 . 
     The system controller  40  is to store data set up by a user operating the operation inputting unit  44  to the setup data memory  42 . The setup data memory  42  is a nonvolatile memory, such as a flash memory, which is rewritable. The unique data memory  43  is a ROM which stores data unique to the image forming apparatus  10 . This unique data stored in the unique data memory  43  includes the above-mentioned unique code  50 . Here, it is also possible to use a predetermined region of hard disc instead of the setup data memory  42  and the unique data memory  43 . 
     Before using the document protection function of the image forming apparatus  10 , a user should operate the operation inputting unit  44  to set up the printing paper with the electronic tags to one of the paper feeding trays  111 ,  112  and  113 .  FIG. 3  and  FIG. 4  show operation screens for this setup. These operation screens are displayed on a touch panel of the image forming apparatus  10 . 
     In  FIG. 3 , reference numbers  1 ,  2  and  3  are identification numbers of the paper feeding trays and correspond with the paper feeding trays  111 ,  112  and  113  shown in  FIG. 1 , respectively. In each of rectangular spaces numbered  1 ,  2  and  3  which represent the paper feeding trays  111 ,  112  and  113 , respectively, a size, direction and type of printing paper is indicated based on data stored in the setup data memory  42 . This particular embodiment shows a case in which setting of the paper feeding trays is changed such that the paper feeding tray  3  is set with sheets of A4 vertical printing paper with electronic tags attached thereto, while in the original setting, the paper feeding trays  1  and  3  should be set with sheets of seize A4 vertical plain paper and the paper feeding tray  2  should be set with sheets of size A3 horizontal recycled paper. 
     When the rectangular space  3  indicating the paper feeding tray  113  is selected by touch, the selected part will be highlighted (in  FIG. 3 , this part is shown in hatched pattern) and a pull-down menu including a size, direction and type of the printing paper to be set to the paper feeding tray  113  will be displayed. When the highlighted rectangular space  3  corresponding to the paper feeding tray  113  is touched again, the pull-down menu will disappear and the display will return to the original state (this sequence will be the same with other selecting operations). When the type of printing paper is selected in a screen shown in  FIG. 3 , the selected part will be highlighted and a further pull-down menu in a lower echelon will be displayed as shown in  FIG. 4 . In fact, there are more possible options for the types of printing paper, but in order to simplify the description of this embodiment,  FIG. 4  shows only three options. In  FIG. 4 , when tagged plain paper is selected, the indication of ‘plain paper’ inside the rectangular space  3  corresponding to the paper feeding tray  113  will change to ‘tagged plain paper’. The setup data set up by such operation will be once stored in the work area of the memory. When an ‘OK’ button in the upper right corner of the screen, for instance, is touched, the setup data will be transferred from the work area to the setup data memory  42 , upon which the set up data will be updated. 
     The host computer HC shown in  FIG. 1  has a printer driver PD for the image forming apparatus  10  installed. When the printer driver PD is run, a print setup dialog will be displayed, and upon selecting one of the tabs, an operation screen for setting up the document protection function, such as the one shown in  FIG. 5 , will be displayed. 
     In case of encoding a text and then have it printed, the document protection function can be turned on by pointing and clicking a radio button ‘ON’ using a pointing device, or in other case, the document protection function can be turned off by pointing and clicking a radio button ‘OFF’ using the pointing device. 
     When a ‘Print’ button shown in  FIG. 5  is touched, image data including the print setup data will be transmitted from the host computer HC to the system controller  40 . As the system controller  40  receives the image data, it will have the data once stored into the image data memory  41 . 
       FIG. 6  is a flowchart showing processes to be conducted by the system controller  40  after the image data is stored into the image data memory  41  as described above. 
     In step S 0 , the system controller  40  checks the contents of the image data memory  41  to determine whether the document protection function is turned on or off. Then the system controller  40  proceeds to step S 1  when it confirms that the document protection function is turned off (No of Step S 0 ), or it proceeds to step S 2  when it confirms that the document protection function is turned on (Yes of Step S 0 ). 
     In step S 1 , the system controller  40  conducts a normal printing process unrelated to the document protection function and terminates the process. 
     In step S 2 , the system controller  40  automatically generates coding keys and coding key correspondence code  51  using a random number generation function etc. Furthermore, the system controller  40  generates decoding keys based on the coding keys generated in this step. 
     In step S 3 , the system controller  40  encodes the image data stored in the image data memory  41  by a triple DES (Data Encryption Standard) system, for instance, and then the system controller  40  transmits the coded image data as a code string to the printing unit controller  46  along with a pattern conversion instruction. 
     In step S 4 , in response to the transmission of the code string and the pattern conversion instruction from the system controller  40 , the printing unit controller  46  converts each code (e.g. each 4-bit code) in the code string into a predetermined small pattern. For instance, as shown in  FIG. 7 , 4-bit hexadecimal codes ‘0’ to ‘F’ are converted into small patterns  520  to  52 F. Broken lines in each small pattern shown in  FIG. 7  are used to facilitate understanding of the small patterns and they show parts which are not for printing. Each small pattern is a 5×5 dot pattern, of which dots in the outmost perimeter are used for discriminating a boundary between each adjacent small pattern and do not include black dots. There are 126 ways of arranging arbitrary four dots in a 5×5 matrix, and 16 of them which are difficult to be misclassified are used for the small patterns. 
     In step S 5 , the system controller  40  starts feeding the printing paper  123  inside the selected paper feeding tray  113  using the conveyance roller controller  45 , and have the unique code  50  and the coding key correspondence code  51  generated in step S 2  written on the electronic tag  33  at predetermined timing via the tag writer  35 . 
     In steps S 6  and S 7 , the printing unit controller  46  forms an electrostatic latent image of a small pattern array image on the photoconductive drum  21  by controlling the laser beam LB on and off based on the small pattern array image data, while having the photoconductive drum  21  scanned by the laser beam LB. On the other hand, the system controller  40  arranges to have a feeding unit be driven via the conveyance roller controller  45  based on status information received from the printing unit controller  46  and information received from a printing paper sensor. As for the second page and pages beyond that, sheets of untagged printing paper are fed for printing. As the printing is finished with all pages (Yes of Step S 7 ), the system controller  40  will proceed to step S 8 . 
     Through the processes described above, coded document is produced. 
     In step S 8 , the system controller  40  transmits the decoding keys generated in Step S 2  to the host computer HC. In response to the transmission of the decoding keys, the printer driver PD will transmits to a predetermined address an e-mail of a text including the decoding keys. 
       FIG. 8  is a pattern diagram representing the image of the manuscript  12  which has been printed out in the coding mode. 
       FIG. 9  is a flowchart showing the processes to be conducted by the system controller  40 . These processes will start by pressing a copy start button in the operation inputting unit  44 . 
     In step S 10 , the system controller  40  arranges that the contents stored in the electronic tag  33  of the manuscript  12  placed on the manuscript table  31  is read out by the tag reader  34 . If the unique code  50  contained in the read out contents does not coincide with the unique code stored in the unique data memory  43  (No of Step S 10 ), the system controller  40  proceeds to step S 11 , or if the unique code  50  coincides with the unique code stored in the unique data memory  43  (Yes of Step S 10 ), the system controller  40  will proceed to step S 12 . 
     In step S 11 , the system controller  40  arranges to have information indicating that the manuscript is not applicable for decoding with the image forming apparatus  10  displayed on the display of the operation inputting unit  44 , and it also rings an electronic buzzer, before terminating the process. 
     In step S 12 , the system controller  40  advises the user to input decoding keys via the operation inputting unit  44 . In response, the user inputs the decoding keys, and touches a ‘Verification’ button (not shown). 
     In step S 13 , the system controller  40  responds to the inputs by the user, encodes the decoding keys and determines whether they match the coding key correspondence code  51 . When they are found not matching (No of Step S 13 ), the system controller  40  proceeds to step S 14 , or when they are found matching (Yes of Step S 13 ), the system controller  40  proceeds to step S 15 . 
     In step S 14 , the system controller  40  arranges to have information indicating that there is an input error displayed on the display of the operation inputting unit  44 , and goes back to step S 12 . 
     In step S 15 , the system controller  40  arranges to have the scanner  32  reads out the image on the manuscript  12 . 
     In step S 16 , the system controller  40  converts each small pattern in the small pattern string of the read out image data (q.v.  FIG. 7 ) into a hexadecimal code based on the correspondence between small patterns and hexadecimal codes. In addition the correspondence between small patterns and hexadecimal codes is controlled by the system controller  40  in advance. 
     In step S 17 , the system controller  40  arranges to have the image data written in PDL restored by decoding the converted codes using the decoding keys, and then the system controller  40  arranges to have the restored image data once stored in a predetermined region inside the image data memory  41 . 
     In step S 18 , the system controller  40  controls the conveyance roller controller  45  and the printing unit controller  46  based on the image data stored in the image data memory  41  and the contents of the setup data memory  42  to have the printing paper delivered and then have the image formed on the printing paper. 
     As described above, according to this embodiment of the present invention, since a manuscript is converted into a coded pattern, it is possible to protect confidential information written on the manuscript from leaking, even if the manuscript happens to be left behind on the manuscript table of the image forming apparatus or other place. Furthermore, according to this embodiment, in the decoding mode, the image forming process will not be conducted unless the decoding keys inputted by the user correspond with the coding key correspondence code  51  read out from the electronic tag  33  attached to the coded document  12 . Therefore, it is possible to prevent unnecessary image forming from being executed on printing paper due to incorrect input of decoding keys, and thereby it is possible to prevent the coded document from being figured out due to misplacing the printed manuscript. 
     Moreover, according to this embodiment of the present invention, in the decoding mode, before the user inputs the decoding keys, the unique code  50  is read out from the electronic tag  33  attached to the coded document  12 , and in case when it is determined that this unique code  50  does not correspond with the unique data stored in the unique data memory  43 , the user will be notified of device nonconformity. Therefore, in attempting decoding, the user will be able to promptly determine whether the image forming apparatus is applicable to the decoding, and thereby can save unnecessary time and efforts. 
     Furthermore, considering a case when the image forming apparatus  10  is provided with hard disc and the coding key correspondence code  51  is stored in this hard disc, if the coded document  12  is to be decoded at multiple separate places (i.e. at multiple image forming apparatuses), it is necessary to coordinate linkage among the image forming apparatuses and synchronize the memory contents, which leads to complicate the system. However, in this embodiment, since the coding key correspondence code  51  is to be written on the electronic tag  33  attached to the coded document  12 , it is possible to prevent the system from becoming complicated and to utilize the document protection function at low cost. 
     In addition, according to this embodiment of the present invention, since the decoding keys and the coding keys are automatically generated by the image forming apparatus  10 , it is possible to prevent the decoding keys from being figured out using simple coding keys. 
     This invention can have other modified forms. 
     In the above embodiment of the present invention, the case of coding the entire image data for printing and decoding the printed image was described. However, the present invention is not limited to this arrangement, and it is possible to apply the following structure, for instance. In this structure, by having only confidential contents within the entire document being designated by an application program of the host computer HC, only the confidential contents are coded by the system controller  40  under the coding mode as shown in  FIG. 10 , and information representing this confidential contents is written to the electronic tag  33 . In the decoding mode, this information is read out and the above described decoding process is conducted only with respect to the confidential contents, and a normal process is conducted with respect to the other contents. 
     Furthermore, it is possible to have a structure in which the information representing the coded contents should not be stored in the electronic tag  33  by having a header and a tooter attached to the beginning and the end of the small pattern string, respectively. 
     Moreover, it is possible to have a structure in which a public key of the asymmetric key cryptosystem is used as the coding key of the coding key correspondence code  51  while the coding key correspondence code  51  is used as the public key itself, and a private key of the asymmetric key cryptosystem is used as the decoding key. 
     Moreover, in an image forming apparatus selectively using more than two kinds of coding systems, it is possible to have more than two kinds of unique data, and in this case, in Step S 10  of  FIG. 9 , it is determined as a mach if unique data corresponding to one of more than two kinds of unique data is stored in the electronic tag  33 . 
     Moreover, although the case of coding image data written in PDL was described in the above embodiment of the present invention, it is also possible to have a structure in which bitmap data or corresponding image compressed data is coded. 
     Moreover, although the case of using the electronic tag  33  as a decoding data recording means was described in the above embodiment of the present invention, it is also possible to have a structure in which the codes of unique code  50  and coding key correspondence code  51  are converted into small patterns as described above and a pattern image is formed in the header  53  in the upper part of the manuscript  12  instead of using the electronic tag  33 , as shown in  FIG. 11 . In this structure, since the electronic tag  33  is unnecessary, a running cost can be made inexpensive. Moreover, since the scanner  32  can be used in place of the tag reader  34  and the printing unit  20  can be used in place of the tag writer  35 , it is not necessary to have the tag reader  34  and the tag writer  35  provided inside the image forming apparatus  10 , and therefore, it is possible to manufacture the image forming apparatus  10  at comparatively low cost. 
     It is also possible to have a structure in which electronic tag  33  or the header is attached to or formed on each page of the manuscript  12 . 
     Moreover, since a display in a compact printer is comparatively small, the setup means in the present invention is not limited to the one having the GUI (Graphical User Interface), but it can be such setup means provided with a means, such as an electrical keyboard, which can display characters and symbols. 
     Moreover, although the case in which the image forming apparatus  10  is a printer or a copier was described in the above embodiment of the present invention, the image forming apparatus  10  may be a facsimile or the like as well. 
     In addition, the system of image forming means is not limited to the laser electrostatic image transferring system, but as long as it is possible to form images on the fed sheet of printing paper, it may be an ink-jet system or the like. 
     While the preferred embodiments of the invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or the scope of the following claims. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments. 
     Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention.