Methods for secure document printing

0001The present invention relates to document printing. More particularly, this invention relates to secure networked document printing.

0002A computer network typically includes one or more personal computers (sometimes called "client computers") coupled together via a communications network. The communications network may link client computers located within a single office, or may couple client computers located over a large geographic area. A computer network also may include one or more print output devices, such as printers, copiers, facsimile machines, and plotters (collectively referred to herein as "printers"), that may be used on a shared basis by network users to print documents created or displayed on the client computers. In particular, when a user issues a "print" command from within an application program on a client computer, a print job is created that typically is routed over the network to a selected printer.

0003If the print job includes confidential or other sensitive information, the security of that information may be compromised in a conventional networked printing environment. For example, a networked computer that includes network snooping software may be able to intercept the print job as it is routed to the selected printer. Previously known techniques for addressing this problem have required special purpose printers that may not be available on existing computer networks, and may be costly to obtain.

0004For example, Slick et al. European Patent Application No. <patcit id="pcit0001" dnum="EP1091285A2"><text>EP 1091285 A2 ("Slick</text></patcit>") describes a secure printing method in which print data are first encrypted with a randomly generated symmetric key, and then the symmetric key is encrypted twice, using public keys of both the intended recipient and the intended printer. To implement this technique, the selected printer includes an embedded "smart chip" that contains the printer's private key, and also includes an interface device for reading the intended recipient's private key from a smart card. Slick does not, however, describe secure document printing techniques that may be implemented using conventional printers that do not include such specialized smart chips and smart card interfaces.

0005<patcit id="pcit0002" dnum="US5633932A"><text>Davis et al. U.S. Patent No. 5,633,932</text></patcit> ("Davis") describes a system and method for preventing a document from printing until the printer locally authenticates the intended recipient. In particular, a sending node first obtains a public key of a selected printing node, encrypts a document using the printer's public key, and then transfers the encrypted document over a network to the printing node. The printing node decrypts the document using the printing node's private key, and then prints the decrypted document only after the intended recipient is authenticated via either a keyboard, keypad, smart card, or biometric input coupled to the printing node. Davis does not describe secure document printing techniques that may be implemented using conventional printers that do not include such specialized authentication interfaces. Also, Davis does not describe systems that include printers that do not include public and private keys provided by the printer manufacturer

0006Accordingly, such previously known techniques may not be useful for existing computer networks that include conventional printers, without costly replacement or retrofit of the printers to include specialized hardware. It therefore would be desirable to provide methods and apparatus for securely printing a document over a communications network using conventional printers.

0007<patcit id="pcit0003" dnum="WO9809209A1"><text>WO 98/09209 A1</text></patcit> describes systems and methods for electronic commerce including secure transaction management and electronic rights protection. Electronic appliances such as computers employed help to ensure that information is accessed and used only in authorized ways, and maintain the integrity, availability, and/or confidentiality of the information. Secure subsystems used with such electronic appliances provide a distributed virtual distribution environment (VDE) that may enforce a secure chain of handling and control, for example, to control and/or meter or otherwise monitor use of electronically stored or disseminated information. Such a virtual distribution environment may be used to protect rights of various participants in electronic commerce and other electronic or electronic-facilitated transactions. Secure distributed and other operating system environments and architectures, employing, for example, secure semiconductor processing arrangements that may establish secure, protected environments at each node. These techniques may be used to support an end-to-end electronic information distribution capability that may be used, for example, utilizing the "electronic highway".

0008In view of the foregoing, it is an object of this invention to provide methods and apparatus for securely printing a document over a communications network using conventional printers. These and other objects of the present invention are accomplished by methods according to the appended claims. Software instructions program the printer to generate and exchange cryptographic keys with the secure client application, and then to decrypt an encrypted print job.

0009The above mentioned objects and features of this invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same structural elements throughout, and in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a schematic diagram of a previously known system for printing a document on a networked printer;</li><li><figref idref="f0001">FIG. 2</figref> is a block diagram of an exemplary system for securely printing a document in accordance with this invention;</li><li><figref idref="f0002">FIG. 3</figref> is a flowchart of an exemplary method for securely printing a document in accordance with this invention;</li><li><figref idref="f0003">FIG. 4</figref> is a block diagram of a more specific exemplary method for securely printing a document in accordance with this invention; and</li><li><figref idref="f0004">FIG. 5</figref> is a block diagram of another more specific exemplary method for securely printing a document in accordance with this invention.</li></ul>

0010In accordance with the present invention, methods and apparatus are described for securely printing a document over a computer network. Referring to <figref idref="f0001">FIG. 1</figref>, a conventional computer network is described. Computer network 10 includes client computer 12, communications path 14 and printer 16. Client computer 12 may be any conventional computer, such as a personal computer, desktop computer, laptop computer, handheld computer, computer workstation, personal digital assistant, or any other similar computer. Communications path 14 may be any conventional communications network such as a local area network, wide area network, public switched telephone network, wireless network, Internet, or other similar communications network. Printer 16 may be any conventional print output device, such as a laser printer, inkjet printer, facsimile device, plotter, or other similar print output device.

0011Client computer 12 typically includes application software that may be used to create, display and print a document, such as a word processing document, spreadsheet, presentation, digital image, or other similar document. Typically, the software application or a print driver formats the document into a print file using a page description language ("PDL"), such as PCL or PostScript. The PDL print file can then be communicated over communications path 14 to printer 16, which receives, interprets and prints the document.

0012Referring now to <figref idref="f0001">FIG. 2</figref>, an exemplary system in accordance with this invention is described for securely printing a document on a computer network, such as the network of <figref idref="f0001">FIG. 1</figref>. As shown in <figref idref="f0001">FIG. 2</figref>, client computer 12 includes central processing unit ("CPU") 18, memory 20, and network interface 22, all coupled to system bus 24. CPU 18 may be any conventional programmable microprocessor or similar processor. Memory 20 may be any conventional computer-readable memory media, such a magnetic memory, optical disk memory, or other similar type of memory media. Memory 20 includes secure client application ("SCA") 26, described in more detail below, that securely communicates documents to a networked printer in accordance with this invention. Network interface 22 is a conventional network interface that is used to interface client computer 12 to communications path 14.

0013Printer 16 is a conventional printer that includes network interface 30, read-only memory ("ROM") 32, random access memory ("RAM") 34, and CPU 36, coupled via system bus 38, and print engine 40. Network interface 30 is a conventional network interface that is used to interface printer 16 to communications path 14. ROM 32 includes program instructions that form the control program for printer 16. CPU 36 may be any conventional programmable microprocessor, or similar processor. CPU 36 processes program instructions in ROM 32, and controls print engine 40, which is a standard part of a conventional printer. Printer 16 also includes variable seed source 42, coupled to system bus 38. Variable seed source 42 may be software or hardware that samples a clock signal within printer 16 to provide a seed S_R for a pseudo random number generator.

0014Printer 16 is a programmable printer. That is, the printer has facilities for downloading programs to it and executing these programs. PostScript is one previously known programming language used in a wide variety of conventional printers. In particular, PostScript is an interpreted language that includes a PDL that may be used to describe how to draw objects on a drawing surface. PostScript also is a powerful programming language that may be used to program the operation of CPU 36 to perform a wide variety of functions.

0015In accordance with this invention, SCA 26 downloads computer program instructions to printer 16 to implement a cryptographic key exchange program and an encryption/decryption program. As used herein, a "key" is an encoding or decoding parameter for a cryptographic algorithm. The key exchange program instructs the printer to generate and exchange an encryption key with SCA 26, which encrypts print data using the encryption key, and transmits the encrypted print data to printer 16. The encryption/decryption program instructs printer 16 to decrypt the encrypted print data using a decryption key. Accordingly, the print data are securely communicated between client computer 12 and conventional printer 16.

0016Referring now to <figref idref="f0002">FIG. 3</figref>, an exemplary key exchange and encryption/decryption method in accordance with this invention is described. In particular, <figref idref="f0002">FIG. 3</figref> describes an exemplary method that may be implemented by SCA 26 and printer 16. Beginning at step 52, SCA 26 downloads a key exchange program to RAM 34 in printer 16. The key exchange program may be, for example, a set of computer program instructions or sequence of PostScript procedures that instruct printer 16 to generate and exchange one or more cryptographic keys with SCA 26 in accordance with a key establishment protocol, such as the Rivest Shamir Adlemann ("RSA") protocol, the Diffie-Hellman key agreement protocol, or any other key establishment protocol.

0017At step 54, the key exchange program instructs printer 16 to generate an encryption key Ep and a decryption key Dp. As part of this step, the key exchange program first instructs printer 16 to generate a pseudo-random number using seed S<b>_R</b> from variable seed source 42. For example, if printer 16 is capable of implementing PostScript commands, the key exchange program may use the PostScript "usertime" or "realtime" operators to generate seed S<b>_R</b>, and may use the PostScript "rand" operator and seed S<b>_R</b> to generate the pseudo random number. Printer 16 uses the pseudo random number to generate encryption key Ep and decryption key Dp. Alternatively, other sources of entropy in the printer can be exploited to generate the seed S<b>_R</b>, or a custom made program can be used to generate the pseudo random number instead of the "rand" operator.

0018Next, at step 56, the key exchange program executes on printer 16 to exchange encryption key Ep with SCA 26, which receives Ep at step 58. At step 66, SCA 26 encrypts print data using encryption key Ep. In particular, SCA 28 may encrypt the print data using encryption key Ep and a cipher, such as RC4, RC5, twofish, blowfish, IDEA, DES, 3DES, Rijndael, or may encrypt the print data using encryption key Ep and an asymmetric public key algorithm. Next, at step 68, SCA 26 downloads a decryption program to RAM 34 in printer 16. The decryption program may be a set of computer program instructions such as a sequence of PostScript procedures that instruct printer 16 to decrypt a received print job using the printer's decryption key Dp and the cryptographic algorithm used by SCA 26 to encrypt the print data. For example, a conventional printer that implements PostScript commands typically reads PostScript data such as print data from an incoming data stream, or from an internal disk drive. In accordance with this invention, decryption programs modify the conventional read operation with a customized operation to decrypt the print data.

0019In particular, at step 70, the decryption program running on printer 16 modifies the conventional file read operation to enable printer 16 to decrypt print data using decryption key Dp. For example, the decryption program may insert PostScript decryption filters or an embedded decryption interpreter in the incoming data stream to decrypt incoming encrypted data using decryption key Dp. Alternatively, the decryption program may override the conventional file read operator with a customized operator to decrypt incoming or stored print data using decryption key Dp. Persons of ordinary skill in the art will understand that other similar techniques may be used to modify the conventional file read operation with an operation or sequence of operations that cause printer 16 to decrypt incoming data using decryption key Dp.

0020At step 72, SCA 26 downloads the encrypted data to printer 16. SCA 26 may download data in a single data block, or in a sequence of multiple smaller data blocks. At step 74, the decryption program instructs printer 16 to implement the modified file read operation to decrypt incoming data. Upon implementing the modified operation, printer 16 decrypts the print data and the renders the print data at step 76. Note that several steps in the above-described method may be interchanged. For example, SCA 26 may download the decryption program to RAM 34 prior to receiving the encryption key from printer 16, or prior to encrypting print data using encryption key Ep. All such variations are within the scope of this invention.

0021Referring now to <figref idref="f0003">FIG. 4</figref>, a more specific exemplary key exchange and encryption/decryption method in accordance with this invention is described. As described above, the key exchange program may be implemented using any suitable programming language, such as PostScript, that may be used to program a programmable printer. Beginning at step 52, SCA 26 downloads a key exchange program to RAM 34 in printer 16. In the embodiment of <figref idref="f0003">FIG 4</figref>, the key exchange program is a sequence of PostScript procedures that instruct printer 16 to generate and exchange cryptographic keys in accordance with the Rivest Shamir Adlemann ("RSA") key exchange algorithm. In particular, at step 54, the key exchange program instructs printer 16 to generate a pseudo random number (as described above) and then generate public key E<b>_P</b> and private key D<b>_P</b>.

0022Next, at step 56, the key exchange program executes on printer 16 to exchange public key E<b>_P</b> with SCA 26. In particular, printer 16 transmits a copy of its public key E<b>_P</b> to SCA 26, which receives E<b>_P</b> at step 58. At step 66, SCA 26 encrypts print data using the printer's public key E<b>_P</b>. In particular, SCA 28 encrypts the print data using public key E<b>_P</b> and an asymmetric public key algorithm. Next, at step 68, SCA 26 downloads a sequence of PostScript procedures that instruct printer 16 to decrypt a received print job using the printer's private key D<b>_P</b>. In particular, at step 70, the decryption program running on printer 16 inserts PostScript decryption filters in the incoming data stream to decrypt incoming encrypted data using the printer's private key D<b>_P</b>.

0023At step 72, SCA 26 downloads the encrypted data to printer 16. SCA 26 may download data in a single data block, or in a sequence of multiple smaller data blocks. At step 74, the decryption program instructs printer 16 to implement the modified file read operation to decrypt incoming data. In particular, printer 16 filters the incoming print data using the PostScript decryption filters. Upon implementing the modified operation, printer 16 decrypts the print data and the renders the print data at step 76. Note that several steps in the above-described method may be interchanged. For example, SCA 26 may download the decryption program to RAM 34 prior to receiving the public key E<b>_P</b> from printer 16, or prior to encrypting print data using the printer's public key E<b>_P</b>. All such variations are within the scope of this invention.

0024Referring now to <figref idref="f0004">FIG. 5</figref>, another more specific exemplary key exchange and encryption/decryption method in accordance with this invention is described. As described above, the key exchange program may be implemented using any suitable programming language, such as PostScript, that may be used to program a programmable printer. In the embodiment of <figref idref="f0004">FIG 5</figref>, the key exchange program is a sequence of PostScript procedures that instruct printer 16 to generate and exchange cryptographic keys in accordance with the Diffie-Hellman key exchange algorithm. Beginning at step 50, SCA 26 generates public key E<b>_S</b> and private key D<b>_S</b>. At step 52, SCA 26 downloads a key exchange program to RAM 34 in printer 16. At step 54, the key exchange program instructs printer 16 to generate a pseudo random number (as described above) and then generate public key Ep and private key Dp.

0025Next, at step 56, the key exchange program executes on printer 16 to exchange public keys Ep and E<b>_S</b> with SCA 26. In particular, printer 16 transmits a copy of its public key Ep to SCA 26, which receives Ep at step 58. In addition, SCA 26 transmits a copy of its public key E<b>_S</b> to printer 16. At steps 60 SCA 26 generates a copy of a shared secret key K<b>_S</b> using the printer's public key Ep and the SCA's private key D_S. At step 62, the key exchange program instructs printer 16 to generate a copy of shared secret key K<b>_S</b> using the SCA's public key E<b>_S</b> and the printer's private key Dp. Thus, at the end of steps 60 and 62, SCA 26 and printer 16 each have a copy of shared secret key K_S.

0026Next, at step 66, SCA 26 encrypts print data using shared secret key K_S. In particular, SCA 28 encrypts the print data using shared secret key K<b>_S</b> and a cipher, such as RC4, RC5, twofish, blowfish, IDEA, DES, 3DES, Rijndael or any other cryptographic algorithm. Next, at step 68, SCA 26 downloads a sequence of PostScript procedures that instruct printer 16 to decrypt a received print job using shared secret key K_S. In particular, at step 70, the decryption program running on printer 16 inserts PostScript decryption filters in the incoming data stream to decrypt incoming encrypted data using shared secret key K_S.

0027At step 72, SCA 26 downloads the encrypted data to printer 16. SCA 26 may download data in a single data block, or in a sequence of multiple smaller data blocks. At step 74, the decryption program instructs printer 16 to implement the the incoming print data using the PostScript decryption filters. Upon implementing the modified operation, printer 16 decrypts the print data and the renders the print data at step 76. Note that several steps in the above-described method may be interchanged. For example, SCA 26 may download the decryption program to RAM 34 prior to exchanging public keys with printer 16, or prior to encrypting print data using shared secret key K_S.

0028Persons of ordinary skill in the art will understand that methods in accordance with this invention may be implemented in computer software and/or hardware. In particular, methods in accordance with this invention may be implemented using any program language that may be used to program a programmable printer. Persons of ordinary skill in the art also will recognize that methods and apparatus in accordance with this invention may be implemented using steps or devices other than those shown and discussed above. All such modifications are within the scope of the present invention, which is limited only by the claims that follow.