Patent Publication Number: US-2010124329-A1

Title: Encrypted communication between printing system components

Description:
FIELD OF THE INVENTION 
     This invention relates generally to encrypted communication, and in particular to encrypted communication between printing system components. 
     BACKGROUND OF THE INVENTION 
     In printing systems, for example inkjet printing systems, information is transmitted between various printing system components. Typically, this information is stored in the memory of one of the printing system components for later retrieval and use. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention a system and method of communicating between a first device and a second device are provided. Communication includes transferring information or data in a secure manner without encrypting the information itself. When appropriate, the transferred information or data is used or stored by at least one of the first device and the second device. 
     According to another aspect of the present invention, a method of communicating between a first device and a second device includes providing unencrypted data to be transmitted in the first device, encrypting at least a portion of the unencrypted data to be transmitted to form encrypted data, using at least a portion of the encrypted data to form a first validation code, appending the first validation code to the unencrypted data to form a packet of data to be transmitted, and transmitting the packet of data from the first device to the second device. 
     According to another aspect of the present invention, a method of communicating between a first device and a second device also includes extracting the unencrypted data from the received packet of data, extracting the first validation code from the received packet of data, encrypting at least a portion of the extracted unencrypted data, using at least a portion of the extracted encrypted data to form a second validation code that is calculated by the second device, and comparing the second validation code to the first validation code. The portion of extracted unencrypted data encrypted by the second device corresponds to the portion of unencrypted data encrypted by the first device and the portion of the extracted encrypted data that forms the second validation code corresponds to the portion of the encrypted data that forms the first validation code. 
     According to another aspect of the present invention, an inkjet printing system includes a first device including unencrypted data to be transmitted, a second device, and a communication link between the first and second devices. The first device is configured to encrypt at least a portion of the unencrypted data to be transmitted to form encrypted data, to use at least a portion of the encrypted data to form a first validation code, and to append the first validation code to the unencrypted data to form a packet of data to be transmitted. The communication link is configured to transmit the packet of data from the first device to the second device. 
     According to another aspect of the present invention, an inkjet printing system also includes the second device being configured to extract the unencrypted data from the received packet of data, extract the first validation code from the received packet of data, encrypt at least a portion of the extracted unencrypted data, use at least a portion of the extracted encrypted data to form a second validation code, and compare the second validation code to the first validation code. The portion of extracted unencrypted data encrypted by the second device corresponds to the portion of unencrypted data encrypted by the first device and the portion of the extracted encrypted data that forms the second validation code corresponds to the portion of the encrypted data that forms the first validation code. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  shows a simplified schematic block diagram of an example embodiment of a printing system made in accordance with the present invention; 
         FIG. 2  is a schematic view of an example embodiment of a continuous printhead made in accordance with the present invention; P  FIG. 3  is a schematic view of an example embodiment of a continuous printhead made in accordance with the present invention; 
         FIG. 4  is a schematic block diagram of an example embodiment of an inkjet printing system made in accordance with the present invention showing the relationship between a jetting module and a host; 
         FIG. 5  is a flowchart of an example embodiment of the present invention showing a communication session for communicating encrypted data using a communication session specific key; and 
         FIG. 6  is a flowchart of an example embodiment of the present invention showing a method of communicating data between a first device and a second device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements. 
     The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of the ordinary skills in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention. 
     As described herein, the example embodiments of the present invention provide a printhead and/or printhead components typically used in inkjet printing systems. However, many other applications are emerging which use inkjet printheads to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. As such, as described herein, the terms “liquid” and/or “ink” refer to any material that can be ejected by the printhead and/or printhead components described below. 
     Typically, inkjet printing is accomplished by one of two technologies, referred to as drop-on-demand inkjet printing and continuous inkjet printing. While the present invention finds application in various types of printing systems, it is particularly well suited for drop-on-demand inkjet printing systems and continuous inkjet printing systems. 
     In drop on demand ink jet printing, ink drops are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink drop through a nozzle bore that strikes a print medium. The formation of printed images is achieved by controlling the individual formation of ink drops and relative movement of the recording medium and the printhead. A slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle bore and also forms a slightly concave meniscus at the nozzle bore. 
     In continuous inkjet printing, a pressurized ink source is used to eject a filament of fluid through a nozzle bore from which a continuous stream of ink drops are formed using a drop forming device. The ink drops are directed to an appropriate location using one of several methods (electrostatic deflection, heat deflection, gas deflection, etc.). When no print is desired, the ink drops are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink drops are not deflected and allowed to strike a print media. Alternatively, deflected ink drops can be allowed to strike the print media, while non-deflected ink drops are collected in the ink capturing mechanism. 
     Referring to  FIG. 1 , an example embodiment of a continuous printing system made in accordance with the present invention is shown. A continuous printing system  20  includes an image source  22  such as a scanner or computer which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. This image data is converted to half-toned bitmap image data by an image processing unit  24  which also stores the image data in memory. A plurality of drop forming mechanism control circuits  26  read data from the image memory and apply time-varying electrical pulses to a drop forming mechanism(s)  28  that are associated with one or more nozzles of a printhead  30 . These pulses are applied at an appropriate time, and to the appropriate nozzle, so that drops formed from a continuous ink jet stream will form spots on a recording medium  32  in the appropriate position designated by the data in the image memory. 
     Recording medium  32  is moved relative to printhead  30  by a recording medium transport system  34 , which is electronically controlled by a recording medium transport control system  36 , and which in turn is controlled by a micro-controller  38 . The recording medium transport system shown in  FIG. 1  is a schematic only, and many different mechanical configurations are possible. For example, a transfer roller could be used as recording medium transport system  34  to facilitate transfer of the ink drops to recording medium  32 . Such transfer roller technology is well known in the art. In the case of page width printheads, it is most convenient to move recording medium  32  past a stationary printhead. However, in the case of scanning print systems, it is usually most convenient to move the printhead along one axis (the sub-scanning direction) and the recording medium along an orthogonal axis (the main scanning direction) in a relative raster motion. 
     Ink is contained in an ink reservoir  40  under pressure. In the non-printing state, continuous ink jet drop streams are unable to reach recording medium  32  due to an ink catcher  42  that blocks the stream and which may allow a portion of the ink to be recycled by an ink recycling unit  44 . The ink recycling unit reconditions the ink and feeds it back to reservoir  40 . Such ink recycling units are well known in the art. The ink pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzles and thermal properties of the ink. A constant ink pressure can be achieved by applying pressure to ink reservoir  40  under the control of ink pressure regulator  46 . As shown in  FIG. 1 , catcher  42  is a type of catcher commonly referred to as a “knife edge” catcher. 
     The ink is distributed to printhead  30  through an ink channel  47 . The ink preferably flows through slots and/or holes etched through a silicon substrate of printhead  30  to its front surface, where a plurality of nozzles and drop forming mechanisms, for example, heaters, are situated. When printhead  30  is fabricated from silicon, drop forming mechanism control circuits  26  can be integrated with the printhead. Printhead  30  also includes a deflection mechanism (not shown in  FIG. 1 ) which is described in more detail below with reference to  FIGS. 2 and 3 . 
     Referring to  FIG. 2 , a schematic view of continuous liquid printhead  30  is shown. A jetting module  48  of printhead  30  includes an array or a plurality of nozzles  50  formed in a nozzle plate  49 . In  FIG. 2 , nozzle plate  49  is affixed to jetting module  48 . However, as shown in  FIG. 3 , nozzle plate  49  can be integrally formed with jetting module  48 . 
     Liquid, for example, ink, is emitted under pressure through each nozzle  50  of the array to form filaments of liquid  52 . In  FIG. 2 , the array or plurality of nozzles extends into and out of the figure. 
     Jetting module  48  is operable to form liquid drops having a first size or volume and liquid drops having a second size or volume through each nozzle. To accomplish this, jetting module  48  includes a drop stimulation or drop forming device  28 , for example, a heater or a piezoelectric actuator, that, when selectively activated, perturbs each filament of liquid  52 , for example, ink, to induce portions of each filament to breakoff from the filament and coalesce to form drops  54 ,  56 . 
     In  FIG. 2 , drop forming device  28  is a heater  51  located in a nozzle plate  49  on one or both sides of nozzle  50 . This type of drop formation is known and has been described in, for example, U.S. Pat. No. 6,457,807 B1, issued to Hawkins et al., on Oct. 1, 2002; U.S. Pat. No. 6,491,362 B1, issued to Jeanmaire, on Dec. 10, 2002; U.S. Pat. No. 6,505,921 B2, issued to Chwalek et al., on Jan. 14, 2003; U.S. Pat. No. 6,554,410 B2, issued to Jeanmaire et al., on Apr. 29, 2003; U.S. Pat. No. 6,575,566 B1, issued to Jeanmaire et al., on Jun. 10, 2003; U.S. Pat. No. 6,588,888 B2, issued to Jeanmaire et al., on Jul. 8, 2003; U.S. Pat. No. 6,793,328 B2, issued to Jeanmaire, on Sep. 21, 2004; U.S. Pat. No. 6,827,429 B2, issued to Jeanmaire et al., on Dec. 7, 2004; and U.S. Pat. No. 6,851,796 B2, issued to Jeanmaire et al., on Feb. 8, 2005, the disclosures of which are incorporated by reference herein. 
     Typically, one drop forming device  28  is associated with each nozzle  50  of the nozzle array. However, a drop forming device  28  can be associated with groups of nozzles  50  or all of nozzles  50  of the nozzle array. 
     When printhead  30  is in operation, drops  54 ,  56  are typically created in a plurality of sizes or volumes, for example, in the form of large drops  56 , a first size or volume, and small drops  54 , a second size or volume. The ratio of the mass of the large drops  56  to the mass of the small drops  54  is typically approximately an integer between 2 and 10. A drop stream  58  including drops  54 ,  56  follows a drop path or trajectory  57 . 
     Printhead  30  also includes a gas flow deflection mechanism  60  that directs a flow of gas  62 , for example, air, past a portion of the drop trajectory  57 . This portion of the drop trajectory is called the deflection zone  64 . As the flow of gas  62  interacts with drops  54 ,  56  in deflection zone  64  it alters the drop trajectories. As the drop trajectories pass out of the deflection zone  64  they are traveling at an angle, called a deflection angle, relative to the undeflected drop trajectory  57 . 
     Small drops  54  are more affected by the flow of gas than are large drops  56  so that the small drop trajectory  66  diverges from the large drop trajectory  68 . That is, the deflection angle for small drops  54  is larger than for large drops  56 . The flow of gas  62  provides sufficient drop deflection and therefore sufficient divergence of the small and large drop trajectories so that catcher  42  (shown in  FIGS. 1 and 3 ) can be positioned to intercept one of the small drop trajectory  66  and the large drop trajectory  68  so that drops following the trajectory are collected by catcher  42  while drops following the other trajectory bypass the catcher and impinge a recording medium  32  (shown in  FIGS. 1 and 3 ). 
     When catcher  42  is positioned to intercept large drop trajectory  68 , small drops  54  are deflected sufficiently to avoid contact with catcher  42  and strike the print media. As the small drops are printed, this is called small drop print mode. When catcher  42  is positioned to intercept small drop trajectory  66 , large drops  56  are the drops that print. This is referred to as large drop print mode. 
     Referring to  FIG. 3 , jetting module  48  includes an array or a plurality of nozzles  50 . Liquid, for example, ink, supplied through channel  47 , is emitted under pressure through each nozzle  50  of the array to form filaments of liquid  52 . In  FIG. 3 , the array or plurality of nozzles  50  extends into and out of the figure. 
     Drop stimulation or drop forming device  28  (shown in  FIGS. 1 and 2 ) associated with jetting module  48  is selectively actuated to perturb the filament of liquid  52  to induce portions of the filament to break off from the filament to form drops. In this way, drops are selectively created in the form of large drops and small drops that travel toward a recording medium  32 . 
     Positive pressure gas flow structure  61  of gas flow deflection mechanism  60  is located on a first side of drop trajectory  57 . Positive pressure gas flow structure  61  includes first gas flow duct  72  that includes a lower wall  74  and an upper wall  76 . Gas flow duct  72  directs gas flow  62  supplied from a positive pressure source  92  at downward angle θ of approximately a 45° relative to liquid filament  52  toward drop deflection zone  64  (also shown in  FIG. 2 ). An optional seal(s)  84  provides an air seal between jetting module  48  and upper wall  76  of gas flow duct  72 . 
     Upper wall  76  of gas flow duct  72  does not need to extend to drop deflection zone  64  (as shown in  FIG. 2 ). In  FIG. 3 , upper wall  76  ends at a wall  96  of jetting module  48 . Wall  96  of jetting module  48  serves as a portion of upper wall  76  ending at drop deflection zone  64 . 
     Negative pressure gas flow structure  63  of gas flow deflection mechanism  60  is located on a second side of drop trajectory  57 . Negative pressure gas flow structure includes a second gas flow duct  78  located between catcher  42  and an upper wall  82  that exhausts gas flow from deflection zone  64 . Second duct  78  is connected to a negative pressure source  94  that is used to help remove gas flowing through second duct  78 . An optional seal(s)  84  provides an air seal between jetting module  48  and upper wall  82 . 
     As shown in  FIG. 3 , gas flow deflection mechanism  60  includes positive pressure source  92  and negative pressure source  94 . However, depending on the specific application contemplated, gas flow deflection mechanism  60  can include only one of positive pressure source  92  and negative pressure source  94 . 
     Gas supplied by first gas flow duct  72  is directed into the drop deflection zone  64 , where it causes large drops  56  to follow large drop trajectory  68  and small drops  54  to follow small drop trajectory  66 . As shown in  FIG. 3 , small drop trajectory  66  is intercepted by a front face  90  of catcher  42 . Small drops  54  contact face  90  and flow down face  90  and into a liquid return duct  86  located or formed between catcher  42  and a plate  88 . Collected liquid is either recycled and returned to ink reservoir  40  (shown in  FIG. 1 ) for reuse or discarded. Large drops  56  bypass catcher  42  and travel on to recording medium  32 . Alternatively, catcher  42  can be positioned to intercept large drop trajectory  68 . Large drops  56  contact catcher  42  and flow into a liquid return duct located or formed in catcher  42 . Collected liquid is either recycled for reuse or discarded. 
     As shown in  FIG. 3 , catcher  42  is a type of catcher commonly referred to as a “Coanda” catcher. However, the “knife edge” catcher shown in  FIG. 1  and the “Coanda” catcher shown in  FIG. 3  are interchangeable and work equally well. Alternatively, catcher  42  can be of any suitable design including, but not limited to, a porous face catcher, a delimited edge catcher, or combinations of any of those described above. 
     Some or all of printing system components or devices described above communicate with each other. Communication includes transferring or transmitting (sending and/or receiving) information or data in a secure manner without encrypting the information itself. When appropriate, the transferred information or data is used or stored by at least one of the printing system devices. 
     The information or data that is transmitted and stored can be of various types. For example, the information can be data used by the printing system controller to optimize settings of various control parameters for one or more printheads to ensure print quality and printhead reliability. The information can include printhead operating history information such as ink types and/or other fluid types that have been used in a printhead. Additionally, the information can include business or billing related information such as printhead usage (printhead hours), billing method, speed limitations of the printhead, and/or other sensitive or proprietary information. 
     The stored information is used for various purposes. For example, the stored history information can be used by the host to block transfer of a fluid to a particular printhead in situations in which it may not be appropriate to mix that fluid with a fluid already present in the printhead. An example of this use of the information includes preventing the transfer of a cyan ink to a printhead being operated with magenta ink. 
     When the information is based at least in part on customer use data and/or used to bill or change a user of the printing system, it is preferable that this type of information be secure so that the information cannot be modified or copied from one printing system component, for example, from one printhead to another printhead. It is also preferable that the information not be destroyed, erased, forged (spoofed), or emulated. However, except in situations where the information itself is confidential or proprietary, there is typically no reason to keep the information itself from being decipherable by users of the printing system. 
     Referring to  FIG. 4 , a schematic block diagram showing the relationship between jetting module  48  and a host of inkjet printing system  20  is shown. Inkjet printing system  20  includes a field replaceable unit, typically, either printhead  30  or jetting module  48 . When printhead  30  is a printhead in a continuous printing system, printhead  30  includes a deflection system and a jetting module. When printhead  30  is a printhead in a drop on demand printing system, printhead  30  is the jetting module. Field replaceable jetting module  48  includes a drop forming mechanism(s), commonly referred to as a drop generator  110 , and module electronics  116 . Additionally, printing system  20  includes a host system  114 , to which the jetting module  48  or printhead  30  connects to, and a print data source  102 . The jetting module  48  or printhead  30  includes printhead memory  108  that can contain information or data. Sometimes this data is needed by the host  114  to optimize jetting module  48  or printhead  30  operation. Sometimes the data is related to the operating history of the jetting module or printhead. In some instances, the at least some of the data contained in printhead memory  108  is originally supplied by the host  114 . 
     Communication between the printhead memory  108  and the host  114  passes through the printhead microprocessor  106 . The printhead memory  108 , the printhead microprocessor  106 , and a print data processor  104  form what is commonly referred to as module electronics  116  of the jetting module  48  or printhead  30 . A print data source  102 , for example, a scanner or computer, provides digital data, for example, raster image data, outline image data in the form of a page description language, or digital image data to the print data processor  104 . Print data processor  104  converts the image data into a data form that can be sent to the drop generator  110  of jetting module  48  or printhead  30 . To help ensure the integrity of the data stored in the printhead memory  108 , host  114  and module electronics  116  should be physically secure and tamper-resistant to reduce the likelihood of these devices being altered, copied, or destroyed. 
     As described below, the host  114  is a fluid system. However, it should be understood that the host can be included in any printing system component. For example, the host  114  can be a fluid system, another printhead, a user interface, a jetting module, a printer controller, or another inkjet system component. When host  114  is a fluid system, the fluid system CPU typically communicates with the printhead microprocessor  106 . 
     Referring to  FIG. 5 , when a first device  118 , for example, the fluid system CPU, wishes to transfer information or data to a second device  120 , for example, the jetting module, the first device initiates a communication session S 10  with the second device. During this communication session, a communication session specific encryption key is generated. The communication session specific encryption key is used along with an encryption algorithm to encrypt the messages sent during the communication session. Each communication session employs a different communication session specific encryption key, making the system more secure when compared to systems employing the same encryption key for all messages sent over the lifetime of the system. 
     After the communication session has been initiated, both the first device  118  and the second device  120  independently generate a random seed value S 12 , S 14 . The first device  118  transmits seed value  1  to the second device S 16  while the second device  120  transmits seed value  2  to the first device S 18 . The seed values are received by the appropriate devices S 20 , S 22 . Each device reads a stored hidden key S 24 , S 26  that is common to the first device  118  and the second device  120  but not transmitted, therefore remaining independently known only to the first device  118  and the second device  120 . Each device now has the seed value it created, the seed value created by the other device, and the stored hidden key. 
     The seed value provided by first device  118 , the seed value provided by the second device  120 , and the hidden key known to both the first device  118  and second device  120  are collectively employed to form a communication session specific key S 28 , S 30 . For example, in one example embodiment of the invention, the seed value provided by the second device  120  (for example, a jetting module) and the seed value provided by the first device  118  (for example, a fluid system) are combined to form a communication session specific number, which is then put through an encryption algorithm using the hidden key as the encryption key. The resultant number is the communication session specific key. Because both devices use the same hidden key and the same encryption algorithm, the communication session specific key created by first device  118  is identical to the communication session specific key created by second device  120 . 
     While any encryption algorithm known in the art can be used, typically, the particular encryption algorithm employed depends on the security level desired for the specific application being contemplated. Examples of encryption algorithm include TDES (or 3DES), AES, Lucifer, Madryga, REDOC, RC 2 , IDEA, MMB, GOST, CAST, as well as other conventional block ciphers. In a preferred example embodiment, the encryption algorithm is TDES. First and second devices  118 ,  120  then communicate or transfer the information or data S 32 . 
     Referring to  FIG. 6 , the information transferring process S 32  is shown. As the communication between devices is bidirectional, the terms “sender” and “receiver” are used herein, and include, for example, a fluid system, a printhead, a user interface, and/or other inkjet system components. 
     The data to be transmitted from the first device  118  (for example, the fluid system) to the second device  120  (for example, the printhead), or vice-versa, is identified S 40 . At least a portion of the data cluster to be transmitted is encrypted using the communication session specific encryption key previously created S 42 . The encryption algorithm used can be the same as the algorithm used to create the communication session specific key or it can be a different algorithm. The portion of the data cluster to be encrypted depends on the specific application contemplated and can be, for example, the entire data cluster that is to be transmitted or a subset of the entire data cluster. 
     At least a portion of the encrypted data cluster is then selected to form a validation code S 44 . Again, the portion of encrypted data selected to form the validation code depends on the specific application contemplated and the system specifics, including how many bytes are available as a validation code, the amount of total encrypted data, and the amount of unencrypted data to be transmitted. For example, if only a small portion of unencrypted data is encrypted, the validation code can be all of the encrypted data. However, when a larger portion of the data is encrypted, the validation code can be only a few bytes of the encrypted data. These bytes can come from the beginning, middle, end, or from multiple locations of the encrypted data string. This validation code is appended to the unencrypted data cluster previously identified S 46  to create a packet of data that is transmitted by the sender to the receiver S 48 . It is preferred that the validation code be at least two bytes or 16 bits long to reduce the risk that one could correctly guess the validation code on any given transfer of a data cluster. 
     The integrity of the data validation method depends on the secrecy of the hidden key. The process of forming a validation key from only a portion of the encrypted data rather than the entire encrypted data actually makes it harder to discover the value of the hidden key. Similarly, encrypting only a portion of the data cluster rather than encrypting the entire data cluster also makes it harder to discover the value of the hidden key. In both of cases, the process or algorithm used to define the data portion should be kept confidential. 
     The packet of data is received by the receiver S 50 . The receiver then extracts, or separates, the received data packet into the original unencrypted data cluster and the validation code S 52 . The receiver encrypts at least a portion of the extracted unencrypted data using the communication session specific encryption key previously created S 54  and selects at least a portion of the encrypted data cluster to form a second validation code S 56 . The validation code calculated by the receiver is compared to the validation code extracted from the data received from the sender S 58 . When there is a match between the two validation codes, the receiver uses the unencrypted data S 60 . For example, the information can be written to the printhead memory or displayed on a user interface. 
     However, when there is not a match of the validation codes, the information is disregarded S 62 . In some cases, such as in the described embodiment where the first device  118  (sender) is a fluid system and the second device  120  (receiver) is a jetting module, disregarding the information takes the form of refusing to write the data to the printhead memory. However, when there is no match of the validation codes during several successive tries, communication can be permanently shut down to the jetting module. In such cases, the customer is typically required to contact the manufacturer to correct the problem and replace the jetting module. When the first device  118  (sender) is, for example, the jetting module and the second device (receiver) is, for example, the fluid system, disregarding the information can take the form of inhibiting operation of the system. 
     In the description above, it was discussed relative to steps S 42  and S 44  that at least a portion of the data cluster is encrypted and that at least a portion of the encrypted data is selected to form a validation code. The portion of unencrypted data that is encrypted in step S 54  and the portion of encrypted data selected to form the second validation code in step S 56  should correspond to the portion of the unencrypted data encrypted in step S 42  and the portion of encrypted data selected to form the first validation code in step S 44 . In other words, if all of the data is encrypted in step S 42 , all of the data should be encrypted in step S 54 , and if, by way of example, the first four bytes of the encrypted data are selected as the first validation code in step S 44 , the first four bytes of encrypted data should be selected as the second validation code in step S 56 . To ensure that matching portions are selected by the sender and the receiver, the portions to be selected are usually fixed and do not vary from one communication session to another. Alternatively, when a communications session is initiated in S 10 , some information can be communicated between the first device  118  and the second device  120  to declare what portions of the unencrypted data are to be selected for encryption and which portions of the encrypted data are selected to form the validation codes. 
     Referring back to  FIG. 5 , when the transfer of information is complete, the communication session is closed S 34 . As such, each time the devices wish to communicate with one another, a new communication session is initiated, and a new encryption key is used. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
         
           
               20  continuous printer system 
               22  image source 
               24  image processing unit 
               26  mechanism control circuits 
               28  device 
               30  printhead 
               32  recording medium 
               34  recording medium transport system 
               36  recording medium transport control system 
               38  micro-controller 
               40  reservoir 
               42  catcher 
               44  recycling unit 
               46  pressure regulator 
               47  channel 
               48  jetting module 
               49  nozzle plate 
               50  plurality of nozzles 
               51  heater 
               52  liquid 
               54  drops 
               56  drops 
               57  trajectory 
               58  drop stream 
               60  gas flow deflection mechanism 
               61  positive pressure gas flow structure 
               62  gas flow 
               63  negative pressure gas flow structure 
               64  deflection zone 
               66  small drop trajectory 
               68  large drop trajectory 
               72  first gas flow duct 
               74  lower wall 
               76  upper wall 
               78  second gas flow duct 
               82  upper wall 
               86  liquid return duct 
               88  plate 
               90  front face 
               92  positive pressure source 
               94  negative pressure source 
               96  wall 
               102  print data source 
               104  print data processor 
               106  printhead microprocessor 
               108  printhead memory 
               110  drop generator 
               114  host system 
               116  module electronics 
               118  first device 
               120  second device