Title of Invention: Secure currency Abstract: A secure document comprises a pliable fabric comprising human-readable information. The secure document further comprises a memory attached to the pliable fabric in which machine-readable information about the secure document is stored. The secure document further comprises an interface attached to the pliable fabric and coupled to the memory that, when a reader device reads the secure document, transmits at least a portion of the machine-readable information stored in the memory to the reader device. Detailed Description: Part general_description_paragraphs: Part TECHNICAL FIELD: The following description relates to secure documents in general and to secure currency in particular. Part BACKGROUND: The increasing availability of low-cost, full-color imaging devices such as laser and inkjet printers and off-the-shelf imaging software has lead to an increase in counterfeiting of paper-based currency. The treasury departments of governments across the world in conjunction with imaging product manufacturers have taken steps to reduce counterfeiting. One approach is to identify the imaging devices used to make counterfeit currency. For example, color copiers and printers typically print an invisible watermark that includes a serial number associated with that particular copier or printer. When a copy or print is made with such a copier or printer, the invisible watermark can be decoded in order to read the serial number. Such an approach, however, typically only helps in identifying the imaging device used to make that copy or print. Part SUMMARY: In one embodiment, a secure document comprises a pliable fabric comprising human-readable information. The secure document further comprises a memory attached to the pliable fabric in which machine-readable information about the secure document is stored. The secure document further comprises an interface attached to the pliable fabric and coupled to the memory that, when a reader device reads the secure document, transmits at least a portion of the machine-readable information stored in the memory to the reader device. In another embodiment, a currency comprises a pliable fabric comprising human-readable currency information. The secure currency further comprises a security module comprising a memory attached to the pliable fabric in which machine-readable currency information is stored and a radio frequency interface attached to the pliable fabric and coupled to the memory. The radio frequency interface transmits at least a portion of the machine-readable currency information to a radio frequency reader device when the radio frequency interface receives a radio frequency field radiated by the radio frequency reader device. In another embodiment, a currency comprises a fabric and a security module attached to the fabric. The security module comprises a memory in which information about the currency is stored and an ink reservoir in which ink is stored. When the security module receives a predetermined command, the security module releases the ink stored in the ink reservoir in order to mark the fabric. Another embodiment is a secure server for tracking a plurality of secure documents, each of the plurality of secure documents comprising human-readable information and a memory in which a machine-readable identifier is stored. The secure server comprises a database and an interface in communication with the database that, when coupled to a network, communicates over the network with at least one client device. When first information related to a first one of the plurality of secure documents is received by the secure server from the client device, the secure server stores at least a portion of the first information in the database. The first information comprises the machine-readable identifier read by the client device from the first one of the plurality of secure documents. Another embodiment is a device for reading a secure document that comprises human-readable information and a memory in which a machine-readable identifier is stored. The device comprises a reader device that, when the device reads the secure document, reads the machine-readable identifier stored in the memory of the secure document. The device further comprises an interface that, when coupled to a network, communicates with a secure server coupled to the network. The device sends first information related to the secure document to the secure server over the network via the interface. The first information includes the machine-readable identifier. Another embodiment is a method of tracking secure currency that comprises a pliable fabric and a security module in which machine-readable currency information is stored. The method comprises, in connection with a physical transfer of the secure currency, reading machine-readable currency information from the security module using a reader device, obtaining information related to the physical transfer of the secure currency, and sending at least a portion of the machine-readable currency information and the obtained information to a database for storage in the database. Another embodiment is a method of manufacturing secure currency that comprises a pliable fabric. The method comprises attaching a security module to the pliable fabric of the secure currency. The method further includes, after attaching the security module to the pliable fabric of the secure currency, putting the secure currency into an inactive state and, when the secure currency is ready to be put into circulation, activating the item of secure currency. Another embodiment is a method of destroying a secure document that comprises a pliable fabric and a security module attached to the pliable fabric. The method comprises, when the secure document is to be destroyed, deactivating the secure document, physically collecting the secure document, and destroying the secure document. Destroying the secure document comprises separating the security module from the pliable fabric of the secure document. The details of one or more embodiments of the claimed invention are set forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description, the drawings, and the claims. Like reference numbers and designations in the various drawings indicate like elements. Part DETAILED DESCRIPTION: FIG. 1 is a block diagram of one embodiment of a secure document 100. In the embodiment shown in FIG. 1, secure document 100 is implemented as an item of currency (referred to here as “secure currency” 100). It is to be understood, however, embodiments of secure document 100 are suitable for use in other applications, for example, where it is desired to have a document (such as a traveler's check, coupon, gift certificate, passport, driver's license, or other personal identification document) documents with security or tracking functionality. Secure currency 100 includes a pliable fabric 102. In one embodiment, the fabric 102 comprises natural and/or synthetic fibers that are weaved (or otherwise formed) into a fabric. In other embodiments, the pliable fabric 102 comprises cloth, paper, and/or laminate formed from or otherwise comprising natural and/or synthetic materials. Pliable fabric 102 is “pliable” in that fabric 102 allows the secure currency 100 to be handled in the same general manner as conventional paper currency including, for example, by allowing the secure currency 100 to be folded or curled without breaking. Various currency-related images and/or other indicia (collectively referred to here as “artwork” 104) are printed on the pliable fabric 102. For example, in one embodiment, the artwork 104 includes human-readable currency information 105. In the embodiment shown in FIG. 1, the human-readable currency information 105 includes a human-readable representation of a denomination 106 of the item of secure currency 100 (for example, indicating that the item of secure currency 100 is a 20 dollar bill). Also, in such an embodiment, the human-readable currency information 105 includes a human-readable representation of a serial number 108 (or other identification information) for the item of secure currency 100. In the embodiment shown in FIG. 1, the artwork 104 also includes a machine-readable representation of one or more of the human-readable items. The artwork 104 includes a bar code 110. The bar code 110, in one implementation of such an embodiment, is embedded in the artwork 104 using watermarking techniques so that the bar code 110 is not readable by a human observing the artwork 104. The watermarked bar code 110 in such an implementation, however, is readable by an optical bar code reader when the artwork 104 (which includes the watermarked bar code 110) is scanned by the optical bar code reader. In another implementation of such embodiment, the bar code 110 is embedded in the artwork 104 by printing the bar code 110 using a magnetic ink that is not perceptible to the unaided human eye. The magnetic bar code 110 in such an implementation, however, is readable by a magnetic bar code reader when the artwork 104 (which includes the magnetic bar code 110) is scanned by the magnetic bar code reader. In another implementation, the bar code 110 is embedded in the artwork 104 using watermarking techniques and using magnetic ink. In the embodiment shown in FIG. 1, the bar code 110 includes machine-readable currency information 111 encoded therein. For example, as shown in FIG. 1, the machine-readable currency information 111 comprises a machine-readable denomination 113 (for example, 20 dollars) and a machine-readable identifier 115 (for example, a machine-readable serial number 115 in the embodiment shown in FIG. 1). The secure currency 100 also includes security module 112. In the embodiment shown in FIG. 1, the security module 112 is implemented using a passive radio frequency identification (RFID) transponder. In one implementation of the embodiment shown in FIG. 1, the security module 112 is fabricated as a single, monolithic component, for example, as a single integrated circuit (also referred to here as a “single chip”) that includes the functionality described here as being included in the security module 112. The security module 112 includes a memory 114. In the embodiment shown in FIG. 1, at least a portion of memory 114 is non-volatile. In one embodiment, memory 114 includes a relatively small (for example, on the order of 1 kilobyte) and low-power read only memory (ROM) or write-once memory (WOM). In such an embodiment, machine-readable currency information 119 is stored in the memory 114 when the currency 100 is manufactured or when the currency 100 is activated and put into circulation (for example, as described below in connection with FIG. 7). In one example, the machine-readable currency information 119 comprises a machine-readable denomination 116 (for example, 20 dollars) and a machine-readable identifier 118 (for example, a machine-readable serial number 118 in the embodiment shown in FIG. 1). In other embodiments, other information or data is stored in memory 114 in addition to or instead of the denomination 116 and serial number 118. For example, in one embodiment, such other information or data includes an indication of whether a particular item of secure currency 100 has been activated or deactivated and/or an indication of whether the particular item of secure currency 100 has been tampered with. Other examples of such other information or data include data generated by one or more sensors included in the secure currency 100 and/or data related to one or more transactions in which the particular item of secure currency 100 has been used. The security module 112 also includes an integrity meter 120. The integrity meter 120 includes a sensor or other mechanism that is used to determine whether the integrity of the security module 112 or the pliable fabric 102 has been compromised due to, for example, tampering or wear. In one embodiment, the integrity meter 120 determines whether the security module 112 is securely attached to the pliable fabric 102. One such embodiment of an integrity meter 120 is described below in connection with FIG. 10. The security module 112 includes a radio frequency (RF) interface 122. The RF interface 122 includes an antenna 124 on which RF signals are received and transmitted. A RF reader device (not shown in FIG. 1) communicates with the security module 112 using a RF signal in order to “read” data stored in the memory 114 of the security module 112. Also, in the embodiment shown in FIG. 1, such a RF reader device communicates with the security module 112 using a RF signal in order to “write” data to the memory 114. In another embodiment, however, the security module 112 can only be read via the RF interface 122 and not written to. In such a read-only embodiment, functionality described here for carrying out such write operations via the RF interface 122 need not be included in the security module 112. In such a read-only embodiment, data is written to memory 114 some other way (for example, by including a programmable read only memory (PROM) in memory 114 that is programmed, for example, using a laser). When a RF signal radiated by a RF reader device is received on the antenna 124, a power extraction circuit 126 included in the RF interface 122 outputs a power signal generated from the received RF signal. When the received RF signal contains sufficient power, the power signal output by the power extraction circuit 126 turns on the components of the security module 112 (also referred to here as “powering on” or “waking up” the security module 112). In one embodiment, the power extraction circuit 126 includes a rectifier that rectifies the received RF signal in order to output a direct current (DC) power signal used to power the security module 112. In one implementation of such an embodiment, the power extraction circuit 126 includes a capacitor (or other power storage element) that is charged by the DC power signal output by such a rectifier. In another implementation, no such power storage element is used. The RF interface 122 also includes a receive circuit 128. The receive circuit 128 extracts any data included in the received RF signal (referred to here as “received data”). In some embodiments, the receive circuit 128 includes a demodulator that demodulates the received RF signal to extract data modulated thereon. A buffer 129 stores a digital representation of the extracted data output by the demodulator. The RF interface 122 also includes a transmit circuit 130. The transmit circuit 130 is used to transmit data over the antenna 124. In one embodiment, the transmit circuit 130 includes a modulator that modulates the received RF signal with the transmitted data. In one implementation of such an embodiment, the transmit circuit 130 damps the received RF signal with an internal load across the antenna 124 in order to modulate the received RF signal with the transmit data. In some embodiments, the transmit data includes at least a portion of the information stored in memory 114. For example, in one such embodiment, the denomination 116 and serial number 118 stored in the memory 114 are transmitted. Also, in one embodiment, the status of the integrity meter 120 (or other sensors included in the security module 112) is transmitted. Although the antenna 124, the power extraction circuit 126, the receive circuit 128, and the transmit circuit 130 are shown separately in FIG. 1, in some embodiments one or more of these items are combined in whole or in part. The security module 112 also includes a decoder 132. In the embodiment shown in FIG. 1, the decoder 132 determines what operation, if any, the security module 112 should perform when the security module 112 is woken up. In one embodiment, an authentication process or operation is performed before any read or write operations are performed by the security module 1112. In one implementation of such an embodiment, the decoder 132 authenticates the RF reader device that radiated the RF field that woke up the security module 112. In another implementation of such an embodiment, the RF reader device authenticates the security module 112. In another implementation, both the security module 112 authenticates the RF reader device and the RF reader device authenticates the security module 112. Where such an authentication process or operation is performed, a subsequent read or write operation is performed only if the authentication process or operation is successful. In one implementation of such an embodiment, authentication data 117 is also stored in memory 114 (for example, during manufacture or activation of the item of secure currency 100). The authentication data 117, for example, includes data indicating which RF reader devices are authorized to communicate with the RF reader device and/or data that is provided to the RF reader device so that the RF reader device can determine if the security module 112 is authorized to communicate with that RF reader device. When the decoder 132 determines that a write operation is to be performed, at least a portion of the received data is written to memory 114. In one embodiment where such received data is written to write once memory included in memory 114, such a write operation is performed only once. Also, in one embodiment where communications between the security module 112 and the reader device are encrypted, the decoder 132 decrypts the received data prior to storage in the memory 114. In another embodiment where communications between the security module 112 and the reader device are encrypted, the decoder 132 stores the received data in memory 114 in encrypted form. When the decoder 132 determines that a read operation is to be performed, at least a portion of the data stored in memory 114 is read and transmitted to the reader device via the transmit circuit 130. Also, in the embodiment shown in FIG. 1, when a read operation is to be performed, the state of the integrity meter 120 is read when a read operation is performed. The state of the integrity meter 120 is included in the data that is transmitted to the RF reader device. In one embodiment where communications between the security module 112 and the RF reader device are encrypted, the decoder 132 encrypts the data read from the memory 114 and the integrity meter 120 prior to transmission. In another embodiment where communications between the security module 112 and the reader device are encrypted and where data stored in the memory 114 is stored in encrypted form, the decoder 132 does not need to encrypt the data read from the memory 114 since the data is already in encrypted form. In one embodiment, the decoder 132 is implemented as a finite state machine that is implemented, for example, as a non-clocked, in-line encoding logic circuit. Using such a non-clocked circuit obviates the need to generate a clock signal. In other embodiments, implemented using circuitry and/or devices that require a clock signal, the RF interface 122 includes a clock recovery circuit that generates a clock signal based on the received RF signal. FIGS. 2A-2B show a flow diagram of one embodiment of a method 200 of writing data to and reading data from a security module included on a secure document. In the embodiment shown in FIGS. 2A-2B, method 200 is implemented using the embodiment of secure currency 100 shown in FIG. 1. In such an embodiment, the functionality of method 200 is performed by the decoder 132 of the security module 112 while the security module 112 communicates with a RF reader device 316 (described below in connection with FIG. 3). It is to be understood, however, that other embodiments of method 200 are implemented using other embodiments of secure currency, other types of secure documents, and/or other reader devices. Method 200 starts when the security module 112 is woken up (checked in block 202 shown in FIG. 2A). The security module 112 is woken up when the RF signal received by the antenna 124 contains sufficient power to power on the security module 112. In one example, this occurs when the item of secure currency 100 is placed in a RF field radiated by a RF reader device. When such a RF signal is received by the antenna 124, the power extraction circuit 126 outputs a power signal. If the power signal contains sufficient power to wake up the security module 112, the security module 112 wakes up. In the embodiment shown in FIGS. 2A-2B, when the security module 112 of the secure currency 100 wakes up, an authentication process is performed (block 204). For example, in one implementation of such an embodiment, the decoder 132 authenticates the RF reader device with which the security module 112 communicates. The decoder 132 authenticates the RF reader device, for example, to determine if that RF reader device is authorized to communicate with that security module 112. In one such implementation, the decoder 132 authenticates the RF reader device using an identifier (or other data) included in data received by the security module 112 from the RF reader device. The identifier is compared to the authentication data 117 stored in memory 114. In another implementation of such an embodiment, the RF reader device authenticates the security module 112. That is, the RF reader device with which the security module 112 communicates determines if that security module 112 is authorized to communicate with that RF read device. In one such implementation, the RF reader device authenticates the security module 112, for example, by having the decoder 132 cause the security module 112 to transmit the serial number of the secure currency 100 (or other data such as authentication data 117) to the RF reader device. The RF reader device receives the serial number (or other data) and compares the serial number to a list (or other data structure) containing authorized items of secure currency 100. In another implementation of such an embodiment, the decoder 132 authenticates the RF reader device and the RF reader device authenticates the security module 112. In another embodiment, no authentication process is performed. In the embodiment shown in FIGS. 2A-2B, if the authentication process is unsuccessful (checked in block 206), the security module 112 powers down (block 208 shown in FIG. 2B) and method 200 is restarted. If the authentication process is successful, the decoder 132 determines if a write operation is to be performed (block 210 shown in FIG. 2A). In one implementation of such an embodiment, when a write operation is to be performed, the RF signal output by the RF reader device includes an amount of power that is above a predetermined write power threshold. Such an approach is suitable for embodiments of the secure currency 100 where the amount of power required by the security module 112 to write data to the memory 114 is higher than that required to read data from the memory 114. In such an implementation, the power extraction circuitry 126 includes power level detection circuitry that detects whether or not the amount of power supplied in the received RF signal is greater than the predetermined write power threshold. In another implementation of such an embodiment, the determination as to whether a write operation is to be performed is made by the decoder 132 by inspecting data received on the antenna 124 from the RF reader device. If the received data includes a predetermined command or other data (also referred to here as a “write command”), a write operation is to be performed. In other embodiments, the determination as to whether a write operation is to be performed is made in other ways. If a write operation is to be performed, the decoder 132 receives data via the antenna 124 (block 212). In the embodiment shown in FIGS. 2A-2B, the data transmitted by the reader device 316 (shown in FIG. 3) and received via antenna 124 is in encrypted form and is decrypted by decoder 132 before writing to memory 114 (block 214). After the received data is decrypted, the decoder 132 writes at least a portion of the decrypted data to memory 114 (block 216). In the embodiment shown in FIGS. 2A-2B, when a RF signal is received via the antenna 124, the receive circuit 128 demodulates and extracts any data encoded in the received RF signal and stores a digital representation of the extracted data in buffer 129. The decoder 132 decrypts the received data and writes at least a portion of the decrypted data to memory 114. In other embodiments, the received data is not decrypted prior to being written to memory 114, for example, because the data stored in memory 114 is stored in encrypted form or because the data transmitted by the RF reader device is not in encrypted form. In one implementation of the embodiment shown in FIGS. 2A-2B, all the data stored in the memory 114 is overwritten when a write operation is performed. In such an implementation, the decoder 132 writes the first unit of data that is written to a predetermined starting address in memory 114 and writes each successive unit of data at successive addresses in memory 114 (for example, by incrementing a counter). Such an approach to writing data is suitable for use in initializing all the data stored in the memory 114 at one time, for example, during manufacture of the secure currency 100 or during activation of the secure currency 100. For example, where memory 114 includes write-once-memory, such a write operation is performed only once to write all data stored in memory 114. After the write operation is complete, the security module 112 powers down (block 208 shown in FIG. 2B) and method 200 is restarted. If a read operation is to be performed, the decoder 132 reads from memory 114 at least a portion of the data stored in memory 114 (block 218 shown in FIG. 2B). In the embodiment shown in FIGS. 2A-2B, the data read from memory 114 is encrypted (block 220) and then transmitted to the RF reader device (block 222). In the embodiment shown in FIGS. 2A-2B, the decoder 132 reads data from memory 114 and encrypts the read data. The transmit circuit 130 encodes and modulates the encrypted data to produce a RF signal that is transmitted over antenna 124. Then, the security module 112 powers down (block 208) and method 200 restarts. In one implementation of the embodiment shown in FIGS. 2A-2B, all the data stored in the memory 114 is read from memory 114 and transmitted over antenna 124 when a read operation is performed. In such an implementation, the decoder 132 reads the first unit of data that is read during the read operation from a predetermined starting address in memory 114 and reads each successive unit of data at successive addresses in memory 114 (for example, by incrementing a counter) until all data is read. FIG. 3 is a block diagram of one embodiment of a secure document system 300. In the embodiment shown in FIG. 3, secure document system 300 is implemented using the embodiment of secure currency 100 shown in FIG. 1. Secure document system 300 is also referred to here as a “secure currency” system 300. It is to be understood, however, that other embodiments of secure document system 300 are implemented using other embodiments of secure currency and other types of secure documents. System 300 includes a secure server 302. Secure server 302 includes a currency database 304. Server software 305 executing on the secure server 302 stores information about items of secure currency 100 in the currency database 304. The server software 305 is executed on one or more programmable processors 307 included in the secure server 302. For example, in one embodiment, for each item of secure currency 100 tracked by the secure server 302, the information stored in the currency database 304 for that item of secure currency 100 includes the serial number (or other identifier) and denomination for that item of secure currency 100. In such an embodiment, the information stored in the currency database 304 for each item of secure currency 100 also includes an indication of whether that item of secure currency 100 has been activated or deactivated. In one embodiment, the information stored in the secure currency database 304 includes where each item of secure currency 100 was printed, where each item of secure currency 100 was activated, and information about one or more transactions in which each item of secure currency 100 was used. As used herein, a transaction occurs when one or more items of secure currency 100 are physically transferred from one party to another party (for example, where one or more items of secure currency 100 are tendered for payment for goods and/or services). For example, in one implementation of such an embodiment, the transaction information stored in the currency database 304 for an item of secure currency 100 includes where and when a transaction involving that item of secure currency 100 took place, who was involved in the transaction, how much money was involved in that transaction, and the serial numbers of other items of secure currency 100 involved in the transaction. In other embodiments, other information is stored in the currency database 304 for each item of secure currency 100 that is tracked by the secure server 302. For example, in embodiments where the secure currency 100 includes a sensor (for example, an integrity meter 120), data generated by the sensors is stored in the currency database 304 for each item of secure currency 100. The secure server 302 includes a network interface 306 that is used to couple the secure server 302 to a network 310. One or more client devices 308 communicate with the secure server 302 using the network 310. Each client device 308 includes a network interface 309 that is used to couple the client device 308 to the network 310 so that the client device 308 can communicate over the network 310. Each client device 308 executes client software 311 that carries out the functionality described here as being performed by each client device 308. The client software 311 is executed on one or more programmable processors 313 included in each client device 308. One example of a client device 308 is a computer workstation. In the embodiment shown in FIG. 3, the network 310 includes a public network such as the Internet. In such an embodiment, the server software 305 includes a server cryptography module 312 and the client software 311 executed on each of the client devices 308 includes a client cryptography module 314. The cryptography modules 312 and 314 are used to encrypt, decrypt, and authenticate communications between the client devices 308 and the secure server 302. In one implementation of such an embodiment, the cryptography modules 312 and 314 are implemented using public key cryptography technology (for example secure sockets layer (SSL) technology). In the embodiment shown in FIG. 3, at least one client device 308 has a RF reader device 316 coupled to that client device 308 (for example, over a serial communication link). The RF reader device 316 includes a RF transceiver 317 and an antenna 319. The operation of the RF reader device 316 is controlled by the client software 311 executing on the client device 308. When the RF reader device 316 is used to read one or more items of secure currency 100, the RF transceiver 317 radiates an RF signal via antenna 319, thereby creating an RF field. When an item of secure currency 100 is placed within the radiated RF field, the antenna 124 of the item of secure currency 100 receives the RF signal radiated by the antenna 319. As described above, the power extraction circuit 126 outputs a power signal based on the received RF signal. If the power signal contains sufficient power, the security module 112 powers on and communicates with the RF reader device 316. In one embodiment, the RF reader device 316 detects the presence of the powered-on security module 112. Once the security module 112 has woken up and the RF reader device 316 has detected the presence of the item of secure currency 100 (more specifically, the security module 112), the RF reader device 316 reads information from or writes information to the security module 112, for example, as described above in connection with the embodiment of method 200 shown in FIGS. 2A-2B. In one example, the RF reader device 316 reads the denomination 116 and serial number 118 stored in memory 114 of the security module 112 and the status of the integrity meter 120 of the security module 112. The client device 308 to which the RF reader device 316 is coupled then uses the data read from the security module 112 for subsequent processing. In another example, the RF reader device 316 is used to write the denomination 116 and serial number 118 to memory 114 of the security module 112 (for example, during manufacture or activation of the item of secure currency 100). In such an example, the RF reader device 316 is sometimes referred to as a “RF writer device” or a “RF reader/write device” (where the device is capable of both reading from and writing to the security module 112). In embodiments of the secure currency 100 where memory 114 is implemented using write-once memory, such a write operation is performed once (for example, during manufacture or activation of the item of secure currency 100). In the embodiment shown in FIG. 3, at least one client device 308 has a bar code reader device 318 coupled to that client device 308. In embodiments where the bar code 110 included in the artwork 104 of an item of secure currency 100 is a watermarked bar code, the bar code reader device 318 includes an optical bar code reader suitable for reading the watermarked bar code printed on the item of secure currency 100. In embodiments where the bar code 110 included in the artwork 104 is a magnetic bar code, the bar code reader device 318 includes a magnetic bar code reader suitable for reading the magnetic bar code printed on the item of secure currency 100. In other embodiments, the bar code reader device 318 includes both optical bar code reader and magnetic bar code reader functionality that allows the bar code reader device 318 to read both watermarked and magnetic bar codes printed on items of secure currency 100. In the embodiment shown in FIG. 3, at least some of the client devices 308 include or are attached to a display device 320 on which the client device 308 displays information read from the item of secure currency 100 and/or received from the secure sever 302. In one implementation, the display device 320 includes a computer monitor. For example, information read from an item of secure currency 100 such as the denomination 116 is displayed on a display device 320, for example, so that a user of the client device 308 can check if the human-readable denomination 106 matches the denomination 116 stored in memory 114 of the item of secure currency 100. In addition (or instead), information returned from the secure server 302 such as an indication of whether the item of secure currency 100 has been activated or reported stolen is displayed on a display device 320. In another implementation, the display device 320 includes one or more light emitting diodes or other indicators that are used to indicate the status of some aspect of the operation of the client device 308, RF reader device 316, and/or bar code reader 318 attached to that client device 308. For example in one implementation, one indicator indicates that the client device 308 is ready to read an item of secure currency 100 and then, after reading the item of secure currency 100, indicates that the read operation was successful or unsuccessful. In the embodiment of a secure document system 300 shown in FIG. 3, server software 305 includes a web server 322. The web server 322 allows a user of a client device 308 to interact with the secure sever 302 using a web browser 324 executing on the client device 308. A user interface for the web browser 324 is displayed on a display device 320 connected to the client device 308. Information is received from the user via an input device 321 coupled to the client device 308 (such as a keyboard). Information is displayed for the reader via the user interface of the web browser 324. Thus, a user need not have a RF reader device 316 or a bar code reader device 318 to query the secure server 302 (for example, to determine if an item of secure currency 100 has been activated) or to supply information about an item of secure currency 100 or a transaction in which the item was used. For example, in such an embodiment, when one or more items of secure currency 100 are tendered in order to pay for goods or services, a user directs the web browser 324 to access a web site provided by the web server 322. A web page is supplied by the web server 322 to the web browser 324 for display by the client device 308 on a display device 320 attached to the client device 308. The web page includes one or more fields (or other user interface elements) in which the user is able to enter information about the item of secure currency 100 and/or the transaction in which the item is used. In one scenario, the user reads the human-readable serial number 108 and human-readable denomination 106 included in the artwork 104 printed on each item of secure currency 100 and enters that information into an appropriate field on the web page displayed by the web browser 324. As a result, the web browser 324 will have the denomination and serial number of each item of secure currency 100 used in the transaction. The web browser 324 communicates the entered information about each item of secure currency 100 to the web server 322. In such an embodiment, the web server 322 provides information to the client device 308 for display by the web browser 324 on the display device 320. More specifically, the web server 322 generates and sends to the web browser 324 an appropriate web page for display on the display device 320 coupled to the client device 308. In the embodiment shown in FIGS. 2A-2B, the server cryptography module 312 of the secure server 302 and the client cryptography module 314 of that client device 308 secure the communications between the web browser 324 and the web server 322 (for example, using SSL technology). FIG. 4 is a flow diagram of one embodiment of a method 400 of tracking an item of secure currency 100. In the embodiment shown in FIG. 4, the functionality of method 400 is implemented and carried out by a client device 308 of the secure system 300 shown in FIG. 3. For example, in one exemplary implementation, the functionality of method 400 is implemented in software that is executed by client device 308. Moreover, the embodiment of method 400 is used to track items of secure currency 100 of the type shown in FIG. 1. The embodiment of method 400 is suitable for use, for example, at locations where items of secure currency 100 are received (for example, at a retail point of sale or a bank teller where a client device 308 is located). Other embodiments of method 400, however, are implemented using other devices, systems, and secure documents. When one or more items of secure currency 100 are tendered in connection with a transaction (checked in block 402), information related to the transaction is read from at least one of the items of secure currency 100 (block 404). Information such as denomination and serial number is read from each item of secure currency 100. Each item of secure currency 100 can be read in a number of ways. A user of the client device 308 can read the human-readable serial number 108 and the human-readable denomination 106 printed on each item of secure currency 100 and then enter the human-readable information into, for example, a web browser 324 executing on the client device 308. Where a RF reader 316 is coupled to the client device 308, the RF reader 316 can be used to read the information (for example, serial number 118 and denomination 116) stored in memory 114 of each item of secure currency 100. Where a bar code reader 318 is coupled to the client device 308, the bar code reader 318 can be used to read the information encoded in the bar code 110 included in the artwork 104 printed on each item of secure currency 100. In the embodiment of method 400 shown in FIG. 4, the client device 308 receives (or otherwise obtains) additional information about the transaction (block 406). In one embodiment, such additional transaction information includes information about the receiving client device 308. For example, the client device information in one embodiment includes a serial number or other identifier of the client device 308 and a location of the client device 308. Such client device information, in one embodiment, is predetermined (that is, the client device 308 is preset with the serial number and/or information about the location of the client device 308) and information typically does not change from transaction to transaction. In one embodiment, such additional transaction information includes information about one or more of the parties to the transaction (for example, a social security number or other identifier), the subject matter of the transaction (for example, the particular goods or services purchased), any taxes or other governmental fees that were paid as a part of transaction, and/or the time of the transaction. Such additional transaction information is provided to the client device 308, for example, by having a user of the client device 308 enter such information into a web browser 324 (or other software) executing on the client device 308, or by receiving the information from, for example, an optical scanner coupled to the client device 308. The scanner is used to scan a universal price code (UPC) bar code affixed to each of one or more goods purchased as a part of the transaction. The information read from the items of secure currency 100 and the additional transaction information are sent from the client device 308 to the secure server 302 over network 310 (block 408). In one embodiment, the server cryptography module 312 and the client cryptography module 314 are used to encrypt, decrypt, and authenticate communications between the client devices 308 and the secure server 302 (for example, using public key encryption technology). The secure server 302 receives the transferred information and processes the transferred information, for example, as described below in connection with FIG. 5. In the embodiment shown in FIG. 4, the client device 308 receives a response from the secure sever 302 about the item of secure currency (block 410) and displays information on the display device 320 based on the received response (block 412). For example, in one implementation of such an embodiment, the response received from the secure server 302 includes an indication if there is any reason why any particular item of secure currency 100 included in the transaction should not be accepted or used in that particular transaction and/or that a particular governmental agency (for example, the police) should be notified in connection with the use of an item of secure currency 100 in this transaction. For example, it may be the case that the secure currency database 304 at the secure server 302 indicates that a particular item of secure currency 100 used in the transaction has been stolen. In such a case, the response sent from the server 302 by the client device 308 includes information indicating that the item of secure currency 100 has been stolen and should not be accepted and that the police (or the party that reported the particular of item of secure currency 100 as stolen) should be contacted in connection with this transaction. Client software 311 (such as web browser 324) executing on the client device 308 displays such information on a display device 320 coupled to the client device 308 for a user of the client device 308 to view. FIG. 5 is a flow diagram of one embodiment of a method 500 of tracking an item of secure currency 100. In the embodiment shown in FIG. 5, the functionality of method 500 is implemented and carried out by a secure server 302 of the secure system 300 shown in FIG. 3. For example, in one exemplary implementation, the functionality of method 500 is implemented in software that is executed by the secure server 302. Moreover, the embodiment of method 500 is used track items of secure currency 100 of the type shown in FIG. 1. The embodiment of method 500 is suitable for use, for example, with client devices 308 that are located where items of secure currency 100 are received (for example, at a retail point of sale or a bank teller where a client device 308 is located). Other embodiments of method 500, however, are implemented using other devices, systems, and secure documents. When information about an item of secure currency 100 is received at the secure server 302 from a client device 308 (checked in block 502), the secure server 302 extracts an identifier of item of secure currency 100 (block 504). For example, in the embodiment shown in FIG. 5, information about the item of secure currency 100 is read from the item of secure currency 100 and transmitted to the secure server 302 over the network 310 in accordance with method 400 of FIG. 4. The secure server 302, for example, extracts a serial number from the information transmitted to the secure server 302 to use as the identifier of that item of secure currency 100. In such an embodiment, the identifier is used as a key into the currency database 304. The secure server 302 stores at least a portion of the received information in the currency database 304 (block 506). In one example, the received information includes information about a transaction in which the item of secure currency is being used. The transaction information is stored in the currency database 304 for that item of secure currency 100. Also, in the embodiment of method 500 shown in FIG. 5, the secure server 302 retrieves information about the item of secure currency 100 from the currency database 304 using the identifier (block 508). In one example, the secure server 302 retrieves status information stored in the currency database 304 for that item of secure currency 100 using the identifier as a key. The status information in one example indicates whether the item of secure currency 100 has been activated or deactivated (for example, in accordance with an embodiment of method 700 described below in connection with FIGS. 7A-7B) and/or whether the item of secure currency 100 has been reported stolen. In other embodiments, other types of information are retrieved from the currency database 304. The secure server 302 transmits a response to the client device 308 derived from at least a portion of the retrieved information (block 510). In the embodiment shown in FIG. 5, the response is transmitted between a server programs executing on secure server 302 to a client program executing on the client device 308 using some type of remote procedure call (RPC) technology. In one example, a web server 322 included in secure server 302 generates and supplies to the client device 308 a web page containing at least a portion of the retrieved information (or information derived from the retrieved information). The web page is transmitted to the client device 308, and a web browser 324 executing on the client device 308 displays the web page on a display device 320 coupled to the client device 308. The server cryptography module 312 and the client cryptography module 314 are used to encrypt, decrypt, and authenticate communications between the secure server 302 and the client devices 308 (for example, using public key encryption technology) in this example. In one such example, the web page generated by the secure server 302 includes an indication if there is any reason why the item of secure currency 100 should not be accepted or used in that transaction and/or that a particular governmental agency (for example, the police) should be notified in connection with the use of that item of secure currency 100. For example, it may be the case that the currency database 304 at the secure server 302 indicates that the item of secure currency 100 used in the transaction has been stolen. In such a case, the web page sent from the server 302 by the client device 308 includes information indicating that the item of secure currency 100 has been stolen and should not be accepted and that the police (or the party that reported the particular of item of secure currency 100 as stolen) should be contacted in connection with this transaction. In one embodiment, the secure server 302 automatically notifies the government agency (for example, by sending an email or other communication to the government agency). FIG. 6 is a block diagram of one embodiment of a system 600 for manufacturing secure documents. In the embodiment of system 600 shown in FIG. 6, items of secure currency 100 of the type shown in FIG. 1 are manufactured.
{'bibliographic_information': "{'country': 'US', 'doc-number': '10781197', 'kind': 'B2', 'date': '20040218', 'extension': '146', 'invention_title': 'Secure currency'}", 'source_file': './grant/2007/ipg070724.xml', 'source_data': 'USPTO-Grant'}