Abstract:
Establishing a chain of trust in a public key infrastructure can be costly, time consuming and requires nearly constant access to the appropriate network-based authorities. Local trust between devices is established using a combination of a personal identification number (PIN) delivered out-of-band and self-signed certificates. The client may present the PIN to an electronic device such as a projector or printer so the electronic device can trust the client. The electronic device may present a self-signed digital certificate with the electronic device UUID based on a hash of the electronic device public key from the certificate.

Description:
BACKGROUND  
       [0001]     Establishing trust between electronic devices is important for the security of transactions and for preventing a variety of annoying, malicious, or destructive attacks. One method of establishing trust is public key infrastructure (PKI). Most often, PKI relies on each participant having a private key and a public key for use in cryptographic functions. The public and private keys are used cooperatively for digital signatures and for encrypting/decrypting. Trust between participants is established using digital certificates for each participant, with each digital certificate traceable to a root certificate authority. The root certificate authority takes responsibility for the integrity of each digital certificate in its hierarchy, as long as each participant routinely verifies other digital certificates against a certificate revocation list. There is a cost, often significant, associated with developing and maintaining a root certificate authority and a hierarchy of intermediate and user certificates. As well, there is a cost associated with routinely verifying participant digital certificates. In some cases, each participant may not have free access to the root certificate authority or its associated certificate revocation list causing the chain of trust to be suspect.  
         [0002]     For some uses, the use of a full public key infrastructure is both prudent and cost-effective compared to the risks associated with fraud or other attacks. However, in other cases, the cost associated with a trusted public key infrastructure cannot be justified based on either the quantity of devices involved or the relatively low risk associated with typical transactions between devices. For example, a series of conference rooms may each have projection equipment available to conference room attendees with portable computers or to electronically connected remote attendees. There is a real, but low, risk that a non-attendee may capture the conference room projector to deny access to the service either maliciously or as a prank. The cost of supplying and maintaining fully trusted public key infrastructure could hardly be justified relative to the low risk and relatively minimal consequences of such an attack. Nonetheless, the risk is real and the cost in inconvenience or correcting such a problem could be locally significant.  
       SUMMARY  
       [0003]     Self-signed certificates are one alternative to the high cost and high maintenance associated with full, trusted, public key infrastructure. Participating electronic devices may generate or be assigned a password or passphrase, and an identifier. In some embodiments, the identifier is a function of the universal unique identifier (UUID) of the electronic device. These two elements, the password and the identifier, may be used together or separately to aid in establishing trust between the electronic device and a requesting party or client. In one embodiment the two parts are distributed together, creating a longer, but more secure personal identification number (PIN). In another embodiment, only the password is used for the PIN. In this embodiment the identifier is a hash of the public key associated with the device which is also used as the UUID of the electronic device. The PIN may be delivered via a channel separate from that used for a subsequent electronic connection with a client and may be used to establish sufficient trust in self-signed certificate applications.  
         [0004]     The PIN may be transported via e-mail, entered from a sticker on the device itself, or displayed on a monitor or other display associated with the device. For example, a projector may display the PIN on the projection or a printer or may display a PIN on a local multi-line display. The PIN can be subsequently used in establishing trust of the client by the electronic device. To establish trust of the electronic device by the client, the public key of electronic device may be operated on, for example, hashed, to give a value that is set equal to the universal unique identifier (UUID) of the electronic device. Since the UUID resolves to the device and the digital certificate supplied by the device is linked to the UUID, trust of the electronic device by the client can also be established. Supplemental techniques described below, such as including a portion of the UUID in the PIN may be useful for increasing confidence in the trusted relationship. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a simplified and representative block diagram of a computer network;  
         [0006]      FIG. 2  is a block diagram of a computer that may be connected to the network of  FIG. 1 ;  
         [0007]      FIG. 3  is flow chart of a method of establishing trust between a client and an electronic device;  
         [0008]      FIG. 4  is a flow chart of an alternate method of establishing trust between a client and an electronic device; and  
         [0009]      FIG. 5  is a flow chart of a detail of the method of  FIG. 4  for calculating a PIN value.  
     
    
     DETAILED DESCRIPTION  
       [0010]     Although the following text sets forth a detailed description of numerous different embodiments, it should be understood that the legal scope of the description is defined by the words of the claims set forth at the end of this disclosure. The detailed description is to be construed as exemplary only and does not describe every possible embodiment since describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims.  
         [0011]     It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘______’ is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term by limited, by implication or otherwise, to that single meaning. Finally, unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. §112, sixth paragraph.  
         [0012]     Much of the inventive functionality and many of the inventive principles are best implemented with or in software programs or instructions and integrated circuits (ICs) such as application specific ICs. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts in accordance to the present invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the preferred embodiments.  
         [0013]      FIGS. 1 and 2  provide a structural basis for the network and computational platforms related to the instant disclosure.  
         [0014]      FIG. 1  illustrates a network  10  that may be used to implement a dynamic software provisioning system. The network  10  may be the Internet, a virtual private network (VPN), or any other network that allows one or more computers, communication devices, databases, etc., to be communicatively connected to each other. The network  10  may be connected to a personal computer  12  and a computer terminal  14  via an Ethernet  16  and a router  18 , and a landline  20 . Other networked resources, such as a projector  13  and printer  15  may also be supported via the Ethernet  16  or another data network. On the other hand, the network  10  may be wirelessly connected to a laptop computer  22  and a personal data assistant  24  via a wireless communication station  26  and a wireless link  28 . Similarly, a server  30  may be connected to the network  10  using a communication link  32  and a mainframe  34  may be connected to the network  10  using another communication link  36 . In one embodiment, the server  30  may function as a presentation server for serving presentation data on the network  10 . In another embodiment, the mainframe  34  may function as a broadcast server to make available data to a large number of users, for example, corporate financial results presentations. The network  10  may be useful for supporting peer-to-peer network traffic. It should be noted that peer-to-peer network traffic may pass through intermediate hosts, including servers, proxies, routers, switches, and other elements whose role is to facilitate the transmission of data between the communicating hosts.  
         [0015]      FIG. 2 . illustrates a computing device in the form of a computer  110 . Components of the computer  110  may include, but are not limited to a processing unit  120 , a system memory  130 , and a system bus  121  that couples various system components including the system memory to the processing unit  120 . The system bus  121  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus.  
         [0016]     The computer  110  may also include a cryptographic unit  125 . Briefly, the cryptographic unit  125  has a calculation function that may be used to verify digital signatures, calculate hashes, digitally sign hash values, and encrypt or decrypt data. The cryptographic unit  125  may also have a protected memory for storing keys and other secret data. In addition, the cryptographic unit  125  may include an RNG (random number generator) which is used to provide random numbers. In other embodiments, the functions of the cryptographic unit may be instantiated in software or firmware and may run via the operating system.  
         [0017]     Computer  110  typically includes a variety of computer readable media. Computer readable media can be any available media that can be accessed by computer  110  and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, FLASH memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by computer  110 . Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer readable media.  
         [0018]     The system memory  130  includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM)  131  and random access memory (RAM)  132 . A basic input/output system  133  (BIOS), containing the basic routines that help to transfer information between elements within computer  110 , such as during start-up, is typically stored in ROM  131 . RAM  132  typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  120 . By way of example, and not limitation,  FIG. 2  illustrates operating system  134 , application programs  135 , other program modules  136 , and program data  137 .  
         [0019]     The computer  110  may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only,  FIG. 2  illustrates a hard disk drive  141  that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive  151  that reads from or writes to a removable, nonvolatile magnetic disk  152 , and an optical disk drive  155  that reads from or writes to a removable, nonvolatile optical disk  156  such as a CD ROM or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive  141  is typically connected to the system bus  121  through a non-removable memory interface such as interface  140 , and magnetic disk drive  151  and optical disk drive  155  are typically connected to the system bus  121  by a removable memory interface, such as interface  150 .  
         [0020]     The drives and their associated computer storage media discussed above and illustrated in  FIG. 2 , provide storage of computer readable instructions, data structures, program modules and other data for the computer  110 . In  FIG. 2 , for example, hard disk drive  141  is illustrated as storing operating system  144 , application programs  145 , other program modules  146 , and program data  147 . Note that these components can either be the same as or different from operating system  134 , application programs  135 , other program modules  136 , and program data  137 . Operating system  144 , application programs  145 , other program modules  146 , and program data  147  are given different numbers here to illustrate that, at a minimum, they are different copies. A user may enter commands and information into the computer  20  through input devices such as a keyboard  162  and cursor control device  161 , commonly referred to as a mouse, trackball or touch pad. A camera  163 , such as web camera (webcam), may capture and input pictures of an environment associated with the computer  110 , such as providing pictures of users. The webcam  163  may capture pictures on demand, for example, when instructed by a user, or may take pictures periodically under the control of the computer  110 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  120  through an input interface  160  that is coupled to the system bus, but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB). A monitor  191  or other type of display device is also connected to the system bus  121  via an interface, such as a graphics controller  190 . In addition to the monitor, computers may also include other peripheral output devices such as speakers  197  and printer  196 , which may be connected through an output peripheral interface  195 .  
         [0021]     The computer  110  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  180 . The remote computer  180  may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  110 , although only a memory storage device  181  has been illustrated in  FIG. 2 . The logical connections depicted in  FIG. 2  include a local area network (LAN)  171  and a wide area network (WAN)  173 , but may also include other networks. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet.  
         [0022]     When used in a LAN networking environment, the computer  110  is connected to the LAN  171  through a network interface or adapter  170 . When used in a WAN networking environment, the computer  110  typically includes a modem  172  or other means for establishing communications over the WAN  173 , such as the Internet. The modem  172 , which may be internal or external, may be connected to the system bus  121  via the input interface  160 , or other appropriate mechanism. In a networked environment, program modules depicted relative to the computer  110 , or portions thereof, may be stored in the remote memory storage device. By way of example, and not limitation,  FIG. 2  illustrates remote application programs  185  as residing on memory device  181 .  
         [0023]     The communications connections  170   172  allow the device to communicate with other devices. The communications connections  170   172  are an example of communication media. The communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. A “modulated data signal” may be a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Computer readable media may include both storage media and communication media.  
         [0024]      FIG. 3  is a flow chart of a method  300  of establishing trust between a representative client  301  and a representative electronic device  302 . The client  301  may be any electronic device or user-oriented, processor-based equipment that may want to establish a trusted relationship with an electronic device  302 . Examples of possible clients include, but are not limited to, a laptop  22 , a personal computer  12 , a personal digital assistant  24 , a cellular telephone (not depicted) and the like. The electronic device  110 , may be a computer  12 , or other processor-based device or peripheral such as a printer  15 , a projector  13 , a cellular telephone (not depicted), a personal digital assistant  24 , a network access point  26 , a scanner (not depicted) and the like. Both the client  301  and the electronic device  302  may have some or all of the functional components of the computer of  FIG. 2 , including, but not limited to, a processing unit  120 , at least one of the memory architectures  130 ,  141   151   155 , a network interface  170 , and a hardware or software implementation of the cryptographic unit  125 .  
         [0025]     The electronic device  302  may acquire a public/private key pair at block  303 . The generation of public/private key pairs is well known in the art of cryptography, particularly asymmetric cryptography. RSA and elliptic curve are both examples of asymmetric cryptographic methods for generating and using public/private key pairs. RSA keys are often in the range of 1 kilobit, while elliptic curve keys are often in the range of 160 bits but may vary based on the cryptographic strength desired. As is known, one of the two keys of the key pair is kept secret and is designated the private key, the other key is made available to other entities and is designated the public key. The key pair may be produced internally using a cryptographic function or may received from an outside source, for example, from a hardware security module in a secure manufacturing environment.  
         [0026]     At block  304 , a cryptographic derivative of the public key may be generated, such as a hash, using any of many known hash algorithms, such as SHA-1 or MD5 or a more complex algorithm as described below. The hash of the public key may be used as the universal unique identifier (UUID) of the electronic device  302  at block  306 . In some networking implementations, such as web services, the UUID is used to identify a device using an address resolution protocol such as Web Services-Discovery. The linking of the UUID of the electronic device  302  to the public key associated with the electronic device  302  may be used later in establishing the trusted relationship.  
         [0027]     A digital certificate may be created that has fields including the public key and the UUID. Digital certificates, or just certificates, are known. One common standard defining the format and fields for digital certificates is the X.509 standard. Other fields may be the expiration date, the signing authority, etc. In one embodiment, the ‘subject’ field may be set to the UUID. As a reminder, the UUID is the hash of the public key, so the two are linked. The certificate may be self-signed, that is a signature field of the certificate may be encrypted using the certificate holders own private key, see block  303 . In one embodiment, the certificate may be fully qualified and signed by a recognized certificate authority.  
         [0028]     At block  308 , a personal identification number (PIN) may be created. In the strictest sense this is not a PIN, since this is not related to a person. In most applications a PIN is a limit number set, usually  4  numbers. As used herein, the PIN could be any phrase, word, character set, or description. However, since the PIN in this application is used to identify a particular endpoint, such as a device, it will be used for convenience. In one embodiment, the PIN may be a concatenation of a number, such as a random number, with at least a portion of the UUID. The random number portion may be thought of as the secret part of the PIN, although it is displayed in the clear in some embodiments. The full UUID may be concatenated with the secret when using the UUID to resolve the address of the electronic device  302 . Shortening the UUID portion may be done to make subsequent entry of the PIN easier, when done manually, but the UUID is no longer available for use as an address and other discovery processes may be necessary to find the electronic device  302 .  
         [0029]     The PIN may then be made available to clients using an out-of-band channel at block  310 . If an in-band channel is the network mechanism ultimately used for communication between the client  301  and the electronic device  302 , then an out-of-band channel is anything else. For example, the out-of-band channel may include but is not limited to electronic mail, a computer display  190  associated with the electronic device  302 , a sticker (not depicted) or label attached to the electronic device  302 , and a sticker or label attached to a remote control (not depicted) associated with the electronic device  302 .  
         [0030]     At block  312 , the client  301  may receive the PIN electronically, for example, by email, or a user may enter the PIN value via a user interface when viewed on one of the locations mentioned above. At block  314 , the PIN may then be parsed into two portions, the secret portion and the UUID portion. The client  301  may use the UUID to resolve the address of the electronic device  302  and a secure channel may be created at block  316 . The electronic device  302  participates in the secure channel creation and may then reply, at block  318 , with the self-signed certificate. The UUID may be embedded in the certificate, for example, in the subject field.  
         [0031]     Proceeding to block  320 , when the client  301  receives the certificate, it may determine if it is a trusted certificate or a self-signed certificate. If the certificate is trusted, the root authority may be checked, and if valid, the certificate accepted and the electronic device  302  given a trusted status. If the certificate is self-signed, the client  301  may extract the public key from the certificate and perform the agreed-to hash function. If the hash of the public key matches the UUID in the certificate, and if the electronic device  302  was discovered at that same UUID, the electronic device  302  may at that point be given trusted status by the client  301 .  
         [0032]     With the electronic device  302  trusted by the client  301 , it remains for the electronic device  302  to establish trust with the client  301 . At block  322  the electronic device  302  may send a message to the client  301  requesting a certificate or other identifier. The client  301  may respond at block  324  with a certificate with the PIN embedded in the certificate, for example, in the header field. The electronic device  302  may analyze the certificate at block  326  to determine if the PIN received in the certificate matches the PIN of the electronic device  302 . If they match, the electronic device  302  has an assurance that the client  301  has received the PIN and trust may be extended by placing the client certificate in a trusted store.  
         [0033]     Referring to  FIG. 4 , a flow chart of an alternate method of establishing trust between a client and an electronic device is discussed and described. As above, the client and electronic device may any combination of a number of devices. This exemplary embodiment allows the use of a potentially shorter PIN than that of the previous example.  
         [0034]     The electronic device  402  may acquire a public/private key pair at block  403 . The generation of public/private key pairs is well known in the art of cryptography, particularly asymmetric cryptography. RSA and elliptic curve are both examples of asymmetric cryptographic methods for generating and using public/private key pairs. RSA keys are often in the range of 1 kilobit, while elliptic curve keys are often in the range of 160 bits but may vary based on the cryptographic strength desired. As is known, one of the two keys of the key pair is kept secret and is designated the private key, the other key is made available to other entities and is designated the public key. The key pair may be produced internally using a cryptographic function or may received from an outside source, for example, a hardware security module in a secure manufacturing environment.  
         [0035]     At block  404 , a cryptographic derivative of the public key may be generated, such as a hash, using any of many known hash algorithms, such as SHA-1 or MD5 or another algorithm, described below. The hash of the public key may be used as the universal unique identifier (UUID) of the electronic device  402  at block  406 . In some networking implementations, such as web services, the UUID is used to identify a device using an address resolution protocol such as Web Services-Discovery. The linking of the UUID of the electronic device  402  to the public key associated with the electronic device  402  may be used later in establishing the trusted relationship.  
         [0036]     A digital certificate may be created that includes the public key and the UUID. Digital certificates, or just certificates, are known. One common standard defining the format and fields for digital certificates is the X.509 standard. Other fields may be the expiration date, the signing authority, etc. In one embodiment, the ‘subject’ field is set to the UUID. As a reminder, the UUID is the hash of the public key, so the two are linked. The certificate may be self-signed, that is the signature field of the cert may be encrypted using the certificate holders own private key, see block  403 . In one embodiment, the certificate may be fully qualified and signed by a recognized certificate authority.  
         [0037]     At block  408 , a PIN may be generated and made available using out-of-band channels the same as or similar to those described above, for example, a displayed value or a sticker attached to the electronic device  402 . The PIN may simply be a random number or other number denoted as a secret for the purpose of this discussion.  
         [0038]     At block  410 , the client  401  receives the PIN either via an out-of-band transmission, such as an email, or may be manually entered by a user viewing the published PIN value. The client may then send a multi-cast probe, as described in the Web Services Discovery protocol, with a setting of secure devices, that is, a probe looking for a response from a secure device. The scope of the multi-cast probe may be confined to a particular area, for example, geographically or by network locale.  
         [0039]     The electronic device  402 , at block  414  may respond to the probe. The response, to use a Web Services embodiment as an example, may include an XML security header with the signature value set to the hash of the PIN. Since the hash of the secret is being sent, and the PIN may subsequently be involved in establishing trust, the PIN is susceptible to a dictionary attack by an entity that does not actually hold the PIN. To increase the difficulty in succeeding with a dictionary attack, an advanced hash algorithm may be used. For example, a combination of a PBKDF2 algorithm may be used with an HMAC-SHA-1 hash function. Salt may be added to increase the calculation time and overall difficulty of performing the dictionary attack.  
         [0040]     Turning briefly to  FIG. 5 , this exemplary hash algorithm is described. The PDBDF2 algorithm is known and documented in the PKSC#5 version 2 standard and is available on both cryptography web sites and in widely available cryptography books. This is an implementation of the key derivation function KDF-2 from PKCS #5: Password-Based Cryptography (PBE). This KDF is essentially a way to transform a password and a salt into a stream of random bytes, which may then be used to initialize a cipher or a MAC, where the PIN is the password and the MAC is the hash output. Briefly, the PIN is generated at block  502 , for example, following the process of  FIG. 4 , block  408 . At block  504 , the PBKDF2 algorithm takes the PIN as input, as well as the salt  506  and an iteration count  508 . A final input, specifying the output length  510 , may vary, but in one embodiment is 128 bits.  
         [0041]     The PBKDF2 algorithm uses the iteration count to specify the number of cycles or turns the internal calculation routine uses to process the inputs. In some password generating applications, the iteration count may be set to 1000. In one exemplary embodiment, the iteration count may be set high, on the order of 50,000, again, to increase the time and difficulty required to guess the PIN. When the difficulty or cost (in terms of time and resources) is increased high enough and the value of the attack is relatively low, most attackers are discouraged from the attempt. The output of the PBKDF2 algorithm may be passed to an HMAC-SHA-1 algorithm increase the hash strength. The combination of PBKDF2 and HMAC-SHA-1 is known. The resultant output may be passed to the client  401 .  
         [0042]     Returning to  FIG. 4 , the probe response, including the hash, and optionally, the salt, and the iteration count may be sent to the client  401  at block  414 . The client  401  may then, at block  416 , use the PIN received at step  410  and calculate its own hash value using the process of  FIG. 5 . If the locally calculated hash matches that received, the client  401  may assume with confidence that the electronic device  402  has the PIN. At block  418 , the client  401  and electronic device  402  may create a secure channel, such as secure sockets library (SSL) over an Internet protocol channel, for further communication. The secure channel in itself does not establish trust, only that future transmissions have integrity. The electronic device  402  may send, at block  420 , a message including a self-signed certificate that includes the UUID of the electronic device  402 , for example, in the subject field. When the client  401  receives the certificate at block  422 , the client  401  may determine if the certificate is self-signed. If the certificate is self-signed then the client  401  may extract the public key and see if it hashes to the UUID of the electronic device  402 . When the values match, in combination with verifying that the electronic device  402  has the PIN at block  416 , then the client  401  may grant trusted status to the electronic device  402 .  
         [0043]     In cases where the certificate is trusted, i.e. signed by a root authority, the client  401  may verify the authenticity using a stored root public key and/or check with the certificate authority for authenticity and possible revocation.  
         [0044]     With the electronic device  402  trusted by the client  401 , it remains for the electronic device  402  to establish trust with the client  401 . At block  424  the electronic device  402  may send a message to the client  401  requesting a certificate or other identifier. The client  401  may respond at block  426  with a certificate with the PIN embedded in the certificate, for example, in the header field. The electronic device  402  may analyze the certificate at block  428  to determine if the PIN received in the certificate matches the PIN of the electronic device  402  (refer to block  408 ). If the two match, the electronic device  402  has an assurance that the client  401  has received the PIN and trust may be extended by placing the client certificate in a trusted store. Depending on the local requirements, the lifetime of the certificates may be set with different terms, for example, one day vs. 1 year, depending on the accessibility of the electronic device  402  from wide area networks, or the amount of visitor traffic common in a given facility.  
         [0045]     By following the authentication process outlined, a client device may fairly easily establish contact with an electronic device for the purpose of communicating data and, in many cases, sharing resources. The example of the conference room projector and client laptop illustrates the usefulness of a quick and easy connection, the value of the trusted relationship with respect to jamming and misuse, and the convenience of retaining trusted status for clients and electronic devices that may be in frequent contact with each other. For example, when a particular group routinely uses the same conference room, long-term trusted status may reduce the time to establish a session and start a meeting. However, in another environment, such as a public conferencing facility at an airport, short-lived certificates and frequently regenerated key pairs and/or PIN numbers may be used to assure availability.  
         [0046]     Although the foregoing text sets forth a detailed description of numerous different embodiments of the invention, it should be understood that the scope of the invention is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possibly embodiment of the invention because describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims defining the invention.  
         [0047]     Thus, many modifications and variations may be made in the techniques and structures described and illustrated herein without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the methods and apparatus described herein are illustrative only and are not limiting upon the scope of the invention.