Abstract:
Disclosed is an authentication mechanism that enables an information recipient to ascertain that the information comes from the sender it purports to be from. This mechanism integrates a private/public key pair with selection by the sender of a portion of its address. The sender derives its address from its public key, for example, by using a hash of the key. The recipient verifies the association between the address and the sender&#39;s private key. The recipient may retrieve the key from an insecure resource and know that it has the correct key because only that key can produce the sender&#39;s address in the message. The hash may be made larger than the sender-selectable portion of the address. The recipient may cache public key/address pairs and use the cache to detect brute force attacks and to survive denial of service attacks. The mechanism may be used to optimize security negotiation algorithms.

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 10/010,352, filed Nov. 13, 2001 which is a continuation-in-part of U.S. patent application Ser. No. 09/833,922, filed on Apr. 12, 2001, which is in incorporated herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to communications in computer networks, and, more particularly, to information authentication in connection with communications between network nodes. 
     BACKGROUND OF THE INVENTION 
     Authentication is an important issue in many types of network communications. Many messages are meaningful only if the recipient can verify the identity of the sender. In some cases, the recipient associates a particular characteristic with a sender&#39;s identity. When a message is received that is known to come from that sender, the recipient refers to the characteristic in deciding what action to take. For example, a company employee associates the characteristic “authority to issue certain orders” with the company&#39;s president. Upon receiving the message “Take tomorrow off,” the employee treats the message with more respect if the message is known to come from the company president than if it came from an anonymous person. In another example, the recipient associates the characteristic “truth in reporting” with the identity of certain news outlets but not with other news outlets nor with the populace in general. Even if a message is received from a sender for whom the recipient has no pre-established association, the sender&#39;s identity may be meaningful in linking multiple messages together. For example, a police department receiving the message “False alarm: I&#39;m not being robbed after all” would presumably accept the message at face value only if it could verify that the sender was indeed the same person who sent the earlier “Help! I&#39;m being robbed!” message. 
     In network communications, an often used form of identity is the network address used by a device to identify itself on the network. Messages are typically tagged with this form of identity, which can be used by a recipient to address a message in response. However, a nefarious party may easily send a message with a deceptive sender&#39;s address. Without an authentication mechanism to verify that the sender&#39;s network address as contained in the message is actually the address where the message originated, this form of identity is vulnerable to fraudulent misrepresentation. 
     Protocols address the problem of fraudulent misrepresentation by implementing authentication services. The recipient of a message uses the authentication services to verify the identity of the sender of the message. The recipient then takes action based on the characteristics associated with the sender&#39;s identity. For example, the Internet Engineering Task Force Request for Comments (IETF RFC) 2401 “Security Architecture for the Internet Protocol” describes the use of IPsec authentication in restricting the acceptance of messages to those originating from authenticated senders. Other protocols provide similar authentication services. However, one perceived difficulty in implementing authentication is that some of these authentication services provide their security by means of quite complicated mechanisms. They come at a heavy perceived price in terms of a significant investment in administrative and communicative overhead. 
     The parent of the present application discloses a unilateral authentication mechanism that provides much of the security of heavyweight authentication mechanisms, but with lower administrative and communicative overhead. The present application discloses implementation options usable with the unilateral authentication mechanism and describes scenarios in which unilateral authentication may be profitably employed. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present application provides a simple unilateral authentication mechanism that enables an information recipient to quickly ascertain that the information comes from the sender it purports to be from. This authentication mechanism integrates a private/public key pair with the selection by the sender of a portion of its network address. The sender derives its network address from its public key, for example, by using a hash of the public key. The recipient verifies the association between the network address and a private key held by the sender, i.e., the information could only have come from a sender for whom the network address was selected. 
     Authentication information may be sent to the recipient in a packet option of a message. Then, if the recipient does not understand the authentication protocol, it may ignore the packet option and accept the message, albeit without the protection provided by the authentication mechanism. 
     The sender may, but need not, send its public key directly to the recipient. The recipient may also retrieve the key from an insecure resource, such as a key posting service. Because this resource is not secure, the recipient may receive several erroneous keys in addition to a correct key. However, this is not a problem as the recipient knows which is a correct public key because incorrect keys cannot produce the sender&#39;s address in the message. 
     The hash may be made larger than the portion of the address generated by the sender. To avoid possible conflicts between the results of the hash and portions of the address not settable by the sender, the sender chooses a modifier, appends it to the public key, hashes the result, and repeats with other modifiers until the results of the hash match the non-selectable portions of the network address. 
     The recipient may cache public key/address pairs. Then, the sender need not send the public key in every message (or in any message, as described above). This cache may be used to detect brute force attacks wherein a malicious attacker tries different keys until it finds one whose hash matches the selectable portion of the sender&#39;s address. Whenever the recipient receives two messages from the same address but with different public keys, it knows that it is under attack. The cache is also useful in surviving denial of service attacks: when flooded with incoming messages, the recipient discards any message whose public key/address is not in the cache. The recipient does this without going through the expense of decoding authentication information in every message. Upon receipt of a public key/address pair not in the cache, the recipient may request the public key from the sender (or from a third party, as described above). 
     If the authentication information is used to sign the entire message, then only the sender with the private key can create a valid message. This prevents “Man in the Middle” attacks wherein a device intercepts a message, alters it, and sends it along: unless the intercepting device has the private key, its altered message will be detected by the recipient as invalid. As another result of the fact that only the holder of the private key can create a valid message, the sender cannot later repudiate a message it has sent. 
     As one more consequence of the fact that signing the entire message prevents anyone but the authenticate sender from creating a valid message, the present invention may be used to optimize the Internet Key Exchange and other security negotiation algorithms. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic diagram showing an exemplary computer architecture on which the mechanisms of the invention may be implemented; 
         FIG. 2  is a schematic diagram showing an exemplary communications network in which the invention may be practiced for authenticating messages; 
         FIG. 3  is a schematic diagram illustrating how portions of a Public Key-Derived (PKD) address are derived from a public key of the sending device; 
         FIG. 4  is a flowchart of a procedure for setting a PKD address of the sending device; 
         FIG. 5  is a block diagram showing a data structure of an exemplary authenticated message; 
         FIG. 6  is a flowchart of a procedure for creating an authenticated message by the sending device; 
         FIG. 7  is a flowchart of a procedure for authenticating a message from the sending device; 
         FIGS. 8   a  and  8   b  are a flowchart of a procedure for building and maintaining a cache of valid public key/network address associations; and 
         FIG. 9  is a flowchart of a procedure for using a cache of valid public key/network address associations, such as may have been built by the procedure of  FIGS. 8   a  and  8   b , to authenticate a message. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable computing environment. The following description is based on embodiments of the invention and should not be taken as limiting the invention with regard to alternative embodiments that are not explicitly described herein. Sections I through III describe the basics of the unilateral authentication mechanism. Section IV presents implementation options useful in particular situations. Section V shows how the authentication mechanism operates in situations beyond those discussed in Sections I through III. 
     I. Exemplary Environment 
     In the description that follows, the invention is described with reference to acts and symbolic representations of operations that are performed by one or more computers, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of the computer of electrical signals representing data in a structured form. This manipulation transforms the data or maintains them at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the computer in a manner well understood by those skilled in the art. The data structures where data are maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the invention is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that various of the acts and operations described hereinafter may also be implemented in hardware. 
     Referring to  FIG. 1 , the present invention relates to communications between network nodes on various connected computer networks. Each of the network nodes may reside in a computer that may have one of many different computer architectures. For descriptive purposes,  FIG. 1  shows a schematic diagram of an exemplary computer architecture usable for these devices. The architecture portrayed is only one example of a suitable environment and is not intended to suggest any limitation as to the scope of use or functionality of the invention. Neither should the computing devices be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in  FIG. 1 . The invention is operational with numerous other general-purpose or special-purpose computing or communications environments or configurations. Examples of well known computing systems, environments, and configurations suitable for use with the invention include, but are not limited to, mobile telephones, pocket computers, personal computers, servers, multiprocessor systems, microprocessor-based systems, minicomputers, mainframe computers, and distributed computing environments that include any of the above systems or devices. 
     In its most basic configuration, a computing device typically includes at least one processing unit  102  and memory  104 . The memory  104  may be volatile (such as RAM), non-volatile (such as ROM, flash memory, etc.), or some combination of the two. This most basic configuration is illustrated in  FIG. 1  by the dashed line  106 . The devices may have additional features and functionality. For example, they may include additional storage (removable and non-removable) including, but not limited to, PCMCIA cards, magnetic and optical disks, and magnetic tape. Such additional storage is illustrated in  FIG. 1  by removable storage  108  and non-removable storage  110 . Computer-storage media include volatile and non-volatile, 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. Memory  104 , removable storage  108 , and non-removable storage  110  are all examples of computer-storage media. Computer-storage media include, but are not limited to, RAM, ROM, EEPROM, flash memory, other memory technology, CD-ROM, digital versatile disks (DVD), other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage, other magnetic storage devices, and any other media that can be used to store the desired information and that can be accessed by the computing device. These devices may also contain communication channels  112  that allow the host to communicate with other devices. Communication channels  112  are examples of communications media. Communications media typically embody 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 include 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, communications media include wired media, such as wired networks and direct-wired connections, and wireless media such as acoustic, radio, infrared, and other wireless media. The term computer-readable media as used herein includes both storage media and communications media. The devices may also have input components  114  such as a keyboard, mouse, pen, a voice-input component, a touch-input device, etc. Output components  116  include screen displays, speakers, printer, etc., and rendering modules (often called “adapters”) for driving them. Each of the devices has a power supply  118 . All these components are well known in the art and need not be discussed at length here. 
     II. The Problem of Authentication 
     The present invention is directed to a simplified, lightweight, authentication mechanism for a recipient of a message to authenticate the message, i.e., to determine whether the message is from the sender it purports to be from. This authentication mechanism may be advantageously used for providing adequate network security with respect to any type of messages sent for any reason. 
     To facilitate an understanding of the need for a lightweight, low-overhead, and easily deployable authentication mechanism such as the one provided by the invention, consider the exemplary network set up of  FIG. 2 . Accompanying a computing device  100  on a local area network (LAN)  200  is a server  202  and a router  204 . The router allows the devices on the LAN to communicate over an internetwork  206  to remote computing devices such as device  208 . The Internet is one example of an internetwork. Any of the devices, including the router, in  FIG. 2  may send a message to any other device. In protocols typically used today, the message contains the network address of the sender (the “from address”) and the network address of the recipient (“to address”). Consider, as a first example, that device  100  is operated by a systems administrator authorized to reconfigure the router  204 . For security&#39;s sake, the router does not accept commands from devices other than device  100 . But when the router receives a reconfiguration command message with its from address set to the address of device  100 , it believes the message is authentic and carries out the command. (Note that this example is for illustrative purposes only: router communications are generally much more secure than is depicted here.) However, it is very easy to create and send a message with a deceptive from address. For example, device  208  could create a router reconfiguration command message and put device  100 &#39;s network address in the from address field. Without a mechanism for authenticating a message, the router cannot know which device actually sent the message. The router does not know that the message is not authentic and carries out the command, possibly compromising the security of all of the devices on the LAN. As a second example, device  100  uses a “Neighbor Discovery” protocol to discover which router it should use when communicating with remote devices such as device  208 . Without an authentication mechanism in place, device  202  may impersonate router  204 . Messages from device  100  intended for device  208  would then go to device  202  which may read their contents, discard them (thus launching a denial of service attack against device  100 ), or alter them before sending them on to device  208 . These two examples are intended to show the ubiquity of the need for authentication. They can be extended to any sort of message sent to any sort of device using any sort of communications method or protocol. 
     To fend off such attacks, a recipient of a message needs to authenticate the sender of the message, that is, determine if the message was actually sent by the “sender” indicated in the message. Then the recipient can choose to act on the message only if it is sent by an appropriate device. The next section details how the present invention enables this authentication. 
     III. Unilaterally Authenticating a Message 
     The invention enables a device, such as the computing device  100 , to write a message in such a way that the message could only have been written by this particular device. This authentication mechanism is unilateral in that the recipient does not need to have further communications with the sender for completing the authentication process. This is because the message contains everything the message recipient, such as the router  204 , needs to decode the message and to determine that it must have come from this particular device. 
     The invention is based on public key/private key cryptography used in combination with the selection of a network address of the message sender based on the public key. This selected network address is called the Public Key-Derived (PKD) address. In the example of  FIG. 2 , the PKD address is an address of the computing device  100 . One way this address may be derived from the public key of the device is described in connection with  FIGS. 3 and 4 . In step  400 , the sending device selects a cryptographic key pair with a private key  300  and a public key  302 . Modern network addresses, such as those used in IPv6, are composed of two parts. The first part, called the route prefix  308 , contains a routable address that can be used to route a message to an appropriate network link. In step  402 , the sending device sets the route prefix  308  of its PKD address  310 , possibly by listening on its network link for advertisements from the local router  204 . For details on how this works on an IPv6 network, see the IETF RFC 2462 “IPv6 Stateless Address Autoconfiguration.” 
     Starting in step  404 , the sending device sets the second part of its PKD address. This part is called the “node-selectable” portion  312  because the sending device is free to set this part as it sees fit. In IPv6, the node-selectable portion is called the “interface identifier” and is often set to the network interface&#39;s Medium Access Control (MAC) address. Here, however, in accordance with one embodiment of the invention, the sending device in step  406  creates a hash  306  of the public key  302  and selects part of the hash to be the node-selectable portion  312  of its PKD address. In IPv6, the interface identifier comprises 64 bits, but two of those bits (the “u” and “g” bits) should be set to zero, leaving the sending device to choose 62 bits of the hash for the remainder of the interface identifier. While it is not intrinsically important which bits the sending device chooses, its procedure should be well known so that a recipient of the message can recreate the PKD address as will be described in greater detail below. For instance, the sending device may choose the lowest-order 62 bits of the hash  306  for use as the interface identifier. In step  408 , the sending device checks whether this address is already in use by another node in the network. In other words, the sending device checks whether there is an address conflict with respect to the PKD address it has generated. Different protocols may provide different ways of determining this. For IPv6, the mechanism is called “duplicate address detection.” If the address is not already in use, the process of constructing the PKD address of the sending device is complete. If the constructed PKD address is in use by another device, however, the sending device in step  410  chooses a modifier  304 , which may be, for example, a two-bit integer. The sending device appends the modifier  304  to the public key  302 , in step  412  creates a hash of the composite number, and again tests to see if the generated address is unique. If necessary, the sending device continues to loop through steps  408 ,  410 , and  412  choosing different modifiers until one is found to produce a PKD address that is not used by another device. In some embodiments, the sending device may choose to always use a modifier in which case it jumps directly from step  404  to step  410 , looping if necessary to find an appropriate modifier. 
     By constructing the unique PKD address of the sending device based on its public key, an association between the PKD address of the sending device and its public/private key pair is created. This association is then used in the authentication mechanism of the invention to allow a recipient of a message to authenticate the sender of the message by verifying the association. 
     Referring now to  FIGS. 5 and 6 , the sending device sends an authenticated message  500  to its recipient. The message contains, among other things such as the standard IP header, message content data  508  and  510 , the PKD address  310 , and the public key  302  of the sending device. The message content data in  FIG. 5  are divided into two fields  508  and  510  because the sender may choose to use the methods of the present invention to authenticate some, all, or none of the message content data. In one embodiment, the message also contains data for preventing a replay attack. Such data may include, for instance, a time stamp  502  and data  504  identifying the intended recipient of the message. The identifier of the recipient may be anything that uniquely identifies the recipient and will often be the recipient&#39;s network address. The message content data, PKD address, public key, and the optional data may be in plain text (i.e., not hashed or encrypted). 
     In addition to the plain text data, the message further includes a digital signature  506  generated from data included in the message. To generate this signature, the sending device in step  602  creates a hash of the data to authenticate. These data typically include the PKD address  310  it set earlier, the authenticated portion of the message content data  508 , and the optional data such as the time stamp  502  and the identifier of the intended recipient  504 . In step  604 , a cryptographic signature  506  of the hash generated in step  602  is created from the hash using the private key  300  associated with the public key  302 , which was used to create the PKD address of the sending device. In step  606 , the authenticated message  500  is populated with the message content data  508  and  510 , the cryptographic signature, the PKD address, any optional data that went into forming the signature, the public key, and the modifier  304  if it was used in creating the PKD address. 
     When the recipient receives the authenticated message  500 , it performs the process in  FIG. 7  to decode the message and to attempt to authenticate the identity of the sender of the message. Note that, for the most part, these steps may be performed in any order. In step  700 , if there is a time stamp  502  in the message, the recipient compares it to its local time, which should be synchronized with the clock of the sending device to within a few seconds. The recipient discards the message if it deems the message to be too old. In step  702 , if the message contains an identifier of the message&#39;s intended recipient  504 , the recipient compares the identifier in the message with its own identification and discards the message if there is no match. In steps  704  and  706 , the recipient uses the public key included in the message to recreate the node-selectable portion of the sender&#39;s PKD address. This operation is identical to that of step  412  of  FIG. 4 . Specifically, the recipient hashes the message&#39;s public key and modifier, if present, and then takes part of the results of the hash to be the node-selectable portion of the sender&#39;s address. Then the recipient compares the value for the node-selectable portion it has generated from the public key with the node-selectable part of the PKD address  310  in the message. If the two values differ, then there is something amiss, and the recipient discards the message. On the other hand, if the two values match, the recipient knows that public key included in the message is the one used to create the PKD address. 
     In step  708 , the recipient uses the public key  302  in the message to validate the cryptographic signature  506  and extracts the hash that was used to form the signature. In steps  710  and  712 , the recipient follows the procedure of step  602  of  FIG. 6  to recreate the hash using the data included in the message. If the hash created by the recipient does not match the hash extracted from the cryptographic signature in the message, then the recipient discards the message. If the hash values match, then the recipient knows that the cryptographic signature was written by a device that knows the private key  300  that corresponds to the public key  302  in the message. The recipient also knows, by the results of step  706 , that this public key  302  is the one used to create the sender&#39;s PKD address  310 . Therefore, the recipient knows that the message must have been written by the device that created the PKD address  310 , the only device with the private key  300 , that is to say, by the sending device. The sending device&#39;s address has been verified, and the recipient is now free to accept the authenticated message content data  508  as authentic and to act on them accordingly. 
     IV. Further Embodiments of Unilateral Authentication 
     The authentication information may be provided to the recipient in any number of ways. For example, the information may be placed in an IPsec Authentication Header or Encapsulating Security Payload, or in a packet option. If a packet option is used, and the recipient does not understand the unilateral authentication mechanism, then it can discard the packet option and accept the remainder of the message. Of course, this recipient does not benefit from the authentication mechanism, but at least it can understand and use the message contents. 
     For clarity&#39;s sake, the discussion so far depicts all of the authentication information as arriving at the recipient in a single message. (See, for example,  FIGS. 5 and 7  and accompanying discussion.) This is not necessary for the mechanism to work. As public keys can be quite large, in some embodiments the sender can shorten its messages by leaving the public key out of each message. The sender delivers its public key to a key publishing service. (Note that the key publishing service may in implemented on the same device as the sender.) When a message is received, the recipient requests the sender&#39;s public from the publishing service. Because communications with the publishing service may not be secure, nefarious agents may intercept the request and attempt to fool the recipient. In response to its request, the recipient may receive several invalid public keys along with the one valid key. That is not a problem, however. The recipient uses each received public key in turn in the procedure of  FIG. 7 . Only the valid public key leads to a successful authentication. Having found the valid key, the recipient can store it for future use. If none of the received keys works, then the recipient discards the message. 
     As a general rule of thumb, the cryptographic strength of an authentication mechanism that uses a hash increases with the number of bits produced by the hash. However, the number of usable bits is often restricted by other considerations. In  FIGS. 3 and 4  and the accompanying discussion, for example, the number of hash bits usable is restricted by the size of the node-selectable portion of the sender&#39;s address. In an IPv6 unicast address, this portion is 62 bits long (excluding the “u” and “g” bits whose values may not be set by the sender). One aspect of the present invention provides a way to increase security while adhering to these bit restrictions. Returning to  FIG. 3  and the procedure of  FIG. 4 , let the hash value  306  be 64 bits long. Then, as an additional part of step  408 , compare two bits of the hash value to the “u” and “g” bits. If there is a match, use 62 bits of the hash value as the node-selectable portion  312 , as before. If there is no match, then choose another modifier  304  and repeat until a match is found. In this example, security is enhanced by producing a 64-bit hash rather than a 62-bit hash, while staying within the 62-bit restraint of the node-selectable portion  312 . This method is quite general, with its scope depending upon the particular addressing scheme in use. As an example of the method&#39;s scope, again consider IPv6. Typically, the lowest-order 16 bits of an IPv6 route prefix define a subnet. A hash of up to 80 bits (62 node-selectable bits, “u” bit, “g” bit, and the 16-bit subnet) may be formed and discarded if the “u” and “g” bits are incorrect or if the generated subnet is invalid. 
     The techniques discussed so far prevent another device from spoofing the node-selectable portion ( 312  in  FIG. 3 ) of a sender&#39;s PKD address  310 . The technique may be extended to prevent spoofing of the route prefix  308  of the PKD address as well. Again consider  FIGS. 3 ,  4 , and  7 . The inputs to the hash functions of steps  406  and  412  are extended to include at least a portion of the route prefix  308  as well as the public key  302  and, possibly, the modifier  304 . The balance of the procedure of  FIG. 4  remains the same. On receiving a message, the recipient uses the same portion of the route prefix to create the hash in step  704  of  FIG. 7 . The balance of the procedure of  FIG. 7  remains the same. If the authentication succeeds in  FIG. 7 , then the recipient knows that the portion of the route prefix used to create the hash is authentic. One would expect that the entire route prefix would normally be used in authentication, but that need not be the case. 
     V. Applications of Unilateral Authentication 
     The methods of the present invention are applicable to several other applications. The authentication mechanism associates an authenticated message with its creator. If the authentication information is used to sign the entire message, then only a message creator with the private key can create a valid message. This aspect prevents “Man in the Middle” attacks wherein an attacker intercepts a message, alters it, and sends it along, presenting the altered message as if it were the product of the original creator. Unless the intercepting attacker has the sender&#39;s private key, its altered message will be detected by the recipient as invalid. As another aspect of the bond between an authenticated message and its creator, the creator cannot later repudiate a message it has sent. Yet another consequence of the fact that signing the entire message prevents anyone but the authenticate sender from creating a valid message, the present invention may be used to optimize the Internet Key Exchange and other security negotiation algorithms. 
     Several advantages follow when a recipient uses aspects of the present invention to create and maintain a cache of authenticated public key/network address associations. The flowchart in  FIGS. 8   a  and  8   b  show one way that a message recipient can create such a cache. The first parts of this flowchart parallel the procedure depicted in  FIG. 7 . When a message arrives at the recipient, the message may have a time stamp and, if so, the recipient discards the message in step  800  if the message is out of date. In steps  802 ,  804 ,  806 ,  808 , and  810  (parallel to steps  704 ,  706 ,  708 ,  710 , and  712  of  FIG. 7 ), the recipient checks the internal consistency of the authentication information in the message. If the authentication fails, then the message is discarded just like in the procedure of  FIG. 7 . If the authentication information is internally consistent, then in step  812  the recipient compares the received public key/network address with the public key/network address associations already in the cache (if any). If in step  814 , the received public key/network address is found to be a duplicate of an association already in the cache, then the new values add no new information to the cache and may be discarded. (Of course, the message contents themselves have already been authenticated and are accepted as shown in the procedure of  FIG. 7 . That is not depicted here because  FIGS. 8   a  and  8   b  deal solely with creating and maintaining the cache, not with processing message contents.) In some embodiments, each association in the cache is discarded after a period of non-use. In these embodiments, step  816  resets the use timer of the association already in the cache that matches the values in the present message. If no duplicate association was found in step  814 , then step  818  looks for a match of just the network address. If one is found, there may be a problem: two public keys (the one already in the cache and the one in the present message) have been found to be associated with the same network address. This may indicate a brute force attack in which an attacker generates public keys until it finds one that, through the procedures of  FIGS. 3 and 4  and the accompanying text, produces a network address to match the one produced by a legitimate sender. The safe course for the recipient is to consider the network address compromised. The message is discarded and the association in the cache with the matching network address is also discarded in step  820 . If the network address in the present message is not found to match that of any association already in the cache, then the public key and network address in the present message are used to create a new association in the authentication cache. Through this procedure, a cache of valid public key/network address associations is built and maintained. The next paragraph describes how this cache may be used. 
     With the cache produced by the procedure of  FIGS. 8   a  and  8   b  in place, the recipient can use it to authenticate incoming messages, especially messages that arrive without public keys. For messages that contain public keys, however, it may seem at first that the cache does not reduce the recipient&#39;s processing load because for these messages, the procedures of  FIGS. 7 ,  8   a , and  8   b  must still be performed. That is often true, but an exception makes the cache very valuable. A recipient may be the target of a “Denial of Service” (DOS) attack in which it is flooded with invalid incoming messages. Because decrypting each incoming message according to the procedures of  FIGS. 7 ,  8   a , and  8   b  consumes significant amounts of processor resources, the recipient may be overwhelmed during the attack and may stop performing any useful work. The cache can be used to prevent this. If the number of incoming messages exceeds the number that the recipient can comfortably process, then the recipient processes incoming messages according to the simplified procedure of  FIG. 9  instead of the procedures of  FIGS. 7 ,  8   a , and  8   b . In  FIG. 9 , after checking a message&#39;s time stamp in step  900 , the message&#39;s public key and network address are compared, in step  902 , with public key/network address associations already in the cache. If no match is found in step  904 , then the message is discarded. If a match is found, then the message is subjected to the full authentication procedure of  FIG. 7 . During the DOS attack, this procedure filters out invalid messages without wasting expensive decryption time on them and thus allows the recipient to keep up with valid incoming messages. Of course, the procedure of  FIG. 9  only decrypts messages whose public key and network address are already in the cache. It does not allow the recipient to accept an incoming message with a valid, but not previously seen, public key and network address. That is a tradeoff a recipient may be willing to make during the course of the DOS attack. Once the recipient weathers the DOS attack successfully, it reverts to the fuller procedures of  FIGS. 7 ,  8   a , and  8   b.    
     In view of the many possible embodiments to which the principles of this invention may be applied, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of invention. For example, for performance reasons the cryptographic operations may be implemented in hardware, such as on a network card, rather than in software. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.