Abstract:
In the fields of data security and system reliability and qualification, this disclosure is of a method, system and apparatus for verifying or authenticating a device to a host using a zero-knowledge based authentication technique which includes a keyed message authentication code such as an HMAC or keyed cipher function and which operates on secret information shared between the host and the device. This is useful both for security purposes and also to make sure that a device such as a computer peripheral or accessory or component is qualified to be interoperable with the host.

Description:
FIELD OF THE INVENTION 
     This disclosure relates to authentication and verification in the computer and data security fields, and more particularly to authentication or qualification of a device. 
     BACKGROUND 
     Authentication is well known in the computer/cryptographic fields; typical applications are to ensure that another party (or entity) in a communications context is properly identified. An example of such authentication is that distributors of music and video content using the Internet or other computer networks do so using a Digital Rights Management system (DRM) to protect the content from illicit copying and use. DRM is used to protect digital content transferred over a network and transferred from a computer to an associated playback device. The DRM system is implemented by software resident in the host and audio/video player or associated computer. It is often desirable to make sure that the playback device is an authenticated device as part of the DRM system. 
     So it is known to first authenticate such a device intended to receive such content (or other valuable information) before transmitting to the device any valuable or important information. More broadly, authentication is a way to verify the identity of another device or entity for purposes of sending information to or receiving information from that other entity. 
     SUMMARY 
     This disclosure is directed to a “lightweight” (meaning relatively fast to compute with limited computing resources) authentication or qualification method and associated system and apparatus for authenticating third-party devices of diverse sources by a host, using prior delivery of shared secret information to the device manufacturer or supplier. (“Host” as used generally refers to a computing apparatus with which the device desires to communicate.) This method employs a zero-knowledge based authentication process. In cryptography, a zero-knowledge protocol is an interactive method for a party to prove to another that some statement is true, without revealing to the party information other than the truth of the statement. The present method is another way to authenticate and so is not zero-knowledge. 
     In this method, the host implementor generates a set of N (N being an integer) batches of randomly generated data, each batch designated D Bi  where i=0 to N−1, along with a set of M (M being an integer) randomly generated fixed-size cryptographic keys each designated D aki  where i=0 to M−1. (The data and keys are the shared secret information so this is not a true zero-knowledge authentication protocol.) The size of each data batch is not limiting but is for instance in the range of a few thousand bytes. 
     A randomly selected data batch designated D B  having an assigned unique identifier id (identification number) designated D Bid  (where 0&lt;=id&lt;N) and a randomly selected key designated D ak  having an assigned unique identifier id (identification number) designated D akid  (where 0&lt;=id&lt;M) is provided to the manufacturer of the third-party device i, where i is an index designating e.g. one device. The association between the device identifier (ID) pair which is designated D Bid ,D akid  and the device is stored at the host, e.g., in computer readable memory. In some embodiments, a single identifier rather than a pair is used for each device. 
     At arbitrary times during the life cycle of the host, the host requests that the third-party device authenticate itself by computing a keyed hash of a selected portion of its data batch and return its computed hash value (digest), along with its identifier pair (D Bid ,D akid ). 
     The host can immediately decide to sever communication with the device should it determine that the received identifier pair (D Bid ,D akid ) has been revoked. Should that identifier pair still be valid, the host, having prior knowledge of the associated device data batch and authentication key, is able to verify the validity of the requested data and hence the device. As long as the verification does not fail, the host has no reason to distrust the third-party device and continues to communicate with it. Should verification (authentication) fail, the host may decide to sever communication with the third-party device. 
     Message authentication codes using hash functions or keyed ciphers are well known in the data security field. The principle is to take data (a digital message, digital signature, etc.) and use it as an entry to e.g. a hash function or keyed cipher, resulting in an output called a “digest” of predetermined length which is intended to uniquely identify (“fingerprint”) the message. A secure (cryptographic) hash or cipher is such that any alteration in the message results in a different digest, even though the digest is much shorter than the message. Such functions are “collision-resistant” and “one-way.” Some “keyed” hash functions (as described here) conventionally are keyed in the way a particular cipher is keyed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a system in accordance with the invention. 
         FIG. 2  shows graphically a method in accordance with the invention. 
         FIG. 3A  shows a device in accordance with the invention. 
         FIG. 3B  shows a host in accordance with the invention. 
         FIG. 4  shows detail of a generic computing device suitable for use as the  FIG. 3A  device or  FIG. 3B  host. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention in some embodiments is used in the exemplary system  10  depicted in  FIG. 1  in which each element is largely conventional. In a first embodiment, host  14  is, e.g., a computer server platform such as a server in a DRM (digital rights management) system for distribution and protection of copyrighted digital content. In a second embodiment, host  14  is e.g. a desktop or laptop computer, Smart Phone, audio/video player or other end user computing apparatus. Host  14  is coupled to a communications link  16  such as the Internet in the first embodiment, but which could be another type of communication network, wired or wireless including Ethernet, cellular telephone, etc. In the second embodiment, the communications link is a more local type wired or wireless link such as a USB (universal serial bus), Fire Wire, or internal computer bus. In both embodiments, also coupled to link  16  are one or more (client) devices  20 ,  22 , . . . ,  26  which are e.g. in the first embodiment consumer electronic devices, other types of computing devices, Smart phones, etc., here respectively designated device  1 , device  2 , . . . , device i. In the second embodiment, devices  20 ,  22 , . . . ,  26  are e.g. computer or consumer electronic device accessories or components or peripherals such as a computer display screen, a computer optical or hard disc drive, a USB key, or other electronic device which is internal or external to the host and communicates therewith. In this second embodiment, in addition to the security aspect, the present method has the commercial and reliability advantages of allowing only authenticated (qualified) devices to be coupled to or installed in the host, preventing interoperability problems. 
     At the time of manufacturing (or initialization) of these third-party devices, the organization or person operating or manufacturing the host  14  conventionally establishes a master array or set of N batches D Bi  of random data, and M random fixed-size cryptographic keys D aki . This master array is stored in memory located in or associated with the host and remains there for the lifetime of the host. 
     As new third-party devices such as  20 ,  22 ,  26  come to life (e.g., are manufactured or initialized), identifier id (identification number) pairs (D Bid ,D akid ) are assigned uniquely to each of these device models and the corresponding data D B  and key D ak  are distributed to the device manufacturer by the system implementer (who typically also maintains or manufactures the host) for inclusion (e.g., storage in memory) into each device. A given device model (instance or unit), therefore, stores only one of the many data batches and one of the many keys known by the host. 
     As the host is updated by the implementer, should the authentication of particular devices be revoked for the purposes of this authentication, the revoked identifier pairs are conventionally recorded at the host as being revoked. 
     The following authentication process then takes place at arbitrary times during the life of each third-party device, at the prerogative of the host. For this process, it is assumed that each device has stored in it a data batch D B , of size (length) designated D Bsz  and the associated data batch identifier designated D Bid  and a fixed-size authentication key D ak  and the associated key identifier designated D akid  as earlier assigned by the host implementer to the manufacturer of the device and as installed into memory in the device as explained above. The key length is, e.g., conventionally about 20 bytes but this (like the other numerical parameters described here) is not limiting. 
     Furthermore, the process assumes that each such device i can perform a MAC computation such as a keyed hash computation (e.g., the well known HMAC-SHA1 function as defined in RFC 2104 or other keyed hash functions, of which many are known) using the data batches and keys or alternatively a keyed cipher-based MAC computation. HMAC stands for Hash Message Authentication Code (a keyed hash function). The present MAC computation is typically done by an appropriately programmed processor or dedicated logic circuitry resident in the device as explained in further detail below. The notation HMAC(K, D) below indicates the HMAC computation of generic data D using generic key K. More generally, the authentication may use any message authentication code process, including a cipher based MAC. 
     The authentication process, as depicted with time running along the vertical axis in  FIG. 2  for the host and device, includes: 
     1. The host (which is a computing apparatus as explained above) generates a fixed-size random number as an authentication nonce H an . (A nonce in cryptography is a random number used once to avoid a replay attack by making each exchange unique.) The nonce is, e.g., of about the same length as the intended hash digest such as about 20 bytes. 
     2. The host also generates a random offset value designated H do  and a random length value designated H dl  such that H do +H dl  is less than or equal to the total size (e.g., in bits or bytes) D Bsz  of the data batch D B  held by the device. These “random” number generations may be performed conventionally, for instance by conventional pseudo random number generator software executed by a processor in the host. 
     3. The host sends (via communications link  16  to which it is conventionally coupled in  FIG. 1  the nonce H an , the data offset H do  and the data length H dl  (which collectively are the data batch selection parameters) generated in steps 1 and 2 to the device. These particular selection parameters are only exemplary. 
     4. The host sends (via the communications link  16 ) a request to the device to return the computed authentication hash digest value D ah . (Steps 3 and 4 may be combined into one transmission or reversed in order.) 
     5. The device (also a computing device, see above) computes, e.g., the MAC digest value D ah =HMAC(D ak , ∥D B [H do  . . . H do +H dl −1]), that is, the predetermined HMAC or keyed cipher MAC function as keyed by the device key D ak , of the data, where the data is the concatenation of the host nonce H an  and the subset of the data batch D B  specified as being offset H do  and of length H dl  in data batch D B . 
     6. The device sends to the host via the communications link the computed authentication digest value D ah  computed in step 5, with its batch identifier which is D Bid  and its key identifier which is D akid  (which together are the ID pair). 
     7. The host verifies whether the received ID pair (B Did , D akid ) has been revoked. If so, the host elects to sever communication with the device immediately and the process stops. An error message may be sent to the device by the host at this point. 
     8. If there is no revocation, then using the received batch identifier D Bid  and key identifier D akid , the host using that received ID pair looks up the associated data batch B D  and key D ak  in its storage and using them and the earlier generated selection parameters H an , H do , H al  independently computes the equivalent MAC digest D ah . Note that the data batches do not need each to be stored as a separate entry in the host. Instead the data batches may overlap in the host memory to economize on host memory, and looked up using an addressing scheme with offsets or other conventional addressing techniques. The host then conventionally compares this computed MAC digest D ah  to the authentication value D ah  received from the device. 
     If the verification of step  8  fails (no match of the two digests), the host determines that the device is not an authenticated device and severs its communication with the device (e.g., sends an error message or just stops communications). But as long as this authentication exchange is completed successfully (the two digests do match), the host has no reason to distrust that the particular device is authenticated and may continue to communicate with it, in other words the authentication is successful. 
       FIG. 3A  shows in a block diagram relevant portions of exemplary device  20 . Non-relevant portions of the device (those not involved in the authentication process) are conventional and not shown, for ease of illustration. Device  20  includes a conventional access port which is adapted to couple to the external communications link  16 . Incoming data or requests are sent to storage (memory such as RAM)  32  or conventional processor  40  as shown. (The processor may be the main processor for the device which also performs other functions, or may be a processor or circuit dedicated to the authentication task.) Also provided is storage  34  (e.g., ROM) which holds the factory installed data, key and ID pair as shown. Also provided is storage  38  (e.g., ROM) storing code (computer software) such as the MAC computation software  40  (“MAC function”) to be executed by processor  40 . The output of the MAC function  44  is stored in storage (e.g., RAM)  48  also coupled to port  30 . 
     An example of host  14  is depicted in similar block diagram form in  FIG. 3B , with many similar elements. Port  31  supports two way communications to link  16 . Incoming data (the digest and ID pair from the device) is stored in memory (e.g., RAM)  33 . Processor (or equivalent)  41  executes code provided from code memory  39  to do the PRNG calculation in PRNG  43  and the MAC computation in calculator  45 . Memory  35  (RAM or ROM) stores the ID pairs and associated data batches and keys. Comparator  47  in processor  41  verifies both the key pairs and the incoming device digests as explained above. Verifier  49  then severs communications or not. Note that the comparator and verifier also me in the form of software executed by the processor. Both are conventional functions. 
       FIG. 4  shows further conventional detail of the  FIG. 3A  device (or the  FIG. 3B  host) in one embodiment.  FIG. 4  illustrates a typical and conventional computing system  60  that may be employed to implement processing functionality in embodiments of the invention. Computing systems of this type may also be used in a computer host (server) or user (client) computer or other computing device or peripheral or accessory or component as described above, for example. Those skilled in the relevant art will also recognize how to implement embodiments of the invention using other computer systems or architectures. Computing system  60  may represent, for example, a desktop, laptop or notebook computer, hand-held computing device (personal digital assistant (PDA), cell phone, consumer electronic device, palmtop, etc.), mainframe, server, client, or any other type of special or general purpose computing device such as a peripheral or accessory or component as explained above as may be desirable or appropriate for a given application or environment. Computing system  60  can include one or more processors, such as a processor  64  (equivalent to processor  40  in  FIG. 3A ). Processor  64  can be implemented using a general or special purpose processing engine such as, for example, a microprocessor, microcontroller or other control logic. In this example, processor  64  is connected to a bus  62  or other communications medium. Note that in some embodiments the present process is carried out in whole or in part by “hardware” (dedicated circuitry) which is equivalent to the above described software embodiments. 
     Computing system  60  can also include a main memory  68  (equivalent to memories  32 ,  48  in  FIG. 3A ), such as random access memory (RAM or read only memory (ROM)) or other dynamic memory, for storing information and instructions to be executed by processor  64 . Main memory  68  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  64 . Computing system  60  may likewise include a read only memory (ROM) or other static storage (equivalent to memories  34 ,  38  in  FIG. 3A ) device coupled to bus  62  for storing static information and instructions for processor  64 . 
     Computing system  60  may also include information storage system  70 , which may include, for example, a media drive  72  and a removable storage interface  80 . The media drive  72  may include a drive or other mechanism to support fixed or removable storage media, such as flash memory, a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a compact disk (CD) or digital versatile disk (DVD) drive (R or RW), or other removable or fixed media drive. Storage media  78  may include, for example, a hard disk, floppy disk, magnetic tape, optical disk, CD or DVD, or other fixed or removable medium that is read by and written to by media drive  72 . As these examples illustrate, the storage media  78  may include a computer-readable storage medium having stored therein particular computer software or data. 
     In alternative embodiments, information storage system  70  may include other similar components for allowing computer programs or other instructions or data to be loaded into computing system  60 . Such components may include, for example, a removable storage unit  82  and an interface  80 , such as a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, and other removable storage units  82  and interfaces  80  that allow software and data to be transferred from the removable storage unit  78  to computing system  60 . 
     Computing system  60  can also include a communications interface  84  (equivalent to port  30  in  FIG. 3A ). Communications interface  84  can be used to allow software and data to be transferred between computing system  60  and external devices. Examples of communications interface  84  can include a modem, a network interface (such as an Ethernet or other network interface card (NIC)), a communications port (such as for example, a USB port), a PCMCIA slot and card, etc. Software and data transferred via communications interface  84  are in the form of signals which can be electronic, electromagnetic, optical or other signals capable of being received by communications interface  84 . These signals are provided to communications interface  84  via a channel  88 . This channel  88  may carry signals and may be implemented using a wireless medium, wire or cable, fiber optics, or other communications medium. Some examples of a channel include a phone line, a cellular phone link, an RF link, a network interface, a local or wide area network, and other communications channels. 
     In this disclosure, the terms “computer program product,” “computer-readable medium” and the like may be used generally to refer to media such as, for example, memory  68 , storage device  78 , or storage unit  82 . These and other forms of computer-readable media may store one or more instructions for use by processor  64 , to cause the processor to perform specified operations. Such instructions, generally referred to as “computer program code” (which may be grouped in the form of computer programs or other groupings), when executed, enable the computing system  60  to perform functions of embodiments of the invention. Note that the code may directly cause the processor to perform specified operations, be compiled to do so, and/or be combined with other software, hardware, and/or firmware elements (e.g., libraries for performing standard functions) to do so. 
     In an embodiment where the elements are implemented using software, the software may be stored in a computer-readable medium and loaded into computing system  60  using, for example, removable storage drive  74 , drive  72  or communications interface  84 . The control logic (in this example, software instructions or computer program code), when executed by the processor  64 , causes the processor  64  to perform the functions of embodiments of the invention as described herein. 
     This disclosure is illustrative and not limiting. Further modifications will be apparent to these skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.