Patent Publication Number: US-6711681-B1

Title: Cryptographic authorization with prioritized authentication

Description:
FIELD OF THE INVENTION 
     This invention relates to use of one or more authentication mechanisms in secure communications. 
     BACKGROUND OF THE INVENTION 
     During the last decade of the Twentieth Century, the Internet has become a vital communication medium for a variety of application domains, including simple e-mail, home banking, electronic trading of stocks, net-based telephonic communications and many other electronic commerce applications. Authentication of a user is becoming a key requirement in allowing or authorizing a legitimate user to execute the user&#39;s privileges in a particular network or sub-network. 
     Presently, many user authentication mechanisms are available, including simple user name/password, one-time password (e.g., S/Key), RSA-based digital signature authentication, Kerberos, challenge-and-response, and Secure Socket Layer SSL v3.0 with user/client authentication. Bruce Schneier, in Applied Cryptographv, John Wiley &amp; Sons, Inc., New York, Second Edition, 1996, pp. 34-74 and 566-572, discusses and characterizes several user and/or key authentication tests that are often based on, or associated with, an underlying encryption procedure. 
     One interesting authentication scheme is the Sun Pluggable Authentication Mechanism (PAM), discussed in more detail in the following, which facilitates integration of several authentication packages or tests without requiring change of the underlying application (e.g., login). Although a system such as PAM provides a framework for integration, such a system often deals with the plurality of authentication mechanisms as if all have the same cryptographic or authentication strength or priority. For example, one enterprise might require both Kerberos (relatively strong) and user password (relatively weak) to be used for user authentication. Use of several authentication modules can be accommodated within PAM, through the use of stacking. If the user fails to pass one of the authentication tests, among many that are applied in stacking, authentication is denied, without indicating which of the many tests the user has failed to pass. PAM treats all authentication tests in an integrated package as equally strong and equally suitable. 
     What is needed is a system that integrates one or more authentication tests but allows assignment of a priority or strength to each of such tests and allows authentication to be treated as a necessary, but not a sufficient, condition for user authorization. Preferably, where authentication tests are integrated, these tests should be executed based on an indicium that is a measure of priority and/or strength for each authentication test. Preferably, the system should allow identification of, and take account of, which authentication test or tests the user has failed to pass and should grant or withhold access to selected subsets of a resource, depending upon which tests are passed. Preferably, the system should be flexible enough to allow assignment of different priorities and/or strengths to tests within an integrated authentication package, based on the application and the current circumstances. 
     SUMMARY OF THE INVENTION 
     These needs are met by the invention, which provides a system that integrates one or more authentication tests and allows assignment of arbitrary (and changeable) relative priority and/or relative strength to each of these tests. In one embodiment, the system allows an integrated electronic authentication system to accept physical objects, such as drivers licenses, birth certificates, passports, social security cards and the like for partial or full authentication of a user, although each of these documents is used for a different primary purpose, and the purposes seldom overlap. 
     In a first embodiment, the system applies one or more authentication tests with increasing or differing numerical priority or strength and grants access to a resource or selected subset thereof (which may be the empty set), depending upon which test or tests are satisfied. In another embodiment, the system withdraws access to a selected subset (which may be the empty set) of a resource for each authentication test the user fails to satisfy. 
     The invention has the following advantages: (1) the invention strengthens an association or linkage between authentication and the authorization process; (2) the invention allows identification of which authentication test(s) is being used; (3) the invention extends an integration procedure, such as PAM, without distorting the procedure; (4) the invention enhances total security of the authorization process; (5) the invention preserves and deals with authentication mechanisms based on their relative merits and can allocate relative priority based on relative cryptographic strength; and (6) the invention allows an entity to classify those with whom it deals (customers, suppliers, etc.) for authorization purposes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Prior Art 
     FIG. 1 illustrates the architecture of a resource access system that requires user authentication. 
     FIGS. 2A-2B and  3 A- 3 B- 3 C are flow charts of procedures for practicing single-threshold and multiple-threshold embodiments of the invention, respectively. 
     FIGS. 4A-4B are a flow chart for practicing a “top down” embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the invention, user authentication is treated as a necessary, but not sufficient, condition for user authorization in this system. Authorization level varies from user to user, based on the user&#39;s role, group membership, privileges, past behavior and the like. If the user satisfies or passes all authentication tests, the user is allowed access to a maximal set, consistent with the user&#39;s status, of domains or privileges. If the user passes some, but not all, of the authentication tests, the user is allowed access to a selected subset of the maximal domain, where the selected subset may be a proper subset or may be the maximal set and will vary according to the tests passed, or not passed. 
     Strength of an authentication test can be objectively evaluated. For example, SSL v3.0 with authentication is believed by many to be a stronger authentication test than is Kerberos, discussed in Schneier, op cit, pp. 566-572; and Kerberos is considered to be a stronger test than a simple user/password test. If these three authentication test are integrated, an assessment of authentication relative strength for use in the invention might run as follows. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Authentication test 
                 Relative strength 
               
               
                   
                   
               
             
            
               
                   
                 SSL v3.0 
                 1 
               
               
                   
                 Kerberos 
                 2 
               
               
                   
                 User/password 
                 3 
               
               
                   
                   
               
            
           
         
       
     
     A weight w i  (0≦w i ≦1) may be assigned to each authentication test, with a higher weight being assigned to a test with higher relative strength. In one embodiment, relative priority of an authentication test is equated with the relative strength of a test. In another embodiment, relative priority is assigned to each of several tests, independently of their relative strengths, based on the circumstances in which the tests will be used in an integrated approach. 
     The Pluggable Authentication Mechanism (PAM) is discussed in detail by Vipin Samar and Charles Lai in “Making Login Services Independent of Authentication Technologies”, presented at the Third ACM Conference on Computer and Communications Security, March 1996, is useful as a guide in implementing the invention. The Samar et al. article notes that most UNIX systems presently use a login procedure based on a modified Data Encryption Standard (DES) algorithm, which assumes that the password cannot be guessed and that the password does not pass over the communications channel in cleartext. These assumptions are acceptable when communications occur only within a trusted network. However, an open network, such as an internet, requires use of more restrictive and stronger authentication mechanisms. Examples of these stronger mechanisms include Kerberos, RSA digital signature, Diffie-Hellman, S/Key and other one-time passwords, and challenge-and-response and mart card authentication systems. 
     One goal of a PAM system is to require a possibly-different methods of authentication, depending upon the application. For example, a site may require S/Key password authorization for telnetd access but allow console login access after presentation of a UNIX password. Another goal of a PAM system is a requirement that a user pass more than one authentication test, such as Kerberos and RSA digital signature tests, to obtain access to a particular resource or application. Another goal is that system-access services should not have to change when an underlying authentication mechanism changes. 
     Core components of a suitable authentication framework include: (1) one or more applications or resources, such as login, telnetd and ftpd, to which a user seeks access; (2) an authentication mechanism library, such as a PAM Application Programming Interface (API) or library (the front end); and (3) specific authentication modules, such as Kerberos, S/Key and UNIX user password (the back end). FIG. 1 illustrates a relationship between these three components. When a user seeks access to a particular application or resource, the application calls a PAM API, which in turn calls one or more authentication modules that are required for access to that application. The appropriate authentication module(s), as determined by the API, is/are loaded and presented to the user. If the user responds correctly to the authentication test(s) in a PAM, access is granted. If the user responds incorrectly, access is denied and, optionally, the user is given another opportunity to respond correctly to the test(s). 
     A resource access system may be divided into four areas of management functionality: authentication, account, session and password. Authentication management authenticates the user and refreshes, upgrades or destroys the user credentials. Account management checks user account expiration and access hour restrictions, if any, and determines whether a user has access to the resource at that particular date and at that particular time. Session management is used for accounting and billing purposes and, optionally, to determine the amount of time the user has had access to the resource in the current session (useful where the user&#39;s contact time is restricted). Password management is used to change the password from time to time. The PAM implements each of these four management items as a separate, pluggable module. A particular user may not need to be interrogated or monitored by all four modules. Alternatively, the user&#39;s access request may be processed in parallel by two or more of the four modules. 
     According to the invention, the authentication system may allocate a strength and/or a priority to each of several authentication mechanisms associated with a particular application or resource, may apply these mechanisms in a particular order, and/or may require that the user satisfy or pass at least a selected number of these tests in order to gain access to the application. Each associated authentication test may have an assigned weight value w i  (0≦w i ≦1;i=1, . . . ,I ; I≧1), which may increase with increasing strength or priority for the associated test, and the system may assign to the user a “test score”                TS   =       ∑     i   =   1     I                       w   i                   ATS                   (   i   )           ,           (   1   )                         
     where ATS(i)=1 if the user passes authentication test number i and ATS(i)=0 otherwise. The system optionally denies user access to the application unless the user&#39;s test score is at least equal to a selected threshold test score value TS thr  (i.e., TS≧TS thr ), even if the user passes at least one of the associated authentication tests. The threshold test score TS thr  may vary with the particular application for which access is sought. 
     FIGS. 2A-2B present a flow chart illustrating a procedure that incorporates this approach. In step  21 , the user seeks access to a particular application or resource. In step  23 , the system determines which authentication mechanisms (i=1, . . . , I) are associated with access to the chosen application. In step  25 , the system determines the test score threshold associated with the chosen resource. In step  27 , the system is initialized, with i=1 and TS( 0 )=0. In step  29 , the system presents the user with authentication mechanism number i, and the user responds to this test number i in step  31 . In step  33 , the system determines whether the user has passed authentication test number i. If the answer to the query in step  33  is “yes, ” the system sets ATS(i)=1, in step  35 , and passes to step  39  (FIG.  2 B). If the answer to the query in step  33  is “no,”, the system sets ATS(i)=0, in step  37 , and passes to step  39 . In step  39  (FIG.  2 B), the system multiplies ATS(i) by a weight w i  assigned to the test number i, adds the quantity w i ATS(i) to the old sum TS(i−1) to form a new sum TS(i), and increments the index i (i→i+1), in step  40 . In step  41 , the system determines whether i satisfies the condition i≧I+1. If the answer to the query in step  41  is “no, ” the system returns to step  29  and repeats steps  29 ,  31 ,  33 ,  39 ,  40  and  41  at least once. If the answer to the query in step  41  is “yes, ” the system moves to step  43  and compares the sum TS(I) with the associated threshold test score TS thr . If TS(I)≧TS thr , user access to the application is granted, in step  45 . If TS(I)&lt;TS thr , user access to a default subset of the application is granted, in step  47 , where the default subset may be the empty set. 
     Alternatively, the system may set a strictly monotonic sequence of test score threshold values, TS thr,1 , TS thr,2 , . . . , TS thr,N  with TS thr,1 &lt;TS thr,2 &lt;. . . &lt;TS thr,N  and N≧1, and may allow the user access to a selected subset of the full resource, depending upon which threshold values the user&#39;s test score equals or exceeds. As the user&#39;s test score TS(I) increases, the user is granted access to more and more subsets of the target application. 
     FIGS.  3 A- 3 B- 3 C illustrate the procedure according to this alternative embodiment. Steps  21 - 41  in FIGS.  3 A- 3 B- 3 C are performed as in FIGS. 2A-2B to compute the sum TS(I). In step  51  (FIG.  3 B), the system provides a monotonic sequence of N threshold values (N≧2), TS thr,1 &lt;TS thr,2 &lt;. . . &lt;TS thr,N , that will be used to determine what access, if any, the user may be granted within the application or resource. In step  53 , the system is initialized by setting a counting index n=1. In step  55 , the system determines whether the sum TS(I) satisfies the condition TS(I)≧TS thr,N . If the answer to the question in step  55  is “no”, the system determines whether n=1, in step  57  (FIG.  3 C). 
     If the answer to the question in step  57  is “yes”, the system grants the user access to a first default subset S 0  of the application, in step  59 . 
     This first default subset can be the empty subset, which effectively denies the user access to any part of the application. If the answer to the question in step  57  is “no”, corresponding to n&gt;1, the system grants the user access to a selected subset S n−1  of the application. 
     If the answer to the question in step  55  is “yes”, the system increments the count index n (n→n+1), in step  63 , and determines whether n satisfies the condition n≧N+1, in step  65 . If the answer to the question in step  65  is “no”, the system returns to and repeats step  55  at least once. If the answer to the question in step  65  is “yes”, the system grants the user access to another default subset S N , which is optionally the entire application, in step  67 . 
     The preceding embodiments may be characterized as “bottom up” approaches, in which the system allows user access to a default subset of the application or resource, which may be the empty set, initially. The system also allows access by the user to more and more of the application or resource as the user satisfies or passes more and more of the authentication tests. 
     In an alternative “top down ” approach, illustrated in a flow chart in FIGS. 4A-4B, the user begins with potential access to the entire resource or application and loses access to particular subsets of the resource as the user fails to satisfy or pass one or more of the authentication tests. In step  71 , the user seeks access to a resource, or to a subset thereof. In step  73 , the system provides I authentication mechanisms, numbered i=1, 2, . . . , I (I≧1) associated with that application. In step  75 , the system is initialized at i=1. In step  77 , the user is presented with authentication test number i, and the user responds to test number i in step  79 . In step  81 , the system determines whether the user has passed test number i. 
     If the answer to the query in step  81  is “yes”, the system grants the user access to a selected resource subset S i , in step  85  (FIG.  4 B). The system then moves to step  87  and increments the count index i (i→i+1). In step  89 , the system determines whether the count index i satisfies the condition i≧I+1. If the answer to the query in step  89  is “yes”, the system moves to step  91  and grants the user access to the full resource set, or a modified or default version thereof. If the answer to the query in step  89  is “no”, the system returns and repeats steps  77 ,  79  and  81  at least once. 
     If the answer to the query in step  81  is “no”, the system grants the user access to a selected default subset S i,def  of the resource subset S i , in step  83 , and optionally continues with step  87 , where the count index i is incremented and tested against I+1 (Step  89 );. The default subset S i,def  is optionally the empty set. 
     At the end of the procedure(s) shown in FIGS. 4A-4B, if the user has failed to satisfy or pass the authentication tests number i=i1, i2, . . . , iM, among the total number I of authentication tests (0≦M≦I; I ≧1), the system allows the user access to one or more of certain default subsets, S i1,def , S i2,def , . . . , S iM,def , so that the user now has access the union of these default subsets of the original “whole ” resource or application set S. Each time the user satisfies or passes an authentication test, the subset of the resource to which the user has access is unchanged (no loss at this stage). 
     Where multiple users are present, first and second users who seek access to different portions of a resource are optionally presented with different sequences of authentication tests to determine the portion of the resource to which each user will be granted access. For example, the first user may be presented with authentication tests number one, two and four for access to a first selected portion of the resource; and the second user may be presented with authentication tests number two, three, four and five for access to a second selected portion of the resource. Alternatively, where the first and second users pass the same authentication test (e.g., test number two), the portion of the resource to which each is granted access may be different for each user. For example, the first and second users may be granted access to different portions of a given confidential document affecting national security, because these two users have different “needs to know.” 
     The resource or application to which a user seeks access may change from time to time. For example, a resource may include a collection of documents of various levels of classification (e.g., company private and confidential, secret and top secret at the federal level), and the level of authentication required for access may be set by the document(s) with the highest level of confidentiality. The federal government downgrades the classification of selected documents from time to time, and the authentication level required may be correspondingly reduced as a result of this downgrade, or as a result of removal of one or more documents from the resource. Conversely, one or more additional documents with a higher classification level may be added to the resource, and this upgrade in classification may require an increase in authentication level for access to the resource. 
     In another alternative embodiment, one or more authentication levels or tests associated with a given resource optionally changes at a given time, possibly as a result of change of characterization of the resource, or of one of more documents or other objects that are part of or associated with the resource. This change would be implemented at a time that is approximately contemporaneous with the change in characterization and would be subject to subsequent changes in characterization. 
     The preceding embodiments may be implemented by presenting the user with a sequence of one or more authentication tests and requiring the user to affirmatively “pass ” one or more of these tests, in order to obtain access to part or all of the resource. 
     Alternatively, the user may be issued a smartcard containing cleartext and/or (preferably) encrypted responses or “keys ” to I authentication tests (I≧2), where each response may, but need not, correspond to passage of an authentication test. In this approach, the user presents his/her smartcard to the system, the system reads the card and determines which, if any, of the entries on the smartcard correspond to passage of an authentication test, and which test. The smartcard is read by a computer, which tracks which authentication tests the smartcard has “passed ” and thereby determines a corresponding subset of the resource (which may the whole resource, a proper subset of the whole resource, or the empty set) to which the user has access, based on the user&#39;s smartcard score. Preferably, the smartcard requires specification of a card owner&#39;s PIN, which must correspond to the smartcard presented, in order to read the smartcard and determine its score on one or more authentication tests. This approach requires possession of both the smartcard and special knowledge (the PIN) before access to (portions of) a resource is granted. 
     The Pluggable Authentication Mechanism (PAM), which provides integration of one or more authentication tests, is compatible with the invention. The PAM need not be altered, only enhanced, in order to implement the invention.