Abstract:
The present invention provides a digital certificate ( 2, 32 ) comprising a plurality of credential attribute properties ( 6, 36 ), and a trust function ( 8, 42 ) embedded within the certificate as an executable file, which trust function can determine as a function of data ( 12, 44 ) available to it a trust value ( 14, 46 ) attributable to at least a part of the certificate at least partly by execution of the executable file. A corresponding method of communication is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to digital certificates and to methods of communication. 
     BACKGROUND OF THE INVENTION 
     A credential is a data structure provided to a bearer for a purpose, with some acknowledged way to verify the bearer&#39;s right to use the credential. A credential relates to an attribute, normally, but not necessarily, of the bearer. A credential is verified by a trusted source (sometimes referred to as the verifier). Often, there will be a chain of credentials and respective trusted sources until a verification is proffered by an organization in which trust is implicit. Credentials are incorporated in a digital certificate for verification. 
     A digital certificate generally comprises a file containing information, which file is transmitted to a recipient together with a digitally signed version thereof. The digitally signed version is a hash of the file encrypted using a secret key (in a public key infrastructure). A hash is a one-way function that generates a substantially unique output from a file and is for all practical purposes irreversible. These concepts are familiar to those skilled in the art. 
     Digital certificates are used in communication using distributed electronic networks, such as the internet, to transmit a credential, typically of the bearer. A known digital certificate is the X.509 standard. 
     A certificate may contain one or more credential attributes. 
     A credential attribute in a certificate can be almost anything. Typical examples relevant to the present invention may be a credit rating, an access authorization (for physical or electronic access), a verification of identity etc. 
     Each attribute has at least one attribute property, such as a value (e.g. a numeric or alphanumeric) or something more complex such as an indication of trust. 
     Generally, known digital certificates are valid for a fixed period of time (e.g. 1 year), during which time they will be used as a means of authentication and for gaining authorized access to services etc. This is referred to as the valid period. Such digital certificates can, however, be revoked at any time by the verifier (terminating the valid period), thus placing a burden on the certificate recipient to check revocation lists or to use online certificate status protocol services. These certificates are generally valid or not valid; there is no middle ground even though the degree of trust the trusted source has in the credential attribute may, in fact, vary over time (or some other variable) or if there is a wish to vary the credential attribute value. 
     A certificate may still be in a valid period even if a credential attribute within it is not. 
     By way of example, a certificate may specify an individual&#39;s credit limit as a credential attribute. In this example, the credential attribute property value is the value of the credit limit. While this may be correct at the time of generation of the certificate, within the typical one year limit of the certificate, the verifier may not wish to attest to the same credit limit for the full period. 
     Even if the certification can be varied, the recipient may still need to assess the trustworthiness of the certificate or parts thereof. In particular, the recipient would wish to know what degree of trustworthiness the certificate issuer would give to the certificate or a part thereof. While it is known from U.S. Pat. No. 4,868,877 to associate a level of trust, in numerical form, to a credential or certificate, this does not address the problem of trust varying subsequent to issuance of the certificate or for other factors. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention aim to address the problems referred to above. 
     According to the present invention in a first aspect, there is provided a digital certificate comprising a plurality of credential attribute properties, and a trust function embedded within the certificate as an executable file, which trust function has data can determine as a function of data available to it a trust value attributable to at least a part of the digital certificate at least partly by execution of the executable file. 
     In embodiments of the present invention the trust function uses data to generate a trust value the recipient can associate with one or more attributes in the certificate or with the certificate as a whole. Generally, but not exclusively, the trust function uses trust values of attributes to generate what can be described as a composite or global trust value. 
     Thus, the digital certificate can be used locally and dynamically to determine a trust value. 
     Suitably, the trust value is of a credential attribute in the certificate. Suitably, the trust value is of the certificate. 
     Suitably, the data is trust value data. 
     Suitably, the data includes data obtained externally of the certificate. Suitably, the obtained data is obtained from a user by the input of data in response to a query generated by the trust function. Suitably, the obtained data is obtained from a digital data store. Suitably, the digital data store is a web site. 
     Suitably, the trust function varies the trust value as a function of time. 
     Suitably, the trust function is configured to determine the trust value automatically. Suitably, execution of the executable file fully can determine the trust value. Suitably, the executable file is a platform portable code, such as Java Script or HTML. 
     Suitably, the certificate had a valid period and the credential function determines the credential attribute property value during the valid period. 
     Suitably, the plurality of credential attribute properties are from a single credential attribute. Suitably, the plurality of credential attribute properties are from a plurality of credential attributes. 
     Suitably, there is at least one attribute trust value, in which the trust function uses an attribute trust value to determine the trust value. Suitably, there is a plurality of credential attributes and a plurality of attribute trust values, in which the trust function uses a plurality of attribute trust values to determine the trust value. 
     Suitably, a credential function is provided in the certificate, which credential function is associated with at least one credential attribute property and which determines the value of the credential attribute property. 
     Suitably, the trust function uses the credential attribute property value determined by the credential function. Suitably, the credential attribute property value determined by the credential function is a trust value. 
     Suitably, the certificate has a valid period and the trust function determines the trust value during the valid period of the certificate. 
     The “trust” value and the “property” value need not be numerical values, though generally they will be so. Numerical property values may relate to a numerical attribute, e.g. a credit rating, or be a numerical representation of a trust value in a particular credential attribute e.g. that of identity of the bearer. Typically, for a trust value, the value will be between a zero trust number (say ‘0’ or ‘−1’) and a full trust number (say ‘1’) attributing a high confidence level to the credential. The attribute function may be monotonically decreasing over time. 
     Other values may be alphanumeric e.g. “YES”/“NO” outputs or relate to preset word based indications such as “HIGH TRUST”, “MEDIUM TRUST” or “LOW TRUST”. 
     Suitably, the credential function varies the credential attribute property value as a function of time. 
     Suitably, the credential function is configured to determine the credential attribute property value automatically. Suitably, execution of the executable file fully can determine the credential attribute property value. Suitably, the executable file is a platform portable code, such as Java Script or HTML. 
     Suitably, the credential attribute property comprises a value operated on by the credential function to determine a credential attribute property value. 
     Suitably, the credential function uses data obtained from outside the certificate to determine the credential attribute property value. Suitably, the obtained data is obtained from a user by the input of data in response to a query generated by the credential function. Suitably, the obtained data is obtained from a digital data store. Suitably, the digital data store is a web site. 
     Suitably, a plurality of the credential attribute properties have respective credential functions. Suitably, each credential attribute property has a respective credential function. 
     By having the trust and, optionally, credential functions within the certificate it can be trusted by the recipient as a verified determination of the trust value of a part or all of the certificate and, optionally, credential attribute property value. 
     According to the present invention in a second aspect, there is provided a digital certificate comprising a plurality of credential attribute properties and a trust function within the certificate, which trust function comprises an executable file, which trust function has data available to it and can determine as a function of the data available to it a trust value attributable to at least a part of the digital certificate at least partly by execution of the executable file. 
     According to the present invention in a third aspect, there is provided a digital certificate comprising a plurality of credential attribute properties, and a trust function embedded within the certificate as an executable program, which trust function has data available to it and can determine as a function of the data available to it a trust value attributable to at least a part of the digital certificate at least partly by execution of the executable program. 
     According to the present invention in a fourth aspect, there is provided a method of communication, which method comprises the steps of communicating from a sender to a recipient a digital certificate according to any of the first to third aspects of the invention. 
     Suitably, the recipient inspects the certificate and the trust value is determined by the trust function. 
     Suitably, the recipient inspects the certificate and the credential attribute property value is determined according to the credential function. 
     Suitably, the communication is via a distributed electronic network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the drawings that follow; in which: 
         FIG. 1  is a schematic representation of a digital certificate according to a first embodiment of the present invention. 
         FIG. 2  is a schematic representation of a distributed electronic network over which the present invention may be used. 
         FIG. 3  is a schematic representation of a digital certificate according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1  of the drawings that follow there is shown, schematically, a digital certificate  2  according to the X.509 standard, the certificate  2  containing credential attributes  4 A– 4 N, which have credential attribute properties  6 A– 6 X and a trust function  8 . The certificate  2  is digitally signed (a hash created, which hash is encrypted using a verifier&#39;s secret key) as indicated schematically at  10 . A source of external data is indicated schematically at  12 . 
     The credential attribute  4 A relates to a bearer&#39;s identity and contains an identity attribute property value  6 A (eg “FRED SMITH”), an address attribute property value  6 B and an indication of trustworthiness attribute property value  6 C (a numerical value between −1 (completely untrustworthy) and +1 (completely trustworthy)). Credential attribute  4 B is for and has a trustworthiness attribute property value  6 D for the certificate as a whole. Credential attribute  4 N relates to a credit limit, having a credit limit numerical attribute property value  6 W and a trustworthiness attribute property value  6 X (for credential attribute  4 N). 
     The trust function  8  is embedded in the certificate  2  as an executable file of platform portable code such as Java script or HTML. 
     The certificate  2  is communicated via a distributed electronic network, such as the internet, as shown schematically in  FIG. 2  of the drawings that follow, in which a sender  16  communicates with a recipient  18  via the internet, indicated schematically at  20 . Communication can be via other distributed electronic networks, such as Wide Area Networks (WANs) or Local Area Networks (LANs). Embodiments of the present invention can also be implemented in other, less preferred, ways, for instance by storing a certificate on a digital storage device (e.g. a floppy disk) and sending this to the recipient  18 . 
     Upon receipt of the digital certificate  2 , the recipient  18  inspects the digital signature  10  to verify the certificate  2 . Having done so, the recipient  18  executes the trust function  6  which operates on some or all of the credential attribute properties  6 A,  6 B,  6 C,  6 D,  6 W and  6 X to determine and output a trust value for the certificate  2 . 
     If external data is required, this is obtained from external data source  12 . 
     By way of example, the certificate may be for a credit rating for a bearer of the certificate. The credit limit in the credential attribute property  6 W may be, say, £10,000. Trust function  8  extracts the trust value credential attribute property values  6 C,  6 D,  6 X and averages these to produce a trust value  14  for the certificate. 
     This is a fairly simple example. Many variations exist, for instance, the trust function  8  need not be a simple average. It could weight one value more than another. Another option is that data is obtained from an external data source  12 , for instance a date or a current account balance. The trust function need not use data from the certificate at all. Further, not just trust values need be used. For instance the trust value may be a function of time (generally trust will decrease over time). 
     Referring to  FIG. 3  of the drawings that follow, there is shown a schematic representation of a digital certificate  32  having a plurality of credential attributes  34 A– 34 N with associated credential attribute properties  36 A– 36 R. The certificate  32  is signed, as indicated at  38 . Digital certificate  32  corresponds to digital certificate  2  of  Figure 1 , except that in digital certificate  32  there is also a plurality of corresponding credential functions  40 A– 40 M. A trust function is indicated at  42  and an external data source at  44 . 
     In this example credential attribute  34 A is a credit limit, having properties of a value  36 A and an indication of trustworthiness  36 B. Other properties  36 C etc. may be included. Credential attribute  34 N is an identity having a value  36 Q and an indication of trustworthiness  36 R. Each function  40 A– 40 M is capable of modifying a respective credential attribute property  36 A– 36 R to determine a respective credential attribute property value obtaining external data as required as indicated at  44 . 
     The credential functions  40 , in this case, may be a modifier of an existing credential attribute value. Pursuing the example of the credit rating, the function  40  may be to reduce the rating by 10% of the original rating for each month. Applying the function  40  to the attribute property  36  above, the function obtains date information and in the second month the credential attribute value  4  is determined as £9,000 and so on. Date information may be obtained from the recipient computer or, for more security, from a trusted source, preferably a trusted source web site. These are digital data sources. 
     Trust function  42  receives the generated credential attribute property values from credential functions  40 A– 40 M and operate a trust value  46  output indicative of the trust in the certificate. External data may be obtained, as required, from external data source  44 . 
     The credential function is embedded in the certificate as an executable file of platform portable code such as Java script or HTML. 
     In another example the credential attribute property  36  may be an access authorization for a building to which the provider of the certificate  32  only wishes to allow the certificate bearer access on specified times, say week days only. The credential attribute property  36  would have a value of “PERMIT ACCESS” in this case. The credential function  40  is, therefore, encoded to determine the day of the week (for instance from a computer on which the certificate  32  is being verified, or from a remote web-site) and generate a modified credential attribute property value which is “DO NOT PERMIT ACCESS” at week ends. It will be appreciated from this that the credential attribute property  36  will not always be modified by function  40 . 
     Alternatively, the credential attribute property  36  may not have an original value in the certificate. Instead, it may solely be generated by a credential function which (generally) obtains data externally of the certificate. 
     There may be a one-to-one correlation between each credential attribute property  34 A– 36 R and its corresponding credential function  40 A– 40 M, though this need not be the case. For instance, one or more, but not necessarily all, of the credential attribute properties  36 A– 36 R need have a credential function  40  for generation thereof. Further, a given credential function  40 A– 40 M may be used for a plurality of credential attribute properties  36 A– 36 R, in which case there may be fewer credential functions  40  than credential attribute properties  36 . 
     In the certificates  2  and  32 , it will be appreciated that many of the fields present in an X.509 certificate are not represented. These may include fields containing data to allow a credential attribute property value to be determined or evaluated according to the second credential function. For instance, these fields may include a credential start date. 
     The certificate  32  may provide the recipient with determined credential attribute property values relevant to one or more attributes therein as well as to the trust function  42 . 
     The trust and credential functions can seek information from elsewhere on which to base its generation of the credential attribute property value. For instance, the functions can access local time data or extract data from a web–site as required, as described above. Alternatively, in a less preferred option, data can be sought from the recipient of the certificate in response to an inquiry generated by the credential attribute function. This option is less preferred as it makes the certificate less self–contained. In some embodiments all data for the credential attribute property value originates externally of the certificate. 
     Thus, the trust function within the certificate can operate automatically to produce a trust value which can vary over time and dynamically according to external data. A certification authority need not be involved in the variation of the trust value after issue, though optionally they may be. 
     The digital certificate may, optionally, be encrypted. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extend to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.