Internet Draft S. Tuecke Document: draft-ietf-pkix-proxy-00.txt D. Engert I. Foster ANL M. Thompson LBNL L. Pearlman C. Kesselman USC/ISI Expires: January 2002 July 2001 Internet X.509 Public Key Infrastructure Proxy Certificate Profile Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document forms a certificate profile for Proxy Certificates, based on X.509 PKI certificates as defined in draft-ietf-pkix-new- part1-08.txt (the draft update to RFC 2459), for use in the Internet. The term Proxy Certificate is used to describe a certificate that is derived from, and signed by, a normal X.509 Public Key End Entity Certificate or by another Proxy Certificate for the purpose of providing restricted impersonation within a PKI based authentication system. This draft replaces draft-ietf-pkix- impersonation-00. Tuecke, et. al. Expires January 2002 1 Internet Draft X.509 Proxy Certificate Profile July 2001 Table of Contents Internet X.509 Public Key Infrastructure Proxy Certificate Profile.1 Status of this Memo................................................1 Abstract...........................................................1 Table of Contents..................................................2 1. Introduction..................................................3 2. Overview of Approach..........................................4 2.1. Terminology.................................................4 2.2. Background..................................................4 2.3. Motivation for Impersonation................................5 2.4. Description Of Approach.....................................7 2.5. Proxy Authority, not Certificate Authority..................8 2.6. Names Versus Subjects.......................................9 2.7. Features Of This Approach..................................10 3. Certificate and Certificate Extensions Profile...............11 3.1. Issuer & Issuer Alternative Name...........................11 3.2. Subject & Subject Alternative Name.........................11 3.3. Key Usage..................................................12 3.4. Extended Key Usage.........................................12 3.5. Basic Constraints..........................................13 3.6. Proxy Certificate Information..............................13 3.6.1. pC........................................................14 3.6.2. pCPathLenConstraint.......................................14 3.6.3. proxyRestriction..........................................14 3.6.4. delegationTrace...........................................16 4. Certificate Path Validation..................................17 5. Relationship to Attribute Certificates.......................19 5.1. Types of Attribute Authorities.............................20 5.2. Delegation Using Attribute Certificates....................20 5.3. Propagation of Authorization Information...................21 5.4. Proxy Certificate as Attribute Certificate Holder..........22 6. Commentary...................................................22 6.1. keyCertSign Bit in the Key Usage Basic Extension...........22 6.2. nonRepudiate Bit in the Key Usage Basic Extension..........23 6.3. Subject Name of a Proxy Certificate........................23 6.4. Carrying Along the End Entity Subject......................24 6.5. Specifying Proxy Restrictions..............................24 6.6. Proxy Restrictions vs. Proxy Rights........................25 6.7. Site Information in Delegation Tracing.....................25 6.8. Certificate Chain in Delegation Tracing....................25 6.9. Another Approach to Delegation Tracing.....................26 6.10. Certificate Policies Extension.............................26 6.11. Kerberos 5 Tickets.........................................26 7. Security Considerations......................................27 8. References...................................................28 9. Acknowledgments..............................................29 10. Change Log...................................................29 11. Contact Information..........................................30 Tuecke, et. al. Expires January 2002 2 Internet Draft X.509 Proxy Certificate Profile July 2001 1. Introduction Use of a proxy credential for impersonation is a common technique used in security systems to allow entity A to grant to another entity B the right for B to authenticate with others as if it were A. In other words, entity B is impersonating entity A. This document forms a certificate profile for Proxy Certificates, based on the draft update to RFC 2459, "Internet X.509 Public Key Infrastructure Certificate and CRL Profile" [7]. In addition to simple, unrestricted impersonation, this profile defines a framework for carrying restriction policies in Proxy Certificates, thus allowing a restriction of the rights an impersonating entity is given. Further, when delegating a Proxy Certificate from one entity to another, this profile defines information that can be optionally included in a Proxy Certificate to allow for tracing of the delegation path. Section 2 provides an overview of the approach. It begins by defining terminology, motivating Proxy Certificates, and giving a brief overview of the approach. It then introduces the notion of a Proxy Authority, as distinct from a Certificate Authority, to describe how end entity signing of a Proxy Certificate is different from end entity signing of another end entity certificate, and therefore why this approach does not violate the end entity signing restrictions contained in the X.509 keyCertSign field of the keyUsage extension. It then continues with discussions of how subject names are used by this impersonation approach, and features of this approach. Section 3 defines requirements on information content in Proxy Certificates. This profile addresses two fields in the basic certificate as well as five certificate extensions. The certificate fields are the subject and issuer fields. The certificate extensions are subject alternative name, issuer alternative name, key usage, basic constraints, and extended key usage. One new certificate extensions, Proxy Certificate Information, is introduced. Section 4 defines path validation rules for Proxy Certificates. Section 5 discusses the relationship of Proxy Certificates to Attribute Certificates. Section 6 provides commentary on various design choices, open issues, related work, and future directions. Section 7 discusses security considerations relating to Proxy Certificates. Section 8 contains the references. Section 9 contains acknowledgements. Tuecke, et. al. Expires January 2002 3 Internet Draft X.509 Proxy Certificate Profile July 2001 Section 10 contains a log of changes made in each version of this draft. Section 11 contains contact information for the authors. This document was written under the auspices of the Global Grid Forum Security Working Group. For more information on this and other related work, see http://www.gridforum.org/security. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [1]. 2. Overview of Approach The goal of this specification is to develop a X.509 Proxy Certificate profile, to facilitate their use within Internet applications for those communities wishing to make use of impersonation within an X.509 PKI authentication based system. This section provides relevant background, motivation, an overview of the approach, and related work. 2.1. Terminology This document uses the following terms: * CA: A "Certificate Authority", as defined by X.509 [7]. * EEC: An "End Entity Certificate", as defined by X.509. That is, it is an X.509 Public Key Certificate issued to an end entity, such as a user or a service, by a CA. * PKC: An end entity "Public Key Certificate". This is synonymous with an EEC. * PC: A "Proxy Certificate", the profile of which is defined by this document. * PA: A "Proxy Authority" is the issuer of a Proxy Certificate, as defined below. * AC: An "Attribute Certificate", as defined by "An Internet Attribute Certificate Profile for Authorization" [4]. * AA: An "Attribute Authority", as defined in [4]. 2.2. Background Computational and Data "Grids" have emerged as a common approach to constructing dynamic, inter-domain, distributed computing environments. As explained in [6], large research and development efforts starting around 1995 have focused on the question of what Tuecke, et. al. Expires January 2002 4 Internet Draft X.509 Proxy Certificate Profile July 2001 protocols, services, and APIs are required for effective, coordinated use of resources in these Grid environments. In 1997, the Globus Project (www.globus.org) introduced the Grid Security Infrastructure (GSI) [5]. This library provides for public key based authentication and message protection, based on standard X.509 certificates and public key infrastructure, the SSL/TLS protocol [3], and delegation using proxy certificates similar to those profiled in this document. GSI has been used, in turn, to build numerous middleware libraries and applications, which have been deployed in large-scale production and experimental Grids [2]. GSI has emerged as the dominant security solution used by Grid efforts worldwide. This experience with GSI has proven the viability of impersonation as a basis for authentication and authorization within Grids, and has further proven the viability of using X.509 Proxy Certificates, as defined in this document, as the basis for that impersonation. This document is one part of an effort to migrate this experience with GSI into standards, and in the process clean up the approach and better reconcile it with existing and recent standards. 2.3. Motivation for Impersonation A motivating example will assist in understanding the role impersonation can play in building Internet based applications. Steve is an engineer, who wants to run a set of simulation jobs on idle workstations on his company's Intranet based Grid. From his laptop he wants to invoke the jobs, and then have an agent process running on his desktop workstation monitor the jobs while he is traveling to a conference. As the jobs complete, the agent should automatically archive the results to the company's mass storage system, and after all the jobs are complete it should run a post- processing job which summarizes the simulation results from all of the archived data sets. Later, Steve will reconnect to the agent to get the results for inclusion in a report. Of course, he wants all of this to happen securely on his company's resources, which requires that he initiate all of this using his PKI smartcard. This scenario requires authentication and delegation in a variety of places: * Steve needs to be able to mutually authenticate with several remote workstations to start the simulation jobs. * Steve needs to be able to mutually authenticate with his desktop workstation to start the agent running. * That agent needs to be delegated the rights to mutually authenticate with the various workstations, in order to monitor the progress of the simulations. Tuecke, et. al. Expires January 2002 5 Internet Draft X.509 Proxy Certificate Profile July 2001 * As simulations complete, the agent needs to move the resulting data from the workstations to the company's mass storage system. In order to perform this move efficiently, it needs to orchestrate a third party data transfer directly between the workstation and the mass storage system. This requires mutual authentication between the agent and the workstations and mass storage system, as well as mutual authentication between the workstations and the mass storage system. * The agent needs to start the post-processing job, which must be delegated rights to mutually authenticate with the mass storage system in order to retrieve the data. * When Steve later reconnects his laptop to the network, a program running on the laptop must mutually authenticate with the agent in order to retrieve the summary of results. Impersonation is a viable approach to solving two (related) problems in this scenario: * Single sign-on: Steve wants to enter his smartcard password (or pin) once, and then run a program that will start all of the simulation jobs and the remote agent. This program needs to be given the rights to be able to perform all of these operations securely, without requiring repeated access to the smartcard or Steve's password. * Delegation: Various remote processes in this scenario need to perform secure operations on Steve's behalf, and therefore must be delegated the necessary rights. For example, the agent needs to be able to authenticate on Steve's behalf with the various workstations and the mass storage system, and must in turn delegate rights to the post-processing job to authenticate on Steve's behalf with the mass storage system. Impersonation can be used to secure all of these interactions: * Impersonation allows for the private key stored on the smartcard to be accessed just once, in order to create the necessary impersonation credential, which allows the starter program to impersonate Steve (that is, authenticate as Steve) when starting the various jobs and the agent. Access to the smartcard and Steve's password is not required after the initial creation of the impersonation credential. * The starter program on the laptop can delegate to the agent the right to impersonate Steve. This, in turn, allows the agent to authenticate to the workstations as if it were Steve in order to start the simulation jobs, and to the mass storage system to archive the data as if it were Steve. * When the agent starts the post-processing job, the agent can delegate to it the right to impersonate Steve. This allows the post-processing job to authenticate as Steve to the mass storage Tuecke, et. al. Expires January 2002 6 Internet Draft X.509 Proxy Certificate Profile July 2001 system in order to gain access to the data sets. * When the laptop reconnects to the agent to get the final results, it can perform mutual authentication. The agent will use its delegated impersonation credential in this interaction. The laptop may use a newly generated impersonation credential, which is just created anew using the smartcard. One concern that arises in such a scenario is what happens if one of the machines that has been given the right to impersonate Steve has been compromised? Can the attacker now do everything that Steve is allowed to do? A solution to this problem is to allow for restrictions to be placed on the impersonation. For example, the machine running the post-processing job above might only be given the right to impersonate Steve for the purpose of reading the simulation output files from the mass storage system. Therefore, if that host is compromised, raw simulation data cannot be changed on the mass storage system, new jobs cannot be started, etc. While this example may seem somewhat contrived, similar applications are already being built today within the Grid community. The Grid Security Infrastructure's single sign-on and delegation capabilities, built on X.509 Proxy Certificates, are being employed to provide authentication services to these applications. 2.4. Description Of Approach This document defines an X.509 "Proxy Certificate" or "PC" as a means of providing for impersonation with an X.509 PKI based authentication system. A Proxy Certificate is an X.509 public key certificate with the following properties: 1) It is signed by either an X.509 End Entity Certificate (EEC), or by another PC. 2) It can sign only another PC. 3) It has its own public and private key pair, distinct from any other EEC or PC. 4) It has no distinct identity of its own. After a PC is used for authentication, the identity that is used for authorization is that of the EEC that signed the PC. The PC effectively inherits the subject or subjectAltName from its signing EEC. 5) It contains a new X.509 extension to identify it as a PC, to place restrictions on the PC, and to carry delegation tracing information about the PC. This new extension, along with other X.509 fields and extensions, are used to enable proper path validation and use of the PC. The process of creating a PC is as follows: Tuecke, et. al. Expires January 2002 7 Internet Draft X.509 Proxy Certificate Profile July 2001 1) A new public and private key pair is generated. 2) An unsigned PC request is created, conforming to the profile described in this document. 3) The PC request is signed by the private key of the EEC, or by another PC. During this process, the unsigned PC is verified to ensure that it is valid (e.g. it is not an EEC, the PC fields are appropriately set, etc). When a PC is created as part of a delegation from entity A to entity B, this process is modified by performing steps #1 and #2 within entity B, then passing the PC request from entity B to entity A over an integrity checked channel, then entity A performs step #3 and passes the PC back to entity B. (Note: There is a related draft that describes how this delegation approach can be incorporated into the TLS protocol [8].) Path validation of a PC is very similar to normal path validation, with a few additional checks to ensure, for example, proper PC signing constraints. In order to make the appropriate PC(s) and EEC available for path validation, the authentication protocol using the PC (e.g. TLS) may pass the entire PC and EEC chain as part of the authentication protocol. 2.5. Proxy Authority, not Certificate Authority A common initial reaction against the approach described in this document is, "You are using the end entity certificate (EEC) as a CA!" However, this is not the case. To understand why, one must first understand what a CA does. In issuing an EEC, a CA performs two primary functions: 1) Naming: The CA assigns a (generally unique) "Name" to the end entity to which it issues an EEC. This Name is contained in the subject or subjectAltName field of the issued EEC. 2) Key to Name binding: By singing an EEC with the CA's private key, the CA is providing a means to allow an authenticating party to verify that the holder of a particular private key should be associated with (bound to) a particular Name. In addition, a CA usually has an associated Registration Authority, which performs the checks necessary to bind the Name to the real world entity (e.g. person, computer, etc) that is to be the bearer of that Name. The reason for doing all of this is to allow for authorization decisions to be made, based at least in part on these CA issued Names. In other words, after the public key authentication operation has determined the Name of the authenticating party, then Tuecke, et. al. Expires January 2002 8 Internet Draft X.509 Proxy Certificate Profile July 2001 that Name can be used as the basis for deciding what the entity is allowed to do. (Note: Attribute certificates are discussed below.) The critical difference between using an EEC to sign a Proxy Certificate, versus using an EEC to sign another EEC, is that a Proxy Certificate does NOT define a new Name. Rather, a Proxy Certificate inherits the name from the EEC that signs it. The next section describes this inheritance in more detail. In effect, the PC simply provides another route to validating the Key to Name binding that the CA has established with an EEC. A PC allow an alternate Key' to bind to the same Name, optionally with restrictions, with this Key' to Name binding vouched for by the holder of the EEC private key. This allows entity A to give to entity B the ability to establish this binding, and thus allows B to establish itself as a proper bearer of A's Name. For this reason, we use the term "Proxy Authority", rather than "Certificate Authority", to refer to the issuer of a Proxy Certificates. A Proxy Authority does not perform the Naming function of a Certificate Authority, but rather just a Key to Name binding. 2.6. Names Versus Subjects In X.509 certificates, the subject (or subjectAltName) is used for two distinct purposes: 1) In an End Entity Certificate, the subject is the Name that the CA has issued, as described in the previous section. This Name is typically used for authorization purposes. 2) In a CA Certificate, the subject is also used for path validation. That is, the issuer field in an EEC or CA Certificate must match the subject field of a CA Certificate, in order for the signing path to be established. As stated previously, a PC does not have its own Name, but rather it inherits its Name from its signing EEC (or more accurately, from the EEC that signed the first PC in the PC chain). In practice what this means is that the subject field of a PC is only used for purpose #2. The only purpose of the subject field of a PC is to establish the signing path that eventually leads to an EEC. The implication of this is that after a PC is used for authentication, the PC subject should not be used for authorization. Instead, the PC signing chain should be followed to find the EEC that signed this PC chain, and the subject from that EEC should be used as the identity (or Name) for authorization purposes. To discourage mistakes in this area, this Proxy Certificate profile defines that the PC subject (actually its subjectAltName) is just a pseudo-randomly generated string. Further, the subject of the EEC Tuecke, et. al. Expires January 2002 9 Internet Draft X.509 Proxy Certificate Profile July 2001 is not maintained anywhere in the PC, which forces the authenticating party to properly retrieve the subject from the EEC. 2.7. Features Of This Approach Using Proxy Certificates to perform delegation has several features that make it attractive: * Ease of integration * Because a PC requires only a minimal change to path validation, it is very easy to incorporate support for Proxy Certificates into existing X.509 based software. For example, SSL/TLS requires no protocol changes to support authentication using a PC, and only small changes to support delegation of a PC [8]. Further, an SSL/TLS implementation requires only minor changes to support PC path validation, and to retrieve the authenticated subject of the signing EEC instead of the subject of the PC. * Many existing authorization systems use the X.509 subject name as the basis for access control. Proxy Certificates require no change to such authorization systems, since a PC inherits its name from the EEC that signed it. * Ease of use * Using PC for single sign-on helps make X.509 PKI authentication easier to use, by allowing users to "login" once and then perform various operations securely. * For many users, properly managing their own EEC private key is a nuisance at best, and a security risk at worst. One option easily enabled with a PC is to manage the EEC private keys and certificates in a centrally managed repository. When a user needs a PKI credential, the user can login to the repository using name/password, one time password, etc. Then the repository can delegate a PC to the user, but continue to protect the EEC private key in the repository. * Protection of private keys * By using the remote delegation approach outlined above, entity A can delegate a PC to entity B, without entity B ever seeing the private key of entity A, and without entity A ever seeing the private key of the newly delegated PC held by entity B. In other words, private keys never need to be shared or communicated by the entities participating in a delegation of a PC. * * When implementing single sign-on, using a PC helps protect the private key of the EEC, because it minimizes the exposure and use of that private key. For example, when an EEC private key is password protected on disk, the password and unencrypted Tuecke, et. al. Expires January 2002 10 Internet Draft X.509 Proxy Certificate Profile July 2001 private key need only be available during the creation of the PC. That PC can then be used for the remainder of its valid lifetime, without requiring access to the EEC password or private key. Similarly, when the EEC private key lives on a smartcard, the smartcard need only be present in the machine during the creation of the PC. * Limiting consequences of a compromised key * When creating a PC, the PA can limit the validity period of the PC, the depth of the PC path that can be created by that PC, and key usage of the PC and its descendents. Further, fine grained restriction policies can be carried by a PC to even further restrict the operations that can be performed using the PC. This permits the PA to limit any damage that could be done by the bearer of the PC, either accidentally or maliciously. * A compromised PC private key does NOT compromise the EEC private key. This makes a short term, or an otherwise restricted PC attractive for day-to-day use, since a compromised PC does not require the user to go through the usually cumbersome and time consuming process of having the EEC with a new private key reissued by the CA. See Section 5 below for more discussion on how Proxy Certificates relate to Attribute Certificates. 3. Certificate and Certificate Extensions Profile This section defines the usage of X.509 certificate fields and extensions in Proxy Certificates, and defines one new extension for Proxy Certificate Information. 3.1. Issuer & Issuer Alternative Name The Proxy Authority (i.e. the issuer) of a Proxy Certificate MUST be either an End Entity Certificate, or another Proxy Certificate. If the Proxy Authority Certificate has a non-empty subject field, then the issuer field of the Proxy Certificate MUST contain the subject of the Proxy Authority Certificate. Otherwise, if the Proxy Authority Certificate has an empty subject field, but non-empty subjectAltName, then the issuer field of the Proxy Certificate MUST be an empty sequence, the issuerAltName MUST be the subjectAltName of the Proxy Authority Certificate, and the issueAltName MUST be critical. 3.2. Subject & Subject Alternative Name The subject field of a Proxy Certificate MUST be an empty sequence. Tuecke, et. al. Expires January 2002 11 Internet Draft X.509 Proxy Certificate Profile July 2001 The subjectAltName extension of a Proxy Certificate MUST be an otherName, using the impersonationCertName OID (?) and an IA5String (?) containing the name of the Proxy Certificate. The subjectAltName extension MUST be critical. The subjectAltName of a Proxy Certificate SHOULD only be used for path validation. As such, the string chosen for the subjectAltName of a Proxy Certificate is arbitrary, but SHOULD be (statistically) unique in order to enable path validation. 3.3. Key Usage If the issuer certificate includes the keyUsage extension, then the Proxy Certificate MUST include a keyUsage extension, which MAY further restrict the issuer's keyUsage. If the issuer certificate does not include a keyUsage extension, then the Proxy Certificate MAY include a keyUsage extension to restrict the key usage of the Proxy Certificate. The keyUsage extension MUST be critical. If the keyUsage extension is present in a Proxy Certificate, it must conform to the following restrictions: The keyCertSign bit MUST NOT be asserted. The following restriction applies to each of these bits: digitalSignature, nonRepudiate, keyEncipherment, dataEncipherment, keyAgreement, cRLSign, encipherOnly, decipherOnly. If this bit in the issuer certificate is not asserted, then this bit in the Proxy Certificate MUST NOT be asserted. If this bit in the issuer certificate is asserted, or if the issuer certificate does not include a keyUsage extension, then this bit in the Proxy Certificate MAY be either asserted or not asserted. See the commentary in section 6 for more information on the keyCertSign and nonRepudiate bits. 3.4. Extended Key Usage If the issuer certificate includes the extKeyUsage extension, then: The Proxy Certificate MUST include an extKeyUsage extension. Any OID that is contained in the Proxy Certificate's extKeyUsage extension MUST be present in the issuer certificate's extKeyUsage extension. The Proxy Certificate's extKeyUsage extension MAY omit any OID that is present in the issuer certificate's extKeyUsage. Tuecke, et. al. Expires January 2002 12 Internet Draft X.509 Proxy Certificate Profile July 2001 If the issuer certificate's extKeyUsage extension is critical, then the Proxy Certificate's extKeyUsage MUST be critical. If the issuer certificate's extKeyUsage extension is not critical, then the Proxy Certificate's extKeyUsage MAY be critical or non-critical. If the issuer certificate does not include the extKeyUsage extension, then the Proxy Certificate MAY include a extKeyUsage extension to restrict the key usage of the Proxy Certificate. In this case, the extKeyUsage extension MAY be critical or non- critical. 3.5. Basic Constraints The cA field in the basic constraints extension MUST NOT be TRUE. 3.6. Proxy Certificate Information One new extension is defined: id-ce-proxy-cert-info OBJECT IDENTIFIER ::= { id-ce ?? } ProxyCertInfo ::= SEQUENCE { pC BOOLEAN DEFAULT TRUE, pCPathLenConstraint INTEGER (0..MAX) OPTIONAL, proxyRestriction ProxyRestriction OPTIONAL, delegationTrace DelegationTrace OPTIONAL } ProxyRestriction ::= SEQUENCE { policyLanguage OBJECT IDENTIFIER, policy OCTET STRING } DelegationTrace ::= SEQUENCE { acceptorCertChain CertChain, acceptorVerifier AcceptorVerifier OPTIONAL } AcceptorVerifier ::= SEQUENCE { acceptorVerifierData AcceptorVerifierData, acceptorVerifierSig Signature } AcceptorVerifierData ::= SEQUENCE { pCPublicKey SubjectPublicKeyInfo, validity Validity } CertChain ::= SEQUENCE OF Certificate Signature ::= SEQUENCE { signatureAlgorithm AlgorithmIdentifier, signatureValue BIT STRING } If a certificate is a Proxy Certificate, then the proxyCertInfo extension MUST be present, the pC field MUST be TRUE, and this extension MUST be marked as critical. Tuecke, et. al. Expires January 2002 13 Internet Draft X.509 Proxy Certificate Profile July 2001 A Proxy Certificate MUST NOT be used to sign an End Entity Certificate or a CA Certificate. If a certificate is not a Proxy Certificate, then the proxyCertInfo extension MAY be present, and MAY appear as a critical or non- critical extension. In this case, if this extension is present, then the pC field MUST be FALSE. The ProxyCertInfo extension consists of one required and three optional fields, which are described in detail in the following subsections. 3.6.1. pC As described above, the pC field indicates whether or not the certificate is a proxy certificate: if the certificate is a proxy certificate, the pC field MUST be TRUE; otherwise, the pC field MUST be FALSE. 3.6.2. pCPathLenConstraint The pCPathLenConstraint field, if present, specifies the maximum depth of the path of Proxy Certificates that can be signed by this End Entity Certificate or Proxy Certificate. A pCPathLenConstraint of 0 means that this certificate MUST not be used to sign a Proxy Certificate. If the proxyCertInfo extension is not present, or if the pCPathLenConstraint is not present, then the proxy path length is unlimited. 3.6.3. proxyRestriction The proxyRestriction field, if present, specifies restrictions on the use of this certificate. If the certificate is not a Proxy Certificate (i.e, if the pC field is FALSE), then the proxyRestriction field MUST NOT be present. An unrestricted proxy is a statement that the PA wishes to delegate all its authority to the bearer (i.e., to anyone who has that proxy certificate and proof of possession of the associated private key). Proxy restrictions are used to limit the amount of authority delegated, for example to assert that the proxy certificate may be used only to make requests to a specific server, or only to authorize specific operations on specific resources. Within the proxyRestriction, the policy field is an expression of policy, and the policyLanguage field indicates the language in which the policy is expressed. Proxy restrictions impose additional requirements on the relying party, because only the relying party is in a position to ensure that those restrictions are met. When making an authorization decision based on a proxy certificate, it is the relying party's responsibility to verify that the requested authority is compatible Tuecke, et. al. Expires January 2002 14 Internet Draft X.509 Proxy Certificate Profile July 2001 with all restrictions in the PC's certificate path. In other words, the relying party MUST verify that the following three conditions are met: 1) If the PC includes a proxy restriction, then the relying party knows how to interpret the policy expressed in the PC's restriction, and the request is allowed under that policy. 2) If the PA is an EEC, then the relying party's local policies authorize the request for the entity named in the EEC. 3) If the PA is another PC, then conditions (1), (2), and (3) are met for the PA. If these conditions are not met, the relying party MUST either deny authorization or ignore the PC entirely when making its authorization decision (i.e., make the same decision that it would have made had the PC never been presented). Note that this verification MUST take place regardless of whether or not the PC itself contains restrictions, as other PCs in the signing chain may contain conditions that must be verified. The relying party MAY impose additional restrictions as to what proxy certificates it accepts. For example, a relying party may choose to reject all proxy certificates, or to accept only those proxy certificates that include delegation tracing information, or to accept proxy certificates only for certain operations, etc. The rights granted to the bearer of a PC will, then, be (at most) the intersection of the set of rights granted to the entity named in the EEC in the PC's certificate path, and the sets of rights authorized by the policies in each proxyRestriction that appears in the certificate path. For example, imagine that Steve is authorized to read and write files A and B on a file server, and that he uses his EEC to create a PC that includes the restriction that it can be used only to read or write files A and C. At most, the rights granted to the bearer of that PC will be the right to read and write file A -- a request to read file B, for example, would be rejected because it would be incompatible with the proxy restriction, and a request to read file C would be rejected because the file server's local policies do not grant Steve any access to file C. If that PC were then used to create a new PC that includes the restriction that it can be used only to read files, then the bearer of that new PC would have, at most, the right to read file A. In many cases, the relying party will not have enough information to evaluate the above criteria at the time that the certificate itself is validated. For example, if a certificate is used to authenticate a connection to some server, that certificate is typically validated during that authentication step, before any requests have been made of the server. In that case, the relying party MUST either have some authorization mechanism in place that will check the proxy restrictions, or reject any certificate that contains proxy Tuecke, et. al. Expires January 2002 15 Internet Draft X.509 Proxy Certificate Profile July 2001 restrictions (or that has a parent certificate that contains proxy restrictions). 3.6.4. delegationTrace The delegationTrace field, if present, enumerates the entities that have participated in the identity delegation. If a certificate is not a Proxy Certificate (i.e., if the pC field is FALSE), then the delegationTrace field MUST NOT be present. The delegationTrace field is a SEQUENCE of the following two subfields: 1) The acceptorCertChain field represents the Acceptor's identity (as understood by the PA). This SHOULD be the certificate path used by the PA to authenticate the Acceptor; the final certificate in the sequence MUST be the identity certificate of the entity accepting the Proxy Certificate. 2) The acceptorVerifier field, if present, is an acknowledgement by the Acceptor that the Acceptor knows the proxy certificate's private key. This consists of: * acceptorVerifierData: The PC's public key (pCPublicKey) and a time period (validity). * acceptorVerifierSig: a signature, using the Acceptor's private key, of acceptorVerifierData. This signature is interpreted as an acknowledgement that the Acceptor knows the private key corresponding to the public key in acceptorVerifierData.pcPublicKey and accepts responsibility for use of that private key during the time period specified in acceptorVerifierData.validity. A relying party could, then, validate the information in acceptorVerifier by checking that all of the following conditions are met: * The Acceptor certificate (the end certificate in acceptorCertChain) is valid. * The pCPublicKey is identical to the certificate's subjectPublicKeyInfo. * acceptorSig is a valid signature, using the private key associated with the Acceptor certificate, of acceptorVerifierData. * The time for which the certificate is being validated falls within the range of acceptorVerifierData.validity. This example sequence of events illustrates one way in which the delegationTrace field could be created: the Acceptor generates the proxy key pair and sends a certificate request to the PA. The public Tuecke, et. al. Expires January 2002 16 Internet Draft X.509 Proxy Certificate Profile July 2001 key in the request is the public key from the newly-generated keypair; the request also includes a ProxyCertInfo extension that has an acceptorVerifier constructed by placing that same public key into acceptorVerifierData and signing that with the Acceptor's public key. The request may include fields of the proxy certificate such as validity time, a proxyRestriction field, other extensions such as key extensions or private extensions. The PA creates the proxy certificate as it sees fit (or possibly negotiates with the Acceptor) and returns the signed PC to the Acceptor. The Acceptor now has the choice to accept or reject the PC. If the PC contains an unacceptable set of rights, or other unacceptable extensions, the Acceptor should reject the PC and destroy the proxy keys. Thus any PC that includes delegationTrace information with an acceptorVerifier should indicate to a relying party that it came from the Acceptor, and that the Acceptor agreed to its contents. 4. Certificate Path Validation [TBD: Consider changing this section to add a second phase to path validation for PC validation, rather than modifying the existing path validation to accommodate the entire chain.] The Certificate Path Validation algorithm described in Section 6 of draft-ietf-pkik-new-part-04 [7] must be modified to accommodate Proxy Certificates. Changes are needed to: 1) check the generalized signing chains involving CAs, End Entity Certificates, and Proxy Certificates; 2) handle the use of subjectAltName and issuerAltName in the certificate path; 3) handle the iCPathLenConstraint in the proxyCertInfo extension. 4) check the key usage and extended key usage extensions. Changes to section 6.1.2, Initialization: (j) This step defines the working_issuer_name to be a distinguished name. However, because a PC uses the issuerAltName, the working_issuer_name variable needs to be generalized to accommodate not just a distinguished name, but any of the valid issuerAltName/subjectAltName types. (new) working_certificate_type: This can be one of CA, EEC, or PC. A certificate type of CA is determined by the basicConstraints extension or as verified out-of-band. A certificate type of PC is determined by the proxyCertInfo extension. Otherwise, the certificate type is EEC. (new) valid_pc_key_usage & pc_key_usage_criticality: These are used to verify that the key usage of a PC is a subset of the key usage of the certificate that signed that PC, and that the Tuecke, et. al. Expires January 2002 17 Internet Draft X.509 Proxy Certificate Profile July 2001 criticality of this extension never diminishes. These variables are not initialized or used until the first EEC or PC is encountered in the path validation algorithm with this extension. (new) valid_pc_ext_key_usage & pc_ext_key_usage_criticality: These are used to verify that the extended key usage OIDs of a PC is a subset of the extended key usage OIDs of the certificate that signed that PC, and that the criticality of this extension never diminishes. These variables are not initialized or used until the first EEC or PC is encountered in the path validation algorithm with this extension. Changes to section 6.1.3, Basic Certificate Processing: (a)(4) The comparison of the certificate issuer name with the working_issuer_name must be generalized to support comparison between any of the valid issuerAltName types. (a)(new) The certificate type is CA and the working_certificate_type is CA, or the certificate type is EEC and the working_certificate_type is CA, or the certificate type is PC and the working_certificate_type is EEC or PC. (b) & (c) This step checks the Name Constraints defined by the CA. However, since a PC does not define a new Name, these checks should be skipped if the certificate type is PC (as specified in a proxyCertInfo extension). (new) If certificate type is PC, and valid_pc_key_usage has been initialized, then verify that: (1) all bits that are asserted in the keyUsage extension of the certificate are also asserted in the valid_pc_key_usage; (2) if pc_key_usage_criticality is true, then the keyUsage extension is critical (new) If certificate type is PC, and valid_pc_ext_key_usage has been initialized, then verify that: (1) all OIDs that are in the extKeyUsage extension in the certificate are also in the valid_pc_ext_key_usage; (2) if pc_ext_key_usage_criticality is true, then the extKeyUsage extension is critical. Changes to section 6.1.4, Preparation for Certificate i+1: (c) Adjust this to assign the subjectAltName to working_issuer_name, if the subject is empty. This is done to accommodate the use of subjectAltName and issuerAltName by PCs. (k) This step verifies that the certificate is a CA certificate. However, it is not general enough to support a PC. So change Tuecke, et. al. Expires January 2002 18 Internet Draft X.509 Proxy Certificate Profile July 2001 this step to simply assign the certificate type to the working_certificate_type. The necessary CA, EEC, and PC signing constraints check has been added to the Basic Certificate Processing section above. (m) This step resets the max_path_length if pathLenConstraint is present in the certificate. This needs to be generalized to support pCPathLengthConstraint from the proxyCertInfo extension, as follows: Reset max_path_length as follows: (1) If certificate type is CA, and pathLenConstraint is present in the certificate and is less than max_path_length, then set max_path_length to the value of pathLenConstraint. (2) If certificate type is EEC, and pCPathLenConstraint is not present in the certificate, then set max_path_length to n. (3) If certificate type is EEC, and pCPathLenConstraint is present in the certificate, then set max_path_length to the value of pCPathLenConstraint. (4) If certificate type is PC, and pCPathLenConstraint is present in the certificate and less than max_path_length, then set max_path_length to the value of pCPathLenConstraint. (n) Since keyCertSign is currently defined to be equivalent to being a CA, this check needs to be changed to accommodate PCs, as follows: If certificate type is CA, and a key usage extension is present and marked critical, verify that the keyCertSign bit is set. (new) If certificate type is EEC or PC, and the key usage extension is present, then set valid_pc_key_usage to keyUsage, and set pc_key_usage_criticality to the keyUsage criticality. (new) If certificate type is EEC or PC, and the extended key usage extension is present, then set valid_pc_ext_key_usage to extKeyUsage, and set pc_ext_key_usage_criticality to the extKeyUsage criticality. At this point we have no plans for a PA (that is, an EEC or PC) to revoke the PCs that it has issued. If this feature is needed in the future, the CRL Distribution Point extension can be used in the PA certificates to locate a CRL. 5. Relationship to Attribute Certificates An Attribute Certificate [4] can be used to grant to one identity, the holder, some attribute such as a role, clearance level, or alternative identity such as "charging identity" or "audit identity". This is accomplished by way of a trusted Attribute Authority (AA), which issues signed Attribute Certificates (AC), Tuecke, et. al. Expires January 2002 19 Internet Draft X.509 Proxy Certificate Profile July 2001 each of which binds an identity to a particular set of attributes. Authorization decisions can then be made by combining information from the authenticated End Entity Certificate providing the identity, with the signed Attribute Certificates providing binding of that identity to attributes. There is clearly some overlap between the capabilities provided by Proxy Certificates and Attribute Certificates. However, the combination of the two approaches together provides a broader spectrum of solutions to authorization in X.509 based systems, than either solution alone. This section seeks to clarify some of the overlaps, differences, and synergies between Proxy Certificate and Attribute Certificates. 5.1. Types of Attribute Authorities For the purposes of this discussion, Attribute Authorities, and the uses of the Attribute Certificates that they produce, can be broken down into two broad classes: 1) End entity AA: An End Entity Certificate may be used to sign an AC. This can be used, for example, to allow an end entity to delegate some of its privileges to another entity. 2) Third party AA: A separate entity, aside from the end entity involved in an authenticated interaction, may sign ACs in order to bind the authenticated identity with additional attributes, such as role, group, etc. For example, when a client authenticates with a server, the third party AA may provide an AC that binds the client identity to a particular group, which the server then uses for authorization purposes. This second type of Attribute Authority, the third party AA, works equally well with an EEC or a PC. For example, Proxy Certificates can be used to delegate the EEC's identity to various other parties. Then when one of those other parties uses the PC to authenticate with a service, that service will receive the EEC's identity via the PC, and can apply any ACs that bind that identity to attributes in order to determine authorization rights. There would appear to be great synergies between the use of Proxy Certificates and Attribute Certificates produced by third party Attribute Authorities. However, the uses of Attribute Certificates that are granted by the first type of Attribute Authority, the end entity AA, overlap considerably with the uses of Proxy Certificates as described in the previous sections. Such Attribute Certificates are generally used for delegation of rights from one end entity to others, which clearly overlaps with the stated purpose of Proxy Certificates, namely single sign-on and delegation. 5.2. Delegation Using Attribute Certificates In the motivating example above, PCs are used to delegate Steve's identity to the various other jobs and agents that need to act on Tuecke, et. al. Expires January 2002 20 Internet Draft X.509 Proxy Certificate Profile July 2001 Steve's behalf. This allows those other entities to authenticate as if they were Steve, for example to the mass storage system. A solution to this example could also be cast using Attribute Certificates that are signed by Steve's EEC, which grant to the other entities in this example the right to perform various operations on Steve's behalf. In this example, the starter program, the agent, the simulation jobs, and the post-processing job would each have their own EECs. Steve's EEC would therefore issue ACs to bind each of those other EEC identities to attributes that grant the necessary privileges allow them to, for example, access the mass storage system. However, this AC based solution to delegation has some disadvantages as compared to the PC based solution: * All protocols, authentication code, and identity based authorization services must be modified to understand ACs. With the PC solution, protocols (e.g. TLS) likely need no modification, authentication code needs minimal modification (e.g. to perform PC aware path validation), and identity based authorization services need no modification. * ACs need to be created by Steve's EEC, which bind attributes to each of the other identities involved in the distributed application (i.e. the agent, simulation jobs, and post-processing job). This implies that Steve must know in advance which other identities may be involved in this distributed application, in order to generate the appropriate ACs which are signed by Steve's ECC. On the other hand, the PC solution allows for much more flexibility, since parties can further delegate a PC without a priori knowledge by the originating EEC. There are many unexplored tradeoffs and implications in this discussion of delegation. However, reasonable arguments can be made in favor of either an AC based solution to delegation or a PC based solution to delegation. The choice of which approach should be taken in a given instance may depend on factors such as the software that it needs to be integrated into, the type of delegation required, and religion. 5.3. Propagation of Authorization Information One possible use of Proxy Certificates is to carry authorization information associated with a particular identity. The merits of placing authorization information into End Entity Certificates (also called a Public Key Certificate or PKC) have been widely debated. For example, Section 1 of "An Internet Attribute Certificate Profile for Authorization" states: "Authorization information may be placed in a PKC extension or placed in a separate attribute certificate (AC). The placement of authorization information in PKCs is usually undesirable for two Tuecke, et. al. Expires January 2002 21 Internet Draft X.509 Proxy Certificate Profile July 2001 reasons. First, authorization information often does not have the same lifetime as the binding of the identity and the public key. When authorization information is placed in a PKC extension, the general result is the shortening of the PKC useful lifetime. Second, the PKC issuer is not usually authoritative for the authorization information. This results in additional steps for the PKC issuer to obtain authorization information from the authoritative source. For these reasons, it is often better to separate authorization information from the PKC. Yet, authorization information also needs to be bound to an identity. An AC provides this binding; it is simply a digitally signed (or certified) identity and set of attributes." ([4], Section 1) Placing authorization information in a PC mitigates the first undesirable property cited above. Since a PC has a lifetime that is mostly independent of (always shorter than) its signing EEC, a PC becomes a viable approach for carrying authorization information. The second undesirable property cited above is true. If a third party AA is authoritative, then using ACs issued by that third party AA is a natural approach to disseminating authorization information. However, this is true whether the identity being bound by these ACs comes from an EEC (PKC), or from a PC. There is one case, however, that the above text does not consider. When performing delegation, it is usually the EEC itself that is authoritative (not the EEC issuer, or any third party AA). That is, it is up to the EEC to decide what authorization rights it is willing to grant to another party. In this situation, including such authorization information into PCs that are generated by the EEC seems a reasonable approach to disseminating such information. 5.4. Proxy Certificate as Attribute Certificate Holder In a system that employs both PCs and ACs, one can imagine the utility of allowing a PC to be the holder of an AC. This would allow for a particular delegated instance of an identity to be given an attribute, rather than all delegated instances of that identity being given the attribute. However, the issue of how to specify a PC as the holder of an AC remains open. 6. Commentary This section provides commentary on various design choices, open issues, related work, and future directions for Proxy Certificates. 6.1. keyCertSign Bit in the Key Usage Basic Extension Tuecke, et. al. Expires January 2002 22 Internet Draft X.509 Proxy Certificate Profile July 2001 This Proxy Certificate profile does not change the definition of the keyCertSign bit of the keyUsage extension. draft-ietf-pkix-new- part1-08 states: "The keyCertSign bit is asserted when the subject public key is used for verifying a signature on public key certificates. If the keyCertSign bit is asserted, then the cA bit in the basic constraints extension (section 4.2.1.10) MUST also be asserted." Nor does this Proxy Certificate profile contradict this keyCertSign definition, since a Proxy Certificate is not an end entity public key certificates, as discussed in section 2 above. 6.2. nonRepudiate Bit in the Key Usage Basic Extension One alternative for the nonRepudiate bit is that it MUST NOT be asserted. It seems, on the surface, and impersonation and non- repudiation are at odds with one another. However, this decision is postponed until further discussion with others who are more familiar with the use of this bit. 6.3. Subject Name of a Proxy Certificate The subject name of a PC is only used for path validation. This PC profile uses a randomly generated subjectAltName to provide a (statistically) unique subject name for the PC. Another possibility for naming the PC is to use a subject field that is derived from the subject of the PA. In fact, this is the approach taken in the current Grid Security Infrastructure implementation. For example, the PC subject field could be the EEC subject field, extended with the addition of a new AttributeType and Value component of proxyLevel:nnnn where proxyLevel is a new AttributeType, and nnnn is the depth of the PC signing path. The issuer field would contain the subject field of the PA that signed the PC. In this scheme the path validation process would check that the subject and issuer names match up the chain and the proxyLevel values increase by one at each subsequent delegation. One advantage of this approach is that some current implementations of path validation, such as OpenSSL-0.9.6, do not support the use of subjectAltName and issuerAltName. Thus for practical purposes it is arguably better to use the subject name and the proxyLevel:nnnn scheme. A disadvantage of this approach is that it is reliant on the DN convention used by the subject field. This limits Proxy Certificates such that they can only be used for EECs that use the subject field. If an EEC instead uses subjectAltName, with a null subject field, then this approach does not work. For this reason, this approach was rejected for this Proxy Certificate profile. Tuecke, et. al. Expires January 2002 23 Internet Draft X.509 Proxy Certificate Profile July 2001 6.4. Carrying Along the End Entity Subject Another suggestion was to include the subject of the signing EEC as an informational field in the PC. This would allow an authorizing process to use only information in the final PC in the chain to determine identity, and not need to walk the chain in order to find out the subject (or subjectAltName) of the EEC that the PC is derived from. This approach was rejected for the following reasons: * It would be easy to spoof this informational field. For example, a PC with an informational subject of "Steve" could be used to create a PC with an informational subject set to "Doug". This leaves us with two alternatives: * We can augment the path validation to check that this informational field of the PC is the same as in the signing PC or EEC. But this is not desirable, as it complicates the path validation. * But if we do not validate this field, we cannot trust the contents of this informational field. So then there is no point in including this informational field. * Upon closer examination, there is a lot of information in the certificate chain that may be needed during authorization, such as the number of levels of delegation, the CA (or multiple levels of CAs) who signed the original EEC, the constraints and keyUsage values of the signing EEC, possibly Certificate Policies associated with CAs or IAs. All of these require essentially the same amount of work as retrieving the subject of the EEC that signed the PC. So why threat the EEC subject specially by including it in an information field? In the end, just including the EEC subject name does not seem to be sufficiently useful to justify the addition of another field and the work of verifying that name during the path validation. Therefore, to determine the identity of a PC for authorization purposes, the subject of the EEC must be retrieved directly from the EEC in the signing chain. This approach also has the beneficial side effect of further stressing that a Proxy Certificate has no identity of its own, but rather inherits it from its signing EEC. 6.5. Specifying Proxy Restrictions The proxyRestriction field in the proxyCertInfo extension does not define a policy language to be used for proxy restrictions; rather, it places the burden on those parties using that extension to define an appropriate language, and to acquire an OID for that language (or to select an appropriate previously-defined language/OID). Because it is essential for the PA that issues a certificate with a Tuecke, et. al. Expires January 2002 24 Internet Draft X.509 Proxy Certificate Profile July 2001 proxyRestriction field and the relying party that interprets that field to agree on its meaning, the policy language OID must correspond to a policy language, not just a policy grammar. Several different approaches were considered regarding how to limit the use of a PC for specific authorization purposes. One of these approaches was to include a list the specific rights granted by the PC (perhaps along with conditions associated with those rights), either as a separate extension or as part of proxyCertInfo. This list of rights would define the subset of the issuer's rights to be granted to the PC holder. But the parties using that extension would still be responsible for ensuring that both the PA and relying party agreed on the meanings of the access rights and conditions appearing in the restriction. Another possible approach is to embed an Attribute Certificate (signed by the EEC issuing the PC) within a PC, which would define a subset of the issuer's attributes to be associated with the PC holder. 6.6. Proxy Restrictions vs. Proxy Rights The proxyRestriction field in the proxyCertInfo extension defines restrictions on the use of the proxy certificate; if that field is not present, the proxy is unrestricted. Another approach would be to require that each proxy certificate explicitly list the rights that it grants. 6.7. Site Information in Delegation Tracing In some cases, it may be desirable to know the hosts involved in a delegation transaction (for example, a relying party may wish to reject proxy certificates that were created on a specific host or domain). The delegationTrace field could be modified to include the PA's and Acceptor's IP addresses; however, IP addresses are typically easy to spoof, and in some cases the two parties to a transaction may not agree on the IP addresses being used (e.g., if the Acceptor is on a host that uses NAT, the Acceptor and the PA may disagree about the Acceptor's IP address). Another suggestion was, in those cases where domain information is needed, to require that the subject names of all End Entities involved (the Acceptor(s) and the End Entity that appears in a PC's certificate path) include domain information. 6.8. Certificate Chain in Delegation Tracing The delegationTrace field includes the Acceptor's certificate chain, which a relying party may use to verify the identity of the Acceptor. However, the set of "most-trusted" certificates trusted by the PA, the Acceptor, and the relying party may be different, in which case the certificate chain in the delegation tracing field may not be sufficient for the relying party to validate the Acceptor's Tuecke, et. al. Expires January 2002 25 Internet Draft X.509 Proxy Certificate Profile July 2001 certificate. Another possible approach would be to include the Acceptor's certificate and a set of additional certificates that the PA believes may be relevant to verifying the Acceptor's certificate. 6.9. Another Approach to Delegation Tracing The acceptorVerifier subfield of the delegation tracing field indicates that the Acceptor accepts responsibility for the use of the certificate. However, there may be cases in which an Acceptor would be willing to accept a particular set of delegated rights but would not be willing to sign an acceptorVerifier (one example would be the case in which the PA and Acceptor are different entities, but both know the private key of the PC). 6.10. Certificate Policies Extension One could imagine some interesting things to do with the Certificate Policies extension. For example: * One could define policies for creation of a Proxy Certificate. For example, was the PC created locally or remotely? * An alternate approach to defining restricted Proxy Certificates would be use the Certificate Policies extension to carry the OIDs of various Proxy Certificate Policies. For example, a Proxy Certificate policy might state that the PC can only be used within a limited scope of machines, or for a limited set of uses. 6.11. Kerberos 5 Tickets The Kerberos Network Authentication Protocol (RFC 1510 [9]) is a widely used authentication system based on conventional (shared secret key) cryptography. It provides support for single sign-on via creation of "Ticket Granting Tickets" or "TGT", and support for delegation of impersonation rights via "forwardable tickets". Kerberos 5 tickets have informed many of the ideas surrounding X.509 Proxy Certificates. For example, the local creation of a short- lived PC can be used to provide single sign-on in an X.509 PKI based system, just as creation of short-lived TGT allows for single sign- on in a Kerberos based system. And just as a TGT can be forwarded (i.e. delegated) to another entity to allow for impersonation in a Kerberos based system, so can a PC can be delegated to allow for impersonation in an X.509 PKI based system. A major difference between a Kerberos TGT and an X.509 PC is that while creation and delegation of a TGT requires the involvement of a third party (the Kerberos Domain Controller), a PC can be unilaterally created without the active involvement of a third party. That is, a user can directly create a PC from an EEC for single sign-on capability, without requiring communication with a third party. And an entity with a PC can delegate the PC to another entity (i.e. by creating a new PC, signed by the first) without requiring communication with a third party. Tuecke, et. al. Expires January 2002 26 Internet Draft X.509 Proxy Certificate Profile July 2001 The method used by Kerberos implementations to protect a TGT can also be used to protect the private key of a PC. For example, some Unix implementations of Kerberos use standard Unix file system security to protect a user's TGT from compromise. Similarly, the Globus Toolkit's Grid Security Infrastructure implementation of Proxy Certificates protects a user's PC private key using this same approach. Looking at developments with Kerberos 5 tickets also can inform us about potential future directions for Proxy Certificates. For example: * Kerberos tickets have two simple mechanisms for allowing their use to be restricted: a time period during which the ticket is valid (the "starttime" and "endtime" fields of a ticket), and a host address which restricts the host on which the ticket may be used (the "caddr" field of a ticket). An X.509 PC also has a validity period, but does not have a host restriction field, though it could be easily added via an X.509 extension. While these particular restrictions have a variety of limitations and problems, they points toward a future of more general restriction policies that might be included in a PC and/or Kerberos 5 ticket. * The Microsoft implementation of Kerberos 5 has (not without controversy) used the "authorization-data" field in the Kerberos ticket to encode authorization information into the ticket. A similar approach could be taken with X.509 Proxy Certificates, by encoding the authorization information into an X.509 extension in a PC. This approach allows for a user's normal, long-lived identity certificate to be used to create a short-lived authorization certificate that can be delegated as necessary. Merits of this approach versus Attribute Certificates are discussed in Section 5. 7. Security Considerations A Proxy Certificate is generally less secure than the EEC that issued it. This is due to the fact that the private key of a PC is generally not protected as rigorously as that of the EEC. For example, the private key of a PC is often protected using only file system security, in order to allow that PC to be used for single sign-on purposes. This makes the PC more susceptible to compromise. However, the risk of a compromised PC is only the misuse of a single user's privileges. Due to the path validation checks made on a PC, a PC cannot be used to sign an EEC or PC for another user. Further, a compromised PC can only be misused for the lifetime of the PC. Therefore, one common way to limit the misuse of a compromised PC is to limit their validity periods to no longer than is needed. Tuecke, et. al. Expires January 2002 27 Internet Draft X.509 Proxy Certificate Profile July 2001 In addition, if a PC is compromised, it does NOT compromise the EEC that created the PC. This property is of great utility in protecting the highly valuable, and hard to replace, public key of the EEC. In other words, the use of Proxy Certificates to provide single sign-on capabilities in an X.509 PKI environment can actually increase the security of the end entity certificates, because creation and use of the PCs for user authentication limits the exposure of the EEC private key to only the creation of the first level PC. The pCPathLenConstraint field of the proxyCertInfo extension can be used by an EEC to limit subsequent delegation of the PC. A service may choose to only authorize a request if a valid PC can be delegated to it. An example of such as service is a job starter, which may choose to reject a job start request if a valid PC cannot be delegated to it. By limiting the pCPathLenConstraint, an EEC can ensure that a compromised PC of one job cannot be used to start additional jobs elsewhere. An EEC or PC can limit what a new PC can be used for by turning off bits in the Key Usage and Extended Key Usage extensions. However, once a key usage or extended key usage has been removed, the path validation algorithm ensures that it cannot added back in a subsequent PC. In other words, key usage can only be decreased in PC chains. The EEC could use the CRL Distribution Points extension and/or OCSP to take on the responsibility of revoking PCs that it had issued, if it felt that they were being misused. The relying party that is going to authorize some actions on the basis of a PC will be aware that it has been presented with a PC, and can determine the depth of the delegation, the time that the delegation took place, and any entities through which the PC was delegated (if the optional delegationTrace field of the proxyCertInfo extension is included in the PCs in the cert chain). It may want to use this information in addition to the information from the signing EEC. Thus a highly secure resource might refuse to accept a PC at all, or maybe only a single level of delegation, or maybe only a PC that has not been delegated through a untrusted host, etc. 8. References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels," BCP 14, RFC 2119, March 1997. [2] Butler, R., D. Engert, I. Foster, C. Kesselman, and S. Tuecke, "A National-Scale Authentication Infrastructure," IEEE Computer, vol. 33, pp. 60-66, 2000. [3] Dierks, T. and C. Allen, "The TLS Protocol, Version 1.0," RFC 2246, January 1999. Tuecke, et. al. Expires January 2002 28 Internet Draft X.509 Proxy Certificate Profile July 2001 [4] Farrell, S. and R. Housley, "An Internet Attribute Certificate Profile for Authorization," Internet Draft draft-ietf-pkix- ac509prof-06.txt, January 2001. [5] Foster, I., C. Kesselman, G. Tsudik, and S. Tuecke, "A Security Architecture for Computational Grids," presented at Proceedings of the 5th ACM Conference on Computer and Communications Security, 1998. [6] Foster, I., C. Kesselman, and S. Tuecke, "The Anatomy of the Grid: Enabling Scalable Virtual Organizations," International Journal of Supercomputer Applications, 2001. [7] Housley, R., W. Ford, W. Polk, and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and CRL Profile," Internet Draft draft-ietf-pkik-new-part1-04.txt (update to RFC 2459), January 1999. [8] Jackson, K., S. Tuecke, and D. Engert, "TLS Delegation Protocol," Internet Draft draft-ietf-tls-delegation-00.txt, 2001. [9] Kohl, J. and C. Neuman, "The Kerberos Network Authentication Service (V5)," RFC 1510, September 1993. 9. Acknowledgments We are grateful to numerous colleagues for discussions on the topics covered in this paper, in particular (in alphabetical order, with apologies to anybody we've missed): Joe Bester, Randy Butler, Keith Jackson, Stephen Kent, Bill Johnston, Marty Humphrey, Sam Meder, Clifford Neuman, Gene Tsudik, Von Welch. This work was supported in part by the Mathematical, Information, and Computational Sciences Division subprogram of the Office of Advanced Scientific Computing Research, U.S. Department of Energy, under Contract W-31-109-Eng-38 and DE-AC03-76SF0098; by the Defense Advanced Research Projects Agency under contract N66001-96-C-8523; by the National Science Foundation; and by the NASA Information Power Grid project. 10. Change Log draft-ietf-pkix-impersonation-00 (February 2001) Initial submission. draft-ietf-pkix-proxy-00 (July 2001) Renamed to "Proxy Certificate", from "Impersonation Certificate", due to overwhelming feedback from IETF and GGF. Tuecke, et. al. Expires January 2002 29 Internet Draft X.509 Proxy Certificate Profile July 2001 Added proxyRestriction field to ProxyCertInfo extension. Added delegationTrace field to ProxyCertInfo extension. Updated to agree with draft-ietf-pkix-part1-08. 11. Contact Information Steven Tuecke Distributed Systems Laboratory Mathematics and Computer Science Division Argonne National Laboratory Argonne, IL 60439 Phone: 630-252-8711 Email: tuecke@mcs.anl.gov Doug Engert Argonne National Laboratory Email: deengert@anl.gov Ian Foster Argonne National Laboratory & University of Chicago Email: foster@mcs.anl.gov Mary Thompson Lawrence Berkeley National Laboratory Email: mrthompson@lbl.gov Laura Pearlman University of Southern California, Information Sciences Institute Email: laura@isi.edu Carl Kesselman University of Southern California, Information Sciences Institute Email: carl@isi.edu Tuecke, et. al. Expires January 2002 30
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