KX509 Kerberized Certificate Issuance Protocol in Use in 2012
draft-hotz-kx509-06.txt

Abstract

This document describes a protocol, called kx509, for using Kerberos tickets to acquire X.509 certificates. These certificates may be used for many of the same purposes as X.509 certificates acquired by other means, but if a Kerberos infrastructure already exists then the overhead of using kx509 may be much less.

While not standardized, this protocol is already in use at several large organizations, and certificates issued with this protocol are recognized by the International Grid Trust Federation.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on December 31, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Protocol Data
• 2.1. Request Packet
• 2.2. Reply Packet
• 3. Protocol Operation
• 4. Acknowledgements
• 5. IANA Considerations
• 6. Security Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Certificate Cacheing and Deployment Considerations
• Appendix B. Historic Extensions
• Appendix C. Example Exchange
• Appendix D. Change History
• Authors' Addresses

1. Introduction

The two primary ways of providing cryptographically secure identification on the Internet are Kerberos tickets [RFC4120], and X.509 [RFC5280] and [X.509] certificates.

In practical IT infrastructure where both are in use, it's highly desirable to deploy their support in a way which guarantees they both authoritatively refer to the same entities. There is already a widely-adopted standard for using X.509 certificates to acquire corresponding Kerberos tickets called PKINIT [RFC4556]. This document describes the kx509 protocol for supporting the symmetric operation of acquiring X.509 certificates using Kerberos tickets.

Preparing and reviewing this document exposed a number of issues which are discussed in the security considerations. Unfortunately, some of them can only be addressed with an incompatible upgrade to this protocol. The IETF's Kerberos working group has an expected work item to address these issues.

The International Grid Trust Federation [IGTF] supports the use of Short Lived Credential Services [SLCS] as a means to authenticate for resource usage based on other, native identity stores which an organization maintains. X.509 certificates issued using the kx509 protocol based on a Kerberos identity is one of the recognized Credential Services. The certificate profile for that use is outside the scope of this RFC, but is described in [GRID-prof].

In normal operation kx509 can be used after a Kerberos ticket-granting-ticket (TGT) is acquired, which is most likely during user login. First, the client generates a RSA public/private key-pair. Next, using the Kerberos ticket-granting-ticket, it acquires a Kerberos service ticket for the KCA (Kerberized Certificate Authority), and uses this to send the public half of its key-pair. The KCA will decrypt the service ticket, verify the integrity of the incoming packet, determine the identity of the user, and use the session key to send back a corresponding X.509 certificate.

1.1. Requirements Language

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 [RFC2119].

2. Protocol Data

The protocol consists of a single request/reply exchange using UDP.

Both the request and the reply packet begin with four bytes of version ID information, followed by a DER encoded ASN.1 message. The first two bytes of the version ID are reserved. They MUST be set to zero when sent, and SHOULD be ignored when received. The third and fourth bytes are the major and minor version numbers, respectively. The version of the protocol described in this document is designated 2.0, so the first four bytes of the packet are 0, 0, 2, 0.

Incompatible variations of this protocol MUST use a different major version number.

2.1. Request Packet

The request consists of a version ID, followed by a DER encoded ASN.1 message containing a Kerberos AP_REQ, integrity check data on the request, and public key generated by the client. The ASN.1 encoding is:

KX509Request ::= SEQUENCE {
        ap-req  OCTET STRING,
        pk-hash OCTET STRING,
        pk-key  OCTET STRING
}

The ap-req is as described in [RFC4120] Section 5.5.1.

The pk-hash is HMAC using SHA-1 as the underlying hash. All 160 bits are sent. The key used is the Kerberos session key. The data to be hashed is the concatenation of

• 4-byte version ID at the beginning of the packet.
• OCTET STRING of the ap-req.
• OCTET STRING of the pk-key.

The pk-key contains a public key. This key and its corresponding private key are generated by the client before contacting the server. Implementations of this protocol MUST support RSA keys, in which case the key is a DER encoded RSAPublicKey as defined in [RFC3447], section A.1.1, and then stored in this octet string in the request. Its encoding as an OCTET STRING starts with the 0x30 byte sequence at the beginning of a DER encoded RSAPublicKey. Use of other public-key types is not defined.

Appendix C shows an example request packet.

2.2. Reply Packet

The reply consists of a version ID, followed by a DER encoded ASN.1 message containing an error code, and an optional authentication hash, optional certificate, and optional error text. The service SHOULD return replies of the same version as the request where possible.

KX509Response ::= SEQUENCE {
        error-code[0]  INTEGER DEFAULT 0,
        hash[1]        OCTET STRING OPTIONAL,
        certificate[2] OCTET STRING OPTIONAL,
        e-text[3]      VisibleString OPTIONAL
}

Although the format of the reply contains independently optional objects, the server MUST only generate replies with one of the following allowed combinations.

The first case is returned when the server successfully generates a certificate for the user. The certificate is a DER encoded Certificate as defined in [RFC5280] Section A, page 116. Its encoding as an OCTET STRING starts with the 0x30 byte sequence that is at the beginning of a DER encoded Certificate.

The second case is returned when the server successfully authenticates the user and their key, but is unable for some other reason to generate a certificate.

The third case MAY be returned if the server is unable to successfully authenticate the user and intends to return some unauthenticated information to the client.

The hash on a response is computed using SHA-1 HMAC as for the request.

The data that is hashed is the concatenation of the following things:

• 4-byte version ID at the beginning of the packet.
• DER representation of the error-code exclusive of the tag and length, if it is present.
• OCTET STRING of the certificate, if it is present.
• VisibleString representation of the e-text exclusive of the tag and length, if it is present.

In other words, the hash is computed on the data in the fields which are present exclusive of the overall ASN.1 wrapping.

The e-text MAY be translated into other character sets for display purposes, but the hash is computed on the e-text in its VisibleString representation. If the e-text contains NUL characters, the client MAY ignore any part of the error message after the first NUL character for display purposes.

As implied by the above table, if the reply does not contain a certificate it MUST contain an error message and a non-zero error code. Conversely, if a certificate is returned then the error code MUST be zero. The server SHOULD use the DEFAULT encoding for a zero error-code value by omitting any explicit error-code from the reply.

The defined error codes are as follows:

If a client error is returned, the client SHOULD NOT retry the request unless some remedial action is first taken, although if error-code 3 is returned, the client MAY retry with other servers before giving up.

If a server error is returned, it is RECOMMENDED that the client retry the request with a different server if one is known.

Since all KCAs serving a Kerberos realm are intended to be equivalent, in accordance with [RFC5280] Section 4.1.2.2, the certificates returned from different KCAs serving the same Kerberos realm MUST NOT contain duplicate serial numbers.

This protocol and document do not address certificate verification or path construction. There are no provisions for returning any additional certificates which might be needed. Any application using a returned certificate must be configured independently to address these issues. An incompatible upgrade to this protocol will provide options to address this issue.

The returned certificate MUST identify the Kerberos client principal from the ap-req in the original KX509Request in the subject of the cert, or in a subjectAltName extension. The identification MUST be unique within the organization's deployed infrastructure. It is RECOMMENDED that a subjectAltName extension be included of type id-pkinit-san as described in [RFC4556] Section 3.2.2. Note that the id-pkinit-san is simply a standard representation of a Kerberos principal, and has no other implications with respect to PKINIT.

Other extensions MAY be added according to local policy.

Appendix C shows an example reply packet.

3. Protocol Operation

Absent errors, the protocol consists of a single request, sent via UDP, and a single reply, also sent via UDP.

There is no special provision for requests or replies which exceed the allowable size of a UDP packet. Also some implementations have imposed hard size limits which are smaller than a typical UDP MTU, and will limit the use of extensions and the supportable key size. Even without hard limits, if the request or reply exceeds the MTU size of a UDP packet for the infrastructure in use, then the reliability of the exchange will decrease significantly.

For “normal” Kerberos ap-req structures, and “normal” X.509 certificates, this is unlikely unless the Kerberos service ticket contains large amounts of authorization data. For this reason, it is RECOMMENDED that service tickets for the KCA be issued without authorization data. If the KCA performs authorization, it should do so by other means.

Before constructing the request, the client must know the canonical name(s) and port(s) of the server(s) to contact. It MAY determine them by looking up the service's SRV record as described in[RFC2782]. The entry to be used is _kca._udp.realm, where realm is the Kerberos realm, used as part of the DNS name.

The client has to acquire a service ticket in order to construct the ap-req for the service. Conventionally, the Kerberos service principal name to use for this service has a first component of "kca_service". Absent local configuration or other external knowledge of the correct principal to use, the second and final component is conventionally the canonical name of the KCA server being contacted, and the realm of the principal is determined following normal Kerberos domain to realm mapping conventions, as discussed in [RFC4120] Section 1.3.

When the server receives a request, it MUST verify the following properties of the request before issuing a certificate:

• The AP-REQ can be decoded and is not expired.
• If the request uses cross-realm authentication, then it satisfies the requirements of local policy and [RFC4120] Sections 1.2 and 2.7.
• The request's hash is valid.

The server SHOULD make other sanity checks, such as a minimum public key length, to the extent feasible.

The server MAY decline to respond to an erroneous request. If it does not receive a response a client MAY retry its request, but the client SHOULD wait at least one second before doing so.

The client MUST verify any hash in the reply, and MUST NOT use any certificate in a reply whose hash does not verify. The client MAY display the e-text if the hash is absent or does not verify, but SHOULD indicate the message is not authenticated.

4. Acknowledgements

The original version of kx509 was implemented using Kerberos 4 at the University of Michigan, and was nicely documented in [KX509]. Many thanks to them for their original work, as well as the subsequent updates.

While developing this document important corrections and comments were provided by Jeffrey Altman, and Love Hornquist Astrand. The following people also provided many helpful comments and corrections: Doug Engert, Jeffrey Hutzelman, Sam Hartman, Timothy J. Miller, Chaskiel Grundman, and Jim Schaad. Alan Sill provided the references to the International Grid Trust Federation and its acceptable credential services. Example network traffic was provided by Doug Engert, Marcus Watts, Matt Crawford, and Chaskiel Grundman from their deployments, and was extremely useful for verifying the reality of this specification.

5. IANA Considerations

This service is conventionally run on UDP port 9878. IANA is requested to register that port in the Service Name and Transport Port Number Registry as follows:

RFC Editor Note: Change RFC XXXX to the assigned RFC number on publication and remove this note.

  Service Name:       kca-service
  Transport Protocol: UDP
  Assignee:           IESG <iesg@ietf.org>
  Contact:            IETF Chair <chair@ietf.org>
  Description:        The KX509 Kerberized Certificate Issuance
                      Protocol in Use in 2012
  Reference:          RFC XXXX
  Port Number:        9878
  Assignment Notes:   Historically, this service has been referred to
                      as "kca_service", but this service name does
                      not meet the registry requirements.
        

The GSSAPI/Kerberos/SASL service name currently in use for this protocol is "kca_service". This does not meet the naming requirements for IANA's GSSAPI/Kerberos/SASL service name registry, so no registration is requested. The conflict between the conventional service name and the registry rules is expected to be addressed in a future version of this protocol. Appropriate registrations will be requested at that time.

6. Security Considerations

The only encrypted information in the protocol is that used by Kerberos itself. The considerations for any Kerberized service apply here.

The public key in the request is sent in the clear, and without any guarantees that the requester actually possesses the corresponding private key. Therefore the only appropriate uses of the returned certificate are those where the identity of the requester is unimportant, or the subsequent use independently guarantees that the user possesses the private key. This issue is expected to be addressed in a future version of this protocol.

For example, if the kx509-issued certificate is used for a digital signature in a way which does not independently demonstrate proof-of-possession of the private key, then an eavesdropper could request their own valid certificate via kx509 and claim to have originated material signed by the legitimate, original requester. [RFC4211], Appendix C contains a more detailed discussion of why proof-of-possession is important.

An example which should be safe is initial client authentication with TLS [RFC5246] connections. If a client certificate is used then a Certificate Verify message (Section 7.4.8 of that RFC) is added to the handshake exchange. It includes a signature of the handshake messages to that point. Those messages depend on data known only to the client and server ends of the specific connection, so computing the signature proves possession of the private key. This application was the original intended use case for kx509.

Some information, such as the public key and certificate, is transmitted in the clear but (as the name implies) were generally intended to be publicly available. However their visibility could still raise privacy concerns. The hash is used to protect their integrity.

The policies for issuing Kerberos tickets and X.509 certificates are usually expressed very differently. An implementation of this protocol should not provide a mechanism for bypassing ticket or certificate policies.

In particular, if the issued certificate can be used with PKINIT, this authentication loop SHOULD NOT bypass policy limits for either X.509 certificates or Kerberos tickets.

X.509 certificates are usually issued with considerably longer validity times than Kerberos tickets. Care should be taken that the issued certificate is not valid for longer than the intended policy should allow. Note that [RFC4556] Section 3.2.3.1 REQUIRES that the lifetime of an issued ticket not exceed the lifetime of the predecessor certificate. By analogy it is RECOMMENDED that the lifetime of an issued certificate not exceed the lifetime of the predecessor Kerberos ticket unless the implications with respect to local policy and supporting infrastructure are clearly understood and allow it.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Certificate Cacheing and Deployment Considerations

As noted in the Security Considerations section, the functional lifetime of the acquired X.509 certificate should usually match the lifetime of its predecessor Kerberos ticket. Therefore, it is likely that X.509 certificates issued with this protocol should be deleted when the supporting Kerberos tickets are deleted. That makes the Kerberos ticket cache a reasonable location to store the certificate (and its private key).

On the other hand applications, such as web browsers, probably expect certificates in different stores.

A widely used solution to this dichotomy is to implement a PKCS11 library which supports the KX509-acquired credentials. The credentials remain stored in the Kerberos credentials cache, but full PKI functionality is still available via a standard interface for PKI credentials.

Appendix B. Historic Extensions

This appendix documents extensions to the kx509 protocol which are either no longer in use, or are expected to be dropped.

A subjectAltName othername extension of type kcaAuthRealm (OID value 1.3.6.1.4.1.250.42.1) is frequently used to include the client's realm as an ASN.1 octet string.

The Microsoft-defined userPrincipalName has frequently been used for the same purpose as the id-pkinit-san.

The historic implementations of this protocol included provisions for DSA keys in place of RSA. DSA does not appear to be in use. A future version of this protocol will use a standard certificate request structure which will provide algorithm agility.

The historic implementations of this protocol allowed an optional client-version field (at the end of the request) of type VisibleString. If present, the KCA copied it into the issued certificate as an extension with the OID 1.3.6.1.4.1.250.42.2. This feature does not appear to be in use.

Appendix C. Example Exchange

The request and reply are from the same exchange. The Ethernet, IP, and UDP headers, and the 4-byte version string at the beginning of the request and reply packets are all omitted. Only the ASN.1-encoded portions are shown.

   0:d=0  hl=4 l= 678 cons: SEQUENCE
   4:d=1  hl=4 l= 509 prim:  OCTET STRING
                        [HEX DUMP]:6E8201F9308201F5A003... (ap-req)
 517:d=1  hl=2 l=  20 prim:  OCTET STRING
                        [HEX DUMP]:ECFF1C922300D0E9DD02... (pk-hash)
 539:d=1  hl=3 l= 140 prim:  OCTET STRING
                        [HEX DUMP]:30818902818100B70F46... (pk-key)

  0:d=0  hl=4 l= 870 cons: SEQUENCE
  4:d=1  hl=2 l=  22 cons:  cont [ 1 ]
  6:d=2  hl=2 l=  20 prim:   OCTET STRING
                     [HEX DUMP]:F3A844834C26D843B6FD... (hash)
 28:d=1  hl=4 l= 842 cons:  cont [ 2 ]
 32:d=2  hl=4 l= 838 prim:   OCTET STRING
                     [HEX DUMP]:308203423082022AA003... (certificate)

Appendix D. Change History

RFC Editor Note: Delete this appendix before final publication.

Changes from Draft -04 to Draft -05:

1. The title, a word in the abstract, and the reference to the IETF Kerberos working group were changed to make it clearer that this is not a standards-track document.
2. Added Appendix C, to clarify the ASN.1 encoding, and specify the byte string that begins the ASN.1 OCTET STRING encoding of certificates.
3. Removed the request for IANA registration of the GSSAPI/Kerberos/SASL name, since the service name registry does not allow the form of name actually in use. Add an IANA registration request for the conventional port number.

Changes from Draft -03 to Draft -04:

1. The list of possible issues was deleted. Either appropriate comments have been added to the text, or the issue is mentioned as something to be addressed in an incompatible future version of this protocol.
2. Clarified the hash computations in sections 2.1 and 2.2.
3. Clarified the procedure for determining the Kerberos principal of the KCA in section 3.
4. Clarified the discussion of the "proof-of-possession" issue in the Security Considerations with appropriate references.

Changes from Draft -02 to Draft -03:

1. The abstract was expanded.
2. Additional information was provided on traditional UDP size restrictions and their effect on reliability and key sizes in section 3.
3. The updates to the security considerations for digital signature usage were incomplete, and have been rewritten.
4. Information on an optional client version feature (which does not appear to be actually in use) was added to the request ASN.1, and Appendix B, and the title of the appendix changed.
5. As before some minor changes to wording were made for clarity, but are not believed to have changed the meaning.

Changes from Draft -01 to Draft -02:

1. The retry behavior was made slightly less specific.
2. The traditionally used SAN extensions were moved to a new appendix, leaving only the id-pkinit-san as the RECOMMENDED SAN.
3. The absolute prohibition against digital signatures in the Security Considerations section was relaxed since there are legitimate situations where a signature based on the KX509 certificate is still useful. (E.g. integrity protection where the actual signing identity is not important.)
4. Reference to TAGPMA in the abstract was replaced with a reference to its parent, the International Grid Trust Federation, and more detailed informative references were expanded in the Introduction.
5. Assorted other wording changes were made for clarity, but are not believed to have changed the meaning.

Authors' Addresses