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Internet Engineering Task Force (IETF)
Request for Comments: 8070
Category: Standards Track
ISSN: 2070-1721
M. Short, Ed.
S. Moore
P. Miller
Microsoft Corporation
February 2017

Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)

Freshness Extension

Abstract

This document describes how to further extend the Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) extension (defined in RFC 4556) to exchange an opaque data blob that a Key Distribution Center (KDC) can validate to ensure that the client is currently in possession of the private key during a PKINIT Authentication Service (AS) exchange.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8070.

Copyright Notice

Copyright © 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Kerberos Message Flow Using KRB_AS_REQ without
           Pre-authentication .........................................3
      1.2. Requirements Language ......................................3
   2. Message Exchanges ...............................................4
      2.1. Generation of KRB_AS_REQ Message ...........................4
      2.2. Generation of KRB_ERROR Message ............................4
      2.3. Generation of KRB_AS_REQ Message ...........................4
      2.4. Receipt of KRB_AS_REQ Message ..............................5
      2.5. Receipt of Second KRB_ERROR Message ........................5
   3. PreAuthentication Data Types ....................................5
   4. Extended PKAuthenticator ........................................6
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................7
   7. Interoperability Considerations .................................7
   8. Normative References ............................................8
   Acknowledgements ...................................................8
   Authors' Addresses .................................................9

1. Introduction

The Kerberos PKINIT extension [RFC4556] defines two schemes for using asymmetric cryptography in a Kerberos pre-authenticator. One uses Diffie-Hellman key exchange and the other depends on public key encryption. The public key encryption scheme is less commonly used for two reasons:

  • Elliptic Curve Cryptography (ECC) Support for PKINIT [RFC5349] only specified Elliptic Curve Diffie-Hellman (ECDH) key agreement, so it cannot be used for public key encryption.
  • Public key encryption requires certificates with an encryption key, which is not deployed on many existing smart cards.

In the Diffie-Hellman exchange, the client uses its private key only to sign the AuthPack structure (specified in Section 3.2.1 of [RFC4556]), which is performed before any traffic is sent to the KDC. Thus, a client can generate requests with future times in the PKAuthenticator, and then send those requests at those future times. Unless the time is outside the validity period of the client's certificate, the KDC will validate the PKAuthenticator and return a Ticket-Granting Ticket (TGT) the client can use without possessing the private key.

As a result, a client performing PKINIT with the Diffie-Hellman key exchange does not prove current possession of the private key being used for authentication. It proves only prior use of that key. Ensuring that the client has current possession of the private key requires that the signed PKAuthenticator data include information that the client could not have predicted.

1.1. Kerberos Message Flow Using KRB_AS_REQ without Pre-authentication

Today, password-based AS exchanges [RFC4120] often begin with the client sending a KRB_AS_REQ without pre-authentication. When the principal requires pre-authentication, the KDC responds with a KRB_ERROR containing information needed to complete an AS exchange, such as the supported encryption types and salt values. This message flow is illustrated below: 

   Client                                          KDC
   
   AS-REQ without pre-authentication     ---->
                                         <----     KRB-ERROR
   
   AS-REQ                                ---->
                                         <----     AS-REP
   
   TGS-REQ                               ---->
                                         <----     TGS-REP
   
                           Figure 1

We can use a similar message flow with PKINIT, allowing the KDC to provide a token for the client to include in its KRB_AS_REQ to ensure that the PA_PK_AS_REQ [RFC4556] was not pre-generated.

1.2. 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 [RFC2119].

2. Message Exchanges

The following summarizes the message flow with extensions to [RFC4120] and [RFC4556] required to support a KDC-provided freshness token during the initial request for a ticket:

  1. The client generates a KRB_AS_REQ, as specified in Section 2.9.3 of [RFC4120], that contains no PA_PK_AS_REQ and includes a freshness token request.
  1. The KDC generates a KRB_ERROR, as specified in Section 3.1.4 of [RFC4120], providing a freshness token.
  1. The client receives the error, as specified in Section 3.1.5 of [RFC4120], extracts the freshness token, and includes it as part of the KRB_AS_REQ as specified in [RFC4120] and [RFC4556].
  1. The KDC receives and validates the KRB_AS_REQ, as specified in Section 3.2.2 of [RFC4556], then additionally validates the freshness token.
  1. The KDC and client continue, as specified in [RFC4120] and [RFC4556].

2.1. Generation of KRB_AS_REQ Message

The client indicates support of freshness tokens by adding a padata element with padata-type PA_AS_FRESHNESS and padata-value of an empty octet string.

2.2. Generation of KRB_ERROR Message

The KDC will respond with a KRB_ERROR [RFC4120] message with the error-code KDC_ERR_PREAUTH_REQUIRED [RFC4120] adding a padata element with padata-type PA_AS_FRESHNESS and padata-value of the freshness token to the METHOD-DATA object.

2.3. Generation of KRB_AS_REQ Message

After the client receives the KRB-ERROR message containing a freshness token, it extracts the PA_AS_FRESHNESS padata-value field of the PA-DATA structure as an opaque data blob. The PA_AS_FRESHNESS padata-value field of the PA-DATA structure SHALL then be added as an opaque blob in the freshnessToken field when the client generates the PKAuthenticator specified in Section 4 for the PA_PK_AS_REQ message. This ensures that the freshness token value will be included in the signed data portion of the KRB_AS_REQ value.

2.4. Receipt of KRB_AS_REQ Message

If the realm requires freshness and the PA_PK_AS_REQ message does not contain the freshness token, the KDC MUST return a KRB_ERROR [RFC4120] message with the error-code KDC_ERR_PREAUTH_FAILED [RFC4120] with a padata element with padata-type PA_AS_FRESHNESS and padata-value of the freshness token to the METHOD-DATA object.

When the PA_PK_AS_REQ message contains a freshness token, after validating the PA_PK_AS_REQ message normally, the KDC will validate the freshnessToken value in the PKAuthenticator in an implementation- specific way. If the freshness token is not valid, the KDC MUST return a KRB_ERROR [RFC4120] message with the error-code KDC_ERR_PREAUTH_EXPIRED [RFC6113]. The e-data field of the error contains a METHOD-DATA object [RFC4120], which specifies a valid PA_AS_FRESHNESS padata-value. Since the freshness tokens are validated by KDCs in the same realm, standardizing the contents of the freshness token is not a concern for interoperability.

2.5. Receipt of Second KRB_ERROR Message

If a client receives a KDC_ERR_PREAUTH_EXPIRED KRB_ERROR message that includes a freshness token, it SHOULD retry using the new freshness token.

3. PreAuthentication Data Types

The following are the new PreAuthentication data types: 

               +----------------------+-------------------+
               | Padata and Data Type | Padata-type Value |
               +----------------------+-------------------+
               |   PA_AS_FRESHNESS    |        150        |
               +----------------------+-------------------+

4. Extended PKAuthenticator

The PKAuthenticator structure specified in Section 3.2.1 of [RFC4556] is extended to include a new freshnessToken as follows: 

   PKAuthenticator ::= SEQUENCE {
      cusec        [0] INTEGER (0..999999),
      ctime        [1] KerberosTime,
                -- cusec and ctime are used as in [RFC4120], for
                -- replay prevention.
      nonce        [2] INTEGER (0..4294967295),
                -- Chosen randomly;  this nonce does not need to
                -- match with the nonce in the KDC-REQ-BODY.
      paChecksum   [3] OCTET STRING OPTIONAL,
                -- MUST be present.
                -- Contains the SHA1 checksum, performed over
                -- KDC-REQ-BODY.
      ...,
      freshnessToken     [4] OCTET STRING OPTIONAL,
                -- PA_AS_FRESHNESS padata value as received from the
                -- KDC. MUST be present if sent by KDC
      ...
   }

5. IANA Considerations

IANA has assigned numbers for PA_AS_FRESHNESS listed in a subregistry of the "Kerberos Parameters" registry titled "Pre-authentication and Typed Data" as follows: 

                  +------+-----------------+-----------+
                  | Type |      Value      | Reference |
                  +------+-----------------+-----------+
                  | 150  | PA_AS_FRESHNESS | [RFC8070] |
                  +------+-----------------+-----------+

6. Security Considerations

The freshness token SHOULD include signing, encrypting, or sealing data from the KDC to determine authenticity and prevent tampering.

Freshness tokens serve to guarantee that the client had the key when constructing the AS-REQ. They are not required to be single use tokens or bound to specific AS exchanges. Part of the reason the token is opaque is to allow KDC implementers the freedom to add additional functionality as long as the tokens expire so that the "freshness" guarantee remains.

7. Interoperability Considerations

Since the client treats the KDC-provided data blob as opaque, changing the contents will not impact existing clients. Thus, extensions to the freshness token do not impact client interoperability.

Clients SHOULD NOT reuse freshness tokens across multiple exchanges. There is no guarantee that a KDC will allow a once-valid token to be used again. Thus, clients that do not retry with a new freshness token may not be compatible with KDCs, depending on how they choose to implement freshness validation.

Since upgrading clients takes time, implementers may consider allowing both freshness-token based exchanges and "legacy" exchanges without use of freshness tokens. However, until freshness tokens are required by the realm, the existing risks of pre-generated PKAuthenticators will remain.

8. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
   
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <http://www.rfc-editor.org/info/rfc4120>.
   
   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556,
              DOI 10.17487/RFC4556, June 2006,
              <http://www.rfc-editor.org/info/rfc4556>.
   
   [RFC5349]  Zhu, L., Jaganathan, K., and K. Lauter, "Elliptic Curve
              Cryptography (ECC) Support for Public Key Cryptography for
              Initial Authentication in Kerberos (PKINIT)", RFC 5349,
              DOI 10.17487/RFC5349, September 2008,
              <http://www.rfc-editor.org/info/rfc5349>.
   
   [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication", RFC 6113,
              DOI 10.17487/RFC6113, April 2011,
              <http://www.rfc-editor.org/info/rfc6113>.

Acknowledgements

Douglas E. Engert, Sam Hartman, Henry B. Hotz, Nikos Mavrogiannopoulos, Martin Rex, Nico Williams, and Tom Yu were key contributors to the discovery of the freshness issue in PKINIT.

Sam Hartman, Greg Hudson, Jeffrey Hutzelman, Nathan Ide, Benjamin Kaduk, Bryce Nordgren, Magnus Nystrom, Nico Williams, and Tom Yu reviewed the document and provided suggestions for improvements.

Authors' Addresses

Michiko Short (editor)
Microsoft Corporation
United States of America

Email: 

          michikos@microsoft.com

Seth Moore
Microsoft Corporation
United States of America

Email: 

          sethmo@microsoft.com

Paul Miller
Microsoft Corporation
United States of America

Email: 

          paumil@microsoft.com