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rfc:rfc8070

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

Short, et al. Standards Track [Page 1] RFC 8070 PKINIT Freshness February 2017

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:
 o  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.
 o  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.

Short, et al. Standards Track [Page 2] RFC 8070 PKINIT Freshness February 2017

 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].

Short, et al. Standards Track [Page 3] RFC 8070 PKINIT Freshness February 2017

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.
 2.  The KDC generates a KRB_ERROR, as specified in Section 3.1.4 of
     [RFC4120], providing a freshness token.
 3.  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].
 4.  The KDC receives and validates the KRB_AS_REQ, as specified in
     Section 3.2.2 of [RFC4556], then additionally validates the
     freshness token.
 5.  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.

Short, et al. Standards Track [Page 4] RFC 8070 PKINIT Freshness February 2017

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        |
             +----------------------+-------------------+

Short, et al. Standards Track [Page 5] RFC 8070 PKINIT Freshness February 2017

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] |
                +------+-----------------+-----------+

Short, et al. Standards Track [Page 6] RFC 8070 PKINIT Freshness February 2017

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.

Short, et al. Standards Track [Page 7] RFC 8070 PKINIT Freshness February 2017

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.

Short, et al. Standards Track [Page 8] RFC 8070 PKINIT Freshness February 2017

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

Short, et al. Standards Track [Page 9]

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