Kitten Working Group M. Short, Ed.
Internet-Draft S. Moore
Intended status: Standards Track P. Miller
Expires: September 7, 2015 Microsoft Corporation
March 6, 2015
Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
Freshness Extension
draft-ietf-kitten-pkinit-freshness-01
Abstract
This document describes how to further extend the Public Key
Cryptography for Initial Authentication in Kerberos (PKINIT)
extension [RFC4556] to exchange an opaque data blob which a KDC can
validate to ensure that the client is currently in possession of the
private key during a PKInit AS exchange.
Status of This Memo
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the Trust Legal Provisions and are provided without warranty as
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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 . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . 4
2.5. Receipt of second KRB_ERROR Message . . . . . . . . . . . 5
3. PreAuthentication Data Types . . . . . . . . . . . . . . . . 5
4. Extended PKAuthenticator . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Interoperability Considerations . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Kerberos PKINIT extension [RFC4556] defines two schemes for using
asymmetric cryptography in a Kerberos preauthenticator. 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
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certificate, the KDC will validate the PKAuthenticator and return a
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 some password-based AS exchanges [RFC4120] depend on the client
sending a KRB_AS_REQ without pre-authentication to trigger the KDC to
provide the Kerberos client with information needed to complete an AS
exchange such as the supported encryption types and salt values (see
the message flow below):
KDC Client
<---- AS-REQ without pre-authentication
KRB-ERROR ---->
<---- AS-REQ
AS-REP ---->
<---- TGS-REQ
TGS-REP ---->
Figure 1
We can use this mechanism in PKInit for KDCs to provide data which
the client returns as part of the KRB_AS_REQ to ensure that the
PA_PK_AS_REQ [RFC4556] was not pregenerated.
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 RFC 2119 [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:
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1. The client generates a KRB_AS_REQ as specified in Section 2.9.3
[RFC4120] without an authenticator which includes the freshness
token request to the KDC.
2. The KDC generates a KRB_ERROR as specified in Section 3.1.3 of
[RFC4120] providing a freshness token.
3. The client receives the error as specified in Section 3.1.4 of
[RFC4120] and includes the freshness token 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 [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
PA_AS_FRESHNESS padata type with an empty octet string as the padata-
value.
2.2. Generation of KRB_ERROR Message
The KDC will respond by adding a PA_AS_FRESHNESS padata type with the
freshness token as the padata-value 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 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
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 KDC_ERR_PREAUTH_FAILED with PA_AS_FRESHNESS.
Since the freshness tokens are validated by KDCs in the same realm,
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standardizing the contents of the freshness token is not a concern
for interoperability.
2.5. Receipt of second KRB_ERROR Message
Clients SHOULD retry in the cases when receiving a
KDC_ERR_PREAUTH_FAILED KRB_ERROR message which includes a freshness
token where there is a possibility that there was too much delay
between the client receiving the freshness token and sending the
PA_PK_AS_REQ message.
3. PreAuthentication Data Types
The following are the new PreAuthentication data types:
+----------------------+-------------------+
| Padata and Data Type | Padata-type Value |
+----------------------+-------------------+
| PA_AS_FRESHNESS | TBD |
+----------------------+-------------------+
4. Extended PKAuthenticator
The PKAuthenticator structure specified in Section 3.2.1 [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 recieved from the
-- KDC. MUST be present if sent by KDC
...
}
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5. Acknowledgements
Henry B. Hotz, Nico Williams, Sam Hartman, Tom Yu, Martin Rex, and
Douglas E. Engert were key contributors to the discover of the
freshness issue in PKINIT.
Greg Hudson, Nathan Ide, Benjamin Kaduk, Magnus Nystrom, Nico
Williams and Tom Yu reviewed the document and provided suggestions
for improvements.
6. IANA Considerations
IANA is requested to assign numbers for PA_AS_FRESHNESS listed in the
Kerberos Parameters registry Pre-authentication and Typed Data as
follows:
+------+-----------------+------------+
| Type | Value | Reference |
+------+-----------------+------------+
| TBD | PA_AS_FRESHNESS | [This RFC] |
+------+-----------------+------------+
7. Security Considerations
The freshness token SHOULD include signing, encrypting or sealing
data from the KDC to determine authenticity and prevent tampering.
Kerberos error messages are not integrity protected unless
authenticated using Kerberos FAST [RFC6113]. Even if FAST is
required to provide integrity protection, a different KDC would not
be able to validate freshness tokens without some kind of shared
database.
8. 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.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[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,
September 2008.
9.2. Informative References
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113, April 2011.
Authors' Addresses
Michiko Short (editor)
Microsoft Corporation
USA
Email: michikos@microsoft.com
Seth Moore
Microsoft Corporation
USA
Email: sethmo@microsoft.com
Paul Miller
Microsoft Corporation
USA
Email: paumil@microsoft.com
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