draft-ietf-kitten-pkinit-freshness-01.txt   draft-ietf-kitten-pkinit-freshness-02.txt 
Kitten Working Group M. Short, Ed. Kitten Working Group M. Short, Ed.
Internet-Draft S. Moore Internet-Draft S. Moore
Intended status: Standards Track P. Miller Intended status: Standards Track P. Miller
Expires: September 7, 2015 Microsoft Corporation Expires: June 13, 2016 Microsoft Corporation
March 6, 2015 December 11, 2015
Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
Freshness Extension Freshness Extension
draft-ietf-kitten-pkinit-freshness-01 draft-ietf-kitten-pkinit-freshness-02
Abstract Abstract
This document describes how to further extend the Public Key This document describes how to further extend the Public Key
Cryptography for Initial Authentication in Kerberos (PKINIT) Cryptography for Initial Authentication in Kerberos (PKINIT)
extension [RFC4556] to exchange an opaque data blob which a KDC can extension [RFC4556] to exchange an opaque data blob that a KDC can
validate to ensure that the client is currently in possession of the validate to ensure that the client is currently in possession of the
private key during a PKInit AS exchange. private key during a PKINIT AS exchange.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 7, 2015. This Internet-Draft will expire on June 13, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 21 skipping to change at page 2, line 21
authentication . . . . . . . . . . . . . . . . . . . . . 3 authentication . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Message Exchanges . . . . . . . . . . . . . . . . . . . . . . 3 2. Message Exchanges . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Generation of KRB_AS_REQ Message . . . . . . . . . . . . 4 2.1. Generation of KRB_AS_REQ Message . . . . . . . . . . . . 4
2.2. Generation of KRB_ERROR Message . . . . . . . . . . . . . 4 2.2. Generation of KRB_ERROR Message . . . . . . . . . . . . . 4
2.3. Generation of KRB_AS_REQ Message . . . . . . . . . . . . 4 2.3. Generation of KRB_AS_REQ Message . . . . . . . . . . . . 4
2.4. Receipt of KRB_AS_REQ Message . . . . . . . . . . . . . . 4 2.4. Receipt of KRB_AS_REQ Message . . . . . . . . . . . . . . 4
2.5. Receipt of second KRB_ERROR Message . . . . . . . . . . . 5 2.5. Receipt of second KRB_ERROR Message . . . . . . . . . . . 5
3. PreAuthentication Data Types . . . . . . . . . . . . . . . . 5 3. PreAuthentication Data Types . . . . . . . . . . . . . . . . 5
4. Extended PKAuthenticator . . . . . . . . . . . . . . . . . . 5 4. Extended PKAuthenticator . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Interoperability Considerations . . . . . . . . . . . . . . . 6 8. Interoperability Considerations . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 9. Normative References . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
The Kerberos PKINIT extension [RFC4556] defines two schemes for using The Kerberos PKINIT extension [RFC4556] defines two schemes for using
asymmetric cryptography in a Kerberos preauthenticator. One uses asymmetric cryptography in a Kerberos preauthenticator. One uses
Diffie-Hellman key exchange and the other depends on public key Diffie-Hellman key exchange and the other depends on public key
encryption. The public key encryption scheme is less commonly used encryption. The public key encryption scheme is less commonly used
for two reasons: for two reasons:
o Elliptic Curve Cryptography (ECC) Support for PKINIT [RFC5349] o Elliptic Curve Cryptography (ECC) Support for PKINIT [RFC5349]
only specified Elliptic Curve Diffie-Hellman (ECDH) key agreement only specified Elliptic Curve Diffie-Hellman (ECDH) key agreement,
so it cannot be used for public key encryption. so it cannot be used for public key encryption.
o Public key encryption requires certificates with an encryption key o Public key encryption requires certificates with an encryption
which is not deployed on many existing smart cards. key, that is not deployed on many existing smart cards.
In the Diffie-Hellman exchange, the client uses its private key only In the Diffie-Hellman exchange, the client uses its private key only
to sign the AuthPack structure specified in Section 3.2.1 of to sign the AuthPack structure (specified in Section 3.2.1 of
[RFC4556] which is performed before any traffic is sent to the KDC. [RFC4556]), that is performed before any traffic is sent to the KDC.
Thus a client can generate requests with future times in the Thus a client can generate requests with future times in the
PKAuthenticator, and then send those requests at those future times. PKAuthenticator, and then send those requests at those future times.
Unless the time is outside the validity period of the client's Unless the time is outside the validity period of the client's
certificate, the KDC will validate the PKAuthenticator and return a certificate, the KDC will validate the PKAuthenticator and return a
TGT the client can use without possessing the private key. TGT the client can use without possessing the private key.
As a result, a client performing PKINIT with the Diffie-Hellman key As a result, a client performing PKINIT with the Diffie-Hellman key
exchange does not prove current possession of the private key being exchange does not prove current possession of the private key being
used for authentication. It proves only prior use of that key. used for authentication. It proves only prior use of that key.
Ensuring that the client has current possession of the private key Ensuring that the client has current possession of the private key
requires that the signed PKAuthenticator data include information requires that the signed PKAuthenticator data include information
that the client could not have predicted. that the client could not have predicted.
1.1. Kerberos message flow using KRB_AS_REQ without pre-authentication 1.1. Kerberos message flow using KRB_AS_REQ without pre-authentication
Today some password-based AS exchanges [RFC4120] depend on the client Today, password-based AS exchanges [RFC4120] often begin with the
sending a KRB_AS_REQ without pre-authentication to trigger the KDC to client sending a KRB_AS_REQ without pre-authentication. When the
provide the Kerberos client with information needed to complete an AS principal requires pre-authentication, the KDC responds with a
exchange such as the supported encryption types and salt values (see KRB_ERROR containing information needed to complete an AS exchange,
the message flow below): such as the supported encryption types and salt values. This message
flow is illustrated below:
KDC Client KDC Client
<---- AS-REQ without pre-authentication <---- AS-REQ without pre-authentication
KRB-ERROR ----> KRB-ERROR ---->
<---- AS-REQ <---- AS-REQ
AS-REP ----> AS-REP ---->
<---- TGS-REQ <---- TGS-REQ
TGS-REP ----> TGS-REP ---->
Figure 1 Figure 1
We can use this mechanism in PKInit for KDCs to provide data which We can use a similar message flow with PKINIT, allowing the KDC to
the client returns as part of the KRB_AS_REQ to ensure that the provide a token for the client to include in its KRB_AS_REQ to ensure
PA_PK_AS_REQ [RFC4556] was not pregenerated. that the PA_PK_AS_REQ [RFC4556] was not pregenerated.
1.2. Requirements Language 1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Message Exchanges 2. Message Exchanges
The following summarizes the message flow with extensions to The following summarizes the message flow with extensions to
[RFC4120] and [RFC4556] required to support a KDC provided freshness [RFC4120] and [RFC4556] required to support a KDC-provided freshness
token during the initial request for a ticket: token during the initial request for a ticket:
1. The client generates a KRB_AS_REQ as specified in Section 2.9.3 1. The client generates a KRB_AS_REQ as specified in Section 2.9.3
[RFC4120] without an authenticator which includes the freshness [RFC4120] that contains no PA_PK_AS_REQ and includes a freshness
token request to the KDC. token request.
2. The KDC generates a KRB_ERROR as specified in Section 3.1.3 of 2. The KDC generates a KRB_ERROR as specified in Section 3.1.3 of
[RFC4120] providing a freshness token. [RFC4120] providing a freshness token.
3. The client receives the error as specified in Section 3.1.4 of 3. The client receives the error as specified in Section 3.1.4 of
[RFC4120] and includes the freshness token as part of the [RFC4120], extracts the freshness token, and includes it as part
KRB_AS_REQ as specified in [RFC4120] and [RFC4556]. of the KRB_AS_REQ as specified in [RFC4120] and [RFC4556].
4. The KDC receives and validates the KRB_AS_REQ as specified in 4. The KDC receives and validates the KRB_AS_REQ as specified in
Section 3.2.2 [RFC4556] then additionally validates the freshness Section 3.2.2 [RFC4556], then additionally validates the
token. freshness token.
5. The KDC and client continue as specified in [RFC4120] and 5. The KDC and client continue as specified in [RFC4120] and
[RFC4556]. [RFC4556].
2.1. Generation of KRB_AS_REQ Message 2.1. Generation of KRB_AS_REQ Message
The client indicates support of freshness tokens by adding a The client indicates support of freshness tokens by adding a padata
PA_AS_FRESHNESS padata type with an empty octet string as the padata- element with padata-type PA_AS_FRESHNESS and padata-value of an empty
value. octet string.
2.2. Generation of KRB_ERROR Message 2.2. Generation of KRB_ERROR Message
The KDC will respond by adding a PA_AS_FRESHNESS padata type with the The KDC will respond by adding a padata element with padata-type
freshness token as the padata-value to the METHOD-DATA object. PA_AS_FRESHNESS and padata-value of the freshness token to the
METHOD-DATA object.
2.3. Generation of KRB_AS_REQ Message 2.3. Generation of KRB_AS_REQ Message
After the client receives the KRB-ERROR message containing a After the client receives the KRB-ERROR message containing a
freshness token, it extracts the PA_AS_FRESHNESS padata-value field 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 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 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 opaque blob in the freshnessToken field when the client generates the
PKAuthenticator for the PA_PK_AS_REQ message. This ensures that 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 freshness token value will be included in the signed data portion of
the KRB_AS_REQ value. the KRB_AS_REQ value.
2.4. Receipt of KRB_AS_REQ Message 2.4. Receipt of KRB_AS_REQ Message
After validating the PA_PK_AS_REQ message normally, the KDC will After validating the PA_PK_AS_REQ message normally, the KDC will
validate the freshnessToken value in the PKAuthenticator in an validate the freshnessToken value in the PKAuthenticator in an
implementation specific way. If the freshness token is not valid, implementation-specific way. If the freshness token is not valid,
the KDC MUST return KDC_ERR_PREAUTH_FAILED with PA_AS_FRESHNESS. the KDC MUST return KDC_ERR_PREAUTH_EXPIRED [RFC6113] with
Since the freshness tokens are validated by KDCs in the same realm, PA_AS_FRESHNESS. Since the freshness tokens are validated by KDCs in
standardizing the contents of the freshness token is not a concern the same realm, standardizing the contents of the freshness token is
for interoperability. not a concern for interoperability.
2.5. Receipt of second KRB_ERROR Message 2.5. Receipt of second KRB_ERROR Message
Clients SHOULD retry in the cases when receiving a If a client receives a KDC_ERR_PREAUTH_EXPIRED KRB_ERROR message that
KDC_ERR_PREAUTH_FAILED KRB_ERROR message which includes a freshness includes a freshness token, it MUST retry using the new freshness
token where there is a possibility that there was too much delay token.
between the client receiving the freshness token and sending the
PA_PK_AS_REQ message.
3. PreAuthentication Data Types 3. PreAuthentication Data Types
The following are the new PreAuthentication data types: The following are the new PreAuthentication data types:
+----------------------+-------------------+ +----------------------+-------------------+
| Padata and Data Type | Padata-type Value | | Padata and Data Type | Padata-type Value |
+----------------------+-------------------+ +----------------------+-------------------+
| PA_AS_FRESHNESS | TBD | | PA_AS_FRESHNESS | TBD |
+----------------------+-------------------+ +----------------------+-------------------+
skipping to change at page 6, line 7 skipping to change at page 5, line 47
-- KDC-REQ-BODY. -- KDC-REQ-BODY.
..., ...,
freshnessToken [4] OCTET STRING OPTIONAL, freshnessToken [4] OCTET STRING OPTIONAL,
-- PA_AS_FRESHNESS padata value as recieved from the -- PA_AS_FRESHNESS padata value as recieved from the
-- KDC. MUST be present if sent by KDC -- KDC. MUST be present if sent by KDC
... ...
} }
5. Acknowledgements 5. Acknowledgements
Henry B. Hotz, Nico Williams, Sam Hartman, Tom Yu, Martin Rex, and Douglas E. Engert, Sam Hartman, Henry B. Hotz, Nikos
Douglas E. Engert were key contributors to the discover of the Mavrogiannopoulos, Martin Rex, Nico Williams, and Tom Yu were key
freshness issue in PKINIT. contributors to the discovery of the freshness issue in PKINIT.
Greg Hudson, Nathan Ide, Benjamin Kaduk, Magnus Nystrom, Nico Sam Hartman,Greg Hudson, Jeffrey Hutzelman, Nathan Ide, Benjamin
Williams and Tom Yu reviewed the document and provided suggestions Kaduk, Bryce Nordgren, Magnus Nystrom, Nico Williams and Tom Yu
for improvements. reviewed the document and provided suggestions for improvements.
6. IANA Considerations 6. IANA Considerations
IANA is requested to assign numbers for PA_AS_FRESHNESS listed in the IANA is requested to assign numbers for PA_AS_FRESHNESS listed in the
Kerberos Parameters registry Pre-authentication and Typed Data as Kerberos Parameters registry Pre-authentication and Typed Data as
follows: follows:
+------+-----------------+------------+ +------+-----------------+------------+
| Type | Value | Reference | | Type | Value | Reference |
+------+-----------------+------------+ +------+-----------------+------------+
| TBD | PA_AS_FRESHNESS | [This RFC] | | TBD | PA_AS_FRESHNESS | [This RFC] |
+------+-----------------+------------+ +------+-----------------+------------+
7. Security Considerations 7. Security Considerations
The freshness token SHOULD include signing, encrypting or sealing The freshness token SHOULD include signing, encrypting or sealing
data from the KDC to determine authenticity and prevent tampering. 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 Freshness tokens serve to guarantee that the client had the key when
required to provide integrity protection, a different KDC would not constructing the AS-REQ. They are not required to be single use
be able to validate freshness tokens without some kind of shared tokens or bound to specific AS exchanges. Part of the reason the
database. token is opaque is to allow KDC implementers the freedom to add
additional functionality as long as the "freshness" guarantee
remains.
8. Interoperability Considerations 8. Interoperability Considerations
Since the client treats the KDC provided data blob as opaque, Since the client treats the KDC provided data blob as opaque,
changing the contents will not impact existing clients. Thus changing the contents will not impact existing clients. Thus
extensions to the freshness token do not impact client extensions to the freshness token do not impact client
interoperability. interoperability.
9. References 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.
9.1. Normative References Since upgrading clients takes time, implementers may consider
allowing both freshness-token based exchanges as well as "legacy"
exchanges without use of freshness tokens. However, until freshness
tokens are required by the realm, existing risks of pre-generated
PKAuthenticators will remain.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 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 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120, Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005. DOI 10.17487/RFC4120, July 2005,
<http://www.rfc-editor.org/info/rfc4120>.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. 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 [RFC5349] Zhu, L., Jaganathan, K., and K. Lauter, "Elliptic Curve
Cryptography (ECC) Support for Public Key Cryptography for Cryptography (ECC) Support for Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)", RFC 5349, Initial Authentication in Kerberos (PKINIT)", RFC 5349,
September 2008. DOI 10.17487/RFC5349, September 2008,
<http://www.rfc-editor.org/info/rfc5349>.
9.2. Informative References
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for [RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113, April 2011. Kerberos Pre-Authentication", RFC 6113,
DOI 10.17487/RFC6113, April 2011,
<http://www.rfc-editor.org/info/rfc6113>.
Authors' Addresses Authors' Addresses
Michiko Short (editor) Michiko Short (editor)
Microsoft Corporation Microsoft Corporation
USA USA
Email: michikos@microsoft.com Email: michikos@microsoft.com
Seth Moore Seth Moore
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