Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) Freshness Extension
A KDC-issued freshness token that proves a Kerberos PKINIT client holds its private key right now, not just that it used it once.
Short, M., Ed., Moore, S., and P. Miller, "Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) Freshness Extension", RFC 8070, DOI 10.17487/RFC8070, February 2017, <https://www.rfc-editor.org/info/rfc8070>.
@misc{rfc8070,
series = {Request for Comments},
number = 8070,
howpublished = {RFC 8070},
publisher = {RFC Editor},
doi = {10.17487/RFC8070},
url = {https://www.rfc-editor.org/info/rfc8070},
author = {M. Short and S. Moore and P. Miller},
title = {{Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) Freshness Extension}},
pagetotal = 9,
year = 2017,
month = feb,
} RFC text
Contents
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.
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.
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.
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.
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
AI ELI5
On this page
§1Why Freshness Is Needed
RFC 4556 (PKINIT) defines two ways to use public-key cryptography in Kerberos pre-authentication: Diffie-Hellman key exchange and public key encryption. The encryption scheme is less common. RFC 5349 specified only ECDH for elliptic curves, so it cannot be used for encryption. Encryption also needs certificates with an encryption key, which many deployed smart cards lack.
With Diffie-Hellman, the client uses its private key only to sign the AuthPack, before any traffic goes to the KDC. A client can therefore generate requests with future times in the PKAuthenticator and send them later. Unless the time falls outside the certificate's validity period, the KDC validates the request and returns a Ticket-Granting Ticket the client can use without the private key.
So the client proves only prior use of the key, not current possession. Proving current possession needs signed data that the client could not have predicted.
Imagine signing a stack of permission slips in advance and handing them out later. Anyone holding a slip looks like you, even if your pen is long gone.
§2The Message Exchange
The design borrows the flow password-based Kerberos often uses: the client first sends an AS request with no pre-authentication, and the KDC answers with an error carrying what is needed to continue. Here the KDC supplies a token.
- The client sends a
KRB_AS_REQwith noPA_PK_AS_REQand a freshness token request. - The KDC replies with a
KRB_ERRORcarrying a freshness token. - The client extracts the token and includes it in a new
KRB_AS_REQ. - The KDC validates the request as in RFC 4556 and also validates the token.
- The exchange continues as in RFC 4120 and RFC 4556.
To ask for a token, the client adds a padata element of type PA_AS_FRESHNESS with an empty octet string. The KDC answers with error code KDC_ERR_PREAUTH_REQUIRED and a PA_AS_FRESHNESS element holding the token in the METHOD-DATA object. The client treats the token as an opaque blob and places it in the freshnessToken field of the PKAuthenticator, so it falls inside the signed data.
You ask the guard for a fresh code, the guard gives you one, you sign it, and you hand it back. A signature made earlier cannot contain a code that did not exist yet.
- KRB_AS_REQ (no PA_PK_AS_REQ, PA_AS_FRESHNESS empty)
- KRB_ERROR (PA_AS_FRESHNESS token)
- KRB_AS_REQ (PA_PK_AS_REQ with freshnessToken)
- AS-REPAfter the KDC validates the request and the token.
§3Padata Type and the Extended PKAuthenticator
The RFC defines one new pre-authentication data type, PA_AS_FRESHNESS, with padata-type value 150.
It also extends the PKAuthenticator from RFC 4556 with an optional freshnessToken field, tagged [4], an OCTET STRING that carries the PA_AS_FRESHNESS value as received from the KDC. The comments in the ASN.1 say it MUST be present if the KDC sent one. The existing fields stay: cusec and ctime for replay prevention, a randomly chosen nonce, and paChecksum, a SHA1 checksum over the KDC-REQ-BODY.
There is a new empty box on the form labelled "fresh code goes here". Because it sits inside the signed part, it cannot be changed or filled in earlier.
§4How the KDC Validates and Retries
If the realm requires freshness and the request has no token, the KDC MUST return a KRB_ERROR with KDC_ERR_PREAUTH_FAILED and a PA_AS_FRESHNESS element holding a token.
If a token is present, the KDC validates the request normally and then checks the token in an implementation-specific way. An invalid token makes the KDC MUST return KDC_ERR_PREAUTH_EXPIRED (RFC 6113), whose e-data holds a METHOD-DATA object with a valid token. A client that receives such an error with a new token SHOULD retry with it.
Since freshness tokens are validated by KDCs in the same realm, standardizing their contents is not an interoperability concern.
If your code is too old, the guard says so and hands you a new one. You try again with that.
§5Security and Interoperability Notes
The token SHOULD include signing, encrypting or sealing of data from the KDC, so it can be authenticated and not tampered with. Tokens guarantee the client had the key when constructing the AS-REQ. They need not be single-use or bound to a specific exchange. The token is opaque so KDC implementers can add functionality, as long as tokens expire so the freshness guarantee holds.
Because clients treat the token as opaque, changing its contents does not affect existing clients. Clients SHOULD NOT reuse tokens across exchanges, and a client that does not retry with a new token may be incompatible with some KDCs.
Implementers may allow both token-based and legacy exchanges while clients are upgraded. Until a realm requires freshness tokens, the existing risk of pre-generated PKAuthenticators remains.
The code has to expire, or it would be just another old slip. Until the guard insists on codes, someone with an old slip might still get in.