oysteikt/heimdal
0
0
Fork 0
Code Issues 5 Pull Requests Releases Activity

x

Browse Source 
git-svn-id: svn://svn.h5l.se/heimdal/trunk/heimdal@12824 ec53bebd-3082-4978-b11e-865c3cabbd6b
This commit is contained in:
Love Hörnquist Åstrand
2003-09-12 02:37:10 +00:00
parent b84f7ecd80
commit 1bc0573626
1 changed files with 329 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

329
doc/standardisation/draft-ietf-krb-wg-hw-auth-03.txt Normal file
View File

@@ -0,0 +1,329 @@
Kerberos Working Group Matt Crawford
Internet Draft Fermilab
10 September 2003
Passwordless Initial Authentication to Kerberos
by Hardware Preauthentication
<draft-ietf-krb-wg-hw-auth-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract
This document specifies an extension to the Kerberos protocol for
performing initial authentication of a user without using that
user's long-lived password. Any "hardware preauthentication" method
may be employed instead of the password, and the key of another
principal must be nominated to encrypt the returned credential.
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 [KWORD].
1. Motivation
Many sites using Kerberos for authentication have users who are
often, or even always, away from the site. Sometimes these users
Expires March 15, 2004 Crawford [Page 1]
Internet Draft Passwordless Hardware Authentication 10 September 2003
may need to connect to their site while they have no immediate
access to a computer with Kerberos software or any other trusted
secure remote-access mechanism. Requiring hardware
preauthentication in addition to a password for all such users is an
incomplete solution because an eavesdropper with access to both the
remote users' path to the host in the site and that host's path to
the KDC can still steal the user's credential.
This document specifies a method by which a Kerberos application
server can request that a KDC authenticate a user using a hardware
preauthentication method and use a key held by the server in the
decryption of the KDC's reply, in place of the user's password.
2. Definitions
The following terms used here are defined in [KRB5] and [KRB5bis]:
KDC_ERR_PREAUTH_FAILED, KDC_ERR_PREAUTH_REQUIRED, KRB_AS_REQ,
KRB_ERROR, PrincipalName, e-data, enc-part, error-code, kdc-
options, padata-type, padata-value.
These terms are defined in [KRB5bis]:
PA-SAM-CHALLENGE, PA-SAM-RESPONSE.
The term "service" denotes some Kerberos service which normally
requires a client/server authentication exchange [KRB5] for access
and which is capable of both communicating with the KDC's
Authentication Service and interacting with the user to the extent
required to carry out a single-use authentication mechanism (SAM).
It must have access to some principal's long-lived key. Telnet and
FTP services are examples.
The Kerberos Authentication Service will be denoted by "AS" to avoid
confusion with the service.
3. Method
This mechanism is intended to be employed when a user connects to a
service which normally allows only Kerberos-authenticated access.
When the service determines that the user will not authenticate (for
example, it receives a telnet "WONT AUTHENTICATION" command
[TELAUTH], or an FTP "USER" command without a preceding "AUTH"
command [FTPSEC]), it may accept a user principal name and attempt
to perform passwordless hardware authentication in the following
manner.
Expires March 15, 2004 Crawford [Page 2]
Internet Draft Passwordless Hardware Authentication 10 September 2003
3.1. Initial AS Request and reply
The service, on behalf of the user, prepares a KRB_AS_REQ [KRB5]
message with the flag OPT-HARDWARE-AUTH set in the kdc-options
field, in addition to any other desired options and lifetimes. The
service sends this message to a KDC. If the KDC's policy permits
this form of authentication for the user named in the request, and
the request is acceptable in all other respects, the KDC determines
what hardware preauthentication methods are available for the user
principal and constructs a KRB_ERROR message with the error-code set
to KDC_ERR_PREAUTH_REQUIRED. The e-data field of this KRB_ERROR
message contains a sequence of PA-DATA which includes an element
with padata-type equal to PA-ALT-PRINC and an empty padata-value.
In addition to that are any elements needed for hardware
preauthentication of the user. Typically this will consist of an
element with padata-type PA-SAM-CHALLENGE and padata-value
appropriate to the authentication method.
3.2. Second AS Request
The service, upon receiving the KRB_ERROR message from the KDC, must
process the PA-ALT-PRINC element by selecting a principal whose
long-lived key it has access to, and which is in the same realm as
the client. This principal will be referred to as the alternate
principal. It processes the PA-SAM-CHALLENGE normally, except that
whenever the user's long-lived (password-derived) encryption key is
called for, it uses the alternate principal's key instead.
The service constructs a second KRB_AS_REQ, again with the OPT-
HARDWARE-AUTH flag set in the kdc-options field, and this time with
a padata field which includes at least these two PA-DATA items, in
this order:
One with padata-type equal to PA-ALT-PRINC and as padata-value
the encoded PrincipalName of the alternate principal,
One with padata-type equal to PA-SAM-RESPONSE and padata-value
constructed as it would be for normal hardware
preauthentication, but with the alternate principal's key used
in place of the user's key.
Other PA-DATA may be present before, between or after these items.
The service sends this second KRB_AS_REQ to a KDC.
Expires March 15, 2004 Crawford [Page 3]
Internet Draft Passwordless Hardware Authentication 10 September 2003
3.3. Final AS Reply
The KDC begins processing the AS request normally. When the PA-ALT-
PRINC field is encountered, the KDC does the following:
First, if this use of the alternate principal named in the
request is against local policy, or if the alternate principal
does not exist in the database, a KRB_ERROR message with error-
code KDC_ERR_PREAUTH_FAILED is returned and processing ends.
Then, the alternate principal's key is fetched from the database
and held for use in subsequent processing. It will be needed to
process the PA-SAM-RESPONSE and to encrypt the enc-part of the
KRB_AS_REP if authentication is successful.
The remainder of the AS request processing is normal, with the noted
substitution of the alternate principal's key for the user's.
The service, upon receiving a KRB_AS_REP, uses the alternate
principal's key to decrypt the enc-part, saves the user's credential
and takes appropriate measures to ensure that the KRB_AS_REP came
from a legitimate KDC and not an imposter.
4. IANA Considerations
As of this writing, management of Kerberos protocol parameters has
not been delegated to IANA. No new naming or numbering spaces are
created by this specification. Two new values from existing spaces
are defined:
The flag OPT-HARDWARE-AUTH is a previously unused bit in the
kdc-options field of a KDC-REQ-BODY [KRB5]. The assignment of
bit 11 is expected [BCN].
The preauthentication type PA-ALT-PRINC is denoted by padata-
type 24 [KRB5bis].
5. Security Considerations
There are no means provided here for protecting the traffic between
the user and the service, so it may be susceptible to eavesdropping,
hijacking and alteration. This authentication mechanism is not
intended to be used as an alternative to the Kerberos client/server
authentication exchange, but as an improvement over making an
unprotected connection with a Kerberos password alone, or a password
plus a single-use authenticator.
Expires March 15, 2004 Crawford [Page 4]
Internet Draft Passwordless Hardware Authentication 10 September 2003
The alternate principal's key MUST be involved in construction of
the PA-SAM-RESPONSE padata-value, to prevent an adversary
constructing a KRB_AS_REQ using that data but a different alternate
principal. In practice, this means that the response data alone
must not determine the encryption key for the padata-value.
A service impersonator can obtain a presumably-valid SAM response
from the user which may (or may not) be usable for impersonating the
user at a later time. And of course in the case of successful
authentication the service obtains access to the user's credentials.
As always, if the service host is compromised, so are the
credentials; but at least the service host never has access to the
user's password.
A service host which accepts a Kerberos password for access
typically protects itself against an impostor KDC by using the
received ticket-granting credential to get a ticket for a service
for which it has the key. This step may be unnecessary when the
service host has already successfully used such a key to decrypt the
ticket-granting credential itself.
Use of this authentication method employs the service's long-term
key, providing more ciphertext in that key to an eavesdropper. This
key is generally of better quality than a password-derived key and
any remaining concerns about the strength of the KRB_AS_REP are
better addressed by a general mechanism applicable to all AS
exchanges.
6. Acknowledgments
The first implementation of this extension grew from a beginning by
Ken Hornstein, which in turn was built on code released by the MIT
Kerberos Team.
7. References
[BCN] Newman, C., private communication.
[FTPSEC] Horowitz, M. and S. Lunt, "FTP Security Extensions", RFC
2228.
[KRB5] Kohl, J., and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510.
[KRB5bis] Neuman, C., T. Yu, S. Hartman, and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", Work in
Expires March 15, 2004 Crawford [Page 5]
Internet Draft Passwordless Hardware Authentication 10 September 2003
progress. (Currently draft-ietf-krb-wg-kerberos-
clarifications-04.txt.)
[KWORD] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119, March 1997.
[TELAUTH] Ts'o, T. and J. Altman, "Telnet Authentication Option",
RFC 2941.
8. Author's Address
Matt Crawford
Fermilab MS 369
PO Box 500
Batavia, IL 60510
USA
Phone: +1 630 840-3461
EMail: crawdad@fnal.gov
Expires March 15, 2004 Crawford [Page 6]