Network Working Group G. Richards
Internet-Draft RSA, The Security Division of EMC
Intended status: Standards Track October 9, 2009
Expires: April 12, 2010
OTP Pre-authentication
draft-ietf-krb-wg-otp-preauth-11
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 12, 2010.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
The Kerberos protocol provides a framework authenticating a client
using the exchange of pre-authentication data. This document
describes the use of this framework to carry out One Time Password
Richards Expires April 12, 2010 [Page 1]
Internet-Draft OTP Pre-authentication October 2009
(OTP) authentication.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Overall Design . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Conventions Used in this Document . . . . . . . . . . . . 4
2. Usage Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. OTP Mechanism Support . . . . . . . . . . . . . . . . . . 4
2.2. Pre-Authentication . . . . . . . . . . . . . . . . . . . . 4
2.3. PIN Change . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Re-Synchronization . . . . . . . . . . . . . . . . . . . . 6
3. Pre-Authentication Protocol Details . . . . . . . . . . . . . 7
3.1. Initial Client Request . . . . . . . . . . . . . . . . . . 7
3.2. KDC Challenge . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Client Response . . . . . . . . . . . . . . . . . . . . . 9
3.4. Verifying the pre-auth Data . . . . . . . . . . . . . . . 11
3.5. Confirming the Reply Key Change . . . . . . . . . . . . . 13
3.6. Reply Key Generation . . . . . . . . . . . . . . . . . . . 13
4. OTP Kerberos Message Types . . . . . . . . . . . . . . . . . . 15
4.1. PA-OTP-CHALLENGE . . . . . . . . . . . . . . . . . . . . . 15
4.2. PA-OTP-REQUEST . . . . . . . . . . . . . . . . . . . . . . 18
4.3. PA-OTP-CONFIRM . . . . . . . . . . . . . . . . . . . . . . 21
4.4. PA-OTP-PIN-CHANGE . . . . . . . . . . . . . . . . . . . . 22
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
6.1. Man-in-the-Middle . . . . . . . . . . . . . . . . . . . . 24
6.2. Reflection . . . . . . . . . . . . . . . . . . . . . . . . 25
6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 25
6.4. Replay . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.5. Brute Force Attack . . . . . . . . . . . . . . . . . . . . 26
6.6. FAST Facilities . . . . . . . . . . . . . . . . . . . . . 26
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Normative References . . . . . . . . . . . . . . . . . . . 27
8.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Examples of OTP Pre-Authentication Exchanges . . . . 31
B.1. Four Pass Authentication . . . . . . . . . . . . . . . . . 31
B.2. Two Pass Authentication . . . . . . . . . . . . . . . . . 34
B.3. PIN Change . . . . . . . . . . . . . . . . . . . . . . . . 35
B.4. Resynchronization . . . . . . . . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38
Richards Expires April 12, 2010 [Page 2]
Internet-Draft OTP Pre-authentication October 2009
1. Introduction
1.1. Scope
This document describes a FAST [ZhHa09] factor that allows One-Time
Password (OTP) values to be used in the Kerberos V5 [RFC4120] pre-
authentication in a manner that does not require use of the user's
Kerberos password. The system is designed to work with different
types of OTP algorithms such as time-based OTPs [RFC2808], counter-
based tokens [RFC4226] and challenge-response systems such as
[RFC2289]. It is also designed to work with tokens that are
electronically connected to the user's computer via means such as a
USB interface.
This FAST factor provides the following facilities (as defined in
[ZhHa09]): client-authentication, replacing-reply-key and KDC-
authentication. It does not provide the strengthening-reply-key
facility.
This proposal is partially based upon previous work on integrating
single-use authentication mechanisms into Kerberos [HoReNeZo04].
1.2. Overall Design
This proposal supports 4-pass and 2-pass variants. In the 4-pass
system, the client sends the KDC an initial AS-REQ and the KDC
responds with a KRB-ERROR containing padata that includes a random
nonce. The client then encrypts the nonce and returns it, along with
its own random value, to the KDC in a second AS-REQ. Finally, the
KDC returns the client's random value encrypted within the padata of
the AS-REP. Note that this variant can only be used for users that
require pre-authentication. In the 2-pass variant, the client
encrypts a timestamp rather than a nonce from the KDC and the
encrypted data is sent to the KDC in the initial AS-REQ. This
variant can be used in cases where the client can determine in
advance that OTP pre-authentication is supported by the KDC, which
OTP key should be used and the encryption parameters required by the
KDC.
In both systems, in order to create the message sent to the KDC, the
client must generate the OTP value and two keys: the standard Reply
Key used to decrypt the KDC's reply and a key to encrypt the data
sent to the KDC. In most cases, the OTP value will be used in the
key generation but in order to support algorithms where the KDC
cannot obtain the value (e.g. [RFC2289]), the system also supports
the option of including the OTP value in the request along with the
encrypted nonce. In addition, in order to support situations where
the KDC is unable to obtain the plaintext OTP value, the system also
Richards Expires April 12, 2010 [Page 3]
Internet-Draft OTP Pre-authentication October 2009
supports the use of hashed OTP values in the key derivation.
The preauth data sent from the client to the KDC is sent within the
encrypted data provided by the FAST padata type of the AS-REQ. The
KDC then obtains the OTP value, generates the same keys and verifies
the pre-authentication data by decrypting the nonce. If the
verification succeeds then it confirms knowledge of the Reply Key by
returning the client's nonce encrypted under one the generated Reply
Key within the encrypted part of the FAST padata of the AS-REP.
1.3. Conventions Used in this Document
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].
This document assumes familiarity with the Kerberos pre-
authentication framework [ZhHa09] and so freely uses terminology and
notation from this document.
The word padata is used as shorthand for pre-authentication data.
2. Usage Overview
2.1. OTP Mechanism Support
As described above, this document describes a generic system for
supporting different OTP mechanisms in Kerberos pre-authentication.
To ensure interoperability, all implementations of this specification
SHOULD provide a mechanism (e.g. a provider interface) to add or
remove support for a particular OTP mechanism.
2.2. Pre-Authentication
The approach uses pre-authentication data in AS-REQ, AS-REP and KRB-
ERROR messages.
In the 4-pass system, the client begins by sending an initial AS-REQ
to the KDC that may contain pre-authentication data such as the
standard Kerberos password data. The KDC will then determine, in an
implementation dependent fashion, whether OTP authentication is
required and if it is, it will respond with a KRB-ERROR message
containing a PA-OTP-CHALLENGE (see Section 4.1) in the PA-DATA.
The PA-OTP-CHALLENGE will contain a KDC generated nonce, an optional
list of hash algorithm identifiers, an optional iteration count and
optional information on how the OTP should be generated by the
Richards Expires April 12, 2010 [Page 4]
Internet-Draft OTP Pre-authentication October 2009
client. The client will then generate the OTP value, its own nonce
and two keys: a Client Key to encrypt the KDC's nonce and a Reply Key
used to decrypt the KDC's reply.
As described in section 6.5.1 of [ZhHa09], the FAST system uses an
Armor Key to set up an encrypted tunnel for use by FAST factors. As
described in Section 3.6 of this document, the Client Key and Reply
Key will be generated from the Armor Key and the OTP value unless the
OTP algorithm does not allow the KDC to obtain the OTP value. If
hash algorithm identifiers were included in the PA-OTP-CHALLENGE then
the client will use the hash of the OTP value rather than the
plaintext value in the key generation. Both keys will have the same
encryption type as the Armor Key.
The generated Client Key will be used to encrypt the nonce received
from the KDC. The encrypted value, a random nonce generated by the
client along with optional information on how the OTP was generated
are then sent to the KDC in a PA-OTP-REQUEST (see Section 4.2)
encrypted within the armored-data of a PA-FX-FAST-REQUEST PA-DATA
element of a second AS-REQ.
In the 2-pass system, the client sends the PA-OTP-REQUEST in the
initial AS-REQ instead of sending it in response to a PA-OTP-
CHALLENGE returned by the KDC. Since no challenge is received from
the KDC, the client includes an encrypted timestamp in the request
rather than the encrypted KDC nonce.
In both cases, on receipt of a PA-OTP-REQUEST, the KDC generates the
keys in the same way as the client, and uses the generated Client Key
to verify the pre-authentication by decrypting the encrypted data
sent by the client (either nonce or timestamp). If the validation
succeeds then the KDC will authenticate itself to the client and
confirm that the Reply Key has been updated by encrypting the
client's nonce under the Reply Key and returning the encrypted value
in the encData of a PA-OTP-CONFIRM (see Section 4.3). The PA-OTP-
CONFIRM is encrypted within the armored-data of a PA-FX-FAST-REPLY
PA-DATA element of the AS-REP as described in [ZhHa09].
2.3. PIN Change
Most OTP tokens involve the use of a PIN in the generation of the OTP
value. This PIN value will be entered by the user on the client and
combined with the value produced by the token in a manner specific to
the algorithm to generate the final OTP value that will be used in
this protocol.
If, following successful validation of a PA-OTP-REQUEST in a AS-REQ,
the KDC determines that the user needs to change their PIN, that the
Richards Expires April 12, 2010 [Page 5]
Internet-Draft OTP Pre-authentication October 2009
PIN has been changed for the user or is about to expire then it MAY
include a PA-OTP-PIN-CHANGE (see Section 4.4) encrypted within the
armored data of the PA-FX-FAST-REPLY PA-DATA element of the AS-REP.
This data can be used to return a new PIN to the user if the PIN has
been updated, to indicate to the user that they must change their
PIN, to inform the user of the PIN's expiration time and to carry
formatting information about that PIN.
If PIN change is to be handled by a PIN-change service that requires
a Kerberos service ticket then the KDC MUST NOT return a TGT when the
user is authenticated and user PIN change is required, the KDC SHOULD
instead return a service ticket to the PIN-change service, in order
for the client to compute an AP-REQ. In order to complicate stealing
service tickets intended for the PIN-change service (and the
corresponding session keys), the lifetime of the PIN-change service
tickets should be just long enough to complete the PIN change,
regardless whether the existing PIN needs to be changed or not. A
1-minute lifetime is RECOMMENDED.
If the user's PIN has not expired but has been changed and the KDC is
returning the new value to the user in a PA-OTP-PIN-CHANGE or PIN
change is not required and the PA-OTP-PIN-CHANGE is being used to
inform the client of the current PIN's expiration time then no such
restrictions are required and the KDC SHOULD return the originally
requested ticket.
If the KDC is returning a ticket for a PIN-change service then it is
RECOMMENDED that any context information, such as key and vendor
identifiers be, that is required by the service be added to the
ticket as authorization data elements.
2.4. Re-Synchronization
It is possible with time and event-based tokens that the OTP server
will lose synchronization with the current token state. For example,
event-based tokens may drift since the counter on the token is
incremented every time the token is used but the counter on the
server is only incremented on an authentication. Similarly, the
clocks on time-based tokens may drift.
If, when processing a PA-OTP-REQUEST, the pre-authentication
validation fails for this reason then the KDC MAY return a KRB-ERROR
message. The KRB-ERROR message MAY contain a PA-OTP-CHALLENGE in the
PA-DATA with a single otp-keyInfo representing the token used in the
initial authentiation attempt but with "nextOTP" flag set. If this
flag is set then the client SHOULD re-try the authentication using an
OTP value generated using the token in the "state" after that used in
the failed authentication attempt. For example, using the next time
Richards Expires April 12, 2010 [Page 6]
Internet-Draft OTP Pre-authentication October 2009
interval or counter value.
3. Pre-Authentication Protocol Details
3.1. Initial Client Request
In the 4-pass mode, the client begins by sending an initial AS-REQ,
possibly containing other pre-authentication data. If the KDC
determines that OTP-based pre-authentication is required and the
request does not contain a PA-OTP-REQUEST then it will respond as
described in Section 3.2.
If the client has all the necessary information, it MAY use the
2-pass system by constructing a PA-OTP-REQUEST as described in
Section 3.3 and including it in the initial request.
3.2. KDC Challenge
If the user is required to authenticate using an OTP then the KDC
SHALL respond to the initial AS-REQ with a KRB-ERROR of type
KDC_ERR_PREAUTH_REQUIRED with a PA-OTP-CHALLENGE contained within the
enc-fast-rep of the armored-data of a PA-FX-FAST-REPLY encrypted
under the current Armor Key as described in [ZhHa09].
If the OTP mechanism is to be carried out as an individual mechanism
then the PA-OTP-CHALLENGE SHALL be carried within the padata of the
KrbFastResponse. Alternatively, if the OTP mechanism is required as
part of an authentication set then the PA-OTP-CHALLENGE SHALL be
carried within a PA-AUTHENTICATION-SET-ELEM as described in section
6.4 of [ZhHa09].
The PA-OTP-CHALLENGE SHALL contain a nonce value to be returned
encrypted in the client's PA-OTP-REQUEST. This nonce string MUST be
as long as the longest key length of the symmetric key types that the
KDC supports and MUST be chosen randomly. In order to allow it to
maintain any state necessary to verify the returned nonce, the KDC
SHOULD use the mechanism described in section 6.3 of [ZhHa09].
Since the OTP mechanism described in this document is replacing the
Reply Key, the standard shared-key system cannot be relied upon to
allow the client to verify the KDC. Therefore, as described in
section 4.4 of [ZhHa09], some other mechanism must be provided to
support this. If the OTP value is used in the Reply Key generation
then the client and KDC have a shared key and kdc-authentication is
provided by the PA-OTP-CONFIRM. However, if the OTP value is sent in
the otp-value element of the PA-OTP-REQUEST then there is no such
shared key and the OTP mechanism does not provide kdc-authentication.
Richards Expires April 12, 2010 [Page 7]
Internet-Draft OTP Pre-authentication October 2009
Therefore, if the OTP mechanism is not being used in an environment
where kdc-authentication is being provided by other means (e.g. by
the use of host key armor) then the KDC MUST set the "must-encrypt-
nonce" flag in the PA-OTP-CHALLENGE.
In order to support cases where the KDC cannot obtain plaintext
values for the OTPs, the KDC MAY include in the supportedHashAlg and
iterationCount elements of the PA-OTP-REQUEST a sequence of one way
hash function algorithm identifiers and a minimum value of the
iteration count. If these elements are included then the client MUST
use hashed OTP values in the generation of the Reply Key and Client
Key as described in Section 3.6.
The KDC MAY use the otp-service field to assist the client in
locating the OTP token to be used by identifying the purpose of the
authentication. For example, the otp-service field could assist a
user in identifying the token to be used when a user has multiple OTP
tokens from that are used for different purposes.
Finally, the KDC MAY also include a sequence of otp-keyInfo elements
containing information on the token or tokens that the user can use
for the authentication and how the OTP value is to be generated using
those tokens. If a single otp-keyInfo element is included then the
client MUST generate the OTP value according to the information
contained within that element. If more than one otp-keyInfo element
is included then the client MUST generate the OTP value according to
the information contained within one of those elements. If no otp-
keyInfo elements are included then the KDC is imposing no
restrictions on token usage.
The KDC MAY include the otp-vendor field in an otp-keyInfo to assist
the client in locating the OTP token to be used by identifying the
vendor of the token.
If the OTP for a token is to be generated using a server generated
challenge then the value of the challenge SHALL be included in the
otp-challenge field of the otp-keyInfo for that token. If the OTP is
to be generated by combining the challenge with the token's current
state (e.g. time) then the "combine" flag SHALL be set within the
otp-keyInfo containing the challenge.
If the KDC can determine which OTP token key (the seed value on the
token used to generate the OTP) is to be used, then the otp-keyID
field MAY be included in the otp-keyInfo to pass that value to the
client.
The otp-algID field MAY be included in an otp-keyInfo to identify the
algorithm that should be used in the OTP calculation for that token.
Richards Expires April 12, 2010 [Page 8]
Internet-Draft OTP Pre-authentication October 2009
For example, it could be used when a user has been issued with
multiple tokens that support different algorithms.
If the KDC can determine that an OTP token that can be used by the
user does not require a PIN as part of the OTP generation then it MAY
set the "pin-not-required" flag in the otp-keyInfo representing that
token. If the token requires a PIN then it MAY set the "pin-
required" flag. If the KDC is unable to determine whether the token
requires a PIN then the "pin-required" and "pin-not-required" flags
MUST NOT be set.
Finally, in order to support connected tokens that can generate OTP
values of varying length, the KDC MAY include the desired length of
the OTP in the otp-length field of an otp-keyInfo.
3.3. Client Response
The client response SHALL be sent to the KDC as a PA-OTP-REQUEST
included within the enc-fast-req of the armored-data within a PA-FX-
FAST-REQUEST encrypted under the current Armor Key as described in
[ZhHa09].
In order to generate its response, the client must generate an OTP
value. If the PA-OTP-CHALLENGE contained one or more otp-keyInfo
elements then the OTP value MUST be based on the information
contained within one of those elements. The PA-OTP-REQUEST returned
by the client SHOULD include any information on the generated OTP
value reported by the OTP token.
If the "nextOTP" flag is set in the otp-keyInfo from the PA-OTP-
CHALLENGE, then the OTP value MUST be generated from the next token
state than that used in the previous PA-OTP-REQUEST for that token.
The "nextOTP" flag MUST also be set in the new PA-OTP-REQUEST.
If the "pin-not-required" flag is set in the otp-keyInfo from the PA-
OTP-CHALLENGE, then the token represented by the otp-keyInfo does not
require a PIN to be collected as part of the OTP value. If the "pin-
required" flag is set then the token requires a PIN to be collected.
If neither flag is set then PIN requirements of the token are
unspecified. If both flags are set then client SHALL regard the
request as invalid.
The otp-time and otp-counter fields MAY be used to return the time
and counter values used by the token. The otp-format field MAY be
used to report the format of the generated OTP. This field SHOULD be
used if a token can generate OTP values in multiple formats. The
otp-algID field MAY be used by the client to report the algorithm
used in the OTP calculation and the otp-keyID MAY be used to report
Richards Expires April 12, 2010 [Page 9]
Internet-Draft OTP Pre-authentication October 2009
the identifier of the OTP token key used.
If an otp-challenge is present in the otp-keyInfo selected by the
client from the PA-OTP-CHALLENGE then the OTP value for the token
MUST be generated based on a challenge if the token is capable of
accepting a challenge. The client MAY ignore the provided challenge
if and only if the token is not capable of including a challenge in
the OTP calculation. If the "combine" flag is not set in the otp-
keyInfo of the PA-OTP-CHALLENGE then the OTP SHALL be calculated
based only the challenge and not the internal state (e.g. time or
counter) of the token. If the "combine" flag is set then the OTP
SHALL be calculated using both the internal state and the provided
challenge. If the flag is set but otp-challenge is not present then
the client SHALL regard the request as invalid.
If the OTP value was generated using a challenge that was not sent by
the KDC then the challenge SHALL be included in the otp-challenge of
the PA-OTP-REQUEST. If the OTP was generated by combining a
challenge (either received from the KDC or generated by the client)
with the token state then the "combine" flag SHALL be set in the PA-
OTP-REQUEST.
The client MUST derive the Client Key and Reply Key as described in
Section 3.6. In order to support OTP algorithms where the KDC cannot
obtain the OTP value, the client MAY include the generated value in
the otp-value field of the PA-OTP-REQUEST. However, the client MUST
NOT include the OTP value if the "must-encrypt-nonce" flag was set in
the PA-OTP-CHALLENGE or if it is not allowed by the algorithm
profile. If the OTP value is included in the PA-OTP-REQUEST then it
MUST NOT be used in the key derivation.
If the client used hashed OTP values in the key derivation process
then it MUST include the hash algorithm and iteration count used in
the hashAlg and iterationCount fields of the PA-OTP-REQUEST. These
fields MUST NOT be included if hashed OTP values were not used. It
is RECOMMENDED that the iteration count used by the client be chosen
in such a way that it is computationally infeasible/unattractive for
an attacker to brute-force search for the given OTP.
If the PA-OTP-REQUEST is being sent in response to a PA-OTP-CHALLENGE
that contained hash algorithm identifiers and the OTP value is to be
used in the key derivation then the client MUST use hashed OTP values
and MUST select the first algorithm from the list that it supports.
However, if the algorithm identifiers do not conform to local policy
restrictions then the authentication attempt MUST NOT proceed. If
the iteration count specified in the PA-OTP-CHALLENGE does not
conform to local policy then the client MAY use a higher value but
MUST NOT use a lower value. That is, the value in the KDC challenge
Richards Expires April 12, 2010 [Page 10]
Internet-Draft OTP Pre-authentication October 2009
is a minimum value.
The generated Client Key is used by the client to encrypt data to be
included in the encData of the PA-OTP-REQUEST to allow the KDC to
authenticate the user. The key usage for this encryption is
KEY_USAGE_OTP_REQUEST.
o If the PA-OTP-REQUEST is being generated in response to a PA-OTP-
CHALLENGE returned by the KDC then the client SHALL encrypt a PA-
OTP-ENC-REQUEST containing the value of nonce from the PA-OTP-
CHALLENGE using the same encryption type as the Armor Key.
o If the PA-OTP-REQUEST is not in response to a PA-OTP-CHALLENGE
then the client SHALL encrypt a PA-ENC-TS-ENC containing the
current time as in the encrypted timestamp pre-authentication
mechanism [RFC4120].
If the client is working in 2-pass mode and so is not responding to
an initial KDC challenge then the values of the iteration count and
hash algorithms cannot be obtained from that challenge. The client
SHOULD use any values obtained from a previous PA-OTP-CHALLENGE or,
if no values are available, it MAY use initial configured values.
Finally, the client generates a nonce value to include in the PA-OTP-
REQUEST that will be returned encrypted by the KDC. This nonce
string MUST be as long as the longest key length of the symmetric key
types that the client supports. This nonce MUST be chosen randomly.
3.4. Verifying the pre-auth Data
The KDC validates the pre-authentication data by generating the
Client Key and Reply Key in the same way as the client and using the
generated Client Key to decrypt the value of encData from the PA-OTP-
REQUEST. The generated Reply Key is used to encrypt data in the AS-
REP and to encrypt the encData value in the PA-OTP-CONFIRM.
If the otp-value field is included in the PA-OTP-REQUEST then the KDC
MUST use that value unless the OTP method is required to support kdc-
authentication (see Section 3.2). If kdc-authentication is required
then a PA-OTP-REQUEST containing an otp-value MUST be rejected. If
the otp-value is not included in the PA-OTP-REQUEST then the KDC will
need to generate or obtain the OTP value.
If the otp-challenge field is present, then the OTP was calculated
using that challenge. If the "combine" flag is also set, then the
OTP was calculated using the challenge and the token's current state.
It is RECOMMENDED that the KDC acts upon the values of otp-time, otp-
Richards Expires April 12, 2010 [Page 11]
Internet-Draft OTP Pre-authentication October 2009
counter, otp-format, otp-algID and otp-keyID if they are present in
the PA-OTP-REQUEST. If the KDC receives a request containing these
values but cannot act upon them then they MAY be ignored.
The KDC generates the Client Key and Reply Key as described in
Section 3.6 from the OTP value using the hash algorithm and iteration
count if present in the PA-OTP-REQUEST. The KDC MUST fail the
request with KDC_ERR_INVALID_HASH_ALG if the KDC requires hashed OTP
values and the hashAlg field was not present in the PA-OTP-REQUEST or
if the value of this field does not conform to local KDC policy.
Similarly, the KDC MUST fail the request with
KDC_ERR_INVALID_ITERATION_COUNT if the value of the iterationCount
included in the PA-OTP-REQUEST does not conform to local KDC policy
or is less than that specified in the PA-OTP-CHALLENGE.
KDC_ERR_INVALID_HASH_ALG 94
KDC_ERR_INVALID_ITERATION_COUNT 95
The generated Client Key is then used to decrypt the encData from the
PA-OTP-REQUEST. If the client response was sent as a result of a PA-
OTP-CHALLENGE then the decrypted data will be a PA-OTP-ENC-REQUEST
and the client authentication MUST fail with KDC_ERR_PREAUTH_FAILED
if the nonce value from the PA-OTP-ENC-REQUEST is not the same as the
nonce value sent in the PA-OTP-CHALLENGE. If the response was not
sent as a result of a PA-OTP-CHALLENGE then the decrypted value will
be a PA-ENC-TS-ENC and the authentication process will be the same as
with standard encrypted timestamp pre-authentication [RFC4120]
The KDC MUST fail the request with KDC_ERR_ETYPE_NOSUPP if the
encryption type used by the client in the encData does not conform to
KDC policy.
If authentication fails due to the hash algorithm, iteration count or
encryption type used by the client then the KDC SHOULD return a PA-
OTP-CHALLENGE with the required values in the error response. If the
authentication fails due to the token state on the server no longer
being synchronized with the token used then the KDC MAY return a PA-
OTP-CHALLENGE with the "nextOTP" flag set as described in
Section 2.4.
If, during the authentication process, the KDC determines that the
user's PIN has been changed then it SHOULD include a PA-OTP-PIN-
CHANGE in the response as described in Section 2.3 containing the new
PIN value. The KDC MAY also include the new PIN's expiration time
and the expiration time of the OTP account within the last-req field
of the PA-OTP-PIN-CHANGE. If the KDC determines that the user's PIN
or OTP account are about to expire, it MAY return a PA-OTP-PIN-CHANGE
with that information. Finally, if the KDC determines that the
Richards Expires April 12, 2010 [Page 12]
Internet-Draft OTP Pre-authentication October 2009
user's PIN has expired then it MAY return a PA-OTP-PIN-CHANGE,
otherwise, it SHOULD return the same response as for a non-OTP
expired password.
3.5. Confirming the Reply Key Change
If the pre-authentication data was successfully verified, then, in
order to support mutual authentication, the KDC SHALL respond to the
client's PA-OTP-REQUEST by including in the AS-REP, a PA-OTP-CONFIRM
containing the client's nonce from PA-OTP-REQUEST encrypted under the
generated Reply Key.
The nonce SHALL be returned within a PA-OTP-ENC-CONFIRM encrypted
within the encData of the PA-OTP-CONFIRM. The key usage SHALL be
KEY_USAGE_OTP_CONFIRM and the encryption type SHOULD be the same as
used by the client in the encData of the PA-OTP-REQUEST.
The PA-OTP-CONFIRM SHALL be sent to the client within the enc-fast-
rep of a PA-FX-FAST-REPLY encrypted under the current Armor Key.
The client then uses its generated Reply Key to decrypt the PA-OTP-
ENC-CONFIRM from the encData of the PA-OTP-CONFIRM. The client MUST
NOT continue with the authentication process if the nonce value in
the PA-OTP-ENC-CONFIRM is not the same as the nonce value sent in the
PA-OTP-REQUEST.
3.6. Reply Key Generation
In order to authenticate the user, the client and KDC need to
generate two encryption keys:
o The Client Key to be used by the client to encrypt and by the KDC
to decrypt the encData in the PA-OTP-REQUEST.
o The Reply Key to be used in the standard manner by the KDC to
encrypt data in the AS-REP but is also to be used by the KDC to
encrypt and by the client to decrypt the encData value in the PA-
OTP-CONFIRM.
The method used to generate the two keys will depend on the OTP
algorithm.
o If the OTP value is included in the otp-value of the PA-OTP-
REQUEST then the two keys SHALL be the same as the Armor Key
(defined in [ZhHa09]).
Richards Expires April 12, 2010 [Page 13]
Internet-Draft OTP Pre-authentication October 2009
o If the OTP value is not included in the otp-value of the PA-OTP-
REQUEST then the two keys SHALL be derived from the Armor Key and
the OTP value as described below.
If the OTP value is not included in the PA-OTP-REQUEST, then the
Reply Key and Client Key SHALL be generated using the KRB-FX-CF2
algorithm from [ZhHa09] as follows:
Client Key = KRB-FX-CF2(K1, K2, O1, O2)
Reply Key = KRB-FX-CF2(K1, K2, O3, O4)
The octet string parameters, O1, O2, O3 and O4, shall be the ASCII
string "Combine1", "Combine2", "Combine3" and "Combine4" as shown
below:
{0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x31}
{0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x32}
{0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x33}
{0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x34}
The first input keys, K1, SHALL be the Armor Key and so, as described
in section 6.1 of [ZhHa09], the enctypes of the generated Client Key
and Reply Key will be the same as the enctype of Armor Key. The
second input key, K2, shall be derived from the OTP value using
string-to-key (defined in [RFC3961]) as follows.
If the hash of the OTP value is to be used then K2 SHALL be derived
as follows:
o An initial hash value, H, is generated:
H = hash(sname|nonce|OTP)
Where:
* "|" denotes concatenation
* hash is the hash algorithm selected by the client.
* sname is the UTF-8 encoding of the KDC's fully qualified domain
name. If the domain name is an Internationalized Domain Name
then the value shall be the output of nameprep [RFC3491] as
described in [RFC3490].
* nonce is the random nonce value generated by the client to be
included in the PA-OTP-REQUEST.
* OTP is the OTP value.
o The initial hash value is then hashed iterationCount-1 times to
produce a final hash value, H'. (Where iterationCount is the
value from the PA-OTP-REQUEST.)
Richards Expires April 12, 2010 [Page 14]
Internet-Draft OTP Pre-authentication October 2009
H' = hash(hash(...(iterationCount-1 times)...(H)))
o The value of K2 is then derived from the Base64 [RFC2045] encoding
of this final hash value.
K2 = string-to-key(Base64(H')||"Krb-preAuth")
If the OTP value is binary and the hash value is not used, then K2
SHALL be derived from the base64 encoding of the OTP value.
K2 = string-to-key(Base64(OTP)||"Krb-preAuth")
If the OTP value is not binary and the hash value is not used, then
K2 SHALL be derived by running the OTP value once through string-to-
key.
K2 = string-to-key(OTP||"Krb-preAuth")
The salt and any additional parameters for string-to-key will be
derived as described in section 3.1.3 of [RFC4120] using preauth data
or default values defined for the particular enctype. The symbol
"||" denotes string concatenation.
4. OTP Kerberos Message Types
4.1. PA-OTP-CHALLENGE
The padata-type PA-OTP-CHALLENGE is returned by the KDC to the client
in the enc-fast-rep of a PA-FX-FAST-REPLY in the PA-DATA of a KRB-
ERROR when OTP pre-authentication is required. The corresponding
padata-value field contains the Distinguished Encoding Rules (DER)
[X.680] [X.690] encoding of a PA-OTP-CHALLENGE containing a server
generated nonce and information for the client on how to generate the
OTP.
Richards Expires April 12, 2010 [Page 15]
Internet-Draft OTP Pre-authentication October 2009
PA-OTP-CHALLENGE 141
PA-OTP-CHALLENGE ::= SEQUENCE {
flags [0] ChallengeFlags,
nonce [1] OCTET STRING,
supportedHashAlg [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
iterationCount [3] Int32 OPTIONAL,
otp-service [4] UTF8String OPTIONAL,
otp-keyInfo [5] SEQUENCE OF OTP-KEYINFO OPTIONAL,
...
}
OTP-KEYINFO ::= SEQUENCE {
flags [0] OTPFlags,
otp-vendor [1] UTF8String OPTIONAL,
otp-challenge [2] OCTET STRING (SIZE(8..MAX)) OPTIONAL,
otp-length [3] Int32 OPTIONAL,
otp-keyID [4] OCTET STRING OPTIONAL,
otp-algID [5] AnyURI OPTIONAL,
...
}
ChallengeFlags ::= KerberosFlags
-- reserved(0),
-- must-encrypt-nonce (1)
OTPFlags ::= KerberosFlags
-- reserved(0),
-- nextOTP(1),
-- combine(2),
-- pin-required(3),
-- pin-not-required(4)
flags
If the "must-encrypt-nonce" flag is set then the OTP value MUST
NOT be included in the otp-value field of the PA-OTP-REQUEST but
instead MUST be used in the generation of the Reply Key and Client
Key as described in Section 3.6.
nonce
A KDC-supplied nonce value to be encrypted by the client in the
PA-OTP-REQUEST. This nonce string MUST be as long as the longest
key length of the symmetric key types that the KDC supports and
MUST be chosen randomly.
Richards Expires April 12, 2010 [Page 16]
Internet-Draft OTP Pre-authentication October 2009
supportedHashAlg
If present then a hash of the OTP value MUST be used in the key
derivation rather than the plain text value. Each
AlgorithmIdentifier identifies a hash algorithm that is supported
by the KDC in decreasing order of preference. The client MUST
select the first algorithm from the list that it supports.
Support for SHA1 by both the client and KDC is REQUIRED. The
AlgorithmIdentifer selected by the client MUST be placed in the
hashAlg element of the PA-OTP-REQUEST.
iterationCount
The minimum value of the iteration count to be used by the client
when hashing the OTP value. This value MUST be present if and
only if supportedHashAlg is present. If the value of this element
does not conform to local policy on the client then the client MAY
use a larger value but MUST NOT use a lower value. The value of
the iteration count used by the client MUST be returned in the PA-
OTP-REQUEST sent to the KDC.
otp-service
Use of this field is OPTIONAL, but MAY be used by the KDC to
assist the client to locate the appropriate OTP tokens to be used.
For example, this field could be used when a user has multiple OTP
tokens for different purposes.
otp-keyInfo
Use of this field is OPTIONAL, but MAY be used by the KDC to carry
information on the tokens that the user may use to carry out the
authentication. Each otp-keyInfo element carries information on a
valid token. If this field is omitted then there are no token
restrictions.
flags
If the "nextOTP" flag is set then the OTP SHALL be based on the
next token "state" rather than the one used in the previous
authentication. As an example, for a time-based token, this
means the next time slot and for an event-based token, this
could mean the next counter value. If the "nextOTP" flag is
set then there MUST only be a single otp-keyInfo element in the
PA-OTP-CHALLENGE.
The "combine flag controls how the challenge included in otp-
challenge shall be used. If the flag is set then OTP SHALL be
calculated using the challenge from otp-challenge and the
internal token state (e.g. time or counter). If the "combine"
flag is not set then the OTP SHALL be calculated based only on
the challenge. If the flag is set and otp-challenge is not
present then the request SHALL be regarded as invalid.
Richards Expires April 12, 2010 [Page 17]
Internet-Draft OTP Pre-authentication October 2009
If the "pin-not-required" flag is set then the token
represented by the current otp-keyInfo does not require a PIN
to be collected as part of the OTP. If the "pin-required" flag
is set then the token requires a PIN. If neither flag is set
then whether or not a PIN is required is unspecified. The
flags are mutually exclusive and so both flags MUST NOT be set
and the client MUST regard the request as invalid.
otp-vendor
Use of this field is OPTIONAL, but MAY be used by the KDC to
identify the vendor of the OTP token to be used.
otp-challenge
The otp-challenge is used by the KDC to send a challenge value
for use in the OTP calculation. The challenge is an optional
octet string that SHOULD be uniquely generated for each request
it is present in, and SHOULD be eight octets or longer when
present. When the challenge is not present, the OTP will be
calculated on the current token state only. The client MAY
ignore a provided challenge if and only if the OTP token the
client is interacting with is not capable of including a
challenge in the OTP calculation. In this case, KDC policies
will determine whether to accept a provided OTP value or not.
otp-length
Use of this field is OPTIONAL, but MAY be used by the KDC to
specify the desired length of the generated OTP in octets. For
example, this field could be used when a token is capable of
producing OTP values of different lengths.
otp-keyID
Use of this field is OPTIONAL, but MAY be used by the KDC to
identify which token key should be used for the authentication.
For example, this field could be used when a user has been
issued multiple token keys by the same server.
otp-algID
Use of this field is OPTIONAL, but MAY be used by the KDC to
identify the algorithm to use when generating the OTP.
4.2. PA-OTP-REQUEST
The padata-type PA-OTP-REQUEST is sent by the client to the KDC in
the KrbFastReq padata of a PA-FX-FAST-REQUEST that is included in the
PA-DATA of an AS-REQ. The corresponding padata-value field contains
the DER encoding of a PA-OTP-REQUEST.
Richards Expires April 12, 2010 [Page 18]
Internet-Draft OTP Pre-authentication October 2009
The message contains pre-authentication data encrypted by the client
using the generated Client Key and optional information on how the
OTP was generated. It may also, optionally, contain the generated
OTP value.
PA-OTP-REQUEST 142
PA-OTP-REQUEST ::= SEQUENCE {
flags [0] OTPFlags,
nonce [1] OCTET STRING,
encData [2] EncryptedData,
-- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
-- Key usage of KEY_USAGE_OTP_REQUEST
hashAlg [3] AlgorithmIdentifier OPTIONAL,
iterationCount [4] Int32 OPTIONAL,
otp-value [5] OCTET STRING OPTIONAL,
otp-challenge [6] OCTET STRING (SIZE(8..MAX)) OPTIONAL,
otp-time [7] KerberosTime OPTIONAL,
otp-counter [8] OCTET STRING OPTIONAL,
otp-format [9] OTPFormat OPTIONAL,
otp-keyID [10] OCTET STRING OPTIONAL,
otp-algID [11] AnyURI OPTIONAL,
...
}
KEY_USAGE_OTP_REQUEST 45
PA-OTP-ENC-REQUEST ::= SEQUENCE {
nonce [0] OCTET STRING,
...
}
OTPFormat ::= INTEGER {
decimal(0),
hexadecimal(1),
alphanumeric(2),
binary(3)
}
flags
If the "nextOTP" flag is set then the OTP was calculated based on
the next token "state" rather than the current one. This flag
MUST be set if and only if it was set in a corresponding PA-OTP-
CHALLENGE.
Richards Expires April 12, 2010 [Page 19]
Internet-Draft OTP Pre-authentication October 2009
If the "combine" flag is set then the OTP was calculated based on
a challenge and the token state.
nonce
A value generated by the client to be returned encrypted by the
KDC in the PA-OTP-CONFIRM and used in the generation of hashed OTP
values as described in Section 3.6 . This nonce string MUST be as
long as the longest key length of the symmetric key types that the
client supports and MUST be chosen randomly.
encData
This field contains the pre-authentication data encrypted under
the Client Key with a key usage of KEY_USAGE_OTP_REQUEST. If the
PA-OTP-REQUEST is sent as a result of a PA-OTP-CHALLENGE then this
MUST contain a PA-OTP-ENC-REQUEST with the nonce from the PA-OTP-
CHALLENGE. If no challenge was received then this MUST contain a
PA-ENC-TS-ENC.
hashAlg
This field MUST be present if and only if a hash of the OTP value
was used as input to string-to-key (see Section 3.6) and MUST
contain the AlgorithmIdentifier of the hash algorithm used. If
the PA-OTP-REQUEST is sent as a result of a PA-OTP-CHALLENGE then
the AlgorithmIdentifer MUST be the first one supported by the
client from the supportedHashAlg of the PA-OTP-CHALLENGE.
iterationCount
This field MUST be present if and only if a hash of the OTP value
was used as input to string-to-key (see Section 3.6) and MUST
contain the iteration count used when hashing the OTP value. If
the PA-OTP-REQUEST is sent as a result of a PA-OTP-CHALLENGE then
the value MUST NOT be less that that specified in the PA-OTP-
CHALLENGE.
otp-value
The generated OTP value. This value MUST NOT be present unless
allowed by the OTP algorithm profile and if the "must-encrypt-
nonce" flag was not set in the PA-OTP-CHALLENGE.
otp-challenge
Value used by the client in the OTP calculation. It MUST be sent
to the KDC if and only if the value would otherwise be unknown to
the KDC. For example, the token or client modified or generated
challenge.
Richards Expires April 12, 2010 [Page 20]
Internet-Draft OTP Pre-authentication October 2009
otp-time
This field MAY be included by the client to carry the time value
as reported by the OTP token. Use of this element is OPTIONAL but
it MAY be used by a client to simplify the OTP calculations of the
KDC. It is RECOMMENDED that the KDC act upon this value if it is
present in the request and it is capable of using it in the
generation of the OTP value.
otp-counter
This field MAY be included by the client to carry the token
counter value, as reported by the OTP token. Use of this element
is OPTIONAL but it MAY be used by a client to simplify the OTP
calculations of the KDC. It is RECOMMENDED that the KDC act upon
this value if it is present in the request and it is capable of
using it in the generation of the OTP value.
otp-format
This field MAY be used by the client to send the format of the
generated OTP as reported by the OTP token. Use of this element
is OPTIONAL but it MAY be used by the client to simplify the OTP
calculation. It is RECOMMENDED that the KDC act upon this value
if it is present in the request and it is capable of using it in
the generation of the OTP value.
otp-keyID
This field MAY be used by the client to send the identifier of the
token key used, as reported by the OTP token. Use of this field
is OPTIONAL but MAY be used by the client to simplify the
authentication process by identifying a particular token key
associated with the user. It is RECOMMENDED that the KDC act upon
this value if it is present in the request and it is capable of
using it in the generation of the OTP value.
otp-algID
This field MAY be used by the client to send the identifier of the
OTP algorithm used, as reported by the OTP token. Use of this
element is OPTIONAL but it MAY be used by the client to simplify
the OTP calculation. It is RECOMMENDED that the KDC act upon this
value if it is present in the request and it is capable of using
it in the generation of the OTP value.
4.3. PA-OTP-CONFIRM
The padata-type PA-OTP-CONFIRM is returned by the KDC in the enc-
fast-rep of a PA-FX-FAST-REPLY in the AS-REP of the KDC. It is used
to return the client's nonce encrypted under the new Reply Key in
order to authenticate the KDC and confirm the Reply Key change.
Richards Expires April 12, 2010 [Page 21]
Internet-Draft OTP Pre-authentication October 2009
The corresponding padata-value field contains the DER encoding of a
PA-OTP-CONFIRM.
PA-OTP-CONFIRM 143
PA-OTP-CONFIRM ::= SEQUENCE {
encData [0] EncryptedData,
-- PA-OTP-ENC-CONFIRM
-- Key usage of KEY_USAGE_OTP_CONFIRM
...
}
KEY_USAGE_OTP_CONFIRM 46
PA-OTP-ENC-CONFIRM ::= SEQUENCE {
nonce [0] OCTET STRING,
...
}
encData
An EncryptedData containing a PA-OTP-ENC-CONFIRM containing the
value of the nonce from the corresponding PA-OTP-REQUEST encrypted
under the current Reply Key. The key usage SHALL be
KEY_USAGE_OTP_CONFIRM and the encryption type SHOULD be the same
as that used by the client in the PA-OTP-REQUEST.
4.4. PA-OTP-PIN-CHANGE
The padata-type PA-OTP-PIN-CHANGE is returned by the KDC in the enc-
fast-rep of a PA-FX-FAST-REPLY in the AS-REP if the user must change
their PIN, if the user's PIN has been changed or to notify the user
of the PIN's expiry time.
The corresponding padata-value field contains the DER encoding of a
PA-OTP-PIN-CHANGE.
Richards Expires April 12, 2010 [Page 22]
Internet-Draft OTP Pre-authentication October 2009
PA-OTP-PIN-CHANGE 144
PA-OTP-PIN-CHANGE ::= SEQUENCE {
flags [0] PinFlags,
pin [1] UTF8String OPTIONAL,
minLength [2] INTEGER OPTIONAL,
maxLength [3] INTEGER OPTIONAL,
last-req [4] LastReq OPTIONAL,
format [5] OTPFormat OPTIONAL,
...
}
PinFlags ::= KerberosFlags
-- reserved(0),
-- systemSetPin(1),
-- mandatory(2),
flags
The "systemSetPin" flag is used to indicate the type of PIN change
that is taking place. If the flag is set then the user's PIN has
been changed for the user by the system. If the flag is not set
then the user's PIN needs to be changed by the user.
If the "systemSetPin" flag is not set and the "mandatory" flag is
set then user PIN change is required before the next
authentication using the current OTP token. If the "mandatory"
flag is not set then the user PIN change is optional. If the
"systemSetPin" flag is set then the "mandatory" flag has no
meaning and SHOULD be ignored by the client.
pin
The pin field is used to carry a new PIN value. If the
"systemSetPin" flag is set then field is used to carry the new PIN
value set for the user and MUST be present. If the "systemSetPin"
flag is not set then this field MAY be used to carry a system
generated PIN that MAY be used by the user when changing the PIN.
minLength and maxLength
If the "systemSetPin" flag is not set then the minLength and
maxLength fields MAY be included to pass restrictions on the size
of the user selected PIN.
last-req
The last-req element (as defined in section 5.4.2 of [RFC4120])
MAY be included with an lr-type of 6 to carry PIN expiration
information.
Richards Expires April 12, 2010 [Page 23]
Internet-Draft OTP Pre-authentication October 2009
* If the "systemSetPin" flag is set then the expiration time MUST
be that of the new system-set PIN.
* If the "systemSetPin" flag is not set then the expiration time
MUST be that of the current PIN of the token used in the
authentication.
The element MAY also be included with an lr-type of 7 to indicate
when the OTP account will expire.
format
The format element MAY be included by the KDC to carry PIN format
restrictions on the new PIN.
* If the "systemSetPin" flag is set then the element MUST
describe the format of the new system-generated PIN.
* If the "systemSetPin" flag is not set then the element MUST
describe restrictions on any new user generated PIN.
5. IANA Considerations
A registry will be required for the URIs to be used as otp-algID
values as introduced in Section 4.1. It is currently anticipated
that the registry being introduced in section 12.4 of [HoPeMa09] can
be used for this purpose and no other IANA actions are anticipated.
6. Security Considerations
6.1. Man-in-the-Middle
In the system described in this document, the OTP pre-authentication
protocol is tunneled within the FAST Armor channel provided by the
pre-authentication framework. As described in [AsNiNy02], tunneled
protocols are potentially vulnerable to man-in-the-middle attacks if
the outer tunnel is compromised and it is generally considered good
practice in such cases to bind the inner encryption to the outer
tunnel.
In order to mitigate against such attacks, the proposed system uses
the outer Armor Key in the derivation of the inner Client and Reply
keys and so achieve crypto-binding to the outer channel.
As described in section 6.5 of [ZhHa09], FAST can use an anonymous
TGT obtained using anonymous PKINIT [ZhLe08] [RFC4556] as the Armor
Key. However, the current anonymous PKINIT proposal is open to man-
Richards Expires April 12, 2010 [Page 24]
Internet-Draft OTP Pre-authentication October 2009
in-the-middle attacks since the attacker can choose a session key
such that the session key between the MITM and the real KDC is the
same as the session key between the client and the MITM.
As described in Section 3.6, if the OTP value is not being sent to
the KDC then the Armor Key is used along with the OTP value in the
generation of the Client Key and Reply Key. If the Armor Key is known
then the only entropy remaining in the key generation is provided by
the OTP value. If the OTP algorithm requires that the OTP value be
sent to the KDC then it is sent encrypted within the tunnel provided
by the FAST armor and so is exposed to the attacker if the attacker
has the Armor Key.
It is therefore recommended that anonymous PKINIT not be used with
OTP algorithms that require the OTP value to be sent to the KDC and
that careful consideration be made of the security implications
before it is used with other algorithms such as those with short OTP
values.
Careful consideration should also be made if host key armor is used
to provide the kdc-authentication facility with OTP algorithms where
the OTP value is sent within the otp-value field of the PA-OTP-
REQUEST since compromised host keys would allow an attacker to
impersonate the KDC.
6.2. Reflection
The 4-pass system described above is a challenge-response protocol
and such protocols are potentially vulnerable to reflection attacks.
No such attacks are known at this point but to help mitigate against
such attacks, the system uses different keys to encrypt the client
and server nonces.
6.3. Denial of Service
The protocol supports the use of an iteration count in the generation
of the Client and Reply keys and the client can send the number of
iterations used as part of the PA-OTP-REQUEST. This could open the
KDC up to a denial of service attack if a large value for the
iteration count was specified by the attacker. It is therefore
particularly important that, as described in Section 3.4, the KDC
reject any authentication requests where the iteration count is above
a maximum value specified by local policy.
6.4. Replay
In the 4-pass version of this protocol, the client encrypts a KDC
generated nonce and so replay can be detected by the KDC. The 2-pass
Richards Expires April 12, 2010 [Page 25]
Internet-Draft OTP Pre-authentication October 2009
version of the protocol does not involve a server nonce but the
client instead encrypts a timestamp and so is not protected from
replay in the same way. In the case of time or event-based tokens, a
replayed OTP can be detected by the OTP server since they keep track
of the last used value.
However, OTP servers may not be able to detect replay of OTPs
generated using only a client generated challenge and since the KDC
would not be able to detect replay in 2-pass mode, it is recommended
that the use of OTPs generated from only a client-generated challenge
(that is, not in combination with some other factor such as time)
should not be supported in 2-pass mode.
6.5. Brute Force Attack
A compromised or hostile KDC may be able to obtain the OTP value used
by the client via a brute force attack. If the OTP value is short
then the KDC could iterate over the possible OTP values until a
Client Key is generated that can decrypt the encData sent in the PA-
OTP-REQUEST.
As described in Section 3.6, an iteration count can be used in the
generation of the Client Key and the value of the iteration count can
be controlled by local client policy. Use of this iteration count
can make it computationally infeasible/unattractive for an attacker
to brute-force search for the given OTP within the lifetime of that
OTP.
6.6. FAST Facilities
The secret used to generate the OTP is known only to the client and
the KDC and so successful decryption of the encrypted nonce by the
KDC authenticates the user. If the OTP value is used in the Reply
Key generation then successful decryption of the encrypted nonce by
the client proves that the expected KDC replied. The Reply Key is
replaced by either a key generated from the OTP and Armor Key or by
the Armor Key. This FAST factor therefore provides the following
facilities: client-authentication, replacing-reply-key and, depending
on the OTP algorithm, KDC-authentication.
7. Acknowledgments
Many significant contributions were made to this document by RSA
employees but special thanks go to Magnus Nystrom, John Linn, Richard
Zhang, Piers Bowness, Robert Polansky and Boris Khoutorski.
Many valuable suggestions were also made by members of the Kerberos
Richards Expires April 12, 2010 [Page 26]
Internet-Draft OTP Pre-authentication October 2009
Working Group but special thanks go to Larry Zhu, Jeffrey Hutzelman,
Tim Alsop, Henry Hotz and Nicolas Williams.
I would also like to thank Tim Alsop and Srinivas Cheruku of
CyberSafe for many valuable review comments.
8. References
8.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[X.680] ITU-T, "Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.".
[X.690] ITU-T, "Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).".
[ZhHa09] Zhu, L. and S. Hartman, "A generalized Framework for
Kerberos Pre-Authentication",
draft-ietf-krb-wg-preauth-framework-14 (work in progress),
August 2009.
Richards Expires April 12, 2010 [Page 27]
Internet-Draft OTP Pre-authentication October 2009
8.2. Informative References
[AsNiNy02]
Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
in Tunneled Authentication Protocols", Cryptology ePrint
Archive Report 2002/163, November 2002.
[HoPeMa09]
Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
Key Container", draft-ietf-keyprov-pskc-03 (work in
progress), June 2009.
[HoReNeZo04]
Horstein, K., Renard, K., Neuman, C., and G. Zorn,
"Integrating Single-use Authentication Mechanisms with
Kerberos", draft-ietf-krb-wg-kerberos-sam-03 (work in
progress), July 2004.
[RFC2289] Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-
Time Password System", RFC 2289, February 1998.
[RFC2808] Nystrom, M., "The SecurID(r) SASL Mechanism", RFC 2808,
April 2000.
[RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
O. Ranen, "HOTP: An HMAC-Based One-Time Password
Algorithm", RFC 4226, December 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[ZhLe08] Zhu, L. and P. Leach, "Anonymity Support for Kerberos",
draft-ietf-krb-wg-anon-10 (work in progress),
October 2008.
Appendix A. ASN.1 Module
OTPKerberos
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
KerberosTime, KerberosFlags, EncryptionKey, Int32,
EncryptedData, LastReq
FROM KerberosV5Spec2 {iso(1) identified-organization(3)
dod(6) internet(1) security(5)
Richards Expires April 12, 2010 [Page 28]
Internet-Draft OTP Pre-authentication October 2009
kerberosV5(2) modules(4) krb5spec2(2)}
-- as defined in RFC 4120.
AlgorithmIdentifier
FROM PKIX1Explicit88 { iso (1) identified-organization (3)
dod (6) internet (1)
security (5) mechanisms (5) pkix (7)
id-mod (0) id-pkix1-explicit (18) };
-- As defined in RFC 3280.
PA-OTP-CHALLENGE ::= SEQUENCE {
flags [0] ChallengeFlags,
nonce [1] OCTET STRING,
supportedHashAlg [2] SEQUENCE OF AlgorithmIdentifier
OPTIONAL,
iterationCount [3] Int32 OPTIONAL,
otp-service [4] UTF8String OPTIONAL,
otp-keyInfo [5] SEQUENCE OF OTP-KEYINFO OPTIONAL,
...
}
OTP-KEYINFO ::= SEQUENCE {
flags [0] OTPFlags,
otp-vendor [1] UTF8String OPTIONAL,
otp-challenge [2] OCTET STRING (SIZE(8..MAX)) OPTIONAL,
otp-length [3] Int32 OPTIONAL,
otp-keyID [4] OCTET STRING OPTIONAL,
otp-algID [5] AnyURI OPTIONAL,
...
}
ChallengeFlags ::= KerberosFlags
-- reserved(0),
-- must-encrypt-nonce (1)
OTPFlags ::= KerberosFlags
-- reserved(0),
-- nextOTP(1),
-- combine(2),
-- pin-required(3),
-- pin-not-required(4)
PA-OTP-REQUEST ::= SEQUENCE {
flags [0] OTPFlags,
nonce [1] OCTET STRING,
encData [2] EncryptedData,
-- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
-- Key usage of KEY_USAGE_OTP_REQUEST
hashAlg [3] AlgorithmIdentifier OPTIONAL,
Richards Expires April 12, 2010 [Page 29]
Internet-Draft OTP Pre-authentication October 2009
iterationCount [4] Int32 OPTIONAL,
otp-value [5] OCTET STRING OPTIONAL,
otp-challenge [6] OCTET STRING (SIZE(8..MAX)) OPTIONAL,
otp-time [7] KerberosTime OPTIONAL,
otp-counter [8] OCTET STRING OPTIONAL,
otp-format [9] OTPFormat OPTIONAL,
otp-keyID [10] OCTET STRING OPTIONAL,
otp-algID [11] AnyURI OPTIONAL,
...
}
PA-OTP-ENC-REQUEST ::= SEQUENCE {
nonce [0] OCTET STRING,
...
}
OTPFormat ::= INTEGER {
decimal(0),
hexadecimal(1),
alphanumeric(2),
binary(3)
}
PA-OTP-CONFIRM ::= SEQUENCE {
encData [0] EncryptedData,
-- PA-OTP-ENC-CONFIRM
-- Key usage of KEY_USAGE_OTP_CONFIRM
...
}
PA-OTP-ENC-CONFIRM ::= SEQUENCE {
nonce [0] OCTET STRING,
...
}
PA-OTP-PIN-CHANGE ::= SEQUENCE {
flags [0] PinFlags,
pin [1] UTF8String OPTIONAL,
minLength [2] INTEGER OPTIONAL,
maxLength [3] INTEGER OPTIONAL,
last-req [4] LastReq OPTIONAL,
format [5] OTPFormat OPTIONAL,
...
}
PinFlags ::= KerberosFlags
-- reserved(0),
-- systemSetPin(1),
Richards Expires April 12, 2010 [Page 30]
Internet-Draft OTP Pre-authentication October 2009
-- mandatory(2)
AnyURI ::= UTF8String
(CONSTRAINED BY {
/* MUST be a valid URI in accordance with IETF RFC 2396 */
})
END
Appendix B. Examples of OTP Pre-Authentication Exchanges
This section is non-normative.
B.1. Four Pass Authentication
In this mode, the client sends an initial AS-REQ to the KDC that does
not contain a PA-OTP-REQUEST and the KDC responds with a KRB-ERROR
containing a PA-OTP-CHALLENGE.
In this example, the user has been issued with a connected, time-
based token and the KDC requires hashed OTP values in the key
generation with SHA-384 as the preferred hash algorithm and a minimum
of 1024 iterations. The local policy on the client supports SHA-256
and requires 100,000 iterations of the hash of the OTP value.
The basic sequence of steps involved is as follows:
1. The client obtains the user name of the user.
2. The client sends an initial AS-REQ to the KDC that does not
contain a PA-OTP-REQUEST.
3. The KDC determines that the user identified by the AS-REQ
requires OTP authentication.
4. The KDC constructs a PA-OTP-CHALLENGE as follows:
nonce
A randomly generated value.
supportedHashAlg
AlgorithmIdentifiers for SHA-384, SHA-256 and SHA-1
Richards Expires April 12, 2010 [Page 31]
Internet-Draft OTP Pre-authentication October 2009
iterationCount
1024
otp-service
A string that can be used by the client to assist the user in
locating the correct token.
5. The KDC returns a KRB-ERROR with an error code of
KDC_ERR_PREAUTH_REQUIRED and the PA-OTP-CHALLENGE in the e-data.
6. The client displays the value of otp-service and prompts the
user to connect the token.
7. The client obtains the current OTP value from the token and
records the time as reported by the token.
8. The client generates Client Key and Reply Key as described in
Section 3.6 using SHA-256 from the list of algorithms sent by
the KDC and the iteration count of 100,000 as required by local
policy.
9. The client constructs a PA-OTP-REQUEST as follows:
flags
0
nonce
A randomly generated value.
encData
An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
under the Client Key with a key usage of
KEY_USAGE_OTP_REQUEST and the encryption type of the Armor
Key. The PA-OTP-ENC-REQUEST contains the nonce from the PA-
OTP-CHALLENGE.
hashAlg
SHA-256
iterationCount
100,000
otp-time
The time used in the OTP calculation as reported by the OTP
token.
Richards Expires April 12, 2010 [Page 32]
Internet-Draft OTP Pre-authentication October 2009
10. The client encrypts the PA-OTP-REQUEST within the enc-fast-req
of a PA-FX-FAST-REQUEST.
11. The client sends an AS-REQ to the KDC containing the PA-FX-FAST-
REQUEST within the padata.
12. The KDC validates the pre-authentication data in the PA-OTP-
REQUEST:
* Generates the Client Key and Reply Key from the OTP value for
the user identified in the AS-REQ, using an iteration count
of 100,000 and hash algorithm of SHA-256 as specified in the
PA-OTP-REQUEST.
* Uses the generated Client Key to decrypt the PA-OTP-ENC-
REQUEST in the encData of the PA-OTP-REQUEST.
* Authenticates the user by comparing the nonce value from the
decrypted PA-OTP-ENC-REQUEST to that sent in the
corresponding PA-OTP-CHALLENGE.
13. The KDC constructs a TGT for the user.
14. The KDC constructs a PA-OTP-CONFIRM as follows:
encData
An EncryptedData containing a PA-OTP-ENC-CONFIRM encrypted
under the Reply Key with a key usage of KEY_USAGE_OTP_CONFIRM
and the same encryption type as used by the client in the PA-
OTP-REQUEST. The PA-OTP-ENC-CONFIRM contains the nonce from
the PA-OTP-REQUEST.
15. The KDC encrypts the PA-OTP-CONFIRM within the enc-fast-rep of a
PA-FX-FAST-REPLY.
16. The KDC returns an AS-REP to the client, encrypted using the
Reply Key, containing the TGT and padata with the PA-FX-FAST-
REPLY.
17. The client authenticates the KDC and verifies the Reply Key
change.
* Uses the generated Reply Key to decrypt the PA-OTP-ENC-
CONFIRM in the encData of the PA-OTP-CONFIRM.
* Authenticates the KDC by comparing the nonce value from the
decrypted PA-OTP-ENC-CONFIRM to that sent in the
corresponding PA-OTP-REQUEST.
Richards Expires April 12, 2010 [Page 33]
Internet-Draft OTP Pre-authentication October 2009
B.2. Two Pass Authentication
In this mode, the client includes a PA-OTP-REQUEST within a PA-FX-
FAST-REQUEST pre-auth of the initial AS-REQ sent to the KDC.
In this example, the user has been issued with a hand-held token and
so none of the OTP generation parameters (otp-length etc) are
included in the PA-OTP-REQUEST. The KDC does not require hashed OTP
values in the key generation.
It is assumed that the client has been configured with the following
information or has obtained it from a previous PA-OTP-CHALLENGE.
o The encryption type of aes128-cts-hmac-sha1-96 to use to encrypt
the encData.
o The fact that hashed OTP values are not required.
The basic sequence of steps involved is as follows:
1. The client obtains the user name and OTP value from the user.
2. The client generates Client Key and Reply Key using unhashed OTP
values as described in Section 3.6.
3. The client constructs a PA-OTP-REQUEST as follows:
flags
0
nonce
A randomly generated value.
encData
An EncryptedData containing a PA-ENC-TS-ENC encrypted under
the Client Key with a key usage of KEY_USAGE_OTP_REQUEST and
an encryption type of aes128-cts-hmac-sha1-96. The PA-ENC-
TS-ENC contains the current client time.
4. The client encrypts the PA-OTP-REQUEST within the enc-fast-req
of a PA-FX-FAST-REQUEST.
5. The client sends an AS-REQ to the KDC containing the PA-FX-FAST-
REQUEST within the padata.
6. The KDC validates the pre-authentication data:
* Generates the Client Key and Reply Key from the unhashed OTP
value for the user identified in the AS-REQ.
Richards Expires April 12, 2010 [Page 34]
Internet-Draft OTP Pre-authentication October 2009
* Uses the generated Client Key to decrypt the PA-ENC-TS-ENC in
the encData of the PA-OTP-REQUEST.
* Authenticates the user using the timestamp in the standard
manner.
7. The KDC constructs a TGT for the user.
8. The KDC constructs a PA-OTP-CONFIRM as follows:
encData
An EncryptedData containing a PA-OTP-ENC-CONFIRM encrypted
under the Reply Key with a key usage of KEY_USAGE_OTP_CONFIRM
and an encryption type of aes128-cts-hmac-sha1-96 (the
encryption type used by the client in the PA-OTP-REQUEST).
The PA-OTP-ENC-CONFIRM contains the nonce from the PA-OTP-
REQUEST.
9. The KDC encrypts the PA-OTP-CONFIRM within the enc-fast-rep of a
PA-FX-FAST-REPLY.
10. The KDC returns an AS-REP to the client, encrypted using the
Reply Key, containing the TGT and padata with the PA-FX-FAST-
REPLY.
11. The client authenticates the KDC and verifies the key change.
* Uses the generated Reply Key to decrypt the PA-OTP-ENC-
CONFIRM in the encData of the PA-OTP-CONFIRM.
* Authenticates the KDC by comparing the nonce value from the
decrypted PA-OTP-ENC-CONFIRM to that sent in the
corresponding PA-OTP-REQUEST.
B.3. PIN Change
This exchange follows from the point where the KDC receives the PA-
OTP-REQUEST from the client in the examples in Appendix B.1 and
Appendix B.2. During the validation of the pre-authentication data
(whether encrypted nonce or encrypted timestamp), the KDC determines
that the user's PIN has expired and so must be changed. The KDC
therefore includes a PA-OTP-PIN-CHANGE along with the PA-OTP-CONFIRM
in the AS-REP.
In this example, the KDC does not generate PIN values for the user
but requires that the user generate a new PIN that is between 4 and 8
characters in length. The actual PIN change is handled by a PIN
change service.
Richards Expires April 12, 2010 [Page 35]
Internet-Draft OTP Pre-authentication October 2009
The basic sequence of steps involved is as follows:
1. The client constructs and sends a PA-OTP-REQUEST to the KDC as
described in the previous examples.
2. The KDC validates the pre-authentication data and authenticates
the user as in the previous examples but determines that the
user's PIN has expired.
3. KDC constructs a ticket for a PIN change service with a one
minute lifetime.
4. KDC constructs a PA-OTP-CONFIRM as in the previous examples.
5. KDC constructs a PA-OTP-PIN-CHANGE as follows:
flags
0
minLength
4
maxLength
8
6. KDC encrypts the PA-OTP-PIN-CHANGE and PA-OTP-CONFIRM within the
enc-fast-rep of a PA-FX-FAST-REPLY.
7. KDC returns an AS-REP to the client containing the ticket to the
PIN change service and padata containing the PA-FX-FAST-REPLY.
8. The client authenticates the KDC as in the previous examples.
9. The client uses the ticket in the AS-REP to call the PIN change
service and change the user's PIN.
10. The client sends a second AS-REQ to the KDC containing a PA-OTP-
REQUEST constructed using the new PIN.
11. The KDC responds with an AS-REP containing a TGT and a PA-OTP-
CONFRIM.
B.4. Resynchronization
This exchange follows from the point where the KDC receives the PA-
OTP-REQUEST from the client. During the validation of the pre-
authentication data (whether encrypted nonce or encrypted timestamp),
Richards Expires April 12, 2010 [Page 36]
Internet-Draft OTP Pre-authentication October 2009
the KDC determines that the local record of the token's state needs
to be re-synchronized with the token. The KDC therefore includes a
KRB-ERROR containing a PA-OTP-CHALLENGE with the "nextOTP" flag set.
The sequence of steps below follows is a variation of the four pass
examples shown in Appendix B.1 but the same process would also work
in the two-pass case.
1. The client constructs and sends a PA-OTP-REQUEST to the KDC as
described in the previous examples.
2. The KDC validates the pre-authentication data and authenticates
the user as in the previous examples but determines that user's
token requires re-synchronizing.
3. KDC constructs a PA-OTP-CHALLENGE as follows:
nonce
A randomly generated value.
supportedHashAlg
AlgorithmIdentifiers for SHA-256 and SHA-1
iterationCount
1024
otp-service
Set to a string that can be used by the client to assist the
user in locating the correct token.
otp-keyInfo
Information about how the OTP should be generated from the
token.
flags
nextOTP bit set
4. KDC returns a KRB-ERROR with an error code of
KDC_ERR_PREAUTH_REQUIRED and the PA-OTP-CHALLENGE in the e-data.
5. The client obtains the next OTP value from the token and records
the time as reported by the token.
6. The client generates Client Key Reply Key as described in
Section 3.6 using SHA-256 from the list of algorithms sent by
the KDC and the iteration count of 100,000 as required by local
policy.
Richards Expires April 12, 2010 [Page 37]
Internet-Draft OTP Pre-authentication October 2009
7. The client constructs a PA-OTP-REQUEST as follows:
flags
nextOTP bit set
nonce
A randomly generated value.
encData
An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
under the Client Key with a key usage of
KEY_USAGE_OTP_REQUEST and the encryption type of the Armor
Key. The PA-OTP-ENC-REQUEST contains the nonce from the PA-
OTP-CHALLENGE.
hashAlg
SHA-256
iterationCount
100,000
otp-time
The time used in the OTP calculation as reported by the OTP
token.
8. The client encrypts the PA-OTP-REQUEST within the enc-fast-req
of a PA-FX-FAST-REQUEST.
9. The client sends an AS-REQ to the KDC containing the PA-FX-FAST-
REQUEST within the padata.
10. Authentication process now proceeds as with the standard
sequence.
Author's Address
Gareth Richards
RSA, The Security Division of EMC
RSA House
Western Road
Bracknell, Berkshire RG12 1RT
UK
Email: gareth.richards@rsa.com
Richards Expires April 12, 2010 [Page 38]
