Network Working Group D. Kuegler
Internet-Draft Bundesamt fuer Sicherheit in der
Intended status: Experimental Informationstechnik (BSI)
Expires: January 2, 2011 Y. Sheffer
Independent
July 1, 2010
Password Authenticated Connection Establishment with IKEv2
draft-kuegler-ipsecme-pace-ikev2-01
Abstract
IKEv2 does not allow secure peer authentication when using short
credential strings, i.e. passwords. Several proposals have been made
to integrate password-authentication protocols into IKE. This
document provides an adaptation of PACE (Password Authenticated
Connection Establishment) to the setting of IKEv2 and demonstrates
the advantages of this integration.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 2, 2011.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 4
3.1. The IKE_SA_INIT Exchange . . . . . . . . . . . . . . . . . 5
3.2. The IKE_PACE Exchange . . . . . . . . . . . . . . . . . . 6
3.3. The IKE_PACE_AUTH Exchange . . . . . . . . . . . . . . . . 6
4. Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Diffie Hellman . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Elliptic Curve Diffie Hellman . . . . . . . . . . . . . . 8
4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Password Processing . . . . . . . . . . . . . . . . . . . 8
5.2. The PACE_SUPPORTED Notification . . . . . . . . . . . . . 8
5.3. The ENONCE Payload . . . . . . . . . . . . . . . . . . . . 9
5.4. The PKE (PKEi/PKEr) Payloads . . . . . . . . . . . . . . . 9
5.5. PACE and Session Resumption . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6.1. Credential Security Assumptions . . . . . . . . . . . . . 10
6.2. Vulnerability to Passive and Active Attacks . . . . . . . 10
6.3. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 10
6.4. Identity Protection . . . . . . . . . . . . . . . . . . . 10
6.5. Denial of Service . . . . . . . . . . . . . . . . . . . . 10
6.6. Choice of Encryption Algorithms . . . . . . . . . . . . . 10
6.7. Security Model and Security Proof . . . . . . . . . . . . 10
6.8. Long-Term Credential Storage . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Protocol Selection Criteria . . . . . . . . . . . . . 12
A.1. Security Criteria . . . . . . . . . . . . . . . . . . . . 12
A.2. Intellectual Property Criteria . . . . . . . . . . . . . . 12
A.3. Miscellaneous Criteria . . . . . . . . . . . . . . . . . . 12
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
PACE [TR03110] is a security protocol that establishes a mutually
authenticated (and encrypted) channel between two parties based on
weak (short) passwords. PACE provides strong session keys that are
independent of the strength of the password. This draft describes
the integration of PACE into IKEv2 ([RFC4306] and
[I-D.ietf-ipsecme-ikev2bis]) as a new authentication mode, analogous
to the existing certificate and PSK authentication modes.
Compared to other protocols aiming at similar goals, PACE has several
advantages. PACE was designed to be free of patents, and to allow
for a high level of flexibility with respect to cryptographic
algorithms, e.g. it can be implemented based on standard Diffie
Hellman as well as Elliptic Curve Diffie Hellman without any
restrictions on the mathematic group to be used other than the
requirement that the group is cryptographically secure. The protocol
itself is also proven to be cryptographically secure [PACEsec].
The integration aims at keeping as much as possible of IKEv2
unchanged, i.e. the mechanisms used to establish session keys as
provided by IKEv2 are completely maintained.
NOTE: Due to the adaptations of the original protocol [TR03110], the
proof [PACEsec] requires some modifications, that will be provided
once the details of the integration are fixed.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Overview
The following notation is used in this draft:
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E() Symmetric encryption
D() Symmetric decryption
KA() Key agreement
Map() Mapping function
Pwd Shared Password
KPwd Symmetric key derived from a password Pwd
G Static group generator
GE Ephemeral group generator
ENONCE Encrypted nonce
PKEi Ephemeral public key of the initiator
SKEi Ephemeral secret key of the initiator
PKEr Ephemeral public key of the responder
SKEr Ephemeral secret key of the responder
AUTH Authentication token
At a high level the following steps are performed by the initiator
and the responder. They result in exchanges IKE_PACE and
IKE_PACE_AUTH as described in Section 3 that are performed directly
after IKE_SA_INIT and fully replace IKE_AUTH.
1. The initiator randomly and uniformly chooses a nonce, encrypts
the nonce, and sends the ciphertext ENONCE to the responder.
2. The responder recovers the plaintext nonce with the help of the
shared password Pwd.
3. The initiator and the responder perform the following steps:
A. A mapping function Map() is used to derive an ephemeral
generator GE = Map(G,s) from the exchanged nonce s and the
generator G of the used group.
B. They perform an anonymous Diffie-Hellman key agreement based
on the ephemeral generator and compute the shared secret
PACESharedSecret = KA(SKEi, PKEi, GE) = KA(SKEr, PKEr,
GE).
C. They generate, exchange, and verify the authentication token
AUTH using the shared secret PACESharedSecret.
3. Protocol Sequence
The protocol consists of three exchanges, IKE_SA_INIT, IKE_PACE, and
IKE_PACE_AUTH as follows:
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Initiator Responder
--------- ---------
IKE_SA_INIT:
HDR, SAi1, KEi, Ni, N(PACE_SUPPORTED) ->
<- HDR, SAr1, KEr, Nr, N(PACE_SUPPORTED)
IKE_PACE:
HDR, SK{IDi, [IDr,], SAi2,
TSi, TSr, ENONCE, PKEi} ->
<- HDR, SK{IDr, PKEr}
IKE_PACE_AUTH:
HDR, SK{AUTH} ->
<- HDR, SK{AUTH, SAr2, TSi, TSr}
3.1. The IKE_SA_INIT Exchange
The initiator sends a PACE_SUPPORTED notification to indicate its
support of this extension, and its wish to authenticate using a
password. If the responder accepts, it responds with the same
notification. Otherwise, it omits the notification to indicate a
preference for a regular IKE exchange. In the case of anti-DOS
cookies (Sec. 2.6 of [RFC4306]), the notification MUST be resent by
each peer every time it sends its IKE_SA_INIT message.
If PACE is supported the algorithms negotiated in SAi1 and SAr1 are
also used for the execution of PACE, i.e. the key agreement protocol
(standard Diffie Hellman or Elliptic Curve Diffie Hellman), the group
to be used, and the authentication algorithm.
In addition, a new transform type is used to negotiate the password-
based encryption of the nonce. This transform includes the cipher
and its mode of operation as well as the bit length of the nonce to
be used.
NOTE: The password-based encryption MUST NOT introduce any redundancy
that allows for excluding possible plaintexts with a brute-force
attack on the password.
An example for a suitable password based encryption is a block cipher
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in ECB mode using key KPwd = prf+(Pwd, "PACE Password") which is
derived from the shared password Pwd. In this case the size of the
nonce SHALL be the block size of the cipher.
3.2. The IKE_PACE Exchange
This new exchange (number TBD by IANA) is the first part of the PACE
authentication of the peers.
The initiator selects a nonce s as binary bit string. The nonce MUST
be chosen randomly and uniformly, the length of the nonce SHALL be
according to the negotiated value. The nonce is encrypted to ENONCE
= E(Pwd, s) using the negotiated password based-encryption using the
password Pwd.
The initiator maps the nonce to an ephemeral generator of the group
as described in Section 4, chooses randomly and uniformly an
ephemeral key pair (SKEi,PKEi) based on the ephemeral generator and
finally generates the payloads ENONCE containing the encrypted nonce
and PKEi containing the ephemeral public key.
The responder decrypts the received encrypted nonce s = D(Pwd,
ENONCE), performs the mapping and randomly and uniformly chooses an
ephemeral key pair (SKEr,PKEr) based on the ephemeral generator. The
responder generates the PKEr payload containing the ephemeral public
key.
During the Diffie-Hellman key agreement, each party MUST check that
the two public keys PKEi and PKEr differ. Otherwise, it MUST abort
the protocol.
The IKE_PACE request is equivalent to the IKE_AUTH request in a
normal IKEv2 exchange, i.e. any payload which is valid in an IKE_AUTH
request is valid (with the same semantics) in the IKE_PACE request.
In particular, certificate-related payloads are allowed, even though
their use may not be practical within this mode.
3.3. The IKE_PACE_AUTH Exchange
This new exchange (number TBD by IANA) is the second part of the PACE
authentication of the peers.
The initiator and the responder calculate the shared secret
PACESharedSecret, derive the authentication key, and calculate the
authentication token AUTH.
The initiator calculates:
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AUTHi = prf(prf+(PACESharedSecret, "PACE Authentication i" | IDr |
IDi | PKEr), <InitiatorSignedOctets>)
See Sec. 2.15 of [RFC4306] (further clarified in Sec. 2.15 of
[I-D.ietf-ipsecme-ikev2bis]) for the definition of signed octets.
The responder calculates:
AUTHr = prf(prf+(PACESharedSecret, "PACE Authentication r" | IDi |
IDr | PKEi), <ResponderSignedOctets>)
The authentication tokens are then exchanged and each of them MUST be
verified by the other party. The behavior when this verification
fails is unchanged from [RFC4306].
Provided authentication was successful, the IKE_PACE_AUTH response is
equivalent to the IKE_AUTH response in a normal IKEv2 exchange, i.e.
any payload which is valid in an IKE_AUTH response is valid (with the
same semantics) in the IKE_PACE_AUTH response.
Following authentication, all temporary values MUST be deleted by the
peers, including in particular s, the ephemeral generator and public
keys, and PACESharedSecret.
4. Mapping
The mapping is based on a second anonymous Diffie-Hellman key
agreement protocol to create a shared secret which is used together
with the exchanged nonce to calculate a common secret generator of
the group.
While in [TR03110] the generation of the shared secret is part of the
mapping, in the setting of IKEv2 a shared secret SASharedSecret has
already been generated as part of the IKE_SA_INIT step. Using the
notation of [RFC4306],
SASharedSecret = g^ir
Let G and GE be the generator of the negotiated DH group, and the
calculated ephemeral generator, respectively.
4.1. Diffie Hellman
The function Map:G->GE is defined as GE = G^s * SASharedSecret.
Note that the protocol will fail if G^s = 1/SASharedSecret. If s is
chosen randomly, this event occurs with negligible probability. In
implementations that detect such a failure, the initiator SHOULD
choose s again.
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4.2. Elliptic Curve Diffie Hellman
The function Map:G->GE is defined as GE = s*G + SASharedSecret.
Note that the protocol will fail if s*G = -SharedSecret. If s is
chosen randomly, this event occurs with negligible probability. In
implementations that detect such a failure, the initiator SHOULD
choose s again.
4.3. Validation
Implementations MUST verify that the shared secrets SASharedSecret
and PACESharedSecret are elements of the group generated by G to
prevent small subgroup attacks.
It is RECOMMENDED to use the public key validation method (or an
Elliptic Curve equivalent) described in Section 2.1.5 of [RFC2631].
For Elliptic Curves compatible cofactor multiplication [TR03111] MAY
be used instead of public key validation. In this case
implementations MUST check that PACESharedSecret is not the point at
infinity.
Any failure in the validation SHALL be interpreted as an attack.
5. Protocol Details
5.1. Password Processing
The input password string SHOULD be processed according to the rules
of the [RFC4013] profile of [RFC3454]. A password SHOULD be
considered a "stored string" per [RFC3454] and unassigned code points
are therefore prohibited. The output is the binary representation of
the processed UTF-8 character string. Prohibited output and
unassigned codepoints encountered in SASLprep preprocessing SHOULD
cause a preprocessing failure and the output SHOULD NOT be used.
5.2. The PACE_SUPPORTED Notification
This protocol defines a new PACE_SUPPORTED notification, with type
number TBD by IANA. This is an empty notification: The Protocol ID
and SPI size fields are set to zero, and there is no additional data
associated with this notification.
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5.3. The ENONCE Payload
This protocol defines a new ENONCE (encrypted nonce) payload, with
payload type TBD by IANA. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (optional, length is block size for encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Nonce ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ENONCE Payload Structure
The length of the nonce MUST be an integer multiple of the selected
encryption algorithm's block size (if any). Note that the payload
format does not support any padding of the encrypted data.
5.4. The PKE (PKEi/PKEr) Payloads
These payloads have an identical format to the IKEv2 KE payload.
However, this protocol defines a new payload type named PKE (Public
Key - Ephemeral), whose value is TBD by IANA.
5.5. PACE and Session Resumption
A session resumption [RFC5723] ticket may be requested during the
IKE_PACE/IKE_PACE_AUTH exchanges. The request MUST be sent in the
IKE_PACE request, and any response MUST be sent in the IKE_PACE_AUTH
response.
PACE should be considered an "authentication method", in the sense of
Sec. 5 of [RFC5723], which means that its use MUST be noted in the
protected ticket.
Note that even if the initial authentication used PACE and its new
exchange types, session resumption will still include the normal
IKE_AUTH exchange.
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6. Security Considerations
TODO: PACE is cryptographically proven secure in [PACEsec]. The
application of PACE in IKEv2 however is however a slightly different
setting that requires a modification of the proof. A new proof will
be provided once the details of the integration are fixed.
What follows is an outline of the security considerations for this
protocol.
6.1. Credential Security Assumptions
6.2. Vulnerability to Passive and Active Attacks
6.3. Perfect Forward Secrecy
6.4. Identity Protection
6.5. Denial of Service
6.6. Choice of Encryption Algorithms
6.7. Security Model and Security Proof
6.8. Long-Term Credential Storage
7. IANA Considerations
IANA is requested to allocate (has allocated) the following values:
o The PACE_SUPPORTED notification type (TBD) from the "IKEv2 Notify
Message Types - Status Types" registry.
o A payload type (TBD) for the Encrypted Nonce (ENONCE) payload from
the "IKEv2 Payload Types" registry.
o A payload type (TBD) for the Ephemeral Public Key (PKE) payload
from the same registry.
o An authentication method (TBD) for Password Authenticated
Connection Establishment (PACE) from the "IKEv2 Authentication
Method" registry.
o An exchange type (TBD) for the IKE_PACE exchange, from the "IKEv2
Exchange Types" registry.
o An exchange type (TBD) for the IKE_PACE_AUTH exchange, from the
same registry.
This document does not define any new registries.
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method",
RFC 2631, June 1999.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
8.2. Informative References
[I-D.harkins-ipsecme-pake-criteria]
Harkins, D., "Password-Based Authentication in IKEv2:
Selection Criteria and Considerations",
draft-harkins-ipsecme-pake-criteria-00 (work in progress),
April 2010.
[I-D.ietf-ipsecme-ikev2bis]
Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol: IKEv2",
draft-ietf-ipsecme-ikev2bis-11 (work in progress),
May 2010.
[PACEsec] "Security Analysis of the PACE Key-Agreement Protocol",
Information Security Conference (ISC) 2009, Lecture Notes
in Computer Science, Volume 5735, pp. 33-48, Springer-
Verlag.", 2009.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
January 2010.
[TR03110] "BSI TR-03110, Advanced Security Mechanisms for Machine
Readable Travel Documents - Extended Access Control (EAC),
Password Authenticated Connection Establishment (PACE),
and Restricted Identification (RI), Version 2.03", 2010.
[TR03111] "BSI TR-03111, "Elliptic Curve Cryptography", Version
1.11", 2009.
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Appendix A. Protocol Selection Criteria
To support the selection of a password-based protocol for inclusion
in IKEv2, a number of criteria are provided in
[I-D.harkins-ipsecme-pake-criteria]. In the following sections,
those criteria are applied to the PACE protocol.
A.1. Security Criteria
SEC1: PACE is a zero knowledge protocol.
SEC2: The protocol supports perfect forward secrecy and is resistant
to replay attacks.
SEC3: The identity protection provided by IKEv2 remains unchanged.
SEC4: Any cryptographically secure Diffie-Hellman group can be used.
SEC5: The protocol is proven secure in the Bellare-Pointcheval-
Rogaway model.
SEC6: Strong session keys are generated.
SEC7: A transform of the password can be used instead of the
password itself.
A.2. Intellectual Property Criteria
IPR1: The first draft of [TR03110] was published on May 21, 2007.
IPR2: BSI has developed PACE aiming to be free of patents. BSI has
not applied for a patent on PACE.
IPR3: The protocol itself is believed to be free of IPR.
A.3. Miscellaneous Criteria
MISC1: One additional exchange is required.
MISC2: The protocol requires the following operations per entity:
* one key derivation from the password,
* one symmetric encryption or decryption,
* one multi-exponentiation for the mapping,
* one exponentiation for the key pair generation,
* one exponentiation for the shared secret calculation, and
* two symmetric authentications (generation and
verification).
MISC3: The performance is independent of the type/size of password.
MISC4: Internationalization of character-based passwords is
supported.
MISC5: The protocol uses the same group as negotiated for IKEv2.
MISC6: The protocol fits into the request/response nature of IKE.
MISC7: The password-based symmetric encryption must be additionally
negotiated.
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MISC8: Neither trusted third parties nor clock synchronization are
required.
MISC9: Only general cryptographic primitives are required.
MISC10: Any secure variant of Diffie Hellman (e.g. standard or
Elliptic Curve) can be used.
MISC11: The protocol can be implemented easily based on existing
cryptographic primitives.
Appendix B. Change Log
Note to RFC Editor: please remove this appendix before publication.
B.1. draft-kuegler-ipsecme-pace-ikev2-01
Formalized the protocol: added payload formats, error behavior etc.
Authors' Addresses
Dennis Kuegler
Bundesamt fuer Sicherheit in der Informationstechnik (BSI)
Postfach 200363
Bonn 53133
Germany
Email: dennis.kuegler@bsi.bund.de
Yaron Sheffer
Independent
Email: yaronf.ietf@gmail.com
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