Network Working Group D. Kuegler
Internet-Draft Bundesamt fuer Sicherheit in der
Intended status: Experimental Informationstechnik (BSI)
Expires: February 18, 2012 Y. Sheffer
Porticor
August 17, 2011
Password Authenticated Connection Establishment with IKEv2
draft-kuegler-ipsecme-pace-ikev2-07
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on February 18, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Kuegler & Sheffer Expires February 18, 2012 [Page 1]
Internet-Draft IKEv2 with PACE August 2011
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Kuegler & Sheffer Expires February 18, 2012 [Page 2]
Internet-Draft IKEv2 with PACE August 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 6
3.1. The IKE_SA_INIT Exchange . . . . . . . . . . . . . . . . . 6
3.2. The IKE_AUTH Exchange, Round #1 . . . . . . . . . . . . . 7
3.3. The IKE_AUTH Exchange, Round #2 . . . . . . . . . . . . . 7
4. Encrypting and Mapping the Nonce . . . . . . . . . . . . . . . 8
4.1. Encrypting the Nonce . . . . . . . . . . . . . . . . . . . 8
4.2. Mapping the Nonce . . . . . . . . . . . . . . . . . . . . 9
4.2.1. MODP Diffie-Hellman . . . . . . . . . . . . . . . . . 10
4.2.2. Elliptic Curve Diffie-Hellman . . . . . . . . . . . . 10
4.2.3. Validation . . . . . . . . . . . . . . . . . . . . . . 10
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Password Processing . . . . . . . . . . . . . . . . . . . 10
5.2. The SECURE_PASSWORD_METHODS Notification . . . . . . . . . 11
5.3. The GSPM(ENONCE) Payload . . . . . . . . . . . . . . . . . 12
5.4. The KE (KEi2/KEr2) Payloads in PACE . . . . . . . . . . . 12
5.5. PACE and Session Resumption . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.1. Credential Security Assumptions . . . . . . . . . . . . . 13
6.2. Vulnerability to Passive and Active Attacks . . . . . . . 13
6.3. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 13
6.4. Randomness . . . . . . . . . . . . . . . . . . . . . . . . 13
6.5. Identity Protection . . . . . . . . . . . . . . . . . . . 14
6.6. Denial of Service . . . . . . . . . . . . . . . . . . . . 14
6.7. Choice of Encryption Algorithms . . . . . . . . . . . . . 14
6.8. Security Model and Security Proof . . . . . . . . . . . . 14
6.9. Long-Term Credential Storage . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Protocol Selection Criteria . . . . . . . . . . . . . 17
A.1. Security Criteria . . . . . . . . . . . . . . . . . . . . 17
A.2. Intellectual Property Criteria . . . . . . . . . . . . . . 17
A.3. Miscellaneous Criteria . . . . . . . . . . . . . . . . . . 18
Appendix B. Password Salting . . . . . . . . . . . . . . . . . . 18
B.1. Solving the Asymmetric Case with Symmetric Cryptography . 19
B.2. Solving the Fully Symmetric Case with Asymmetric
Cryptography . . . . . . . . . . . . . . . . . . . . . . . 19
B.3. Generation of a Long Shared Secret . . . . . . . . . . . . 20
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Kuegler & Sheffer Expires February 18, 2012 [Page 3]
Internet-Draft IKEv2 with PACE August 2011
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. PACE belongs to a
family of protocols often referred to as Zero-Knowledge Password
Proof (ZKPP) protocols, all of which amplify weak passwords into
strong session keys. This draft describes the integration of PACE
into IKEv2 [RFC5996] as a new authentication mode, analogous to the
existing certificate and PSK authentication modes.
Some of the advantages of our approach, compared to the existing
IKEv2, include:
o The current best practice to implement password authentication in
IKE involves certificate-based authentication of the server plus
some EAP method to authenticate the client. This involves two
non-trivial infrastructure components (PKI and EAP/AAA).
Moreover, certificate authentication is hard to get right, and
often depends for its security on unreliable user behavior.
o Alternatively, native IKEv2 shared secret authentication can be
used with passwords. This usage however is insecure, specifically
it is vulnerable to active attackers.
o Some newer EAP methods can be used for mutual authentication, and
combined with [RFC5998] can be well integrated into IKEv2. This
is certainly an option in some cases, but the current proposal may
be simpler to implement.
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 mathematical 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
PACE protocol is currently used in an international standard for
digital travel documents [ICAO].
The integration aims at keeping as much as possible of IKEv2
unchanged, e.g. the mechanisms used to establish Child SAs as
provided by IKEv2 are maintained with no change.
The PAKE Framework document
[I-D.kivinen-ipsecme-secure-password-framework] defines a set of
payloads for different types of PAKE methods within IKEv2. This
document reuses this framework. Note that the current document is
self-contained, i.e. all relevant payloads and semantics are
redefined here.
Kuegler & Sheffer Expires February 18, 2012 [Page 4]
Internet-Draft IKEv2 with PACE August 2011
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 [RFC2119].
The following notation is used in this draft:
E() Symmetric encryption
D() Symmetric decryption
KA() Key agreement
Map() Mapping function
Pwd Shared password
SPwd Stored 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 payload
Any other notation used here is defined in [RFC5996].
2. Overview
At a high level the following steps are performed by the initiator
and the responder. They result in a two-round IKE_AUTH exchange,
described in Section 3 below.
1. The initiator randomly and uniformly chooses a nonce, encrypts
the nonce using the password, 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.
Kuegler & Sheffer Expires February 18, 2012 [Page 5]
Internet-Draft IKEv2 with PACE August 2011
C. They generate, exchange, and verify the authentication token
AUTH using the shared secret PACESharedSecret.
3. Protocol Sequence
The protocol consists of three round trips, IKE_SA_INIT, and a
2-round IKE_AUTH exchange, as follows:
Initiator Responder
--------- ---------
IKE_SA_INIT:
HDR, SAi1, KEi, Ni, N(SECURE_PASSWORD_METHODS) ->
<- HDR, SAr1, KEr, Nr, N(SECURE_PASSWORD_METHODS)
IKE_AUTH round #1:
HDR, SK{IDi, [IDr,], SAi2,
TSi, TSr, GSPM(ENONCE), KEi2} ->
<- HDR, SK{IDr, KEr2}
IKE_AUTH round #2:
HDR, SK{AUTH} ->
<- HDR, SK{AUTH, SAr2, TSi, TSr}
Figure 1: IKE SA Setup with PACE
3.1. The IKE_SA_INIT Exchange
The initiator sends a SECURE_PASSWORD_METHODS notification that
indicates its support of this extension, and its wish to authenticate
using a password. The following text assumes that the responder sent
back a SECURE_PASSWORD_METHODS notification that indicates its
preference for PACE.
If PACE was chosen, 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 encryption algorithm.
Kuegler & Sheffer Expires February 18, 2012 [Page 6]
Internet-Draft IKEv2 with PACE August 2011
3.2. The IKE_AUTH Exchange, Round #1
This is the first part of the PACE authentication of the peers. This
exchange MUST NOT be used unless both peers indicated support of this
protocol.
The initiator selects a random nonce s and encrypts it to form ENONCE
using the password Pwd, as described in Section 4.1. Then the
initiator maps the nonce to an ephemeral generator GE of the group as
described in Section 4.2, chooses randomly and uniformly an ephemeral
key pair (SKEi,PKEi) based on the ephemeral generator and finally
generates the payloads GSPM(ENONCE) containing the encrypted nonce
and KEi2 containing the ephemeral public key.
The responder decrypts the received encrypted nonce s = D(KPwd,
ENONCE), performs the mapping and randomly and uniformly chooses an
ephemeral key pair (SKEr,PKEr) based on the ephemeral generator GE.
The responder generates the KEr2 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 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 this round's request. In
particular, certificate-related payloads are allowed, even though
their use may not be practical within this mode.
3.3. The IKE_AUTH Exchange, Round #2
This is the second part of the PACE authentication of the peers.
The initiator and the responder calculate the shared secret
PACESharedSecret:
PACESharedSecret = KA(SKEi, PKEr, GE) = KA(SKEr, PKEi, GE),
where KA denotes the Diffie-Hellman key agreement, e.g. (for MODP
groups) modular exponentiation. Then they calculate the
authentication tokens AUTHi and AUTHr.
The initiator calculates:
AUTHi = prf(prf+(Ni | Nr, PACESharedSecret),
<InitiatorSignedOctets> | PKEr)
Kuegler & Sheffer Expires February 18, 2012 [Page 7]
Internet-Draft IKEv2 with PACE August 2011
See Sec. 2.15 of [RFC5996] for the definition of signed octets.
The responder calculates:
AUTHr = prf(prf+(Ni | Nr, PACESharedSecret),
<ResponderSignedOctets> | PKEi)
Both AUTH payloads MUST indicate the Secure Password Authentication
Method [I-D.kivinen-ipsecme-secure-password-framework], whose value
is TBD by IANA. The authentication tokens are exchanged and each of
them MUST be verified by the other party. The behavior when this
verification fails is unchanged from [RFC5996].
This round's 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 second round's
response.
Following authentication, all temporary values MUST be deleted by the
peers, including in particular s, the ephemeral generator, the
ephemeral key pairs, and PACESharedSecret.
4. Encrypting and Mapping the Nonce
4.1. Encrypting the Nonce
The shared password is not used as-is. Instead, it SHOULD be
converted into a "stored password" SPwd, so that the plaintext
password does not need to be stored for long periods. SPwd is
defined as:
SPwd = prf("IKE with PACE", Pwd)
where the literal string consists of ASCII characters with no zero
terminator. If the negotiated prf requires a fixed-size key, the
literal string is either truncated or padded with zero octets on the
right, as needed.
KPwd = prf+(Ni | Nr, SPwd)
where Ni and Nr are the regular IKE nonces, stripped of any headers.
If the negotiated prf takes a fixed-length key and the lengths of Ni
and Nr do not add up to that length, half the bits must come from Ni
and half from Nr, taking the first bits of each. "prf+" is defined in
Sec. 2.13 of [RFC5996]. The length of KPwd is determined by the key
length of the negotiated encryption algorithm.
Kuegler & Sheffer Expires February 18, 2012 [Page 8]
Internet-Draft IKEv2 with PACE August 2011
A nonce s is randomly selected by the initiator (see Section 6.4 for
additional considerations). The length of s MUST be exactly 32
octets.
Note: Padding MUST NOT be used when encrypting the nonce. The size
of the nonce has been chosen such that it can be encrypted with block
ciphers having block sizes of 32, 64, and 128 bit without any
padding.
If an authenticated encryption cipher [RFC5282] has been negotiated
for the IKE SA, it MUST NOT be used as-is because such use would be
vulnerable to dictionary attacks. Instead, the corresponding
unauthenticated mode MUST be used. All GCM and all CCM encryption
algorithms are mapped to the corresponding counter-mode algorithm.
For example, if the negotiated encryption algorithm (Transform Type
1) is "AES-GCM with a 8 octet ICV", then ENCR_AES_CTR (with the same
key length) is used to encrypt the nonce. If such a mapping does not
exist for a particular cipher, then it MUST NOT be used within the
current protocol.
KPwd is now used with the encryption transform to encrypt the nonce:
ENONCE = E(KPwd, s)
If an Initialization Vector (IV) is required by the cipher, it MUST
be included in the GSPM(ENONCE) payload. It is RECOMMENDED to choose
the IV randomly and uniformly distributed, even though this condition
is not necessary for the cryptographic security of the protocol.
4.2. Mapping the Nonce
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 [RFC5996],
SASharedSecret = g^ir
Let G and GE be the generator of the negotiated DH group, and the
calculated ephemeral generator, respectively. The following
subsections describe the mapping for different Diffie-Hellman
variants.
Kuegler & Sheffer Expires February 18, 2012 [Page 9]
Internet-Draft IKEv2 with PACE August 2011
4.2.1. MODP 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.
4.2.2. 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.
4.2.3. 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 the
compatible cofactor exponentiation described in Section 3.1 and
Section 3.4, respectively, of [RFC2785]. The Elliptic Curve
equivalents of those methods are described in more detail in
[TR03111].
Any failure in the validation MUST be interpreted as an attack, and
the protocol SHALL be aborted.
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.
Kuegler & Sheffer Expires February 18, 2012 [Page 10]
Internet-Draft IKEv2 with PACE August 2011
5.2. The SECURE_PASSWORD_METHODS Notification
[I-D.kivinen-ipsecme-secure-password-framework] defines a new type of
Notify payload to indicate support for Secure Password Methods (SPM)
in the IKE_SA_INIT exchange. The SPM Notify payload is defined as
follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol ID | SPI Size | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Parameter Index (SPI) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Notification Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SECURE_PASSWORD_METHODS Payload Structure
The Protocol ID is zero, and the SPI Size is also zero, indicating
that the SPI field is empty. The Notify Message Type is
SECURE_PASSWORD_METHODS (value TBD) from the IANA IKEv2 registry.
The Notification Data contains the list of the 16-bit secure password
method numbers:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secure Password Method #1 | Secure Password Method #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secure Password Method #3 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SECURE_PASSWORD_METHODS Payload Data
For the current method, the list of proposed methods MUST include the
value PACE_AUTH_METHOD, which is TBD by IANA.
Kuegler & Sheffer Expires February 18, 2012 [Page 11]
Internet-Draft IKEv2 with PACE August 2011
5.3. The GSPM(ENONCE) Payload
This protocol defines the ENONCE (encrypted nonce) payload, which
reuses the GSPM payload type
[I-D.kivinen-ipsecme-secure-password-framework] (value TBD). 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PACE-RESERVED | Initialization Vector |
+-+-+-+-+-+-+-+-+ +
| (optional, length depends on the encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Nonce ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ENONCE Payload Structure
See Section 4.1 for further details about the encrypted nonce. Note
that the protocol, and in particular this payload's format, does not
support any padding of the encrypted data.
The PACE-RESERVED field must be sent as zero, and must be rejected by
the receiver if it is not 0.
5.4. The KE (KEi2/KEr2) Payloads in PACE
PACE reuses the KE payload for its Diffie-Hellman exchange, with the
new payloads being sent within the IKE_AUTH exchange. Since only one
Diffie-Hellman group is negotiated, the group denoted by these
payloads MUST be identical to the one used in the "regular" KE
payloads in IKE_SA_INIT.
5.5. PACE and Session Resumption
A session resumption [RFC5723] ticket may be requested during the
IKE_AUTH exchange. The request MUST be sent in the request of the
first round, and any response MUST be sent in the response of the
second one.
Kuegler & Sheffer Expires February 18, 2012 [Page 12]
Internet-Draft IKEv2 with PACE August 2011
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. The format of the ticket is not standardized,
however we RECOMMEND that this indication should distinguish between
the different secure password authentication methods defined for IKE.
Note that even if the initial authentication used PACE and its
extended IKE_AUTH, session resumption will still include the normal
IKE_AUTH exchange.
6. Security Considerations
A major goal of this protocol has been to maintain the level of
security provided by IKEv2. What follows is an analysis of this
protocol. The reader is referred to [RFC5996] for the generic IKEv2
security considerations.
6.1. Credential Security Assumptions
This protocol makes no assumption on the strength of the shared
credential. Best common practices regarding minimal password length,
use of multiple character classes etc. SHOULD be followed.
6.2. Vulnerability to Passive and Active Attacks
The protocol is secure against both passive and active attackers.
See Section 6.8 for a security proof.
While not attacking the cryptography, an attacker can still perform a
standard password guessing attack. To mitigate such attacks, an
implementation MUST include standard protections, such as rate
limiting the number of allowed password guessing attempts, possibly
locking identities out after a certain number of failed attempts etc.
Note that the protocol is symmetric and therefore this guidance
applies to client-side implementations as well.
6.3. Perfect Forward Secrecy
The key derivation for the IKE SA and any Child SAs is performed as
part of IKEv2 and remains unchanged. It directly follows that
perfect forward security is provided independent of the
authentication additionally performed by PACE.
6.4. Randomness
The security of this protocol depends on the quality generation of
random quantities, and see Sec. 5 of [RFC5996] for more details.
Kuegler & Sheffer Expires February 18, 2012 [Page 13]
Internet-Draft IKEv2 with PACE August 2011
Specifically, any deviation from randomness of the nonce s might
compromise the password. Therefore, it is strongly RECOMMENDED that
the initiator passes the raw random material through a strong prf to
ensure the statistical qualities of the nonce.
6.5. Identity Protection
This protocol is identical to IKEv2 in the quality of identity
protection it provides. Both peers' identities are secure from
passive attackers, and both peers' identities are exposed to active,
man-in-the-middle attackers.
6.6. Denial of Service
We are not aware of any new denial-of-service attack vector enabled
by this protocol.
6.7. Choice of Encryption Algorithms
Any transforms negotiated for IKEv2 may be used by this protocol.
Please refer to Section 4.1 for the considerations regarding
authenticated encryption ("combined mode") algorithms.
6.8. Security Model and Security Proof
PACE is cryptographically proven secure in [PACEsec] in the model of
Bellare, Pointcheval, and Rogaway [BPRmodel]. The setting in which
PACE is proven secure is however slightly different from the setting
used in IKEv2. The differences are described in the following:
o Part of the mapping is already performed within IKEv2 before PACE
is started. This rearrangement does not affect the proof as the
resulting PACESharedSecret remains close to uniformly distributed
in the group generated by G.
o The keys for the IKE SA and any Child SAs are already generated
within IKEv2 before PACE is started. While those session keys
could also be derived in PACE, only the keys for the
authentication token are considered in the proof, which explicitly
recommends a separate key for this purpose.
o IKEv2 allows the negotiation of a stream cipher for PACE while the
proven variant always uses a block cipher. The ideal cipher is
replaced in the proof by lazy-sampling technique which is
similarly applicable to the stream cipher based construction.
The differences in the setting therefore have no impact on the
validity of the proof.
Kuegler & Sheffer Expires February 18, 2012 [Page 14]
Internet-Draft IKEv2 with PACE August 2011
6.9. Long-Term Credential Storage
This protocol does not require peers to store the plaintext password.
Instead, the value SPwd SHOULD be stored by both peers.
It has been suggested to generate a "long" shared secret after the
initial authentication, such that the peers can use it later for
standard preshared-secret authentication, in lieu of the short
password. This approach is described in more detail in Appendix B.3,
but it is not clear whether its security benefits justify the added
complexity.
The main usage scenario for the current protocol is for
authentication between IKE peers, in a symmetric setting. We believe
that IKEv2 with EAP takes care quite well of the asymmetric, remote
access case. Each of the peers might need to initiate the IKE SA,
which makes traditional password salting less than ideal.
A future version of this protocol might cater to the scenario where
one of the peers is more vulnerable than the other to theft of the
password database, using a salt exchanged in IKE_SA_INIT.
Alternatively, public key techniques can be used to salt the password
differently on each of the peers, however this would introduce
additional computational effort into the protocol. See Appendix B
for a more detailed discussion of potential solutions.
7. IANA Considerations
IANA is requested to allocate (has allocated) the following value:
o A PACE_AUTH_METHOD value from the "IKEv2 Secure Password Methods"
registry [I-D.kivinen-ipsecme-secure-password-framework].
This document does not define any new registries.
8. Acknowledgments
We would like to thank Dan Harkins for pointing out a security issue
with our use of combined-mode algorithms, in a previous version of
the protocol. We thank Tero Kivinen for his generic framework
document, and for a thorough and fruitful review.
9. References
Kuegler & Sheffer Expires February 18, 2012 [Page 15]
Internet-Draft IKEv2 with PACE August 2011
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2785] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
Attacks on the Diffie-Hellman Key Agreement Method for
S/MIME", RFC 2785, March 2000.
[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.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
9.2. Informative References
[BPRmodel]
Bellare, M., Pointcheval, D., and P. Rogaway,
"Authenticated Key Exchange Secure against Dictionary
Attacks, EUROCRYPT 2000, LNCS 1807, pp. 139-155, Springer-
Verlag", 2000.
[I-D.harkins-ipsecme-pake-criteria]
Harkins, D., "Password-Based Authentication in IKEv2:
Selection Criteria and Considerations",
draft-harkins-ipsecme-pake-criteria-01 (work in progress),
October 2010.
[I-D.kivinen-ipsecme-secure-password-framework]
Kivinen, T., "Secure Password Framework for IKEv2",
draft-kivinen-ipsecme-secure-password-framework-01 (work
in progress), July 2011.
[ICAO] ISO/IEC JTC1 SC17 WG3/TF5 for ICAO, "Supplemental Access
Control for Machine Readable Travel Documents, version
1.00", March 2010.
[PACEsec] Bender, J., Fischlin, M., and D. Kuegler, "Security
Analysis of the PACE Key-Agreement Protocol. The extended
abstract appeared in Information Security Conference (ISC)
2009, LNCS 5735, pp. 33-48, Springer-Verlag", 2009.
Kuegler & Sheffer Expires February 18, 2012 [Page 16]
Internet-Draft IKEv2 with PACE August 2011
[RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282,
August 2008.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
January 2010.
[RFC5998] Eronen, P., Tschofenig, H., and Y. Sheffer, "An Extension
for EAP-Only Authentication in IKEv2", RFC 5998,
September 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.
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
Kuegler & Sheffer Expires February 18, 2012 [Page 17]
Internet-Draft IKEv2 with PACE August 2011
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.
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. Password Salting
This protocol requires that passwords should not be stored in
plaintext. Instead, we store a hash of the password with a fixed
hash. This value is then used in the ZKPP protocol, replacing the
original password and acting as a "password equivalent". The main
benefit of this solution is that a system administrator or an
undetermined attacker does not get immediate access to the passwords.
We believe this is sufficiently secure for the main usage scenario of
the protocol.
However the common practice of password salting is clearly more
powerful, and this appendix presents a few ideas on how password
salting can be applied and/or adopted to fit into a symmetric
protocol such as IKE. First, let us list the threats that we expect
salting to handle, as well as the non-threats:
Kuegler & Sheffer Expires February 18, 2012 [Page 18]
Internet-Draft IKEv2 with PACE August 2011
o The plain password should not be visible to a casual onlooker, as
noted above. It is assumed that very often the same password is
used for multiple applications, and so a password exposed allows
an attacker a starting point for further attacks.
o An attacker must not be able to construct lookup tables (such as
the famous "rainbow tables") that enable her to discover the plain
password.
o IKE is a symmetric protocol, in the sense that any of the peers
might initiate an IKE exchange to another peer. As a result all
peers must have stored credentials (passwords or password
equivalents) that would enable them to set up an IKE exchange. So
an attacker that reaches the credential store would in fact be
able to impersonate IKE to another peer. We believe that this
reduces, but does not invalidate the importance of salting,
because of the other threats that remain.
Below we present different scenarios and solutions that support
password salting in this setting.
We assume that each credential is used to authenticate exactly two
peers to one another, i.e. (as per the best practice) group
credentials are not allowed.
B.1. Solving the Asymmetric Case with Symmetric Cryptography
Despite the protocol's symmetry, there are use cases that are
somewhat asymmetric. Consider the case of an organization that
consists of a headquarters and branches, using a hub-and-spoke
architecture. Communication sessions can be initiated by the center
or by any of the branches, but only the center holds a large
credential database.
Here it would be possible to use traditional password salting,
stored password = hash(salt, password),
where the hash function is a symmetric hash (e.g. HMAC-SHA-256), and
the salt is picked at random for each password. The salt would need
to be sent in the first exchange of the protocol, regardless of which
side initiates the session. Unlike the normal use of salted
passwords, here it is the stored password, rather then the original
password, that is used by the follow-on ZKPP protocol.
B.2. Solving the Fully Symmetric Case with Asymmetric Cryptography
For the fully symmetric case, we propose a salting method based on a
commutative one-way function. This is essentially a novel variant of
the RSA protocol.
The implementation proposed here requires a composite number n that
is common to all peers. The composite number n can be either
Kuegler & Sheffer Expires February 18, 2012 [Page 19]
Internet-Draft IKEv2 with PACE August 2011
generated by a trusted (third) party as n = p * q, where p and q are
strong primes (i.e. p = 2 * p' + 1 and q = 2 * q' + 1, where p' and
q' are also primes), and the trusted party promises not to retain a
copy of the primes. Alternatively, n can be chosen randomly and
tested for "small" prime factors. In the latter case it is certainly
not guaranteed that n is composed of only two primes. While this has
the advantage that no one knows the factorization of n, the
disadvantage is that n is likely to be significantly easier to
factor.
Each peer then chooses a public encrytion key e. In a simple
implementation the encryption key is generated randomly by each peer,
picking a different value for each of the passwords that it stores.
Note that although the pair (n,e) is similar to an RSA public key,
the usual rules for generating "e" for the RSA protocol do not apply
here, and a random "e" is sufficient. The password is hashed by a
symmetric hash function H (e.g. SHA-256). Each peer i stores the
two values
e_i, H(P)^e_i (mod n)
where P is the original password. The values e_i are exchanged by
the peers before the ZKPP protocol commences (in IKEv2-PACE, this
would be in IKE_SA_INIT) and the following value is used in the ZKPP
protocol run that follows, in lieu of the original password:
H(P) ^ (e_i * e_j) (mod n).
This transformation is used as a salting mechanism only and the
salted values themselves are never sent on the wire.
This scheme can be enhanced by basing the value "e" on each peer's
identity (IDi, IDr), e.g. making it a simple hash of the identity.
This eliminates the need to send "e" explicitly, and additionally
binds the identity of the peer with its secret.
B.3. Generation of a Long Shared Secret
An alternative to salting is to store the plain passwords, but only
for a short while. As soon as the first IKE SA is set up between two
peers, the peers exchange nonces and generate a long shared secret,
based on IKE's SK_d. They now destroy the short password and replace
it with the new secret.
Such a protocol extension would need to be architected so that a
protocol failure does not result in the two peers losing the ability
to communicate. Schematically, this could be done as follows:
Kuegler & Sheffer Expires February 18, 2012 [Page 20]
Internet-Draft IKEv2 with PACE August 2011
Initiator Responder
--------- ---------
Generates nonce1
---> Nonce1
Generates nonce2
Computes and stores the shared secret
<--- Nonce2
(The above two messages can be piggybacked onto round #2 of IKE_AUTH)
Computes and stores the shared secret
---> Secret_OK
Deletes the short password
<--- Secret_OK
Deletes the short password
Figure 5: Password Hardening
In the above, it is critical that the responder should only store the
shared secret after it has fully authenticated the initiator.
Moreover, this method should be optional, because we do not expect
all IKE implementations to have write-access into their credential
database.
Appendix C. Change Log
Note to RFC Editor: please remove this appendix before publication.
C.1. -07
Adopted the framework proposed in
[I-D.kivinen-ipsecme-secure-password-framework]. However this
document is self-contained. Changed the PKEi/r payloads to reuse the
normal KE payloads; they are disambiguated by the context: which
exchange they are used in. Added an appendix on password salting for
symmetric protocols.
Kuegler & Sheffer Expires February 18, 2012 [Page 21]
Internet-Draft IKEv2 with PACE August 2011
C.2. -06
Defined how authenticated-encryption algorithms can be used. Updated
references.
C.3. -05
Editorial corrections.
C.4. -04
Editorial corrections.
C.5. -03
Completed the security considerations (security proof). Reordered
some sections for clarity.
C.6. -02
Added security considerations. Changed encryption of the nonce.
Simplified the derivation of the AUTH payloads.
C.7. 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
Porticor
Email: yaronf.ietf@gmail.com
Kuegler & Sheffer Expires February 18, 2012 [Page 22]
