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Internet Engineering Task Force (IETF)
Request for Comments: 6631
Category: Experimental
ISSN: 2070-1721
D. Kuegler
BSI
Y. Sheffer
Porticor
June 2012

Password Authenticated Connection Establishment

with the Internet Key Exchange Protocol version 2 (IKEv2)

Abstract

The Internet Key Exchange protocol version 2 (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 Password Authenticated Connection Establishment (PACE) to the setting of IKEv2 and demonstrates the advantages of this integration.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.

This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6631.

Copyright Notice

Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
   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
      3.4. Public Key Validation ......................................8
      3.5. Creating a Long-Term Shared Secret .........................9
      3.6. Using the Long-Term Shared Secret .........................11
   4. Encrypting and Mapping the Nonce ...............................11
      4.1. Encrypting the Nonce ......................................11
      4.2. Mapping the Nonce .........................................12
           4.2.1. Modular Diffie-Hellman .............................13
           4.2.2. Elliptic Curve Diffie-Hellman ......................13
   5. Protocol Details ...............................................13
      5.1. Password Processing .......................................13
      5.2. The SECURE_PASSWORD_METHODS Notification ..................14
      5.3. The PSK_PERSIST Notification ..............................15
      5.4. The PSK_CONFIRM Notification ..............................15
      5.5. The GSPM(ENONCE) Payload ..................................15
      5.6. The KE (KEi2/KEr2) Payloads in PACE .......................16
      5.7. PACE and Session Resumption ...............................16
   6. Security Considerations ........................................16
      6.1. Credential Security Assumptions ...........................16
      6.2. Vulnerability to Passive and Active Attacks ...............16
      6.3. Perfect Forward Secrecy ...................................17
      6.4. Randomness ................................................17
      6.5. Identity Protection .......................................17
      6.6. Denial of Service .........................................17
   
      6.7. Choice of Encryption Algorithms ...........................17
      6.8. Security Model and Security Proof .........................18
      6.9. Long-Term Credential Storage ..............................18
   7. IANA Considerations ............................................19
   8. Acknowledgments ................................................19
   9. References .....................................................19
      9.1. Normative References ......................................19
      9.2. Informative References ....................................20
   Appendix A. Protocol Selection Criteria ...........................22
     A.1. Security Criteria ..........................................22
     A.2. Intellectual Property Criteria .............................22
     A.3. Miscellaneous Criteria .....................................22
   Appendix B. Password Salting ......................................23
     B.1. Solving the Asymmetric Case with Symmetric Cryptography ....24
     B.2. Solving the Fully Symmetric Case with Asymmetric
          Cryptography ...............................................25
     B.3. Generation of a Strong, Long-Term, Shared Secret ...........26

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 document describes the integration of PACE into IKEv2 [RFC5996] as a new authentication mode, analogous to the existing certificate and Pre-Shared Key (PSK) authentication modes.

Some of the advantages of our approach, compared to the existing IKEv2, include the following:

  • The current best practice to implement password authentication in IKE involves certificate-based authentication of the server plus some Extensible Authentication Protocol (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 on unreliable user behavior for its security.
  • Alternatively, native IKEv2 shared secret authentication can be used with passwords. However, this usage is insecure; specifically, it is vulnerable to active attackers.
  • 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 allow for a high level of flexibility with respect to cryptographic algorithms; e.g., it can be implemented based on Modular 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 be 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 IKEv2 unchanged as much as possible; e.g., the mechanisms used to establish Child security associations (SAs) as provided by IKEv2 would be maintained with no change.

The Password-Authenticated Key Exchange (PAKE) framework document [RFC6467] 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.

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 document: 

      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 s, encrypts the nonce using the password, and sends the ciphertext

ENONCE = E(KPwd, s)

to the responder. The responder recovers the plaintext nonce s with the help of the shared password Pwd.

  1. The nonce s is mapped to an ephemeral generator

GE = G^s * SASharedSecret,

where G is the generator of the selected Modular Exponential (MODP) group and SASharedSecret is a shared secret that has been generated in the IKE_SA_INIT step.

  1. Both the initiator and the responder each calculate an ephemeral key pair

(SKEi, PKEi = GE^SKEi) and (SKEr, PKEr=GE^SKEr),

respectively, based on the ephemeral generator GE, and exchange the public keys.

  1. Finally, they compute the shared secret

PACESharedSecret = PKEi^SKEr = PKEr^SKEi

and generate, exchange, and verify the IKE authentication token AUTH using the shared secret PACESharedSecret.

The encryption function E() must be carefully chosen to prevent dictionary attacks that would otherwise allow an attacker to recover the password. Those restrictions are described in Section 4.1. Details on the mapping function, including the elliptic curve variant, can be found in Section 4.2.

To avoid the risks inherent in storing a short password (e.g., the fact that passwords are often reused for different applications), this protocol allows the peers to jointly convert the password into a cryptographically stronger shared secret. This shared secret can then be stored by both peers, in lieu of the original password or its salted variants.

3. Protocol Sequence

The protocol consists of three round trips -- an IKE_SA_INIT exchange and a 2-round IKE_AUTH exchange -- as shown in the next figure. An optional Informational exchange may follow (see Section 3.5). 

     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 [, N(PSK_PERSIST)] } ->
     
                   <- HDR, SK{AUTH, SAr2, TSi, TSr [, N(PSK_PERSIST)] }

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 (Modular Diffie-Hellman or Elliptic Curve Diffie-Hellman), the group to be used, and the encryption algorithm.

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.

The request is equivalent to the IKE_AUTH request in a normal IKEv2 exchange; i.e., any payload that 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)

See Section 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 as their authentication method the Generic Secure Password Authentication Method [RFC6467], whose value is 12. 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].

Each of the peers MAY generate a long-term credential at this point, after it has verified the opposite peer's identity. The shared secret is

LongTermSecret = prf(Ni | Nr, "PACE Generated PSK" | PACESharedSecret),

where the literal string is ASCII-encoded, with no zero terminator. The generated secret MUST be persisted to stable memory before sending the response. See Section 3.5 for more details about this facility.

This round's response is equivalent to the IKE_AUTH response in a normal IKEv2 exchange; i.e., any payload that 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.

3.4. Public Key Validation

The security of the protocol relies on the entanglement of a weak password with cryptographically strong shared secrets, SASharedSecret and PACESharedSecret, mutually and randomly generated by the initiator and the responder. If an attacker can influence the randomness of those shared secrets, the confidentiality of the password may be directly affected.

Implementations MUST therefore verify that the shared secrets SASharedSecret and PACESharedSecret are random elements of the group generated by G to prevent small subgroup attacks. This can be achieved by a validation of the public keys (i.e., KEi, PKEi, and KEr, PKEr).

First of all, each party MUST check that the public keys PKEi, PKEr, KEi, and KEr differ. Otherwise, it MUST abort the protocol.

For each received public key PK, the following tests SHOULD be performed. Any failure in the validation MUST be interpreted as an attack, and the protocol SHALL be aborted.

  • Verify that PK is an element of the Diffie-Hellman Group.
  • For Modular Diffie-Hellman, check that PK lies within the interval [2,p-2].
  • For Elliptic Curve Diffie-Hellman, check that PK is a point on the Elliptic Curve and not the point at infinity.
  • Verify that PK is an element of the cryptographic subgroup of order q.
  • For Modular Diffie-Hellman, check that PK^q = 1 (mod p).
  • For Elliptic Curve Diffie-Hellman, check that q * PK = 0.

Note that for most of the MODP groups, the order q = (p-1)/2. This applies in particular to the standard groups #2, #5, and #14, commonly used in IKE. For ECP and MODP groups not based on safe primes, the order q is explictly stated in the parameters.

As an alternative to the public key validation, the compatible cofactor exponentiation/multiplication may be used, which is often more efficient but requires changes to the implementation of the key agreement. Details on the implementation can be found in [RFC2785] and in [TR03111] for Modular Diffie-Hellman and Elliptic Curve Diffie-Hellman, respectively.

3.5. Creating a Long-Term Shared Secret

To reduce the time that the peers store a hashed password, it is RECOMMENDED that the password be replaced by a dedicated shared secret, according to the method described in this section. See Appendix B for more discussion of the security threats involved.

Both peers generate the value LongTermSecret during round #2 of IKE_AUTH, as shown above. Later on, they exchange a PERSIST_PSK notification. Assume that both peers support this mechanism (e.g., the IKE implementation is able to modify its own credential store). Then, each of the peers, when receiving the notification, permanently deletes the stored password and replaces it with LongTermSecret. These credentials are stored in the Peer Authorization Database (PAD) [RFC4301] and are associated with the identity of the opposite peer.

This solution is designed as a two-phase commitment, so that failure at any time cannot result in the peers not having any shared secret. 

     Initiator                      Responder
     ---------                      ---------

IKE_AUTH round #2: 

     HDR, SK{..., N(PSK_PERSIST)} ---------->
                                 Responder computes and stores PSK
     
                           <------- HDR, SK{..., N(PSK_PERSIST)}

Initiator computes and stores PSK 

     HDR, SK{N(PSK_CONFIRM)} -------------->

Responder deletes the short password 

                           <-------------- HDR, SK{N(PSK_CONFIRM)}

Initiator deletes the short password

Figure 2: IKE SA Setup with PACE and PSK Generation

In the second round of IKE_AUTH, the initiator MAY send a PSK_PERSIST notification if it wishes to use this mechanism. If the responder agrees, and only after it has authenticated the initiator, it MUST generate a new PSK, save it to stable storage (e.g., to disk), and MUST respond with a PSK_PERSIST notification. Otherwise, it simply does not include the notification in its reply. When receiving the reply, and after authenticating the responder, the initiator MUST also generate the PSK and save it in stable storage.

If the peers have negotiated this mechanism, the initiator MUST send the PSK_CONFIRM notification in an Informational exchange shortly after the IKE SA has been set up. When the responder receives it, it MUST delete the stored short password from its credential database and respond with a PSK_CONFIRM notification. Upon receiving this notification, the initiator deletes its copy of the short password.

If not saved to persistent storage, the LongTermSecret MUST be deleted when the IKE SA is rekeyed or when it is torn down. It SHOULD be deleted 1 hour after the initial IKE SA has been set up.

3.6. Using the Long-Term Shared Secret

The LongTermSecret MUST be used as a regular IKE Pre-Shared Key (PSK), rather than with PACE or any other password-based authentication method.

Normally, at the completion of this protocol, both peers will have either a shared password or a shared PSK. The protocol is designed so that the peers will have a shared credential, regardless of any protocol failures. However, in some failure cases, the initiator may find itself with both a short password and a PSK for a particular peer. In that case, it MUST first try to authenticate with a password and, upon success, MUST attempt to convert it to a PSK. If password authentication fails, it MUST use the PSK and upon successful setup of the IKE SA MUST permanently delete the password.

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 pseudorandom function (prf) requires a fixed-size key, the literal string is either truncated or padded with zero octets on the right, as needed. Multiple copies of SPwd MAY be stored, if the prf function is not known in advance. 

      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 Section 2.13 of [RFC5996]. The length of KPwd is determined by the key length of the negotiated encryption algorithm.

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.

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 that the IV be chosen both randomly and uniformly distributed, even though this condition is not necessary for the cryptographic security of the protocol.

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 bits 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 Galois/Counter Mode (GCM) and all Counter with CBC-MAC (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 an 8-octet Integrity Check Value (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.

4.2. Mapping the Nonce

The mapping is based on a second anonymous Diffie-Hellman key agreement protocol to create a shared secret that 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 Diffie-Hellman group, and the calculated ephemeral generator, respectively. The following subsections describe the mapping for different Diffie-Hellman variants.

4.2.1. Modular 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.

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]; therefore, unassigned code points are 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. A compliant implementation MUST NOT apply any other form of processing to the input password, other than as described in this section.

See Section 3 of [RFC4013] for examples of SASLprep processing.

5.2. The SECURE_PASSWORD_METHODS Notification

[RFC6467] defines a new type of Notify payload to indicate support for Secure Password Methods (SPMs) 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 3: 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 16424).

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 4: SECURE_PASSWORD_METHODS Payload Data

For the current method, the list of proposed methods MUST include the value PACE (1).

5.3. The PSK_PERSIST Notification

This document defines the PSK_PERSIST notification type, whose value is 16425. This notification MUST be sent with no data. However, for future extensibility, the receiver MUST ignore any notification data if such data is present.

5.4. The PSK_CONFIRM Notification

This document defines the PSK_CONFIRM notification type, whose value is 16426. This notification MUST be sent with no data. However, for future extensibility, the receiver MUST ignore any notification data if such data is present.

5.5. The GSPM(ENONCE) Payload

This protocol defines the ENONCE (encrypted nonce) payload, which reuses the Generic SPM (GSPM) payload type [RFC6467] (value 49). 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 5: 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 it must be rejected by the receiver if it is not 0.

5.6. The KE (KEi2/KEr2) Payloads in PACE

PACE reuses the Key Exchange (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.

5.7. 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.

PACE should be considered an "authentication method", in the sense of Section 5 of [RFC5723], which means that its use MUST be noted in the protected ticket. The format of the ticket is not standardized; however, it is RECOMMENDED that this indication 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; 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; see Section 5 of [RFC5996] for more details. Specifically, any deviation from randomness of the nonce s might compromise the password. Therefore, it is strongly RECOMMENDED that the initiator pass 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:

  • 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.
  • 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.
  • 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 a lazy-sampling technique that is similarly applicable to the stream-cipher-based construction.

The differences in the setting therefore have no impact on the validity of the proof.

6.9. Long-Term Credential Storage

This protocol does not require that peers store the plaintext password. Instead, the value SPwd SHOULD be stored by both peers.

In addition, the protocol allows both peers to replace the password by a crypto-strength shared secret. This solution improves the system's security (since passwords are often used for multiple applications), but at the cost of implementation complexity. In particular, if this optional mechanism is to be used, the credential database would need to be writable by the key management subsystem.

See Appendix B for alternatives to this approach.

7. IANA Considerations

IANA has allocated the following values:

  • A PACE value of 1 from the "IKEv2 Secure Password Methods" registry [RFC6467].
  • A PSK_PERSIST value of 16425 and a PSK_CONFIRM value of 16426 from the "IKEv2 Notify Message Types - Status Types" registry. We note that these notification types are generic and that other password authentication methods may also choose to use them.

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. Hugo Krawczyk proposed that the password be amplified into a persistent shared secret.

9. References

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.
   
   [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
                 Internet Protocol", RFC 4301, December 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, <http://www.iacr.org/cryptodb/
                 archive/2000/EUROCRYPT/18070139.pdf>.
   
   [ICAO]        ISO/IEC JTC1 SC17 WG3/TF5 for the International Civil
                 Aviation Organization (ICAO), "Supplemental Access
                 Control for Machine Readable Travel Documents",
                 Version 1.01, November 2010.
   
   [IKEv2-CONS]  Harkins, D., "Password-Based Authentication in IKEv2:
                 Selection Criteria and Considerations", Work
                 in Progress, October 2010.
   
   [PACEsec]     Bender, J., Fischlin, M., and D. Kuegler, "Security
                 Analysis of the PACE Key-Agreement Protocol",
                 LNCS 5735, pp. 33-48, Springer-Verlag (the extended
                 abstract appeared in Information Security Conference
                 (ISC) 2009), December 2009,
                 <http://eprint.iacr.org/2009/624>.
   
   [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.
   
   [RFC6467]     Kivinen, T., "Secure Password Framework for Internet
                 Key Exchange Version 2 (IKEv2)", RFC 6467,
                 December 2011.
   
   [TR03110]     BSI, "TR-03110, Advanced Security Mechanisms for
                 Machine Readable Travel Documents, Part 2 - Extended
                 Access Control Version 2 (EACv2), Password
                 Authenticated Connection Establishment (PACE), and
                 Restricted Identification (RI)", Version 2.10,
                 March 2012.
   
   [TR03111]     BSI, "TR-03111, Elliptic Curve Cryptography",
                 Version 1.11, April 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 [IKEv2-CONS]. 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 version 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 that 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., Modular 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 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 adapted 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:

  • 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.
  • An attacker must not be able to construct lookup tables (such as the famous "rainbow tables") that enable her to discover the plain password.
  • 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, using the salt as its key), 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 than 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. Using this solution, all protocol peers can store the password in a salted form.

The implementation proposed here requires a composite number n that is common to all peers. The composite number n can be 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 encryption 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 Strong, Long-Term, 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 strong shared secret, based on IKE's SK_d. They now destroy the short password and replace it with the new secret.

This method has been added to the current protocol as an optional mechanism.

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