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Versions: 00 01 02 03 04 05 06 07 08 RFC 6617

Network Working Group                                         D. Harkins
Internet-Draft                                            Aruba Networks
Intended status: Experimental                              July 27, 2011
Expires: January 28, 2012


                   Secure PSK Authentication for IKE
                   draft-harkins-ipsecme-spsk-auth-05

Abstract

   This memo describes a secure pre-shared key authentication method for
   IKE.  It is resistant to dictionary attack and retains security even
   when used with weak pre-shared keys.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 January 28, 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Keyword Definitions  . . . . . . . . . . . . . . . . . . .  3
   2.  Usage Scenarios  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Notation . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Discrete Logarithm Cryptography  . . . . . . . . . . . . . . .  5
     4.1.  Elliptic Curve Cryptography (ECP) Groups . . . . . . . . .  6
     4.2.  Finite Field Cryptography (MODP) Groups  . . . . . . . . .  7
   5.  Random Numbers . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Using Passwords and Raw Keys For Authentication  . . . . . . .  8
   7.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Secure PSK Authentication Message Exchange . . . . . . . . . .  9
     8.1.  Negotiation of Secure PSK Authentication . . . . . . . . . 10
       8.1.1.  IKEv1 Negotiation  . . . . . . . . . . . . . . . . . . 10
       8.1.2.  IKEv2 Negotiation  . . . . . . . . . . . . . . . . . . 11
     8.2.  Fixing the Secret Element, SKE . . . . . . . . . . . . . . 11
       8.2.1.  ECP Operation to Select SKE  . . . . . . . . . . . . . 12
       8.2.2.  MODP Operation to Select SKE . . . . . . . . . . . . . 13
     8.3.  Encoding and Decoding of Group Elements and Scalars  . . . 14
       8.3.1.  Encoding and Decoding of Scalars . . . . . . . . . . . 14
       8.3.2.  Encoding and Decoding of ECP Elements  . . . . . . . . 14
       8.3.3.  Encoding and Decoding of MODP Elements . . . . . . . . 15
     8.4.  Message Generation and Processing  . . . . . . . . . . . . 15
       8.4.1.  Generation of a Commit . . . . . . . . . . . . . . . . 15
       8.4.2.  Processing of a Commit . . . . . . . . . . . . . . . . 16
         8.4.2.1.  Validation of an ECP Element . . . . . . . . . . . 16
         8.4.2.2.  Validation of a MODP Element . . . . . . . . . . . 16
         8.4.2.3.  Commit Processing Steps  . . . . . . . . . . . . . 16
       8.4.3.  Authentication of the Exchange . . . . . . . . . . . . 17
     8.5.  Payload Format . . . . . . . . . . . . . . . . . . . . . . 17
       8.5.1.  Commit Payload . . . . . . . . . . . . . . . . . . . . 18
       8.5.2.  Notify Payload . . . . . . . . . . . . . . . . . . . . 18
     8.6.  IKEv1 Messaging  . . . . . . . . . . . . . . . . . . . . . 19
     8.7.  IKEv2 Messaging  . . . . . . . . . . . . . . . . . . . . . 20
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     12.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26









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1.  Introduction

   Both [RFC2409] and [RFC5996] allow for authentication of the IKE
   peers using a pre-shared key.  The exchanges, though, are susceptible
   to dictionary attack and are therefore insecure.  In addition,
   [RFC2409] requires that a pre-shared key be identified by IP address
   and this severely constrains its usefulness.  These are obvious
   drawbacks to using pre-shared key authentication in IKEv1 and IKEv2.

   To address the security issue, [RFC5996] recommends that the pre-
   shared key used for authentication "contain as much unpredictability
   as the strongest key being negotiated".  That means any non-
   hexidecimal key would require over 100 characters to provide enough
   strength to generate a 128-bit key for AES.  This is an unrealistic
   requirement because humans have a hard time entering a string over 20
   characters without error.  Consequently, pre-shared key
   authentication in [RFC2409] and [RFC5996] are used insecurely today.

   A pre-shared key authentication method built on top of a zero-
   knowledge proof will provide resistance to dictionary attack and
   still allow for security when used with weak pre-shared keys, such as
   user-chosen passwords.  Such an authentication method is described in
   this memo.

   Resistance to dictionary attack is achieved when an attacker gets
   one, and only one, guess at the secret per active attack (see for
   example, [BM92], [BMP00] and [BPR00]).  Another way of putting this
   is that any advantage the attacker can realize is through interaction
   and not through computation.  This is demonstrably different than the
   technique from [RFC5996] of using a large, random number as the pre-
   shared key.  That can only make a dictionary attack less likely to
   succeed, it does not prevent a dictionary attack.  And, as [RFC5996]
   notes, it is completely insecure when used with weak keys like user-
   generated passwords.

1.1.  Keyword Definitions

   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.  Usage Scenarios

   [RFC5996] describes usage scenarios for IKEv2.  These are:

   1.  "Security Gateway to Security Gateway Tunnel": the endpoints of
       the IKE (and IPsec) communication are network nodes that protect



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       traffic on behalf of connected networks.  Protected traffic is
       between devices on the respective protected networks.

   2.  "Endpoint-to-Endpoint Transport": the endpoints of the IKE (and
       IPsec) communication are hosts according to [RFC4301].  Protected
       traffic is between the two endpoints.

   3.  "Endpoint to Securty Gateway Tunnel": one endpoint connects to a
       protected network through a network node.  The endpoints of the
       IKE (and IPsec) communication are the endpoint and network node,
       but the protected traffic is between the endpoint and another
       device on the protected network behind the node.

   The authentication and key exchange described in this memo is
   suitable for all the usage scenarios described in [RFC5996].  In the
   "Security Gateway to Security Gateway Tunnel" scenario and the
   "Endpoint-to-Endpoint Transport" scenario it provides a secure method
   of authentication without requiring a certificate.  For the "Endpoint
   to Security Gateway Tunnel" scenario it provides for secure username+
   password authentication that is popular in remote access VPN
   situations.

   [RFC2409] does not describe usage scenarios for IKEv1 but IKEv1 has,
   traditionally, been used in the same "Security Gateway to Security
   Gateway Tunnel" scenario and the "Endpoint-to-Endpoint Transport"
   scenario.  Its pre-shared key-based authentication method is
   constrained to only allow keys identified by IP address and therefore
   it lacks a robust way to do user authentication using a password,
   prompting the definition of different insecure ways to do password
   authentication.  Therefore, a secure pre-shared key-based
   authentication method in IKEv1 will obviate the need to do insecure
   password-based authentication, such as [XAUTH], and remove the
   requirement that a pre-shared key in IKEv1 needs to be based on IP
   address.


3.  Notation

   The following notation is used in this memo:

   PSK
       A shared, secret and potentially low-entropy word, phrase, code
       or key used as a credential to mutually authenticate the peers.

   a = prf(b, c)
       The string "b" and "c" are given to a pseudo-random function to
       produce a fixed-length output "a".




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   a | b
       denotes concatenation of string "a" with string "b".

   [a]b
       indicates a string consisting of the single bit "a" repeated "b"
       times.

   len(a)
       indicates the length in bits of the string "a".

   LSB(a)
       returns the least-significant bit of the bitstring "a".

   The convention for this memo to represent an element in a finite
   cyclic group is to use an upper-case letter or acronym, while a
   scalar is indicated with a lower-case letter or acronym.


4.  Discrete Logarithm Cryptography

   This protocol uses Discrete Logarithm Cryptography to achieve
   authentication.  Each party to the exchange derives ephemeral public
   and private keys with respect to a particular set of domain
   parameters (referred to here as a "group").  Groups can be either
   based on finite field cryptography (MODP groups) or elliptic curve
   cryptography (ECP groups).

   This protocol uses the same group as the IKE exchange in which it is
   being used for authentication, with the exception of characteristic-
   two elliptic curve groups (EC2N).  Use of such groups is undefined
   for this authentication method and an IKE exchange that negotiates
   one of these groups MUST NOT use this method of authentication.

   For each group the following operations are defined:

   o   "scalar operation"-- taking a scalar and an element in the group
       producing another element-- Z = scalar-op(x, Y).

   o   "element operation"-- taking two elements in the group to produce
       a third-- Z = element-op(X, Y).

   o   "inverse operation"-- take an element and returns another element
       such that the element operation on the two produces the identity
       element of the group-- Y = inverse(X).







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4.1.  Elliptic Curve Cryptography (ECP) Groups

   The key exchange defined in this memo uses fundamental algorithms of
   ECP groups as described in [RFC6090].

   Domain parameters for ECP elliptic curves used for secure pre-shared
   key-based authentication include:

   o  A prime, p, determining a prime field GF(p).  The cryptographic
      group will be a subgroup of the full elliptic curve group which
      consists points on an elliptic curve-- elements from GF(p) that
      satisfy the curve's equation-- together with the "point at
      infinity" (denoted here as "O") that serves as the identity
      element.

   o  Elements a and b from GF(p) that define the curve's equation.  The
      point (x,y) is on the elliptic curve if and only if y^2 = x^3 +
      a*x + b.

   o  A prime, r, which is the order of G, and thus is also the size of
      the cryptographic subgroup that is generated by G.

   The scalar operation is multiplication of a point on the curve by
   itself a number of times.  The point Y is multiplied x-times to
   produce another point Z:

       Z = scalar-op(x, Y) = x*Y

   The element operation is addition of two points on the curve.  Points
   X and Y are summed to produce another point Z:

       Z = element-op(X, Y) = X + Y

   The inverse function is defined such that the sum of an element and
   its inverse is "0":

       Q + inverse(Q) = "O"

   Elliptic curve groups require a mapping function, q = F(Q), to
   convert a group element to an integer.  The mapping function used in
   this memo returns the x-coordinate of the point it is passed.

   scalar-op(x, Y) can be viewed as x iterations of element-op() by
   defining:

       Y = scalar-op(1, Y)





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       Y = scalar-op(x, Y) = element-op(Y, scalar-op(x-1, Y)), for x > 1

   A definition of how to add two points on an elliptic curve (i.e.
   element-op(X, Y)) can be found in [RFC6090].

   Note: There is another ECP domain parameter, a co-factor, h, that is
   defined by the requirement that the size of the full elliptic curve
   group (including "O") be the product of h and r.  ECP groups used for
   secure pre-shared key-based authentication MUST have a co-factor of
   one (1).  At the time of publication of this memo, all ECP groups in
   the IANA registry used by IKE had a co-factor of one (1).

4.2.  Finite Field Cryptography (MODP) Groups

   Domain parameters for MODP groups used for secure pre-shared key-
   based authentication include:

   o  A prime, p, determining a prime field GF(p), the integers modulo
      p.

   o  A prime, r, which is the multiplicative order of G, and thus also
      the size of the cryptographic subgroup of GF(p)* that is generated
      by G.

   The scalar operation is exponentiation of a generator modulus a
   prime.  An element Y is taken to the x-th power modulo the prime
   returning another element, Z:

       Z = scalar-op(x, Y) = Y^x mod p

   The element operation is modular multiplication.  Two elementx, X and
   Y, are multiplied modulo the prime returning another element, Z:

       Z = element-op(X, Y) = (X * Y) mod p

   The inverse function for a MODP group is defined such that the
   product of an element and its inverse modulo the group prime equals
   one (1).  In other words,

       (Q * inverse(Q)) mod p = 1

   Unlike ECP groups, MODP groups do not require a mapping function to
   convert an element into a scalar.  But for the purposes of notation
   in protocol definition, the function F, when used below, shall just
   return the value that was passed to it-- i.e.  F(i) = i.

   Some MODP groups in the IANA registry for use by IKE (and the secure
   pre-shared key authentication method) are based on safe primes and



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   the order is not included in the group's domain parameter set.  In
   this case only, the order, r, MUST be computed as the prime minus one
   divided by two-- (p-1)/2.  If an order is included in the group's
   domain parameter set that value MUST be used in this exchange when an
   order is called for.  If a MODP group does not include an order in
   its domain parameter set and is not based on a safe prime it MUST NOT
   be used with this exchange.


5.  Random Numbers

   As with IKE itself, the security of the secure pre-shared key
   authentication method relies upon each participant in the protocol
   producing quality secret random numbers.  A poor random number chosen
   by either side in a single exchange can compromise the shared secret
   from that exchange and open up the possibility of dictionary attack.

   Producing quality random numbers without specialized hardware entails
   using a cryptographic mixing function (like a strong hash function)
   to distill entropy from multiple, uncorrelated sources of information
   and events.  A very good discussion of this can be found in
   [RFC4086].


6.  Using Passwords and Raw Keys For Authentication

   The PSK used as an authentication credential with this protocol can
   be either a character-based password or passphrase, or it could be a
   binary or hexidecimal string.  Regardless though, this protocol
   requires both the Initiator and Responder to have identical binary
   representations of the shared credential.  If the PSK is a character
   string in a character set other than US-ASCII, ambiguities may arise
   due to internationalization.  In that case, the PSK MUST be pre-
   processed to remove any ambiguity before it is used in this protocol.
   When the PSK can be unambigouously represented as a binary string--
   i.e. it is already a raw hexidecimal string, or it is restricted to
   the US-ASCII character set-- it can be used directly without any pre-
   processing.

   This memo describes two ways to pre-process the shared, secret
   credential depending on the type of credential:

   o   None/Raw Key: The input credential SHALL be treated as an ASCII
       string or a binary/hexadecimal string with no pre-processing or
       normalization performed.  The output SHALL be the binary
       representation of the input string.





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   o   SASLprep/Character String: The input credential is processed
       according to the rules of the [RFC4013] profile of [RFC3454].
       The credential SHALL be considered a "stored string" per
       [RFC3454] and unassigned code points are therefore prohibited.
       The output SHALL be the binary representation of the processed
       UTF-8 character string.  Prohibited output and unassigned
       codepoints encountered in SASLprep pre-processing SHALL cause a
       failure of pre-processing and the output SHALL NOT be used with
       Secure Password Authentication.

   During negotiation of Secure PSK Authentication (see Section 8.1),
   the Initiator indicates the type of credential it will be using--
   either a raw key (no pre-processing required) or a character-based
   key (SASLprep pre-processing required)-- and the Responder
   acknowledges it will do the same.

   For the purposes of interoperability, pre-processing of "None"-- i.e.
   using raw keys or the ASCII character set-- MUST be supported and
   SASLprep SHOULD be supported.


7.  Assumptions

   The security of the protocol relies on certain assumptions.  They
   are:

   1.  The pseudo-random function, prf, defined in IKE (either [RFC2409]
       or [RFC5996]) acts as an "extractor" (see [RFC5869]) by
       concentrating the entropy from a secret input into a short,
       fixed, string.  The output of prf is indistinguishable from a
       random source.

   2.  The discrete logarithm problem for the chosen finite cyclic group
       is hard.  That is, given G, p and Y = G^x mod p it is
       computationally infeasible to determine x.  Similarly for an
       elliptic curve group given the curve definition, a generator G,
       and Y = x * G it is computationally infeasible to determine x.

   3.  The pre-shared key is drawn from a finite pool of potential keys.
       Each possible key in the pool has equal probability of being the
       shared key.  All potential attackers have access to this pool of
       keys.


8.  Secure PSK Authentication Message Exchange

   The key exchange described in this memo is based on the "Dragonfly"
   key exchange which has also been proposed in 802.11 wireless networks



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   (see [SAE]) and as an EAP method (see [RFC5931]).  "Dragonfly" is
   patent-free and royalty-free.  It has been defined here for use in
   both IKEv1 ([RFC2409]) and IKEv2 ([RFC5996]).  It makes use of the
   same pseudo-random function (prf) and the same Diffie-Hellman group
   that are negotiated for use in the IKE exchange that "dragonfly" is
   authenticating.

   A pseudo-random function which uses a block cipher is NOT RECOMMENDED
   for use with Secure PSK Authentication due to its poor job operating
   as an "extractor" (see Section 7).  Pseudo-random functions based on
   hash functions using the HMAC construct from [RFC2104] SHOULD be
   used.

   To perform secure pre-shared key authentication each side must
   generate a shared and secret element in the chosen group based on the
   pre-shared key.  This element, called the Secret Key Element, or SKE,
   is then used in the "Dragonfly" authentication and key exchange
   protocol.  "Dragonfly" consists of each side exchanging a "Commit"
   payload and then proving knowledge of the resulting shared secret.

   The "Commit" payload contributes ephemeral information to the
   exchange and binds the sender to a single value of the pre-shared key
   from the pool of potential pre-shared keys.  An authentication
   payload (either the HASH or AUTH payload depending on whether IKEv1
   or IKEv2, respectively, is being used) proves that the pre-shared key
   is known and completes the zero-knowledge proof.

8.1.  Negotiation of Secure PSK Authentication

   The technique used for negotiating whether, and how, to use Secure
   PSK Authentication depends on whether IKEv1 or IKEv2 is being used.
   Secure PSK Authentication in IKEv2 MUST be implemented to claim
   conformance to this memo.  Secure PSK Authentication in IKEv1 is NOT
   RECOMMENDED.

8.1.1.  IKEv1 Negotiation

   With IKEv1, the Initiator indicates its desire to use Secure PSK
   Authentication, and the pre-processing it will apply to the shared
   credential (see Section 6), by setting the Authentication Method in
   the SA payload to either TBD1 or TBD2 indicating SPSK with a raw or
   ASCII key (no pre-processing) or SPSK with a character string
   (SASLprep pre-processing), respectively.

   The Responder indicates its desire to use Secure PSK Authentication
   and its agreement on the pre-processing applied to the shared
   credential by echoing back an SA payload with the same Authentication
   Method.



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8.1.2.  IKEv2 Negotiation

   With IKEv2, the Initiator indicates its desire to use Secure PSK
   Authentication, and the type of pre-processing to perform on the PSK
   (see Section 6), by adding a Notify payload of type
   SECURE_PASSWORD_METHODS (see
   [I-D.kivinen-ipsecme-secure-password-framework]) to the first message
   of the IKE_SA_INIT exchange and by putting TBD3 or TBD4 in the
   notification data field of the Notify payload, indicating SPSK with a
   raw or ASCII key (no preprocessing) or SPSK with a character string
   (SASLprep pre-processing), respectively.

   The Responder indicates its desire to perform Secure PSK
   Authentication, and agrees on the type of PSK pre-processing, by
   adding a Notify payload of type SECURE_PASSWORD_METHODS to its
   response in the IKE_SA_INIT exchange and by echoing back the pre-
   processing technique in the notification data field of the Notify
   payload.  If the Responder does not agree with the pre-processing
   technique indicated by the Initiator it MUST abort the exchange.

8.2.  Fixing the Secret Element, SKE

   The method of fixing SKE depends on the type of group, either MODP or
   ECP.  The function "prf+" from [RFC5996] is used as a key derivation
   function.  This is true even if performing secure pre-shared key
   authentication with IKEv1.

   Fixing SKE involves an iterative hunting-and-pecking technique using
   the prime from the negotiated group's domain parameter set and an
   ECP- or MODP-specific operation depending on the negotiated group.
   This technique requires the pre-shared key to be a binary string,
   therefore any pre-processing transformation (see Section 6) MUST be
   performed on the pre-shared key prior to fixing SKE.

   First, an 8-bit counter is set to the value one (1).  Then, the
   pseudo-random function is used to generate a secret seed using the
   counter, the pre-shared key, and two nonces (without the fixed
   headers) exchanged by the Initiator and the Responder (see
   Section 8.6 and Section 8.7):

      ske-seed = prf(Ni | Nr, psk | counter)

   Then, the ske-seed is expanded using prf+ to create an ske-value:

      ske-value = prf+(ske-seed, "IKE SKE Hunting And Pecking")

   where len(ske-value) is the same as len(p), the length of the prime
   from the domain parameter set of the negotiated group.



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   If the ske-seed is greater than or equal to the prime, p, the counter
   is incremented and a new ske-seed is generated and the hunting-and-
   pecking continues.  If ske-seed is less than the prime, p, it is
   passed to the group-specific operation to select the SKE or fail.  If
   the group-specific operation fails, the counter is incremented, a new
   ske-seed is generated and the hunting-and-pecking continues.

   Note: The probability that more than "n" iterations of the "hunting-
   and-pecking" loop are required to find SKE is roughly (1-(r/2p))^n
   which rapidly approaches zero (0) as "n" increases.

8.2.1.  ECP Operation to Select SKE

   The group-specific operation for ECP groups uses ske-value, ske-seed
   and the equation of the curve to produce SKE.  First ske-value is
   used directly as the x-coordinate, x, with the equation of the
   elliptic curve, with parameters a and b from the domain parameter set
   of the curve, to solve for a y-coordinate, y.

   If there is no solution to the equation the operation fails (and the
   hunting-and-pecking continues).  If a solution is found then an
   ambiguity exists as there are technically two solutions to the
   equation, and ske-seed is used to unambiguously select one of them.
   If the low-order bit of ske-seed is equal to the low-order bit of y
   then a candidate SKE is defined as the point (x,y); if the low-order
   bit of ske-seed differs from the low-order bit of y then a candidate
   SKE is defined as the point (x, p-y) where p is the prime from the
   negotiated group's domain parameter set.  The candidate SKE becomes
   the SKE and the ECP-specific operation completes successfully.

   Algorithmically, the process looks like this:




















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         found = 0
         counter = 1
         do {
           ske-seed = prf(Ni | Nr, psk | counter)
           ske-value = prf+(ske-seed, "IKE SKE Hunting And Pecking")
           if (ske-value < p)
           then
             x = ske-value
             if ( (y = sqrt(x^3 + ax + b)) != FAIL)
             then
               if (LSB(y) == LSB(ske-seed))
               then
                 SKE = (x,y)
               else
                 SKE = (x, p-y)
               fi
               found = 1
             fi
           fi
           counter = counter + 1
         } while (found == 0)

                    Figure 1: Fixing SKE for ECP Groups

8.2.2.  MODP Operation to Select SKE

   The group-specific operation for MODP groups takes ske-value, and the
   prime, p, and order, r, from the group's domain parameter set to
   directly produce a candidate SKE by exponentiating the ske-value to
   the value ((p-1)/r) modulo the prime.  If the candidate SKE is
   greater than one (1) the candidate SKE becomes the SKE and the MODP-
   specific operation completes successfully.  Otherwise, the MODP-
   specific operation fails (and the hunting-and-pecking continues).

   Algorithmically, the process looks like this:
















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         found = 0
         counter = 1
         do {
           ske-seed = prf(Ni | Nr, psk | counter)
           ske-value = prf+(swd-seed, "IKE SKE Hunting And Pecking")
           if (ske-value < p)
           then
             SKE = ske-value ^ ((p-1)/r) mod p
             if (SKE > 1)
             then
               found = 1
             fi
           fi
           counter = counter + 1
         } while (found == 0)

                   Figure 2: Fixing SKE for MODP Groups

8.3.  Encoding and Decoding of Group Elements and Scalars

   The payloads used in the secure pre-shared key authentication method
   contain elements from the negotiated group and scalar values.  To
   ensure interoperability, scalars and field elements MUST be
   represented in payloads in accordance with the requirements in this
   section.

8.3.1.  Encoding and Decoding of Scalars

   Scalars MUST be represented (in binary form) as unsigned integers
   that are strictly less than r, the order of the generator of the
   agreed-upon cryptographic group.  The binary representation of each
   scalar MUST have a bit length equal to the bit length of the binary
   representation of r.  This requirement is enforced, if necessary, by
   prepending the binary representation of the integer with zeros until
   the required length is achieved.

   Scalars in the form of unsigned integers are converted into octet-
   strings and back again using the technique described in [RFC6090].

8.3.2.  Encoding and Decoding of ECP Elements

   Elements in ECP groups are points on the negotiated elliptic curve.
   Each such element MUST be represented by the concatenation of two
   components, an x-coordinate and a y-coordinate.

   Each of the two components, the x-coordinate and the y-coordinate,
   MUST be represented (in binary form) as an unsigned integer that is
   strictly less than the prime, p, from the group's domain parameter



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   set.  The binary representation of each component MUST have a bit
   length equal to the bit length of the binary representation of p.
   This length requirement is enforced, if necessary, by prepending the
   binary representation of the integer with zeros until the required
   length is achieved.

   The unsigned integers that represent the coordinates of the point are
   converted into octet-strings and back again using the technique
   described in [RFC6090].

   Since the field element is represented in a payload by the
   x-coordinate followed by the y-coordinate it follows, then, that the
   length of the element in the payload MUST be twice the bit length of
   p.

8.3.3.  Encoding and Decoding of MODP Elements

   Elements in MODP groups MUST be represented (in binary form) as
   unsigned integers that are strictly less than the prime, p, from the
   group's domain parameter set.  The binary representation of each
   group element MUST have a bit length equal to the bit length of the
   binary representation of p.  This length requirement is enforced, if
   necessary, by prepending the binary representation of the interger
   with zeros until the required length is achieved.

   The unsigned integer that represents a MODP element is converted into
   an octet-string and back using the technique described in [RFC6090].

8.4.  Message Generation and Processing

8.4.1.  Generation of a Commit

   Before a Commit can be generated, the SKE must be fixed using the
   process described in Section 8.2.

   A Commit has two components, a scalar and an Element.  To generate a
   Commit, two random numbers, a "private" value and a "mask" value, are
   generated (see Section 5).  Their sum modulo the order of the group,
   r, becomes the scalar component:

       scalar = (private + mask) mod r

   If the scalar is not greater than one (1), the private and mask
   values MUST be thrown away and new values randomly generated.  If the
   scalar is greater than one (1), the inverse of the scalar operation
   with the mask and SKE becomes the Element component.





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       Element = inverse(scalar-op(mask, SKE))

   The Commit payload consists of the scalar followed by the Element and
   the scalar and Element are encoded in the Commit payload according to
   Section 8.3.

8.4.2.  Processing of a Commit

   Upon receipt of a peer's Commit the scalar and element MUST be
   validated.  The processing of an element depends on the type, either
   an ECP element or a MODP element.

8.4.2.1.  Validation of an ECP Element

   Validating a received ECP Element involves: 1) checking whether the
   two coordinates, x and y, are both greater than zero (0) and less
   than the prime defining the underlying field; and 2) checking whether
   the x- and y-coordinates satisfy the equation of the curve (that is,
   that they produce a valid point on the curve that is not "0").  If
   either of these conditions are not met the received Element is
   invalid, otherwise the received Element is valid.

8.4.2.2.  Validation of a MODP Element

   A received MODP Element is valid if: 1) it is between one (1) and the
   prime, p, exclusive; and 2) if modular exponentiation of the Element
   by the group order, r, equals one (1).  If either of these conditions
   are not true the received Element is invalid.

8.4.2.3.  Commit Processing Steps

   Commit validation is accomplished by the following steps:

   1.  The length of the Commit payload is checked against its
       anticipated length (the anticipated length of the scalar plus the
       anticipated length of the element, for the negotiated group).  If
       it is incorrect, the Commit is invalidated, otherwise processing
       continues.

   2.  The peer's scalar is extracted from the Commit payload according
       to Section 8.3.1 and checked to ensure it is between one (1) and
       r, the order of the negotiated group, exclusive.  If it is not,
       the Commit is invalidated, otherwise processing continues.

   3.  The peer's Element is extracted from the Commit payload according
       to Section 8.3.2 and checked in a manner that depends on the type
       of group negotiated.  If the group is ECP the element is
       validated according to Section 8.4.2.1, if the group is MODP the



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       element is validated according to Section 8.4.2.2.  If the
       Element is not valid then the Commit is invalidated, otherwise
       the Commit is validated.

   4.  The Initiator of the IKE exchange has an added requirement to
       verify that the received element and scalar from the Commit
       payload differ from the element and scalar sent to the Responder.
       If they are identical, it signifies a reflection attack and the
       Commit is invalidated.

   If the Commit is invalidated the payload MUST be discarded and the
   IKE exchange aborted.

8.4.3.  Authentication of the Exchange

   After a Commit has been generated and a peer's Commit has been
   processed a shared secret used to authenticate the peer is derived.
   Using SKE, the "private" value generated as part of Commit
   generation, and the peer's scalar and Element from its Commit, named
   here peer-scalar and peer-element, respectively, a preliminary shared
   secret, skey, is generated as:

        skey = F(scalar-op(private,
                           element-op(peer-element,
                                      scalar-op(peer-scalar, SKE))))

   For the purposes of subsequent computation, the bit length of skey
   SHALL be equal to the bit length of the prime, p, used in either a
   MODP or ECP group.  This bit length SHALL be enforced, if necessary,
   by prepending zeros to the value until the required length is
   achieved.

   A shared secret, ss, is then computed from skey and the nonces
   exchanged by the Initiator (Ni) and Responder (Nr) (without the fixed
   headers) using prf():

        ss = prf(Ni | Nr, skey | "Secure PSK Authentication in IKE")

   The shared secret, ss, is used in an authentication payload (either
   HASH or AUTH payload depending on whether IKEv1 or IKEv2,
   respectively, is being used) to prove possession of the shared
   secret, and therefore knowledge of the pre-shared key.

8.5.  Payload Format







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8.5.1.  Commit Payload

   [I-D.kivinen-ipsecme-secure-password-framework]) defines a Generic
   Secure Password Method (GSPM) payload which is used to convey
   information that is specific to a particular secure password method.
   This memo uses the GSPM payload as a "Commit Payload" to contain the
   Scalar and Element used in the SPSK exchange:


    The Commit 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        !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                            Scalar                             ~
       |                                                               |
       ~                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
       |                                                               |
       ~                           Element                             ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Scalar and Element SHALL be encoded in the Commit payload
   according to Section 8.3.

8.5.2.  Notify Payload

   [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:















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                            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)                 ~
       !                                                               !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ! Secure PSK Authentication     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the SPM Notify payload the Protocol ID and SPI Size SHALL be set
   to zero and, therefore, the SPI field SHALL be empty.  The Notify
   Message Type SHALL be SECURE_PASSWORD_METHODS from [IKEV2-IANA].  The
   body of the SPM Notify payload is defined to be a list of 16-bit
   numbers indicating the particular Secure Password Method to use.  For
   SPSK those values SHALL be either TBD3 or TBD4, indicating support
   for SPSK with a raw key requiring no processing or SPSK with a
   character string requiring SASLprep processing, respectively (see
   Section 6).

8.6.  IKEv1 Messaging

   Secure PSK Authentication can be used in either Main Mode (see
   Figure 3) or Aggressive Mode (see Figure 4) with IKEv1 and SHALL be
   indicated by negotiation of an Authentication Method of either TBD1
   or TBD2 from [IKEV1-IANA], in the SA payload.  When using IKEv1 the
   "C" (critical) bit in the Commit payload (Section 8.5.1) MUST be
   clear (i.e. a value of zero).


              Initiator                          Responder
             -----------                        -----------
              HDR, SAi                 -->
                                       <--    HDR, SAr
              HDR, KEi, Ni             -->
                                       <--    HDR, KEr, Nr
              HDR*, IDii, COMi         -->
                                       <--    HDR*, IDir, COMr
              HDR*, HASH_I             -->
                                       <--    HDR*, HASH_R

   where COMi is the Commit payload sent by the Initiator and COMr is
   the Commit payload sent by the Responder.




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                     Figure 3: Secure PSK in Main Mode



           Initiator                          Responder
          -----------                        -----------
           HDR, SAi, KEi,
                Ni, IDii, COMi      -->
                                    <--    HDR, SAr, KEr, Nr,
                                                IDir, COMr, HASH_R
           HDR, HASH_I              -->

   where COMi is the Commit payload sent by the Initiator and COMr is
   the Commit payload sent by the Responder.

                  Figure 4: Secure PSK in Aggressive Mode

   For Secure PSK Authentication with IKEv1 the SKEYID value is computed
   as follows:

       SKEYID = prf(Ni_b | Nr_b, g^xy)

   Note that in Main Mode, SKEYID_a and SKEYID_e are used to protect the
   messages containing the identities and Commit payloads.  HASH_I and
   HASH_R are computed as follows:

          HASH_I = prf(SKEYID, ss | g^xi | g^xr | CKY-I | CKY-R |
                       SA_ib | IDii_b | COMi_b | COMr_b)

          HASH_R = prf(SKEYID, ss | g^xr | g^xi | CKY-R | CKY-I |
                       SA_ib | IDir_b | COMr_b | COMi_b)

   Where "ss" is the shared secret derived in Section 8.4.3, and COMi_b
   and COMr_b are the scalar and Element from the Commit payloads (i.e.
   without the header) sent by the Initiator and Responder,
   respectively.

8.7.  IKEv2 Messaging

   SPSK authentication modifies the IKE_AUTH exchange by adding one
   additional round trip to exchange Commit payloads to perform the
   Secure PSK Authentication exchange, and by changing the calculation
   of the AUTH payload data to bind the IKEv2 exchange to the outcome of
   the Secure PSK Authentication exchange (see Figure 5).







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

   IKE_SA_INIT:

    HDR, SAi1, KEi, Ni,
         N(SPM-SPSK)  -->

                                  <--    HDR, SAr1, KEr, Nr,
                                              N(SPM-SPSK)

   IKE_AUTH:

    HDR, SK {IDi, COMi, [IDr,]
             SAi2, TSi, TSr}      -->
                                  <--    HDR, SK {IDr, COMr}
    HDR, SK {AUTHi}               -->
                                  <--    HDR, SK {AUTHr, SAr2, TSi, TSr}

   where N(SPM-SPSK) indicates the Secure Password Methods Notify
   payloads (with SPSK-specific pre-processing) used to negotiate the
   use of SPSK authentication (see Section 8.1.2), COMi and AUTHi are
   the Commit payload and AUTH payload, respectively, sent by the
   Initiator and COMr and AUTHr are the Commit payload and AUTH payload,
   respectively, sent by the Responder.

                       Figure 5: Secure PSK in IKEv2

   The AUTH payloads when doing SPSK authentication SHALL be computed as

       AUTHi = prf(ss, <InitiatorSignedOctets> | COMi | COMr)

       AUTHr = prf(ss, <ResponderSignedOctets> | COMr | COMi)

   Where "ss" is the shared secret derived in Section 8.4.3, COMi and
   COMr are the entire Commit payloads (including the fixed headers)
   sent by the Initiator and Responder, respectively, and
   <InitiatorSignedOctets> and <ResponderSignedOctets> are defined in
   [RFC5996].  The Authentication Method indicated in both AUTH payloads
   SHALL be "Secure Password Authentication Method" from [IKEV2-IANA].


9.  IANA Considerations

   IANA SHALL assign a value for "SPSK Authentication with a raw key",
   replacing TBD1 above, from the IPSEC Authentication Method registry
   in [IKEV1-IANA] with the method name of "SPSK Authentication Method
   with a raw key."



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   IANA SHALL assign a value for "SPSK Authentication with a character
   string", replacing TBD2 above, from the IPSEC Authentication Method
   registry in [IKEV1-IANA] with the method name of "SPSK Authentication
   Method with a character string."

   IANA SHALL assign a value for "SPSK Authentication with a raw key"
   replacing TBD3 from the IKEv2 Secure Password Authentication Methods
   registry in [IKEV2-IANA] with the method name of "SPSK Authentication
   with a raw key".

   IANA SHALL assign a value for "SPSK Authentication with a character
   string" replacing TBD4 from the IKEv2 Secure Password Authentication
   Methods registry in [IKEV2-IANA] with the method name of "SPSK
   Authentication with a character string".

   The RFC Editor SHALL remove these IANA Considerations after values
   have been obtained from IANA and the TBD placeholders replaced by the
   actual values.


10.  Security Considerations

   Both the Initiator and Responder obtain a shared secret, "ss" (see
   Section 8.4.3) based on a secret group element and their own private
   values contributed to the exchange.  If they do not share the same
   pre-shared key they will be unable to derive the same secret group
   element and if they do not share the same secret group element they
   will be unable to derive the same shared secret.

   Resistance to dictionary attack means that the attacker must launch
   an active attack to make a single guess at the pre-shared key.  If
   the size of the pool from which the key was extracted was D, and each
   key in the pool has an equal probability of being chosen, then the
   probability of success after a single guess is 1/D. After X guesses,
   and removal of failed guesses from the pool of possible keys, the
   probability becomes 1/(D-X).  As X grows so does the probability of
   success.  Therefore it is possible for an attacker to determine the
   pre-shared key through repeated brute-force, active, guessing
   attacks.  This authentication method does not presume to be secure
   against this and implementations SHOULD ensure the size of D is
   sufficiently large to prevent this attack.  Implementations SHOULD
   also take countermeasures, for instance refusing authentication
   attempts for a certain amount of time, after the number of failed
   authentication attempts reaches a certain threshold.  No such
   threshold or amount of time is recommended in this memo.

   An active attacker can impersonate the Responder of the exchange and
   send a forged Commit payload after receiving the Initiator's Commit.



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   The attacker then waits until it receives the authentication payload
   from the Responder.  Now the attacker can attempt to run through all
   possible values of the pre-shared key, computing SKE (see
   Section 8.2), computing "ss" (see Section 8.4.3), and attempting to
   recreate the Confirm payload from the Responder.

   But the attacker committed to a single guess of the pre-shared key
   with her forged Commit.  That value was used by the Responder in his
   computation of "ss" which was used in the authentication payload.
   Any guess of the pre-shared key which differs from the one used in
   the forged Commit would result in each side using a different secret
   element in the computation of "ss" and therefore the authentication
   payload could not be verified as correct, even if a subsequent guess,
   while running through all possible values, was correct.  The attacker
   gets one guess, and one guess only, per active attack.

   An attacker, acting as either the Initiator or Responder, can take
   the Element from the Commit message received from the other party,
   reconstruct the random "mask" value used in its construction and then
   recover the other party's "private" value from the Scalar in the
   Commit message.  But this requires the attacker to solve the discrete
   logarithm problem which we assumed was intractable above (Section 7).

   Instead of attempting to guess at pre-shared keys an attacker can
   attempt to determine SKE and then launch an attack.  But SKE is
   determined by the output of the pseudo-random function, prf, which is
   assumed to be indistinguishable from a random source (Section 7).
   Therefore, each element of the finite cyclic group will have an equal
   probability of being the SKE.  The probability of guessing SKE will
   be 1/r, where r is the order of the group.  This is the same
   probability of guessing the solution to the discrete logarithm which
   is assumed to be intractable (Section 7).  The attacker would have a
   better chance of success at guessing the input to prf, i.e. the pre-
   shared key, since the order of the group will be many orders of
   magnitude greater than the size of the pool of pre-shared keys.

   The implications of resistance to dictionary attack are significant.
   An implementation can provision a pre-shared key in a practical and
   realistic manner-- i.e. it MAY be a character string and it MAY be
   relatively short-- and still maintain security.  The nature of the
   pre-share key determines the size of the pool, D, and countermeasures
   can prevent an attacker from determining the secret in the only
   possible way: repeated, active, guessing attacks.  For example, a
   simple four character string using lower-case English characters, and
   assuming random selection of those characters, will result in D of
   over four hundred thousand.  An attacker would need to mount over one
   hundred thousand active, guessing attacks (which will easily be
   detected) before gaining any significant advantage in determining the



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   pre-shared key.

   For a more detailed discussion of the security of the key exchange
   underlying this authentication method see [SAE] and [RFC5931].


11.  Acknowledgements

   The author would like to thank Scott Fluhrer and Hideyuki Suzuki for
   their insight in discovering flaws in earlier versions of the key
   exchange that underlies this authentication method and for their
   helpful suggestions in improving it.  Thanks to Lily Chen for useful
   advice on the hunting-and-pecking technique to "hash into" an element
   in a group and to Jin-Meng Ho for a discussion on countering a small
   sub-group attack.  Rich Davis suggested several checks on received
   messages that greatly increase the security of the underlying key
   exchange.  Hugo Krawczyk suggested using the prf as an extractor.


12.  References

12.1.  Normative References

   [IKEV1-IANA]
              "Internet Assigned Numbers Authority, Internet Key
              Exchange (IKE) Attributes",
              <http://www.iana.org/assignments/ipsec-registry>.

   [IKEV2-IANA]
              "Internet Assigned Numbers Authority, IKEv2 Parameters",
              <http://www.iana.org/assignments/ikev2-parameters>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [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.



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   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

12.2.  Informative References

   [BM92]     Bellovin, S. and M. Merritt, "Encrypted Key Exchange:
              Password-Based Protocols Secure Against Dictionary
              Attack", Proceedings of the IEEE Symposium on Security and
              Privacy, Oakland, 1992.

   [BMP00]    Boyko, V., MacKenzie, P., and S. Patel, "Provably Secure
              Password Authenticated Key Exchange Using Diffie-Hellman",
              Proceedings of Eurocrypt 2000, LNCS 1807 Springer-Verlag,
              2000.

   [BPR00]    Bellare, M., Pointcheval, D., and P. Rogaway,
              "Authenticated Key Exchange Secure Against Dictionary
              Attacks", Advances in Cryptology -- Eurocrypt '00, Lecture
              Notes in Computer Science Springer-Verlag, 2000.

   [I-D.kivinen-ipsecme-secure-password-framework]
              Kivinen, T., "Secure Password Framework for IKEv2",
              draft-kivinen-ipsecme-secure-password-framework (a work in
              progress), May 2011.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869, May 2010.

   [RFC5931]  Harkins, D. and G. Zorn, "Extensible Authentication
              Protocol (EAP) Authentication Using Only a Password",
              RFC 5931, August 2010.

   [SAE]      Harkins, D., "Simultaneous Authentication of Equals: A
              Secure, Password-Based Key Exchange for Mesh Networks",
              Proceedings of the 2008 Second International Conference on
              Sensor Technologies and Applications Volume 00, 2008.

   [XAUTH]    Pereira, R. and S. Beaulieu, "Extended Authentication



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              within ISAKMP/Oakley (XAUTH)",
              draft-ietf-ipsec-isakmp-xauth-06.txt (a work in progress),
              December 1999.


Author's Address

   Dan Harkins
   Aruba Networks
   1322 Crossman Avenue
   Sunnyvale, CA  94089-1113
   United States of America

   Email: dharkins@arubanetworks.com





































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