Network Working Group D. Harkins Internet-Draft Aruba Networks Intended status: Standards Track November 11, 2009 Expires: May 15, 2010 Secure PSK Authentication for IKE draft-harkins-ipsecme-spsk-auth-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 15, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract 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. Harkins Expires May 15, 2010 [Page 1]

Internet-Draft Secure PSK Authentication for IKE November 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Keyword Definitions . . . . . . . . . . . . . . . . . . . 3 2. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Finite Cyclic Groups . . . . . . . . . . . . . . . . . . . . . 5 4.1. Elliptic Curve Groups . . . . . . . . . . . . . . . . . . 6 4.2. Prime Modulus Groups . . . . . . . . . . . . . . . . . . . 6 5. Random Numbers . . . . . . . . . . . . . . . . . . . . . . . . 7 6. The Random Function . . . . . . . . . . . . . . . . . . . . . 7 7. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. Secure Pre-Shared Key Authentication Message Exchange . . . . 8 8.1. Fixing the Secret Element, SKE . . . . . . . . . . . . . . 9 8.1.1. Elliptic Curve SKE . . . . . . . . . . . . . . . . . . 9 8.1.2. Prime Modulus SKE . . . . . . . . . . . . . . . . . . 11 8.2. Encoding of Elements . . . . . . . . . . . . . . . . . . . 11 8.3. Generation of a Commit . . . . . . . . . . . . . . . . . . 11 8.4. Generation of a Confirm . . . . . . . . . . . . . . . . . 12 8.5. Commit Payload . . . . . . . . . . . . . . . . . . . . . . 13 8.6. Confirm Payload . . . . . . . . . . . . . . . . . . . . . 13 8.7. IKEv1 Messaging . . . . . . . . . . . . . . . . . . . . . 14 8.8. IKEv2 Messaging . . . . . . . . . . . . . . . . . . . . . 15 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 12.1. Normative References . . . . . . . . . . . . . . . . . . . 18 12.2. Informative References . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Harkins Expires May 15, 2010 [Page 2]

Internet-Draft Secure PSK Authentication for IKE November 2009 1. Introduction Both [RFC2409] and [RFC4306] allow for authentication of the IKE peers using a pre-shared key. The exchanges, though, are susceptible to dictionary attack and are therefore insecure. To address this security issue, [RFC4306] 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. 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 [RFC4306] of using a large, random number as the pre- shared key. That can only make a dictionary attack less likely to suceed, it does not prevent a dictionary attack. And, as [RFC4306] 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. Scenarios [RFC4306] 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 traffic on behalf of connected networks. Protected traffic is between devices on the respective protected networks. Harkins Expires May 15, 2010 [Page 3]

Internet-Draft Secure PSK Authentication for IKE November 2009 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. [RFC4306] also defines an EAP authentication method which can use a pre-shared key or password in a manner that is resistant to dictionary attack. But this requires the IKE Responder to have a certificate. Also, EAP is strictly a client-server protocol used for network access where one side is, typically, has a human behind it and the other side is a network node. And, for EAP to scale a server that terminates the EAP conversation is typically located on the protected network behind the network node. Therefore EAP authentication is really only applicable to the "Endpoint to Security Gateway Tunnel" usage scenario. The authentication and key exchange described in this memo is therefore suitable for both the "Security Gateway to Security Gateway Tunnel" scenario and the "Endpoint-to-Endpoint Transport" scenario. In both of those, there is no defined roles. Either party could initiate an IKE connection to the other and there isn't necessarily a human involved. Also, both sides will have access to the pre-shared key (i.e. no external authentication server) and neither side is required to have a certificate. While it is certainly possible to use EAP authentication in these cases with an EAP method such as [EAPPWD], it will be a pointless and problematic encapsulation-- it requires implementation of both the EAP client and EAP server state machines, requires support of at least one EAP method, requires support for EAP fragmentation, etc. [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. Also, 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 mitigate the need to do insecure password-based authentication and remove the requirement that a pre- shared key in IKEv1 needs to be based on IP address. There is a need to do secure pre-shared key-based authentication in Harkins Expires May 15, 2010 [Page 4]

Internet-Draft Secure PSK Authentication for IKE November 2009 IKE and it makes sense to do it as part of IKE and not by requiring additional authentication protocols. 3. Notation The following notation is used in this memo: psk The key shared between the two protocol participants. a = H(b) The binary string "b" is given to a function H which produces a fixed-length output "a". 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(x) indicates the length in bits of the string x. LSB(x) returns the least-significant bit of the bitstring "x". The convention for this memo to represent an element in a finite cyclic group is to use an upper-case letter while a scalar is indicated with a lower-case letter. 4. Finite Cyclic Groups This protocol uses the same group (from the IANA repository created by [RFC2409]) as the IKE exchange in which this authentication method is used. Groups can be either based on exponentiation modulo a prime or on an elliptic curve. These groups all define the following operations: 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). Harkins Expires May 15, 2010 [Page 5]

Internet-Draft Secure PSK Authentication for IKE November 2009 o "inverse operation"-- take an element an return another element such that the element operation on the two produces the identity element of the group-- Y = inverse(X). 4.1. Elliptic Curve Groups ECP elliptic curves are defined by a curve equation, y^2 = x^3 + ax + b, for a defined "a" and "b". ECP groups have a generator G, a prime p, an order r, and, optionally, a co-factor f. The scalar operation is multiplication of a point on the curve by itself a number of times, and the element operation is addition of two points on the curve: Z = scalar-op(x, Y) = x*Y the point Y is multiplied x-times to produce another point on the curve, Z. Z = element-op(X, Y) = X + Y points X and Y are summed to produce another point on the curve, Z. 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. If a co-factor is given in the group definition it MUST be used as the co-factor, f, when a co-factor is called for, otherwise the co- factor, f, MUST be 1. The inverse function for an elliptic group is defined such that the sum of an element and its inverse is the "point at infinity", denoted here as "O". In other words, Q + inverse(Q) = "O" Only ECP elliptic curve can be used by the secure pre-shared key authentication method. EC2N elliptic curves SHALL NOT be used. While such groups exist in the IANA registry their use is not defined. 4.2. Prime Modulus Groups Groups formed by a prime modulus have a generator G, a prime modulus p, and an order r. The scalar operation is exponentiation of a generator modulus a Harkins Expires May 15, 2010 [Page 6]

Internet-Draft Secure PSK Authentication for IKE November 2009 prime, and the element operation is modular multiplication: Z = scalar-op(x, Y) = Y^x mod p an element, Y taken to the x-th power modulo the prime returning another element in the group, Z. Z = element-op(X, Y) = (X * Y) mod p two elements, X and Y, are multiplied modulo the prime returning another element, Z. If the order of the generator of the group is part of the group definition that value MUST be used as the order of the group, r, when an order is called for, otherwise the order, r, MUST be computed as the prime minus one divided by two-- (p-1)/2. Unlike elliptic curve groups, prime modulus 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 integer representation of an element in a prime modulus group. The inverse function for a prime modulus 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 5. Random Numbers As with IKE itself, the security of the secure pre-shared key authenticaiton 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. The Random Function The protocol described in this memo uses a random function, H. This Harkins Expires May 15, 2010 [Page 7]

Internet-Draft Secure PSK Authentication for IKE November 2009 is a "random oracle" as defined in [RANDOR]. At first glance one may view this as a hash function. As noted in [RANDOR], though, hash functions are too structured to be used directly as a random oracle. But they can be used to instantiate a random oracle. The random function, H, in this memo is instantiated by HMAC-SHA256 (see [RFC4634]) with a key whose length is 32 octets and whose value is zero. In other words, H(x) = HMAC-SHA-256([0]32, x) 7. Assumptions The security of the protocol relies on certain assumptions. They are: 1. Function H maps a binary string of indeterminate length onto a fixed binary string that is x bits in length. H: {0,1}^* --> {0,1}^x 2. Function H is a "random oracle" (see [RANDOR]). Given knowledge of the input to H an adversary is unable to distinguish the output of H from a random data source. 3. 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. 4. 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 Pre-Shared Key Authentication Message Exchange 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 an authentication and key exchange protocol. The key exchange protocol consists of each side exchanging two data, a "Commit" and a "Confirm". The "Commit" contributes ephemeral information to the exchange and Harkins Expires May 15, 2010 [Page 8]

Internet-Draft Secure PSK Authentication for IKE November 2009 binds the sender to a single value of the pre-shared key from the pool of potential pre-shared keys. The "Confirm" proves that the pre-shared key is known and completes the zero-knowledge proof. 8.1. Fixing the Secret Element, SKE The method of fixing SKE depends on the type of group, either MODP or ECP. For the sake of convenience, we will use a single notation of prf+() to denote the function prf+() from [RFC4306] as well as the function prf() from [RFC2409], depending on whether the exchange is being performed in IKEv2 or IKEv1, respectively. 8.1.1. Elliptic Curve SKE For a finite cyclic group based on an elliptic curve it is necessary to use an iterative hunt-and-peck technique to fix the secret key element. First, an 8-bit counter is set to the value one (1). Then, the random function is used to generate a secret seed using the counter, the pre-shared key, and the two nonces exchange by the Initiator and the Responder: ske-seed = H(psk | counter | Ni | Nr) Then, the swe-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 of the curve. If the ske-value is greater than or equal to the prime, p, the counter is incremented, and a new ske-seed is gnerated and the hunting-and-pecking continues. If ske-value is less than the prime, p, it is used as the x-coordinate, x, with the equation for the elliptic curve, with parameters a and b from the definition of the curve, to solve for a y-coordinate, y. If there is no solution to the quadratic equation the counter is incremented, a new ske-seed is generated 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 over which the curve is defined. If the co-factor equals 1 then the candidate SKE becomes the SKE and hunting and pecking Harkins Expires May 15, 2010 [Page 9]

Internet-Draft Secure PSK Authentication for IKE November 2009 terminates. If the co-factor of the curve is not equal to one the order of the candidate SKE is checked to make sure it can safely be used as a generator in the protocol. If the co-factor of the curve multiplied by the candidate SKE equals the point-at-infinity then the candidate SKE is discarded, the counter is incremented, a new ske- seed is generated and the hunting and pecking continues. If it does not equal the point-at-infinity the candidate SKE becomes the SKE and hunting and pecking terminates. (Note: the point multiplied by the co-factor does not become SKE, it is only used to determine the order of the group defined with SKE as a generator). Algorithmically, the process looks like this: found = 0 counter = 1 do { ske-seed = H(psk | counter | Ni | Nr) ske-value = prf+(swd-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 if (f == 1) then found = 1 else P = f*SKE if (P != "O") then found = 1 fi fi fi fi counter = counter + 1 } while (found == 0) Figure 1: Fixing SKE for Elliptic Curves Harkins Expires May 15, 2010 [Page 10]

Internet-Draft Secure PSK Authentication for IKE November 2009 8.1.2. Prime Modulus SKE For a finite cyclic group based on exponentiation of a generator, G, modulo a large prime p it is not necessary to hunt-and-peck to find SKE. An SKE can be computed in a sub-field of the group directly using the prime, p, and order r. First, the random function is used to generate a seed: ske-seed = H(psk | Ni | Nr) Then the ske-seed is expanded using prf+ to the length of the prime, modulo the prime. ske-gen = prf+(ske-seed, "IKE Affixing the SKE") ske-gen = ske-gen mod p The ske is then computed by exponentiating the ske-gen to the value ((p-1)/r) modulo the prime. SKE = ske-gen ^ ((p-1)/r) mod p 8.2. Encoding of Elements Encoding of an element in a finite cyclic group depends on the type of group. For MODP groups the element is treated as an unsigned integer less than the prime that defines the group. The length of each MODP element MUST have a bit length equal to the bit length of the prime. This length is enforced, if necessary, by prepending the integer with zeros until the required length is achieved. For ECP groups the element is treated as two unsigned integers, each less than the prime that defines the group, representing the coordinates of the point. The first is the x-coordinate and the second is the y-coordinate. Each of these two integers MUST have a bit length equal to the bit length of the prime. This length is enforced, if necessary, by prepending the integer with zeros until the required length is achieved. 8.3. Generation of a Commit 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: Harkins Expires May 15, 2010 [Page 11]

Internet-Draft Secure PSK Authentication for IKE November 2009 Scalar = (private + mask) mod r and the inverse of the scalar operation with the mask and SWE becomes the Element component. Element = inverse(scalar-op(mask, SKE)) The Commit payload consists of the Scalar followed by the Element. 8.4. Generation of a Confirm After receipt of a peer's Commit and generation of one's own Commit a candidate shared secret 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, peer-scalar and peer- element, respectively, the shared secret, ss, is generated as: ss = scalar-op(private, element-op(peer-element, scalar-op(peer- scalar, SKE))) For the purposes of subsequent computation, the bit length of ss 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. Using the shared secret, ss, and the generated Scalar and Element, self-scalar and self-element, respectively, and the received Scalar and Element, peer-scalar and peer-element, respectively, an authenticating Tag is generated as: Tag = H(self-scalar | peer-scalar | F(self-element) | F(peer- element) | ss) The Commit payload consists of the authenticating Tag. Harkins Expires May 15, 2010 [Page 12]

Internet-Draft Secure PSK Authentication for IKE November 2009 8.5. Commit Payload 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 Commit Payload SHALL be indicated in both IKEv1 and IKEv2 with TBD1 from the [IKEV2-IANA] registry maintained by IANA. The Scalar SHALL be encoded as an unsigned integer with a bit length equal to the bit length of the order of the group used in the exchange. This length is enforced, if necessary, by prepending the integer with zeros until the required length is achieved. The Element is encoded according to Section 8.2. 8.6. Confirm Payload The Confirm 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 ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ! ! ~ Tag ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Confirm Payload SHALL be indicated in both IKEv1 and IKEv2 with TBD2 from the [IKEV2-IANA] registry maintained by IANA. Harkins Expires May 15, 2010 [Page 13]

Internet-Draft Secure PSK Authentication for IKE November 2009 8.7. IKEv1 Messaging Secure pre-shared key authentication can be used in either Main Mode (see Figure 2) or Aggressive Mode (see Figure 3) with IKEv1 and SHALL be indicated by negotiation of the TBD3 Authentication Method from the [IKEV1-IANA] registry maintained by IANA, in the SA payload. When using IKEv1 the "C" (critical) bit from Section 8.5 and Section 8.6 MUST be clear (i.e. a value of zero). Initiator Responder ----------- ----------- HDR, SAi --> <-- HDR, SAr HDR, KEi, Ni --> <-- HDR, KEr, Nr HDR*, IDii, Commit --> <-- HDR*, IDir, Commit, Confirm HDR*, Confirm, HASH_I --> <-- HDR*, HASH_R Figure 2: Secure PSK in Main Mode Initiator Responder ----------- ----------- HDR, SAi, KEi, Ni, IDii, Commit --> <-- HDR, SAr, KEr, Nr, IDir, Commit, Confirm HDR, Confirm, HASH_I --> <-- HDR, HASH_R Figure 3: Secure PSK in Aggressive Mode For secure pre-shared key authentication with IKEv1 the SKEYID value is computed as follows: SKEYID = prf(Ni_b | Nr_b, g^xy) And 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) HASH_R = prf(SKEYID, ss | g^xr | g^xi | CKY-R | CKY-I | SA_ib | IDir_b) Harkins Expires May 15, 2010 [Page 14]

Internet-Draft Secure PSK Authentication for IKE November 2009 Where "ss" is the shared secret derived in Section 8.4. 8.8. IKEv2 Messaging The specific authentication method being employed in IKEv2 is not negotiated, like in IKEv1. It is inferred from the components of the message. The presence of a Commit payload in second message sent by the Initiator indicates an intention to perform secure pre-shared key authentication (see Figure 4). The critical bit is used in both the Commit and Confirm payloads to allow for backwards compatibility and MUST be set (i.e. a value of one). Initiator Responder ----------- ----------- HDR, SAi1, KEi, Ni --> <-- HDR, SAr1, KEr, Nr, [CERTREQ] HDR, SK {IDi, Commit, [IDr,] SAi2, TSi, TSr} --> <-- HDR, SK {IDr, Commit, Confirm} HDR, SK {Confirm, AUTH} --> <-- HDR, SK {AUTH, SAr2, TSi, TSr} Figure 4: Secure PSK in IKEv2 In the case of secure pre-shared key authentication the AUTH value is computed as: AUTH = prf(ss, <msg octets>) Where "ss" is the shared secret derived in Section 8.4. The Authentication Method indicated in the AUTH payload SHALL be TBD4 from the [IKEV2-IANA] registry maintained by IANA. 9. IANA Considerations IANA SHALL assign values for the Commit payload (Section 8.5) and the Confirm payload (Section 8.6), and replace TBD1 and TBD2, respectively, above, from the [IKEV2-IANA] of "IKEv2 Payload Types". IANA SHALL assign a value for "Secure Shared Key Authentication", replacing TBD3 above, from the IPSEC Authentication Method registry in [IKEV1-IANA]. IANA SHALL assign a value for "Secure Shared Key Authentication", replacing TBD4 above, from the IKEv2 Authentication Method registry in [IKEV2-IANA]. Harkins Expires May 15, 2010 [Page 15]

Internet-Draft Secure PSK Authentication for IKE November 2009 10. Security Considerations Both the Initiator and Responder obtain a shared secret, "ss" (see Section 8.4) 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 Initiator of the exchange and send a forged Commit payload. The attacker then waits until it receives a Commit and a Confirm from the Responder. Now the attacker can attempt to run through all possible values of the pre-shared key, computing SKE (see Section 8.1), computing "ss" (see Section 8.4), 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 to construct his Confirm. 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 Confirm 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 Harkins Expires May 15, 2010 [Page 16]

Internet-Draft Secure PSK Authentication for IKE November 2009 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 random function, H, 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 H, 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 pre-shared key. For a more detailed discussion of the security of the key exchange underlying this authentication method see [SAE] and [EAPPWD]. 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. Lily Chen provided useful advice on how to "hash into an element" in a finite cyclic group. Hugo Krawczyk suggested the particular instantiation of the random function, H. 12. References Harkins Expires May 15, 2010 [Page 17]

Internet-Draft Secure PSK Authentication for IKE November 2009 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>. [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. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006. 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. [EAPPWD] Harkins, D. and G. Zorn, "EAP Authentication Using Only A Password", draft-harkins-emu-eap-pwd-12 (work in progress), October 2009. [RANDOR] Bellare, M. and P. Rogaway, "Random Oracles are Practical: Harkins Expires May 15, 2010 [Page 18]

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Internet-Draft Secure PSK Authentication for IKE November 2009
A Paradigm for Designing Efficient Protocols", Proceedings
of the 1st ACM Conference on Computer and Communication
Security, ACM Press, 1993,
<http://www.cs.ucsd.edu/~mihir/papers/ro.pdf>.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[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.
Author's Address
Dan Harkins
Aruba Networks
1322 Crossman Avenue
Sunnyvale, CA 94089-1113
United States of America
Email: dharkins@arubanetworks.com
Harkins Expires May 15, 2010 [Page 19]
```