--- 1/draft-fluhrer-qr-ikev2-02.txt 2016-10-28 07:16:43.435828607 -0700 +++ 2/draft-fluhrer-qr-ikev2-03.txt 2016-10-28 07:16:43.455829098 -0700 @@ -1,19 +1,19 @@ Internet Engineering Task Force S. Fluhrer Internet-Draft D. McGrew Intended status: Informational P. Kampanakis -Expires: February 5, 2017 Cisco Systems - August 4, 2016 +Expires: May 1, 2017 Cisco Systems + October 28, 2016 Postquantum Preshared Keys for IKEv2 - draft-fluhrer-qr-ikev2-02 + draft-fluhrer-qr-ikev2-03 Abstract This document describes an extension of IKEv2 to allow it to be resistant to a Quantum Computer, by using preshared keys Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. @@ -21,21 +21,21 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on February 5, 2017. + This Internet-Draft will expire on May 1, 2017. Copyright Notice Copyright (c) 2016 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 @@ -43,31 +43,30 @@ 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3 - 2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 3 + 2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 3.1. Computing SKEYSEED . . . . . . . . . . . . . . . . . . . 6 - 3.2. Verifying preshared key . . . . . . . . . . . . . . . . . 7 - 3.3. Child SAs . . . . . . . . . . . . . . . . . . . . . . . . 7 - 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 - 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 - 5.1. Normative References . . . . . . . . . . . . . . . . . . 8 - 5.2. Informational References . . . . . . . . . . . . . . . . 9 - Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 9 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 + 4. Creating Child SA Keying Material . . . . . . . . . . . . . . 5 + 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 + 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 6.1. Normative References . . . . . . . . . . . . . . . . . . 7 + 6.2. Informational References . . . . . . . . . . . . . . . . 7 + Appendix A. Discussion and Rationale . . . . . . . . . . . . . . 7 + Appendix B. Acknowledgement . . . . . . . . . . . . . . . . . . 9 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction It is an open question whether or not it is feasible to build a quantum computer, but if it is, many of the cryptographic algorithms and protocols currently in use would be insecure. A quantum computer would be able to solve DH and ECDH problems, and this would imply that the security of existing IKEv2 systems would be compromised. IKEv1 when used with preshared keys does not share this vulnerability, because those keys are one of the inputs to the key @@ -80,35 +79,46 @@ This document describes a way to extend IKEv2 to have a similar property; assuming that the two end systems share a long secret key, then the resulting exchange is quantum resistant. By bringing postquantum security to IKEv2, this note removes the need to use an obsolete version of the Internet Key Exchange in order to achieve that security goal. The general idea is that we add an additional secret that is shared between the initiator and the responder; this secret is in addition to the authentication method that is already provided within IKEv2. - We stir in this secret when generating the IKE keys (along with the - parameters that IKEv2 normally uses); this secret adds quantum - resistance to the exchange. + We stir in this secret when generating the key material (KEYMAT) keys + for the child SAs (along with the parameters that IKEv2 normally + uses); this secret provides quantum resistance to the IPsec SAs. It was considered important to minimize the changes to IKEv2. The existing mechanisms to do authentication and key exchange remain in place (that is, we continue to do (EC)DH, and potentially a PKI authentication if configured). This does not replace the authentication checks that the protocol does; instead, it is done as a parallel check. 1.1. Changes Changes in this draft from the previous versions + draft-02 + + - Simplified the protocol by stirring in the preshared key into the + child SAs; this avoids the problem of having the responder decide + which preshared key to use (as it knows the initiator identity at + that point); it does mean that someone with a Quantum Computer can + recover the initial IKE negotation. + + - Removed positive endorsements of various algorithms. Retained + warnings about algorithms known to be weak against a Quantum Computer + draft-01 - Added explicit guidance as to what IKE and IPsec algorithms are Quantum Resistant draft-00 - We switched from using vendor ID's to transmit the additional data to notifications @@ -128,416 +138,266 @@ 1.2. Requirements Language 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. Assumptions We assume that each IKE peer (both the initiator and the responder) has an optional Postquantum Preshared Key (PPK) (potentially on a - per-peer basis), and also has a configurable flag that determines - whether this postquantum preshared key is mandatory. This preshared - key is independent of the preshared key (if any) that the IKEv2 - protocol uses to perform authentication. - - In addition, we assume that the initiator knows which PPK to use with - the peer it is initiating to (for instance, if it knows the peer, - then it can determine which PPK will be used). + per-peer basis, selected by peer identity), and also has a + configurable flag that determines whether this postquantum preshared + key is mandatory. This preshared key is independent of the preshared + key (if any that the IKEv2 protocol uses to perform authentication. 3. Exchanges If the initiator has a configured postquantum preshared key (whether or not it is optional), then it will include a notify payload in its - initial exchange as follows: + initial encrypted exchange as follows: Initiator Responder ------------------------------------------------------------------ - HDR, SAi1, KEi, Ni, N(PPK_REQUEST) ---> + HDR, SK {IDi, [CERT,] [CERTREQ,] + [IDr,] AUTH, SAi2, + TS, TSr, N(PPK_NOTIFY)} ---> - N(PPK_REQUEST) is a status notification payload with the type [TBA]; + N(PPK_NOTIFY) is a status notification payload with the type [TBA]; it has a protocol ID of 0, and no SPI and no notification data associated with it. - When the responder recieves the initial exchange with the notify - payload, then (if it is configured to support PPK), it responds with: - - Initiator Responder - ------------------------------------------------------------------ - <--- HDR, N(COOKIE), N(PPK_ENCODE) - - If it is not configured to support PPK, the responder continues with - the standard IKEv2 protocol. - - In other words, it asks for the responder to generate and send a - cookie in its responses (as listed in section 2.6 of RFC7296), and in - addition, include a notify that gives details of how the initiator - should indicate what the PPK is. This notification payload has the - type [TBA}; it has a protocol ID of 0, and no SPI; the notification - data is of the format: - - 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 - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | PPK Indicator Algorithm | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | PPK Indicator Input (variable) | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - The PPK Indicator Algorithm is a 4 byte word that states which PPK - indicator to use. That is, it gives the encoding format for the PPK - that should be used is given to the responder. At present, the only - assigned encoding is 0x00000001, which indicates that AES256_SHA256 - will be used (as explained below). - - PPK Indicator Input is a data input to the PPK indicator Algorithm; - its length will depend on the PPK indicator; for the indicator - AES256_SHA256, this PPK Indicator Input is 16 bytes. - - The contents of this PPK Indicator Input is selected by responder - policy; below we give trade-offs of the various possibilities - - When the initiator receives this notification, it responds as - follows: + When the responder receives the initial encrypted exchange, it checks + to see if it received a notify within that exchange, is configured to + support PPK with the initiator's identity, and whether that use is + mandatory. If the notify was received, and the responder does have a + PPK for that identity, then it responds with the standard IKE + response with the PPK_NOTIFY notify message included, namely: Initiator Responder ------------------------------------------------------------------ - HDR, N(COOKIE), SAi1, KEi, Ni, N(PPK_REQUEST) ---> - - This is the standard IKEv2 cookie response, with a PPK_REQUEST - notification added - - N(PPK_REQUEST) is a status notification payload with the type [TBA]; - it has a protocol ID of 0, and no SPI; however this time, the - notification data as 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 - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | PPK Indicator Algorithm | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | PPK Indicator Input (variable) | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | PPK Indicator (variable) | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - The PPK Indicator Algorithm and PPK Indicator Input are precisely the - same as was given in the PPK_ENCODE format (as is repeated in case - the responder ran this cookie protocol in a stateless manner). The - PPK Indicator is the encoded version of the PPK that the initiator - has. The idea behind this is to allow the responder to select which - PPK it should use when it derives the IKEv2 keys. + <--- HDR, SK {IDr, [CERT,] AUTH, + SAr2, TSi, TSr, N(PPK_NOTIFY)} - For the AES256_SHA256 PPK indicator, the PPK Indicator is 16 bytes. - To compute it, we use HMAC_SHA256(PPK, "A") as the 256 bit AES key to - encrypt the 16 bytes on PPK Indicator Input (in ECB mode), where "A" - is a string consisting of a single 0x41 octet. + If the responder is not configured to support PPK with that identity, + it continues with the standard IKE protocol, not including the + notification. - When the responder receives this notification payload, it verifies - that the PPK Indicator Algorithm is as it has specified, and it MAY - verify that the PPK Indicator Input is as it has specified. If - everything is on the level, it scans through its list of configured - postquantum preshared keys, and determines which one it is (possibly - (assuming AES256_SHA256_PPK) by computing AES256(HMAC_SHA256(PPK, - "A"), PPK_Indicator_Input) and comparing that value to the 16 bytes - within the payload. Alternatively, it may have preselected a PPK - Indicator Input, and has precomputed (again assuming - AES256_SHA256_PPK) AES256(HMAC_SHA256(PPK, "A"), PPK_Indicator_Input) - for each PPK it knows about (in which case, this is a simple search). + If the responder is configured to support PPK with that identity, and + it does not receive the notification, then if the PPK usage is + configured as mandatory, it MUST abort the exchange. If the PPK + usage is configured as optional, it continues with the standard IKE + protocol, not including the notification. - If the responder finds a value that matches the payload for a - particular PPK, that indicates that the intiator and responder share - a PPK and can make use of this extension. Upon finding such a - preshared key, the responder includes a notification payload with the - response: + This table summarizes the above logic by the responder - Initiator Responder + Received Nonce Have PPK PPK Mandatory Action ------------------------------------------------------------------ - <--- HDR, SAr1, Ker, Nr, [CERTREQ], N(PPK_ACK) - - N(PPK_ACK) is a status notification payload with the type [TBA]; it - has a protocol ID of 0, and no SPI and no notification data - associated with it. This notification serves as a postquantum - preshared key confirmation. - - If the responder does not find such a PPK, then it MAY continue with - the protocol without including a notification ID (if it is configured - to not have mandatory preshared keys), or it MAY abort the exchange - (if it configured to make preshared keys mandatory). - - When the initiator receives the response, it MUST check for the - presence of the notification. If it receives one, it marks the SA as - using the configured preshared key; if it does not receive one, it - MAY either abort the exchange (if the preshared key was configured as - mandatory), or it MAY continue without using the preshared key (if - the preshared key was configured as optional). - -3.1. Computing SKEYSEED - - When it comes time to generate the keying material during the initial - Exchange, the implementation (both the initiator and the responder) - checks to see if there was an agreed-upon preshared key. If there - was, then both sides use this alternative formula: - - SKEYSEED = prf(prf(PPK, Ni) | prf(PPK, Nr), g^ir) - (SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) = - prf+(SKEYSEED, prf(PPK, Ni) | prf(PPK, Nr) | - SPIi | SPIr) + No No * Standard IKE protocol + No Yes No Standard IKE protocol + No Yes Yes Abort negotiation + Yes No * Standard IKE protocol + Yes Yes * Include PPK_NOTIFY Nonce - where PPK is the postquantum preshared key, Ni, Nr are the nonces - exchanged in the IKEv2 exchange, and prf is the pseudorandom function - that was negotiated for this SA. + When the initiator receives the response, then (if it is configured + to use a PPK with the responder), then it checks for the presense of + the notification. If it receives one, it marks the SA as using the + configured PPK; if it does not receive one, it MUST either abort the + exchange (if the PPK was configured as mandatory), or it MUST + continue without using the PPK (if the PPK was configured as + optional). - We reuse the negotiated PRF to transform the received nonces. We use - this PRF, rather than negotiating a separate one, because this PRF is - agreed by both sides to have sufficient security properties - (otherwise, they would have negotiated something else), and so that - we don't need to specify a separate negotiation procedure. + The protocol continues as standard until it comes time to compute the + child SA keying material. -3.2. Verifying preshared key +4. Creating Child SA Keying Material - Once both the initiator and the responder have exchanged identities, - they both double-check with their policy database to verify that they - were configured to use those preshared keys when negotiating with the - peer. If they are not, they MUST abort the exchange. + When it comes time to generate the keying material for a child SA, + the implementation (both the initiator and the responder) checks to + see if they agreed to use a PPK. If they did, then they look up + (based on the peer's identity) the configured PPK, and then both + sides use one of these alternative formula (based on whether an + optional Diffie-Hellman was included): -3.3. Child SAs + Ni' = prf(PPK, Ni) + Nr' = prf(PPK, Nr) + KEYMAT = prf+(SK_d, Ni' | Nr') - When you create a child SA, the initiator and the responder will - transform the nonces using the same PPK as they used during the - original IKE SA negotiation. That is, they will use one of the - alternative derivations (depending on whether an optional Diffie- - Hellman was included): + or - KEYMAT = prf+(SK_d, prf(PPK, Ni) | prf(PPK, Nr)) + Ni' = prf(PPK, Ni) + Nr' = prf(PPK, Nr) + KEYMAT = prf+(SK_d, g^ir (new) | Ni' | Nr') - or + where PPK is the configured postquantum preshared key, Ni, Nr are the + nonces from the IKE_SA_INIT exchange if this require is the first + Child SA created or the fresh Ni and Nr from the CREATE_CHILD_SA + exchange if this is a subsequent creation, and prf is the + pseudorandom function that was negotiated for this SA. - KEYMAT = prf+(SK_d, g^ir (new) | - prf(PPK, Ni) | prf(PPK, Nr)) + This is the standard IKE KEYMAT generation, except that the nonces + are transformed (via the negotiated PRF function) using the preshared + PPK value + We use this negotiated PRF, rather than negotiating a separate one, + because this PRF is agreed by both sides to have sufficient security + properties (otherwise, they would have negotiated something else), + and so that we don't need to specify a separate negotiation + procedure. When you rekey an IKE SA (generating a fresh SKEYSEED), the initiator and the responder will transform the nonces using the same PPK as they used during the original IKE SA negotiation. That is, they will use the alternate derivation: - SKEYSEED = prf( SK_d (old), g^ir (new) | - prf(PPK, Ni) | prf(PPK, Nr)) + Ni' = prf(PPK, Ni) + Nr' = prf(PPK, Nr) + SKEYSEED = prf( SK_d (old), g^ir (new) | Ni' | Nr' ) (SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) = - prf+(SKEYSEED, prf(PPK, Ni) | prf(PPK, Nr) | - SPIi | SPIr) + prf+(SKEYSEED, Ni' | Nr' | SPIi | SPIr) -4. Security Considerations + An implementation MAY rekey the initial IKE SA immediately after + negotiating it; this would reduce the amount of data available to an + attacker with a Quantum Computer - The PPK Indicator Input within the PPK_ENCODE notification are there - to prevent anyone from deducing whether two different exchanges use - the same PPK values. To prevent such a leakage, servers are - encouraged to vary them as much as possible (however, they may want - to repeat values to speed up the search for the PPK). Repeating - these values places the anonymity at risk; however it has no other - security implication. +5. Security Considerations Quantum computers are able to perform Grover's algorithm; that effectively halves the size of a symmetric key. Because of this, the user SHOULD ensure that the postquantum preshared key used has at least 256 bits of entropy, in order to provide a 128 bit security level. - In addition, the policy SHOULD be set to negotiate only quantum- - resistant symmetric algorithms; here is a list of defined IKEv2 (and - IPsec) algorithms which are believed to be Quantum Resistant - - IKE Encryption algorithm: assuming that the negotiated keysize is >= - 256, then all of: ENCR_AES_CBC, ENCR_AES_CTR, ENCR_AES_CCM_*, - ENCR_AES-GCM, ENCR_CHACHA20_POLY1305, ENCR_CAMELLIA, ENCR_RC5, - ENCR_BLOWFISH - - IKE PRF: PRF_HMAC_SHA2_256, PRF_HMAC_SHA2_384, PRF_SHA2_512. Note - that PRF_AES128_XCBC and PRF_AES128_CBC are not on this list, even - though they can use larger keys, because they use a 128 bit key - internally - - IKE Integrity algorithm: AUTH_HMAC_SHA2_256, AUTH_HMAC_SHA2_384, - AUTH_HMAC_SHA2_512, AUTH_AES_256_GMAC + Although this protocol preserves all the security properties of IKE + against adversaries with conventional computers, this protocol allows + an adversary with a Quantum Computer to decrypt all traffic encrypted + with the initial IKE SA. In particular, it allows the adversary to + recover the identities of both sides. If there is IKE traffic other + than the identities that need to be protected against such an + adversary, one suggestion would be to form an initial IKE SA (which + is used to exchange identities), perhaps by using the protocol + documented in RFC6023. Then, you would immediately create a child + IKE SA (which is used to exchange everything else). Because the + child IKE SA keys are a function of the PPK (among other things), + traffic protected by that SA is secure against Quantum capable + adversaries. - AH Transforms: AH-SHA2-256, AH-SHA2-384, AH-SHA2-512, AH-AES-256-GMAC + In addition, the policy SHOULD be set to negotiate only quantum- + resistant symmetric algorithms; while this RFC doesn't claim to give + advise as to what algorithms are secure (as that may change based on + future cryptographical results), here is a list of defined IKEv2 and + IPsec algorithms that should NOT be used, as they are known not to be + Quantum Resistant - ESP Transforms: assuming that the negotiated keysize is >= 256, then - all of: ESP_AES-CBC, ESP_AES-CR, ESP_AES-CCM, ESP_AES-GCM, - ESP_CAMELLIA, ESP_RC5, ESP_BLOWFISH, ESP_NULL_AUTH_AES-GMAC + Any IKE Encryption algorithm, PRF or Integrity algorithm with key + size <256 bits - ESP Authentication algorithms: HMAC-SHA2-256, HMAC-SHA2-384, HMAC- - SHA2-512, AES-256-GMAC + Any ESP Transform with key size <256 bits -5. References + PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined to + be able to use an arbitrary key size, they convert it into a 128 bit + key internally -5.1. Normative References +6. References - [AES] National Institute of Technology, "Specification for the - Advanced Encryption Standard (AES)", 2001, . +6.1. Normative References [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014, . -5.2. Informational References +6.2. Informational References + + [RFC6023] Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A + Childless Initiation of the Internet Key Exchange Version + 2 (IKEv2) Security Association (SA)", RFC 6023, + DOI 10.17487/RFC6023, October 2010, + . [SPDP] McGrew, D., "A Secure Peer Discovery Protocol (SPDP)", 2001, . Appendix A. Discussion and Rationale The idea behind this is that while a Quantum Computer can easily reconstruct the shared secret of an (EC)DH exchange, they cannot as easily recover a secret from a symmetric exchange this makes the - SKEYSEED depend on both the symmetric PPK, and also the Diffie- - Hellman exchange. If we assume that the attacker knows everything - except the PPK during the key exchange, and there are 2**n plausible - PPK's, then a Quantum Computer (using Grover's algorithm) would take - O(2**(n/2)) time to recover the PPK. So, even if the (EC)DH can be - trivially solved, the attacker still can't recover any key material - unless they can find the PPK, and that's too difficult if the PPK has - enough entropy (say, 256 bits). + IPsec KEYMAT and any child SA's SKEYSEED depend on both the symmetric + PPK, and also the Diffie-Hellman exchange. If we assume that the + attacker knows everything except the PPK during the key exchange, and + there are 2**n plausible PPK's, then a Quantum Computer (using + Grover's algorithm) would take O(2**(n/2)) time to recover the PPK. + So, even if the (EC)DH can be trivially solved, the attacker still + can't recover any key material (except for the SK values for the + initial IKE exchange) unless they can find the PPK, and that's too + difficult if the PPK has enough entropy (say, 256 bits). Note that + we do allow an attacker with a Quantum Computer to rederive the + keying material for the initial IKE SA; this was a compromise to + allow the responder to select the correct PPK quickly. Another goal of this protocol is to minimize the number of changes within the IKEv2 protocol, and in particular, within the cryptography of IKEv2. By limiting our changes to notifications, and translating the nonces, it is hoped that this would be implementable, even on systems that perform much of the IKEv2 processing is in hardware. A third goal was to be friendly to incremental deployment in operational networks, for which we might not want to have a global shared key, and also if we're rolling this out incrementally. This is why we specifically try to allow the PPK to be dependent on the peer, and why we allow the PPK to be configured as optional. A fourth goal was to avoid violating any of the security goals of - IKEv2. One such goal is anonymity; that someone listening into the - exchanges cannot easily determine who is negotiating with whom. + IKEv2. The third and fourth goals are in partial conflict. In order to achieve postquantum security, we need to stir in the PPK when the keys are computed, however the keys are computed before we know who we're talking to (and so which PPK we should use). And, we can't just tell the other side which PPK to use, as we might use different PPK's for different peers, and so that would violate the anonymity goal. If we just (for example) included a hash of the PPK, someone listening in could easily tell when we're using the same PPK for different exchanges, and thus deduce that the systems are related. - The compromise we selected was to allow the responder to make the - trade-off between anonymity and efficiency (by including the PPK - Indicator Input, which varies how the PPK is encoded, and allowing - the responder to specify it). - - A responder who values anonymitity may select a random PPK Indicator - Input each time; in this case, the responder needs to do a linear - scan over all PPK's it has been configured with - - A responder who can't afford a linear scan could precompute a small - (possibly rolling) set of the PPK Indicator Inputs; in this case, it - would precompute how each PPK would be indicated. If it reissues the - same PPK Indicator Input to two different exchanges, someone would be - able to verify whether the same PPK was used; this is some loss of - anonymity; but is considerably more efficient. - - An alternative approach to solve this problem would be to do a normal - (non-QR) IKEv2 exchange, and when the two sides obtain identities, - see if they need to be QR, and if so, create an immediate IKEv2 child - SA (using the PPK). One issue with this is that someone with a - quantum computer could deduce the identities used; another issue is - the added complexity required by the IKE state machines. - - A slightly different approach to try to make this even more friendly - to IKEv2-based cryptographic hardware might be to use invertible - cryptography when we present the nonces to the kdf. The idea here is - in case we have IKEv2 hardware that insists on selecting its own - nonces (and so we won't be able to give a difference nonce to the - KDF); instead, we encrypt the nonce that we send (and decrypt the - nonce that we get). Of course, this means that the responder will - need to figure out which PPK we're using up front (based on the - notifications); we're not sure if this idea would be a net - improvement (especially since the transform we're proposing now is - cryptographically secure and simple). - - The reasoning behind the cryptography used: the values we use in the - AES256_SHA256 PPK Indicator Algorithm are cryptographically - independent of the values used during the SKEYSEED generation - (because, even if we use HMAC_256 as our PRF, HMAC_SHA256(PPK, A) is - independent of HMAC_SHA256(PPK, B) if A and B are different strings - (and as any real nonce must be longer than a single byte, there is - never a collision between that and "A". This independent stems from - the assumption that HMAC_SHA256 is a secure MAC. - - The method of encoding the PPK within the notification (using AES- - 256) was chosen as it met two goals: - - o Anonymity; given A, AES256_K1(A), B, AES256_K2(B), it's fairly - obvious that gives someone (even if they have a quantum computer) - no clue about whether K1==K2 (unless either A==B or AES256_K1(A)== - AES256_K2(B); both highly unlikely events if A and B are chosen - randomly). - - o Performance during the linear search; a responder could preexpand - the AES keys, and so comparing a potential PPK against a - notification from the initiator would amount to performing a - single AES block encryption and then doing a 16 byte comparison. - - The first goal is considered important; one of the goals of IKEv2 is - to provide anonymity. The second is considered important because the - linear scan directly affects scalability. While this draft allows - the server to gain performance at the cost of anonymity, it was - considered useful if we make the fully-anonymous method as attractive - as possible. This use of AES makes this linear scan as cheap as - possible (while preserving security). + The compromise we selected was to stir in the PPK in all the derived + keys except the initial IKE SA keys, While this allows an attacker + with a Quantum Computer to recover the identities, a poll on the + IPsecME mailing list indicated that the majority of the people on the + list did not think anonymity was an important property within IKE. + We stir in the shared secret within the Child SA keying material; + this allows an implementation that wants to protect the other IKE- + based traffic to create an initial IKE SA to exchange identities, and + then immediately create a Child SA, and use that Child SA to exchange + the rest of the negotiation. - We allow the responder to specify the PPK Indicator Algorithm; this - was in response to requests for algorithm agility. At present, it - appears unlikely that there would be a need for an additional - encoding (as the current one is extremely conservative - cryptographically); however the option is there. + In addition, when we stir in the PPK, we always use it to modify a + nonce (using the negotiated PRF). We modify the nonce (rather than, + say, including the PPK in with the prf or prf+ computation directly) + so that this would be easier to implement on an hardware-based IKE + implementation; the prf computations might be built-in, but the + nonces would be external inputs, and so modifying those would + minimize the changes. - The current draft forces a cookie exchange, and hence adds a round - trip over the normal IKEv2 operation. This was done to allow the - server to specify the PPK Indicator algorithm. While as additional - round trip may seem costly, it does not invalidate this proposal, The - reason for this proposal is to give an alternative to IKEv1 with - preshared keys. While this additional round trip may seem costly, it - is important to note that, even with the additional round trip, this - proposal is still cheaper than IKEv1. Thus the mechanisms specified - in this note meet the goal of providing a better alternative than - relying on an obsolete version of the protocol for post quantum - security. +Appendix B. Acknowledgement - One issue that is currently open: what should happen if the initiator - guesses at the PPK Indicator Algorithm, selects a random PPK - Indicator Input, and includes that in the initial message? After - all, if the server follows the recommendation that the cookie - exchange is stateless, and if the server chooses the PPK Indicator - Input In randomly, it has no way to know that the client isn't - running this protocol as specified. If the responder supports that - PPK Indicator Algorithm, it could very well respond without forcing a - cookie exchange (which would eliminate a message exchange round). - It's not clear is whether we should endorse this mode of operation, - and explicitly state that if the server recieves such an initial - request, and it doesn't recognize the PPK Indicator Input, it should - act like it recieved an iniital PPK_REQUEST. + The idea of stirring in the PPK into the IPsec key generation process + was originally suggested on the list by Tero Kivinen. Authors' Addresses Scott Fluhrer Cisco Systems Email: sfluhrer@cisco.com David McGrew Cisco Systems