Internet DRAFT - draft-krywaniuk-ipsec-properties

draft-krywaniuk-ipsec-properties








Internet Engineering Task Force                         Andrew Krywaniuk
IP Security Working Group                                        Alcatel
Internet Draft                                              July 9, 2001



            Security Properties of the IPsec Protocol Suite
               <draft-krywaniuk-ipsec-properties-00.txt>

Status of this Memo

   This document is a submission to the IETF Internet Protocol Security
   (IPsec) Working Group. Comments are solicited and should be addressed
   to the working group mailing list (ipsec@lists.tislabs.com) or to the
   editor.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
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Copyright Notice


   This document is a product of the IETF's IPsec Working Group.
   Copyright (C) The Internet Society (2001).  All Rights Reserved.










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Abstract


   This document describes the "security properties" of the IPsec
   architecture and protocols, including ESP, AH, and IKE.

   By documenting these properties, we aim to provide a guide for users
   who wish to understand the abilities and limitations of the IPsec
   protocol suite. We also hope to provide motivation for future work in
   this area.








































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Table of Contents

   1.   Introduction.................................................4
   2.   Specification of Requirements................................4
   3.   General Approach.............................................4
   3.1 Terminology...................................................5
   4.   Confidentiality..............................................5
   4.1 Encryption Coverage...........................................5
   4.2 Traffic Flow..................................................6
   4.3 Cryptographic Mumbo-Jumbo.....................................6
   4.4 Identity Protection...........................................7
   5.   Authentication...............................................8
   5.1 Authentication Coverage.......................................8
   5.2 Cryptographic Mumbo-Jumbo.....................................8
   5.3 Host Authentication...........................................9
   5.4 User Authentication...........................................9
   6.   Key Generation...............................................9
   6.1 Rekeying.....................................................10
   6.2 Independence of Keying Material..............................10
   6.3 Cryptographic Mumbo-Jumbo....................................10
   6.4 Perfect Forward Secrecy......................................11
   6.5 Weak Keys....................................................11
   6.6 Ad Hoc Groups................................................12
   6.7 Key Strength.................................................12
   7.   Denial of Service...........................................13
   7.1 ESP Packet Spoofing..........................................13
   7.2 Memory Consumption...........................................13
   7.3 Time Consumption.............................................13
   7.4 Synchronization..............................................14
   8.   Miscellaneous...............................................14
   8.1 Replay.......................................................14
   9.   IANA Considerations.........................................15
   10.  Security Considerations.....................................15
   11.  References..................................................15
















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1.   Introduction

   Before you can know where you are going, you must first know where
   you have been.

   An analysis of IPsec by Counterpane researchers [Counterpane]
   complained that IPsec has a lack of clearly expressed design goals,
   and shows evidence of design by committee. We concur with these
   observations, in the sense that some features appear incomplete or
   are not used for the purpose for which they were intended. Part of
   the confusion comes from the fact that [ISAKMP] defines a large set
   of features; [IKE] only uses a subset of these features, but it does
   not clearly state which ones.

   The IPsec working group has undertaken a project to redesign the IKE
   protocol in order to "simplify" it; there has also been talk of
   reducing the number of IPsec usage permutations by deprecating AH
   and/or tunnel mode. We believe that it is inappropriate to redesign a
   protocol until the existing protocol is well understood.

   Perhaps IPsec is well understood by some, but frequent questions on
   the developers' mailing list confirm that one cannot become an IPsec
   expert merely by reading the RFCs. Much valuable information is
   buried deep in the list archives or in the minds of its designers.

   Other protocol designers depend on IPsec for transport security; if
   they cannot clearly understand what security properties IPsec
   provides, they may use it incorrectly. The same could be said for
   IPsec users.


2.   Specification of Requirements

   This document shall use the keywords "MUST", "MUST NOT",
   "REQUIRED","SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED, "MAY", and "OPTIONAL" to describe requirements. They
   are to be interpreted as described in [Bradner].


3.   General Approach

   This document is not an introduction to IPsec, nor is it cryptography
   101. It is merely a description of the security properties associated
   with one particular security protocol, IPsec.

   Our intention is merely to document what exists today. In the few
   places where we discuss alternatives, they relate specifically to


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   known issues concerning the security properties of IPsec.

   Some of this information in this memo is already available in the
   RFCs, some is not; this document collects it all into one place.
   Sometimes the RFCs are ambiguous, for example in the case where a
   feature is described in ISAKMP, but not used in IKE; here, we attempt
   to resolve that ambiguity.

   The amount of space devoted to a particular property does not
   necessarily reflect on the importance of that property in the context
   of IPsec. For example, identity protection is discussed in some
   detail, even though its applicability is limited, precisely because
   the issues are complicated.


3.1   Terminology

   For the purposes of this document, "the IPsec protocol suite" shall
   consist of RFCs 2401 through 2412, plus any other documents which we
   consider relevant. We assume the use of ESP and/or AH SAs, negotiated
   by IKE, and used according to the rules prescribed by [ARCH]. We do
   not cover specialized applications, such as multicast and alternate
   key exchange protocols.

   The "security properties" we discuss include properties such as
   confidentiality, authentication, and resistance to DoS. We only
   attempt to define properties that can be measured objectively. As
   such, we do not discuss such issues as technical merit, ease of use,
   or level of complexity. The document focuses more on IKE than on
   ESP/AH, since IKE appears to have more intricate security properties.


4.   Confidentiality

   Traffic confidentiality is one of the main reasons for using IPsec.
   For better or for worse, IPsec provides two completely independent
   implementations of encryption: one in IKE and one in ESP.

   Obviously, in a network scenario not all data can be encrypted.
   Otherwise, it would be impossible to create SAs and route traffic.
   What data is encrypted and what data is not is a matter of some
   interest.


4.1   Encryption Coverage

   ESP encryption covers protocol layers 4 and above (and, potentially,
   some of the so-called layer 3.5 protocols). The IP header and any


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   additional lower-level headers are sent in the clear. If tunnel mode
   is used, the data in the inner header can be concealed, but some of
   that information will be copied into the outer header anyway, since
   it is needed for routing.

   In IKE, a large portion of the data must be sent in the clear, simply
   to bootstrap the negotiation. For example, an attacker can see which
   transforms are being used in IKE. Modern cryptological thinking
   postulates that revealing this kind of data is not a security
   weakness. Once the key exchange is complete, subsequent IKE data is
   encrypted.


4.2   Traffic Flow

   ESP hides such information as the layer 4 port and protocol, however
   some information about the traffic flow is leaked due to packet
   sizes. ESP allows an implementation to add padding to packets in
   order to conceal packet lengths; this is constrained to a maximum of
   255 bytes.

   In tunnel mode, when an edge device is applying the encryption, a
   snooper is generally unable to determine which end nodes the router
   is proxying for.


4.3   Cryptographic Mumbo-Jumbo

   Encryption in ESP and ISAKMP is typically deployed using the CBC
   (Cipher Block Chaining) mode of operation. CBC uses the previous
   block as an IV to the next block, which ensures that the IV is always
   random. A random IV makes it next to impossible that two blocks of
   ciphertext will be the same.

   CBC does not have the infinite error propagation property, which
   means that it is vulnerable to known-plaintext attacks. This is not
   to say that IPsec is vulnerable to known-plaintext attacks; all it
   means is that the chosen cipher must itself be secure against these
   attacks.

   Other modes of operation could be used and they would have different
   security properties. Encryption should not be used as a substitute
   for authentication. With block ciphers, an attacker is generally
   prevented from making a predictable change to the plaintext. This is
   not necessarily true for other types of ciphers, such as stream
   ciphers.




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4.4   Identity Protection

   IKE main mode purports to deliver a feature called identity
   protection, which means that the identities are not sent in the
   clear. However, in order to complete the authentication, one side
   must reveal its identity first. In main mode with public key
   signatures, the initiator reveals his identity first; therefore, an
   active attacker who impersonates the responder can determine the
   initiatorĘs identity.

   Even when the identity is protected, a host may need to send a
   "certificate request" in order to force the sender to include a
   certificate in a later message. The certificate request payload
   typically contains the name of the CA to be used, which reveals some
   limited information about the sender's identity to a passive snooper.

   This limited form of identity protection can only be used with public
   key signature authentication. Due to the particular construction of
   SKEYID_e in the case of preshared keys, the identity must be sent in
   the clear in order to generate the encryption key. In the case of
   public key encryption, the initiator's identity is protected but the
   responder's identity must be assumed in order to choose the correct
   public key. (IKE specifies that a hash of the certificate may be sent
   in order to identify the certificate, but this hash is an identity-
   equivalent, in the sense that it uniquely correlates to an identity.)

   In many cases, the IP address of the host implicitly describes the
   identity (e.g. the identity can be found by a DNS lookup); in these
   cases, identity protection is moot. Since the initiatorĘs identity is
   less likely to be implicit from an IP address than the responderĘs
   is, itĘs a shame that signature-based authentication provides higher
   protection to the responderĘs id. However, if the host is a gateway
   that is proxying for multiple end nodes, at least the identity of the
   end host is protected by tunnel mode encapsulation.

   In the case where the identity is sent in the clear, it could be a
   random binary string; IKE allows the transmission of unformatted
   identities using the ID_KEY_ID type. However, it would be desirable
   that the obfuscated identity not be an identity-equivalent, so that
   multiple logins by the same user could not be correlated. IKE does
   not provide this feature.

   IKE also provides a feature called Identity PFS, in which every quick
   mode exchange uses a new phase 1 SA. [IKE] doesnĘt specify why one
   might want to do this, but it does theoretically allow the host to
   delete the identity of the peer from memory, thus ensuring that it is
   not revealed even if the physical security of the box is compromised.



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   (Although it may be difficult to apply policy rules if the identity
   of the peer is not remembered.)


5.   Authentication

   Traffic encryption is not much use if the user at the other end is
   unknown or if the data could be forged. IPsec provides several types
   of authentication: packet authentication, exchange authentication,
   and host authentication.


5.1   Authentication Coverage

   Each AH packet contains an HMAC; likewise for ESP, assuming that ESP
   authentication is being used. ESP authentication covers the entire
   payload of the IP packet. AH also covers the non-mutable fields in
   the header.

   When tunnel mode is being used, AH has the same effective coverage as
   ESP, because the outer header is merely a transient routing header.
   If AH is being used to ensure that the header of the IP packet
   remains uncorrupted during transit, this is really only useful if any
   of the intermediate routers which interpret the header are also privy
   to the AH key.

   The IKE phase 1 hash covers sufficient material to bind the identity
   of the peer to unforgeable session data, such as the DH secret.
   However, phase 1 does not have full authentication coverage (a
   shortcoming which will most likely be fixed in a future version of
   the protocol). Consequently, optional payloads, such as notify
   messages and vendor ids, are not authenticated by the exchange. Even
   if these payloads are part of an encrypted message, an attacker can
   still corrupt them without being detected.

   Phase 2 messages are fully authenticated by an HMAC, with the
   exception of the ISAKMP header. Modifying the commit bit (in the
   header) is a potential DoS vulnerability.


5.2   Cryptographic Mumbo-Jumbo

   The HMAC that is used for packet authentication is truncated. This
   limits the amount of information an attacker can gather by analyzing
   the output.

   The HMAC functions that are used in IKE are also used as PRFs.
   Therefore, the output of the HMAC should always appear statistically


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   random.


5.3   Host Authentication

   The host is authenticated in IKE phase 1 by the generation of a
   public key signature, an HMAC signature, or by a proof of possession
   of a decryption key. The obvious man in the middle attack is thwarted
   by including the Diffie-Hellman public keys in the HASH_I/HASH_R
   values which are generated, signed, or encrypted.

   Authentication using public key signatures provides the non-
   repudiation property, which means that one party could potentially
   prove (possibly in a court of law) that a phase 1 negotiation between
   the two parties took place. However, this does not prove to a third
   party that any data sent with the negotiated keys is genuine. The
   other authentication methods do not have this property.

   When authenticating with preshared keys, the strength of the
   authentication is based on the effective entropy of the secret. When
   authenticating with public key encryption, the strength of the
   authentication is based on the length of the public key. Likewise for
   public key signatures, but as an additional wrinkle the strength of
   the MAC algorithm is also important. Since all the inputs to the MAC
   are sent on the wire except possibly the IDs (which can be guessed),
   the strength of the PK signature is limited by the difficulty of
   finding a collision in the MAC function.


5.4   User Authentication

   While IKE provides a number of ways to identify the peer, this
   identity is not communicated up to the application level. This means
   that all traffic-level authorization must ultimately be applied to
   IPs and ports.

   The application layer will typically have to perform its own
   authentication stage since there is no well-defined interface to
   extract the authenticated id from the IKE process. The upside of this
   limitation is that a simple username/password protocol is obviously
   more secure when it is sent across an ESP tunnel than when it is
   performed in the clear.


6.   Key Generation

   Key generation is the most important function of the quick mode
   exchange. Key generation is also part of phase 1, since ISAKMP needs


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   its own session keys.

   The properties of key generation are more complicated and harder to
   explain than most other security properties. Nonetheless, we will
   make an attempt.


6.1   Rekeying

   One purpose of rekeying is to thwart cryptanalysis by limiting the
   amount of ciphertext that an attacker can examine, but the main
   purpose is simply to limit the consequences of a compromised key.

   [IKE] defines two different types of lifetimes, time-based and
   traffic-based. Time-based lifetimes protect against the possibility
   that a key will be compromised by brute force; traffic-based
   lifetimes guard against attacks based on gathering ciphertext.

   [IKE] also proposes a third lifetime for phase 1 SAs, based on the
   number of quick modes used with this SA. The justification given for
   this lifetime is suspect because a PRF can provide keying material
   for a large number of random keys, and these keys are not revealed to
   an attacker for analysis. Nonetheless, this lifetime makes sense
   because it correlates strongly with the volume of IPsec traffic.


6.2   Independence of Keying Material

   The keys negotiated by IKE are derived from the Diffie-Hellman
   secret, some random session data, and possibly a preshared key. This
   information is run through a pseudo-random function in order to
   generate a key.

   The keys generated by IKE are never derived directly from each other,
   nor are they reused for multiple purposes. Each encryption or
   authentication key is created by an HMAC-based PRF, which is keyed by
   a shared primitive key that is never revealed.

   The PRF used in IKE must be a strong one-way function. This means
   that even if one key is compromised, other keys created from the same
   DH secret cannot be cracked unless the PRF is reversed.


6.3   Cryptographic Mumbo-Jumbo

   All the components of the key material, including the DH secret are
   HMAC'ed before they are used. This ensures that any analytical attack



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   on the key exchange function will not directly translate into an
   analytical attack on the key generation function.

   Wherever possible, the SKEYID is derived from a secret value other
   than g^xy (this is not possible in the case of public key
   signatures). In the case where keys are generated from each other
   (e.g. SKEYID_d -> SKEYID_a -> SKEYID_e), g^xy is reintroduced at
   every stage so that the key is always directly based on a shared
   primitive.

   A more detailed description of the motivation for SKEYID construction
   is given in [Krawczyk].


6.4   Perfect Forward Secrecy

   IKE also provides a mode called perfect forward secrecy, in which a
   separate DH secret is generated during each quick mode exchange. The
   term PFS is a bit misleading, since "forward secrecy" generally
   refers to the property that compromise of the long-term credential
   does not reveal the session keys; IKE phase 1 already provides
   forward secrecy because the key is based on a DH secret.

   Why might one want to use PFS?

   a) If you donĘt fully trust the one-way property of the PRF.

   b) In order to limit the consequences if the box is physically
      compromised.

   The point of item (b) above is that PFS does not significantly
   increase the security of IKE against long-term cryptanalysis. An
   attacker who can crack the phase 1 DH exchange can presumably crack a
   second DH exchange with equivalent work.

   PFS provides additional security, but the work factor is linear.
   Alternate countermeasures, such as simply increasing the modulus of
   the DH group from phase 1, would provide better than linear security.
   What PFS does provide is a faster (but less secure) alternative to
   phase 1 rekeying.


6.5   Weak Keys

   IKE mandates the use of weak key checks. In practice, weak keys are
   very unlikely to be generated randomly, and an attacker wonĘt be able
   to detect them if they are used.



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6.6   Ad Hoc Groups

   IKE allows the negotiation of ad hoc groups, either during phase 1,
   or after phase 1 using new group mode. To use new group mode, an
   implementation would have to trust the phase 1 group enough to use it
   in the short term, but not trust it for long term security.

   According to [IKE], implementations are meant to verify the primality
   of a proposed group before using it. The implications of this
   statement are interesting. Presumably, this is to detect
   implementation errors in the peer, rather than malfeasance.
   Otherwise, it would also be pertinent to send an attribute describing
   the algorithm by which the group was chosen (e.g. a seed, which is
   hashed, and then used as the starting point in a search for a prime).


6.7   Key Strength

   All keys in IKE are derived from a PRF output. A PRF provides a
   theoretically completely random key. Assuming that the cipher
   algorithm is strong against analysis, the most significant attack is
   brute force. Therefore, the strength of a key is approximately
   proportional to the key length.

   But the length of a key is not the only factor in determining its
   strength. The length of the encryption key must be large enough to
   thwart a direct attack, but the length of the DH secret used to
   generate the key is also important, as described in [Orman].

   [Beaulieu] notes that a third factor comes into play if one attempts
   to derive a large encryption key from a small hash output. As
   described in [IKE], the key material for an ISAKMP SA may be
   "stretched" using the following algorithm:

     Ka = K1 | K2 | K3

   and

     K1 = prf(SKEYID_e, 0)
     K2 = prf(SKEYID_e, K1)
     K3 = prf(SKEYID_e, K2)

   This definition does not fully utilize the entropy of the DH secret
   and further constrains the strength of the key to the length of the
   HMAC output. A similar limitation applies to the keys generated by
   quick mode when PFS is not being used.




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7.   Denial of Service

   IPsec provides some protection against denial of service attacks but
   also creates some new holes.


7.1   ESP Packet Spoofing

   IPsec ESP/AH authentication provides strong protection against DoS
   because any spoofed packets will be identified and discarded. The
   time lost to DoS is limited to the length of time required to verify
   an HMAC. ESP without authentication has less DoS protection because
   spoofed packets will pass IPsec processing.

   The fact that SPIs are random, and therefore unknown to an
   unprivileged attacker, provides additional protection against
   spoofing. If an authenticated user sends encrypted packets which
   cause DoS, the source of the attack will be obvious.

   However, packets that are sent in the clear can still cause DoS.
   Obviously, some packets, most notably the first few packets of IKE,
   must still be sent in the clear.


7.2   Memory Consumption

   ISAKMP reuses the stateless cookie idea from Photuris, but IKE does
   not provide a mode in which they can be used. This is odd because the
   elaborate construction of the phase 1 authentication hash is quite
   clearly intended to facilitate stateless operation. (Anti-clogging
   cookies are meant to prevent state clogging attacks akin to the TCP
   SYN attack.)

   [Huttunen] describes a proposal for enabling stateless operation
   (without lengthening main mode) by requiring the initiator to include
   an encrypted "state payload" which the responder generates in the
   third message of phase 1.


7.3   Time Consumption

   The key exchange and public key authentication operations of IKE
   phase 1 provide the greatest vehicle for DoS. An attacker can
   generate a fake key exchange or signature payload, forcing the
   responder to perform a time-consuming modular exponentiation
   operation (without significant work by the attacker).




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   IKE provides some protection against this attack in main mode by
   requiring an initial cookie exchange. Even though the cookie cannot
   be used for stateless operation, it performs a similar function as a
   nonce, proving the liveness of the initiator.

   Aggressive Mode is vulnerable because the signature must be generated
   before the cookie exchange is complete. The DH exponentiation can be
   delayed until the third packet is received, but at the cost of
   latency. Quick mode with PFS has a similar vulnerability if the
   exchange is replayed; again, the attack can be avoided (at the cost
   of latency) by delaying the computation.

   IKE does not provide protection against DDoS zombies. Countermeasures
   such as client puzzles exist, but there is no mechanism for using
   them with IKE.


7.4   Synchronization

   The lack of full authentication coverage in some IKE messages can
   allow an active attacker to exploit synchronization issues. For
   example, he can set the commit bit in the ISAKMP header, causing one
   side to wait for a CONNECTED notification that may never come.

   Alternately, he could add a vendor id to an IKE phase 1 message that
   would cause one side to enable a non-standard behaviour. Since the
   vendor id is not authenticated, this could cause one host to behave
   in a non-interoperable manner.

   An attacker can potentially prevent the delivery of delete
   notifications or forge invalid SPI/cookie messages, which could cause
   one side to delete an SA or to believe that an SA has been deleted by
   the peer. By forcing a connection to be repeatedly torn down, the
   attacker can cause a host to waste CPU in frequent renegotiations, to
   deny service to a legitimate user, or to waste memory maintaining an
   SA after the peer has disconnected.


8.   Miscellaneous

   There are a few additional security properties that do not fall into
   the above categories.


8.1   Replay

   Replay of ESP data is a vulnerability that depends on the upper layer
   protocol. Many session-based protocols will reject replayed data. ESP


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   and AH packets contain an anti-replay counter which may optionally be
   checked. This counter is not time-based, so it does not prevent an
   attacker from intercepting all packets, storing them, and then
   selectively delivering them at a future time. Replay protection can
   only be used in conjunction with packet authentication.

   ISAKMP does not provide explicit replay protection. Replay protection
   is accomplished in some exchanges (main mode, aggressive mode, quick
   mode) by sending a random value (e.g. a nonce) to the peer and having
   them return that value in a subsequent message. This technique is not
   possible with 1 or 2 message exchanges (new group mode, info mode).
   Also, replaying a quick mode exchange with PFS can cause DoS, as
   noted in section 7.3.

   It has been suggested that an implementation needs to remember all
   message ids it receives in order to avoid replayed messages; we
   believe that a better general-purpose solution is to add a message
   counter to ISAKMP SAs as described in [REPLAY].


9.   IANA Considerations

   This document does not require any assigned numbers.


10.  Security Considerations

   The focus of this document is security; hence security considerations
   permeate this specification.


11.  References

   [ARCH]  Kent, S., and R. Atkinson, "Security Architecture for the
           Internet Protocol", RFC 2401, November 1998.

   [Beaulieu] Beaulieu, S., "Generating 3DES keys from SKEYID_e",
           http://www.vpnc.org/ietf-ipsec/00.ipsec/msg01288.html, July
           2001.

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

   [Counterpane] Ferguson, Niels, and Schneier, Bruce, "A Cryptographic
           Evaluation of IPSec", http://www.counterpane.com, April 1999.

   [Huttunen] Huttunen, A., "Re: Future ISAKMP Denial of Service
           Vulnerablity Needs Addressing", http://www.vpnc.org/ietf-


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           ipsec/00.ipsec/msg00160.html, January 2000.

   [ISAKMP]Maughan, D., Schertler, M., Schneider, M., and J. Turner,
           "Internet Security Association and Key Management Protocol
           (ISAKMP)", RFC 2408, November 1998.

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

   [Krawczyk] Krawczyk, H., "Rationale for the definitions of SKEYID",
           http://www.vpnc.org/ietf-ipsec/mail-archive/msg00844.html,
           June 2001.

   [Orman] Orman, H., Hoffman, P., "Determining Strengths for Public
           Keys Used For Exchanging Symmetric Keys", draft-orman-public-
           key-lengths-02.txt, March 19, 2001 (work in progress)

   [REPLAY]Krywaniuk, A., "Using Isakmp Message Ids for Replay
           Protection", draft-krywaniuk-ipsec-antireplay-00.txt, July 9,
           2001 (work in progress)



   Authors' Addresses

     Andrew Krywaniuk
     Alcatel Networks Corporation
     600 March Road
     Kanata, ON
     Canada, K2K 2E6
     +1 (613) 784-4237
     E-mail: andrew.krywaniuk@alcatel.com




   The IPsec working group can be contacted via the IPsec working
   group's mailing list (ipsec@lists.tislabs.com) or through its chairs:

     Theodore Y. Ts'o
     tytso@MIT.EDU
     Massachusetts Institute of Technology

     Barbara Fraser
     byfraser@cisco.com
     Cisco Systems




IPsec Working Group                                            [Page 16]

Internet Draft             IPsec Properties                July 12, 2001


Expiration


   This document expires February 9th, 2002.














































IPsec Working Group                                            [Page 17]