Internet Engineering Task Force Andrew Krywaniuk
IP Security Working Group Alcatel
Internet Draft June 30, 2002
Security Properties of the IPsec Protocol Suite
<draft-ietf-ipsec-properties-02.txt>
Status of this Memo
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Krywaniuk Expires January 30th, 2003 [Page 1]
Internet Draft IPsec Properties June 30, 2002
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.....................................9
5.3 Host Authentication...........................................9
5.4 User Authentication..........................................10
6. Key Generation..............................................10
6.1 Rekeying.....................................................10
6.2 Independence of Keying Material..............................11
6.3 Cryptographic Mumbo-Jumbo....................................11
6.4 Perfect Forward Secrecy......................................12
6.5 Weak Keys....................................................13
6.6 Ad Hoc Groups................................................13
6.7 Key Strength.................................................14
7. Denial of Service...........................................14
7.1 ESP Packet Spoofing..........................................14
7.2 Memory Consumption...........................................15
7.3 Time Consumption.............................................15
7.4 Synchronization..............................................16
8. Miscellaneous...............................................16
8.1 Replay.......................................................16
8.2 Repudiation..................................................17
9. IANA Considerations.........................................18
10. Security Considerations.....................................18
11. Notes.......................................................18
12. References..................................................18
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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 documented.
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 suite of security protocols.
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 Denial of Service
(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
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ESP encryption covers protocol layers 4 and above (and, potentially,
some of the so-called layer 3.5 protocols). The IP header and any
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. A future version of ESP may allow extra padding, and even
completely bogus packets.
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. This situation is improved if a single tunnel mode
SA is used to carry all traffic between two protected networks,
rather than using separate SAs for each traffic flow.
Sending multiple traffic flows on a single SA allows a privileged
attacker who is behind one of the IPsec gateways to launch an
adaptive chosen plaintext attack against the encryption key, thus
compromising all traffic sent between the two networks. However,
modern ciphers are assumed to be resistant to this type of
cryptanalysis. The IPsec RFCs (e.g. [DES]) suggest that you may reuse
a block of ciphertext from another packet as an IV, but it is better
to use a completely random IV for each packet because of the attack
described in [Fluhrer].
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
pseudorandom. A random IV makes it next to impossible that two blocks
of ciphertext will be the same.
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CBC does not have the infinite error propagation property, which
means that it does not protect against known-plaintext analysis. 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 (as all modern ciphers are). Other modes of operation
could be used and they would have different security properties.
In general, encryption should not be used as a substitute for
authentication, although some new modes propose to combine the two.
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.
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. This it does, but with some caveats. 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 initiatorÆs 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. The drawback is
that mobile clients using preshared keys donÆt get identity
protection. We recommend that this be fixed in a future version of
IKE.
In the case of public key encryption, both identities are protected,
but protection for the responder is weak for two reasons: Firstly, in
order for the initiator to know where to send the packet, the
responder's identity must be linked to an IP address (this is true of
all the authentication methods). Secondly, in the case where the
initiator sends a hash of the identity, this hash is an identity-
equivalent, in the sense that it uniquely correlates to an identity.
This means that a snooper can correlate multiple SAs negotiated with
the same identity because the hash will be the same. Also, if the
snooper can guess which possible identities might correspond to the
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responder, he can test his assumption because the hash does not
contain any secret material.
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 initiatorÆs identity is
less likely to be implicit from an IP address than the responderÆs
is, itÆs a shame that signature-based authentication provides higher
protection to the responderÆs id. On the other hand, it is much
easier to mount an attack against the responderÆs id than against the
initiatorÆs.
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] doesnÆt 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.
(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
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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 should 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
random. Uri Blumenthal has stated that HMAC has never been proven to
be an adequate PRF, although we have no specific reason to believe it
isn't. [IKE] allows alternate PRFs to be used, but IANA has yet to
assign any.
5.3 Host Authentication
Depending on the chosen authentication method, 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.
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
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finding a collision in the MAC function.
5.4 User Authentication
While IKE provides a number of ways to identity the peer, there is no
standardized interface to communicate this identity up to the
application layer. Ultimately, all traffic-level authorization must
be applied to IPs and ports. If an application doesn't have a
mechanism to extract the authenticated id from the IKE process then
the application layer will have to perform its own, separate
authentication stage.
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, especially since the
use of IKE authentication rules out the possibility of a man-in-the-
middle attack.
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
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. Both
lifetimes limit the amount of data that is vulnerable if a key is
compromised.
[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
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an attacker for analysis. Nonetheless, this lifetime makes sense
because it correlates strongly with the volume of IPsec traffic.
When using strong ciphers with small block sizes (e.g. 3DES), the use
of rekeying to thwart cryptanalysis becomes more important. Due to
the birthday paradox, an attacker has a statistically significant
chance of detecting a collision in the output stream after he
collects about 2^(block size/2) blocks of encrypted data. If each
block is 64 bits, this works out to 32 gigabytes of encrypted
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 not 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 sent on the wire.
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.
In all cases, entropy for the key derivation is added explicitly by
means of a random nonce. The size of the nonce is not all that
important, but it should be larger than the square of the number of
keys that will be derived from the raw key material. (It does no good
to make the nonce larger than the HMAC output.)
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
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. One exception to this rule is noted below in section 6.7.
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A more detailed description of the motivation for SKEYID construction
is given in [Krawczyk]. Whether or not this elaborate derivation was
entirely necessary is a contentious issue.
6.4 Perfect Forward Secrecy
The description of PFS in IKE is complicated because there are
actually two different types of forward secrecy. PFS of the first
kind means that compromise of the long-term credential (e.g. an RSA
key) will not reveal any session keys. PFS of the second kind means
that an active attack on the system (e.g. the box is hacked) will not
reveal all of the expired session keys.
The original requirement for the IPsec WG was PFS of the first kind
(an earlier protocol called SKIP did not have this property). IKE
phase 1 automatically provides this feature because the key is based
on a per-session DH secret. This is the most important type of
forward secrecy, and it is not optional in IKE. Therefore, in the
context of IKE, the term PFS usually refers to the second type of
forward secrecy.
PFS of the second kind can be used to reduce the window of
vulnerability even further. In the case of a break-in, it is
desirable that only a limited amount of data should be compromised;
we call this the forward secrecy window. If the forward secrecy
window is 1 hour, then no session key should ever be kept in memory
for more than 1 hour after it is first used (and this includes the
key material from which this session key was derived).
Unfortunately, [IKE] does not explain the intended usage of the PFS
feature. I suspect that the intention was that it should be off for
the first quick mode exchange(s) and on for subsequent exchanges
(e.g. rekeys). Why? Because you may have deleted SKEYID_D from memory
in order to reduce the consequences of a break-in. In practice, most
people implement PFS as an on/off setting, and the user must
configure which setting is to be used for each connection.
Note 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. And deriving each session key from a separate shared
primitive is overkill because independence of keying material is
already guaranteed by using a strong PRF. If you want to increase
your resistance to cryptanalysis, a better solution would be to
lengthen the modulus of the phase 1 DH group and the block size of
the PRF.
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What PFS does provide is a faster alternative to phase 1 rekeying.
Repetition of the authentication stage may detect if a user's
certificate is revoked, but this could be accomplished by other means
(e.g. periodic CRL retrieval). The only case where this would not be
possible is identity PFS, where the host deletes the peer's identity
after the SAs are created. IKE also specifies a method for bulk
negotiation of keys. This can be used to accomplish PFS without
further DH negotiations because it allows the peers to rekey even
after SKEID_D has been deleted. (In practice, this feature is not
widely implemented.)
Whether or not phase 1 rekeying actually provides any additional
security over PFS depends on how much information an attacker can
gather if the box is physically compromised or otherwise hacked into.
If the box is entirely compromised and the attacker can learn the
host's RSA private key (or preshared key table), then all is lost and
the host will be insecure until the private key is replaced. If the
compromise is less atomic, and the attacker merely discovers SKEYID
(or, more precisely, both SKEYID_E and SKEYID_A), then he can act as
a man in the middle during subsequent quick mode negotiations.
6.5 Weak Keys
IKE mandates the use of weak key checks. In practice, this does not
provide a significant benefit because weak keys are very unlikely to
be generated randomly (and an attacker wonÆt be able to detect them
if they are used).
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).
New Group mode is not often used in practice, but it provides a
theoretical extra level of security. If everyone uses the same group,
then an attacker can build a large database of known keys for that
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group, thus amortizing the cost of a brute force search over many
keys. In practice this attack is not as devastating as it sounds,
since Diffie-Hellman keys are already large enough to defend against
birthday attacks, such as the Pollard-Rho method.
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.
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
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time lost to DoS is limited to the length of time required to verify
an HMAC. ESP without authentication has less DoS protection because
encryption is generally slower than authentication; also, with the
CBC mode of operation, it is quite easy to form a corrupt packet
which will pass ESP processing.
IKE does not dictate how SPI values should be chosen, but many
implementations choose SPIs randomly. 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. (Anti-clogging cookies
are meant to prevent state clogging attacks akin to the TCP SYN
attack.)
There are multiple ways to extend IKE to allow stateless operation.
One method is to add an optional cookie exchange when a DoS attack is
detected. Another technique is to repeat the information from message
1 in message 3 of the exchange. [Huttunen] describes a proposal for
optimizing this technique with the use of an encrypted "state
payload."
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).
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
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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.
These comments mainly apply to cases where authentication is tightly
bound to authorization. In cases (e.g. a publicly accessible server
on the Internet) where authorization is not important and the
authentication stage is performed merely to ensure the
confidentiality of the negotiated key, the user is essentially
unauthenticated and he is free to launch any of a myriad of DoS
attacks which we won't describe here.
IKE does not provide explicit 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
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Replay of ESP data is a vulnerability that depends on the upper layer
protocol. Many session-based protocols will reject replayed data. ESP
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 replay
counter to ISAKMP packets. A logical way to do this would be to
simply convert the random message id into a counter (the randomness
of the message id is only needed for collision-resistance, and not
for any essential security feature).
8.2 Repudiation
Authentication using either pre-shared keys or public key encryption
has the repudiation property. Either side is capable of forging the
entire exchange; therefore there is no reliable way to prove that the
transaction took place.
Authentication using public key signatures does not provide full
repudiation, but it doesnÆt provide explicit non-repudiation either.
When Bob generates a signature, it proves that he talked to somebody,
but not necessarily Alice. It is possible for Alice to encode a
signed hash of her identity into a payload that will be signed by Bob
during the course of the exchange. This would prove that Bob talked
to Alice (or someone colluding with Alice), although not necessarily
on purpose. Note that this does not prove to a third party that any
data sent with the negotiated keys is genuine.
So for all intents and purposes, IKE provides repudiation of the
phase 1 exchange, no matter which mode of authentication you use.
IPsec Working Group [Page 17]
Internet Draft IPsec Properties June 30, 2002
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. Notes
The authors would like to thank Radia Perlman, Olivier Paradiens, and
Angelos Kerymitis for their comments which were incorporated into
this draft.
12. 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", IPsec
mailing list, 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.
[DES] Madson, C., and N. Doraswamy, "The ESP DES-CBC Cipher
Algorithm With Explicit IV", RFC 2405, November 1998
[Fluhrer]Fluhrer, S., "Suggested modification to AES privacy draft",
IPsec mailing list, http://www.vpnc.org/ietf-ipsec/mail-
archive/msg02598.html, January 2002.
[Huttunen] Huttunen, A., "Re: Future ISAKMP Denial of Service
Vulnerablity Needs Addressing", IPsec mailing list,
http://www.vpnc.org/ietf-ipsec/00.ipsec/msg00160.html,
January 2000.
[IKE] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)",
RFC 2409, November 1998
IPsec Working Group [Page 18]
Internet Draft IPsec Properties June 30, 2002
[ISAKMP]Maughan, D., Schertler, M., Schneider, M., and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[Krawczyk] Krawczyk, H., "Rationale for the definitions of SKEYID",
IPsec mailing list, 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)
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
Expiration
This document expires January 30th, 2003.
IPsec Working Group [Page 19]
