Network working group Dacheng Zhang
Internet Draft Huawei Technologies Co.,Ltd
Category: Informational Dmitriy Kuptsov
Created: May 29, 2010 Helsinki Institute for Information Technology
Expires: November 2010 Sean Shen
CNNIC
Host Identifier Revocation in HIP
draft-irtf-hiprg-revocation-00
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Abstract
This document mainly analyzes the key revocation issue with host
identities (HIs) in the Host Identity Protocol (HIP). Generally, key
revocation is an important functionality of key management systems;
it is concerned with the issues of removing antique cryptographic
keys from operational usages when they are not secure or not secure
enough any more. This functionality is particularly important for
the security systems expected to execute for long periods. This
document also attempts to investigate several key issues that a
designer of HI revocation mechanisms need to carefully consider.
Conventions used in this document
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].
Table of Contents
1. Introduction...................................................2
2. Terminologies..................................................3
3. Key Management.................................................3
4. Key Revocation.................................................4
4.1. Revocation of Transient and Permanent Keys................4
4.2. Classification of permanent Key Revocation Mechanisms.....5
5. Implicit HI Revocation in HIP..................................7
6. Explicit HI Revocation........................................12
7. Related Discussions...........................................14
7.1. Influence of HI revocation on Already Generated HIP
Associations..................................................14
7.2. HI Refreshment...........................................14
8. Conclusions...................................................16
9. IANA Considerations...........................................16
10. Acknowledgments..............................................16
11. References...................................................16
11.1. Normative References....................................16
11.2. Informative References..................................17
Authors' Addresses...............................................17
1. Introduction
In a HIP architecture [RFC5201], a HIP host needs to generate a
"permanent" public key pair before it communicates with other HIP
hosts. The public key is used as its HI while the private key is
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kept securely by the host. When two HIP hosts attempt to initiate a
communication (e.g., a TCP session), they can take advantage of
their HI key pairs to perform mutual authentication and distribute
keying materials for securing subsequent data and signaling packets.
Therefore, the security of HIP architectures largely relies on the
security of those HI key pairs. If the HI key pair of a HIP host is
revealed, an attacker can easily impersonate the victim to carry out
malicious attacks without being detected.
It has been widely recognized that a cryptographic key (which can be
either a symmetric key or a public key) should have a reasonable
valid period [Recommendations]. After having been employed for a
certain period, a cryptographic key will be in more dangers of
compromise. As time elapses, an attacker can collect more materials
(e.g., encrypted data, signatures and associated plain texts, etc.)
and obtain more time to compromise the key. In addition, unexpected
key disclosure is a common practical issue, which may be caused by,
e.g., improper key management policies or hardware stealing.
Consequently, in the design of a security system which is expected
to execute for a long period, the issues with revoking the
cryptographic keys which do not have enough security strengths must
be considered.
In current HIP architectures, the key revocation issues with
transient (session) keys have been well discussed. HIP allows two
communicating hosts to update their transient keys securely at run
time. However, the key revocation issues with permanent keys (i.e.,
HIs) have not been well explored yet. No facility is provided for HI
revocation either.
2. Terminologies
BEX: Base Exchange
HIP: Host Identity Protocol
PKI: Public Key Infrastructure
3. Key Management
Key management aims at guaranteeing the security of cryptographic
keys during the period of their application and includes all of the
provisions made in a security system design which are related to
generation, validation, exchange, storage, safeguard, application,
and replacement of cryptographic keys. Appropriate key management is
critical to security mechanisms providing confidentiality, entity
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authentication, data origin authentication, data integrity, and
digital signatures. Specifically, a full-fledged key management
system should be able to support [Menezes et al. 1996]:
1. Initialization of system users within a domain;
2. Generation, distribution, and installation of keying material;
3. Controlling the use of keying material;
4. Update, revocation, and destruction of keying material; and
5. Storage, backup/recovery, and archival of keying material.
4. Key Revocation
Key revocation is an essential functionality of a security system.
By refreshing antique cryptographic keys, a security system can
reduce the dangers of being compromised. Key revocation is also an
important step when a security system attempts to confine and
recover from the damages caused by attacks. The criteria measuring a
key revocation mechanism should include security, efficiency,
latency, overheads in terms of communication, and etc.
4.1. Revocation of Transient and Permanent Keys
Cryptographic keys adopted in a security system can be classified
into permanent keys and transient keys according to their life
periods. As indicated by the name, permanent keys are maintained by
holders for relatively long periods which can be various from months
to years. Because frequent usages of permanent keys can damage their
security strength and reduce their valid periods, in many security
mechanisms, permanent keys are employed to generate and distribute
transient keys which are only valid in relatively short periods
(e.g., within a single TCP session). Key revocation issues with
transient keys have been taken account of in most authentication
mechanisms (e.g., Kerberos, IPSec, SSL, etc.). For instance, in
Kerberos, a user can use her password to obtain a session key from a
KDC; the session key then can be further used to securely discard
and update antique sub-session keys. The revocation of transient
keys is also considered in the design of HIP. A basic handshaking
protocol (i.e., the HIP Base Exchange) has been proposed. Using it,
two communicating HIP hosts can employ the authenticated Diffie-
Hellman algorithm to securely distribute keying materials which will
be used to generate new cryptographic keys in the following
communications. After a handshake, the hosts are able to refresh
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their transient keys and the corresponding HIP associations, using
Update packets.
The revocation issues with permanent keys are also taken into
account in lots of key management mechanisms (e.g., PGP, PKI, Peer-
to-Peer Key Management for Mobile Ad Hoc Networks [Merwe et al.
2007]). Particularly, in PKI, key revocation issues are addressed in
certificate revocation mechanisms.
4.2. Classification of permanent Key Revocation Mechanisms
This draft focuses on the issues with permanent key revocation in
HIP. In the remainder of this draft, key revocation indicates
permanent key revocation, without mentioned otherwise.
Mechanisms for key revocation can be classified in different ways,
according to:
Whether additional operations are needed. If a key revocation
mechanism does not need any additional operation in the revoking
process of a cryptographic key, it is called an implicit key
revocation mechanism. The basic idea of an implicit HI revocation
mechanism is to associate a key with a valid period and use
cryptographic methods to prove the binding between the key and
its valid period. Therefore, after the pre-defined period expires,
the key is obsolete automatically. For instance, in PKI, a
Certificate Authority (CA) can issue a certificate for a user in
order to assert the association between the user and its public
key. The certificate is associated with a life period. When the
period expires, the user's public key is revoked automatically.
If a key revocation mechanism needs to carry out additional
operations (e.g., notifications) to revoke a cryptographic key,
it is called an explicit key revocation mechanism. In different
explicit key revocation mechanisms, such operations can be
performed either by a dedicated server or by the owner of the key.
Compared with implicit key revocation mechanisms, an explicit key
revocation mechanism has the capability to revoke a cryptographic
key before its life period expires. For instance, in X.509
[RFC2459] based systems, an issuer can generate a list of
certificates, which were revoked for some reasons before their
expiring dates, for users to consult.
Whether a secure third party is needed. In some revocation
mechanisms, the status information of a cryptographic key is
provided by a secure third party. A proof of validity is
performed during each request from users, and the secure third
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party provides up-to-date information. Online Certificate Status
Protocol (OCSP) for X.509 certificate is such a mechanism. An
OSPF client generates an OCSP request that primary contains the
information of one or more queried certificates and send it to a
trusted OCSP server. After receiving the OCSP request, the server
creates an OCSP response containing the updated status
information of the queried certificates. In some other revocation
mechanism, validity information is distributed to the requester
by a non-secured server. For example, in PGP, a principal can use
its revoked key to sign a key revocation certificate and uploaded
it to a key repository server. The server is regarded as "non-
secured" only because the server only provides a repository
service and does not make any assertion. Certificates themselves
are individually secured by the signatures thereon, and need not
be transferred over secured channels. In fact, authorization
policies to a repository server in the form of write and delete
protection is mandatory so as to enable maintenance and update
without denial of service
The list is adopted. According to the information provided, key
revocation mechanisms can be classified into black list
mechanisms and white list mechanisms. A black list mechanism can
provide the information of the keys which are not valid anymore.
The Certificate Revocation List (CRL) is an example of this kind
of mechanism. In a CRL, revoked certificates are listed in a
signed list, so that users can query the information about the
revoked keys whenever it is convenient. White list mechanisms,
instead, only provide information of valid keys. For example, SSH
specify a kind of resource record (RR) called SSHFP [RFC4255]. A
SSHFP RR contains the information of the fingerprint of a valid
cryptographic key. If a key needs to be revoked, the associated
SSHFP RR is removed. If a user cannot find the associated SSHFP
RR from DNS, she will believe that the key inquired about is no
longer valid.
The way of distributing revocation information. In a key
revocation mechanism applying the push model, upon a key is
revoked, a server proactively contacts the related users to
inform the case. In contrast, in a key revocation mechanism
applying the pull model, a client needs to query a server for
particular revocation information. OCSP, CRL, and the key
revocation mechanisms adopted in PGP and SSH all belong to this
category.
There are few discussions about the HI revocation issues with HIP.
In the current HIP architecture, hosts are allowed to update their
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identifiers arbitrarily without notifying others. The lack of HI
revocation mechanism can be taken advantage of by attackers to, for
instance, escape tracking, bypass ACLs (Access Control Lists),
impersonate others using the compromised HIs, etc. In remainder of
this document, candidate approaches and related issues are discussed.
5. Implicit HI Revocation in HIP
Implicit key revocation is the simplest key revocation approach. By
associating an HI with a life period, the holder of the HI needs to
update the HI periodically so as to reduce the risk that the HI is
compromised. In addition, life periods of HIs can help users to
verify how long an HI has been used and how long the HI will still
be valid. This is enable host managers to define more specific
security policies.
Note that the HI and the HIT of a host are cryptographic associated.
A revocation of an HI will cause the revocation of the correspondent
HIT, and vice versa. Therefore, without losing generality, in this
document we assume that the life periods of an HI and its HIT are
identical and they are generated and revoked concurrently in a same
process.
The life period of an HI can be specified either by the holder of
the HI or by a trusted authority. During HIP BEXs, such life period
information can be encapsulated in parameters and transported within
HIP packets. If the life period of the HI is specified by its holder,
the holder needs to use the associated private key to sign the
parameter. If the life period of the HI is specified by a trusted
authority, the authority needs to use its private key to sign a life
period certificate for the HI. The certificate can be encapsulated
within a CERT parameter and transported in HIP packets.
Figure 1 illustrates an extended HOST_ID parameter which is able to
transport an HI and the associated life period. This parameter can
be adopted in the cases where the life period of the HI is specified
by its holder. Similar with the live periods of X.509 certificates,
the life period of an HI is specified by a Not Before Time and a Not
After Time. In this parameter, the NB Length and NA Length fields
indicate the lengths of Not Before Time and Not After Time fields
respectively. The Not-Before-Time and the Not-After-Time can be in a
format of either UTCTime or GeneralizedTime defined in [RFC2459].
During a HIP base exchange, the parameter containing Initiator's HI
and the associated life period information is transported in the I2
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packet, while the parameter containing Responder's HI and the
associated life period information is transported in the R1 packet.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HI Length |DI-type| DI Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NB Length | NA Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Identity /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Domain Identifier /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Not Before Time /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Not After Time /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1 An extension of HOST_ID parameter
The approach to enabling a holder to specify the life period of its
HI does not rely on any trusted third party and introduces little
performance penalty in verifying the life period. However, a concern
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about this approach is how to ensure that HIP hosts will
appropriately define and manage the life periods of their HIs. In
practice, the revocation and refreshment of an HI can be quite
complex. Apart from updating the key material locally, additional
operations also need to be performed (e.g., updating the associated
HIP resource record in DNS, proactively informing the partners which
may be affected by the revocation, etc.). Therefore, a lazy manager
of a HIP host may attempt to avoid refreshing the HI and HIT of her
host. If the manager assigns an extremely long life period for its
HI, other HIP hosts can easily detect the problem and refuse to
communicate with the host. However, if the manager selects to assign
a new life period with a reasonable length for her HI whenever the
old life period has expired, the renewal of the life period can be
difficult to be detected in current HIP architectures. For instance,
in practice a HIP host normally does not maintain the HIs and other
related information of its communicating partners for a long period,
in order to reduce memory consumption and foil deny-of-service
attacks. Moreover, because HITs are treated by applications as
ordinary IP addresses which have no expire date, In referral
scenarios the receiver of a HIT may not be able to obtain the
knowledge of the life period of a HIT from the referrer. In the
current HIP resolution solutions (e.g., HIP RR), there is no concern
about the life periods of HIs. On such occasions, a host can only
obtain the life period information from its communicating partner
(i.e., the holder of the HI). Therefore, in current HIP
architectures, the approach which allows a holder to specify the
life period of its HI can only be feasible in the environments where
there has already been a certain level of trust between two HIP
hosts beforehand, that is, a HIP host can believe its communicating
partner has specified an appropriate life period for its HI and will
only attempt to use it within the valid period.
The issues mentioned above can be largely addressed by assigning a
trusted authority to manage the life periods of HIs. However, a
dedicated trust third party may introduce complexity into the
current HIP architecture, impose additional communications (e.g.,
registration process, generation of certificate chain, etc.), and
cause issues in terms of scalability and trust. The details of the
issues imposed by such dedicated authorities are discussed in
section 6.
In the remainder of this sub-section, we introduce two complementary
approaches to mitigating the issues of arbitrarily modifying HI life
periods while imposing little performance penalty to HIP hosts.
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The first approach is to extend resolution systems (e.g., DNS
servers) to provide trustable life-period information of HIs. In
this approach, the life-period information can be encapsulated in
the same packet with other mapping information and sent back to
users so as to eliminate addition communicating overheads between
users and resolutions systems.
In order to achieve this, spaces for the life period information
needs to be allocated in the resource records sent back to users. In
Figure 2, an extension of the HIP RR with life period information is
illustrated. Same as the extended HOST_ID parameter in Figure 1, the
NB Length and NA Length fields indicate the lengths of Not Before
Time and Not After Time fields respectively. The Not-Before-Time and
the Not-After-Time can be in a format of either UTCTime or
GeneralizedTime defined in [RFC2459].
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HIT Length | PK algorithm | PK Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NB Length | NA Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HIT /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Public Key /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Rendezvous Server /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Not Before Time /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ | Not After Time /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |
+-+-+-+-+
Figure 2 An Extension of HIP RR
The basic functionality of a resolution server is to provide mapping
information for users. In practice, it is normally the
responsibility of authorized users to maintain and update the
contents of RRs while resolution severs can verify the contents of
RRs against certain security policies. Therefore, in this approach,
information of the life period of an HI, just like the other
information in the RR, can be provided by an authorized user at the
registration time. After the registration, the life period
information is only allowed to be updated by the ones who have
higher privileges (e.g., server managers).
Let us use DNS servers as an example. After a user uploads the
information of a HIP host in an authoritative DNS server, the user
is not allowed to modify the Not Before Time and Not After Time
fields of the HI any more. Moreover, after the life period of the HI
has expired, the associated RRs needs to be removed.
Until now, the ID to Locator mapping solutions in HIP has not been
standardized yet. We argue that it is desired to integrate the
implicit key revocation functionality into such systems.
The second approach is to introduce the life periods of HIs into the
generating process of HITs. For instance, the life period of an HI
can be used as a part of the input for generating the associated HIT.
This approach makes it computational difficult for the holder of an
HI to modify the life period without modifying the associated HIT.
Therefore, after a host advertises its contacting information in
resolution servers, any attempts to modify the life period of the HI
can be easily detected. For instance, in the case a host obtains a
HIT from its referrer. It needs to first obtain the knowledge to
access the host holding the HIT from resolution servers. Then it can
get the associated HI and the life period from the HIT holder, and
re-calculate the HIT to verify whether the life period of the HIT is
valid. This approach needs little modification on the resolution
servers and can be applied independently. A disadvantage of this
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approach is its inflexibility in the cases where the life periods of
HIs need to be extended.
6. Explicit HI Revocation
As mentioned previously, in many typical scenarios a cryptographic
key should not be used any more even when it is still in its valid
life period. For instance, when a key is detected to be compromised,
it must to be revoked immediately even if it has not reached its
expiration date. In such a case, explicit key revocation is needed.
When an HI needs to be removed from operational use prior to its
originally scheduled expiry, the revocation of the HI needs to be
informed to all the hosts which might be affected. If there is no
dedicated third party to rely on, the holder of the HI needs to
deliver the revocation certificate signed by the associated private
key to all the affected partners. The poor scalability of this
solution is always a subject of debates. First of all, this solution
requires the holder an HI to maintain a long list of information
about the partner which may be affected by the revocation; this job
can be onerous and error prone. In addition, because HIP does not
support multicast, the holder has to generate a notification packet
for each of its partners, and send them out during the revocation.
When the number of related partners increases, the holder may have
to spend a large amount of bandwidth, memory and computing resources
in generating and delivering the notification packets. In order to
improve the performance of this solution, the holder can send the
certificate to a limited set of partners. These partners then relay
the certificate to others. However, this solution may introduce
additional latency and make the delivery of the certificate un-
reliable. Besides the above issues, this solution requires all the
involved partners to be online during an HI revocation process,
which can be hardly fulfilled on many occasions. Basically, this
solution is only suitable in the circumstances where the number of
involved hosts is relatively small and stable.
The experiences in PKI demonstrate that pull models can be more
scalable in dealing with a large amount of users, and as a result,
most of the certification revocation mechanisms (e.g., Certification
Revocation Lists (CRLs) and the On-Line Certificate Status Protocol
(OCSP)) proposed in PKI are based on pull models. In these
mechanisms, the revocation information is maintained in a third
party for users to query whenever it is convenient.
PKI has provided a set of certificate management mechanisms. On many
occasions, it is feasible for HIP to take advantage of PKI style
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solutions to address the issues with HI management. However, it
should be realized that PKI oriented solutions are not silver
bullets and cannot be utilized to address all the issues that HIP
has to encounter. After HIP has been globally deployed, it is
expected that there will be billions of HIP users which may belong
to different organizations and attach to the Internet through
different ISPs. Due to the poor scalability of PKI and lack of trust,
it is extremely difficult (if possible) to put such a big amount of
geographically distributed users under the control of a unique PKI
security domain. Therefore, it is reasonable to assume that there
will be many different security domains all over the world. When two
HIP hosts belong to two different security domains, it may be
difficult for a host to verify the assertion made by the security
server in the domain of the other one. Although there have been
solutions of generating trust relationship across various security
domains, all of them impose additional overheads with respect to the
construction and verification of credential chains, communications
with remote security servers, which negatively influences the
performance of HIP. Therefore, the HIP community argues that two
HIP-aware hosts should be able to communicate without any additional
security facilities. Actually, the only third party server
introduced in the base-line HIP architecture is the Rendezvous
Server (RVS)[RFC5204]. A RVS only relays messages for the hosts
which attempts to communicate with mobile hosts and provides little
security functionality. The HIP hosts intending to communicate with
each other still need to use the HIP Base Exchange protocol to carry
out authentication and exchange keying material for future
communications.
Again, resolution servers can be potentially adopted to construct a
global explicit HI revocation mechanism applying a pull model. For
instance, when a host intends to revoke its HI, it can send a
revocation certificate signed by its private key to an authoritative
DNS server. After receiving the certificate, the correspondent RR
will be removed, and thus users will not obtain the information
about the revoked HI any more. Therefore, DNS servers can perform as
a white list HI revocation mechanism, just similar with what
specified in SSH. To avoid the long delay in the spread of
revocation information caused by caching RRs on DNS resolvers, the
TTL (Time To Life) of RRs can be set to zero.
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7. Related Discussions
7.1. Influence of HI Revocation on Already Generated HIP Associations
In this sub-section, we investigate the possibility of using already
generated HIP associations to transport the update information after
the correspondent HI key pair is no longer valid.
In a BEX, HI key pairs of the both communicating partners are used
to carry out mutual authentication while the key materials for
securing subsequent communication are generated by the Diffie-
Hellman algorithm. Therefore, if an HI key pair is secure at the
time when a HIP association is generated, the later revocation of
the HI key pair will not affect the security of the keying materials.
Assume there is an attacker which has compromised the HI key pair.
It is still computational difficult for the attacker to decrypt the
packets transported between the communicating partners. Because the
Update packets are under the protection of HMAC, the attacker cannot
forge them to interfere with the communications. Note that the
attacker can try to forge Notify packets. However, according to [RFC
5201] Notify packets are only informative, which will not affect the
state of the communicating partners. Therefore, if no explicit key
revocation occurs, the expiry of an HI will not affect the security
of HIP associations generated using the HI when it is still valid.
They still can be used until they reach their expiring time. However,
if an HI is found to be compromised, the security of the keying
materials of the already generated HIP associations cannot be
guaranteed. In practice, the compromise of a cryptographic key can
be perceived only after the attacks employing the key are detected.
It is difficult for one to identify the exact time from which the
key is no longer secure. Hence, under this circumstance, the pre-
generated HIP associations can only be used to deliver revocation
certificates, as it is difficult for the communicating partners to
know whether the HI is still secure when the HIP associations were
generated.
7.2. HI Refreshment
In key management mechanisms, key refreshment is concerned with the
issues of using new cryptographic keys to take place of "old" ones.
Therefore, it closely related with key revocation. A refreshment
procedure of a key can occur either before or after the revocation
of the key (Note that in the first case the key is still valid). In
this section, we briefly discuss the issues with HI refreshment in
HIP.
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Ideally, the refreshment of an operational HI should be performed
before its crypto-period is expired. That is, when an HI refreshment
process is performed, the HI expected to be updated is still valid.
The holder then can use the old HI to establish secure channels, and
use Update packets to transport the refreshment information to
related partners (in a push model) or to trusted third parties (in a
pull model). In the Update packets, the new HI and other related
information are encapsulated. Therefore, before the old HI expires,
both HIs are valid, and the HIP associations generated with the old
HI can still be applied.
In practice, the third parties deployed for HI revocation can also
be used to support HI refreshment. For instance, when using a pull
model, a host can transport the HI revoking and the refreshing
information to a third party. Therefore, when a user inquires of the
third party about the status information of an HI, the user can get
the status of the HI inquired about as well as the associated
refreshment information.
If an HI needs to be revoked due to accident disclosure or
compromise, the update of the HI can be a little more complex, as a
host cannot use the invalid key to securely transport the
refreshment information any more. If a host has multiple HIs, it can
also select a valid HI to generate secure channels to transport the
refreshment information of another HI. The refreshment information
can be transported in an Update packet in which both the new HI and
the old HI are contained. This solution require that the partner
communicating with the host can ensure that the HI used to generate
secure channel and the HI going to be refreshed are possessed by the
same HIP host. Such knowledge can be obtained from resolution
systems or provided by the host. In the cases where all the HIs of a
host become invalid (e.g., the host is found to compromised), the
host only can distribute the refreshment information using an out-
of-band way.
A host can also implement a pull model by directly transporting the
update information to resolution servers. If the information is
forwarded to a DNS server, users can query the latest HI using FQDN
of the host. In a resolution system providing ID to locator mapping
services (e.g., DHT), users can only try to query the resolution
systems using old HITs. In this case, besides the IP addresses
inquired, the resolution system should also provide the latest HIs
and other useful information. Note that it is assumed that no two
HITs of different hosts are identical, even if they are adopted in
different period. In practice, because the length of HITs is long,
the possibility that two hosts select a same HI can be very low. In
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order to further reduce the possibility, a user can also provide the
life period of the inquired HIT in a query.
8. Conclusions
Key revocation is critical for HIP to be secure, practical and
manageable. Particularly, HIP hosts are expected to keep working
securely for a relatively long period, proper key revocation
mechanisms for HIs must be provided. This document focuses on
con/pro of different key revocations and analyzes their performances
in different practical scenarios. Although key management has been
an active research area for a long period and lots of successful
key-management systems (e.g., PKI) are widely adopted in practice,
many issues (e.g., scalability, lack of trust) still exist. There is
no solution being found to meet the timeliness and performance
requirements of all applications and environments that HIP is
expected to support [McDaniel et al. 2001]. Therefore, it is
predicable that various HI revocation approaches will be adopted
after HIP has been globally adopted.
9. IANA Considerations
No such considerations.
10. Acknowledgments
Many Thanks to Thomas.R.Henderson for his kindly revision and
precious comments.
11. References
11.1. Normative References
[RFC2459] Internet X.509 Public Key Infrastructure: Certificate and
CRL Profile January 1999.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
Henderson, "Host Identity Protocol", RFC 5201, April 2008.
[RFC5204] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", RFC 5204, April 2008.
[RFC5205] Nikander, P. and J. Laganier, "Host Identity Protocol
(HIP) Domain Name System (DNS) Extensions", RFC 5205,
April 2008.
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11.2. Informative References
[Recommendations]
Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
"Recommendation for Key Management - Part1 -
General(Revised)", NIST Special Publication 800-57, March
2007, <http://csrc.nist.gov/publications/nistpubs/800-
57/SP800-57-Part1.pdf>.
[Menezes et al. 1996]
MENEZES, A., VAN OORSCHOT, P., AND VANSTONE, S. 1996.
Handbook in Applied Cryptography. CRC Press,Boca Raton, FL.
[Merwe et al. 2007]
Merwe, J., Dawoud, D., and McDONALD, S., "A Survey on
Peer-to-Peer Key Management for Mobile Ad Hoc
Networks" ,2007.
[McDaniel et al. 2001]
McDaniel, P., and Rubin, A., "A Response to "can we
eliminate certificate revocation list?"", 2001.
Authors' Addresses
Dacheng Zhang
Huawei Technologies Co.,Ltd
KuiKe Building, No.9 Xinxi Rd.,
Hai-Dian District
Beijing, 100085
P.R. China
Email: zhangdacheng@huawei.com
Dmitriy Kuptsov
Helsinki Institute for Information Technology
PO. Box 9800
TKK FI-02015
Finland
Email: dmitriy.kuptsov@hiit.fi
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Internet-Draft Host Identifier Revocation in HIP May 2010
Sean Shen
CNNIC
4, South 4th Street, Zhongguancun
Beijing 100190
P.R. China
Email: shenshuo@cnnic.cn
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