One document matched: draft-tschofenig-rsvp-sec-properties-00.txt
Hannes Tschofenig
Internet Draft Siemens AG
Document: draft-tschofenig-rsvp-sec-
properties-00.txt
Expires: November, 2002
June, 2002
RSVP Security Properties
<draft-tschofenig-rsvp-sec-properties-00.txt>
Status of this Memo
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Tschofenig Informational - Expires December 2000 1
RSVP Security Properties June 2002
Abstract
As the work of the NSIS working group has begun there are also
concerns about security and its implication for the design of a
signaling protocol. In order to understand the security properties
and available options of RSVP a number of documents have to be read.
This document tries to summarize the security properties of RSVP and
to view them from a different point of view. This work in NSIS is
part of the overall process of analyzing other protocols and to
learn from their design considerations. This document should also
provide a starting point for further discussions.
Table of Contents
1 Introduction...................................................2
2 Terminology....................................................3
3 Overview.......................................................5
3.1 The RSVP INTEGRITY Object....................................5
3.2 Security Associations........................................6
3.3 RSVP Key Management Assumptions..............................7
3.4 Identity Representation......................................7
3.5 RSVP Integrity Handshake....................................11
4 Detailed Security Property Discussion.........................12
4.1 Discussed Network Topology..................................12
4.2 Host/Router.................................................13
4.3 User to PEP/PDP.............................................17
4.4 Communication between RSVP aware routers....................25
4.5 Miscellaneous Issues........................................28
4.5.1 Dictionary Attacks and Kerberos............................28
4.5.2 Example of User-to-PDP Authentication......................30
4.5.3 Open Issues................................................30
5 Conclusions...................................................31
6 Security Considerations.......................................32
7 IANA considerations...........................................32
8 Acknowledgments...............................................32
9 References....................................................32
10 Author's Contact Information..................................36
11 Full Copyright Statement......................................36
1 Introduction
As the work of the NSIS working group has begun there are also
concerns about security and its implication for the design of a
signaling protocol. In order to understand the security properties
and available options of RSVP a number of documents have to be read.
This document tries to summarize the security properties of RSVP and
to view them from a different point of view. This work in NSIS is
part of the overall process of analyzing other protocols and to
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RSVP Security Properties June 2002
learn from their design considerations. This document should also
provide a starting point for further discussions.
The content of this document is organized as follows:
Section 3 provides an overview of the security mechanisms provided
by RSVP including the INTEGRITY object, a description of the
identity representation within the POLICY_DATA object (i.e. user
authentication) and the RSVP Integrity Handshake mechanism.
Section 4 provides a more detailed discussion of the used mechanism
and tries to describe the mechanisms provided in detail.
Finally the last Section briefly addresses issues like the
discussion of the vulnerability of Kerberos against dictionary
attacks and open issues in the context of RSVP and issues for
further investigation.
2 Terminology
To begin with the description of the security properties of RSVP it
is natural to describe some basic building-blocks.
- Chain-of-Trust
The security mechanisms supported by RSVP [RFC2747] heavily relies
on optional hop-by-hop protection using the built-in INTEGRITY
object. Hop-by-hop security with the INTEGRITY object inside the
RSVP message thereby refers to the protection between RSVP
supporting network elements. Additionally there is the notion of
policy aware network elements that additionally understand the
POLICY_DATA element within the RSVP message. Since this element also
includes an INTEGRITY object there is an additional hop-by-hop
security mechanism that provides security between policy aware
nodes. Policy ignorant nodes are not affected by the inclusion of
this object in the POLICY_DATA element since they do not try to
interpret it.
To protect signaling messages that are possibly modified by each
RSVP router along the path it must be assumed that each incoming
request is authenticated, integrity and replay protected. This
provides protection against unauthorized nodes injecting bogus
messages. Furthermore each RSVP-router is assumed to behave in the
expected manner. Outgoing messages transmitted to the next hop
network element experience protection according RSVP security
processing.
Using the above described mechanisms a chain-of-trust is created
whereby a signaling message transmitted by router A via router B and
received by router C is supposed to be secure if router A and B and
router B and C share a security association and all routers behave
expectedly. Hence router C trusts router A although router C does
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not have a direct security association with router A. We can
therefore conclude that the protection achieved with this hop-by-hop
security for the chain-of-trust is as good as the weakest link in
the chain.
If one router is malicious (for example because an adversary has
control over this router) then it can arbitrarily modify messages
and cause unexpected behavior and mount a number of attacks not only
restricted to QoS signaling. Additionally it must be mentioned that
some protocols demand more protection than others (this depends
between which nodes these protocols are executed). For example edge
devices, where end-users are attached, may more likely be attacked
in comparison to the more secure core network of a service provider.
In some cases a network service provider may choose not to use the
RSVP provided security mechanisms inside the core network because a
different security protection is deployed.
Section 6 of [RFC2750] mentions the term chain-of-trust in the
context of RSVP integrity protection. In Section 6 of [HH01] the
same term is used in the context of user authentication with the
INTEGRITY object inside the POLICY_DATA element. Unfortunately the
term is not explained in detail and the assumption is not clearly
specified.
- Host and User Authentication
The presence of the RSVP protection and a separate user identity
representation leads to the fact that both user- and the host-
identities are used for RSVP protection. Therefore user and host
based security is investigated separately because of the different
authentication mechanisms provided. To avoid confusion about the
different concepts Section 3.4 will describe the concept of user
authentication in more detail.
- Key Management
For most of the security associations required for the protection of
RSVP signaling messages it is assumed that they are already
available and hence key management was done in advance. There is
however an exception with the support for Kerberos. Using Kerberos
an entity is able to distribute a session key used for RSVP
signaling protection.
- RSVP INTEGRITY and POLICY_DATA INTEGRITY Object
RSVP uses the INTEGRITY object in two places of the message. The
first usage is in the RSVP message itself and covers the entire RSVP
message as defined in [RFC2747] whereas the latter is included in
the POLICY_DATA object and defined in [RFC2750]. In order to
differentiate the two objects regarding their scope of protection
the two terms RSVP INTEGRITY and POLICY_DATA INTEGRITY object are
used. The data structure of the two objects however is the same.
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3 Overview
3.1 The RSVP INTEGRITY Object
The RSVP INTEGRITY object is the major component of the RSVP
security protection. This object is used to provide integrity and
replay protect the content of the signaling message between two RSVP
participating router. Furthermore the RSVP INTEGRITY object provides
data origin authentication. The attributes of the object are briefly
described:
- Flags field
The Handshake Flag is the only defined flag and is used to
synchronize sequence numbers if the communication gets out-of-sync
(i.e. for a restarting host to recover the most recent sequence
number). Setting this flag to one indicates that the sender is
willing to respond to an Integrity Challenge message. This flag can
therefore be seen as a capability negotiation transmitted within
each INTEGRITY object.
- Key Identifier
The Key Identifier selects the key used for verification of the
Keyed Message Digest field and hence must be unique for the sender.
Its length is fixed with 48-bit. The generation of this Key
Identifier field is mostly a decision of the local host. [RFC2747]
describes this field as a combination of an address, the sending
interface and a key number. We assume that the Key Identifier is
simply a (keyed) hash value computed over a number of fields with
the requirement to be unique if more than one security association
is used in parallel between two hosts (i.e. as it is the case with
security association that have overlapping lifetimes). A receiving
system uniquely identifies a security association based on the Key
Identifier and the sender's IP address. The sender's IP address may
be obtained from the RSVP_HOP object or from the source IP address
of the packet if the RSVP_HOP object is not present. The sender uses
the outgoing interface to determine which security association to
use. The term outgoing interface might be confusing. The sender
selects the security association based on the receiver's IP address
(of the next RSVP capable router). To determine which node is the
next capable RSVP router is not further specified and is likely to
be statically configured.
- Sequence Number
The sequence number used by the INTEGRITY object is 64-bits in
length and the starting value can be selected arbitrarily. The
length of the sequence number field was chosen to avoid exhaustion
during the lifetime of a security association as stated in Section 3
of [RFC2747]. In order for the receiver to distinguish between a new
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and a replayed sequence number each value must be monotonically
increasing modulo 2^64. We assume that the first sequence number
seen (i.e. the starting sequence number) is stored somewhere. The
modulo-operation is required because the starting sequence number
may be an arbitrary number. The receiver therefore only accepts
packets with a sequence number larger (modulo 2^64) than the
previous packet. As explained in [RFC2747] this process is started
by handshaking and agreeing on an initial sequence number. If no
such handshaking is available then the initial sequence number must
be part of the establishment of the security association.
The generation and storage of sequence numbers is an important step
in preventing replay attacks and is largely determined by the
capabilities of the system in presence of system crashes, failures
and restarts. Section 3 of [RFC2747] explains some of the most
important considerations.
- Keyed Message Digest
The Keyed Message Digest is an RSVP built-in security mechanism used
to provide integrity protection of the signaling messages. Prior to
computing the value for the Keyed Message Digest field the Keyed
Message Digest field itself must be set to zero and a keyed hash
computed over the entire RSVP packet. The Keyed Message Digest field
is variable in length but must be a multiple of four octets. If
HMAC-MD5 is used then the output value is 16 bytes long. The keyed
hash function HMAC-MD5 [RFC2104] is required for a RSVP
implementation as noted in Section 1 of [RFC2747]. Hash algorithms
other than MD5 [RFC1321] like SHA [SHA] may also be supported.
The key used for computing this Keyed Message Digest may be obtained
from the pre-shared secret which is either manually distributed or
the result of a key management protocol. No key management protocol,
however, is specified to create the desired security associations.
3.2 Security Associations
Different attributes are stored for security associations of sending
and receiving systems (i.e. unidirectional security associations).
The sending system needs to maintain the following attributes in
such a security association [RFC2747]:
- Authentication algorithm and algorithm mode
- Key
- Key Lifetime
- Sending Interface
- Latest sequence number (sent with this key identifier)
The receiving system has to store the following fields:
- Authentication algorithm and algorithm mode
- Key
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- Key Lifetime
- Source address of the sending system
- List of last n sequence numbers (received with this key
identifier)
Note that the security associations need to have additional fields
to indicate their state. It is necessary to have an overlapping
lifetime of security associations to avoid interrupting an ongoing
communication because of expired security associations. During such
a period of overlapping lifetime it is necessary to authenticate
either one or both active keys. As mentioned in [RFC2747] a sender
and a receiver might have multiple active keys simultaneously.
If more than one algorithm is supported then the algorithm used must
be specified for a security association.
3.3 RSVP Key Management Assumptions
[RFC2205] assumes that security associations are already available.
Manual key distribution must be provided by an implementation as
noted in Section 5.2 of [RFC2747]. Manual key distribution however
has different requirements to a key storage û a simple plaintext
ASCII file may be sufficient in some cases. If multiple security
associations with different lifetimes should be supported at the
same time then a key engine, for example PF_KEY [RFC2367], would be
more appropriate. Further security requirements listed in Section
5.2 of [RFC2747] are the following:
- The manual deletion of security associations must be supported.
- The key storage should persist a system restart.
- Each key must be assigned a specific lifetime and a specific Key
Identifier.
3.4 Identity Representation
In addition to host-based authentication with the INTEGRITY object
inside the RSVP message user-based authentication is available as
introduced with [RFC2750]. Section 2 of [RFC3182] stated that
ôProviding policy based admission control mechanism based on user
identities or application is one of the prime requirements.ö To
identify the user or the application, a policy element called
AUTH_DATA, which is contained in the POLICY_DATA object, is created
by the RSVP daemon at the userÆs host and transmitted inside the
RSVP message. The structure of the POLICY_DATA element is described
in [RFC2750]. Network nodes like the PDP then use the information
contained in the AUTH_DATA element to authenticate the user and to
allow policy-based admission control to be executed. As mentioned in
[RFC3182] the policy element is processed and the policy decision
point replaces the old element with a new one for forwarding to the
next hop router.
A detailed description of the POLICY_DATA element can be found in
[RFC2750]. The attributes contained in the authentication data
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policy element AUTH_DATA, which is defined in [RFC3182], are briefly
explained in this Section. Figure 1 shows the abstract structure of
the RSVP message with its security relevant objects and the scope of
protection. The RSVP INTEGRITY object (outer object) covers the
entire RSVP message whereas the POLICY_DATA INTEGRITY object only
covers objects within the POLICY_DATA element.
+--------------------------------------------------------+
| RSVP Message |
+--------------------------------------------------------+
| INTEGRITY +-------------------------------------------+|
| Object |POLICY_DATA Object ||
| +-------------------------------------------+|
| | INTEGRITY +------------------------------+||
| | Object | AUTH_DATA Object |||
| | +------------------------------+||
| | | Various Authentication |||
| | | Attributes |||
| | +------------------------------+||
| +-------------------------------------------+|
+--------------------------------------------------------+
Figure 1: Security relevant Objects and Elements within the RSVP
message
The AUTH_DATA object contains information for identifying users and
applications together with credentials for those identities. The
main purpose of those identities seems to be the usage for policy
based admission control and not for authentication and key
management. As noted in Section 6.1 of [RFC3182] an RSVP may contain
more than one POLICY_DATA object and each of them may contain more
than one AUTH_DATA object. As indicated in the Figure above and in
[RFC3182] one AUTH_DATA object contains more than one authentication
attribute. A typical configuration for a Kerberos-based user
authentication includes at least the Policy Locator and an attribute
containing the Kerberos session ticket.
A successful user authentication is the basis for doing policy-based
admission control. Additionally other information such as time-of-
day, application type, location information, group membership etc.
may be relevant for a policy.
The following attributes are defined for the usage in the AUTH_DATA
object:
a) Policy Locator
The policy locator string that is a X.500 distinguished name (DN)
used to locate the user and/or application specific policy
information. The following types of X.500 DNs are listed:
- ASCII_DN
- UNICODE_DN
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- ASCII_DN_ENCRYPT
- UNICODE_DN_ENCRYPT
The first two types are the ASCII and the Unicode representation of
the user or application DN identity. The two ôencryptedö
distinguished name types are either encrypted with the Kerberos
session key or with the private key of the userÆs digital
certificate (i.e. digitally signed). The term encrypted together
with a digital signature is easy to misconceive. If user identity
confidentiality shall be provided then the policy locator has to be
encrypted with the public key of the recipient. How to obtain this
public key is not described in the document. Such an issue may be
specified in a concrete architecture where RSVP is used.
b) Credentials
Two cryptographic credentials are currently defined for a user:
Authentication with Kerberos V5 [RFC1510], and authentication with
the help of digital signatures based on X.509 [RFC2495] and PGP
[RFC2440]. The following list contains all defined credential types
currently available and defined in [RFC3182]:
+--------------+--------------------------------+
| Credential | Description |
| Type | |
+===============================================|
| ASCII_ID | User or application identity |
| | encoded as an ASCII string |
+--------------+--------------------------------+
| UNICODE_ID | User or application identity |
| | encoded as an Unicode string |
+--------------+--------------------------------+
| KERBEROS_TKT | Kerberos V5 session ticket |
+--------------+--------------------------------+
| X509_V3_CERT | X.509 V3 certificate |
+--------------+--------------------------------+
| PGP_CERT | PGP certificate |
+--------------+--------------------------------+
Table 1: Credentials Supported in RSVP
The first two credentials only contain a plaintext string and
therefore they do not provide cryptographic user authentication.
These plaintext strings may be used to identify applications, which
are included for policy-based admission control. Note that these
plain-text identifiers may, however, be protected if either the RSVP
INTEGRITY and/or the INTEGRITY object of the POLICY_DATA element is
present. Note that the two INTEGRITY objects can terminate at
different entities depending on the network structure. The digital
signature may also provide protection of application identifiers. A
protected application identity (and the entire content of the
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POLICY_DATA element) cannot be modified as long as no policy
ignorant nodes are used in between.
A Kerberos session ticket, as previously mentioned, is the ticket of
a Kerberos AP_REQ message [RFC1510] without the Authenticator.
Normally, the AP_REQ message is used by a client to authenticate to
a server. The INTEGRITY object (e.g. of the POLICY_DATA element)
provides the functionality of the Kerberos Authenticator, namely
replay protection and shows that the user was able to retrieve the
session key following the Kerberos protocol. This is, however, only
the case if the Kerberos session was used for the keyed message
digest field of the INTEGRITY object. Section 7 of [RFC2747]
discusses some issues for establishment of keys for the INTEGRITY
object. The establishment of the security association for the RSVP
INTEGRITY object with the inclusion of the Kerberos Ticket within
the AUTH_DATA element may be complicated by the fact that the ticket
can be decrypted by node B whereas the RSVP INTEGRITY object
terminates at a different host C. The Kerberos session ticket
contains, among many other fields, the session key. The Policy
Locator may also be encrypted with the same session key. The
protocol steps that need to be executed to obtain such a Kerberos
service ticket are not described in [RFC3182] and may involve
several roundtrips depending on many Kerberos related factors. The
Kerberos ticket does not need to be included in every RSVP message
as an optimisation as described in Section 7.1 of [RFC2747]. Thus
the receiver must store the received service ticket. If the lifetime
of the ticket is expired then a new service ticket must be sent. If
the receiver lost his state information (because of a crash or
restart) then he may transmit an Integrity Challenge message to
force the sender to re-transmit a new service ticket.
If either the X.509 V3 or the PGP certificate is included in the
policy element then a digital signature must be added. The digital
signature computed over the entire AUTH_DATA object provides
authentication and integrity protection. The SubType of the digital
signature authentication attribute is set to zero before computing
the digital signature. Whether or not a guarantee of freshness with
the replay protection (either timestamps or sequence numbers) is
provided by the digital signature is an open issue as discussed in
Section 4.3.
c) Digital Signature
The digital signature computed over the data of the AUTH_DATA object
must be the last attribute. The algorithm used to compute the
digital signature depends on the authentication mode listed in the
credential. This is only partially true since for example PGP again
allows different algorithms to be used for computing a digital
signature. The algorithm used for computing the digital signature is
not included in the certificate itself. The algorithm identifier
included in the certificate only serves the purpose to allow the
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verification of the signature computed by the certificate authority
(except for the case of self-signed certificates).
d) Policy Error Object
The Policy Error Object is used in the case of a failure of the
policy based admission control or other credential verification.
Currently available error messages allow to notify if the
credentials are expired (EXPIRED_CREDENTIALS), if the authorization
process disallowed the resource request (INSUFFICIENT_PRIVILEGES)
and if the given set of credentials is not supported
(UNSUPPORTED_CREDENTIAL_TYPE). The last error message allows the
user's host to discover the type of credentials supported although
by very inefficient means. Furthermore it is unlikely that a user
supports different types of credentials. The purpose of the error
message IDENTITY_CHANGED is unclear. The protection of the error
message is not discussed in [RFC3182].
3.5 RSVP Integrity Handshake
The Integrity Handshake is a protocol that was designed to allow a
crashed or restarted host to obtain the latest valid challenge value
stored at the receiving host. A host stores the latest sequence
number of a fresh and correctly authenticated packet. An adversary
can replay eavesdropped packets if the crashed host has lost its
sequence numbers. A signaling message from the real sender with a
new sequence number would therefore allow the crashed host to update
the sequence number field and prevent further replays. Hence if
there is a steady flow of RSVP protected messages between the two
hosts an attacker may find it difficult to inject old messages since
new authenticated packets with high sequence numbers arrive and get
stored immediately.
The following description explains the details of the RSVP Integrity
Handshake that is started by Node A after recovering from a
synchronization failure:
Integrity Challenge
(1) Message (including
+----------+ a Cookie) +----------+
| |-------------------------->| |
| Node A | | Node B |
| |<--------------------------| |
+----------+ Integrity Response +----------+
(2) Message (including
the Cookie and the
INTEGRITY object)
Figure 2: RSVP Integrity Handshake
The details of the messages are described below:
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CHALLENGE= (Key Identifier, Challenge Cookie)
Integrity Challenge Message:=(Common Header, CHALLENGE)
Integrity Response Message:=(Common Header, INTEGRITY, CHALLENGE)
The ôChallenge Cookieö is suggested to be a MD5 hash of a local
secret and a timestamp [RFC2747].
The Integrity Challenge message is not protected with an INTEGRITY
object as show in the protocol flow above. As explained in Section
10 of [RFC2747] this was done to avoid problems in situations where
both communication parties do not have a valid starting sequence
number.
Whether or not to use the RSVP Integrity Challenge/Response
mechanism is a site-local decision since it may not be needed in all
network environments. It is however recommended to use the RSVP
Integrity Handshake protocol.
4 Detailed Security Property Discussion
The purpose of this section is to describe the security protection
of the RSVP provided mechanisms individually for authentication,
authorization, integrity and replay protection, user identity
confidentiality, confidentiality of the signaling messages.
4.1 Discussed Network Topology
The main purpose of this paragraph is to show the basic interface of
a simple RSVP network architecture. The architecture below assumes
that there is only a very single domain and that two routers are
RSVP and policy aware. These assumptions are relaxed in the
individual paragraphs as necessary. Layer 2 devices between the
clients and their corresponding first hop routers are not shown.
Other network elements like a Kerberos Key Distribution Center and
for example an LDAP server where the PDP retrieves his policies are
also omitted. The security of various interfaces to the individual
servers (KDC, PDP, etc.) depends very much on the security policy of
a specific network service provider.
+--------+
|Policy |
|Decision|
+----+Point +---+
| +--------+ |
| |
| |
| |
+------+ +-+----+ +---+--+ +------+
|Client| |Router| |Router| |Client|
| A +-------+ 1 +--------+ 2 +----------+ B |
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+------+ +------+ +------+ +------+
Figure 3: Simple RSVP Architecture
4.2 Host/Router
When talking about authentication in RSVP it is very important to
make a distinction between user and host authentication of the
signaling messages. By using the RSVP INTEGRITY object the host is
authenticated while credentials inside the AUTH_DATA object can be
used to authenticate the user. In this Section the focus is on host
authentication whereas the next Section covers user authentication.
a) Authentication
We use the term host authentication above since the selection of the
security association is bound to the hostÆs IP address as mentioned
in Section 3.1 and 3.2. Depending on the key management protocol used
to create this security association and the identity used it is also
possible to bind a user identity to this security association. Since
the key management protocol is not specified it is difficult to
evaluate this part and hence we speak about data origin
authentication based on the hostÆs identity for RSVP INTEGRITY
objects. The fact that the host identity is used for selecting the
security association has already been described in Section 3.1.
Data origin authentication is provided with the keyed hash value
computed over the entire RSVP message excluding the keyed message
digest field itself. The security association used between the
userÆs host and the first-hop router is, as previously mentioned,
not established by RSVP and must therefore be available before the
signaling is started. Although not mentioned in [RFC2747] it is also
possible to use IPSec [RFC2401] to protect the RSVP signaling
traffic from the client to the first-hop router. If we use IPSec to
protect the interface between the userÆs host and the first hop
router then the optional RSVP INTEGRITY object may not be required.
It may also be possible (which requires a further investigation)
whether an existing IPSec security association may also be (re-)used
for RSVP. IPSec allows the key exchange protocol IKE [RFC2409] to be
used to dynamically negotiate IPSec security associations. Note that
KINK [FH+01] and other protocols are available that are also able to
establish an IPSec security association. This text mainly refers to
IKE since it is the most frequently used protocol for this purpose.
A detailed description of IPSec and IKE is outside the scope of this
document. Since IKE is computationally expensive it might create a
computational burden to re-establish a new IPSec SA based of the
movement of a mobile user host. Work at the SEAMOBY group tries to
tackle this problem by using IPSec Context Transfer protocols. Hence
in this case we would avoid triggering a separate key exchange
protocol run for RSVP to protect messages at each layer if they
terminate at the same node.
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It is an open issue whether it is enough to provide IPSec protection
of messages between the userÆs host and the first-hop router
although different protocols (i.e. protocols executed at different
protocol layers) (possibly) terminate at different endpoints.
- Kerberos for the RSVP INTEGRITY object
As described in Section 7 of [RFC2747] Kerberos may be used to
create the key for the RSVP INTEGRITY object. How to learn the
principal name (and realm information) of the other node is outside
the scope of [RFC2747]. Section 4.2.1 of [RFC2747] states that the
required identities can be obtained statically or dynamically via a
directory service or DHCP. [HA01] describes a way to distribute
principal and realm information via DNS which can be used for this
purpose (assuming that the FQDN or the IP address of the other node
is known for which this information is desired). It is only required
to encapsulate the Kerberos ticket inside the policy element. It is
furthermore mentioned that Kerberos tickets with expired lifetime
must not be used and the initiator is responsible for requesting and
exchanging a new service ticket before expiration.
RSVP multicast processing in combination with Kerberos requires
additional thoughts:
Section 7 of [RFC2747] states that in the multicast case all
receivers must share a single key with the Kerberos Authentication
Server i.e. a single principal used for all receivers). From a
personal discussion with Rodney Hess it seems that there is
currently no other solution available in the context of Kerberos.
An additional protocol needs to be executed after each user is
authenticated via Kerberos to establish a session key and to allow
multicast specific functionality like entering a group, leaving a
group to be executed securely. This would additionally allow
accounting and billing to be used efficiently and on a per-user
basis. This session key is then used to protect RSVP signaling
messages. These issues definitely need further investigation and are
not fully described in this version of the document.
In case that one entity crashed the established security association
is lost and therefore the other node must retransmit the service
ticket. The crashed entity can use an Integrity Challenge message to
request a new Kerberos ticket to be retransmitted by the other node.
If a node receives such a request then a reply message must be
returned.
b) Integrity Protection
Integrity protection between the userÆs host and the first hop
router is based on the RSVP INTEGRITY object. Since the RSVP
Integrity object is an optional element of the RSVP message IPSec
protection of the signaling message to the router may also provide
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integrity protection either with IPSec AH [RFC2402] or IPSec ESP
[RFC2406] as mentioned already in the previous paragraph.
Furthermore it is stated that other keyed hash functions apart from
HMAC-MD5 may be used within the RSVP INTEGRITY object and it is
obvious that both communicating entities must have security
associations indicating the algorithm used. This may be however
difficult since there is no negotiation protocol defined to agree on
a specific algorithm. Hence it is very likely that HMAC-MD5 is the
only usable algorithm for the RSVP INTEGRITY object if RSVP is used
in a mobile environment and only in local environments it may be
useful to switch to a different keyed hash algorithm. The other
possible alternative is that every implementation must support the
most important keyed hash algorithms for example MD5, SHA-1, RIPEMD-
160 etc. HMAC-MD5 was mainly chosen because of the performance
characteristics. The weaknesses of MD5 [DBP96] are known and
described in [Dob96]. Other algorithms like SHA-1 [SHA] and RIPEMD-
160 [DBP96] instead are known to provide better security properties.
c) Replay Protection
The main mechanism used for replay protection in RSVP are sequence
numbers whereby the sequence number is included in the RSVP
INTEGRITY object. The properties of this sequence number mechanisms
are described in Section 3.1. The fact that the receiver stores a
list of sequence numbers is an indicator for a window mechanism.
This somehow conflicts with the requirement that the receiver only
has to store the highest number given in Section 3 of [RFC2747]. We
assume that this is a typo. Section 4.1 of [RFC2747] gives a few
comments about the out-of-order delivery and the ability of an
implementation to specify the replay window.
If IPSec is used to protect RSVP messages then the optional IPSec
replay protection mechanism may be used which is also based on
sequence numbers with a window mechanism. This window mechanism may
(theoretically) also cause problems whereby an adversary reorders
messages. This is however very difficult to exploit since the
signaling messages are exchanged at a relatively low rate compared
to regular data traffic that may also be protected with IPSec.
- Integrity Handshake
The mechanism of the Integrity Handshake is explained in Section
3.5. The Cookie value is suggested to be hash of a local secret and
a timestamp. The Cookie value is not verified by the receiver. The
mechanism used by the Integrity Handshake is a simple
Challenge/Response message which assumes that the key shared between
the two hosts survives the crash. If the security association is
however dynamically created then this assumption may not be true.
In Section 10 of [RFC2747] the authors note that an adversary can
create faked Integrity Handshake message including challenge
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cookies. Subsequently he would store the received response. Later he
tries to replay these responses while a responder recovers from a
crash or restart. If this replayed Integrity Response value is valid
and has a lower sequence number than actually used then this value
is stored at the recovering host. In order for this attack to be
successful the adversary must either have collected a large number
of challenge/response value pairs or the adversary ôdiscoveredö the
cookie generation mechanism (for example by knowing the local
secret). The collection of Challenge/Response pairs is even more
difficult since they depend on the Cookie value, on sequence number
included in the response message and on the shared key which is used
by the INTEGRITY object.
d) Confidentiality
Confidentiality is not considered to be a security requirement for
RSVP. Hence it is not directly supported by RSVP. However, IPSec can
provide confidentiality by encrypting the transmitted signaling
traffic with IPSec ESP.
e) Authorization
The task of authorization consists of two subcategories: Network
access authorization and RSVP request authorization. Access
authorization is provided when a node is authenticated to the
network e.g. via AAA protocols (for example using RADIUS [RFC2865]
or DIAMETER [CA+02]) and authorization information is downloaded to
one or more network elements for example to the access router/first
hop router to modify filter rules to enable the IP traffic
forwarding. The access router is therefore acting as a firewall with
dynamically created filter rules based on a successful host or user
authentication. Issues related to network access authorization are
outside the scope of RSVP.
The second authorization refers to RSVP itself. Depending on the
network configuration
- the router either forwards the received RSVP request to the policy
decision point e.g. by using COPS (see [RFC2748] and [RFC2749]) and
to request admission control procedure to be executed or
- the router supports the functionality of a PDP and therefore there
is no need to forward the request or
- the router may already be configured with the appropriate policy
information to decide locally whether to grant this request or not.
Based on the result of the admission control the request may be
granted or rejected. Without a policy element being embedded inside
the RSVP message no policy-based admission control can be done.
The interaction between the two access authorization procedures (and
the filter-installation at the various network devices) will likely
be investigated in more detail in the MIDCOM working group.
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f) Performance
The computation of the keyed message digest for a RSVP INTEGRITY
object does not represent a performance problem. The same is true
for IPSec AH (or IPSec ESP). The protection of signaling messages is
usually not a problem since these messages are transmitted at a low
rate. Even a high number of messages does not cause performance
problems for a RSVP routers because of the characteristics of the
keyed message digest routine.
The key management which is computationally more demanding is more
important for scalability. Since RSVP does not specify a particular
key exchange protocol to be used it is difficult to estimate the
effort to create the required security associations. Furthermore the
number of key exchanges to be triggered depends on security policy
issues like lifetime of a security association, required security
properties of the key exchange protocol, authentication mode used by
the key exchange protocol etc. In a stationary environment with a
single administrative domain the manual security association
distribution may be acceptable and provides the best performance
characteristics. In a mobile environment asymmetric authentication
methods are likely to be used with a key exchange protocol and some
sort of certificate verification needs to be supported.
4.3 User to PEP/PDP
As noted in the previous section both user and host based
authentication is supported by RSVP. Using RSVP, a user may
authenticate to the first hop router or to the PDP as specified in
[RFC2747] depending on the infrastructure provided by the network
domain or on the architecture used (e.g. the integration of RSVP and
Kerberos V5 into the Windows 2000 Operating System [MADS01]).
Another architecture where RSVP is tightly integrated is the one
specified by the PacketCable organization. The interested reader is
referred to [PKTSEC] for a discussion of the security architecture.
a) Authentication
When a user sends a RSVP PATH or RESV message then this message may
include some information to authenticate the user. [RFC3182]
describes how user and application information is embedded into the
RSVP message (AUTH_DATA object) and how to protect it. A router
receiving such a message can use this information to authenticate the
client and forward the user/application information to the policy
decision point (PDP). Optionally the PDP itself can authenticate the
user, which is described in the next section. In order to be able to
authenticate the user, to verify the integrity and to check for
replays the entire POLICY_DATA element has to be forwarded from the
router to the PDP e.g. by including the element into a COPS message.
It is assumed that the INTEGRITY object within the POLICY_DATA
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element is sent to the PDP along with all other attributes although
not clearly specified in [RFC3182].
Certificate Verification
Using the policy element as described in [RFC3182] it is not
possible to provide a certificate revocation list or other
information to proof the validity of the certificate inside the
policy element. A specific mechanism for certificate verification is
not discussed in [RFC3182] and hence a number of them can be used
for this purpose. For certificate verification the network element
(a router or the policy decision point), which has to authenticate
the user, could frequently download certificate revocation lists or
should use a protocol like the Online Certificate Status Protocol
(OCSP) [RFC2560] and the Simple Certificate Validation Protocol
(SCVP) [MHHF01] to determine the current status of a digital
certificate.
User Authentication to the PDP
This alternative authentication procedure uses the PDP to
authenticate the user instead of the first hop router. In Section
4.2.1 in [RFC3182] the choice is given for the user to either obtain
a session ticket for the next hop router or for the PDP. As noted in
the same Section the identity of the PDP or the next hop router is
statically configured or dynamically retrieved. Subsequently user
authentication to the PDP is considered.
Kerberos-based Authentication to the PDP
If Kerberos is used to authenticate the user then first a session
ticket for the PDP needs to be requested. If the user roams between
different routers in the same administrative domain then he does not
need to request a new service ticket since the PDP is likely to be
used by most or all first-hop routers within the same administrative
domain. This is different if a session ticket for a router has to be
obtained and authentication to a router is required. The router
therefore plays a passive role of forwarding the request only to the
PDP and executing the policy decision returned by the PDP.
Section 4.5.3 describes one example of user-to-PDP authentication.
User authentication with the policy element only provides unilateral
authentication where the client authenticates to the router or to
the PDP. If a RSVP message is sent to the userÆs host and public
keyed based authentication is used then the message does not contain
a certificate and digital signature. Hence no mutual authentication
can be assumed. In case of Kerberos mutual authentication may be
accomplished if the PDP or the router transmits a policy element
with an INTEGRITY object computed with the session key retrieved
from the Kerberos ticket or if the Kerberos ticket included in the
policy element is also used for the RSVP INTEGRITY object as
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RSVP Security Properties June 2002
described in Section 4.2. This procedure only works if a previous
message was transmitted from the end-host to the network and such
key is already established. [RFC3182] does not discuss this issue
and therefore there is no particular requirement dealing with
transmitting network specific credentials back to the end-user's
host.
b) Integrity Protection
The integrity protection of the RSVP message and the POLICY_DATA
element are protected separately as shown in Figure 1. In case of a
policy ignorant node along the path the RSVP INTEGRITY object and
the INTEGRITY object inside the policy element terminate at
different nodes. Basically the same is true for the credentials of
the user if they are verified at the policy decision point instead
of the first hop router.
- Kerberos
If Kerberos is used to authenticate the user to the first hop router
then the session key included in the Kerberos ticket may be used to
compute the INTEGRITY object of the policy element. It is the keyed
message digest that provides the authentication. The existence of
the Kerberos service ticket inside the AUTH_DATA object does not
provide authentication and a guarantee of freshness for the
receiving host. Authentication and guarantee of freshness is
provided by the keyed hash value of the INTEGRITY object inside the
POLICY_DATA element. The user thereby shows that he actively
participated in the Kerberos protocol and that he was able to obtain
the session key to compute the keyed message digest. The
Authenticator used in the Kerberos V5 protocol provides similar
functionality but replay protection is based on timestamps (or based
on sequence number if the optional seq-number field inside the
Authenticator is used for KRB_PRIV/KRB_SAFE messages as described in
Section 5.3.2 of [RFC1510]) .
- Digital Signature
If public key based authentication is provided then user
authentication is accomplished with the digital signature. As
explained in Section 3.3.3 of [RFC3182] the DIGITAL_SIGNATURE
attribute must be the last attribute in the AUTH_DATA object and the
digital signature covers the entire AUTH_DATA object. Which hash
algorithm and public key algorithm is used for the digital signature
computation is described in [RFC2440] in case that PGP is used. In
case of X.509 credentials the situation is more complex since
different mechanisms like CMS [RFC2630] or PKCS#7 [RFC2315] may be
used for the digitally signing the message element. X.509 only
provides the standard for the certificate layout which seems to
provide insufficient information for this purpose. Therefore X.509
certificates are supported for example by CMS and PKCS#7. [RFC3182],
however, does not make any statements about the usage of CMS and
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RSVP Security Properties June 2002
PKCS#7. Currently there is no support for CMS or PKCS#7 described in
[RFC3182], which provides more than only public key based
authentication (e.g. CRL distribution, key transport, key agreement,
etc.). Furthermore the usage of PGP in RSVP is vague since there are
different versions of PGP (including a OpenPGP [RFC2440]) and there
has been no indication which version should be used. When thinking
about CMS support for RSVP the main question that has to be answered
is whether a public key based authentication (and key agreement
mechanism) should be supported for a QoS signaling protocol.
Especially the risks of denial of service attacks, large processing,
memory and bandwidth utilization should be considered.
If the INTEGRITY object is not included in the POLICY_DATA element or
not sent to the PDP then we have to make the following observation:
a) For the digital signature case only the replay protection provided
by the digital signature algorithm can be used. It is however not
clear whether this usage was anticipated or not. Hence we might
assume that the replay protection is based on the availability of
RSVP INTEGRITY object used with a security association that is
established by other means.
b) If a Kerberos session ticket is included but without using the
Kerberos session key then the analogon of the Kerberos Authenticator
is missing. Obviously there is no guarantee that the user actually
followed the Kerberos protocol and was able to decrypt the received
TGS_REP (or in rare cases the AS_REP if a session ticket is requested
with the initial AS_REQ).
c) Replay Protection
Figure 4 below shows the interfaces relevant for replay protection
of signaling messages in a more complicated architecture. The client
therefore uses the policy data element with PEP2 since PEP1 is not
policy aware. The interfaces between the client and the PEP1 and
between the PEP1 and PEP2 are protected with the RSVP INTEGRITY
object. The link between the PEP2 and the PDP is protected for
example by using the COPS built-in INTEGRITY object. The dotted line
between the Client and the PDP indicates the protection provided by
the AUTH_DATA element which has no RSVP INTEGRITY object included.
AUTH_DATA +----+
+- - - - - - - - - - - - - - - - - - - - - - - - - -+PDP +-+
+----+ |
| |
|
| COPS |
INTEGRITY|
| |
|
| |
+--+---+ RSVP INTEGRITY +----+ RSVP INTEGRITY +----+ |
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RSVP Security Properties June 2002
|Client+-------------------+PEP1+----------------------+PEP2+-+
+--+---+ +----+ +-+--+
| |
+-----------------------------------------------------+
POLICY_DATA INTEGRITY
Figure 4: Replay Protection
Host authentication with the RSVP INTEGRITY object and user
authentication with the INTEGRITY object inside the POLICY_DATA
element both use the same replay mechanism. The length of the
Sequence Number field, sequence number rollover and the Integrity
Handshake is already explained in Section 3.1.
Section 9 in [RFC3182] states ôRSVP INTEGRITY object is used to
protect the policy object containing user identity information from
security (replay) attacks.ö. Hence the public key based
authentication does not support the RSVP based replay protection
since the digital signature does not cover the POLICY_DATA INTEGRITY
object with its Sequence Number field. The digital signature covers
the entire AUTH_DATA object.
The use of public key systems within the AUTH_DATA object
complicates replay protection. Digital signature computation with
PGP is described in [PGP] and in [RFC2440]. The data structure
preceding the signed message digest includes information about the
message digest algorithm used and a 32-bit timestamp when the
signature was created ("Signature creation time"). The timestamp is
included in the computation of the message digest. The IETF
standardized OpenPGP version [RFC2440] contains more information and
describes the different hash algorithms (MD2, MD5, SHA-1, RIPEMD-
160) provided. [RFC3182] does not make any statements whether the
"Signature creation time" field is used for replay protection. Using
timestamps for replay protection requires different synchronization
mechanisms in case of clock-screws. Traditionally "loosely"
synchronized clocks are assumed in those cases but also requires
specifying a replay-window.
If the "Signature creation time" is not used for replay protection
then a malicious policy ignorant node can use this weakness to
replace the user's credentials without destroying the digital
signature. Additionally the RSVP initiating host, where multiple
users may have access, must be trustworthy even if a smartcard is
used since otherwise, replay attacks with a recorded AUTH_DATA
object are possible. Note that this however violates the hop-by-hop
security assumption. It is therefore assumed that replay protection
of the user credentials is not considered as an important security
requirement since the hop-by-hop processing of the RSVP message
protects the message against modification by an adversary between
two communicating nodes.
There are two additional issues related to a Kerberos based user
authentication in the context of replay protection. The lifetime of
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RSVP Security Properties June 2002
the Kerberos ticket is based on the fields starttime and endtime of
the EncTicketPart structure of the ticket as described in Section
5.3.1 of [RFC1510]. Since the ticket is created by the KDC located
at the network of the verifying entity it is not difficult to have
the clocks roughly synchronized for the purpose of lifetime
verification. Additional information about clock-synchronization and
Kerberos can be found at [DG96].
If we assume that the Kerberos session key is used for RSVP then
there may be a need for rekeying. If we assume that a policy at the
user's host indicates when to rekey then the next RSVP message
includes a new Kerberos session ticket that is then used by the
verifying entity. If the lifetime of the Kerberos ticket or other
policies do not affect rekeying then an RSVP security association
may never require rekeying at all because of the large sequence
number space.
d) (User Identity) Confidentiality
This Section discusses the privacy protection of the identity
information transmitted inside the policy element. Especially the
user identity confidentiality is of interest because there is no
built-in RSVP mechanism for encryption of the POLICY_DATA or the
AUTH_DATA elements. The encryption of one of the attributes inside
the AUTH_DATA element - of the POLICY_LOCATOR attribute is discussed
in the next section.
There has often been the discussion whether the effort for
protecting user identity is worth the additional complexity. With
the increasing privacy awareness there must be at least a discussion
on the mechanisms provided by the given protocol. The main question
in this context is about the threat model i.e. against which entity
the user identity should be protected. Since RSVP does not make any
assumptions about the underlying key management protocol for most
parts it is difficult to make a judgment. However for the identity
representation part of the protocol it is possible to make some
observations. We assume that the most important threat for a user is
to reveal his identity to an adversary located between the userÆs
host and the first-hop router. Identities should furthermore not be
transmitted outside the domain of the visited network provider i.e.
the user identity information inside the policy data element should
be removed or modified by the PDP to prevent revealing information
to other (non-authorized) entities along the signaling path. We
cannot however provide user identity confidentiality against the
network provider to which the user is attached. Different mechanisms
must be deployed to disallow the network provider to create a
profile of the user. These mechanisms are outside the scope of this
document since there is a strong involvement with the initial
authentication and key agreement protocol executed between the user
and the visited network.
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RSVP Security Properties June 2002
If the link between the userÆs host and the first hop router is
protected with IPSec ESP then confidentiality of the entire
signaling messages is provided. Note however that the IPSec
protection may terminate at the different node than the RSVP policy
aware signaling does. The focus of this Section is, however, the
functionality provided by RSVP.
The ASCII or Unicode distinguished name of user or application
inside the POLICY_LOCATOR attribute of the AUTH_DATA element may be
encrypted as specified in Section 3.3.1 of [RFC3182]. The user (or
application) identity is then encrypted with either the Kerberos
session key or with the private key in case of public key based
authentication. Since the private key is used we usually speak of a
digital signature which can be verified by everyone possessing the
public key. Since the certificate with the public key is included in
the message itself this is no obstacle. Furthermore the included
certificate provides enough identity information for an eavesdropper
together with the additional (unencrypted) information provided in
the RSVP message. Hence the possibility of encrypting the policy
locator in case of public key based authentication is less obvious.
To encrypt the identities using asymmetric cryptography the userÆs
host must be able to somehow retrieve the public key of the entity
verifying the policy element (i.e. the first policy aware router or
the PDP). Currently no such mechanism is defined in [RFC3182].
There is no option to encrypt the user or application identity
without Kerberos or public key mechanisms are used since the
selection of an appropriate security association is not possible.
The algorithm used to encrypt the POLICY_LOCATOR with the Kerberos
session key is assumed to be the same as the one used for encrypting
the service ticket. The information about the used algorithm is
available in the etype field of the EncryptedData ASN.1 encoded
message part. Section 6.3 of [RFC1510] lists the supported
algorithms. [Rae01] defines new encryption algorithms (Rijndael,
Serpent, and Twofish) that were published in the context of the AES
competition.
The task of evaluating the confidentiality provided for the user
requires to look at protocols executed outside of RSVP (for example
to look at the Kerberos protocol). The ticket included in the
CREDENTIAL attribute may provide user identity protection by not
including the optional cname attribute inside the unencrypted part
of the Ticket. Since the Authenticator is not transmitted with the
RSVP message the cname and the crealm of the unencrypted part of the
Authenticator are not revealed. In order for the user to request the
Kerberos session ticket, for inclusion in the CREDENTIAL attribute,
the Kerberos protocol exchange must be executed. Then the
Authenticator sent with the TGS_REQ reveals the identity of the
user. The AS_REQ must also include the user identity to allow the
Kerberos Authentication Server to respond with an AS_REP message
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RSVP Security Properties June 2002
that is encrypted with the user's secret key. Using Kerberos, it is
therefore only possible not to reveal content of the encrypted
policy locator, which is only useful if this value differs from the
user identity used with Kerberos. Hence using Kerberos it is not
"entirely" possible to provide user identity confidentiality.
It is important to note that information stored in the policy
element may be changed by a policy aware router or by the policy
decision point. Which parts are changed depends upon whether
multicast or unicast is used, how the policy server reacts, where
the user is authenticated and whether he needs to be re-
authenticated in other network nodes etc. Hence user and application
specific information can leak after the messages leave the first hop
within the network where the user's host is attached. As mentioned
at the beginning of this Section this information leakage is assumed
to be intentional.
e) Authorization
Additional to the description of the authorization steps of the
Host/Router interface, user based authorization is added with the
policy element providing user credentials. The inclusion of user and
application specific information enables policy-based admission
control with special user policies that are likely to be stored at a
dedicated server. Hence a Policy Decision Point can query for
example a LDAP server for a service level agreement stating the
amount of resources a certain user is allowed to request. Additional
to the user identity information group membership and other non-
security related information may contribute to the evaluation of the
final policy decision. If the user is not registered to the
currently attached domain then there is the question of how much
information the home domain of the user is willing to exchange. This
also impacts the users privacy policy. In general the user may not
want to distribute much of his policy information. Furthermore the
missing standardized authorization data format may create
interoperability problems when exchanging policy information. Hence
we can assume that the policy decision point may use information
from an initial authentication and key agreement protocol which may
already required cross-realm communication with the user's home
domain to only assume that the home domain knows the user and that
the user is entitled to roam and to be able to forward accounting
messages to this domain. This represents the traditional subscriber
based accounting scenario. Non-traditional or alternative means of
accounting might be deployed in the near future that do not require
the any type of inter-domain communication. Obviously there is a
strong interrelationship between the authorization and the process
of accounting. Note that the term accounting in this context is not
only related to process of metering. Metering is only the process of
measuring and collecting resource usage information. Instead the
term unites metering, pricing, charging and billing.
f) Performance
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RSVP Security Properties June 2002
If Kerberos is used for user authentication then a Kerberos ticket
must be included in the CREDENTIAL Section of the AUTH_DATA element.
The Kerberos ticket has a size larger than 500 bytes but only needs
to be sent once since a performance optimization allows the session
key to be cached as noted in Section 7.1 of [RFC2747]. It is assumed
that subsequent RSVP messages only include the POLICY_DATA INTEGRITY
object with a keyed message digest that uses the Kerberos session
key. This however assumes that the security association required for
the POLICY_DATA INTEGRITY object is created after (or modified) to
allow the selection of the correct key. Otherwise it difficult to
say which identifier is used to index the security association.
When Kerberos is used as an authentication system then, from a
performance perspective, then the message exchange to obtain the
session key needs to be considered although the exchange only needs
to be done once in a long time frame depending on the lifetime of
the session ticket. This is particularly true in a mobile
environment with a fast roaming user's host.
Public key based authentication usually provides the best
scalability characteristics for key distribution but the protocols
are performance demanding. A major disadvantage of the public key
based user authentication in RSVP is the non-existing possibility to
derive a session key. Hence every RSVP PATH or RESV message includes
the certificate and a digital signature, which is a huge performance
and bandwidth penalty. For a mobile environment with low performance
devices, high latency and low bandwidth links this seems to be less
encouraging. Note that a public key infrastructure is required to
allow the PDP (or the first-hop router) to verify the digital
signature and the certificate. To check for revoked certificates,
certificate revocation lists or protocols like the Online
Certificate Status Protocol [RFC2560] and the Simple Certificate
Validation Protocol [MHHF01]. Then the integrity of the AUTH_DATA
object via the digital signature is verified.
4.4 Communication between RSVP aware routers
a) Authentication
RSVP signaling messages are data origin authenticated and protected
against modification and replay using the RSVP INTEGRITY object.
IPSec may also provide RSVP signaling message protection. The RSVP
message flow between routers is protected based on the chain of trust
and hence each router only needs to have a security association with
its neighboring routers. This assumption was made because of
performance advantages and because of special security
characteristics of the core network where no user hosts are directly
attached. In the core network the network structure does not change
frequently and the manual distribution of shared secrets for the RSVP
INTEGRITY object may be acceptable. The shared secrets may be either
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RSVP Security Properties June 2002
manually configured or distributed by using network management
protocols like SNMP.
If IPSec is used in a hop-by-hop fashion then the required security
associations may be manually created or dynamically distributed with
IKE by either using symmetric or asymmetric authentication modes. A
description of the existing IKE authentication modes and IKE security
properties is outside the scope of this document. The reader is
referred to the relevant documents at the IPSec working group.
Independent of the key distribution mechanism host authentication
with RSVP built-in mechanisms is accomplished with the keyed message
digest in the RSVP INTEGRITY object computed using the previously
exchanged symmetric key. In case of IPSec host authentication is
accomplished with the keyed message digest included in the
Authentication Data field of the IPSec Authentication Header
included in every IP packet.
b) Integrity Protection
Integrity protection is either accomplished with the RSVP INTEGRITY
object with the variable length Keyed Message Digest field or with
the IPSec Authentication Header. A description of the IPSec AH is
found in [RFC2402] and IPSec ESP [RFC2406] with null encryption is
found in [RFC2410]. The main difference between IPSec and RSVP
protection is the layer at which the security is applied.
c) Replay Protection
Replay protection with the RSVP INTEGRITY object is extensively
described in previous Sections. IPSec provides an optional window-
based replay protection, which may cause problems if a strict
message ordering of RSVP messages is required. This problem was
already discussed in a previous Section and a possible solution is
to include the RSVP INTEGRITY object without a key, which reduces
the RSVP integrity protection to a simple MD5 hash. This
modification must however be integrated into an existing
implementation and it is not clear whether the RSVP standard allows
this modification. If the RSVP implementation is able to access the
IPSec Security Association Database and retrieve the required
security association then no such modification to RSVP is required
and IKE is only used to distribute the security associations. This
however requires the RSVP implementation to trigger the IKE
exchange.
To enable crashed hosts to learn the latest sequence number used the
Integrity Handshake mechanism is used in RSVP as explained in a
Section above. IPSec does not provide such a mechanism since a
crashed host looses its negotiated security associations and
therefore has to re-negotiate them using IKE. Note that manually
configured IPSec security associations do not provide replay
protection because a sequence number rollover would require the
Tschofenig Informational - Expires August 2002 26
RSVP Security Properties June 2002
establishment of a new SA. This is obviously not possible when using
manually configured IPSec SAs. Using IKE with pre-shared secrets is
therefore a simple solution.
d) Confidentiality
Confidentiality is not provided by RSVP but using IPSec ESP in a hop-
by-hop mode can provide it. The usage of IPSec ESP for RSVP is not
recommended because of the additional overhead for little additional
security benefit if we think of the underlying assumed trust model of
chain of trust. Hence there must be a good reason why to require
confidentiality in a hop-by-hop fashion in the core network of the
same administrative domain. If the RSVP network spawns different
provider networks then it is possible to encapsulate RSVP messages
between RSVP networks over a non-RSVP cloud similar to a VPN. Such a
configuration is mainly determined by the network structure of a
provider.
e) Authorization
Depending on the RSVP network QoS resource authorization at
different routers may need to contact the PDP again. Since the PDP
is allowed to modify the policy element, a token may be added to the
policy element to increase the efficiency of the re-authorization
procedure. This token is used to refer to an already computed policy
decision. The communications interface from the PEP to the PDP must
be properly secured.
f) Performance
The performance characteristics the protection of the RSVP signaling
messages is largely determined by the key exchange protocol since
the RSVP INTEGRITY object or IPSec AH are only used to compute a
keyed message digest of the transmitted messages. Furthermore only
RSVP signaling messages are protected and the protection of the
application data stream is outside the scope of RSVP. IPSec ESP
provides a performance penalty but may only be rarely used. A
network administrator may however use IPSec ESP in transport mode
with NULL encryption to provide the same functionality as IPSec AH
but with the chance of better hardware support.
The security associations within the core network i.e. between
individual routers (in comparison to the security association
between the userÆs host and the first-hop router or with the
attached network in general) can be established more easily because
of the strong trust assumptions. Furthermore it is possible to use
security associations with an increased lifetime to avoid too
frequent rekeying. Hence there is less impact for the performance
compared to the user to network interface. The security association
storage requirements are also less problematic.
Tschofenig Informational - Expires August 2002 27
RSVP Security Properties June 2002
4.5 Miscellaneous Issues
4.5.1 Dictionary Attacks and Kerberos
This Section addresses issues related to Kerberos and its
vulnerability against dictionary attacks since there often seems to
be a misunderstanding. The reason for including this discussion in
this document is that Kerberos seems to be one of the most widely
supported authentication and key distribution systems available.
The initial Kerberos AS_REQ request (without pre-authentication,
various extensions and without PKINIT) is unprotected. The response
message AS_REP is encrypted with the client's long-term key. An
adversary can take advantage of this fact by requesting AS_REP
messages to mount an off-line dictionary attack. Using pre-
authentication ([Pat92]) can be used to reduce this problem.
However pre-authentication does not entirely prevent dictionary
attacks by an adversary since he can still eavesdrop Kerberos
messages if being located at the path between the mobile node and
the KDC. With mandatory pre-authentication for the initial request
an adversary cannot request a Ticket Granting Ticket for an
arbitrary user. On-line password guessing attacks are still possible
by choosing a password (e.g. from a dictionary) and then
transmitting an initial request including pre-authentication data
field. An unsuccessful authentication by the KDC results in an error
message and the gives the adversary a hint to try a new password and
restart the protocol again.
There are however some proposals that prevent dictionary attacks
from happening. The use of Public Key Cryptography for initial
authentication [TN+01] (PKINIT) is one such solution. Other
proposals use strong-password based authenticated key agreement
protocols like the Encrypted Key Exchange protocol (EKE) to avoid
leaking of user password information. B. Jaspan investigated the use
of EKE for Kerberos V5 called ôDual-workfactor Encrypted Key
Exchangeö [Jas96] which is described below.
With the PA-ENC-DH pre-authentication Jaspan included the Diffie-
Hellman ôpublic keyö of the client encrypted with the user password
in the initial AS_REQ to the Authentication Server. Additionally the
modulus m is included since the client can choose this value
dynamically.
It is interesting to note that pre-authentication was orginally
introduced to allow the user to authenticate to the AS with the
inital AS_REQ message . The use of the Encrypted Key Exchange
protocol [BM92] as a pre-authentication mechanism does not allow the
Authentication Server to authenticate the client since this would
require the client to include verifiable data (e.g. a keyed message
digest for data origin authentication) but this destroys the
properties of EKE. EKE was designed to create a strong-password
based authentication protocol that is resistant against dictionary
Tschofenig Informational - Expires August 2002 28
RSVP Security Properties June 2002
attacks. Hence after the second message the Authentication Server
is authenticated to the client by showing that he was able to
compute the shared key k(a,as) used to encrypt the first part of
message (2). The client is not authenticated to the Authentication
Server.
It is obvious that both the client and the Authentication Server
must be able to provide good random numbers for the creation of the
Diffie-Hellman key pair. Jaspan additionally noted that the
timestamp in the response from the Authentication Server (AS_REP
message) can be used to eliminate the dependency on time
synchronization of the Kerberos protocol. The client can use this
value to adjust his clock after successful authentication of the
Authentication Server.
The vulnerability against denial of service attacks is a
disadvantage common to many strong-password based authenticated key
agreement protocols. Nothing prevents an adversary from flooding the
Authentication Server with bogus AS_REQ messages using the pre-
authentication method PA-ENC-DH. This forces the Authentication
Server to create a Diffie-Hellman public/private key pair, to
decrypt the received response and to compute the session key k(a,as)
and to return a message to the source IP address of the previously
received message. Even if the Authentication Server does not re-
create a new public/private key pair with every session he still has
to compute the session key which requires multiprecision operations
and this is time consuming.
Jaspan furthermore noted that the missing client authentication can
be used by an undetectable on-line password guessing attack as
described in [DH95]. An adversary sends an AS_REQ for a user B
encrypted with a password k(bÆ). The Authentication Server decrypts
the value of the pre-authentication field with the real user
password k(b) and encrypts his response to the adversary. If the
adversary correctly guessed the password of user B then the receive
response verifies correctly. Jaspan proposed to modify the KDC to
allow only a certain number of requests per day but this can be used
by an attacker to mount a denial of service attack against such
users to lock their accounts by sending a number of incorrect
requests to the KDC. The KDC would then reject Ticket Granting
Ticket or even a service ticket from legitimate users.
Tom Wu mentioned in [Wu99] the use of a variant of SRP [Wu98] and
the use of SPEKE [Jab96] to be used in the pre-authentication
process as possible candidates to prevent dictionary attacks.
Unfortunately Wu does not explain the proposals in detail.
Currently only PKINIT is available for preventing off-line
dictionary attacks. Other proposals described above like SPEKE, SRP
etc. are not included in the current Kerberos version. IPR issues
may be one of the reasons.
Tschofenig Informational - Expires August 2002 29
RSVP Security Properties June 2002
4.5.2 Example of User-to-PDP Authentication
The following Section describes an example of user-to-PDP
authentication. Note that the description below is not fully covered
by the RSVP specification and hence it should only be seen as an
example.
Windows 2000, which integrates Kerberos into RSVP, uses a
configuration with the user authentication to the PDP as described
in [MADS01]. The steps for authenticating the user to the PDP in an
intra-realm scenario are the following:
- Windows 2000 requires the user to contact the KDC and to request a
Kerberos service ticket for the PDP account AcsService in the local
realm.
- This ticket is then embedded in the AUTH_DATA element and included
in either the PATH or the RESV message. In case of MicrosoftÆs
implementation the user identity encoded as a distinguished name is
encrypted with the session key provided with the Kerberos ticket.
The Kerberos ticket is sent without the Kerberos authdata element
that contains authorization information as explained in [MADS01].
- The RSVP message is then intercepted by the PEP who forwards it to
the PDP. [MADS01] does not state which protocol is used to forward
the RSVP message to the PDP.
- The PDP who finally receives the message decrypts the received
service ticket. The ticket contains the session key which was used
by the user's host to
a) Encrypt the principal name inside the policy locator field of the
AUTH_DATA object and to
b) Create the integrity protected Keyed Message Digest field in the
INTEGRITY object of the POLICY_DATA element. The protection
described here is between the user's host and the PDP. The RSVP
INTEGRITY object on the other hand is used to protect the path
between the users host and the first-hop router since the two
message parts terminate at a different node and a different security
association must be used. The interface between the message
intercepting first-hop router and the PDP must be protected as well.
c) The PDP does not maintain a user database and [MADS01] describes
that the PDP may query the Active Directory (a LDAP based directory
service) for user policy information.
4.5.3 Open Issues
The following issues have often been mentioned in the context of
RSVP. However a design decision with regard to end-to-end security
and a framework for accounting and charging cannot be found in the
main RSVP documents.
a) End-to-End Security Issues and RSVP
Tschofenig Informational - Expires August 2002 30
RSVP Security Properties June 2002
End-to-end security for RSVP has not been discussed throughout the
document. In this context end-to-end security refers to credentials
transmitted between the two end-hosts using RSVP. It is obvious that
care must be taken to ensure that routers along the path are able to
process and modify the signaling messages according to the
processing procedure. Some objects however could be used for end-to-
end protection. The main question however is what the benefit of
such an end-to-end security is. First there is the question how to
establish the required security association which turned out to be
quite difficult between two arbitrary hosts. Furthermore it depends
on an architecture where RSVP is deployed whether it is useful to
provide end-to-end security. If RSVP is only used to signal QoS
information into the network and other protocols have to be executed
beforehand to negotiate the parameters and to decide which entity
actually has to pay for the reservation then no end-to-end security
is likely to be required. End-to-end security if introduced into
RSVP would then cause problem with extensions like RSVP proxy
[GD+02], Localized RSVP [MS+02] and others which terminate RSVP
signaling somewhere along the path without reaching the destination
end-host. Such a behavior could then be interpreted as a man-in-the-
middle attack.
b) Accounting/Charging Framework
Many documents have been published in the context of accounting and
charging for RSVP/IntServ, pricing, business models etc. The reasons
for large number of proposals and the ôrareö number of used
mechanisms are manifold. The lack of a defined framework makes it
difficult to argument whether the processing of credentials within
the policy element and a possible forwarding to other network
domains is required. Forwarding user credentials would allow other
networks to authenticate the identity acting as a signaling source.
If credentials are however removed then no such behavior can be
achieved and each neighboring domain only exchanges accounting data
to the next domain without taking the length of the real number of
visited domains into consideration. Scalability problems in the core
network speak against solutions that verify the user credentials by
every network along the path or solutions that create an analogon to
a long-distance call. A long-distance call in terms of RSVP can be
simulated by adding a monetary value for the requested resource at
each network along the path. Issues related to accounting will
receive further attention in the NSIS framework discussion.
5 Conclusions
It is often argued that RSVP cannot be used in particular
environments. Whether this is true or not cannot be answered by the
author but what can be observed is the following: RSVP should be
seen as a building block that has to be adapted to provide the
desired services for a given architecture. The point to stress is
"architecture". Hence it is difficult to state whether RSVP provides
Tschofenig Informational - Expires August 2002 31
RSVP Security Properties June 2002
the adequate security for a given architecture without a particular
framework. The author represents the opinion that the RSVP designers
and architects did a good job in providing the necessary blocks
(including security relevant parts) that allows RSVP to be easily
adapted to most architectures. By including some RSVP extensions
additional flexibility and features are provided.
This document aims to provide more insights into the security of
RSVP explained with different words from a different view. It must
not be interpreted as a pass or fail evaluation of the security
provided by RSVP.
Certainly this document is not complete to describe all issues
related to RSVP but it serves as a starting point. Some issues that
require further considerations are RSVP extensions (for example
[RFC2207]), multicast issues and other security properties like
traffic analysis etc. Additionally the interaction with mobility
protocols (micro- and macro-mobility) from a security point of view
demands further investigation. As stated in the previous Section the
interaction with accounting/charging issues are worth a closer look.
What can be learned from a practical protocol experience and from
the increased awareness regarding security is that some of the
available credential types have received more acceptance. Kerberos
is such a system which is integrated in many IETF protocols today.
Public key based authentication techniques are however still
considered to be too heavy-weight (computationally and from a
bandwidth perspective) to be used for a per-flow signaling. The
increased focus on denial of service attacks additionally demands a
closer look on public key based authentication.
6 Security Considerations
This document discusses security properties of RSVP and as such, it
is concerned entirely with security.
7 IANA considerations
This document does not address any IANA considerations.
8 Acknowledgments
I would like to thank Jorge Cuellar, Robert Hancock, Xiaoming Fu and
Guenther Schaefer for their valuable comments. Additionally I would
like to thank Robert and Jorge for their time to discuss various
issues with me. Furthermore I would like to thank Marc De Vuyst for
his comments to the draft.
9 References
[BM92] Bellovin, B., Merrit, M.: ôEncrypted Key Exchange:
Password-based protocols secure against dictionary
Tschofenig Informational - Expires August 2002 32
RSVP Security Properties June 2002
attacksö, in ôProceedings of the IEEE Symposium on
Research in Security and Privacyö, May, 1992.
[CA+02] Calhoun, P., Arkko, J., Guttman, E., Zorn, G., Loughney,
J.: "DIAMETER Base Protocol", <draft-ietf-aaa-diameter-
09.txt>, (work in progress), March, 2002.
[DBP96] Dobbertin, H., Bosselaers, A., Preneel, B.: "RIPEMD-160:
A strengthened version of RIPEMD", in ôFast Software
Encryption, LNCS Vol 1039, pp. 71-82ö, 1996.
[DG96] Davis, D., Geer, D.: ôKerberos With Clocks Adrift:
History, Protocols and Implementationö, in ôUSENIX
Computing Systems Volume 9 no. 1, Winterö, 1996.
[DH95] Ding, Y., Horster, P.: ôUndetectable On-line Password
Guessing Attacksö, Operating Systems Review, 29(No. 4),
pp. 77-86, 1995.
[Dob96] Dobbertin, H.: "The Status of Md5 After a Recent
Attack," RSA Laboratories' CryptoBytes, Volume 2, Number
2, 1996.
[FH+01] Thomas, M., Froh, M., Hur, M., McGrew, D., Vilhuber, J.,
Medvinsky, S.: "Kerberized Internet Negotiation of Keys
(KINK)", <draft-ietf-kink-kink-02.txt>, (work in
progress), October, 2001.
[GD+02] Gai, S., Dutt, D., Elfassy, N., Bernet, Y.: "RSVP
Proxy", <draft-ietf-rsvp-proxy-03.txt>, (work in
progress), March, 2002.
[HA01] Hornstein, K., Altman, J.: "Distributing Kerberos KDC
and Realm Information with DNS", <draft-ietf- krb-wg-
krb-dns-locate-02.txt>, (work in progress), August,
2001.
[HH01] Hess, R., Herzog, S.: "RSVP Extensions for Policy
Control", <draft-ietf-rap-new-rsvp-ext-00.txt>,
(expired), June, 2001.
[Jab96] Jablon, D.: ôStrong password-only authenticated key
exchangeô, Computer Communication Review, 26(5), pp. 5-
26, October, 1996.
[Jas96] Jaspan, B.: ôDual-workfactor Encrypted Key Exchange:
Efficiently Preventing Password Chaining and Dictionary
Attacksö, in ôProceedings of the Sixth Annual USENIX
Security Conferenceö, pp. 43-50, July, 1996.
[MADS01] ôMicrosoft Authorization Data Specification v. 1.0 for
Microsoft Windows 2000 Operating Systemsö, April, 2000,
Tschofenig Informational - Expires August 2002 33
RSVP Security Properties June 2002
available at:
http://www.microsoft.com/technet/security/kerberos/defau
lt.asp, February, 2001.
[MHHF01] Malpani, A., Hoffman, P., Housley, R., Freeman, T.:
ôSimple Certificate Validation Protocol (SCVP)ö, <draft-
ietf-pkix-scvp-04.txt>, (work in progress), July, 2001.
[MS+02] Manner, J., Suihko, T., Kojo, M., Liljeberg, M.,
Raatikainen, K.: "Localized RSVP", <draft-manner-lrsvp-
00.txt>, (work in progress), May, 2002.
[Pat92] Pato, J., "Using Pre-Authentication to Avoid Password
Guessing Attacks", Open Software Foundation DCE Request
for Comments 26, December, 1992.
[PGP] "Specifications and standard documents",
http://www.pgpi.org/doc/specs/, March, 2002.
[PKTSEC] PacketCable Security Specification, PKT-SP-SEC-I01-
991201, Cable Television Laboratories, Inc., December 1,
1999, http://www.PacketCable.com/.
[Rae01] Raeburn, K.: "Rijndael, Serpent, and Twofish
Cryptosystems for Kerberos 5", <draft-raeburn-krb-
rijndael-krb-01.txt>, (work in progress), July, 2001.
[RF2367] McDonald, D., Metz, C., Phan, B.: ôPF_KEY Key Management
API, Version 2ö, RFC 2367, July, 1998.
[RFC1321] Rivest, R.: "The MD5 Message-Digest Algorithm", RFC
1321, April, 1992.
[RFC1510] Kohl, J., Neuman, C.: "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[RFC2104] Krawczyk, H., Bellare, M., Canetti, R.: ôHMAC: Keyed-
Hashing for Message Authenticationö, RFC 2104, February,
1997.
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., Jamin,
S.: äResource ReSerVation Protocol (RSVP) û Version 1
Functional Specificationô, RFC 2205, September 1997.
[RFC2207] Berger, L., OÆMalley, T.: äRSVP Extensions for IPSEC
Data Flowsô, RFC 2207, September 1997.
[RFC2315] Kaliski, B.: " PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March, 1998.
[RFC2367] McDonald, D., Metz, C., Phan, B.: "PF_KEY Key Management
API, Version 2", RFC 2367, July, 1998.
Tschofenig Informational - Expires August 2002 34
RSVP Security Properties June 2002
[RFC2401] Kent, S., Atkinson, R.: "Security Architecture for the
Internet Protocol", RFC 2401, November, 1998.
[RFC2402] Kent, S., Atkinson, R.: "IP Authentication Header", RFC
2402, November, 1998.
[RFC2406] Kent, S., Atkinson, R.: "IP Encapsulating Security
Payload (ESP)", RFC 2406, November, 1998.
[RFC2409] Harkins, D., Carrel, D.: ôThe Internet Key Exchange
(IKE)ö, RFC 2409, November, 1998.
[RFC2410] Glenn, R., Kent, S.: "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November, 1998.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., Thayer, R.:
"OpenPGP Message Format", RFC 2440, November, 1998.
[RFC2495] Housley, R., Ford, W., Polk, W., Solo, D.: "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, January, 1999.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., Adams,
C.: ôX.509 Internet Public Key Infrastructure Online
Certificate Status Protocol û OCSPö, RFC 2560, June,
1999.
[RFC2630] Housley, R.: ôCryptographic Message Syntaxö, RFC 2630,
June, 1999.
[RFC2747] Baker, F., Lindell, B., Talwar, M.: ôRSVP Cryptographic
Authenticationö, RC 2747, January, 2000.
[RFC2748] Boyle, J., Cohen, R., Durham, D., Herzog, S., Rajan, R.,
Sastry, A.: ôThe COPS(Common Open Policy Service)
Protocolö, RFC 2748, January, 2000.
[RFC2749] Boyle, J., Cohen, R., Durham, D., Herzog, S., Rajan, R.,
Sastry, A.: ôCOPS usage for RSVPö, RFC 2749, January,
2000.
[RFC2750] Herzog, S.: "RSVP Extensions for Policy Control", RFC
2750, January, 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A., Simpson, W.:
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June, 2000.
[RFC3182] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore,
T., Herzog, S., Hess, R.: ôIdentity Representation for
RSVPö, RFC 3182, October, 2001.
Tschofenig Informational - Expires August 2002 35
RSVP Security Properties June 2002
[SHA] NIST, FIPS PUB 180-1, "Secure Hash Standard", April,
1995.
[TN+01] Tung, B., Neuman, C., Hur, M., Medvinsky, A., Medvinsky,
S., Wray, J., Trostle, J.: ôPublic Key Cryptography for
Initial Authentication in Kerberosö, < draft-ietf-cat-
kerberos-pk-init-13.txt>, (work in progress), March,
2001.
[Wu98] Wu, T.: ôThe Secure Remote Password Protocolô, in
ôProceedings of the Internet Society Network and
Distributed System Security Symposiumö, pp. 97-111,
March, 1998.
[Wu99] Wu, T.: ôA Real-World Analysis of Kerberos Password
Securityö, in ôProceedings of the 1999 Network and
Distributed System Securityö, February, 1999.
10 Author's Contact Information
Hannes Tschofenig
Siemens AG
Otto-Hahn-Ring 6
81739 Munchen
Germany
Email: Hannes.Tschofenig@mchp.siemens.de
11 Full Copyright Statement
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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Tschofenig Informational - Expires August 2002 36
RSVP Security Properties June 2002
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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Tschofenig Informational - Expires August 2002 37
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