Draft RSVP Cryptographic Authentication June 1996
RSVP Cryptographic Authentication |
draft-ietf-rsvp-md5-02.txt
Status of this Memo
This document is an Internet Draft. Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
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Internet Drafts are valid for a maximum of six months and may
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material or to cite them other than as a "work in progress".
Abstract
This document describes the format and use of RSVP's INTEGRITY
object to provide hop-by-hop integrity and authentication of
RSVP messages.
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1. Introduction
The Resource ReSerVation Protocol RSVP [1] is a protocol for
setting up distributed state in routers and hosts, and in
particular for reserving resources to implement integrated
service. RSVP allows particular users to obtain preferential
access to network resources, under the control of an admission
control mechanism. Permission to make a reservation will
depend both upon the availability of the requested resources
along the path of the data, and upon satisfaction of policy
rules.
To protect the integrity of this admission control mechanism,
RSVP requires the ability to protect its messages against
corruption and spoofing. This document proposes a mechanism
to protect RSVP message integrity hop-by-hop. The proposed
scheme transmits the result of applying a cryptographic
algorithm to a one-way function or ``digest'' of the message
together with a secret Authentication Key. This scheme
affords protection against forgery or message modification,
but not replays. It is possible to replay a message until the
sequence number changes, but the sequence number makes replays
less of an issue. The proposed mechanism does not afford
confidentiality, since messages stay in the clear; however,
the mechanism is also exportable from most countries, which
would be impossible were a privacy algorithm to be used.
The proposed mechanism is independent of a specific
cryptographic algorithm, but the document describes the use of
Keyed MD5 [2] for this purpose.
The cost of computing a Keyed MD5 message digest far exceeds
the cost of computing an RSVP checksum; therefore the RSVP |
checksum should be disabled (set to zero) if MD5 |
authentication is used, as the MD5 digest is a much stronger
integrity check.
Two uses are envisioned: authentication of RSVP messages or |
message fragments (should a fragmentation procedure be defined |
in the future), and authentication of sessions. The INTEGRITY |
object used in both is the same, and is defined in this |
document. The use of the INTEGRITY object for those purposes |
is defined in other more appropriate documents [1] [9].
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1.1. Why not use the Standard IPSEC Authentication Header? |
One obvious question is why, since there exists a standard |
mechanism for authentication, we would choose to not use it.
This was discussed at length in the working group, and was
rejected due to the operational impact of manually opening a
new security association among the routers that a flow
traverses for each flow making reservations. |
It is also not clear that RSVP messages are well defined for
the security associations, as a router must forward PATH and
PATH TEAR messages using the same source address as the sender
listed in the SENDER TEMPLATE, as in RSVP tunnels traffic may
not follow exactly the same IP path otherwise.
These matters are simplified if a secure key management |
protocol exists which can be used to open and key the security |
associations; should such a protocol come into existence, it |
may be worthwhile reviewing this decision. However, the |
addressing considerations conspire against using the same |
solution as one which would work for IPSEC. Therefore, this |
consideration cannot be understood as a promise that this |
procedure will go away.
2. Data Structures
2.1. INTEGRITY Object Format
The RSVP Message consists of a sequence of "objects," which
are type-length-value encoded fields having specific purposes.
The information required for hop-by-hop integrity checking is
carried in an INTEGRITY object.
The contents of INTEGRITY object are defined as a "Keyed
Message Digest" structure, with one of the following formats: |
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IP4 Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 1|
+-------------+-------------+-------------+-------------+
| Key Identifier |
+-------------+-------------+-------------+-------------+
| Sequence Number | |
+-------------+-------------+-------------+-------------+
| Sending System IP4 Address | |
+-------------+-------------+-------------+-------------+ |
| | |
+ + |
| | |
+ Keyed Message Digest | |
| | |
+ + |
| | |
+-------------+-------------+-------------+-------------+ |
IP6 Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 2|
+-------------+-------------+-------------+-------------+ |
| Key Identifier | |
+-------------+-------------+-------------+-------------+ |
| Sequence Number |
+-------------+-------------+-------------+-------------+
| |
+ +
| |
+ Sending System IP6 Address + |
| |
+ +
| |
+-------------+-------------+-------------+-------------+
| | |
+ + |
| | |
+ Keyed Message Digest + |
| | |
+ + |
| | |
+-------------+-------------+-------------+-------------+ |
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(1) Key Indentifier
An unsigned 32-bit number that acts as a key selector. |
With the key, the system stores an algorithm for its |
application.
(2) Sending System Address
This is the same address as would be carried in the
Next Hop or Previous Hop object, the address of the |
interface of the RSVP system that sent this message.
(3) Sequence Number
An unsigned 32-bit non-decreasing sequence number.
Any non-decreasing sequence of numbers may be used as
Sequence Number values. For example, a timestamp on
the message's creation or a simple message counter
might be used.
This sequence number is reset to zero upon any key
change.
(4) Keyed Message Digest |
The digest must be a multiple of 4 octets long. For
MD5, it will be 16 bytes long.
2.2. Keyed MD5 Message Trailer
The Keyed MD5 algorithm requires appending the following
message trailer to the message to be sent, before the hash is
computed. However, this trailer is not transmitted, since the
receiver can reconstruct it knowing the message length and
hash algorithm.
The trailer consists of bytes to pad the length appropriately
followed by the a 64-bit unsigned integer equal to the length
of the RSVP message without the trailer.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more pad bytes (defined by [2] or[8] when MD5 is used)||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length MSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 64 bit message length LSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3. Message Processing Rules
3.1. Message Generation
An RSVP message is created as usual, with these exceptions:
(1) The RSVP checksum is not calculated, but it set to zero.
(2) The INTEGRITY object is inserted in the appropriate
place, and its location in the message is remembered for
later use.
(3) The current sequence number is placed in the Sequence
Number field of the INTEGRITY object.
If several messages are being created simultaneously (for
example, in a periodic refresh generated by a router),
the messages should all use the same sequence number.
This is to assure that message reordering between RSVP |
peers (in non-FIFO queues or in an RSVP tunnel) does not |
cause authentication to fail for some of them.
(4) The appropriate Authentication Key is selected and placed
in the Keyed Message Digest field of the INTEGRITY |
object.
(5) The Key Identifier is placed into the INTEGRITY object. |
(6) The Keyed MD5 message trailer is appended to the end of
the message in memory.
(7) A Keyed Message Digest of the augmented message is |
calculated using the appropriate hash algorithm. When
the Keyed MD5 algorithm is used, the hash calculation is |
described in [2] and [8].
(8) The digest is written into the Cryptographic Digest field
of the INTEGRITY object, overlaying the Authentication
Key.
In the sender, Authentication Key selection is based on the |
interface through which the message is sent, there being a key |
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configured per interface. While administrations may configure |
all the routers and hosts on a subnet (or for that matter, in |
their network) with the same key, implementations should |
assume that each sender may send with a different key on each |
numbered interface, and that they keys are simplex - the key |
that a system uses to sign its messages need he same key that |
its recievers use to sign theirs. Implementations SHOULD |
maintain a separate key per interface that they sign with.
This restriction to numbered interfaces is intentional; if an |
RSVP system peers with another through a set of non-RSVP |
routers, and it might be able to reach systems through that |
domain from either a numbered interface or an unnumbered |
interface using the same address as a router id, the choice of |
key would otherwise be ambiguous. Therefore, on unnumbered |
interfaces, an RSVP router must use the same key as it uses on |
the related numbered interface. User interfaces SHOULD |
provide convenient ways to configure these keys. |
3.2. Message Reception
When the message is received, the process is reversed:
(1) The RSVP checksum is not calculated.
(2) The Cryptographic Digest field of the INTEGRITY object is
set aside.
(3) The Key Identifer field and Sending System Address are
used to determine the Authentication Key and the hash
algorithm to be used. Implementations SHOULD maintain a |
key per neighboring RSVP system address or CIDR prefix, |
as the keys used by neighbors to sign their messages need |
not be the same key that the recieving system uses.
(4) If the received sequence number is less than the last |
sequence number received from the sending system with |
that key identifier, the message is discarded |
unprocessed. |
(5) The Cryptographic Digest field of the INTEGRITY object is
overlaid with the Authentication Key.
(6) The Keyed MD5 message trailer is reconstructed at the end
of the message.
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(7) A new digest calculated using the indicated algorithm.
(8) If the calculated digest does not match the received
digest, the message is discarded unprocessed. |
If a system detects the loss of a neighbor or interface, or |
the RSVP process is restarted on a system, the system should |
start with a new key if possible. In this way, the sequence |
number may be reset without exposure to a replay attack. In |
the event that no other key is available, the sequence number |
should be stored in non-volatile memory around failures, so |
that it may continue without decreasing.
4. Key Management
It is likely that the IETF will define a standard key
management protocol. It is strongly desirable to use that key
management protocol to distribute RSVP Authentication Keys
among communicating RSVP implementations. Such a protocol
would provide scalability and significantly reduce the human
administrative burden. The Key ID can be used as a hook
between RSVP and such a future protocol. Key management
protocols have a long history of subtle flaws that are often
discovered long after the protocol was first described in
public. To avoid having to change all RSVP implementations
should such a flaw be discovered, integrated key management
protocol techniques were deliberately omitted from this
specification.
4.1. Key Management Procedures
Each key has a lifetime associated with it. No key is ever
used outside its lifetime. If more than one key is currently
alive, then the youngest key (the key whose lifetime most
recently started) should be used.
Possible mechanisms for managing key lifetime include: the
use of the Network Time Protocol, hardware time-of-day clocks,
or waiting some time before emitting the first message to
determine what key other systems are signing with. The matter
is left for the implementor. Note that the concept of a "key
lifetime" does not require a hardware time-of-day clock or the
use of NTP, although one or the other is advised; it merely
requires that the earliest and latest times that the key is
valid must be programmable in a way the system understands.
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To maintain security, it is necessary to change the RSVP
Authentication Key on a regular basis. It must be possible to
switch the RSVP Authentication Key without loss of RSVP state
or denial of reservation service, and without requiring people
to change all the keys at once. This requires the RSVP
implementation to support the storage and use of more than one
RSVP Authentication Key on a given interface at the same time.
For each key there will be a locally-stored Key Identifier.
The combination of the Key Identifier and the interface
associated with the message uniquely identifies the
cryptographic algorithm and Authentication Key in use by RSVP.
As noted above, the party creating the RSVP message will
select a valid key from the set of valid keys for that
interface. The receiver will use the Key Identifier and
interface to determine which key to use for authentication of
the received message. More than one key may be associated
with an interface at the same time.
To ensure a smooth switch-over, each communicating RSVP system
must be updated with the new key several minutes before the
current key will expire and several minutes before the new key
lifetime begins. The new key should have a lifetime that
starts several minutes before the old key expires. This gives
time for each system to learn of the new RSVP Authentication
Key before that key will be used. It also ensures that the
new key will begin being used and the current key will go out
of use before the current key's lifetime expires. For the
duration of the overlap in key lifetimes, a system may receive
messages using either key and authenticate the message.
There are four important times for each key:
+ KeyStartReceive: the time the system starts accepting
received packets signed with the key.
+ KeyStartSign: the time the system starts signing packets
with the key.
+ KeyStopSign: the time the system stops signing packets
with the key, which implies that it starts signing with
the next key, if any.
+ KeyStopReceive: the time the system stops accepting
received packets signed with the key.
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The times in the order listed SHOULD form a non-decreasing
sequence. There needs to be some distance between start times
and stop times, to achieve a seamless transition. Each system
sends using the key with the most recent "start" time and
makes its first attempt at validation of incoming traffic with
this same key. If this validation fails and another (older)
key is also active, the system should attempt to validate with
any other active keys it may possess.
4.2. Key Management Requirements
Requirements on an implementation are as follows.
(1) It is strongly desirable that a hypothetical security
breach in one Internet protocol not automatically
compromise other Internet protocols. The Authentication
Key of this specification SHOULD NOT be stored using
protocols or algorithms that have known flaws.
(2) An implementation MUST support the storage of more than
one key at the same time, although normally only one key
will be active on an interface.
(3) An implementation MUST associate a specific lifetime
(i.e., KeyStartSign and KeyStopSign) with each key and
corresponding Key Identifier.
(4) An implementation MUST support manual key distribution
(e.g., the privileged user manually typing in the key,
key lifetime, and key identifier on the console). The
lifetime may be infinite.
(5) If more than one algorithm is supported, then the
implementation MUST require that the algorithm be
specified for each key at the time the other key
information is entered.
(6) Keys that are out of date MAY be deleted at will by the
implementation without requiring human intervention.
(7) Manual deletion of active keys SHOULD also be supported.
(8) Key storage SHOULD persist across a system restart, warm
or cold, to avoid operational issues, and the sequence
number in use SHOULD be stored with it. Implementations
should note, however, that systems with no non-volatile
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storage may reset the sequence number to zero when
restarted.
4.3. Pathological Cases
An implementation of this document must handle two
pathological cases. Both of these should be exceedingly rare.
(1) During key switch-over, devices may exist which have not
yet been successfully configured with the new key.
Therefore, systems MAY implement (and would be well
advised to implement) an algorithm that detects the set
of keys being used by its neighbors, and transmits its
messages using both the new and old keys until all the
neighbors are using the new key or the lifetime of the
old key expires. Under normal circumstances, this
elevated transmission rate will exist for a single
refresh interval.
(2) It is possible that the last key associated with an
interface may expire.
When this happens, it is unacceptable to revert to an
unauthenticated condition, and not advisable to disrupt
current reservations. Therefore, the system should send
a "last authentication key expiration" notification to
the network manager and treat the key as having an
infinite lifetime until the lifetime is extended, the key
is deleted by network management, or a new key is
configured.
5. Conformance Requirements
To conform to this specification, an implementation MUST
support all of its aspects. The MD5 authentication algorithm |
defined in [2] and [8] MUST be implemented by all conforming
implementations. A conforming implementation MAY also support
other authentication algorithms such as NIST's Secure Hash
Algorithm (SHA). Manual key distribution as described above
MUST be supported by all conforming implementations. All
implementations MUST support the smooth key rollover described
under "Key Change Procedures."
The user documentation provided with the implementation MUST
contain clear instructions on how to ensure that smooth key
rollover occurs.
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Implementations SHOULD support a standard key management
protocol for secure distribution of RSVP Authentication Keys
once such a key management protocol is standardized by the
IETF.
6. Acknowledgment
This document is derived directly from similar work done for
OSPF and RIP Version II, jointly by Ran Atkinson and Fred
Baker, with modifications by Dino Farinacci for IDMR.
7. References
[1] Braden, R., Ed., Zhang, L., Estrin, D., Herzog, S., and
S. Jamin, "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specificationq. Internet Draft |
draft-ietf-rsvp-spec-12.ps, May 1996.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[3] S. Bellovin, "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Volume 19,
Number 2, pp.32-48, April 1989.
[4] N. Haller, R. Atkinson, "Internet Authentication
Guidelines", RFC-1704, October 1994.
[5] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of
IAB Workshop on Security in the Internet Architecture",
RFC-1636, June 1994.
[6] R. Atkinson, "IP Authentication Header", RFC-1826, August
1995.
[7] R. Atkinson, "IP Encapsulating Security Payload", RFC-
1827, August 1995.
[8] P. Metzger, W. Simpson, "IP Authentication using Keyed
MD5", RFC-1828, August 1995. |
[9] S. Herzog, "Building Blocks for Accounting and Access |
Control", draft-ietf-rsvp-lpm-arch-00.ps, March 1996.
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8. Security Considerations
This entire memo describes and specifies an authentication
mechanism for RSVP that is believed to be secure against
active and passive attacks. Passive attacks are clearly
widespread in the Internet at present. Protection against
active attacks is also needed even though such attacks are not
currently widespread.
Users need to understand that the quality of the security
provided by this mechanism depends completely on the strength
of the implemented authentication algorithms, the strength of
the key being used, and the correct implementation of the
security mechanism in all communicating RSVP implementations.
This mechanism also depends on the RSVP Authentication Keys
being kept confidential by all parties. If any of these are |
incorrect or insufficiently secure, then no real security will |
be provided to the users of this mechanism.
Confidentiality is not provided by this mechanism. Work is
underway within the IETF to specify a standard mechanism for
IP-layer encryption. That mechanism might be used to provide
confidentiality for RSVP in the future. Protection against
traffic analysis is also not provided. Mechanisms such as
bulk link encryption might be used when protection against
traffic analysis is required.
9. Author's Address
Fred Baker
Cisco Systems
519 Lado Drive
Santa Barbara, California 93111
Phone: (408) 526-4257
Email: fred@cisco.com
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Table of Contents
1 Introduction .......................................... 2
1.1 Why not use the Standard IPSEC Authentication
Header? ........................................... 3
2 Data Structures ....................................... 3
2.1 INTEGRITY Object Format ............................. 3
2.2 Keyed MD5 Message Trailer ........................... 5
3 Message Processing Rules .............................. 6
3.1 Message Generation .................................. 6
3.2 Message Reception ................................... 7
4 Key Management ........................................ 8
4.1 Key Management Procedures ........................... 8
4.2 Key Management Requirements ......................... 10
4.3 Pathological Cases .................................. 11
5 Conformance Requirements .............................. 11
6 Acknowledgment ........................................ 12
7 References ............................................ 12
8 Security Considerations ............................... 13
9 Author's Address ...................................... 13
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