DRAFT OSPF MD5 Authentication September 1994
OSPF MD5 Authentication
draft-ietf-ospf-md5-01.txt
Tue Sep 6 14:53:55 PDT 1994
Fred Baker
Advanced Computer Communications
fbaker@acc.com
Randall Atkinson
Information Technology Division
Naval Research Laboratory
atkinson@itd.nrl.navy.mil
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 groups may also distribute working
documents as Internet Drafts.
Internet Drafts are valid for a maximum of six months and may be
updated, replaced, or obsoleted by other documents at any time. It is
inappropriate to use Internet Drafts as reference material or to cite
them other than as a "work in progress".
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1. Introduction
Growth in the Internet has made us aware of the need for improved
authentication of routing information. OSPF provides two authentication
mechanisms for use in an area: "No Authentication" and "Simple
Password". Both are vulnerable to passive attacks currently widespread
in the Internet. Well-understood security issues exist in routing
protocols [4]. Clear text passwords, currently specified for use with
OSPF, are no longer considered sufficient [5].
If authentication is disabled, then only simple misconfigurations are
detected. Simple passwords transmitted in the clear will further
protect against the honest neighbor, but are useless in the general
case. By simply capturing information on the wire - straightforward
even in a remote environment - a hostile process can learn the password
and overcome the network.
We propose that OSPF use an authentication algorithm, as in SNMP Version
2, augmented by a sequence number. MD5 is proposed as the standard
authentication algorithm for OSPF, but the mechanism is intended to be
algorithm-independent. While this mechanism is not unbreakable (no
known mechanism is), it provides a greatly enhanced probability that a
system being attacked will detect and ignore hostile messages. This is
because we transmit the output of an authentication algorithm (e.g.,
MD5) rather than the secret OSPF Authentication Key. This output is a
one-way function of a message and a secret OSPF Authentication Key.
This OSPF Authentication Key is never sent over the network in the
clear, thus providing protection against the passive attacks now
commonplace in the Internet.
In this way, protection is afforded against forgery or message
modification. It is possible to replay a message until the sequence
number changes, but the sequence number makes replay in the long term
less of an issue. The mechanism does not afford confidentiality, since
messages stay in the clear; however, the mechanism is also exportable
from most countries, which test a privacy algorithm would fail.
Other relevant rationales for the approach are that MD5 is used in SNMP
Version 2, and is therefore present in routers already, as is some form
of password management. A similar approach has been proposed for
authentication in IP version 6 (IPv6).
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2. Implementation Approach
Implementation requires three issues to be addressed:
(1) A changed packet format,
(2) Authentication procedures, and
(3) Management controls.
2.1. OSPF PDU Format
The basic OSPF message format provides for a 24 byte header with an
arbitrary data content. When MD5 is used, the same header and content
are used, except that the eight byte "authentication key" field is
reused to describe a "Keyed Message Digest" trailer. This consists in
five fields:
(1) The "Authentication Type" is Keyed Message Digest Algorithm,
indicated by the value 2.
(2) A 16 bit offset from the OSPF header to the MD5 digest (if no
other trailer fields are ever defined, this value equals the OSPF
Data Length).
(3) An unsigned 8-bit field that contains the Key Identifier or Key-
ID. This identifies the key used to create the Authentication
Data for this OSPF message. A key is associated with an
interface.
(4) An unsigned 8-bit field that contains the length in octets of the
trailing Authentication Data field. The presence of this field
permits other algorithms (e.g., SHA) to be substituted for MD5 if
desired.
(5) An unsigned 32 bit non-decreasing sequence number.
The trailer consists of the Authentication Data, which is the output of
the Keyed Message Digest Algorithm. When the Authentication Algorithm
is MD5, the output data is 16 bytes; during digest calculation, this is
effectively followed by a pad field and a length field as defined by RFC
1321.
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2.2. Processing Algorithm
When the authentication type is "Keyed Message Digest", message
processing is changed in message creation and reception.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | type | OSPF Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType=Keyed Message Digest Fn|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| offset to MD5 digest | Key ID | Auth Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| non-decreasing sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ (OSPF Data Length-24) bytes Data /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Authentication Data (var. length; 16 bytes when MD5 is used) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In memory, the following trailer is appended by the MD5 algorithm and
treated as though it were part of the message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more pad bytes (defined by RFC 1321 when MD5 is used) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length MSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 bit message length LSW |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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2.2.1. Message Generation
The OSPF Packet is created as usual, with these exceptions:
(1) The OSPF checksum is not calculated, but is set to zero.
(2) The authentication type field indicates the Keyed Message Digest
Algorithm (2).
(3) The authentication "password" field is reused to store a packet
offset to the Authentication Data, a Key Identifier, the
Authentication Data Length, and a non-decreasing sequence number.
The value used in the sequence number is arbitrary, but two suggestions
are the time of the message's creation or a simple message counter.
The OSPF Authentication Key is selected by the sender based on the
outgoing interface. 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. Since the key's algorithm is an attribute of
the key, stored in the sender and receiver along with it, the Key ID
effectively indicates which authentication algorithm is in use if the
implementation supports more than one authentication algorithm.
(1) The OSPF header's packet length field indicates the standard OSPF
portion of the packet.
(2) The Authentication Data Offset, Key Identifier, and
Authentication Data size fields are filled in appropriately.
(3) The OSPF Authentication Key, which is 16 bytes long when the MD5
algorithm is used, is now appended to the data. For all
algorithms, the OSPF Authentication Key is never longer than the
output of the algorithm in use.
(4) Trailing pad and length fields are added and the digest
calculated using the indicated algorithm. When MD5 is the
algorithm in use, these are calculated per RFC 1321.
(5) The digest is written over the OSPF Authentication Key. When MD5
is used, this digest will be 16 bytes long.
The trailing pad is not actually transmitted, as it is entirely
predictable from the message length and algorithm in use.
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2.2.2. Message Reception
When the message is received, the process is reversed:
(1) The OSPF Checksum is not calculated,
(2) The digest is set aside,
(3) The appropriate algorithm and key are determined from the value
of the Key Identifier field,
(4) The OSPF Authentication Key is written into the appropriate
number (16 when MD5 is used) of bytes starting at the offset
indicated,
(5) Appropriate padding is added as needed, and
(6) A new digest calculated using the indicated algorithm.
If the calculated digest does not match the received digest, the message
is discarded unprocessed. If the neighbor is in a state other than DOWN
or ATTEMPT and the received sequence number is less than the last one
received, the message likewise is discarded unprocessed. The received
sequence number must, of course, be stored by neighbor and zeroed upon a
"neighbor down" event. Acceptable messages are now truncated to "OSPF
Data Length" and treated normally.
3. Management Procedures
3.1. Use of SNMP in Key Management
It is strongly desirable that a hypothetical security breach in one
Internet protocol not automatically compromise other Internet protocols.
Also, the default method for changing SNMP Version 2 encryption keys
does not provide the property of perfect forward secrecy. Therefore,
the OSPF Authentication key of this specification SHOULD NOT be stored
using SNMP.
3.2. Key Management Requirements
Implementations MUST support the storage of more than one key at the
same time. They MUST associate a specific lifetime (i.e., data/time
first valid and data/time no longer valid) with each key, the key
identifier, and MUST support manual key distribution (e.g., the
privileged user manually typing in the key, key lifetime, and key
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identifier on the router console). 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.
Keys that are out of date MAY be deleted at will by the implementation
without requiring human intervention.
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 OSPF Authentication Keys among communicating OSPF
implementations. Such a protocol would provide scalability and
significantly reduce the human administrative burden. The Key ID can be
used as a hook between OSPF 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 OSPF implementations should such a flaw be discovered,
integrated key management protocol techniques were deliberately omitted
from this specification.
3.3. Key Management Procedures
As with all security methods using keys, it is necessary to change the
OSPF Authentication Key on a regular basis. To maintain routing
stability during such changes, implementations are required to store and
support the use of more than one OSPF Authentication Key on a given
interface at the same time.
Each key will have its own Key Identifier, which is stored locally. The
combination of the Key Identifier and the interface associated with the
message uniquely identifies the Authentication Algorithm and OSPF
Authentication Key in use.
As noted above in Section 2.2.1, the party creating the OSPF 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.
Hence it is possible to have fairly smooth OSPF Authentication Key
rollovers without losing legitimate OSPF messages because the stored key
is incorrect and without requiring people to change all the keys at
once. To ensure a smooth rollover, each communicating OSPF system must
be updated with the new key several minutes before they 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 OSPF
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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.
4. Conformance Requirements
To conform to this specification, an implementation MUST support all of
its aspects. The MD5 authentication algorithm defined in RFC-1321 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.S
The user documentation provided with the implementation MUST contain
clear instructions on how to ensure that smooth key rollover occurs.
Implementations SHOULD support a standard key management protocol for
secure distribution of OSPF Authentication Keys once such a key
management protocol is standardized by the IETF.
5. Acknowledgments
This work was done by the OSPF Working Group, of which John Moy is the
Chair. This suggestion was originally made by Christian Huitema on
behalf of the IAB. Jeff Honig (Cornell) and Dennis Ferguson (ANS) built
the first operational prototype, proving out the algorithms. The
authors gladly acknowledge significant inputs from each of these
sources.
6. References
[1] Moy, John, "OSPF Version 2", RFC 1583, March 1994.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[3] F. Baker, R. Coltun, "OSPF Version 2 Management Information
Base", RFC 1253, August 1991
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[4] S. Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM
Computer Communications Review, Volume 19, Number 2, pp.32-48,
April 1989.
[5] N. Haller, R. Atkinson, "Internet Authentication Guidelines",
RFC-XXXX (already submitted to RFC Editor), September 1994.
7. Security Considerations
This entire memo describes and specifies an authentication mechanism for
the OSPF routing protocol 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
OSPF implementations. This mechanism also depends on the OSPF
Authentication Key being kept confidential by all parties. If any of
these 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 OSPF 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.
The memo is written to address a security consideration in OSPF Version
2 that was raised during the IAB's recent security review [cite RFC
here].
8. Author's Address
Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California 93117
Phone: (805) 685 4455
Email: fbaker@acc.com
Randall Atkinson
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DRAFT OSPF MD5 Authentication September 1994
Information Technology Division
Naval Research Laboratory
Washington, DC 20375-5320
Voice: (DSN) 354-8590
Fax: (DSN) 354-7942
Email: atkinson@itd.nrl.navy.mil
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Table of Contents
1 Introduction .................................................... 2
2 Implementation Approach ......................................... 3
2.1 OSPF PDU Format ............................................... 3
2.2 Processing Algorithm .......................................... 4
2.2.1 Message Generation .......................................... 5
2.2.2 Message Reception ........................................... 6
3 Management Procedures ........................................... 6
3.1 Use of SNMP in Key Management ................................. 6
3.2 Key Management Requirements ................................... 6
3.3 Key Management Procedures ..................................... 7
4 Conformance Requirements ........................................ 8
5 Acknowledgments ................................................. 8
6 References ...................................................... 8
7 Security Considerations ......................................... 9
8 Author's Address ................................................ 9
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