INTERNET DRAFT T. Polk
Intended Status: Informational NIST
R. Housley
Vigil Security
Expires: January 29, 2011 July 28, 2010
Routing Authentication Using A Database of Long-Lived Cryptographic Keys
draft-polk-saag-rtg-auth-keytable-03.txt
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Abstract
This document describes the application of a database of long-lived
cryptographic keys to establish session-specific cryptographic keys
to support authentication services in routing protocols. Keys may be
established between two peers for pair-wise communications, or
between groups of peers for multicast traffic.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Architecture and Design . . . . . . . . . . . . . . . . . . . . . 2
3 Pair-wise Application . . . . . . . . . . . . . . . . . . . . . . 3
4 Identifier Mapping . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Selected Range Reservation . . . . . . . . . . . . . . . . . 6
4.2 Protocol Specific Mapping Tables . . . . . . . . . . . . . . 6
5 Database Maintenance . . . . . . . . . . . . . . . . . . . . . . 6
6 Worked Example: TCP-AO . . . . . . . . . . . . . . . . . . . . . 6
6.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2 Protocol Operation: Xp Initiates a Connection . . . . . . . 8
6.3 Protocol Operation: Yp Initiates a Connection . . . . . . . 8
7 Security Considerations . . . . . . . . . . . . . . . . . . . 10
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1 Normative References . . . . . . . . . . . . . . . . . . 10
9.2 Informative References . . . . . . . . . . . . . . . . . 10
Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 12
1 Introduction
This document describes the application of a database of long-lived
cryptographic keys, as defined in [KEYTAB], to establish session-
specific cryptographic keys to provide authentication services in
routing protocols. Keys may be established between two peers for
pair-wise communications, or between groups of peers for multicast
traffic.
1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2 Architecture and Design
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Figure 1 illustrates the establishment and use of cryptographic keys
for authentication in routing protocols. Long-lived cryptographic
keys are inserted in a database manually. In the future, we
anticipate an automated key management protocol to insert these keys
in the database. (While this future environment conceivably includes
automated key management protocols to negotiate short-lived
cryptographic session keys, such keys are out of scope for this
database.) The structure of the database of long-lived cryptographic
keys is described in [KEYTAB].
The cryptographic keying material for individual sessions is derived
from the keying material stored in the database of long-lived
cryptographic keys. A key derivation function (KDF) and its inputs
are named in the database of long-lived cryptographic keys; session
specific values based on the routing protocol are input the the KDF.
Protocol specific key identifiers may be assigned to the
cryptographic keying material for individual sessions if needed.
+--------------+ +----------------+
| | | |
| Manual Key | | Automated Key |
| Installation | | Mgmt. Protocol |
| | | |
+------+-------+ +--+----------+--+
| | |
| | |
V V |<== Out of scope for this model.
+------------------------+ | Often used in other
| | | protocol environments
| Long-lived Crypto Keys | | like IPsec and TLS.
| | |
+------------+-----------+ |
| |
| |
V V
+---------------------------------+
| |
| Short-lived Crypto Session Keys |
| |
+---------------------------------+
Figure 1. Cryptographic key establishment and use.
3 Pair-wise Application
Figure 2 illustrates how the long-lived cryptographic keys are
accessed and employed when an entity wishes to establish a protected
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session with a peer. As one step in the initiation process, the
initiator requests the set of long term keys associated with the peer
for the particular protocol. If the set contains more than one key,
the initiator selects one long-term key based on the local policy.
The long-term key is provided as an input, along with session-
specific information (e.g., ports or initial counters), to a key
derivation function. The result is session-specific key material
which is used to generate cryptographic authentication.
Where the initiator is establishing a multicast session, the Peer in
the key request identifies the set of systems that will receive this
information.
+-------------------------+
| |
| Long-Lived |
| Crypto Keys |
| |
+-+---------------------+-+
^ |
| |
| V
+-------+-------+ +-------+-------+
| | | |
| Lookup Keys | | Select Key |
| By Peer | | By Policy |
| and Protocol | | |
| | +-------+-------+
+-------+-------+ |
^ |
| V
| +-------+-------+
| | |
| | Session Key |
| | Derivation |
| | |
| +-------+-------+
| |
| |
+-------+-------+ V
| | +-------+-------+
| Initiate | | |
| Session | |Authentication |
| with Peer | | Mechanism |
| | | |
+---------------+ +---------------+
Figure 2. Session Initiation
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Figure 3 illustrates how the long-lived cryptographic keys are
accessed and employed when an entity receives a request establish a
protected session with a peer. As step one in the session
establishment process, the receiver extracts the keyID for the long-
term keyID from the received data. The receiver then requests the
specified long-term key from the table. The long-term key is provided
as an input, along with session-specific information (e.g., ports or
initial counters), to a key derivation function. The result is
session-specific key material which is used to verify the
cryptographic authentication information.
+-------------------------+
| |
| Long-Lived |
| Crypto Keys |
| |
+-+---------------------+-+
^ |
| |
| V
+-------+-------+ +-------+-------+
| | | |
| Lookup Key | | Session Key |
| By KeyID | | Derivation |
| | | |
+-------+-------+ +-------+-------+
^ |
| |
| V
+-------+-------+ +-------+-------+
| | | |
| Receive Data | |Authentication |
| From Peer | | Mechanism |
| | | |
+---------------+ +---------------+
Figure 3. Session Acceptance
4 Identifier Mapping
[KEYTAB] specifies a 16-bit identifier, but protocols already exist
with key identifiers of various sizes. Where the identifiers are of
different sizes, an extra mapping step may be required. Note that
mapping mechanisms are local - that is, different mapping mechanisms
could be employed on different peers.
In practice, the mapping process need only be applied to the
LocalKeyID, whose value must be unique in the context of the
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database, as defined in [KEYTAB]. Uniqueness is not required for the
PeerKeyID, so mapping is generally restricted to truncation. Mapping
would only be needed to expand PeerKeyID's value beyond 16 bits.
4.1 Selected Range Reservation
Where a protocol uses an index of less than 16 bits, a selected range
of the local index space can be reserved for a particular protocol.
For example, consider two protocols P1 and P2 that each use 8 bit key
identifiers. Without identifier mapping these protocols would share
the space {0x0000 through 0x00ff} which would limit the pair of
protocols to 256 keys in total. By reserving the ranges {0x7f00
through 0x7fff} and {0x7e00 through 0x7eff} for P1 and P2
respectively permits each protocol to use the full 256 key
identifiers and establishes an unambiguous mapping for the protocol
key identifiers and local table identifiers.
When an initiator selects a key from the set in the table, the given
key identifier needs to be masked or shifted to the on-the-wire
range. Before requesting a specific key, the receiver would use a
shim layer to map the on-the-wire identifier into the reserved range.
4.2 Protocol Specific Mapping Tables
Each protocol can also maintain a simple mapping table with two
fields: the 16 bit index and the protocol specific value:
KEYTAB index (16 bits) | Protocol specific index (8 bits)
In this case, the host system would maintain separate mapping tables
for protocols P1 and P2.
5 Database Maintenance
The previous sections focus upon installing and using the
cryptographic keys in the database. A mechanism or mechanisms to
remove unneeded keys is also needed to ensure that the key material
up-to-date. [KEYTAB] provides mechanisms for expiration of entries;
such key management could be performed in a fully automated fashion.
Other reasons for key removal, such as severing a business
relationship, or deciding a long lived key has been compromised
before its expiration date, would inherently require a manual key
removal process.
6 Worked Example: TCP-AO
This section describes the way a TCP-AO implementation could use the
database. [tcpao] TCP-AO protocol is an example where the key
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identifier is limited to 8 bits, so an identifier mapping is needed.
We will assume two peers Xp and Yp. Xp employs the range reservation
method for mapping and has reserved the range {0x7f00 ... 0x7fff} for
LocalKeyIDs for TCP-AO, mapping to {0x00 ... 0xff}. Yp employs a
protocol specific mapping table in its TCP-AO implementation.
The following subsections describe how peers Xp and Yp make use of
the database of long-lived cryptographic keys when Xp and Yp
respectively initiate a session. (Note: Rollover to new sessions
keys during a session is described in [tcpao].)
6.1 Setup
The owners of Xp and Yp determine a need for authenticated
communication using TCP-AO. They decide to use AES-CMAC-128 for
authentication, so a 128 bit key is needed. They decide to use the
same key for both directions (inbound and outbound), and that the key
will be available from 12/31/2010 through 12/31/2011. Through an out-
of-band channel, the administrators establish the shared secret:
0x0123456789ABCDEF0123456789ABCDEF
Peer Xp selects the first available TCP-AO identifier in the reserved
range, which is 0x7f05 and maps to an eight-bit identifier 0x05.
Peer Yp selects the next available TCP-AO identifier, 0x12, and the
next available LocalKeyID, which is 0x0107. Peer Yp also adds an
entry to its TCP-AO mapping table mapping the LocalKeyID to the TCP-
AO identifier, as shown in Figure 5:
LocalKeyID TCP-AO identifier
--------------------------------
0x001a | 0x01
0x004d | 0x02
... ...
0x0107 | 0x12
Figure 5. Protocol Specific KeyID Mapping Table for TCP-AO
After exchanging the TCP-AO identifiers, the peers have sufficient
information to establish their [KEYTAB] entries. Peer Xp's [KEYTAB]
entry is shown as Figure 6:
LocalKeyID 0x7f05
PeerKeyID 0x0012
KDFInputs none
AlgID AES-CMAC-128
Key 0x0123456789ABCDEF0123456789ABCDEF
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Direction both
NotBefore 12/31/2010
NotAfter 12/31/2011
Peers yp.example.com
Protocol TCP-AO
Figure 6. Key Table Entry on Xp
Peer Yp's [KEYTAB] entry is shown as Figure 6:
LocalKeyID 0x0107
PeerKeyID 0x0005
KDFInputs none
AlgID AES-CMAC-128
Key 0x0123456789ABCDEF0123456789ABCDEF
Direction both
NotBefore 12/31/2010
NotAfter 12/31/2011
Peers xp.example.com
Protocol TCP-AO
Figure 7. Key Table Entry on Yp
6.2 Protocol Operation: Xp Initiates a Connection
Peer Xp wishes to initiate a connection with Peer Yp.
(1) Xp performs a key lookup for {Peer=Yp, Protocol=TCP-AO}, and the
entry with LocalKeyID 0x7f05 is returned.
(2) The LocalKeyID 0x7f05 is range mapped by Xp to the TCP-AO
identifier 0x05.
(3) Xp performs the session key derivation using the mechanism
specified for the TCP-AO protocol in [ao-crypto].
(4) Xp generates the AES-CMAC-128 MACs for the outgoing traffic using
the derived key, and asserts the key identifier 0x05 in the packets.
(5) Yp receives a protected packet from Xp, and extracts the key
identifier 0x05.
(6) Yp performs a a key lookup for {Peer=Xp, Protocol=TCP-AO,
PeerKeyID=0x05}, and the entry with LocalKeyID 0x0107 is returned.
(7) Yp performs the session key derivation using the mechanism
specified for the TCP-AO protocol in [ao-crypto].
(8) Yp verifies the MACs for the incoming traffic using the derived
key.
6.3 Protocol Operation: Yp Initiates a Connection
Where Peer Yp establishes the connection, the same process is
followed, except that the range mapping process from step (2) is
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replaced by a table lookup.
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7 Security Considerations
<Security considerations text>
8 IANA Considerations
This document requires no actions by IANA.
9 References
9.1 Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[KEYTAB] R. Housley and Polk, T. "Database of Long-Lived
Cryptographic Keys", draft-housley-saag-crypto-key-table-
02.txt, September 2010.
9.2 Informative References
[tcpao] J. Touch, Mankin A., and Bonica R. "The TCP Authentication
Option", draft-ietf-tcpm-tcp-auth-opt-08.txt, October
2009.
[ao-crypto] Lebovitz, G., "Cryptographic Algorithms, Use, &
Implementation Requirments for TCP Authentication
Option", draft-lebovitz-ietf-tcpm-tcp-ao-crypto-02.txt,
July 2009.
Author's Addresses
Tim Polk
National Institute of Standards and Technology
100 Bureau Drive, Mail Stop 8930
Gaithersburg, MD 20899-8930
USA
EMail: tim.polk@nist.gov
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
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USA
EMail: housley@vigilsec.com
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