Network Working Group G. Zorn
Internet-Draft H. Zhou
Updates: 2865 (if approved) J. Salowey
Expires: January 9, 2006 Cisco Systems
July 8, 2005
Session Key Transport in RADIUS
draft-zorn-radius-keyreq-04.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an extension to the RADIUS protocol designed
to support requests for, and delivery of, session key material
between RADIUS clients and servers.
The messages described in this document may be used for a wide
variety of keying applications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Requirements . . . . . . . . . . . . . . . . 3
3. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Packet Types . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 Key-Request . . . . . . . . . . . . . . . . . . . . . . . 7
4.2 Key-Response . . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
AAA servers are a central repository of authentication credentials.
Since authentication is typically a prerequisite to key distribution,
it is natural for AAA servers to be able to deliver keys to clients
for various purposes. One example of this is the provisioning of the
link layer encryption keys used in IEEE 802.11 and 802.1X. There is a
wide variety of additional uses for key distribution in RADIUS.
This document describes an extension to the RADIUS protocol designed
to support requests for, and delivery of, session key material
between RADIUS clients and servers. One example of the applicability
of these extensions is the case where the end of a session key's
lifetime [KEYWRAP] is approaching, but the messages described in this
document may be used for a wide variety of keying applications.
Discussion of this draft may be directed to the authors.
2. Specification of Requirements
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 [RFC2119].
3. Packet Format
Exactly one RADIUS packet is encapsulated in the UDP Data field
[RFC0768] where the UDP Destination Port field indicates 1812
(decimal).
When a reply is generated, the source and destination ports are
reversed.
A summary of the RADIUS data format is shown below. The fields are
transmitted from left to right.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
The Code field is one octet, and identifies the type of RADIUS
packet.
If a RADIUS server receives a packet with an unrecognized Code
field, the server SHOULD respond with an Error-Notification
message containing instances of the Error-Code and Unrecognized-
Packet-Type Attributes [ERRMSG]; if the server does not support
the Error-Notification message, then the invalid packet MUST be
silently discarded.
If a RADIUS client receives a packet with an unrecognized Code
field, the client SHOULD post a RADIUS Accounting-Request message
with the Acct-Status-Type Attribute set to Acct-Error-Notification
containing instances of the Error-Code and Unrecognized-Packet-
Type Attributes [ERRMSG]. The invalid packet MUST be discarded.
The RADIUS Codes (decimal) defined in this document are as
follows:
<MSG1> Key-Request
<MSG2> Key-Response
Identifier
The Identifier field is one octet, and aids in matching requests
and replies. The RADIUS server can detect a duplicate request if
it has the same client source IP address, source UDP port and
Identifier within a short span of time.
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Length
The Length field is two octets. It indicates the length of the
packet including the Code, Identifier, Length, Authenticator and
Attribute fields. Octets outside the range of the Length field
MUST be treated as padding and ignored on reception. If the
packet is shorter than the Length field indicates, it MUST be
silently discarded. The minimum length is 20 and maximum length
is 4096.
Authenticator
The Authenticator field is sixteen (16) octets. The most
significant octet is transmitted first. This value is used to
authenticate the reply from the RADIUS server.
Since the Message-Authentication-Code Attribute [KEYWRAP] is
required to be present in both the Key-Request and the Key-
Response messages, the client and server MAY ignore the contents
of the Authenticator field. In any case, the validity of the of
the Authenticator field MUST NOT affect the evaluation of the
integrity of the message.
Request Authenticator
In Key-Request packets, the Authenticator value is a 16 octet
random number, called the Request Authenticator. The value
SHOULD be unpredictable and unique over the lifetime of a
secret (the password shared between the client and the RADIUS
server), since repetition of an authenticator value in
conjunction with the same secret would permit an attacker to
reply with a previously intercepted response. Since it is
expected that the same secret MAY be used to authenticate with
servers in disparate geographic regions, the Request
Authenticator field SHOULD exhibit global and temporal
uniqueness.
The Authenticator value in a Key-Request packet SHOULD also be
unpredictable, lest an attacker trick a server into responding
to a predicted future request, and then use the response to
masquerade as that server to a future notification packet.
Although protocols such as RADIUS are incapable of protecting
against theft of an authenticated session via real-time active
wiretapping attacks, generation of unique unpredictable
requests can protect against a wide range of active attacks
against authentication.
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Response Authenticator
The value of the Authenticator field in the Key-Response
packet is called the Response Authenticator, and contains a
one-way MD5 hash calculated over a stream of octets consisting
of: the RADIUS packet, beginning with the Code field, including
the Identifier, the Length, the Request Authenticator field
from the Key-Request packet, and the response Attributes,
followed by the shared secret. That is,
Acknowledgement Auth =
MD5(Code+ID+Length+RequestAuth+Attributes+Secret)
where '+' denotes concatenation.
Administrative Note
The secret shared between the client and the RADIUS server SHOULD
be at least as large and unguessable as a well- chosen password.
It is preferred that the secret be at least 16 octets. This is to
ensure a sufficiently large range for the secret to provide
protection against exhaustive search attacks. The secret MUST NOT
be empty (length 0) since this would allow packets to be trivially
forged.
A RADIUS server MUST use the source IP address of the RADIUS UDP
packet to decide which shared secret to use, so that RADIUS
requests can be proxied.
When using a forwarding proxy, the proxy must be able to alter the
packet as it passes through in each direction - when the proxy
forwards the request, the proxy MAY add a Proxy-State Attribute,
and when the proxy forwards a response, it MUST remove its Proxy-
State Attribute if it added one. Proxy-State is always added or
removed after any other Proxy-States, but no other assumptions
regarding its location within the list of attributes can be made.
Since Key-Response packets are authenticated on the entire packet
contents, the stripping of the Proxy-State attribute invalidates
the signature in the packet - so the proxy has to re-sign it.
Further details of RADIUS proxy implementation are outside the
scope of this document.
4. Packet Types
The RADIUS Packet type is determined by the Code field in the first
octet of the Packet.
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4.1 Key-Request
Description
Key-Request packets are sent to a RADIUS server to request the
delivery of a session key. A RADIUS client wishing to request the
delivery of a session key MUST transmit a RADIUS packet with the
Code field set to <MSG1> (Key-Request).
Upon receipt of an Key-Request packet from a valid client, the
server MUST reply using either a Key-Response message or a Error-
Notification message [ERRMSG].
A Key-Request message MUST contain either a NAS-IP-Address
Attribute [RFC2865] or a NAS-Identifier Attribute [RFC2865] or
both. To help defeat spoofing attacks, a Key-Request message MUST
contain either a Message-Authenticator Attribute [RFC2869] or a
Message-Authentication-Code Attribute [KEYWRAP].
A Key-Request message SHOULD contain a Session-Id Attribute
[LOGOFF] if one was returned from the server in the Access-Accept
message for the session; if no Session-Id Attribute is included,
the packet MUST contain a User-Name Attribute and such additional
Attributes as are necessary to positively identify a given user
session (e.g., Service-Type [RFC2865], Calling-Station-Id
[RFC2865], etc.).
A Key-Request packet MAY contain one or more Key Attributes
[KEYWRAP] in order to request a key with a particular identifier
or encrypted using a particular algorithm.
A summary of the Key-Request packet format is shown below. The
fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Request Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
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Code
<MSG1> for Key-Request
Identifier
The Identifier field MUST be changed whenever the content of the
Attributes field changes, and whenever a valid reply has been
received for a previous request. For retransmissions, the
Identifier MUST remain unchanged.
Request Authenticator
The Request Authenticator value MUST be changed each time a new
Identifier is used.
Implementation Note
Since the Message-Authentication-Code Attribute [KEYWRAP] is
required to be present in the Key-Request message, the MAY
ignore the contents of the Request Authenticator. In any case,
the validity of the of the Request Authenticator field MUST NOT
affect the evaluation of the integrity of the message.
Attributes
The Attribute field is variable in length, and contains the list
of required Attributes, as well as any desired optional
Attributes.
4.2 Key-Response
Description
A Key-Response packet is sent by a RADIUS server in response to a
Key-Request packet. A RADIUS server wishing to transfer keying
material to a client in response to a Key-Request packet MUST
transmit a RADIUS packet with the Code field set to <MSG2> (Key-
Response).
At least one Key Attribute [KEYWRAP] MUST be included in a Key-
Response packet.
A summary of the Key-Response packet format is shown below. The
fields are transmitted from left to right.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Response Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
<MSG2> for Key-Response
Identifier
The Identifier field is a copy of the Identifier field of the Key-
Request packet which caused this Key-Response packet to be
created.
Response Authenticator
The Response Authenticator value is calculated from the Key-
Request packet, as described above.
Implementation Note
Since the Message-Authentication-Code Attribute [KEYWRAP] is
required to be present in the Key-Response message, the client
MAY ignore the contents of the Response Authenticator. In any
case, the validity of the of the Response Authenticator field
MUST NOT affect the evaluation of the integrity of the message.
Attributes
The Attribute field is variable in length, and MAY contain any
desired optional Attributes in addition to the required
Attributes.
5. IANA Considerations
The criteria to be used by the Internet Assigned Numbers Authority
(IANA) for assignment of numbers within namespaces defined within
this document are identical to those given in [RFC3575].
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6. Security Considerations
If the key used in the generation of the Message-Authentication-Code
Attribute is compromised, an attacker might be able to modify or
replace the keying material.
7. Normative References
[ERRMSG] Zorn, G., "RADIUS Error Messages",
draft-zorn-radius-err-msg-04.txt (work in progress),
July 2005.
[KEYWRAP] Zorn, G., Zhang, T., Walker, J., and J. Salowey, "RADIUS
Attributes for Key Delivery",
draft-zorn-radius-keywrap-06.txt (work in progress),
July 2005.
[LOGOFF] Zorn, G., "User Session Tracking in RADIUS",
draft-zorn-radius-logoff-06.txt (work in progress),
July 2005.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC2869] Rigney, C., Willats, W., and P. Calhoun, "RADIUS
Extensions", RFC 2869, June 2000.
[RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575,
July 2003.
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Authors' Addresses
Glen Zorn
Cisco Systems
2901 Third Avenue
Suite 600
Seattle, WA 98121
US
Phone: +1 (425) 344-8113
Email: gwz@cisco.com
Hao Zhou
Cisco Systems
4125 Highlander Parkway
REQ01/3/
Richfield, OH 44286
US
Phone: +1 (330) 523-2132
Email: hzhou@cisco.com
Joseph Salowey
Cisco Systems
2901 Third Avenue
SEA1/6/
Seattle, WA 98121
US
Phone: +1 (206) 256-3380
Email: jsalowey@cisco.com
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