One document matched: draft-ietf-radext-crypto-agility-requirements-05.txt
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RADEXT Working Group D. Nelson (Editor)
INTERNET-DRAFT Elbrys Networks, Inc.
Category: Informational
Expires: October 16, 2011
16 April 2011
Crypto-Agility Requirements for Remote Dial-In User Service (RADIUS)
draft-ietf-radext-crypto-agility-requirements-05.txt
Abstract
This memo describes the requirements for a crypto-agility solution
for Remote Authentication Dial-In User Service (RADIUS).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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This Internet-Draft will expire on October 16, 2011.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Requirements Language . . . . . . . . . . . . . . . . . . . 3
1.3. The Charge . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Publication Process . . . . . . . . . . . . . . . . . . . . 4
2. A Working Definition of Crypto-Agility . . . . . . . . . . . . 4
3. The Current State of RADIUS Security . . . . . . . . . . . . . 5
4. The Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Overall Solution Approach . . . . . . . . . . . . . . . . . 6
4.2. Security Services . . . . . . . . . . . . . . . . . . . . . 6
4.3. Backwards Compatibility . . . . . . . . . . . . . . . . . . 8
4.4. Interoperability and Change Control . . . . . . . . . . . . 9
4.5. Scope of Work . . . . . . . . . . . . . . . . . . . . . . . 9
4.6. Applicability of Automated Key Management Requirements . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
1.1. General
This memo describes the requirements for a crypto-agility solution
for Remote Authentication Dial-In User Service (RADIUS). This memo,
when approved, reflects the consensus of the RADIUS Extensions
(RADEXT) Working Group of the IETF as to the features, properties and
limitations of the crypto-agility work item for RADIUS. It also
defines the term "crypto-agility" as used in this context, and
provides the motivations for undertaking and completing this work.
The requirements defined in this memo have been developed based on e-
mail messages posted to the RADEXT WG mailing list, which may be
found in the archives of that list. The purpose of framing the
requirements in this memo is to formalize and memorialize them for
future reference, and to bring them explicitly to the attention of
the IESG and the IETF Community, as we proceed with this work.
1.2. Requirements Language
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].
A RADIUS crypto-agility solution is not compliant with this
specification if it fails to satisfy one or more of the MUST or MUST
NOT statements. A solution that satisfies all the MUST, MUST NOT,
SHOULD, and SHOULD NOT statements is said to be "unconditionally
compliant"; one that satisfies all the MUST and MUST NOT statements
but not all the SHOULD or SHOULD NOT requirements is said to be
"conditionally compliant".
1.3. The Charge
At the IETF-66 meeting, the RADEXT WG was asked by members of the
Security Area Directorate to undertake the action item to prepare a
formal description of a crypto-agility work item, and corresponding
milestones in the RADEXT Charter. After consultation with one of the
Security Area Directors, Russ Housley, text was initially proposed on
the RADEXT WG mailing list on October 26, 2006. That text reads as
follows:
The RADEXT WG will review the security requirements for crypto-
agility in IETF protocols, and identify the deficiencies of the
existing RADIUS protocol specifications against these
requirements. Specific attention will be paid to RFC 4962
[RFC4962].
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The RADEXT WG will propose one or more specifications to remediate
any identified deficiencies in the crypto-agility properties of
the RADIUS protocol. The known deficiencies include the issue of
negotiation of substitute algorithms for the message digest
functions, the key-wrap functions, and the password-hiding
function. Additionally, at least one mandatory to implement
cryptographic algorithm will be defined in each of these areas, as
required.
1.4. Publication Process
RADIUS [RFC2865] is a widely deployed protocol that has attained
Draft Standard status based on multiple independent interoperable
implementations. It is therefore highly desirable that a high level
of interoperability and security be maintained for crypto-agility
solutions.
To ensure that crypto-agility solutions published on the standards
track are well specified, secure and interoperable, the RADEXT WG has
adopted a two phase process for publication of crypto-agility
solutions.
In the initial phase, crypto-agility solutions adopted by the working
group will be published on the Experimental Track. Experimental
Track documents should contain a description of experimental
deployments and implementations in progress, as well as an evaluation
of the proposal against the requirements described in this document.
Based on the proposal evaluations, implementation and deployment
experience, and the results of interoperability tests, initial
proposals will be evaluated for publication on the standards track.
2. A Working Definition of Crypto-Agility
A generalized definition of crypto-agility was offered up at the
RADEXT WG session during IETF-68. Crypto-Agility is the ability of a
protocol to adapt to evolving cryptography and security requirements.
This may include the provision of a modular mechanism to allow
cryptographic algorithms to be updated without substantial disruption
to fielded implementations. It may provide for the dynamic
negotiation and installation of cryptographic algorithms within
protocol implementations (think of Dynamic-Link Libraries (DLL)).
In the specific context of the RADIUS protocol and RADIUS
implementations, crypto-agility may be better defined as the ability
of RADIUS implementations to automatically negotiate cryptographic
algorithms for use in RADIUS exchanges, including the algorithms used
to integrity protect and authenticate RADIUS packets and to hide
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RADIUS Attributes. This capability covers all RADIUS message types:
Access-Request/Response, Accounting-Request/Response, CoA/Disconnect-
Request/Response, and Status-Server. Negotiation of cryptographic
algorithms MAY occur within the RADIUS protocol, or within a lower
layer such as the transport layer.
Since RADIUS is a request/response protocol, the ability to negotiate
cryptographic algorithms within a single RADIUS exchange is
inherently limited. While a RADIUS request can provide a list of
supported cryptographic algorithms which can selected for use within
a response, prior to the receipt of a response, the cryptographic
algorithms utilized to provide security services within the request
will need to be determined a-priori.
Proposals MUST NOT introduce new capabilities negotiation features
into the RADIUS protocol and crypto-agility solutions SHOULD NOT
require changes to the RADIUS operational model as defined in "RADIUS
Design Guidelines" [RFC6158] Section 3.1 and Appendix A.4.
Similarly, a proposal SHOULD focus on the crypto-agility problem and
nothing else. For example, proposals SHOULD NOT require new
attribute formats and SHOULD be compatible with the guidance provided
in [RFC6158] Section 2.3.
Acceptable solutions for determining the mechanisms to be used within
a request include manual configuration as well as backward compatible
negotiation techniques such as those described in Section 4.3.
Solutions for determining the mechanisms to be used in a response
include manual configuration and "advertise and select" mechanisms
(e.g. selection within the response from mechanisms advertised in a
request).
3. The Current State of RADIUS Security
RADIUS packets, as defined in [RFC2865], are protected by an MD5
message integrity check (MIC), within the Authenticator field of
RADIUS packets other than Access-Request [RFC2865] and Status-Server
[RFC5997]. The Message-Authenticator Attribute utilizes HMAC-MD5 to
authenticate and integrity protect RADIUS packets.
While RADIUS does not support confidentiality of entire packets,
various RADIUS attributes support encrypted (also known as "hidden")
values, including: User-Password (defined in [RFC2865] Section 5.2),
Tunnel-Password (defined in [RFC2868] Section 3.5), and various
Vendor-Specific Attributes, such as the MS-MPPE-Send-Key and MS-MPPE-
Recv-Key attributes (defined in [RFC2548] Section 2.4). Generally
speaking, the hiding mechanism uses a stream cipher based on a key
stream from an MD5 digest. Attacks against this mechanism are
described in "RADIUS Support for EAP" [RFC3579] Section 4.3.4.
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"Updated Security Considerations for the MD5 Message-Digest and the
HMAC-MD5 Algorithms" [RFC6151] discusses security considerations for
use of the MD5 and HMAC-MD5 algorithms. While the advances in MD5
collisions do not immediately compromise the use of MD5 or HMAC-MD5
for the purposes used within RADIUS absent knowledge of the RADIUS
shared secret, the progress toward compromise of MD5's basic
cryptographic assumptions has resulted in the deprecation of MD5
usage in a variety of applications. As noted in [RFC6151] Section 2:
MD5 is no longer acceptable where collision resistance is required
such as digital signatures. It is not urgent to stop using MD5 in
other ways, such as HMAC-MD5; however, since MD5 must not be used
for digital signatures, new protocol designs should not employ
HMAC-MD5.
4. The Requirements
4.1. Overall Solution Approach
RADIUS crypto-agility solutions are not restricted to utilizing
technology described in existing RFCs. Since RADIUS over IPsec is
already described in "RADIUS and IPv6" [RFC3162] Section 5 and
[RFC3579] Section 4.2, this technique is already available to those
who wish to use it. Therefore, it is expected that proposals will
utilize other techniques.
4.2. Security Services
Proposals MUST support the negotiation of cryptographic algorithms
for per-packet integrity/authentication protection. Proposals also
MUST support per-packet replay protection for all RADIUS message
types. Crypto-agility solutions MUST specify mandatory-to-implement
cryptographic algorithms for each defined mechanism.
Crypto-agility solutions MUST avoid security compromise, even in
situations where the existing cryptographic algorithms utilized by
RADIUS implementations are shown to be weak enough to provide little
or no security (e.g. in event of compromise of the legacy RADIUS
shared secret). Included in this would be protection against bidding
down attacks. In analyzing the resilience of a crypto-agility
solution, it can be assumed that RADIUS requesters and responders can
be configured to require the use of new secure algorithms in the
event of a compromise of existing cryptographic algorithms or the
legacy RADIUS shared secret.
Guidance on acceptable algorithms can be found in [NIST-SP800-131A];
it is RECOMMENDED that mandatory-to-implement cryptographic
algorithms be chosen from among those classified as "Acceptable" with
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no known deprecation date.
It is RECOMMENDED that solutions provide support for confidentiality,
either by supporting encryption of entire RADIUS packets or by
encrypting individual RADIUS attributes. Proposals supporting
confidentiality MUST support the negotiation of cryptographic
algorithms for encryption.
Solutions providing for encryption of entire RADIUS packets need not
also provide support for encryption of individual RADIUS attributes.
Solutions providing for encryption of individual RADIUS attributes
are REQUIRED to provide support for improving the confidentiality of
existing encrypted (sometimes referred to as "hidden") attributes as
well as encrypting attributes (such as location attributes) that are
currently transmitted in cleartext.
In addition to the goals referred to above, [RFC4962] Section 2
describes additional security requirements, which translate into the
following requirements for RADIUS crypto-agility solutions:
Strong, fresh session keys
RADIUS crypto-agility solutions are REQUIRED to generate fresh
session keys for use between the RADIUS client and server. In
order to prevent the disclosure of one session key from aiding an
attacker in discovering other session keys, RADIUS crypto-agility
solutions are RECOMMENDED to support Perfect Forward Secrecy (PFS)
with respect to session keys negotiated between the RADIUS client
and server.
Limit key scope
It is RECOMMENDED that solutions enable a NAS and RADIUS server to
exchange confidential information such as keying material without
disclosure to third parties. In order to accomplish this, it is
RECOMMENDED that a RADIUS crypto-agility solution be compatible
with NAI-based Dynamic Peer Discovery [RADYN] as well as that it
support the use of public key credentials for authentication
between the NAS and RADIUS server.
For compatibility with existing operations, RADIUS crypto-agility
solutions SHOULD also support pre-shared key credentials. However,
support for end-to-end confidentiality of attributes or direct
communications between the NAS and RADIUS server is not required
when pre-shared key credentials are used.
Prevent the Domino effect
In order to prevent the domino effect, RADIUS crypto-agility
solutions MUST enable each RADIUS client and server pair to
authenticate utilizing unique credentials.
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4.3. Backwards Compatibility
Solutions to the problem MUST demonstrate backward compatibility with
existing RADIUS implementations. That is, an implementation that
supports both the crypto-agility solution and legacy mechanisms MUST
be able to talk with legacy RADIUS clients and servers (using the
legacy mechanisms).
While backward compatibility is needed to ease the transition between
legacy RADIUS and crypto-agile RADIUS, use of legacy mechanisms is
only appropriate prior to the compromise of those mechanisms. After
legacy mechanisms have been compromised, secure algorithms MUST be
used, so that backward compatibility is no longer possible.
In order to enable a request to be handled both by legacy as well as
crypto-agile implementations, a request can be secured with legacy
algorithms and in addition attributes providing security services
using more secure algorithms can be included. This approach allows a
RADIUS packet to be processed by legacy implementations as well as by
crypto-agile implementations, and does not result in additional
response delays.
In this approach to backward compatibility, legacy mechanisms are
initially used between crypto-agile implementations. However, if the
responder indicates support for crypto-agility, future requests can
omit use of legacy mechanisms.
Probing techniques can be used avoid the use of legacy algorithms
between crypto-agile implementations. An initial request can omit
use of legacy mechanisms, and if a response is received, then the
recipient can be assumed to be crypto-agile and future requests to
that recipient can utilize secure mechanisms. Similarly, the
responder can assume that the requester supports crypto-agility and
can prohibit use of legacy mechanisms in future requests.
If a response is not received, in the absence of information
indicating responder support for crypto-agility (such as pre-
configuration or previous receipt of a crypto-agile response), a new
request can be composed utilizing legacy mechanisms.
Since legacy implementations not supporting crypto-agility will
silently discard requests not protected by legacy algorithms rather
than returning an error, repeated requests may be required to
distinguish lack of support for crypto-agility from packet loss or
other failure conditions. As a result, probing techniques can delay
initial communication between crypto-agile requesters and legacy
responders. This can be addressed by upgrading the responders (e.g.
RADIUS servers) first.
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4.4. Interoperability and Change Control
Proposals MUST indicate a willingness to cede change control to the
IETF.
Crypto-agility solutions MUST be interoperable between independent
implementations based purely on the information provided in the
specification.
4.5. Scope of Work
Crypto-agility solutions MUST apply to all RADIUS packet types,
including Access-Request, Access-Challenge, Access-Reject, Access-
Accept, Accounting-Request, Accounting-Response, Status-Server and
CoA/Disconnect messages.
Since it is expected that the work will occur purely within RADIUS or
in the transport, message data exchanged with Diameter SHOULD NOT be
affected.
Proposals MUST discuss any inherent assumptions about, or limitations
on, client/server operations or deployment and SHOULD provide
recommendations for transition of deployments from legacy RADIUS to
crypto-agile RADIUS. Issues regarding cipher-suite negotiation,
legacy interoperability and the potential for bidding down attacks,
SHOULD be among these discussions.
4.6. Applicability of Automated Key Management Requirements
"Guidelines for Cryptographic Key Management" [RFC4107] provides
guidelines for when automated key management is necessary. At the
IETF-70 meeting, and leading up to that meeting, the RADEXT WG
debated whether or not RFC 4107 would require a RADIUS Crypto-Agility
solution to feature Automated Key Management (AKM). The working
group determined that AKM was not inherently required for RADIUS
based on the following points:
o RFC 4107 requires AKM for protocols that involve O(n^2) keys.
This does not apply to RADIUS deployments, which require O(n)
keys.
o Requirements for session key freshness can be met without AKM,
for example, by utilizing a pre-shared key along with an exchange
of nonces.
o RADIUS does not require the encryption of large amounts of data in
a short time.
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o Organizations already have operational practices to manage
existing RADIUS shared secrets to address key changes required
as a result of personnel changes.
o The crypto-agility solution can avoid use cryptographic modes of
operation such as a counter mode cipher that require frequent key
changes.
However, the same time, it is recognized that features recommended in
Section 4.2 such as support for PFS and direct transport of keys
between a NAS and RADIUS server can only be provided by a solution
supporting AKM. As a result, support for Automated Key Management is
RECOMMENDED within a RADIUS crypto-agility solution.
Also, automated key management is REQUIRED for RADIUS crypto-agility
solutions that use cryptographic modes of operation that require
frequent key changes.
5. IANA Considerations
This document makes no request of IANA.
6. Security Considerations
Potential attacks against the RADIUS protocol are described in
[RFC3579] Section 4.1, and details of known exploits as well as
potential mitigations are discussed in [RFC3579] Section 4.3.
This specification describes the requirements for new cryptographic
protection mechanisms, including the modular selection of algorithms
and modes. Therefore, the subject matter of this memo is all about
security.
7. Acknowledgments
Thanks to all the reviewers and contributors, including Bernard
Aboba, Joe Salowey and Glen Zorn.
8. Informative References
[NIST-SP800-131A]
Barker, E. and A. Roginsky, "Transitions: Recommendation for
Transitioning the Use of Cryptographic Algorithms and Key
Lengths", NIST SP-800-131A, January 2011.
[RADYN] Winter, S. and M. McCauley, "NAI-based Dynamic Peer Discovery
for RADIUS over TLS and DTLS", Internet draft (work in
progress), draft-ietf-radext-dynamic-discovery-02, March 2010.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.
and I. Goyret, "RADIUS Attributes for Tunnel Protocol
Support", RFC 2868, June 2000.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC
3162, August 2001.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
In User Service) Support For Extensible Authentication
Protocol (EAP)", RFC 3579, September 2003.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key
Management", BCP 107, RFC 4107, June 2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management", BCP 132,
RFC 4962, July 2007.
[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dialin User Service (RADIUS) Protocol", RFC
5997, August 2010.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for
the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151,
March 2011.
[RFC6158] DeKok, A., "RADIUS Design Guidelines", BCP 158, RFC 6158,
March 2011.
Author's Address
David B. Nelson
Elbrys Networks, Inc.
282 Corporate Drive, Unit 1
Portsmouth, NH 03801
USA
Email: d.b.nelson@comcast.net
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