draft-ietf-radext-crypto-agility-requirements-06.txt   rfc6421.txt 
RADEXT Working Group D. Nelson (Editor) Internet Engineering Task Force (IETF) D. Nelson, Ed.
INTERNET-DRAFT Elbrys Networks, Inc. Request for Comments: 6421 Elbrys Networks, Inc.
Category: Informational Category: Informational November 2011
Expires: November 2, 2011 ISSN: 2070-1721
1 May 2011
Crypto-Agility Requirements for Remote Dial-In User Service (RADIUS) Crypto-Agility Requirements
draft-ietf-radext-crypto-agility-requirements-06.txt for Remote Authentication Dial-In User Service (RADIUS)
Abstract Abstract
This memo describes the requirements for a crypto-agility solution This memo describes the requirements for a crypto-agility solution
for Remote Authentication Dial-In User Service (RADIUS). for Remote Authentication Dial-In User Service (RADIUS).
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. published for informational purposes.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction ....................................................2
1.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. General ....................................................2
1.2 Requirements Language . . . . . . . . . . . . . . . . . . . 3 1.2. Requirements Language ......................................3
1.3. The Charge . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3. Publication Process ........................................3
1.4 Publication Process . . . . . . . . . . . . . . . . . . . . 4 2. A Working Definition of Crypto-Agility ..........................4
2. A Working Definition of Crypto-Agility . . . . . . . . . . . . 4 3. The Current State of RADIUS Security ............................5
3. The Current State of RADIUS Security . . . . . . . . . . . . . 5 4. The Requirements ................................................5
4. The Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Overall Solution Approach ..................................5
4.1. Overall Solution Approach . . . . . . . . . . . . . . . . . 6 4.2. Security Services ..........................................6
4.2. Security Services . . . . . . . . . . . . . . . . . . . . . 6 4.3. Backwards Compatibility ....................................7
4.3. Backwards Compatibility . . . . . . . . . . . . . . . . . . 8 4.4. Interoperability and Change Control ........................9
4.4. Interoperability and Change Control . . . . . . . . . . . . 9 4.5. Scope of Work ..............................................9
4.5. Scope of Work . . . . . . . . . . . . . . . . . . . . . . . 9 4.6. Applicability of Automated Key Management Requirements .....9
4.6. Applicability of Automated Key Management Requirements . . 9 5. Security Considerations ........................................10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 6. Acknowledgments ................................................10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7. References .....................................................10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References ......................................10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References ....................................11
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
1.1. General 1.1. General
This memo describes the requirements for a crypto-agility solution At the IETF 66 meeting, the RADIUS Extensions (RADEXT) Working Group
for Remote Authentication Dial-In User Service (RADIUS). This memo, (WG) was asked by members of the Security Area Directorate to prepare
when approved, reflects the consensus of the RADIUS Extensions a formal description of a crypto-agility work item and corresponding
(RADEXT) Working Group of the IETF as to the features, properties and charter milestones. After consultation with one of the Security Area
limitations of the crypto-agility work item for RADIUS. It also Directors (Russ Housley), text was initially proposed on the RADEXT
defines the term "crypto-agility" as used in this context, and WG mailing list on October 26, 2006. The following summarizes that
provides the motivations for undertaking and completing this work. proposal:
The requirements defined in this memo have been developed based on e- The RADEXT WG will review the security requirements for crypto-
mail messages posted to the RADEXT WG mailing list, which may be 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].
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.
This document describes the features, properties, and limitations of
RADIUS crypto-agility solutions; defines the term "crypto-agility" as
used in this context; and provides the motivations for this work.
The requirements defined in this memo have been developed based on
email messages posted to the RADEXT WG mailing list, which may be
found in the archives of that list. The purpose of framing the found in the archives of that list. The purpose of framing the
requirements in this memo is to formalize and memorialize them for requirements in this memo is to formalize and archive them for future
future reference, and to bring them explicitly to the attention of reference and to bring them explicitly to the attention of the IESG
the IESG and the IETF Community, as we proceed with this work. and the IETF community as we proceed with this work.
1.2. Requirements Language 1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
A RADIUS crypto-agility solution is not compliant with this A RADIUS crypto-agility solution is not compliant with this
specification if it fails to satisfy one or more of the MUST or MUST 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, NOT statements. A solution that satisfies all the MUST, MUST NOT,
SHOULD, and SHOULD NOT statements is said to be "unconditionally SHOULD, and SHOULD NOT statements is said to be "unconditionally
compliant"; one that satisfies all the MUST and MUST NOT statements compliant"; one that satisfies all the MUST and MUST NOT statements
but not all the SHOULD or SHOULD NOT requirements is said to be but not all the SHOULD or SHOULD NOT requirements is said to be
"conditionally compliant". "conditionally compliant".
1.3. The Charge 1.3. Publication Process
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].
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 RADIUS [RFC2865] is a widely deployed protocol that has attained
Draft Standard status based on multiple independent interoperable Draft Standard status based on multiple independent interoperable
implementations. Therefore it is desirable that a high level of implementations. Therefore, it is desirable that a high level of
interoperability be maintained for crypto-agility solutions. interoperability be maintained for crypto-agility solutions.
To ensure that crypto-agility solutions published on the standards To ensure that crypto-agility solutions published on the standards
track are well specified and interoperable, the RADEXT WG has adopted track are well specified and interoperable, the RADEXT WG has adopted
a two phase process for standards-track publication of crypto-agility a two phase process for standards-track publication of crypto-agility
solutions. solutions.
In the initial phase, crypto-agility solutions adopted by the working In the initial phase, crypto-agility solutions adopted by the working
group will be published as Experimental. These documents should group will be published as Experimental. These documents should
contain a description of the implementations and experimental contain a description of the implementations and experimental
deployments in progress, as well as an evaluation of the proposal deployments in progress as well as an evaluation of the proposal
against the requirements described in this document. against the requirements described in this document.
The working group will then select proposals to advance on the The working group will then select proposals to advance on the
standards track. Criteria to be used include evaluation of the standards track. Criteria to be used include evaluation of the
proposal against the requirements, summary of the experimental proposal against the requirements, summary of the experimental
deployment experience and evidence of multiple interoperable deployment experience, and evidence of multiple interoperable
implementations. implementations.
2. A Working Definition of Crypto-Agility 2. A Working Definition of Crypto-Agility
A generalized definition of crypto-agility was offered up at the Crypto-agility is the ability of a protocol to adapt to evolving
RADEXT WG session during IETF-68. Crypto-Agility is the ability of a cryptography and security requirements. This may include the
protocol to adapt to evolving cryptography and security requirements. provision of a modular mechanism to allow cryptographic algorithms to
This may include the provision of a modular mechanism to allow be updated without substantial disruption to fielded implementations.
cryptographic algorithms to be updated without substantial disruption It may provide for the dynamic negotiation and installation of
to fielded implementations. It may provide for the dynamic cryptographic algorithms within protocol implementations (think of
negotiation and installation of cryptographic algorithms within Dynamic-Link Libraries (DLL)).
protocol implementations (think of Dynamic-Link Libraries (DLL)).
In the specific context of the RADIUS protocol and RADIUS In the specific context of the RADIUS protocol and RADIUS
implementations, crypto-agility may be better defined as the ability implementations, crypto-agility may be better defined as the ability
of RADIUS implementations to automatically negotiate cryptographic of RADIUS implementations to automatically negotiate cryptographic
algorithms for use in RADIUS exchanges, including the algorithms used algorithms for use in RADIUS exchanges, including the algorithms used
to integrity protect and authenticate RADIUS packets and to hide to integrity protect and authenticate RADIUS packets and to hide
RADIUS Attributes. This capability covers all RADIUS message types: RADIUS attributes. This capability covers all RADIUS message types:
Access-Request/Response, Accounting-Request/Response, CoA/Disconnect- Access-Request/Response, Accounting-Request/Response, CoA/Disconnect-
Request/Response, and Status-Server. Negotiation of cryptographic Request/Response, and Status-Server. Negotiation of cryptographic
algorithms MAY occur within the RADIUS protocol, or within a lower algorithms MAY occur within the RADIUS protocol, or within a lower
layer such as the transport layer. layer such as the transport layer.
Since RADIUS is a request/response protocol, the ability to negotiate Proposals MUST NOT introduce generic new capability negotiation
cryptographic algorithms within a single RADIUS exchange is features into the RADIUS protocol or require changes to the RADIUS
inherently limited. While a RADIUS request can provide a list of operational model as defined in "RADIUS Design Guidelines" [RFC6158],
supported cryptographic algorithms which can selected for use within Section 3.1 and Appendix A.4. A proposal SHOULD focus on the crypto-
a response, prior to the receipt of a response, the cryptographic agility problem and nothing else. For example, proposals SHOULD NOT
algorithms utilized to provide security services within the request require new attribute formats and SHOULD be compatible with the
will need to be determined a-priori. guidance provided in [RFC6158], Section 2.3. Issues of backward
compatibility are described in more detail in Section 4.3.
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 3. The Current State of RADIUS Security
RADIUS packets, as defined in [RFC2865], are protected by an MD5 RADIUS packets, as defined in [RFC2865], are protected by an MD5
message integrity check (MIC), within the Authenticator field of message integrity check (MIC) within the Authenticator field of
RADIUS packets other than Access-Request [RFC2865] and Status-Server RADIUS packets other than Access-Request [RFC2865] and Status-Server
[RFC5997]. The Message-Authenticator Attribute utilizes HMAC-MD5 to [RFC5997]. The Message-Authenticator Attribute utilizes HMAC-MD5 to
authenticate and integrity protect RADIUS packets. authenticate and integrity protect RADIUS packets.
While RADIUS does not support confidentiality of entire packets, While RADIUS does not support confidentiality of entire packets,
various RADIUS attributes support encrypted (also known as "hidden") various RADIUS attributes support encrypted (also known as "hidden")
values, including: User-Password (defined in [RFC2865] Section 5.2), values, including User-Password (defined in [RFC2865], Section 5.2),
Tunnel-Password (defined in [RFC2868] Section 3.5), and various Tunnel-Password (defined in [RFC2868], Section 3.5), and various
Vendor-Specific Attributes, such as the MS-MPPE-Send-Key and MS-MPPE- Vendor-Specific Attributes, such as the MS-MPPE-Send-Key and
Recv-Key attributes (defined in [RFC2548] Section 2.4). Generally MS-MPPE-Recv-Key attributes (defined in [RFC2548], Section 2.4).
speaking, the hiding mechanism uses a stream cipher based on a key Generally speaking, the hiding mechanism uses a stream cipher based
stream from an MD5 digest. Attacks against this mechanism are on a key stream from an MD5 digest. Attacks against this mechanism
described in "RADIUS Support for EAP" [RFC3579] Section 4.3.4. are described in "RADIUS Support for EAP" [RFC3579], Section 4.3.4.
"Updated Security Considerations for the MD5 Message-Digest and the "Updated Security Considerations for the MD5 Message-Digest and the
HMAC-MD5 Algorithms" [RFC6151] discusses security considerations for HMAC-MD5 Algorithms" [RFC6151] discusses security considerations for
use of the MD5 and HMAC-MD5 algorithms. While the advances in MD5 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 collisions do not immediately compromise the use of MD5 or HMAC-MD5
for the purposes used within RADIUS absent knowledge of the RADIUS for the purposes used within RADIUS absent knowledge of the
shared secret, the progress toward compromise of MD5's basic RADIUS shared secret, the progress toward compromise of MD5's basic
cryptographic assumptions has resulted in the deprecation of MD5 cryptographic assumptions has resulted in the deprecation of MD5
usage in a variety of applications. As noted in [RFC6151] Section 2: usage in a variety of applications. As noted in [RFC6151],
Section 2:
MD5 is no longer acceptable where collision resistance is required MD5 is no longer acceptable where collision resistance is required
such as digital signatures. It is not urgent to stop using MD5 in 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 other ways, such as HMAC-MD5; however, since MD5 must not be used
for digital signatures, new protocol designs should not employ for digital signatures, new protocol designs should not employ
HMAC-MD5. HMAC-MD5.
4. The Requirements 4. The Requirements
4.1. Overall Solution Approach 4.1. Overall Solution Approach
RADIUS crypto-agility solutions are not restricted to utilizing RADIUS crypto-agility solutions are not restricted to utilizing
technology described in existing RFCs. Since RADIUS over IPsec is technology described in existing RFCs. Since RADIUS over IPsec is
already described in "RADIUS and IPv6" [RFC3162] Section 5 and already described in Section 5 of "RADIUS and IPv6" [RFC3162] and
[RFC3579] Section 4.2, this technique is already available to those Section 4.2 of [RFC3579], this technique is already available to
who wish to use it. Therefore, it is expected that proposals will those who wish to use it. Therefore, it is expected that proposals
utilize other techniques. will utilize other techniques.
4.2. Security Services 4.2. Security Services
Proposals MUST support the negotiation of cryptographic algorithms Proposals MUST support the negotiation of cryptographic algorithms
for per-packet integrity/authentication protection. Proposals also for per-packet integrity/authentication protection. Proposals also
MUST support per-packet replay protection for all RADIUS message MUST support per-packet replay protection for all RADIUS message
types. Crypto-agility solutions MUST specify mandatory-to-implement types. Crypto-agility solutions MUST specify mandatory-to-implement
cryptographic algorithms for each defined mechanism. cryptographic algorithms for each defined mechanism.
Crypto-agility solutions MUST avoid security compromise, even in Crypto-agility solutions MUST avoid security compromise, even in
situations where the existing cryptographic algorithms utilized by situations where the existing cryptographic algorithms utilized by
RADIUS implementations are shown to be weak enough to provide little RADIUS implementations are shown to be weak enough to provide little
or no security (e.g. in event of compromise of the legacy RADIUS or no security (e.g., in the event of compromise of the legacy RADIUS
shared secret). Included in this would be protection against bidding shared secret). Included in this would be protection against
down attacks. In analyzing the resilience of a crypto-agility bidding-down attacks. In analyzing the resilience of a crypto-
solution, it can be assumed that RADIUS requesters and responders can agility solution, it can be assumed that RADIUS requesters and
be configured to require the use of new secure algorithms in the responders can be configured to require the use of new secure
event of a compromise of existing cryptographic algorithms or the algorithms in the event of a compromise of existing cryptographic
legacy RADIUS shared secret. algorithms or the legacy RADIUS shared secret.
Guidance on acceptable algorithms can be found in [NIST-SP800-131A]; Guidance on acceptable algorithms can be found in [NIST-SP800-131A].
it is RECOMMENDED that mandatory-to-implement cryptographic It is RECOMMENDED that mandatory-to-implement cryptographic
algorithms be chosen from among those classified as "Acceptable" with algorithms be chosen from among those classified as "Acceptable" with
no known deprecation date. no known deprecation date from within this or successor documents.
It is RECOMMENDED that solutions provide support for confidentiality, It is RECOMMENDED that solutions provide support for confidentiality,
either by supporting encryption of entire RADIUS packets or by either by supporting encryption of entire RADIUS packets or by
encrypting individual RADIUS attributes. Proposals supporting encrypting individual RADIUS attributes. Proposals supporting
confidentiality MUST support the negotiation of cryptographic confidentiality MUST support the negotiation of cryptographic
algorithms for encryption. algorithms for encryption.
Solutions providing for encryption of entire RADIUS packets need not Support for encryption of individual RADIUS attributes is OPTIONAL
also provide support for encryption of individual RADIUS attributes. for solutions that provide encryption of entire RADIUS packets.
Solutions providing for encryption of individual RADIUS attributes Solutions providing for encryption of individual RADIUS attributes
are REQUIRED to provide support for improving the confidentiality of are REQUIRED to provide support for improving the confidentiality of
existing encrypted (sometimes referred to as "hidden") attributes as existing encrypted (sometimes referred to as "hidden") attributes as
well as encrypting attributes (such as location attributes) that are well as encrypting attributes (such as location attributes) that are
currently transmitted in cleartext. currently transmitted in cleartext.
In addition to the goals referred to above, [RFC4962] Section 2 In addition to the goals referred to above, [RFC4962] Section 3
describes additional security requirements, which translate into the describes additional security requirements, which translate into the
following requirements for RADIUS crypto-agility solutions: following requirements for RADIUS crypto-agility solutions:
Strong, fresh session keys 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 RADIUS crypto-agility solutions are REQUIRED to generate fresh
In order to enable a NAS and RADIUS server to exchange confidential session keys for use between the RADIUS client and server. In
information such as keying material without disclosure to third order to prevent the disclosure of one session key from aiding an
parties, it is RECOMMENDED that a RADIUS crypto-agility solution attacker in discovering other session keys, RADIUS crypto-agility
support X.509 certificates for authentication between the NAS and solutions are RECOMMENDED to support Perfect Forward Secrecy (PFS)
RADIUS server. Manual configuration as well as automated discovery with respect to session keys negotiated between the RADIUS client
mechanisms such as NAI-based Dynamic Peer Discovery [RADYN] can be and server.
used to enable direct NAS-RADIUS server communications. Support
for end-to-end confidentiality of RADIUS attributes is not
required.
For compatibility with existing operations, RADIUS crypto-agility Limit key scope:
solutions SHOULD also support pre-shared key credentials. However,
support for direct communications between the NAS and RADIUS server In order to enable a Network Access Server (NAS) and RADIUS server
is not required when pre-shared key credentials are used. to exchange confidential information such as keying material
without disclosure to third parties, it is RECOMMENDED that a
RADIUS crypto-agility solution support X.509 certificates for
authentication between the NAS and RADIUS server. Manual
configuration or automated discovery mechanisms such as NAI-based
Dynamic Peer Discovery [RADYN] can be used to enable
direct NAS-RADIUS server communications. Support for end-to-end
confidentiality of RADIUS attributes is OPTIONAL.
For compatibility with existing operations, RADIUS crypto-agility
solutions SHOULD also support pre-shared key credentials.
However, support for direct communications between the NAS and
RADIUS server is OPTIONAL when pre-shared key credentials are
used.
4.3. Backwards Compatibility 4.3. Backwards Compatibility
Solutions MUST demonstrate backward compatibility with existing Solutions MUST demonstrate backward compatibility with existing
RADIUS implementations. That is, an implementation that supports RADIUS implementations. That is, an implementation that supports
both crypto-agility and legacy mechanisms MUST be able to talk with both crypto-agility and legacy mechanisms MUST be able to talk with
legacy RADIUS clients and servers (using the legacy mechanisms). legacy RADIUS clients and servers (using the legacy mechanisms).
While backward compatibility is needed to ease the transition between While backward compatibility is needed to ease the transition between
legacy RADIUS and crypto-agile RADIUS, use of legacy mechanisms is legacy RADIUS and crypto-agile RADIUS, use of legacy mechanisms is
only appropriate prior to the compromise of those mechanisms. After only appropriate prior to the compromise of those mechanisms. After
legacy mechanisms have been compromised, secure algorithms MUST be legacy mechanisms have been compromised, secure algorithms MUST be
used, so that backward compatibility is no longer possible. used so that backward compatibility is no longer possible.
Since RADIUS is a request/response protocol, the ability to negotiate
cryptographic algorithms within a single RADIUS exchange is
inherently limited. Prior to receipt of a response, a requester will
not know what algorithms are supported by the responder. Therefore,
while a RADIUS request can provide a list of supported cryptographic
algorithms that can be selected for use within a response, prior to
the receipt of a response, the cryptographic algorithms utilized to
provide security services within an initial request will need to be
predetermined.
In order to enable a request to be handled both by legacy as well as 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 crypto-agile implementations, a request can be secured with legacy
algorithms and in addition attributes providing security services algorithms was well as with attributes providing security services
using more secure algorithms can be included. This approach allows a using more secure algorithms. This approach allows a RADIUS packet
RADIUS packet to be processed by legacy implementations as well as by to be processed by legacy implementations as well as by crypto-agile
crypto-agile implementations, and does not result in additional implementations, and it does not result in additional response
response delays. delays. If this technique is used, credentials used with legacy
algorithms MUST be cryptographically independent of the credentials
used with the more secure algorithms, so that compromise of the
legacy credentials does not result in compromise of the credentials
used with more secure algorithms.
In this approach to backward compatibility, legacy mechanisms are In this approach to backward compatibility, legacy mechanisms are
initially used between crypto-agile implementations. However, if the initially used in requests sent between crypto-agile implementations.
responder indicates support for crypto-agility, future requests can However, if the responder indicates support for crypto-agility,
omit use of legacy mechanisms. future requests can use more secure mechanisms. Note that if a
responder is upgraded and then subsequently needs to be downgraded
(e.g., due to bugs), this could result in requesters being unable to
communicate with the downgraded responder unless a mechanism is
provided to configure the requester to re-enable use of legacy
algorithms.
Probing techniques can be used avoid the use of legacy algorithms Probing techniques can be used to avoid the use of legacy algorithms
between crypto-agile implementations. An initial request can omit in requests sent between crypto-agile implementations. For example,
use of legacy mechanisms, and if a response is received, then the an initial request can omit use of legacy mechanisms. If a response
recipient can be assumed to be crypto-agile and future requests to is received, then the recipient can be assumed to be crypto-agile and
that recipient can utilize secure mechanisms. Similarly, the future requests to that recipient can utilize secure mechanisms.
responder can assume that the requester supports crypto-agility and Similarly, the responder can assume that the requester supports
can prohibit use of legacy mechanisms in future requests. crypto-agility and can prohibit use of legacy mechanisms in future
requests. Note that if a requester is upgraded and then subsequently
needs to be downgraded (e.g., due to bugs), this could result in the
requester being unable to interpret responses, unless a mechanism is
provided to configure the responder to re-enable use of legacy
algorithms.
If a response is not received, in the absence of information If a response is not received, in the absence of information
indicating responder support for crypto-agility (such as pre- indicating responder support for crypto-agility (such as pre-
configuration or previous receipt of a crypto-agile response), a new configuration or previous receipt of a crypto-agile response), a new
request can be composed utilizing legacy mechanisms. request can be composed utilizing legacy mechanisms.
Since legacy implementations not supporting crypto-agility will Since legacy implementations not supporting crypto-agility will
silently discard requests not protected by legacy algorithms rather silently discard requests not protected by legacy algorithms rather
than returning an error, repeated requests may be required to than returning an error, repeated requests can be required to
distinguish lack of support for crypto-agility from packet loss or distinguish lack of support for crypto-agility from packet loss or
other failure conditions. As a result, probing techniques can delay other failure conditions. Therefore, probing techniques can delay
initial communication between crypto-agile requesters and legacy initial communication between crypto-agile requesters and legacy
responders. This can be addressed by upgrading the responders (e.g. responders. This can be addressed by upgrading the responders (e.g.,
RADIUS servers) first. RADIUS servers) first.
4.4. Interoperability and Change Control 4.4. Interoperability and Change Control
Proposals MUST indicate a willingness to cede change control to the Proposals MUST indicate a willingness to cede change control to the
IETF. IETF.
Crypto-agility solutions MUST be interoperable between independent Crypto-agility solutions MUST be interoperable between independent
implementations based purely on the information provided in the implementations based purely on the information provided in the
specification. specification.
4.5. Scope of Work 4.5. Scope of Work
Crypto-agility solutions MUST apply to all RADIUS packet types, Crypto-agility solutions MUST apply to all RADIUS packet types,
including Access-Request, Access-Challenge, Access-Reject, Access- including Access-Request, Access-Challenge, Access-Reject,
Accept, Accounting-Request, Accounting-Response, Status-Server and Access-Accept, Accounting-Request, Accounting-Response, Status-Server
CoA/Disconnect messages. and CoA/Disconnect messages.
Since it is expected that the work will occur purely within RADIUS or Since it is expected that the work will occur purely within RADIUS or
in the transport, message data exchanged with Diameter SHOULD NOT be in the transport, message data exchanged with Diameter SHOULD NOT be
affected. affected.
Proposals MUST discuss any inherent assumptions about, or limitations Proposals MUST discuss any inherent assumptions about, or limitations
on, client/server operations or deployment and SHOULD provide on, client/server operations or deployment and SHOULD provide
recommendations for transition of deployments from legacy RADIUS to recommendations for transition of deployments from legacy RADIUS to
crypto-agile RADIUS. Issues regarding cipher-suite negotiation, crypto-agile RADIUS. Issues regarding cipher-suite negotiation,
legacy interoperability and the potential for bidding down attacks, legacy interoperability, and the potential for bidding-down attacks
SHOULD be among these discussions. SHOULD be among these discussions.
4.6. Applicability of Automated Key Management Requirements 4.6. Applicability of Automated Key Management Requirements
"Guidelines for Cryptographic Key Management" [RFC4107] provides "Guidelines for Cryptographic Key Management" [RFC4107] provides
guidelines for when automated key management is necessary. At the guidelines for when automated key management is necessary.
IETF-70 meeting, and leading up to that meeting, the RADEXT WG Consideration was given as to whether or not RFC 4107 would require a
debated whether or not RFC 4107 would require a RADIUS Crypto-Agility RADIUS crypto-agility solution to feature Automated Key Management
solution to feature Automated Key Management (AKM). The working (AKM). It was determined that AKM was not inherently required for
group determined that AKM was not inherently required for RADIUS RADIUS based on the following points:
based on the following points:
o RFC 4107 requires AKM for protocols that involve O(n^2) keys. o RFC 4107 requires AKM for protocols that involve O(n^2) keys.
This does not apply to RADIUS deployments, which require O(n) This does not apply to RADIUS deployments, which require O(n)
keys. keys.
o Requirements for session key freshness can be met without AKM, o Requirements for session key freshness can be met without AKM, for
for example, by utilizing a pre-shared key along with an exchange example, by utilizing a pre-shared key along with an exchange of
of nonces. nonces.
o RADIUS does not require the encryption of large amounts of data in o RADIUS does not require the encryption of large amounts of data in
a short time. a short time.
o Organizations already have operational practices to manage o Organizations already have operational practices to manage
existing RADIUS shared secrets to address key changes required existing RADIUS shared secrets to address key changes required as
as a result of personnel changes. a result of personnel changes.
o The crypto-agility solution can avoid use cryptographic modes of o The crypto-agility solution can avoid the use of cryptographic
operation such as a counter mode cipher that require frequent key modes of operation, such as a counter mode cipher, that require
changes. frequent key changes.
However, the same time, it is recognized that features recommended in However, at the same time, it is recognized that features recommended
Section 4.2 such as support for perfect forward secrecy and direct in Section 4.2 such as support for perfect forward secrecy and direct
transport of keys between a NAS and RADIUS server can only be transport of keys between a NAS and RADIUS server can only be
provided by a solution supporting AKM. As a result, support for provided by a solution supporting AKM. As a result, support for
Automated Key Management is RECOMMENDED within a RADIUS crypto- Automated Key Management is RECOMMENDED within a RADIUS crypto-
agility solution. agility solution.
Also, automated key management is REQUIRED for RADIUS crypto-agility Also, automated key management is REQUIRED for RADIUS crypto-agility
solutions that use cryptographic modes of operation that require solutions that use cryptographic modes of operation that require
frequent key changes. frequent key changes.
5. IANA Considerations 5. Security Considerations
This document makes no request of IANA.
6. Security Considerations
Potential attacks against the RADIUS protocol are described in Potential attacks against the RADIUS protocol are described in
[RFC3579] Section 4.1, and details of known exploits as well as [RFC3579], Section 4.1, and details of known exploits as well as
potential mitigations are discussed in [RFC3579] Section 4.3. potential mitigations are discussed in [RFC3579], Section 4.3.
This specification describes the requirements for new cryptographic This specification describes the requirements for new cryptographic
protection mechanisms, including the modular selection of algorithms protection mechanisms, including the modular selection of algorithms
and modes. Therefore, the subject matter of this memo is all about and modes. Therefore, all the subject matter of this memo is related
security. to security.
7. Acknowledgments 6. Acknowledgments
Thanks to all the reviewers and contributors, including Bernard Thanks to all the reviewers and contributors, including Bernard
Aboba, Pasi Eronen, Joe Salowey and Glen Zorn. Aboba, Mary Barnes, Pasi Eronen, Dan Romascanu, Joe Salowey, and Glen
Zorn.
8. References 7. References
8.1. Normative References 7.1. Normative References
[NIST-SP800-131A] [NIST-SP800-131A]
Barker, E. and A. Roginsky, "Transitions: Recommendation for Barker, E. and A. Roginsky, "Transitions: Recommendation
Transitioning the Use of Cryptographic Algorithms and Key for Transitioning the Use of Cryptographic Algorithms and
Lengths", NIST SP-800-131A, January 2011. Key Lengths", NIST SP-800-131A, January 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
Authentication Dial In User Service (RADIUS)", RFC 2865, June "Remote Authentication Dial In User Service (RADIUS)", RFC
2000. 2865, June 2000.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Management", BCP 107, RFC 4107, June 2005. Key Management", BCP 107, RFC 4107, June 2005.
[RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication, [RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication,
Authorization, and Accounting (AAA) Key Management", BCP 132, Authorization, and Accounting (AAA) Key Management", BCP
RFC 4962, July 2007. 132, RFC 4962, July 2007.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
March 2011. RFC 6151, March 2011.
[RFC6158] DeKok, A., "RADIUS Design Guidelines", BCP 158, RFC 6158, [RFC6158] DeKok, A., Ed., and G. Weber, "RADIUS Design Guidelines",
March 2011. BCP 158, RFC 6158, March 2011.
8.2. Informative References 7.2. Informative References
[RADYN] Winter, S. and M. McCauley, "NAI-based Dynamic Peer Discovery [RADYN] Winter, S. and M. McCauley, "NAI-based Dynamic Peer
for RADIUS over TLS and DTLS", Internet draft (work in Discovery for RADIUS/TLS and RADIUS/DTLS", Work in
progress), draft-ietf-radext-dynamic-discovery-02, March 2010. Progress, July 2011.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
2548, March 1999. RFC 2548, March 1999.
[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. [RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege,
and I. Goyret, "RADIUS Attributes for Tunnel Protocol M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol
Support", RFC 2868, June 2000. Support", RFC 2868, June 2000.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC [RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC
3162, August 2001. 3162, August 2001.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial [RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
In User Service) Support For Extensible Authentication Dial In User Service) Support For Extensible
Protocol (EAP)", RFC 3579, September 2003. Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote [RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dialin User Service (RADIUS) Protocol", RFC Authentication Dial In User Service (RADIUS) Protocol",
5997, August 2010. RFC 5997, August 2010.
Author's Address Author's Address
David B. Nelson David B. Nelson (editor)
Elbrys Networks, Inc. Elbrys Networks, Inc.
282 Corporate Drive, Unit 1 282 Corporate Drive, Unit 1
Portsmouth, NH 03801 Portsmouth, NH 03801
USA USA
Email: d.b.nelson@comcast.net EMail: d.b.nelson@comcast.net
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