Internet-Draft R. Housley
Intended Status: Best Current Practice Vigil Security
Expires: 2 November 2015 1 May 2015
Guidelines for Cryptographic Algorithm Agility
<draft-iab-crypto-alg-agility-03.txt>
Abstract
Many IETF protocols use cryptographic algorithms to provide
confidentiality, integrity, authentication or digital signature.
Communicating peers must support a common set of cryptographic
algorithms for these mechanisms to work properly. This memo provides
guidelines to ensure that protocols have the ability to migrate from
one algorithm suite to another over time.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright and License Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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1. Introduction
Many IETF protocols use cryptographic algorithms to provide
confidentiality, integrity, authentication, or digital signature.
For interoperability, communicating peers must support a common set
of cryptographic algorithms. In most cases, a combination of
compatible cryptographic algorithms will be used to provide the
desired security services. The set of cryptographic algorithms being
used at a particular time is often referred to as a cryptographic
algorithm suite or cipher suite.
Cryptographic algorithms age; they become weaker with time. As new
cryptanalysis techniques are developed and computing capabilities
improve, the work factor to break a particular cryptographic
algorithm will reduce or become more feasible for more attackers.
Algorithm agility is achieved when a protocol can easily migrate from
one algorithm suite to another, more desirable one, over time. For
the protocol implementer, this means that implementations should be
modular to easily accommodate the insertion of new algorithms or
suites of algorithms. For the protocol designer, this means that one
or more algorithm identifier must be carried, the set of mandatory-
to-implement algorithms will change over time, and an IANA registry
of algorithm identifiers will be needed. These algorithm identifiers
might name a single cryptographic algorithm or a suite of algorithms.
Algorithm identifiers by themselves are not sufficient to ensure easy
migration. Action by people that maintain implementations and
operate services is needed to develop, deploy, and adjust
configuration settings to enable the new more desirable algorithms
and to deprecate or disable older, less desirable ones. Ideally,
this takes place before the older algorithm or suite of algorithms is
catastrophically weakened. Experience shows that many people are
unwilling to disable older weaker algorithms; it seems that these
people prefer to live with weaker algorithms, sometimes seriously
flawed ones, to maintain interoperability with older software well
after experts recommend migration.
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1.1. Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
2. Algorithm Agility Guidelines
These guidelines are for use by IETF working groups and protocol
authors for IETF protocols that make use of cryptographic algorithms.
2.1. Algorithm Identifiers
IETF protocols that make use of cryptographic algorithms MUST carry
one or more algorithm or suite identifier.
Some approaches carry one identifier for each algorithm that is used.
Other approaches carry one identifier for a suite of algorithms.
Both approaches are used in IETF protocols. Designers are encouraged
to pick one of these approaches and use it consistently throughout
the protocol or family of protocols. Suite identifiers make it
easier for the protocol designer to ensure that the algorithm
selections are complete and compatible for future assignments.
However, suite identifiers inherently face a combinatoric explosion
as new algorithms are defined. Algorithm identifiers, on the other
hand, impose a burden on implementations by forcing a determination
at run-time regarding which algorithm combinations are acceptable.
Regardless of the approach used, protocols historically negotiate the
symmetric cipher and cipher mode together to ensure that they are
completely compatible.
In the IPsec protocol suite, IKE [RFC2409][RFC4306] carries the
algorithm identifiers for AH and ESP [RFC4302][RFC4303]. Such
separation is a completely fine design choice. In contrast, TLS
[RFC5246] carries cipher suite identifiers, which is also a
completely fine design choice.
An IANA registry SHOULD be used for these algorithm or suite
identifiers.
2.2. Mandatory-to-Implement Algorithms
For secure interoperability, BCP 61 [RFC3365] recognizes that
communicating peers that use cryptographic mechanisms must support a
common set of strong cryptographic algorithms. For this reason, the
protocol MUST specify one or more mandatory-to-implement algorithm or
suite. Note that this is not done for protocols that are embedded in
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other protocols, where the system-level protocol specification
identifies the mandatory-to-implement algorithm or suite. For
example, S/MIME [RFC5751] makes use of the cryptographic message
Syntax (CMS) [RFC5652], and S/MIME specifies the mandatory-to-
implement algorithms, not CMS. This approach allows other protocols
can make use of CMS and make different mandatory-to-implement
algorithm choices.
The IETF needs to be able to change the mandatory-to-implement
algorithms over time. It is highly desirable to make this change
without updating the base protocol specification. To achieve this
goal, the base protocol specification includes a reference to a
companion algorithms document, allowing the update of one document
without necessarily requiring an update to the other. This division
also facilitates the advancement of the base protocol specification
on the standards maturity ladder even if the algorithm document
changes frequently.
Some cryptographic algorithms are inherently tied to a specific key
size, but others allow many different key sizes. Likewise, some
algorithms support parameters of different sizes, such as integrity
check values or nonces. The algorithm specification MUST identify
the specific key sizes and parameter sizes that are to be supported.
When more than one key size is available, expect the mandatory-to-
implement key size to increase over time.
Guidance on cryptographic key size for asymmetric keys can be found
in BCP 86 [RFC3766].
Symmetric keys used for protection of long-term values SHOULD be at
least 128 bits.
2.3. Transition from Weak Algorithms
Transition from an old algorithm that is found to be weak can be
tricky. It is of course straightforward to specify the use of a new,
better algorithm. And then, when the new algorithm is widely
deployed, the old algorithm ought no longer be used. However,
knowledge about the implementation and deployment of the new
algorithm will always be imperfect, so one cannot be completely
assured of interoperability with the new algorithm.
To facilitate transition, protocols MUST be able to advertise which
algorithms are supported. This may naturally occur as part of an
algorithm selection or negotiation mechanism, and other times a
mechanism is needed to determine whether the new algorithm has been
deployed. For example, the DNSSEC EDNS0 option [RFC6975] measures
the acceptance and use of new digital signing algorithms.
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In the worst case, the old algorithm may be found to be tragically
flawed, permitting a casual attacker to download a simple script to
break it. Sadly, this has happened when a secure algorithm is used
incorrectly or used with poor key management, resulting in a weak
cryptographic algorithm suite. In such situations, the protection
offered by the algorithm is severely compromised, perhaps to the
point that one wants to stop using the weak suite altogether,
rejecting offers to use the weak suite well before the new suite is
widely deployed.
In any case, there comes a point in time where one refuses to use the
old, weak algorithm or suite. This can happen on a flag day, or each
installation can select a date on their own.
2.4. Balance Security Strength
When selecting a suite of cryptographic algorithms, the strength of
each algorithm SHOULD be considered. It needs to be considered at
the time a protocol is designed. It also needs to be considered at
the time a protocol implementation is deployed and configured.
Advice from from experts is useful, but in reality, it is not often
available to system administrators that are deploying and configuring
a protocol implementation. For this reason, protocol designers
SHOULD provide clear guidance to implementors, leading to balanced
options being available at the time of deployment and configuration.
Cipher suites include Diffie-Hellman or RSA without specifying a
particular public key length. If the algorithm identifier or suite
identifier named a particular public key length, migration to longer
ones would be more difficult. On the other hand, inclusion of a
public key length would make it easier to migrate away from short
ones when computational resources available to attacker dictate the
need to do so. Therefore, flexibility on asymmetric key length is
both desirable and undesirable at the same time.
In CMS [RFC5652], a previously distributed symmetric key-encryption
key can be used to encrypt a content-encryption key, which is in turn
used to encrypt the content. The key-encryption and content-
encryption algorithms are often different. If, for example, a
message content is encrypted with 128-bit AES key and the content-
encryption key is wrapped with a 256-bit AES key, then at most 128
bits of protection is provided. In this situation, the algorithm and
key size selections should ensure that the key encryption is at least
as strong as the content encryption. In general, wrapping one key
with another key of a different size yields the security strength of
the shorter key.
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2.5. Opportunistic Security
Despite the guidance in Section 2.4, opportunistic security [RFC7435]
SHOULD also be considered, especially at the time a protocol
implementation is deployed and configured. While RSA with a 2048-bit
public key is quite a bit stronger than SHA-1, it is quite reasonable
to use them together if the alternative is no authentication
whatsoever. That said, the use of strong algorithms is always
preferable.
3. Algorithm Agility in Protocol Design
Some attempts at algorithm agility have not been completely
successful. This section provides some of the insights based on
protocol designs and deployments.
3.1. Algorithm Identifiers
If a protocol does not carry an algorithm identifier, then the
protocol version number or some other major change is needed to
transition from one algorithm to another. The inclusion of an
algorithm identifier is a minimal step toward cryptographic algorithm
agility. In addition, an IANA registry is needed to pair the
identifier with an algorithm specification.
Sometimes a combination of protocol version number and explicit
algorithm or suite identifiers is appropriate. For example, the TLS
version number names the default key derivation function and the
cipher suite identifier names the rest of the needed algorithms.
Sometimes application layer protocols can make use of transport layer
security protocols, such as TLS or DTLS. This insulates the
application layer protocol from the details of cryptography, but it
is likely to still be necessary to handle the transition from
unprotected traffic to protected traffic in the application layer
protocol. In addition, the application layer protocol may need to
handle the downgrade from encrypted communication to plaintext
communication.
3.2. Migration Mechanisms
Cryptographic algorithm selection or negotiation SHOULD be integrity
protected. If selection is not integrity protected, then the
protocol will be subject to a downgrade attack. Without integrity
protection of algorithm or suite selection, the attempt to transition
to a new algorithm or suite may introduce new opportunities for
downgrade attack.
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If a protocol specifies a single mandatory-to-implement integrity
algorithm, eventually that algorithm will be found to be weak.
Extra care is needed when a mandatory-to-implement algorithm is used
to provide integrity protection for the negotiation of other
cryptographic algorithms. In this situation, a flaw in the
mandatory-to-implement algorithm may allow an attacker to influence
the choices of the other algorithms.
Performance is always a factor is selecting cryptographic algorithms.
In many algorithms, shorter keys offer higher performance, but less
security. Performance and security need to be balanced. Yet, all
algorithms age, and the advances in computing power available to the
attacker will eventually make any algorithm obsolete. For this
reason, protocols need mechanisms to migrate from one algorithm suite
to another over time, including the algorithm used to provide
integrity protection for algorithm negotiation.
3.3. Preserving Interoperability
Cryptographic algorithm deprecation is very hard. People do not like
to introduce interoperability problems, even to preserve security.
As a result, flawed algorithms are supported for far too long. The
impacts of legacy software an long support tails on security can be
reduced by making it easy to develop, deploy, and configure new
algorithms.
3.4. Cryptographic Key Management
Traditionally, protocol designers have avoided more than one approach
to key management because it makes the security analysis of the
overall protocol more difficult. When frameworks such as EAP and
GSSAPI are employed, the key management is very flexible, often
hiding many of the details from the application. This results in
protocols that support multiple key management approaches. In fact,
the key management approach itself may be negotiable, which creates a
design challenge to protect the negotiation of the key management
approach before it is used to produce cryptographic keys.
Protocols can negotiate a key management approach, derive an initial
cryptographic key, and then authenticate the negotiation. However,
if the authentication fails, the only recourse is to start the
negotiation over from the beginning.
Some environments will restrict the key management approaches by
policy. Such policies tend to improve interoperability within a
particular environment, but they cause problems for individuals that
need to work in multiple incompatible environments.
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4. Cryptographic Algorithm Specifications
There are tradeoffs between the number of cryptographic algorithms
that are supported, time to deploy a new algorithm, and protocol
complexity. This section provides some of the insights about the
tradeoff faced by protocol designers.
4.1. Choosing Mandatory-to-Implement Algorithms
It seems like the ability to use an algorithm of one's own choosing
is very desirable; however, the selection is often better left to
experts. Further, any and all cryptographic algorithm choices ought
not be available in every implementation. Mandatory-to-implement
algorithms ought to be well studied, giving rise to significant
confidence. The selected algorithms need to be resistant to side-
channel attacks as well as meeting the performance, power, and code
size requirements on a wide variety of platforms. In addition,
inclusion of too many alternatives may add complexity to algorithm
selection or negotiation.
Sometime more than one mandatory-to-implement algorithm is needed to
increase the likelihood of interoperability among a diverse
population. For example, authenticated encryption is provided by
AES-CCM [RFC3610] and AES-GCM [GCM]. Both of these algorithms are
considered to be secure. AES-CCM is available in hardware used by
many small devices, and AES-GCM is parallelizable and well suited
high-speed devices. Therefore an application needing authenticated
encryption might specify one of these algorithms or both of these
algorithms, depending of the population.
4.2. Too Many Choices Can Be Harmful
It is fairly easy to specify the use of any arbitrary cryptographic
algorithm, and once the specification is available, the algorithm
gets implemented and deployed. Some people say that the freedom to
specify algorithms independently from the rest of the protocol has
lead to the specification of too many cryptographic algorithms. Once
deployed, even with moderate uptake, it is quite difficult to remove
algorithms because interoperability with some party will be impacted.
As a result, weaker ciphers stick around far too long. Sometimes
implementors are forced to maintain cryptographic algorithm
implementations well beyond their useful lifetime.
In order to manage the proliferation of algorithm choices and provide
an expectation of interoperability, many protocols specify mandatory-
to-implement algorithms or suites. All implementors are expected to
support the mandatory-to-implement cryptographic algorithm, and they
can include any others algorithms that they desire. The mandatory-
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to-implement algorithms are chosen to be highly secure and follow the
guidance in RFC 1984 [RFC1984]. Of course, many other factors,
including intellectual property rights, have an impact on the
cryptographic algorithms that are selected by the community.
Generally, the mandatory-to-implement algorithms ought to be
preferred, and the other algorithms ought to be selected only in
special situations. However, it can be very difficult for a skilled
system administrator to determine the proper configuration to achieve
these preferences.
In some cases, more than one mandatory-to-implement cryptographic
algorithm has been specified. This is intended to ensure that at
least one secure cryptographic algorithm will be available, even if
other mandatory-to-implement algorithms are broken. To achieve this
goal, the selected algorithms must be diverse, so that a
cryptoanalytic advance against one of the algorithms does not also
impact the other selected algorithms. The idea is to have an
implemented and deployed algorithm as a fallback. However, all of
the selected algorithms need to be routinely exercised to ensure
quality implementation. This is not always easy to do, especially if
the various selected algorithms require different credentials.
Obtaining multiple credentials for the same installation is an
unacceptable burden on system administrators. Also, the manner by
which system administrators are advised to switch algorithms or
suites is at best ad hoc, and at worst entirely absent.
4.3. Picking One True Cipher Suite Can Be Harmful
After careful study and evaluation, the protocol designer could
select the one true cipher suite. Since algorithms age, such a
decision cannot be stable forever, so a version number is needed to
signal which algorithm that is being used. This has at least two
desirable consequences. First, the protocol is simpler since there
is no need for algorithm negotiation. Second, system administrators
do not need to make any algorithm-related configuration decisions.
However, the only way to respond to news that the an algorithm that
is part of the one true cipher suite has been broken is to update the
protocol specification to the next version, implement the new
specification, and then get it deployed.
The first IEEE 802.11 [WiFi] specification included the Wired
Equivalent Privacy (WEP) as the only encryption technique. WEP was
found to be quite weak [WEP], and a very large effort was needed to
specify, implement, and deploy the alternative encryption techniques.
Experience with the transition from SHA-1 to SHA-256 indicates that
the time from protocol specificate to widespread use takes more than
five years. In this case, the protocol specifications and
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implementation were straightforward and fairly prompt. In many
software products, the new algorithm was not considered an update to
existing release, so the roll out of the next release, subsequent
deployment, and finally adjustment of the configuration by system
administrators took many years. In many consumer hardware products,
firmware to implement the new algorithm were difficult to locate and
install, or the were simply not available. Further, infrastructure
providers were unwilling to make the transition until all of their
potential clients were able to use the new algorithm.
4.4. National Cipher Suites
Some nations specify cryptographic algorithms, and then require their
use through legislation or regulations. These algorithms may not
have wide public review, and they can have limited reach of
deployments. Yet, the legislative or regulatory mandate creates a
captive market. As a result, the use of such algorithms get
specified, implemented, and deployed. The default server-side
configuration SHOULD disable such algorithms; in this way, explicit
action by the system administrator is needed to enable them where
they are actually required.
4.5. Balance Protocol Complexity
Protocol designers MUST be prepared for the supported cryptographic
algorithm set to change over time. As shown by the discussion in the
previous two sections, there is a spectrum of ways to enable the
transition.
Keep implementations as simple as possible. Complex protocol
negotiation provides opportunities for attack, such as downgrade
attacks. Support for many algorithm alternatives is also harmful, as
discussed in Section 4.1. Both of these can lead to portions of the
implementation that are rarely used, increasing the opportunity for
undiscovered exploitable implementation bugs.
5. Security Considerations
This document provides guidance to working groups and protocol
designers. The security of the Internet is improved when broken or
weak cryptographic algorithms can be easily replaced with strong
ones.
From a software development and maintenance perspective,
cryptographic algorithms can often be added and removed without
making changes to surrounding data structures, protocol parsing
routines, or state machines. This approach separates the
cryptographic algorithm implementation from the rest of the code,
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which makes it easier to tackle special security concerns such as key
exposure and constant-time execution.
The situation is different for hardware, for both tiny devices and
very high-end data center equipment. Many tiny devices do not
include the ability to update the firmware at all. Even if the
firmware can be updated, tiny devices are often deployed in places
that make it very inconvenient to do so. High-end data center
equipment may use special-purpose chips to achieve very high
performance, which means that board-level replacement may be needed
to change the algorithm. Cost and down-time are both factors in such
an upgrade.
In most cases, the cryptographic algorithm remains strong, but an
attack is found against the way that the strong algorithm is used in
a particular protocol. In these cases, a protocol change will
probably be needed. For example, the order of cryptographic
operations in the TLS protocol has evolved as various attacks have
been discovered. Originally, TLS performed encryption after
computation of the message authentication code (MAC). This order of
operations is called MAC-then-encrypt, and it is no longer considered
secure [BN][K]. As a result, a mechanism was specified to use
encrypt-then-MAC instead [RFC7366]. Future versions of TLS are
expected to use exclusively authenticated encryption algorithms
[RFC5166], which should resolve the ordering discussion altogether.
After discovery of such attacks, updating the cryptographic
algorithms is not likely to be sufficient to thwart the new attack.
It may necessary to make significant changes to the protocol.
Some protocols are used to protected stored data. For example,
S/MIME [RFC5751] can protect a message kept in a mailbox. To recover
the protected stored data, protocol implementations need to support
older algorithms, even when they no longer use the older algorithms
for the protection of new stored data.
Support for too many algorithms can lead to implementation
vulnerabilities. When many algorithms are supported, some of them
will be rarely used. Any code that is rarely used can contain
undetected bugs, and algorithm implementations are no different.
Section 2.3 talks about algorithm transition without considering any
other aspects of the protocol design. In practice, there are
dependencies between the cryptographic algorithm and other aspects of
the protocol. For example, the BEAST attack [BEAST] against TLS
[RFC5246] caused many sites to turn off modern cryptographic
algorithms in favor of older and clearly weaker algorithms.
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6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public
Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
April 2004.
7. Informative References
[BEAST] http://en.wikipedia.org/wiki/
Transport_Layer_Security#BEAST_attack.
[BN] Bellare, M. and C. Namprempre, "Authenticated Encryption:
Relations among notions and analysis of the generic
composition paradigm", Proceedings of AsiaCrypt '00,
Springer-Verlag LNCS No. 1976, p. 531, December 2000.
[GCM] Dworkin, M, "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC", NIST
Special Publication 800-30D, November 2007.
[K] Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (or: How Secure Is SSL?)",
Proceedings of Crypto '01, Springer-Verlag LNCS No. 2139,
p. 310, August 2001.
[RFC1984] IAB and IESG, "IAB and IESG Statement on Cryptographic
Technology and the Internet", RFC 1984, August 1996.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61, RFC
3365, August 2002.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
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Guidelines for Cryptographic Algorithm Agility May 2015
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5166] Floyd, S., Ed., "Metrics for the Evaluation of Congestion
Control Mechanisms", RFC 5166, March 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic Algorithm
Understanding in DNS Security Extensions (DNSSEC)",
RFC 6975, July 2013.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer Security
(TLS) and Datagram Transport Layer Security (DTLS)",
RFC 7366, September 2014.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most
of the Time", RFC 7435, December 2014.
[WEP] http://en.wikipedia.org/wiki/Wired_Equivalent_Privacy
[WiFi] IEEE , "Wireless LAN Medium Access Control (MAC) And
Physical Layer (PHY) Specifications, IEEE Std 802.11-1997,
1997.
Acknowledgements
Thanks to Bernard Aboba, Derek Atkins, David Black, Randy Bush, Jon
Callas, Andrew Chi, Steve Crocker, Viktor Dukhovni, Stephen Farrell,
Tony Finch, Ian Grigg, Peter Gutmann, Wes Hardaker, Joe Hildebrand,
Christian Huitema, Watson Ladd, Paul Lambert, Ben Laurie, Eliot Lear,
Nikos Mavrogiannopoulos, Yoav Nir, Rich Salz, Kristof Teichel,
Jeffrey Walton, Nico Williams, and Peter Yee for their review and
insightful comments. While some of these people do not agree with
some aspects of this document, the discussion that resulted for their
comments has certainly resulted in a better document.
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Author's Address
Russ Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Housley [Page 14]
