wdiff rfc9416v1.txt rfc9416.txt

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 9416 SI6 Networks
BCP: 72 I. Arce
Updates: 3552 Quarkslab
Category: Best Current Practice May July 2023
ISSN: 2070-1721

Security Considerations for Transient Numeric Identifiers Employed in
Network Protocols

Abstract

Poor selection of transient numerical identifiers in protocols such
as the TCP/IP suite has historically led to a number of attacks on
implementations, ranging from Denial of Service (DoS) or data
injection to information leakages that can be exploited by pervasive
monitoring. Due diligence in the specification of transient numeric
identifiers is required even when cryptographic techniques are
employed, since these techniques might not mitigate all the
associated issues. This document formally updates RFC 3552,
incorporating requirements for transient numeric identifiers, to
prevent flaws in future protocols and implementations.

Status of This Memo

This memo documents an Internet Best Current Practice.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9416.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Issues with the Specification of Transient Numeric Identifiers
4. Common Flaws in the Generation of Transient Numeric Identifiers
5. Requirements for Transient Numeric Identifiers
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

Network

Networking protocols employ a variety of transient numeric
identifiers for different protocol entities, ranging from DNS Transaction IDs
(TxIDs) to transport-protocol numbers (e.g., TCP ports) or objects, such as IPv4 and IPv6
Identification values [RFC0791] [RFC8200], IPv6 Interface Identifiers (IIDs).
(IIDs) [RFC4291], transport-protocol ephemeral port numbers
[RFC6056], TCP Initial Sequence Numbers (ISNs) [RFC9293], NTP
Reference IDs (REFIDs) [RFC5905], and DNS IDs [RFC1035]. These
identifiers usually typically have specific properties requirements (e.g., uniqueness
during a specified period of time) that must be satisfied such that
they do not result in negative interoperability implications (e.g., uniqueness
during a specified period of time) implications, and an
associated failure severity when such properties requirements are not met.

The TCP/IP protocol suite alone has met
[RFC9415].

| NOTE: Some documents refer to the DNS ID as the DNS "Query ID"
| or "TxID".

For more than 30 years, a large number of implementations of IETF
protocols have been subject to a variety of
attacks on its transient numeric identifiers over the past 30 years
or more, attacks, with effects
ranging from Denial of Service (DoS) or data injection to information
leakages that could be exploited for pervasive monitoring [RFC7258].
The root cause of these issues has been, in many cases, the poor
selection of transient numeric identifiers in such protocols, usually
as a result of insufficient or misleading specifications. While it
is generally trivial to identify an algorithm that can satisfy the
interoperability requirements for of a given transient numeric
identifier,
there exists practical empirical evidence [RFC9415] exists that doing so without
negatively affecting the security and/or privacy properties of the
aforementioned protocols is prone to error. error [RFC9414].

For example, implementations have been subject to security and/or
privacy issues resulting from:

* predictable TCP sequence numbers (e.g., see [Morris1985],
[Bellovin1989], and [RFC6528]),

* predictable transport-protocol numbers (e.g., see [Silbersack2005]
and [RFC6056]),

* predictable IPv4 or IPv6 Fragment Identifiers Identification values (e.g., see
[Sanfilippo1998a], [RFC6274], and [RFC7739]),

* predictable IPv6 IIDs (e.g., see [RFC7217], [RFC7707], and
[RFC7721]),

* predictable transport-protocol ephemeral port numbers (e.g., see
[RFC6056] and [Silbersack2005]),

* predictable DNS TxIDs TCP Initial Sequence Numbers (ISNs) (e.g., see
[Morris1985], [Bellovin1989], and [RFC6528]),

* predictable initial timestamps in TCP timestamps options (e.g.,
see [TCPT-uptime] and [RFC7323]), and

* predictable DNS IDs (see, e.g., [Schuba1993] and [Klein2007]).

Recent history indicates that, when new protocols are standardized or
new protocol implementations are produced, the security and privacy
properties of the associated transient numeric identifiers tend to be
overlooked, and inappropriate algorithms to generate such identifiers
are either suggested in the specification specifications or selected by
implementers. As a result, advice in this area is warranted.

We note that the use of cryptographic techniques for confidentiality
and authentication might not eliminate all the issues associated with
predictable transient numeric identifiers. Therefore, due diligence
in the specification of transient numeric identifiers is required
even when cryptographic techniques are employed.

Note:

| NOTE: For example, cryptographic authentication can readily
| mitigate data injection attacks even in the presence of
| predictable transient numeric identifiers (such as "sequence
| numbers"). However, use of flawed algorithms (such as global
| counters) for generating transient numeric identifiers could
| still result in information leakages even when cryptographic
| techniques are employed. These information leakages could in
| turn be leveraged to perform other devastating attacks (please
| see [RFC9414] [RFC9415] for further details).

Section 3 provides an overview of common flaws in the specification
of transient numeric identifiers. Section 4 provides an overview of
common flaws in the implications generation of predictable transient numeric identifiers. identifiers and
their associated security and privacy implications. Finally,
Section 5 provides key guidelines for protocol designers.

2. Terminology

Transient Numeric Identifier:
A data object in a protocol specification that can be used to
definitely distinguish a protocol object (a datagram, network
interface, transport-protocol endpoint, session, etc.) from all
other objects of the same type, in a given context. Transient
numeric identifiers are usually defined as a series of bits and
represented using integer values. These identifiers are typically
dynamically selected, as opposed to statically assigned numeric
identifiers (e.g., see [IANA-PROT]). We note that different
transient numeric identifiers may have additional requirements or
properties depending on their specific use in a protocol. We use
the term "transient numeric identifier" (or simply "numeric
identifier" or "identifier" as short forms) as a generic term to
refer to any data object in a protocol specification that
satisfies the identification property stated above.

Failure Severity:
The interoperability consequences of a failure to comply with the
interoperability requirements of a given identifier. Severity
considers the worst potential consequence of a failure, determined
by the system damage and/or time lost to repair the failure. In
this document, we define two types of failure severity: "soft" and
"hard".

Soft Failure:
A recoverable condition in which a protocol does not operate in
the prescribed manner but normal operation can be resumed
automatically in a short period of time. For example, a simple
packet-loss event that is subsequently recovered with a
retransmission can be considered a soft failure.

Hard Failure:
A non-recoverable condition in which a protocol does not operate
in the prescribed manner or it operates with excessive degradation
of service. For example, an established TCP connection that is
aborted due to an error condition constitutes, from the point of
view of the transport protocol, a hard failure, since it enters a
state from which normal operation cannot be recovered.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Issues with the Specification of Transient Numeric Identifiers

A recent survey of

Recent work on transient numeric identifier usage in protocol
specifications and implementations [RFC9414] [RFC9415] revealed that
most of the issues discussed in this document arise as a result of
one of the following conditions:

* protocol specifications that under specify the requirements for their transient numeric
identifiers

* protocol specifications that over specify their transient numeric
identifiers

* protocol implementations that simply fail to comply with the
specified requirements

Both under specifying and over specifying transient numeric
identifiers is hazardous. TCP port numbers and sequence numbers [RFC0793] and local ports [RFC0793], as well as DNS TxID
[RFC1035]
IDs [RFC1035], were originally under specified, leading to
implementations that used resulted in predictable values and thus were
vulnerable to numerous off-path attacks. Over specification, as for
IPv6 Interface Identifiers (IIDs) [RFC4291] and Fragment IPv6 Identification
values [RFC2460], left implementations unable to respond to security
and privacy issues stemming from the mandated or recommended
algorithms -- IPv6 IIDs need not expose privacy-sensitive link-layer
addresses, and predictable IPv6 Fragment Identifiers Header Identification values
invite the same off-path attacks that plague TCP.

Finally, there are protocol implementations that simply fail to
comply with existing protocol specifications. That is, appropriate
guidance is provided by the protocol specification (whether it be the
core specification or an update to it), but an implementation simply
fails to follow such guidance. For example, some popular operating
systems still fail to implement transport-protocol port
randomization, as specified in [RFC6056].

Clear specification of the interoperability requirements for the
transient numeric identifiers will help identify possible algorithms
that could be employed to generate them and also make evident if such
identifiers are being over specified. A protocol specification will
usually also benefit from a vulnerability assessment of the transient
numeric identifiers they specify to prevent the corresponding
considerations from being overlooked.

4. Common Flaws in the Generation of Transient Numeric Identifiers

This section briefly notes common flaws associated with the
generation of transient numeric identifiers. Such common flaws
include, but are not limited to:

* employing trivial algorithms (e.g., global counters) that result
in predictable identifiers,

* employing the same identifier across contexts in which constancy
is not required,

* reusing identifiers across different protocols or layers of the
protocol stack,

* initializing counters or timers to constant values when such
initialization is not required,

* employing the same increment space across different contexts, and

* use of flawed Pseudorandom Number Generators (PRNGs).

Employing trivial algorithms for generating the identifiers means
that any node that is able to sample such identifiers can easily
predict future identifiers employed by the victim node.

When one identifier is employed across contexts where such constancy
is not needed, activity correlation is made possible. For example,
employing an identifier that is constant across networks allows for
node tracking across networks.

Reusing identifiers across different layers or protocols ties the
security and privacy properties of the protocol reusing the
identifier to the security and privacy properties of the original
identifier (over which the protocol reusing the identifier may have
no control regarding its generation). Besides, when reusing an
identifier across protocols from different layers, the goal of
isolating the properties of a layer from that those of another layer is
broken, and the vulnerability assessment may be harder to perform
since the combined system, rather than each protocol in isolation,
will have to be assessed.

At times, a protocol needs to convey order information (whether it be
sequence, timing, etc.). In many cases, there is no reason for the
corresponding counter or timer to be initialized to any specific
value, e.g., at system bootstrap. Similarly, there may not be a need
for the difference between successive, counted successive counter values to be a
predictable.

A node that implements a per-context linear function may share the
increment space among different contexts (please see the "Simple
Hash-Based PRF-
Based Algorithm" section in [RFC9414]). [RFC9415]). Sharing the same increment
space allows an attacker that can sample identifiers in other context
to, e.g., learn how many identifiers have been generated between two
sampled values.

Finally, some implementations have been found to employ flawed PRNGs
(e.g., see [Klein2007]).

5. Requirements for Transient Numeric Identifiers

Protocol specifications that employ transient numeric identifiers
MUST explicitly specify the interoperability requirements for the
aforementioned transient numeric identifiers (e.g., required
properties such as uniqueness, along with the failure severity if
such properties requirements are not met).

A vulnerability assessment of the aforementioned transient numeric
identifiers MUST be performed as part of the specification process.
Such vulnerability assessment should cover, at least, spoofing,
tampering, repudiation, information disclosure, DoS, and elevation of
privilege.

Note:

| NOTE: Sections 8 and 9 of [RFC9414] [RFC9415] provide a general
| vulnerability assessment of transient numeric identifiers,
| along with a vulnerability assessment of common algorithms for
| generating transient numeric identifiers. Please see
| [Shostack2014] for further guidance on threat modelling. modeling.

Protocol specifications SHOULD NOT employ predictable transient
numeric identifiers, except when such predictability is the result of
their interoperability requirements.

Protocol specifications that employ transient numeric identifiers
SHOULD recommend an algorithm for generating the aforementioned
transient numeric identifiers that mitigates the vulnerabilities
identified in the previous step, such as those discussed in
[RFC9414].
[RFC9415].

As discussed in Section 1, use of cryptographic techniques for
confidentiality and authentication might not eliminate all the issues
associated with predictable transient numeric identifiers.
Therefore, the advice from this section MUST still be applied for
cases where cryptographic techniques are employed for confidentiality or
authentication of the associated transient numeric identifiers. are employed.

6. IANA Considerations

This document has no IANA actions.

7. Security Considerations

This entire document is about the security and privacy implications
of transient numeric identifiers and formally updates [RFC3552] such
that the security and privacy implications of transient numeric
identifiers are addressed when writing the "Security Considerations"
section of future RFCs.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.

8.2. Informative References

[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.

[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.

[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<https://www.rfc-editor.org/info/rfc7707>.

[RFC6274] Gont, F., "Security Assessment of the Internet Protocol
Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
<https://www.rfc-editor.org/info/rfc6274>.

[RFC7739] Gont, F., "Security Implications

[RFC8174] Leiba, B., "Ambiguity of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2016, <https://www.rfc-editor.org/info/rfc7739>.

[Sanfilippo1998a]
Sanfilippo, S., "about the ip header id", message to the
Bugtraq mailing list, 14 December 1998,
<https://seclists.org/bugtraq/1998/Dec/48>.

[RFC0793] Postel, J., "Transmission Control Protocol", Uppercase vs Lowercase in RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks",
2119 Key Words", BCP 14, RFC 6528, 8174, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>. 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2. Informative References

[Bellovin1989]
Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", Computer Communications Review, vol. 19, no. 2,
pp. 32-48, April 1989,
<https://www.cs.columbia.edu/~smb/papers/ipext.pdf>.

[IANA-PROT]
IANA, "Protocol Registries",
<https://www.iana.org/protocols>.

[Klein2007]
Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
Predictable IP ID Vulnerability", October 2007,
<https://dl.packetstormsecurity.net/papers/attack/OpenBSD_
DNS_Cache_Poisoning_and_Multiple_OS_Predictable_IP_ID_Vuln
erability.pdf>.

[Morris1985]
Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP
Software", CSTR 117, AT&T Bell Laboratories, Murray Hill,
NJ, February 1985,
<https://pdos.csail.mit.edu/~rtm/papers/117.pdf>.

[PREDICTABLE-NUMERIC-IDS]
Gont, F. and I. Arce, "Security and Privacy Implications
of Numeric Identifiers Employed in Network Protocols",
Work in Progress, Internet-Draft, draft-gont-predictable-
numeric-ids-03, 11 March 2019,
<https://datatracker.ietf.org/doc/html/draft-gont-
predictable-numeric-ids-03>.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC0793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.

[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.

[Silbersack2005]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability",
EuroBSDCon 2005 Conference, January 2005,
<https://citeseerx.ist.psu.edu/viewdoc/
download?doi=10.1.1.91.4542&rep=rep1&type=pdf>.

[PREDICTABLE-NUMERIC-IDS]

[RFC6274] Gont, F. and I. Arce, F., "Security and Privacy Implications Assessment of Numeric the Internet Protocol
Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
<https://www.rfc-editor.org/info/rfc6274>.

[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>.

[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers Employed in Network Protocols",
Work in Progress, Internet-Draft, draft-gont-predictable-
numeric-ids-03, 11 March 2019,
<https://datatracker.ietf.org/doc/html/draft-gont-
predictable-numeric-ids-03>. with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.

[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.

[RFC1035] Mockapetris, P., "Domain names - implementation

[RFC7323] Borman, D., Braden, B., Jacobson, V., and
specification", STD 13, R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 1035, 7323, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[RFC4291] Hinden, R. 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.

[RFC7707] Gont, F. and S. Deering, "IP Version 6 Addressing
Architecture", T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 4291, 7707, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[Klein2007]
Klein, 10.17487/RFC7707, March 2016,
<https://www.rfc-editor.org/info/rfc7707>.

[RFC7721] Cooper, A., "OpenBSD DNS Cache Poisoning Gont, F., and Multiple O/S
Predictable IP ID Vulnerability", October 2007,
<https://dl.packetstormsecurity.net/papers/attack/OpenBSD_
DNS_Cache_Poisoning_and_Multiple_OS_Predictable_IP_ID_Vuln
erability.pdf>.

[Schuba1993]
Schuba, C., "Addressing Weakness in the Domain Name System
Protocol", August 1993,
<http://ftp.cerias.purdue.edu/pub/papers/christoph-schuba/
schuba-DNS-msthesis.pdf>.

[Shostack2014]
Shostack, A., "Threat Modeling: Designing D. Thaler, "Security and Privacy
Considerations for Security",
Wiley, 1st edition, IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.

[RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
February 2014.

[RFC9415] 2016, <https://www.rfc-editor.org/info/rfc7739>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.

[RFC9414] Gont, F. and I. Arce, "Unfortunate History of Transient
Numeric Identifiers", RFC 9415, 9414, DOI 10.17487/RFC9415, May 10.17487/RFC9414, July
2023, <https://www.rfc-editor.org/info/rfc9415>.

[RFC9414] <https://www.rfc-editor.org/info/rfc9414>.

[RFC9415] Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", RFC 9414, 9415, DOI 10.17487/RFC9414, May 10.17487/RFC9415, July
2023, <https://www.rfc-editor.org/info/rfc9414>.

[IANA-PROT]
IANA, "Protocol Registries",
<https://www.iana.org/protocols>. <https://www.rfc-editor.org/info/rfc941v>.

[Sanfilippo1998a]
Sanfilippo, S., "about the ip header id", message to the
Bugtraq mailing list, December 1998,
<https://seclists.org/bugtraq/1998/Dec/48>.

[Schuba1993]
Schuba, C., "Addressing Weakness in the Domain Name System
Protocol", August 1993,
<http://ftp.cerias.purdue.edu/pub/papers/christoph-schuba/
schuba-DNS-msthesis.pdf>.

[Shostack2014]
Shostack, A., "Threat Modeling: Designing for Security",
Wiley, 1st edition, February 2014.

[Silbersack2005]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability",
EuroBSDCon 2005 Conference, January 2005,
<http://www.silby.com/eurobsdcon05/
eurobsdcon_silbersack.pdf>.

[TCPT-uptime]
McDanel, B., "TCP Timestamping - Obtaining System Uptime
Remotely", message to the Bugtraq mailing list, March
2001, <https://seclists.org/bugtraq/2001/Mar/182>.

Acknowledgements

The authors would like to thank (in alphabetical order) Bernard
Aboba, Brian Carpenter, Roman Danyliw, Theo de Raadt, Lars Eggert,
Russ Housley, Benjamin Kaduk, Charlie Kaufman, Erik Kline, Alvaro
Retana, Joe Touch, Michael Tüxen, Robert Wilton, and Paul Wouters for
providing valuable comments on draft versions of this document.

The authors would like to thank (in alphabetical order) Steven
Bellovin, Joseph Lorenzo Hall, and Gre Norcie for providing valuable
comments on [PREDICTABLE-NUMERIC-IDS], on which the present document
is based.

The authors would like to thank Diego Armando Maradona for his magic
and inspiration.

Authors' Addresses

Fernando Gont
SI6 Networks
Segurola y Habana 4310 7mo piso
Ciudad Autonoma de Buenos Aires
Argentina
Email: fgont@si6networks.com
URI: https://www.si6networks.com

Ivan Arce
Quarkslab
Segurola y Habana 4310 7mo piso
Ciudad Autonoma de Buenos Aires
Argentina
Email: iarce@quarkslab.com
URI: https://www.quarkslab.com