Internet DRAFT - draft-kent-opportunistic-security
draft-kent-opportunistic-security
Network Working Group S. Kent
Internet-Draft BBN Technologies
Intended status: Informational April 8, 2014
Expires: October 10, 2014
Opportunistic Security as a Countermeasure to Pervasive Monitoring
draft-kent-opportunistic-security-01
Abstract
This document was prepared as part of the IETF response to concerns
about "pervasive monitoring" (PM) as articulated in
[I-D.farrell-perpass-attack]. It begins by describing the current
criteria (discussed at the STRINT workshop [STRINT]) for addressing
concerns about PM. It then examines terminology that has been used
in IETF standards (and in academic publications) to describe
encryption and key management techniques, with a focus on
authentication vs. anonymity. Based on this analysis, it propose a
new term, "opportunistic security" to describe a goal for IETF
security protocols, one countermeasure to pervasive monitoring.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 10, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Removing Impediments to Using Encryption in the Internet . . 2
2. Why not "Opportunistic Encryption"? . . . . . . . . . . . . . 4
3. Authentication, Key Management and Existing IETF Protocols . 6
4. Anonymous, Pseudonymous, and Unauthenticated . . . . . . . . 8
5. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Removing Impediments to Using Encryption in the Internet
Recent discussions in the IETF about countering pervasive monitoring
(PM) have focused on increasing the use of encryption. In many
contexts, it is perceived that requiring authentication as part of
establishing an encrypted session is the major impediment to more
widespread use of encryption. Many IETF security protocols commonly
call for such authentication as part of establishing an encrypted
session. Thus much of the current flurry of activity focuses on
removing this impediment.
The term "opportunistic encryption" has been used frequently to refer
to newly proposed techniques for encouraging more widespread use of
encryption. However, this term has not always been used
consistently, and the term already has a precise meaning in the IETF
[RFC4322]. The next section of this document examines terminology
relevant to the topic, and suggests use of a new term: "opportunistic
security", a compromise based on the many terms that have been
offered. It also proposes a definition for this term, based on
principles adopted during the STRINT Workshop.
Opportunistic Security (for realtime communication) is defined as a
set of mechanisms for a security protocol that exhibit the following
characteristics:
1. It is invisible to users, and, more broadly, to applications that
initiate sessions. (Lack of visibility is considered critical,
so that users do not become confused by the variability in the
set of security services they are being afforded. Similarly, an
application that has not mandated explicit use of security
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protocol benefits from opportunistic security, but OUGHT NOT
[RFC6919] make decisions on how to behave based on the success or
failure of OS mechanisms).
2. Opportunistic security is not intended to be a substitute for
authenticated, integrity-protected encryption when that set of
security services can be provided to a user. For example, if a
user can establish a server-authenticated (see definition later)
TLS session with a financial institution today, the user should
continue to do so. This suggests that users (applications) MUST
still be able to explicitly invoke security protocols.
3. Opportunistic security will make use of perfect forward secrecy
(PFS) for key agreement. (PFS is desired because it affords
protection against a range of attacks that go beyond simple,
passive wiretapping. IKE [RFC5996] has offered this capability,
though it does not appear to be commonly used.)
4. Crypto-based authentication is a desired, though not mandatory,
feature. (Authentication comes in many flavors, as discussed in
Section 2. Authentication is desirable because it offers
protection against a range of active attacks, including MiTM,
that could cause a user to communicate with a party impersonating
the intended communication target. Some security protocols
mandate two-way crypto-based authentication, by default, e.g.,
IKE. TLS [RFC5246] and SSH [RFC4253] typically make use of 1-way
(server-based) crypto-based authentication, although they also
support client-based crypto-based authentication. Because the
success or failure of opportunistic security is not to be
signaled to the initiator of a session, the user will not know
whether the target of the session has been authenticated.
5. Detection of a man-in-the-middle (MiTM) attacks is a desired,
thought not mandatory, feature. If crypto-based authentication
cannot be achieved, it may still be possible to detect a MiTM.
MiTM detection is considered a lower priority than (crypto-based)
authentication, and is to be pursued only if the former security
service is not available (or fails).
6. In some contexts, human-perceptible delays in session/connection
establishment might discourage use of OS. In such contexts,
authentication and MiTM detection SHOULD take place after an
encrypted session is established. This ordering implies that
some data may be transmitted prior to authentication or detection
of a MiTM. In a context where delays imposed by performing
authentication are not considered onerous, authentication MAY be
attempted prior to enabling transmission. In such contexts all
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application traffic will be afforded authentication (and,
typically, integrity) if available.
7. Because opportunistic security entails a new key management
paradigm, it requires new capabilities by peers. Thus, until OS
is universally adopted, an attempt to execute opportunistic
security may fail, and the session will fallback to plaintext
communication. Since an attempt to use opportunistic security is
not communicated to users or applications, falling back to the
status quo, i.e., plaintext communication, is a reasonable
strategy.
2. Why not "Opportunistic Encryption"?
The term "opportunistic encryption" has become very widely used to
describe a range of key management (for encryption) techniques in the
IETF since the second half of 2013. However, it is not a new term.
The term was coined by H. Spencer and D. Redelmeier in 2001
[FreeSwanOE], and entered into the IETF vocabulary by Michael
Richardson in "Opportunistic Encryption using the Internet Key
Exchange (IKE)" an Informational RFC [RFC4322]. In this RFC the term
is defined as:
... the process of encrypting a session with authenticated
knowledge of who the other party is without prearrangement.
This definition above is a bit opaque. The introduction to [RFC4322]
provides a clearer description of the term, by stating the following
goal:
The objective of opportunistic encryption is to allow encryption
without any pre-arrangement specific to the pair of systems
involved.
Later the RFC notes:
Opportunistic encryption creates tunnels between nodes that are
essentially strangers. This is done without any prior bilateral
arrangement.
The reference to "prior bilateral arrangement" is relevant to IPsec
but not to most other IETF security protocols. If every pair of
communicating entities were required to make prior bilateral
arrangements to enable encryption between them, a substantial
impediment would exist to widespread use of encryption. However,
other IETF security protocols define ways to enable encryption that
do not require prior bilateral arrangements. Some of these protocols
require that the target of a communication make available a public
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key, for use by any initiator of a communication; an example of a
prior unilateral arrangement. The essential difference between IPsec
and most other IETF security protocols is that IPsec intrinsically
incorporates access control; other IETF security protocols do not.
The definition provided in [RFC4322] is specific to the IPsec
[RFC4301] context and ought not be used to describe the goals noted
in Section 1 above, as a countermeasure to PM. Because IPsec
implements access controls, it requires explicit specification (by
each peer) of how to process all traffic that crosses an "IPsec
boundary" (inbound and outbound). Traffic is either discarded,
permitted to pass w/o IPsec protection, or protected using IPsec.
The goal of opportunistic encryption (as per [RFC4322]) is to enable
IPsec protected communication without a priori configuration of
access control database entries at each peer (hence, bilateral).
Opportunistic encryption still calls for each party to identify the
other, using IKE v2 [RFC5996] (equivalently, IKE v1 [RFC2409])
authentication mechanisms, so it is not an unauthenticated key
management approach. Also note that [RFC4322] describes
opportunistic encryption relative to IKE, as it should; IPsec
implements encryption using ESP [RFC4303]. ESP usually provides data
integrity and authentication, as well as confidentiality, thus the
phrase opportunistic encryption is unduly narrow relative to the
anti-PM goal.
[RFC4322] also defines anonymous encryption:
Anonymous encryption: the process of encrypting a session without
any knowledge of who the other parties are. No authentication of
identities is done.
Thus, in [RFC4322], the term anonymous encryption refers to encrypted
communication where neither party is authenticated to the other.
Also note that the definition above refers to "the process of
encrypting a session ..." In fact, it is the key management process
that causes an encrypted session to be authenticated, or not, based
on credentials such as public keys, public key certificates, etc.
An examination of about 70 papers published in ACM, IEEE, and other
security conference proceedings identified numerous uses of the terms
opportunistic and anonymous encryption. Most, though not all, of the
papers used the terms opportunistic encryption and anonymous
encryption as defined in [RFC4322], but in some papers the
terminology was unclear or inconsistent with the [RFC4322]
definition.
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Wikipedia [wikipedia] uses a somewhat different definition for
opportunistic encryption. Wikipedia [wikipedia] provides the
following definition:
Opportunistic encryption refers to any system that, when
connecting to another system, attempts to encrypt the
communications channel otherwise falling back to unencrypted
communications. This method requires no pre-arrangement between
the two systems.
This definition shares some aspects of the [RFC4322] definition, but
it is not equivalent; it makes no mention of authentication or access
control, two essential aspects of opportunistic encryption as per
[RFC4322]. The definition is similar to some of the goals listed in
Section 1, but not to all of them. The article goes on to cite
examples of what it considers to be opportunistic encryption (citing
use of self-signed certificates in TLS), and in so doing contradicts
the concise definition above. Given the questionable scholarship of
the article, and its inconsistent use of the term with a range of
examples, it does not merit consideration when choosing a term to
describe the anti-PM mechanisms the IETF is developing.
Thus, the recent penchant for using the term opportunistic encryption
to refer to mechanisms that yield unauthenticated sessions is
inaccurate, even if popular. Although opportunistic encryption, as
described in [RFC4322], did not see widespread use, the effort has
resumed (as briefed in late 2013 [OErevisited]) and thus it makes
sense to reserve the term for that well-defined context.
The adjective "opportunistic" has caught the imagination of many (at
least in the IETF), so it seems desirable to retain that word when
selecting a new phrase to describe the goals cited in Section 1. It
was observed that these goals encompass more than just encryption.
PFS is a key management feature, and the optional (crypto-based)
authentication and MiTM detection features are security services
[ISO.7498-2.1988]. Thus the term "opportunistic security" is
proposed here as the (more accurate) term to replace opportunistic
encryption.
3. Authentication, Key Management and Existing IETF Protocols
As noted above, many IETF security protocols incorporate (crypto-
based) authentication as an intrinsic part of key management. IKE
normally requires two-way (mutual) authentication of the peers that
establish security associations. TLS normally affords server
authentication (based on X.509 certificates and the so-called Web
PKI), and offers optional support for client authentication based on
use of certificates. SSH ([RFC4251], [RFC4252], [RFC4253]) makes use
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of a trust on first use (TOFU) approach (aka a leap of faith, see
below) for server authentication. Because SSH is most often employed
in an enterprise context, reliance on this initial authentication
mechanism (for severs) represents a reasonable risk-based design
tradeoff. (User authentication in SSH is supported via a wide range
of techniques, some of which are cryptographic-based.)
Although, as noted above, many IETF security protocols incorporate
1-way or 2-way crypto-based authentication as part of key management,
most also offer options to enable creation of an encrypted session
based on 1-way or 2-way unauthenticated key management. For example,
TLS typically is used in a fashion that provides server, but not
client, authenticated communication. TLS also supports
"establishment of sessions" in which neither party (client or server)
asserts an identity during the handshake protocol (based on Diffie-
Hellman or ECDH key agreement). Thus TLS offers 2-way
unauthenticated communication in addition to the common, server-
authenticated communication. (The same analysis applies to DTLS
[RFC6347].)
In the store-and-forward environment, encrypted S/MIME messages are
usually signed. Moreover, the recipient of an S/MIME message is
typically identified by a certificate, so the originator specifies to
whom the message is directed. Thus, in common use, S/MIME provides
2-way authentication of traffic. However, S/MIME allows transmission
of originator-anonymous encrypted messages. First, note that signing
of a message by the originator is optional (see Section 3.3 of
[RFC5751]). Also, an originator may employ a key agreement algorithm
(e.g., Diffie-Hellman), to preserve originator anonymity.
(Section 6.2.2 of [RFC5652] notes: "The originatorKey alternative
inclues the algorithm identifier and sender's key agreement public
key. This alternative permits originator anonymity since the public
key is not certified.")
The originator of an S/MIME message directs an encrypted message to a
specific recipient (or set of recipients), and typically makes use of
a public key associated with the intended recipient to encrypt the
content encryption key for the message. If the recipient is
identified by a certificate, as is commonly the case, one would view
the communication as recipient-authenticated. However, if the public
key associated with a recipient is not conveyed via a validated
certificate, then the recipient would not be (crypto) authenticated
in the traditional sense. S/MIME calls for implementations to cache
capabilities information about senders (section 2.7.2 of [RFC5751]),
to facilitate this form of inband cryptographic data transfer. This
represents an alternative way for a prospective recipient to convey
public key info. (Note, this procedure is at odds with the
definition of opportunistic encryption, as it calls for a priori,
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per-peer configuration of data to enable later encrypted
communication!)
4. Anonymous, Pseudonymous, and Unauthenticated
The discussion above used the terms "authenticated" and
"unauthenticated" when describing various modes of key management.
These different modes achieve different results with respect to
identification of participants in a communication. We avoided the
term "anonymous" in part because unauthenticated communication, in
many contexts does not confer anonymity, per se. If a user does not
employ a key management technique that authenticate his/her identity,
the user may be required to employ some other form of authentication
later in the communication. In such cases the user clearly is not
anonymous. Also, the IP address and other characteristics of the
user may be gleaned from a communication, independent of the use of
explicit authentication mechanisms, including those associated with
key management. Finally, we avoid using the term "unauthenticated
encryption" because "authenticated encryption" is a well-defined term
in the crypto community. Instead we use the terms "unauthenticated
encrypted communication" and "authenticated encrypted communication"
as appropriate.
Another reason to avoid the term "anonymous" here is because it is
often confused with mechanisms that offer pseudonymous communication.
Pseudonymity [merriam-webster] implies use of an identifier, but one
that represents a "false name" for an entity. Use of pseudonyms is
common in some Internet communication contexts. Many Gmail, Yahoo,
and Hotmail mail addresses likely represent pseudonyms. A pseudonym
is an attractive way to provide unauthenticated communication. A
pseudonym typically makes use of the same syntax as a verified
identity in authenticated communication, and thus protocols designed
to make use of authenticated identities are compatible with use of
pseudonyms, to first order.
"Traceable Anonymous Certificate", is an Experimental RFC [RFC5636]
that describes a specific mechanism for a Certification Authority
(CA) [RFC5280] to issue an X.509 certificate with a pseudonym. The
goal of the mechanisms described in that RFC is to conceal a user's
identity in PKI-based application contexts (for privacy), but to
permit authorities to reveal the true identity (under controlled
circumstances). This appears to be the only RFC that explicitly
addresses pseudonymous key management; although it uses the term
"pseudonym" extensively, it also uses the term "anonymous" more
often, treating the two as synonyms.
Self-signed certificates [RFC6818] are often used with TLS in both
browser and non-browser contexts. In the HTTPS (browser) context, a
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self-signed certificate typically is accepted after a warning has
been displayed to a user; the HTTPS ([RFC2818], [RFC6797])
requirement to match a server DNS name against a certificate Subject
name does not apply when TLS is employed in non-browser contexts.
The Subject name in a self-signed certificate is completely under the
control of the entity that issued it, thus this is a trivial way to
generate a pseudonymous certificate, without using the mechanisms
specified in [RFC5636]. Thus support for pseudonymous encrypted
communication is supported in web browsing, as a side effect of this
deviation from [RFC2818]. (Some speculate that most self-signed
certificates contain accurate user or device IDs; the certificates
are used to avoid the costs associated with issuance of certificates
by Web PKI CAs.)
Pseudonymous encrypted communication is the result of applying
techniques to distribute keys when an authentication exchange is
based on a pseudonym, e.g., a self-signed certificate containing a
pseudonym. As with unauthenticated encrypted communication,
pseudonymous encrypted communication may apply to one or both parties
in an encrypted communication. One also can imagine mixed mode
communications, e.g., in which unauthenticated encrypted
communication is employed by one party and pseudonymous encrypted
communication is employed by the other.
As noted earlier, the model for opportunistic security (for realtime
communications) is to first establish an encrypted session, using key
management that affords PFS, and then attempt to "upgrade" it to an
authenticated communication. This is analogous to what IKE v2
[RFC5996] does. As experience with IKE has shown, this creates a DoS
vulnerability, i.e., an attacker can cause the target of a session/
connection to expend resources performing key agreement operations
prior to authenticating the initiator of the communication.
Implementations of opportunistic security will have to address this
concern. Opportunistic security designs also will have to address
various flavors of downgrade attacks, since opportunistic security
will allow unauthenticated or plaintext communication. Even though
opportunistic security assumes that a user is not alerted to its use,
it may be appropriate to alert a user to such attacks, or provide a
means by which a system administrator can become aware of them. The
details of how these concerns are addressed probably will be specific
to the protocol context in which opportunistic security is
implemented.
5. Terminology
The following definitions are derived from the Internet Security
Glossary [RFC4949], where applicable.
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Authenticated Encrypted Communication An encrypted communication
session based on using a key management technique that provides
crypto-based authentication, of one or both parties to the
communication. The communication may be 1-way or 2-way
authenticated.
Authentication The process of verifying a claim that a system or
entity has a certain attribute value. In the IETF context,
authentication typically refers to verification of an identity
claim, relative to some identifier's space. Typical identifier
spaces in the Internet include DNS names and [RFC0822] names.
(Data) Confidentiality The security service that prevents
information becoming available to unauthorized entities.
Encryption is the security mechanism typically used to implement
confidentiality.
Content encryption key (CEK) A symmetric cryptographic key used to
encrypt/decrypt the content of an S/MIME message. (Sometimes
referred to as a message encryption key.)
(Data) Integrity The security service that enables a recipient of a
message or a packet to determine if the data has been modified or
destroyed in an unauthorized manner.
Key agreement algorithm A key establishment method based on
asymmetric cryptography, in which a pair of entities engage in a
public exchange of data (public keys and associated data), to
generate the same shared secret value. (Thus both entities
contribute secret values to the resulting key.) This value is
later used to create symmetric keys used for encryption and/or
integrity checking. Diffie-Hellman and Elliptic Curve Diffie-
Hellman (ECDH) are the most common algorithms used for key
agreement as specified in RFCs.
Key transport A key establishment method by which a secret
(symmetric) key is generated by one entity and securely sent to
another entity. (Thus only one entity contributes secret values
to the resulting key.) Key transport may make use of either
symmetric or asymmetric cryptographic algorithms. The RSA
algorithm is most commonly cited in RFCs as a basis for a public
key, key transport mechanism.
Man-in-the-Middle attack (MiTM) A form of active wiretapping attack
in which an attacker intercepts and may selectively modify
communicated data to masquerade as one of the entities involved in
a communication. Masquerading enables the MiTM to violate the
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confidentiality and/or the integrity of communicated data passing
through it.
Opportunistic Encryption A key management technique that enables
authenticated communication between parties, and that does not
require a priori, bilateral arrangements. This term is defined
only for IPsec.
Opportunistic Security The result of employing a key management
technique that attempts to establish an encrypted communication
automatically and invisibly to a user. Opportunistic security may
attempt to upgrade an encrypted communication to provide
authentication (one or two way), and/or to detect MiTM attacks.
If opportunistic security is unable to create an encrypted
communication, e.g., because the other communicant does not
support opportunistic security, unencrypted (plaintext)
communication results.
Perfect Forward Secrecy (PFS) For a key management protocol, the
property that compromise of long-term keys does not compromise
session/traffic/content keys that are derived from or distributed
using the long-term keys.
Private key The secret component of a pair of cryptographic keys
used for asymmetric cryptography.
Public key The publicly disclosed component of a pair of
cryptographic keys used for asymmetric cryptography. The phrase
"public key data" includes a public key and any additional
parameters required to perform computation using the public key.
Pseudonymous Communication A key management technique that enables
pseudonymous communication between parties, e.g., based on use of
a self-signed certificate. Pseudonymous communication may be one-
way or two-way, depending on details of the key management
mechanism employed.
Session A realtime communication between entities.
Shared secret A value derived from a key agreement algorithm and
used as an input to generate a content encryption key or traffic
encryption key.
Symmetric cryptography A type of cryptography in which the
algorithms employ the same key for encryption and decryption, and
the key is not publicly disclosed.
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Traffic (encryption) key (TEK) A symmetric key used to encrypt/
decrypt traffic carried via an association.
Trust on First Use (TOFU) In a protocol, TOFU typically consists of
accepting an asserted identity, without authenticating that
assertion, and caching a key or credential associated with the
identity. Subsequent communication using the cached key/
credential is secure against a MiTM attack, if such an attack did
not succeed during the (vulnerable) initial communication or if
the MiTM is not present for all subsequent communications. The
SSH protocol makes use of TOFU. The phrase "leap of faith" (LoF)
is sometimes used as a synonym.
Unauthenticated Encrypted Communication An encrypted communication
session based on using a key management technique that enables
unauthenticated communication between parties. The communication
may be 1-way or 2-way unauthenticated. If 1-way, the initiator
(client) or the target (server) may be anonymous.
6. Acknowledgements
I want to thank David Mandelberg and Edric Barnes for their help in
generating this document.
7. Security Considerations
[TBS]
8. References
[FreeSwanOE]
Spencer, H. and D. Redelmeier, "Opportunistic Encryption",
May 2001.
[I-D.farrell-perpass-attack]
Farrell, S. and H. Tschofenig, "Pervasive Monitoring is an
Attack", draft-farrell-perpass-attack-06 (work in
progress), February 2014.
[ISO.7498-2.1988]
International Organization for Standardization,
"Information Processing Systems - Open Systems
Interconnection Reference Model - Security Architecture",
ISO Standard 7498-2, 1988.
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[OErevisited]
Wouters, P., "Opportunistic Encryption revisited",
November 2013, <http://www.ietf.org/proceedings/88/slides/
slides-88-saag-3.pdf>.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[RFC4322] Richardson, M. and D. Redelmeier, "Opportunistic
Encryption using the Internet Key Exchange (IKE)", RFC
4322, December 2005.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5636] Park, S., Park, H., Won, Y., Lee, J., and S. Kent,
"Traceable Anonymous Certificate", RFC 5636, August 2009.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
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[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012.
[RFC6818] Yee, P., "Updates to the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 6818, January 2013.
[RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", RFC 6919,
April 1 2013.
[STRINT] "A W3C/IAB workshop on Strengthening the Internet Against
Pervasive Monitoring (STRINT)", March 2014, <https://
www.w3.org/2014/strint/>.
[merriam-webster]
"pseudonymity", March 2014,
<http://www.merriam-webster.com/dictionary/pseudonymity>.
[wikipedia]
"Opportunistic encryption", November 2013,
<http://en.wikipedia.org/w/
index.php?title=Opportunistic_encryption&oldid=581222222>.
Author's Address
Stephen Kent
BBN Technologies
10 Moulton St.
Camridge, MA 02138
US
Email: kent@bbn.com
Kent Expires October 10, 2014 [Page 14]
