Network Working Group S. Hartman
Internet-Draft MIT
Intended status: Informational July 25, 2007
Expires: January 26, 2008
Requirements for Web Authentication Resistant to Phishing
draft-hartman-webauth-phishing-05.txt
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Abstract
This memo proposes requirements for protocols between web identity
providers and users and for requirements for protocols between
identity providers and relying parties. These requirements minimize
the likelihood that criminals will be able to gain the credentials
necessary to impersonate a user or be able to fraudulently convince
users to disclose personal information. To meet these requirements
browsers must change. Websites must never receive information such
as passwords that can be used to impersonate the user to third
parties. Browsers should perform mutual authentication and flag
situations when the target website is not authorized to accept the
identity being offered as this is a strong indication of fraud.
These requirements may serve as a basis for requirements for
preventing fraud in environments other than the web.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Capabilities of Attackers . . . . . . . . . . . . . . . . 6
3.2. Attacks of Interest . . . . . . . . . . . . . . . . . . . 7
4. Requirements for Preventing Phishing . . . . . . . . . . . . . 9
4.1. Support for Passwords and OTher Methods . . . . . . . . . 9
4.2. Trusted UI . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. No Password Equivelents . . . . . . . . . . . . . . . . . 10
4.4. Mutual Authentication . . . . . . . . . . . . . . . . . . 11
4.5. Authentication Tied to Resulting Page . . . . . . . . . . 11
4.6. Restricted Identity Providers . . . . . . . . . . . . . . 12
4.7. Protecting Enrollment . . . . . . . . . . . . . . . . . . 13
5. Is it the right Server? . . . . . . . . . . . . . . . . . . . 14
6. Iana Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Trusted UI Mechanisms . . . . . . . . . . . . . . . . 20
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 21
B.1. Changes since 02 . . . . . . . . . . . . . . . . . . . . . 21
B.2. Changes since 01 . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
Typically, web sites ask users to send a user name and password in
order to log in and authenticate their identity to the website. The
user name and plaintext password are often sent over a TLS [RFC4346]
encrypted connection. As a result of this, the server learns the
password and can pretend to be the user to any other system where the
user has used the same user name and password. The security of
passwords over TLS depends on making sure that the password is sent
to the right, trusted server. TLS implementations typically confirm
that the name entered by the user in the URL corresponds to the
certificate as described in [RFC2818].
One serious security threat on the web today is phishing. Phishing
is a form of fraud where an attacker convinces a user to provide
confidential information to the attacker believing they are providing
the information to a party they trust with that information. For
example, an email claiming to be from a user's bank may direct the
user to go to a website and verify account information. The attacker
captures the user name and password and potentially other sensitive
information. Domain names that look like target websites, links in
email, and many other factors contribute to phishers' ability to
convince users to trust them.
It is useful to distinguish two targets of phishing. Sometimes
phishing is targeting web authentication credentials such as user
name and password. Sometimes phishing is targeting other
confidential information, such as bank account numbers. This memo
presents requirements that significantly reduce the effectiveness of
the first category of phishing: these requirements guarantee that
even if the user authenticates to the wrong server, that server
cannot impersonate the user to a third party. However, to combat
phishing targeted at other confidential information, the best we can
do is help the user detect fraud before they release confidential
information.
The approach taken by this memo is to handle these two types of
phishing differently. Users are given some trusted mechanism to
determine whether they are typing their password into a secure
component that will authenticate them to the server or whether they
are typing their password into an HTML form that will send their
password to the server. If the user types a password into the
trusted component, they have strong assurances that their password
has not been disclosed and that the ensuing data returned from the
server was generated by a party that either knows their password or
who is authenticated by an identity provider who knows their
password. The server can then use confidential information known to
the user and server to enhance the user's trust in its identity
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beyond what is available given the social engineering attacks against
TLS server authentication. If a user enters their password into the
wrong server but discovers this before they give that server any
other confidential information, then there exposure is very limited.
The requirements presented in this memo are intended to be useful to
browser designers, designers of other HTTP applications and designers
of future HTTP authentication mechanisms.
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2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Threat Model
This section describes the assumed capabilities of phishers,
describes assumptions about web security and describes what
vulnerabilities exist.
We assume that the implementations of authentication systems can be
trusted sufficiently to meet their design objectives. This does not
mean that the entire local system and browser need to be trusted.
However if there is a component that has access to users' passwords,
that component needs to be secure enough to be trusted not to divulge
passwords to attackers. Similarly in a system that used smart cards,
the smart cards would need to be trusted not to give attackers access
to private keys or other authentication material. Designing
implementations to limit the size and complexity of the most trusted
components and to layer trust will be important to the security of
implementations. Designing protocols to enable good implementation
will be critical to the utility of the protocols. As a consequence
of this assumption, these requirements are insufficient to provide
protection against phishing if malicious browser extensions, Trojan
software or other malicious software is installed into a sufficiently
trusted part of the local computer or authentication tokens.
We assume that users have limited motivation to combat phishing.
Users cannot be expected to read the source of web pages, understand
how DNS works well enough to look out for spoofed domains, or
understand URI encoding. Users do not typically understand
certificates and cannot make informed decisions about whether the
subject name in a certificate corresponds to the entity they are
attempting to communicate with. As a consequence of this assumption,
users will likely be fooled by strings either in website names or
certificates that look visually similar but that are composed of
different code points.
3.1. Capabilities of Attackers
We assume attackers can convince the user to go to a website of their
choosing. Since the attacker controls the web site and since the
user chose to go to the website the TLS certificate will verify and
the website will appear to be secure. The certificate will typically
not be issued to the entity the user thinks they are communicating
with, but as discussed above, the user will not notice this.
Mechanisms attackers use to accomplish this include links where the
link name and URI target are misleading; email with links; DNS
attacks; and on-path network attacks.
The attacker can convincingly replicate any part of the UI of the
website being spoofed. The attacker can also spoof trust markers
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such as the security lock, URL bar and other parts of the browser UI.
There is one limitation to the attacker's ability to replicate UI.
The attacker cannot replicate a UI that depends on information the
attacker does not know. For example, an attacker could generally
replicate the UI of a banking site's login page. However the
attacker probably could not replicate the account summary page until
the attacker learned the user name and password because the attacker
would not know what accounts to list or approximate balances that
will look convincing to a user. Of course attackers may know some
personal information about a user. Websites that want to rely on
attackers not knowing certain information need to maintain the
privacy of that information.
The attacker can convince the user to do anything with the phishing
site that they would do with the real target site. As a consequence,
when passwords are used, if we want to avoid the user giving the
attacker their password, the web site must prove that it has an
established authentic relationship with the user without requiring a
static password to do so, and in a way that cannot be visually
mimicked so as to trick a user. One approach could be to transition
to a solution where the user could not give the real target site
their password if they are using a new mechanism. Instead they will
need to cryptographically prove that they know their password without
revealing it.
3.2. Attacks of Interest
The ultimate goal of these requirements is to provide protection
against disclosure of confidential information to unintended parties.
These requirements focus on two such disclosures and handle them
separately. The first category is disclosure of credentials that
could allow an unintended party to impersonate the user, possibly
gaining access to additional confidential information. The second
attack is disclosure of confidential information not directly related
to authentication. The second class of attack cannot be directly
defeated, but we can help users know when they are communicating with
an unintended party.
Note that some authentication systems such as Kerberos [RFC4120]
provide a facility to delegate the ability to act as the user to the
target of the authentication. Such a facility when used with an
inappropriately trusted target would be an instance of the first
class of attack. Solutions to these requirements with similar
facilities MUST discuss the security considerations surrounding use
of these facilities.
Of less serious concerns at the present time are attacks on data
integrity where a phisher provides false or misleading information to
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a user.
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4. Requirements for Preventing Phishing
This section describes requirements for web authentication solutions.
These solutions are intended to prevent phishing targeted at
obtaining web authentication credentials. These requirements will
make it more difficult for phishers to target other confidential
information.
The requirements discussed here are similar to the principles
outlined in "Limits to Anti-Phishing" [ANTIPHISHING]. Most of this
work was discovered independently but work from that paper has been
integrated where appropriate. It is desirable that these
requirements are similar to the principles outlined by someone facing
phishing as an operational reality.
4.1. Support for Passwords and OTher Methods
The web authentication solution MUST support passwords and MUST be
secure even when passwords are commonly used. In many environments,
users need the ability to walk up to a computer they have never used
before and log in to a website. Carrying a smart card or USB token
significantly increases the deployment cost of the website and
decreases user convenience. The smart card is costly to deploy
because it requires a process for replacing smart cards, requires
support staff to be trained in answering questions regarding smart
cards and requires a smart card to be issued when an identity is
issued. Smart cards are less convenient because users cannot gain
access to protected resources without having their card physically
with them. Many public access computers do not have smart cards
available and do not provide access to USB ports; when they do they
tend not to support smart cards. It is important not to
underestimate the training costs (either in money or user
frustration) of teaching people used to remembering a user name and
password about a new security technology. Sites that aggregate
identity--for example allowing a user to log into an identity
provider and then gain access to other resources may be a significant
part of a solution. However we cannot assume that a given user will
have only one such website: there are valid and common reasons a user
(or the relying party) would not trust all identity information to
one such site.
A solution to these requirements MUST also support smart cards and
other authentication solutions. Some environments have security
requirements that are strong enough that passwords simply are not a
viable option. Many efforts are under way to reduce the deployment
costs of token-based authentication mechanisms and to address some of
the concerns that make passwords a requirement today.
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4.2. Trusted UI
Users need the ability to trust components of the UI in order to know
that the UI is being presented by a trusted component of the device.
The primary concern is to make sure that the user knows any password
is being given to trusted software rather than being filled into an
HTML form element that will be sent to the server.
There are three basic approaches to establishing a trusted UI. The
first is to use a dynamic UI based on a secret shared by the user and
the local UI; the paper [ANTIPHISHING] recommends this approach. A
second approach is to provide a UI action that highlights trusted or
non-trusted components in some way. This could work similarly to the
Expose feature in Apple's OS X where a keystroke visually
distinguishes structural elements of the UI. Of course such a
mechanism would only be useful if users actually used it. Finally,
the multi-level security community has extensive research in
designing UIs to display classified, compartmentalized information.
It is critical that these UIs be able to label information and that
these labels not be spoofable.
See Section 5 for another case where confidential information in a UI
can be used to build trust.
4.3. No Password Equivelents
A critical requirement is that when a user authenticates to a
website, the website MUST NOT receive a strong password equivalent
[IABAUTH]. A strong password equivalent is anything that would allow
a phisher to authenticate as a user with a different identity
provider. Weak password equivalents (quantities that act as a
password for a given service but cannot be reused with other services
) MAY only be sent when a new identity is being enrolled or a
password is changed. A weak password equivalent allows a party to
authenticate to a given identity provider as the user.
There are two implications of this requirement. First, a strong
cryptographic authentication protocol needs to be used instead of
sending the password encrypted over TLS. The zero-knowledge class of
password protocols such as those discussed in section 8 of the IAB
authentication mechanisms document [IABAUTH] seem potentially useful
in this case. Note that mechanisms in this space tend to have
significant deployment problems because of intellectual property
issues.
The second implication of this requirement is that if an
authentication token is presented to a website, the website MUST NOT
be able to modify the token to authenticate as the user to a third
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party. The party generating the token must bind it to either the
website that will receive the token or to a key known only to the
user. Binding could include cryptographic binding or mechanisms such
as issuing a one-time password for use with a specific website. If
tokens are bound to keys, the user MUST prove knowledge of this key
as part of the authentication process. The key MUST NOT be disclosed
to the server unless the token is bound to the server and the key is
only used with that token.
4.4. Mutual Authentication
[ANTIPHISHING] describes a requirement for mutual authentication. A
common phishing practice is to accept a user name and password as
part of an attempt to make the phishing site authentic. The real
target is some other confidential information. The user name and
password are captured, but are not verified. After the user name and
password are entered, the phishing site collects other confidential
information.
Authentication of the server and client at the TLS level is
sufficient to meet the requirement of mutual authentication. If
authentication is based on a shared secret such as a password, then
the authentication protocol MUST prove that the secret or a suitable
verifier is known by both parties. Interestingly the existence of a
shared secret will provide better protection that the right server is
being contacted than if public key credentials are used. By their
nature, public key credentials allow parties to be contacted without
a prior security association. In protecting against phishing
targeted at obtaining other confidential information, this may prove
a liability. However public key credentials provide strong
protection against phishing targeted at obtaining authentication
credentials because they are not vulnerable to dictionary attacks.
Such dictionary attacks are a significant weakness of shared secrets
such as passwords intended to be remembered by humans. For public
key protocols, this requirement would mean that the server typically
needs to sign an assertion of what identity it authenticated.
4.5. Authentication Tied to Resulting Page
Users expect that whatever party they authenticate to will be the
party that generates the content they see. One possible phishing
attack is to insert the phisher between the user and the real site as
a man-in-the-middle. On today's websites, the phisher typically
gains the user's user name and password. Even if the other
requirements of this specification are met, the phisher could gain
access to the user's session on the target site. This attack is of
particular concern to the banking industry. A man-in-the-middle may
gain access to the session which may give the phisher confidential
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information or the ability to execute transactions on the user's
behalf.
The authentication system MUST guarantee to the user and the target
server that the response is generated by the target server and the
contents of the response will only be seen by parties authorized by
either the target server or the user. This can be done in several
ways:
1. Assuming that only certificates from trusted CAs are accepted and
the user has not bypassed server certificate validation, it is
sufficient to confirm that the identity of the server at the TLS
level is the same at the HTTP authentication level. In the case
of TLS client authentication this is trivially true.
2. Another alternative is to bind the authentication exchange to the
channel created by the TLS session. The general concept behind
channel binding is discussed in section 2.2.2 of [BTNS]. Work is
ongoing to adapt this concept to HTTP over TLS [TLS-CB]
3. Redirect based schemes in which the identity provider is told
what site to return the user to meets this requirement provided
again that certificate validation is done at the TLS layer.
4.6. Restricted Identity Providers
Some identity providers will allow anyone to accept their identity.
However particularly for financial institutions and large service
providers it will be common that only authorized business partners
will be able to accept the identity. The confirmation that the the
relying party is such a business partner will often be a significant
part of the value provided by the identity provider, so it is
important that the protocol enable this. For such identities, the
user MUST be assured that the target server is authorized by the
identity provider to accept identities from that identity provider.
Several mechanisms could be used to accomplish this:
1. The target server can provide a certificate issued by the
identity provider as part of the authentication.
2. The identity provider can explicitly approve the target server.
For example in a redirect-based scheme the identity provider
knows the identity of the relying party before providing claims
of identity to that party. A similar situation happens with
Kerberos.
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4.7. Protecting Enrollment
One area of particular vulnerability to phishing is enrollment of a
new identity in an identity provider. Protecting against phishing
targeted at obtaining other confidential information as a new service
is established is outside the scope of this document. If
confidential information such as credit card numbers are provided as
part of account setup, then this may be a target for phishing.
However there is one critical aspect in which enrollment impacts the
security of authentication. During enrollment, a password is
typically established for an account at an identity provider. The
process of establishing a password MUST NOT provide a strong password
equivalent (a quantity such as the password itself that could be used
to log into another service where the same password is used as the
user) to the identity provider. That is, the identity provider MUST
NOT gain enough information to impersonate the user to a third party
while establishing a password.
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5. Is it the right Server?
In Section 4, requirements were presented for web authentication
solutions to minimize the risk of phishing targeted at web access
information. This section discusses in a non-normative manner
various mechanisms for determining that the right server has been
contacted. Authenticating to the right party is an important part of
reducing the risk of phishing targeted at other confidential
information.
Validation of the certificates used in TLS and verification that the
name in the URI maps to these certificates can be useful. As
discussed in Section 3, attackers can spoof the name in the URI.
However the TLS checks do defeat some attacks. Also, as discussed in
Section 4.5, TLS validation may be important to higher-level checks.
A variety of initiatives propose to assign trust to servers. This
includes proposals to allow users to indicate certain servers are
trusted based on information they enter. Also, proposals to allow
third parties including parties established for this purpose and
existing certificate authorities to indicate trust have been made.
These proposals will almost certainly make phishing more difficult.
In the case where there is an existing relationship, these
requirements provide a way that information about the relationship
can be used to provide assurance that the right party has been
contacted.
In Section 4.2, we discuss how a secret between the user and their
local computer can be used to let the user know when a password will
be handled securely. A similar mechanism can be used to help the
user once they are authenticated to the website. The website can
present information based on a secret shared between the user and
website to convince the user that they have authenticated to the
correct site. This depends critically on the requirements of
Section 4.5 to guarantee that the phisher cannot obtain the secret.
It is tempting to use this form of trusted UI before authentication.
For example, a website could request a user name and then display
information based on a secret for that user before accepting a
password. The problem with this approach is that phishers can obtain
this information, because it can be obtained without knowing the
password. However if the secret is displayed after authentication
then phishers could not obtain the secret. This is one of the many
reasons why it is important to prevent phishing targeted at
authentication credentials.
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6. Iana Considerations
This document requests no action of IANA.
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7. Acknowledgments
I'd like to thank Nicolas Williams, Matt Knopp and David Blumenthal
for helping me walk through these requirements and make sure that if
a solution met them it would actually protect against the real world
attacks consumers of our technology are facing. I was particularly
focusing on attacks that financial institutions are seeing and their
help with this was greatly appreciated.
I'd like to thank Eric Rescorla and ben Laurie for their significant
comments on this draft.
Eliot Lear provided many last call comments and helped work through
several long standing issues with the document.
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8. Security Considerations
This memo discusses the security of web authentication and how to
minimize the risk of phishing in web authentication systems. This
section discusses the security of the overall system and discusses
how components of the system that are not directly within the scope
of this document affect the security of web transactions. This
section also discusses residual risks that remain even when the
requirements proposed here are implemented.
The approach taken here is to separate the problem of phishing into
phishing targeted at web authentication credentials and phishing
targeted at other information. Users are given some trusted
mechanism to determine whether they are typing their password into a
secure browser component that will authenticate them to the web
server or whether they are typing their password into a legacy
mechanism that will send their password to the server. If the user
types a password into the trusted browser component, they have strong
assurances that their password has not been disclosed and that the
page returned from the web server was generated by a party that
either knows their password or who is authenticated by an identity
provider who knows their password. The web server can then use
confidential information known to the user and web server to enhance
the user's trust in its identity beyond what is available given the
social engineering attacks against TLS server authentication. If a
user enters their password into the wrong server but discovers this
before they give that server any other confidential information, then
there exposure is very limited.
This model assumes that the browser and operating system are a
trusted component. As discussed in Section 3, there are numerous
attacks against host security. Appropriate steps should be taken to
minimize these risks. If the host security is compromised, the
password can be captured as it is typed by the user.
This model assumes that users will only enter their passwords into
trusted browser components. There are several potential problems
with this assumption. First, users need to understand the UI
distinction and know what it looks like when they are typing into a
trusted component and what a legacy HTML form looks like. Users must
care enough about the security of their passwords to only type them
into trusted components. The browser must be designed in such a way
that the server cannot create a UI component that appears to be a
trusted component but is actually a legacy HTML form; Section 4.2
discusses this requirement.
In addition, a significant risk that users will type their password
into legacy HTML forms comes from the incremental deployment of any
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web authentication technology. Websites will need a way to work with
older web browsers that do not yet support mechanisms that meet these
requirements. Not all websites will immediately adopt these
mechanisms. Users will sometimes browse from computers that have
mechanisms meeting these requirements and sometimes from older
browsers. They only gain protection from phishing when they type
passwords into trusted components. If the same password is sometimes
used with websites that meet these requirements and sometimes with
legacy websites, and if the password is captured by a phisher
targeting a legacy website, then that captured password can be used
even on websites meeting these requirements. Similarly, if a user is
tricked into using HTML forms when they should not, passwords can be
exposed. Users can significantly reduce this risk by using different
passwords for websites that use trusted browser authentication than
for those that still use HTML forms.
The risk of dictionary attack is always a significant concern for
password systems. Users can (but typically do not) minimize this
risk by choosing long, hard to guess phrases for passwords. The risk
of offline dictionary attack can be removed once a password is
already established by using a zero-knowledge password protocol. The
risk of online dictionary attack is always present. The risk of
offline dictionary attack is always present when setting up a new
password or changing a password. Minimizing the number of services
that use the same password can minimize this risk. When zero-
knowledge password protocols are used, being extra careful to make
sure the right server is used when establishing a password can
significantly reduce this risk.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[ANTIPHISHING]
Nelson, J. and D. Jeske, "Limits to Anti Phishing",
January 2006.
Proceedings of the W3c Security and Usability Workshop; ht
tp://www.w3.org/2005/Security/usability-ws/papers/
37-google/'
[BTNS] Touch, J., "Problem and Applicability Statement for Better
Than Nothing Security",
draft-ietf-btns-prob-and-applic-02.txt (work in progress),
February 2006.
[IABAUTH] Rescorla, E., "A Survey of Authentication Mechanisms",
draft-iab-auth-mech-05.txt (work in progress),
February 2006.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[TLS-CB] Johansson , L., "Channel bindings for HTTP+TLS transport",
draft-johansson-http-tls-cb-00.txt (work in progress),
July 2006.
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Appendix A. Trusted UI Mechanisms
There are three basic approaches to establishing a trusted UI. The
first is to use a dynamic UI based on a secret known by the user;
[ANTIPHISHING] recommends this approach. A second approach is to
provide a UI action that highlights trusted or non-trusted components
in some way. This could work similarly to the Expose feature in
Apple's OS X where a keystroke visually distinguishes structural
elements of the UI. Of course such a mechanism would only be useful
if users actually used it. Finally, the multi-level security
community has extensive research in designing UIs to display
classified, compartmentalized information. It is critical that these
UIs be able to label information and that these labels not be
spoofable.
See Section 5 for another case where confidential information in a UI
can be used to build trust.
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Appendix B. Change History
B.1. Changes since 02
Updated discussion of TLS authentication to point out that it does
meet the requirement of mutual authentication.
Added pointer to HTTP TLS channel bindings work
B.2. Changes since 01
Updated threat model to give examples of attacks that are in scope
and to more clearly discuss host security based on comments from
Chris Drake.
Clarify attacks of interest to be consistent with the
introduction.
Fix ups regarding one-time passwords. I'm not sure that OTPs can
meet all the requirements but clean things up where they clearly
can meet a requirement.
Clarify that in the mutual authentication case I'm concerned about
authentication of client to the server.
Clean up bugs in security considerations
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Author's Address
Sam Hartman
Massachusetts Institute of Technology
Email: hartmans-ietf@mit.edu
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