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Versions: 00 01 02 03 04 05 06 07 08 09                    Informational
Network Working Group                                         S. Hartman
Internet-Draft                                         Painless Security
Intended status: Informational                           August 18, 2008
Expires: February 19, 2009

       Requirements for Web Authentication Resistant to Phishing

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 19, 2009.

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   This memo proposes requirements for protocols between web browsers
   and relying parties at websites; these requirements also impact third
   parties involved in the authentication process.  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 authenticate the website
   to the browser as part of authenticating the user to the website.
   Browsers MUST flag situations when this authentication fails 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Purpose of this Memo . . . . . . . . . . . . . . . . . . .  5
     1.2.  Progress to Date . . . . . . . . . . . . . . . . . . . . .  6
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Passwords and Interface  . . . . . . . . . . . . . . . . .  7
     2.2.  Requirements notation  . . . . . . . . . . . . . . . . . .  7
   3.  Threat Model . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Capabilities of Attackers  . . . . . . . . . . . . . . . .  9
     3.2.  Attacks of Interest  . . . . . . . . . . . . . . . . . . . 10
   4.  Requirements for Authentication that Protects Credentials  . . 11
     4.1.  Support for Passwords and OTher Methods  . . . . . . . . . 11
     4.2.  Trusted UI . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.3.  No Password Equivelents  . . . . . . . . . . . . . . . . . 12
     4.4.  Mutual Authentication  . . . . . . . . . . . . . . . . . . 13
     4.5.  Authentication Tied to Request and Response  . . . . . . . 14
     4.6.  Restricted Identity Providers  . . . . . . . . . . . . . . 15
     4.7.  Protecting Enrollment  . . . . . . . . . . . . . . . . . . 15
   5.  Is it the right Server?  . . . . . . . . . . . . . . . . . . . 17
   6.  Iana Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 23
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Trusted UI Mechanisms . . . . . . . . . . . . . . . . 25
   Appendix B.  Change History  . . . . . . . . . . . . . . . . . . . 26
     B.1.  Changes Since 08 . . . . . . . . . . . . . . . . . . . . . 26
     B.2.  Changes since 07 . . . . . . . . . . . . . . . . . . . . . 26
     B.3.  Changes since 06 . . . . . . . . . . . . . . . . . . . . . 27
     B.4.  Changes since 05 . . . . . . . . . . . . . . . . . . . . . 27
     B.5.  Changes since 02 . . . . . . . . . . . . . . . . . . . . . 28
     B.6.  Changes since 01 . . . . . . . . . . . . . . . . . . . . . 28
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30

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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 plaintext password
   protocol, 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 and on
   that server not exposing the password to third parties.  HTTPs
   [RFC2818] implementations typically confirm that the name entered by
   the user in the URL corresponds to the certificate.

   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.

   Typically the user names and password are not directly valuable to
   the phisher.  However they can be used to access resources of value.
   For example a bank password may permit money transfer or access to
   information useful in identity theft.

   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 can be part of a solution to significantly
   reduce the effectiveness of the first category of phishing: provided
   that a user uses an authentication mechanism that meets these
   requirements, 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 know how to 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.  The user is given new authentication
   mechanisms.  If the user uses these mechanisms,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

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   either knows their password or who is authenticated by an identity
   provider (a third party involved in the authentication exchange in
   order to allow credentials to be used across a wider variety of
   websites) 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 beyond what is available given the
   social engineering attacks against TLS server authentication.  If a
   user authenticates to the wrong server but discovers this before they
   give that server any other confidential information, then there
   exposure is very limited.  The success of this solution depends
   heavily on whether the user uses the new authentication mechanisms;
   designing ways for users to tell if they are using the authentication
   mechanisms and encouraging users to use these mechanisms will be
   critical to achieving any security benefit from these requirements.
   The success of a solution to preventing the disclosure of other
   confidential information based on giving users information about
   whether they are authenticated to the right server depends on the
   user being able to take advantage of this information and choosing to
   do so.

   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.

   These requirements and mechanisms that meet these requirements are
   not sufficient to stop phishing; at best, they form part of a
   solution.  The World Wide Web Consortium proposes recommendations on
   user interface guidelines for web security context [WSCUIG].  These
   guidelines propose mechanisms that will make it more likely that
   users will detect fraud before authentication.  Efforts to limit the
   effect of malicious software and to provide trustable software for
   authentication are also important.  Efforts to track known frauds and
   alert users when they encounter fraudulent sites are also critical.
   Together, all these efforts may significantly reduce phishing.

1.1.  Purpose of this Memo

   In publishing this memo, the IETF recommends making available
   authentication mechanisms that meet the requirements outlined in
   Section 4 in HTTP user agents including web browsers.  It is hoped
   that these mechanisms will prove a useful step in fighting phishing.
   However this memo does not restrict work either in the IETF or any
   other organization.  In particular, new authentication efforts are
   not bound to meet the requirements posed in this memo unless the
   charter for those efforts chooses to make these binding requirements.
   Less formally, the IETF presents this memo as an option to pursue
   while acknowledging that there may be other promising paths both now
   and in the future.

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1.2.  Progress to Date

   This non-normative section describes the author of this memo's
   impressions of the current state of HTTP authentication with regard
   to these requirements.

   In the spring of 2008, Microsoft demonstrated that with no change to
   the spec, GSS-API and NTLM HTTP authentication could be extended to
   support channel binding [RFC5056] [RFC4559].  At first glance, the
   Microsoft extension appears to meet all the requirements outlined in
   this memo for an authentication mechanism.  In addition, Microsoft
   has outlined extensions to HTTP digest authentication that also
   appear to meet these requirements [DIGEST-BIND].  The Microsoft
   extensions do not provide the client with information on whether the
   server supports the extension; so the client may not know whether it
   is strongly authenticated or not.  Also, the Microsoft extensions are
   focused for enterprise deployment and so concerns regarding upgrade
   negotiation and other issues that would be important in a wider
   deployment are not covered.  However Microsoft's efforts underscore
   that new security mechanisms are not needed in order to meet these
   requirements.  Originally, I had expected that changes to meet these
   requirements would be more extensive, but still expected they would
   be incremental changes to existing mechanisms.

   However there is still work that needs to be done in order to make
   mechanisms meeting these requirements available in a usable manner
   across the Internet.  Most of that work concerns usability and falls
   outside the IETF.  Results of the usability work may fall within the
   IETF; mechanisms for picking the right credentials to use for a given
   site may require minor extensions to security mechanisms .
   Mechanisms to provide smoothe upgrades from plaintext password
   protocols to mechanisms meeting these requirements may require
   additional HTTP headers, particularly for non-browser agents.  In
   addition, these requirements may be useful to efforts that are
   designing HTTP authentication mechanisms for unrelated reasons.

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2.  Terminology

2.1.  Passwords and Interface

   There are two related concepts: the user interface of passwords and
   plaintext password protocols.  A plaintext password protocol is a
   protocol where the server receives credentials sufficient to
   impersonate a user to third parties.  A password interface provides a
   user experience where a user types a password into any computer,
   including one they have never used before and that is sufficient to
   authenticate.  The requirements in this memo require support for
   password user interfaces as one option for authentication.  The
   requirements of this memo are incompatible with plaintext password

2.2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.  Human factors issues contribute significantly
   to these vulnerabilities.  For example, security information
   dialogues in web browsers can provide information on the subject of a
   certificate.  However, users rarely examine this information, so an
   attacker could be successful even if examining the security dialogue
   would show an attack.  This threat model is intended to include these
   sorts of attacks and so it is broader than the technical threats
   against protocols.  Efforts are under way to improve these human
   factors issues [WSCUIG].  However these efforts only reduce the risk
   that a user will be confused; even given improved user experience for
   dealing with security context information, users will make mistakes
   and believe that an attacker's site is the site they intended to
   communicate with.

   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.

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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 with a
   misleading name or URI, which they may distribute in emails; attacks
   against DNS; and man-in-the-middle attacks against a TLS handshake.
   The former two attacks allow the attacker to pass authentication
   because the victim user can be tricked into accepting the attacker's
   certificate.  The latter attack will typically create a warning on
   the victim user's side, but many users do not make informed decisions
   on how to respond to such a warning, making them inclined to accept
   the bogus certificate.

   The attacker can convincingly replicate any part of the UI of the
   website being spoofed.  The attacker can also spoof trust markers
   such as the security lock, URL bar and other parts of the browser UI
   sufficiently that a significant class of users will not treat the
   spoofed security indicators as a problem.  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.

   It's not clear how valuable this limitation on the attacker's ability
   will prove in practice.  Research into the effectiveness of security
   indicators [SECIND] suggests that users do not pay attention to
   security indicators.  One difference between the security indicators
   tested in today's research and using private information to detect
   fraud is that the private information may be directly related to the
   task the user is trying to perform.  However the attacker can attempt
   to come up with a convincing explanation such as a partial outage or
   system upgrade for why the private information is not available.

   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

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   established authentic relationship with the user without requiring a
   plaintext password protocol.  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 give information to users that they could use to
   help 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
   a user.

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4.  Requirements for Authentication that Protects Credentials

   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

4.1.  Support for Passwords and OTher Methods

   The web authentication solution MUST support the password user
   interface and MUST be secure even when the password interface is
   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.

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 as part of a

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   plaintext password protocol.

   There are many approaches to establishing a trusted UI.  One example
   is to use a dynamic UI based on a secret shared by the user and the
   local UI; the paper [ANTIPHISHING] recommends this approach.  The W3C
   recommends this approach for security indicators in section 7.1 of
   its user interface guidelines [WSCUIG].  However, the W3C notes that
   research suggests users may not pay attention to these trust
   indicators.  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 Mac 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, another potential approach is to benefit from extensive
   research in the multi-level security community 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.  These approaches are not exhaustive and may not even
   be good; they are provided to demonstrate that thought into how to
   design trusted UIs is ongoing.  However, designing a user interface
   that allows users of the web to distinguish trusted components from
   components potentially controlled by an attacker is an open problem.
   It is likely that transitioning to many new security protocols will
   depend on a solution to this problem.

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 website.
   Consequently, plaintext password protocols are incompatible with
   these requirements.  Weak password equivalents (quantities that act
   as a password for a given service but cannot be reused with other
   services ) are problematic outside of the context of enrolling a user
   or changing a password.  The requirement for mutual authentication
   Section 4.4 is incompatible with sending weak password equivalents in
   every authentication.  Even if that requirement is relaxed, the scope
   of a particular weak password equivalent needs to be carefully
   considered.  Consider for example a protocol that hashes a password
   and the host name component of a URI together to form a weak password
   equivalent.  The same password equivalent is used regardless of which
   certificate authority certifies the public key of the website.  If an
   attacker mounted a man-in-the-middle attack, presenting a self-signed
   certificate, and the user accepted the certificate when asked by the
   browser, then the attacker would receive the same weak password
   equivalent needed to access the legitimate website.  Such a protocol
   would not do a good job of addressing the threats outlined in the

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   threat model.  However if mutual authentication were not a
   requirement, a protocol that hashed a password and the public key
   from the TLS certificate of the website to form a weak password
   equivalent might meet the other requirements.  In any event, weak
   password equivalents MUST NOT be sent without confidentiality

   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 at a first glance.  However, 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
   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 or server.

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.  When mutual authentication fails, there is a strong
   indication of a problem: either the user supplied the wrong
   credential or the website is not the one the user intended to
   communicate with.

   Requiring mutual authentication excludes a class of mechanisms where
   a weak password equivalent is generated for the server and is sent.
   One prominent member of this class is [PWDHASH]; this mechanism has
   the desirable property that it requires no change to the server and
   can be implemented locally on the browser.  These mechanisms provide
   better security than plaintext password protocols.  However attacks
   where the server ignores authentication in order to obtain

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   confidential information are important enough that it is desirable to
   develop mechanisms that provide this assurance.  The desire to
   develop these new mechanisms is not intended to discourage the
   deployment of mechanisms like Pwdhash that improve security today.

   Typically one protocol performs authentication of both parties.
   There tend to be opportunities for a man-in-the-middle attack when
   one protocol authenticates one direction and another protocol
   authenticates the opposite direction.  Sometimes, as in the case of
   TLS and plaintext password protocols, the opportunity for attacks
   depends on human factors issues or certificate management.  In other
   cases, attacks may be more direct.  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
   confidence that the right server is being contacted than if public
   key credentials are used in their typical mode.  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, the mutual authentication requirement would mean that the
   server typically needs to sign an assertion of what identity it
   authenticated or of the request as a whole.

4.5.  Authentication Tied to Request and Response

   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
   information or the ability to execute transactions on the user's

   The authentication system MUST guarantee to the user and the target
   server that the request was generated by the authenticated user and
   the response is generated by the target server .  This can be done in

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   several ways including:

   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.  Note
       however that [WSCUIG] recommends accepting self-signed
       certificates in some cases, so relying on this approach for cases
       other than TLS authentication may be problematic.

   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 [RFC5056].  Channel binding has
       been added to HTTP authentication mechanisms based on digest
       authentication and on GSS-API, suggesting that support for
       channel binding is workable for future HTTP authentication

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
   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 or Digest Authentication in a AAA infrastructure

4.7.  Protecting Enrollment

   One area of particular vulnerability to phishing is enrollment of a
   new identity in an authentication system.  Protecting against
   phishing targeted at obtaining other confidential information as a
   new service is established is outside the scope of this document.  If

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   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 or other security credentials
   are associated with an account.  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).  That is,
   parties other than the user and web browser 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

   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.  The W3C user
   interface guidelines may significantly increase the value of these
   checks [WSCUIG].  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

   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.

   Various schemes have used a secret shared between the server and the
   web browser before authentication.  Cookies or some other state
   management mechanism are used to select the right secret to display
   as the user logs into the site.  Unfortunately these schemes have
   proven ineffective in practice [SECIND].  However, the set of
   information that can be used as contextual clues to evaluate whether
   the right server has been reached after authentication is much
   greater.  For example, a bank server knows what accounts a user has
   and knows their balances.  A business partner may have information

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   about past transactions or the current state of transactions.  If
   this information is related to the task that the user is trying to
   perform, they may be more likely to evaluate it and notice problems
   than they are to notice a missing security indicator before login.
   Strong authentication mechanisms enable this type of evaluation after
   the user has logged in.  However it is not known how effective this
   will be in practice.

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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 the MIT Kerberos Consortium for its funding of work
   on this memo prior to April 2008.

   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.

   Christian Vogt provided text and review comments.

   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 seems good that these requirements
   are similar to the principles outlined by someone facing phishing as
   an operational reality.

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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--a component that presents a password interface--or whether
   they are typing their password into a legacy mechanism that will send
   their password to the server as part of a plaintext password
   protocol.  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 parts of the browser and operating system
   with access to passwords or other long-term credentials are trusted
   software.  As discussed in Section 3, there are numerous attacks
   against host security.  Appropriate steps should be taken to minimize
   these risks.  If the security of the trusted software 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.  It is not
   clear that we have yet developed a solution to this user interface
   problem.  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; the W3C user interface guidelines [WSCUIG] provides

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   requirements that are designed to prevent security sensitive user
   interface from being spoofed by attacker-supplied content.  The W3C
   guidelines provide requirements for a more limited context focused
   around security context but not authentication information.  However
   starting from these requirements may be a successful approach.

   In addition, a significant risk that users will type their password
   into legacy HTML forms comes from the incremental deployment of any
   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.

   [WSCUIG]   Roessler , T. and A. Saldhana , "Web Security Context:
              User Interface Guidelines", W3C Working Draft, July 2008,

              Publication of this draft needs to block until and unless
              this references is approved as some form of W3C

9.2.  Informative References

              Nelson, J. and D. Jeske, "Limits to Anti Phishing",
              January 2006.

              Proceedings of the W3c Security and Usability Workshop; ht

              Santesson, S., Damour, K., and P. Hallin, "Channel Binding
              for HTTP Digest Authentication",
              draft-santesson-digestbind-01.txt (work in progress),
              July 2008.

   [IABAUTH]  Rescorla, E., "A Survey of Authentication Mechanisms",
              draft-iab-auth-mech-05.txt (work in progress),
              February 2006.

   [PWDHASH]  Ross, B., Jackson, C., Miyake, N., Boneh, D., and J.
              Mitchell, "Stronger Password Authentication Using Browser
              Extensions", Proceedings 14th Usenix Security Symposium,
              2005, <http://crypto.stanford.edu/PwdHash/pwdhash.pdf>.

   [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.

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   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, June 2006.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5090]  Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
              and W. Beck, "RADIUS Extension for Digest Authentication",
              RFC 5090, February 2008.

   [SECIND]   Schechter , S., Dhamija , R., Ozment, A., and I. Fischer,
              "The Emperor's New Security Indicators: An evaluation of
              website authentication and the effect of role playing on
              usability studies", IEEE Symposium on Security and
              Privacy, May 2007, <http://www.deas.harvard.edu/~rachna/

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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

   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

   Note to rfc editor: This section should be removed prior to

B.1.  Changes Since 08

   Propose a new purpose section.  Also, add a note describing what has
   been done to date on these issues.

B.2.  Changes since 07

      Reword the abstract not to talk about identity providers

      Define identity provider.  I'm moving away from using it except
      where necessary, but I think that there a couple of cases where
      the term is helpful rather than confusing.

      Add a paragraph to the introduction helping to define how this
      work fits in with other work.

      Significantly rework the mutual authentication requirement to
      describe why pwdhash is excluded, to give more motivation and to
      try and clarify that authentication at different layers is

      Rework the requirement for binding authentication to requests and
      responses.  The discussion of channel binding was obsolete and has
      been updated based on advances in that area.  Drop the comment
      about redirect based schemes, because that depends on certificate
      validation and the W3C guidelines recommend accepting self-signed
      certificates in some cases.

      Remove most references to identity providers from restricted
      identities section and protecting enrollment section.  The
      concepts don't actually depend on whether an identity provider is

      Rework the section on finding the right server to provide a more
      accurate description of image hints prior to login and to discuss
      the uncertainty surrounding the effectiveness of strategies

      Rephrase terminology in security considerations to be consistent
      with changes throughout the rest of the document.  Refer to the
      W3C guidelines as appropriate.

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B.3.  Changes since 06

      Much expanded description of concerns about weak password
      equivalents.  They are not excluded except by the mutual
      authentication requirement.  However there are significant scoping
      issues with them.

      Clarify that the effectiveness of confidential information being
      used to strengthen mutual authentication depends on users taking
      advantage of that.

      Continue to clarify differences between plaintext password
      protocols and the password user interface

      Reduce the use of the term identity provider; it's not entirely
      clear that concept needs to be worked in here and right now
      identity provider is an undefined term

      The text on how to make trusted UIs sounded very authoritative;
      that was not the intent, so rework that text.

B.4.  Changes since 05

      Clarified introduction to distinguish what happens at the TLS
      layer and what at the HTTP layer.  Discuss motivation of phishing

      In the introduction, restate claims to be more accurate.  These
      requirements are useful if users actually use the authentication
      mechanisms; convincing them to do so and making it obvious whether
      they are is a significant risk.  Also, we may give them the
      theoretical information necessary to detect fraud, but whether
      they act on that is open.

      Add a purpose of this memo section.  Whatever text ends up there
      after community discussion needs to be called out in the last

      Add a section calling out the difference between plaintext
      password protocols and password interface.  This needs to be
      worked into the rest of the document.

      Update the threat model.  Significant hopefully clarifying

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B.5.  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.6.  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

      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
   Painless Security, LLC

   Email: hartmans-ietf@mit.edu

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