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Versions: 00 01 02 draft-ietf-httpauth-hoba

Network Working Group                                         S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Standards Track                              P. Hoffman
Expires: April 8, 2013                                    VPN Consortium
                                                               M. Thomas
                                                               Phresheez
                                                         October 5, 2012


                HTTP Origin-Bound Authentication (HOBA)
                     draft-farrell-httpbis-hoba-02

Abstract

   HTTP Origin-Bound Authentication (HOBA) is a design for an HTTP
   authentication method with credentials that are not vulnerable to
   phishing attacks, and that does not require a server-side password
   database.  The design can also be used in Javascript-based
   authentication embedded in HTML.  HOBA is an alternative to HTTP
   authentication schemes that require passwords with all the negative
   attributes that come with password-based systems.  HOBA can be
   integrated with account management and other applications running
   over HTTP and supports portability, so a user can associate more than
   one device or origin-bound key with the same service.  We also
   describe a way in which the HOBA design can be used from a Javascript
   web client.  When deployed, HOBA will be a drop-in replacement for
   password-based HTTP authentication or JavaScript authentication.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 8, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Comparison of HOBA and Current Password Authentication . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  HOBA for Both HTTP Authentication and JavaScript . . . . . . .  6
   3.  HOBA HTTP Authentication Mechanism . . . . . . . . . . . . . .  7
   4.  Using HOBA-http  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  CPK Preparation Phase  . . . . . . . . . . . . . . . . . .  8
     4.2.  Signing Phase  . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Authentication Phase . . . . . . . . . . . . . . . . . . .  8
     4.4.  Logging in on a New User Agent . . . . . . . . . . . . . .  9
   5.  Using HOBA-js  . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Key Storage  . . . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  User Join  . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.3.  User Login . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Enrolling a New User Agent . . . . . . . . . . . . . . . . 11
     5.5.  Replay Protection  . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Signature Parameters . . . . . . . . . . . . . . . . . . . 12
     5.7.  Session Management . . . . . . . . . . . . . . . . . . . . 13
     5.8.  Multiple Accounts on One User Agent  . . . . . . . . . . . 14
     5.9.  Oddities . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Additional Services  . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Registration . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Associating Additional Keys to an Exiting Account  . . . . 15
     6.3.  Logging Out  . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  Mandatory-to-Implement Algorithms  . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     8.1.  localStorage Security for Javascript . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  HOBA Authentication Scheme . . . . . . . . . . . . . . . . 17
     9.2.  .well-known URLs . . . . . . . . . . . . . . . . . . . . . 17
     9.3.  Hash names . . . . . . . . . . . . . . . . . . . . . . . . 18
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Problems with Passwords . . . . . . . . . . . . . . . 19



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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19


















































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1.  Introduction

   [[ Commentary is in double-square brackets, like this.  As you'll see
   there are a bunch of details still to be figured out.  Feedback on
   those is very welcome.  Also note that the authors fully expect that
   the description of HOBA-http and HOBA-js to be mostly merged in the
   draft; they're both here now so readers can see some alternatives and
   maybe support particular proposals. ]]

   HTTP Origin-Bound Authentication (HOBA) is a proposal for a new
   authentication design that can be used as an HTTP authentication
   scheme and for Javascript-based authentication embedded in HTML.  The
   main goal of HOBA is to offer an easy-to-implement authentication
   scheme that is not based on passwords.  If deployment of HOBA reduces
   the number of password entries in databases by any appreciable
   amount, then it would be worthwhile.  As an HTTP authentication
   scheme, it would work in the current HTTP 1.0 and HTTP 1.1
   authentication framework, and will very likely work with whatever
   changes are made to the HTTP authentication scheme in HTTP 2.0.  As a
   JavaScript design, HOBA demonstrates a way for clients and servers to
   interact using the same credentials that are use by the HTTP
   authentication scheme.

   The HTTP specification defines basic and digest authentication
   methods for HTTP that have been in use for many years, but which,
   being based on passwords, are susceptible to theft of server-side
   databases.  (See [RFC2617] for the original specification, and
   [I-D.ietf-httpbis-p7-auth] for clarifications and updates to the
   authentication mechanism.)  Even though few large web sites use basic
   and digest authentication, they still use username/password
   authentication and thus have large susceptible server-side databases
   of passwords.

   Instead of passwords, HOBA uses digital signatures as an
   authentication mechanism.  HOBA also adds useful features such as
   credential management and session logout.  In HOBA, the client
   creates a new public-private key pair for each host ("web-origin") to
   which it authenticates; web-origins are defined in [RFC6454].  These
   keys are used in HOBA for HTTP clients to authenticate themselves to
   servers in the HTTP protocol or in a Javascript authentication
   program.  HOBA keys need not be stored in public key certificates,
   but instead in subjectPublicKeyInfo structures from PKIX [RFC5280].
   Because these are generally "bare keys", there is none of the
   semantic overhead of PKIX certificates, particularly with respect to
   naming and trust anchors.  Thus, client public keys ("CPKs") do not
   have any publicly-visible identifier for the user who possesses the
   corresponding private key, nor the web-origin with which the client
   is using the CPK.



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   HOBA also defines some services that are required for modern HTTP
   authentication:

   o  Servers can bind a CPK with an identifier, such as an account
      name.  HOBA allows servers to define their own policies for
      binding CPKs with accounts during account registration.

   o  Users are likely to use more than one device or user agent (UA)
      for the same HTTP based service, so HOBA gives a way to associate
      more than one CPK to the same account, but without having to
      register for each separately.

   o  Users are also likely to lose a private key, or the client's
      memory of which key pair is associated with which origin.  For
      example if a user loses the computer or mobile device in which
      state is stored.  HOBA allows for clients to tell servers to
      delete the association between a CPK and an account.

   o  Logout features can be useful for user agents, so HOBA defines a
      way to close a current HTTP "session", and also a way to close all
      current sessions, even if more than one session is currently
      active from different user agents for the same account.

1.1.  Comparison of HOBA and Current Password Authentication

   [[ This will be a few paragraphs explaining how HOBA can be used as a
   drop-in replacement for the common form-and-cookie authentication
   used today.  It will show how similar many of the concepts are, and
   also point out some of the advantages sites will get by changing to
   HOBA. ]]

1.2.  Terminology

   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 RFC 2119 [RFC2119].

   A client public key ("CPK") is the public key and associated
   cryptographic parameters needed for a server to validate a signature.

   The term "account" is (loosely) used to refer to whatever data
   structure(s) the server maintains that are associated with an
   identity.  That will contain of at least one CPK and a web-origin; it
   will also optionally include an HTTP "realm" as defined in the HTTP
   authentication specification.  It might also involve many other non-
   standard pieces of data that the server accumulates as part of
   account creation processes.  An account may have many CPKs that are
   considered equivalent in terms of being usable for authentication,



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   but the meaning of "equivalent" is really up to the server and is not
   defined here.

   When describing something that is specific to HOBA as an HTTP
   authentication mechanism or HOBA as a JavaScript implementation, this
   document uses the terms "HOBA-http" and "HOBA-js", respectively.

   Web client: the content and javascript code that run within the
   context of a single user agent instance (such as a tab in a web
   browser).

   User agent (UA): typically, but not always, a web browser doing HOBA.

   User: a person who is running a UA.  In this document, "user" does
   not mean "user name" or "account name".

   [[This specification may later use the Augmented Backus-Naur Form
   (ABNF) notation of [RFC5234].  Or maybe not. ]]


2.  HOBA for Both HTTP Authentication and JavaScript

   A UA that implements HOBA maintains a list of web-origins and realms.
   The UA also maintains one or more client credentials for each web-
   origin/realm combination for which it has created a CPK.

   [[We've discussed whether or not realms are needed.  They may
   disappear if they're not.]]

   On receipt of a challenge from a server, the client marshals a to-be-
   signed blob that includes the web-origin name, the realm, and the
   challenge string; and signs that hashed blob using the hash algorithm
   identified with the challenge and the private key corresponding to
   the CPK for that web-origin.  The client concatenates the signed blob
   with the CPK identifier that the client and host agreed on for the
   client.  This is called the "client response". [[ Note: this is just
   an illustrative first cut at a challenge-response protocol, real
   design and analysis is needed for this, e.g. for security, algorithm
   agility, etc.  Ideally we can just adopt something that already has
   some security proofs.  Expect changes here.]]

   HOBA will support the idea of multiple users on the same user agent.
   This will be useful for the problem of "can I use your browser to
   check my mail..." and so on.  It is [[ currently ]] described only in
   the HOBA-js section, but will apply equally to HOBA-http.  [[There
   are implications beyond the discussion in HOBA-js here in that there
   would only be a single CPK for a set of users for a given origin
   since normative HOBA-http has no clue at all about users and the



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   like.  This needs more thought.]]


3.  HOBA HTTP Authentication Mechanism

   An HTTP server that supports HOBA authentication includes the "hoba"
   auth-scheme value in a WWW-Authenticate header field when it wants
   the client to authenticate with HOBA.

   o  If the "hoba" scheme is listed, it MUST be followed by two or more
      auth-param values.  The auth-param attributes defined by this
      specification are below.  Other auth-param attributes MAY be used
      as well.  Unknown auth-param attributes MUST be ignored by
      clients, if present.

   o  The "challenge" attribute MUST be included.  The challenge is a
      string of characters that the server wants the client to sign in
      its response.  The challenge SHOULD be unique for every HTTP 401
      response in order to prevent replay attacks from passive
      observers.  [[How or if replay detection is specified is TBD.]]

   o  The "hash" attribute MUST be included.  This is the name of the
      hash algorithm that the server wants the client to use in signing
      challenge.  The valid names for hash algorithms are listed in [[
      some IANA registry, maybe same as DKIM ]].

   o  Note that a hash here is sufficient, given the assumption that the
      client and server have already agreed (or are about to agree) the
      public key and asymmetric algorithm for the CPK.

   o  A "realm" attribute MAY be included to indicate the scope of
      protection in the manner described in HTTP/1.1, Part 7
      [I-D.ietf-httpbis-p7-auth].  The "realm" attribute MUST NOT appear
      more than once.

   When the "client response" is created, the HOBA-http client encodes
   the result as a b64token and returns that b64token in the
   Authorization header.

   The HOBA-http authentication mechanism allows for the use of cookies
   for preserving state between protected resources in one HTTP realm.
   This means that the server need only send the WWW-Authenticate header
   field once, and can rely on cookie management for keeping state.


4.  Using HOBA-http

   [[A lot of this is similar to the HOBA-js discussion below.  At some



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   point some nuclear fusion might be nice, but for now it might be best
   to keep them separate until we understand better what can be merged,
   and what is different.]]

   The interaction between an HTTP client and HTTP server using HOBA
   happens in three phases: the CPK preparation phase, the signing
   phase, and the authentication phase.  The first and second phase are
   done in a standard fashion; the third is done using site-specific
   methods.

   [[ Need to describe what happens if the user bails half way through
   the flow. ]]

4.1.  CPK Preparation Phase

   In the CPK preparation phase, the client determines if it already has
   a CPK for the web-origin it is going to.  If the has a CPK, the
   client will use it; if the client does not have a CPK, it generates
   one in anticipation of the server asking for one.

4.2.  Signing Phase

   In the signing phase, the client connects to the server, the server
   asks for HOBA-based authentication, and the client authenticates by
   signing a blob of information as described in the previous sections.

   The user agent tries to access a protected resource on the server.
   The server sends the HOBA WWW-Authenticate challenge.  The user agent
   receives the challenge and signs the challenge using the CPK it
   either already had or just generated.  The server validates the
   signature.  If validation fails, the server aborts the transaction.
   [[ Or maybe it asks again? ]]

4.3.  Authentication Phase

   In the authentication phase, the server extracts the CPK from the
   signing phase and decides if it recognizes the CPK.  If the server
   recognizes the CPK, the server may finish the client authentication
   process.  If the process involves a second factor of authentication,
   such as asking the user which account it wants to use (in the case
   where a user agent is used for multiple accounts on a site), the
   server may prompt the user for the account identifying information.
   None of this is standardized: it all follows the server's security
   policy and session flow.  At the end of this, the server probably
   assigns or updates a session cookie for the client.

   If the server does not recognize the CPK the server might send the
   client through a either a join or login-new-user-agent (see below)



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   process.  This process is completely up to the server, and probably
   entails using HTML, JavaScript and CSS to ask the user some questions
   in order to assess whether or not the server wants to give the client
   an account.  Completion of the joining process might entail require
   confirmation by email, SMS, Captcha, and so on.

   Note that there is no necessity for the server to initiate a joining
   or login process upon completion of the signing phase.  Indeed, the
   server may desire to challenge the user agent even for unprotected
   resources and carry along the CPK in a session cookie for later use
   in a join or login process as it becomes necessary.  For example, a
   server might only want to offer an account to someone who had been to
   a few pages on the web site; in such a case, the server could use the
   CPK from an associated session cookie as a way of building reputation
   for the user until the server wants the user to join.

   After the UA is authenticated (if the user had to join, this could be
   the last step of joining), the server gives the UA access to the
   protected resource that was originally requested at the beginning of
   the signing phase.  It is quite likely that the server would also
   update the UA's session cookie for the web site.

4.4.  Logging in on a New User Agent

   When a user wants to use a new user agent for an existing account,
   the flows are similar to logging in with an already-joined UA or
   joining for the first time.  In fact, the CPK preparation phase (with
   the UA knowing that it needs to create a new CPK) and the signing
   phase are identical.

   During the authentication phase, the server could use HTML,
   JavaScript and CSS to ask the user if they are really a new user or
   want to associate this new CPK with an already-joined CPK.  The
   server can then use some out-of-band method (such as a confirmation
   email round trip, SMS, or an UA that is already enrolled) to verify
   that the "new" user is the same as the already-enrolled one.


5.  Using HOBA-js

   [[ A description of how to use the same HOBA semantics, but doing
   everything in Javascript in a web page.  This is more of a
   demonstration that you could get the similar semantics via JS rather
   than a normative section.]]

   Web sites using javascript can also perform origin-bound
   authentication without needing to involve the http layer, and by
   inference not needing HOBA-specific support in browsers.  One element



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   is required: localStorage (see http://www.w3.org/TR/webstorage/), and
   one when it is available will be highly desirable: WebCrypto (see
   http://www.w3.org/TR/WebCryptoAPI).  In lieu of WebCrypto, javascript
   crypto libraries can be employed with the known deficiencies of PRNG,
   and the general immaturity of those libraries.  The following section
   outlines a mechanism for Javascript HOBA clients to initially enroll,
   subsequent enrollment on new clients, login, and how HOBA-js relates
   to web based session management.  As with HOBA-http, a pure
   Javascript implementation retains the property that only CPKs are
   stored on the server, so that server compromise doesn't suffer the
   multiplier affect that the various recent password exposure debacles
   have vividly demonstrated.

5.1.  Key Storage

   We use the new HTML 5 webstorage feature that is now widely
   available.  Conceptually an implementation stores in the origin's
   localStorage dictionary account identifier, public key, private key
   tuples for subsequent authentication requests.  How this is actually
   stored in localStorage is an implementation detail.  We rely on the
   security properties of the same-origin policy that localStorage
   enforces.  See the security considerations for discussion about
   attacks on localStorage.

5.2.  User Join

   To join a web site, the HOBA-js client generates a public/private key
   pair and takes as input the account identifier to which the key pair
   should be bound.  The key pair and account identifier are stored in
   localStorage for later use.  The user agent then signs the join
   information (see below) using the private key, and forms a message
   with the public key (CPK) and the signed data.  The server receives
   the message and verifies the signed data using the supplied key.  The
   server creates the account and adds the public key to a list of
   public keys associated with this account.

5.3.  User Login

   Each time the user needs to log in to the server, it creates a login
   message (see below) and signs the message using the relevant private
   key stored in localStorage.  The signed login message along with the
   associated CPK identifier is sent to the server.  The server receives
   the message and verifies the signed data.  If the supplied public key
   is amongst the set of valid public keys for the supplied account,
   then the login proceeds.  See below for a discussion about replay.






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5.4.  Enrolling a New User Agent

   When a user wants to start using a different UA, the website has two
   choices: use a currently enrolled UA to permit the enrollment or use
   a trusted out of band mechanism (eg email, sms, etc).  To enroll a
   new UA using an existing UA, the web site can display a one-time
   password on the currently enrolled UA.  This password is a one-time
   password and expires in a fixed amount of time (say, 30 minutes).  It
   doesn't need to be an overly fussy password since it's one-time and
   times out quickly.  The user then inputs the one-time password and
   the new UA generates a new asymmetric key pair and includes the one-
   time password in the login message to the server (see below).

   Alternatively if an enrolled UA is not available, and the site has an
   out of band communication mechanism (eg, sms, email, etc) a user can
   request that a one-time password be sent to the user.  The server
   generates and stores the one-time password as above.  The user
   receives the one-time password, inputs as above on the new UA, and
   the HOBA-js client forms the login message as above.

   In both cases, when the server receives a login message with a one-
   time password, it checks to see if the password supplied is in a list
   of unexpired one-time passwords associated with that account.  If the
   password matches, the server verifies the signature, expires or
   deletes the one-time password and adds the supplied public key to the
   list of public keys associated with the user assuming the signature
   verified correctly.  Subsequent logins proceed as above in User
   Login.

5.5.  Replay Protection

   To guard against replay of a legitimate login/join message, we use
   Kerberos-like timestamps in the expectation of synchronization
   between the browser's and server's clocks is sufficiently reliable.
   This saves an HTTP round trip which is desirable, though a challenge-
   response mechanism as in HOBA-http could also be used.  The client
   puts the current system time into the URL, and the server side vets
   it against its system time.  Like Kerberos, a replay cache covering a
   signature timeout window is required on the server.  This can be done
   using a database table that is keyed (in the database sense of the
   term) using the signature bits.  If the signature is in the replay
   table, it ought be rejected.  If the timestamp in the signature is
   outside the current replay cache window then it also gets rejected.

   [[ An addition of the ability for the server to reject a client with
   potential time skew and give it a nonce (as with HOBA-http) would
   allow the size of the replay cache to be set to just a few minutes
   rather than a much longer period.  Or the HOBA server could always



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   use a nonce method.  This is worthy of more discussion. ]].

5.6.  Signature Parameters

   Since we only require agreement between the server and the client
   where the client is under the control of the server, the actual url
   parameter names here are only advisory.  For each signed url, the
   client forms a url with the necessary login/join information.  For
   example, suppose example.com has login and join scripts with various
   parameters:

   o  http://example.com/site/login.php?username=Mike

   o  http://example.com/site/
      join.php?username=Mike&email=mike@example.com&sms=555.1212

   The client then appends a signature parameter block to the url:

   o  curtime: the time in milliseconds since unix epoch (ie, new Date
      ().getTime ()).

   o  pubkey: the url encoded public key.  See DKIM for the format of
      the base64 encoded PEM formated key.

   o  temppass: an optional url encoded one-time password for subsequent
      enrollment.

   o  keyalg: currently RSA. 2048 bit keys should be use if WebCrypto is
      available

   o  digestalg: currently SHA1.  SHA256 should be used if WebCrypto is
      available.

   o  signature: empty for signing canonicalization purposes

   [[ Signing the full url is problematic with PHP; we should take a
   clue from what OAUTH does here; we almost certainly need to add some
   host identifying information...]]  To create the signature, the
   canonical text includes the path portion, the site-specific url
   parameters and appends a signature block onto the end of the url.
   The signature block consists of the parameters listed above with an
   empty signature parameter (ie, signature=), eg:

   o  Login: /site/
      login.php?username=Mike&curtime=1234567890.1234&keyalg=RSA&
      digestalg=SHA1&signature=





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   o  Join: /site/
      join.php?username=Mike&email=mike@
      example.com&curtime=1234567890.1234&keyalg=RSA&digestalg=SHA1&
      signature=

   o  Login New User Agent: /site/
      login.php?username=Mike&curtime=1234567890.1234&temppass=1239678&
      keyalg=RSA&digestalg=SHA1&signature=

   The canonical signature text is then signed with the private key
   associated with the account.  The signature is then base64 encoded
   and appended to the full url, and sent to the server using
   XMLHttpRequest as usual.  On receipt of the login request, the server
   first extracts the timestamp (curtime) and determines whether the
   timestamp is fresh (see above) rejecting the request if stale.  The
   server then removes the scheme and domain:port portion of the
   incoming url, and removes the signature value only to create the
   canonical signature text.  The server then extracts the public key
   along with the account and verifies the signature.  If the signature
   verifies, the server then determines whether this is an enrolled
   public key for the user.  If it is, login/join succeeds.  If the key
   is not enrolled, the server then checks to see if a one-time password
   was supplied.  If not, login/join fails.  If a one-time password was
   supplied, the server checks to see if a one-time password is valid
   and fails if not.  If valid, the server disables the one-time
   password (eg, deletes it from its database) and adds the new public
   key to the list of enrolled public keys for this user.

   Once verified, the server may start up normal cookie-based session
   management (see below).  The server should send back status to the
   HOBA-js client to determine whether the login/join was successful.
   The details are left as an implementation detail.

   Note: the client SHOULD use an HTTP POST for the XMLHttpRequest as
   both the public key and signature blocks may exhaust the maximum size
   for a GET request (typically around 2KB).

5.7.  Session Management

   Session Management is identical to username/password session
   management.  That is, the session management tool (such as PHP,
   Python CGI, and so on) inserts a session cookie into the output to
   the browser, and logging out simply removes the session cookie.
   HOBA-js does nothing to help or hurt session cookie hijacking -- TLS
   is still our friend.






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5.8.  Multiple Accounts on One User Agent

   A shared UA with multiple accounts is possible if the account
   identifier is stored along with the asymmetric key pair binding them
   to one another.  Multiple entries can be kept, one for each account,
   and selected by the current user.  This, of course, is fraught with
   the possibility for abuse, since you're enrolling the device
   potentially long-term.  A couple of things can possibly be done to
   combat that.  First, the user can request that the credential be
   erased from keystore.  Similarly, in the enrollment phase, a user
   could request that the key pair only be kept for a certain amount of
   time, or that it not be stored at all.  Last, it's probably best to
   just not use shared devices at all since that's never especially
   safe.

5.9.  Oddities

   With the same-origin policy, subdomains do not have access to the
   same localStorage as parent domains do.  For larger/more complex
   sites this could be an issue that requires enrollment into subdomains
   with the requisite hassle for users.  One way to get around this is
   to use session cookies as they can be used across subdomains.  That
   is, login using a single well-known domain, and then use session
   cookies to navigate around a site.


6.  Additional Services

   HOBA uses a well-known URL [RFC5785] "hoba" as a base URI for
   performing many tasks: "https://www.example.com/.well-known/hoba".
   These URLs are based on the name of the host that the HTTP client is
   accessing.  There are many use cases for these URLs to redirect to
   other URLs: a site that does registration through a federated site, a
   site that only does registration under HTTPS, and so on.  Like any
   HTTP client, HOBA clients MUST be able to handle redirection of these
   URLs.  [[There are a bunch of security issues to consider related to
   cases where a re-direct brings you off-origin.]]

   All additional services MUST be done in TLS-protected sessions
   ([RFC5246]).

6.1.  Registration

   Normally, a registration is expected to happen after a UA receives a
   WWW-Authenticate for a web-origin and realm for which it has no
   associated CPK.  The (protocol part of the) process of registration
   for a HOBA account on a server is relatively light-weight.  The UA
   generates a new key pair, and associates it with the web-origin/realm



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   in question.  The UA sets up a TLS-protected session, goes to the
   registration URL ".well-known/hoba/register", and submits the CPK
   using a POST message. [[ More description is clearly needed here. ]]
   It is up to the server to decide what kind of user interaction is
   required before the account is finally set up.

   If the UA has a CPK associated with the web-origin, but not for the
   realm concerned, then a new registration is REQUIRED.  If the server
   did not wish for that outcome, then it ought not use a different
   realm.

   The POST message sent to the registration URL has one parameter,
   called "cpksubmit", which contains the CPK that the UA will use for
   the origin/realm combination.  The CPK MUST be sent base64 encoded.
   The value that is base64 encoded is the DER encoding of the
   subjectPublicKeyInfo structure that is the CPK.  See [RFC5280] for
   details of that data structure.

6.2.  Associating Additional Keys to an Exiting Account

   It is common for a user to have multiple UAs, and to want all those
   UAs to be able to authenticate to a single account.  One method to
   allow a user who has an existing account to be able to authenticate
   on a second device is to securely transport the private and public
   keys and the origin information from the first device to the second.
   Previous history with such key transport has been spotty at best.  As
   an alternative, HOBA allows associating a CPK from the second device
   to the account created on the first device.

   Instead of registering on the new device, the UA generates a new key
   pair, associates it with the web-origin/realm in question, goes to
   the URL for starting an association, ".well-known/hoba/
   associate-start" in a TLS-protected session, and submits the new CPK
   using a POST message. [[ More description is clearly needed here. ]]
   The server's response to this request is a nonce with at least 128
   bits of entropy.  That nonce SHOULD be easy for the user to copy and
   type, such as using Base32 encoding (see [RFC4648]).  The user then
   uses the first UA to log into the origin, goes to the URL for
   finishing an association, ".well-known/hoba/associate-finish", and
   submits the nonce using a POST message. [[ More description is
   clearly needed here. ]].  The server then knows that the
   authenticated user is associated with the second CPK.  The server can
   choose to associate the two CPKs with one account.  Whether to do so
   is entirely at the server's discretion however, but the server SHOULD
   make the outcome clear to the user.






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6.3.  Logging Out

   When the user wishes to logout, the UA simply goes to ".well-known/
   hoba/logout".  The UA MAY also delete session cookies associated with
   the session.  [[Is that right?, maybe a SHOULD- or MUST-delete would
   be better]]

   The server-side MUST NOT allow TLS session resumption for any logged
   out session and SHOULD also revoke or delete any cookies associated
   with the session.


7.  Mandatory-to-Implement Algorithms

   [[ We should list two signature schemes (most likely RSA and ECDSA
   with P256).  We should list two hash algorithms (most likely SHA-256
   and SHA-384). ]]


8.  Security Considerations

   If key binding was server-selected then a bad actor could bind
   different accounts belonging to the user from the network with
   possible bad consequences, especially if one of the private keys was
   compromised somehow.

   Binding my CPK with someone else's account would be fun and
   profitable so SHOULD be appropriately hard.  In particular the string
   generated by the server MUST be hard to guess, for whatever level of
   difficulty is chosen by the server.  The server SHOULD NOT allow a
   random guess to reveal whether or not an account exists.

   [[The potential impact on privacy of HOBA needs to be addressed.  If
   a site can use a 401 and a CPK to track users without permission that
   would be not-so-nice so some guidance on how a UA could indicate to a
   user that HOBA stuff is going on might be needed.]]

   [[lots more TBD, be nice to your private keys etc. etc.]]

8.1.  localStorage Security for Javascript

   Our use of localStorage will undoubtedly be a cause for concern.
   localStorage uses the same-origin model which says that the scheme,
   domain and port define a localStorage instance.  Beyond that, any
   code executing will have access to private keying material.  Of
   particular concern are XSS attacks which could conceivably take the
   keying material and use it to create user agents under the control of
   an attacker.  But XSS attacks are in reality across the board



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   devastating since they can and do steal credit card information,
   passwords, perform illicit acts, etc, etc.  It's not clear that we
   introduce unique threats from which clear text passwords don't
   already suffer.

   Another source of concern is local access to the keys.  That is, if
   an attacker has access to the UA itself, they could snoop on the key
   through a javascript console, or find the file(s) that implement
   localStorage on the host computer.  Again it's not clear that we are
   worse in this regard because the same attacker could get at browser
   password files, etc too.  One possible mitigation is to encrypt the
   keystore with a password/pin the user supplies.  This may sound
   counter intuitive, but the object here is to keep passwords off of
   servers to mitigate the multiplier effect of a large scale compromise
   ala LinkedIn because of shared passwords across sites.

   It's worth noting that HOBA uses asymmetric keys and not passwords
   when evaluating threats.  As various password database leaks have
   shown, the real threat of a password breach is not just to the site
   that was breached, it's all of the sites a user used the same
   password on too.  That is, the collateral damage is severe because
   password reuse is common.  Storing a password in localStorage would
   also have a similar multiplier effect for an attacker, though perhaps
   on a smaller scale than a server-side compromise: one successful
   crack gains the attacker potential access to hundreds if not
   thousands of sites the user visits.  HOBA does not suffer from that
   attack multiplier since each asymmetric key pair is unique per site/
   useragent/user.


9.  IANA Considerations

9.1.  HOBA Authentication Scheme

   Authentication Scheme Name: hoba

   Pointer to specification text: [[ this document ]]

   Notes (optional): The HOBA scheme can be used with either HTTP
   servers or proxies.  [[But we need to figure out the proxy angle;-)]]

9.2.  .well-known URLs

   TBD







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9.3.  Hash names

   TBD, hopefully re-use and existing registry

   We probably want a new registry for the labels beneath .well-known/
   hoba so that other folks can add additional features in a controlled
   way, e.g. for CPK/account revocation or whatever.


10.  Acknowledgements

   [[TBD]]


11.  References

11.1.  Normative References

   [I-D.ietf-httpbis-p7-auth]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-21
              (work in progress), October 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              December 2011.

11.2.  Informative References

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.



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   [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.


Appendix A.  Problems with Passwords

   By far the most common mechanism for web authentication is passwords
   that can be remembered by the user, called "memorizable passwords".
   There is plenty of good research on how users typically use
   memorizable passwords ([[ handful of citations goes here ]]), but
   some of the highlights are that users typically try hard to reuse
   passwords on as many web sites as possible, and that web sites often
   use either email addresses or users' names as the identifier that
   goes with these passwords.

   If an attacker gets access to the database of memorizable passwords,
   that attacker can impersonate any of the users.  Even if the breach
   is discovered, the attacker can still impersonate users until every
   password is changed.  Even if all the passwords are changed or at
   least made unusable, the attacker now possesses a list of likely
   username/password pairs that might exist on other sites.

   Using memorizable passwords on unencrypted channels also poses risks
   to the users.  If a web site uses either the HTTP Plain
   authentication method, or an HTML form that does no cryptographic
   protection of the password in transit, a passive attacker can see the
   password and immediately impersonate the user.  If a hash-based
   authentication scheme such as HTTP Digest authentication is used, a
   passive attacker still has a high chance of being able to determine
   the password using a dictionary of known passwords.

   [[ Say a bit about non-memorizable passwords.  Still subject to
   database attack, although that doesn't give the attacker knowledge
   for other systems.  Safe if digest authentication is used, but that's
   rare. ]]














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Authors' Addresses

   Stephen Farrell
   Trinity College Dublin
   Dublin,   2
   Ireland

   Phone: +353-1-896-2354
   Email: stephen.farrell@cs.tcd.ie


   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


   Michael Thomas
   Phresheez

   Email: mike@phresheez.com






























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