Network Working Group                                         S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Experimental                                 P. Hoffman
Expires: October 20, 2014                                 VPN Consortium
                                                               M. Thomas
                                                          April 18, 2014

                HTTP Origin-Bound Authentication (HOBA)


   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 any 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.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 October 20, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Interfacing to Applications (Cookies) . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  The HOBA Authentication Scheme  . . . . . . . . . . . . . . .   5
   3.  Introduction to the HOBA-http Mechanism . . . . . . . . . . .   7
   4.  Introduction to the HOBA-js Mechanism . . . . . . . . . . . .   8
   5.  HOBA's Authentication Process . . . . . . . . . . . . . . . .   9
     5.1.  CPK Preparation Phase . . . . . . . . . . . . . . . . . .   9
     5.2.  Signing Phase . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Authentication Phase  . . . . . . . . . . . . . . . . . .   9
   6.  Other Parts of the HOBA Process . . . . . . . . . . . . . . .  10
     6.1.  Registration  . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Associating Additional Keys to an Exiting Account . . . .  13
       6.2.1.  Moving private keys . . . . . . . . . . . . . . . . .  13
       6.2.2.  One time password . . . . . . . . . . . . . . . . . .  13
       6.2.3.  Out of band . . . . . . . . . . . . . . . . . . . . .  14
     6.3.  Logging Out . . . . . . . . . . . . . . . . . . . . . . .  14
     6.4.  Getting a Fresh Challenge . . . . . . . . . . . . . . . .  14
   7.  Mandatory-to-Implement Algorithms . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     8.1.  localStorage Security for Javascript  . . . . . . . . . .  15
     8.2.  Multiple Accounts on One User Agent . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  HOBA Authentication Scheme  . . . . . . . . . . . . . . .  16
     9.2.  .well-known URLs  . . . . . . . . . . . . . . . . . . . .  16
     9.3.  Algorithm Names . . . . . . . . . . . . . . . . . . . . .  16
     9.4.  Key Identifier Types  . . . . . . . . . . . . . . . . . .  17
     9.5.  Device Identifier Types . . . . . . . . . . . . . . . . .  17
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  17
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Problems with Passwords  . . . . . . . . . . . . . .  19
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   [[ Commentary is in double-square brackets, like this.  Fewer things
   this time.]]

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   HTTP Origin-Bound Authentication (HOBA) is an authentication design
   that can be used as an HTTP authentication scheme
   [I-D.ietf-httpbis-p7-auth] 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, but
   that can easily replace HTTP or HTML forms-based password
   authentication.  Deployment of HOBA can reduce or eliminate password
   entries in databases, with potentially significant security benefits.

   HOBA is an HTTP authentication mechanism that complies with the
   framework for such schemes [I-D.ietf-httpbis-p7-auth].  As a
   JavaScript design, HOBA demonstrates a way for clients and servers to
   interact using the same credentials that are used by the HTTP
   authentication scheme.

   Current HTTP authentication methods (Basic and Digest), as well as
   username/password authentication using web forms, have been in use
   for many years, but being based on passwords, are susceptible to
   theft of server-side databases.  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" [RFC6454]) to which it authenticates.  These keys are
   used in HOBA for HTTP clients to authenticate themselves to servers
   in the HTTP protocol or in a Javascript authentication program.

   Session management with HOBA is identical to username/password
   session management.  Typically, the session management tool (such as
   PHP, Python CGI, and so on) inserts a session cookie into the output
   to the browser.  HOBA does nothing to help or hurt session cookie
   hijacking; TLS is still required for that.  If the authentication is
   not bound to HTTP sessions, even TLS cannot help against all attacks.

   HOBA keys are "bare keys", so there is no need for the semantic
   overhead of PKIX certificates, particularly with respect to naming
   and trust anchors.  The client public key ("CPK") structures in HOBA
   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.

   HOBA also defines some services that are required for modern HTTP

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

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   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, such as
      when 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 an existing CPK and an account.

   o  Logout features can be useful for UAs, 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 UAs for the same account.

   o  Since there are always devices and applications in which state of
      the art digital signature mechanism runtimes are significant, and
      since HTTP authentication in theory requires that every HTTP
      request to a given realm have a signature in an "Authorization"
      header field, and since HOBA is a challenge response scheme, we
      also define a way in which HTTP servers can indicate the duration
      for which they will consider a given challenge value to be valid.
      As a consequence we also define a way for UAs to fetch a fresh

1.1.  Interfacing to Applications (Cookies)

   HOBA can be used as a drop-in replacement for password-based user
   authentication schemes used in common web applications.  The simplest
   way in which this can be done is to (re-)direct the UA to a HOBA
   "Login" URL and for the response to a successful HTTP request
   containing a HOBA signature to set a session cookie [RFC6265].
   Further interactions with the web application will then be secured
   via the session cookie, as is commonly done today.

   While cookies are bearer tokens, and thus weaker than HOBA
   signatures, they are currently ubiquitously used.  If non-bearer
   token session continuation schemes are developed in future in the
   IETF or elsewhere, then those can interface to HOBA as easily as with
   any password based authentication scheme.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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   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,
   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 UA instance (such as a tab in a web browser).

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

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

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234]

2.  The HOBA Authentication Scheme

   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.

   On receipt of a challenge (and optional realm) from a server, the
   client marshals an HOBA to-be-signed (TBS) blob that includes a
   client generated nonce, the web-origin, the realm, an identifier for
   the CPK and the challenge string; and signs that hashed blob with the
   private key corresponding to the CPK for that web-origin.  The
   formatting chosen for this TBS blob is chosen so as to make server-
   side signature verification as simple as possible for a wide range of
   current server tooling.

   Figure 1 specifies the ABNF for the signature input.  The term
   "unreserved" means that the field does not have a specific format

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   HOBA-TBS = nonce alg origin [ realm ] kid challenge
   nonce = unreserved
   alg = 1*2DIGIT
   origin = scheme "://" authority ":" port
   realm = unreserved
   kid = unreserved
   challenge = unreserved

                   Figure 1: To-be-signed data for HOBA

   The fields above contain the following:

   o  nonce: is a random value chosen by the UA and MUST be base64url
      encoded before being included in the HOBA-TBS value. (base64url
      encoding is defined in [RFC4648].)  UAs MUST be able to use at
      least 32 bits of randomness in generating a nonce.  UAs SHOULD be
      able to use 64 or more bits of randomness for nonces.

   o  alg: specifies the signature algorithm being used encoded as an
      ASCII character as defined in Section 9.3.  RSA-SHA256 MUST be
      supported, RSA-SHA1 MAY be supported.  The IANA registered
      algorithm values are encoded as ASCII numbers; for example, the
      encoding of RSA-SHA256 is 0x30.

   o  origin: is the web origin expressed as the concatenation of the
      scheme, authority and port from [RFC3986].  These are not base64
      encoded as they will be most readily available to the server in
      plain text.  For example, if accessing the URL "https://" then the bytes input to the signature
      process will be "".  There is no
      default for the port number, and the port number MUST be present.

   o  realm: is similarly just a string with the syntactic restrictions
      defined in [I-D.ietf-httpbis-p7-auth].  If no realm is specified
      for this authentication then this is absent.  (A missing field
      here is no problem since both sides know when it needs to be

   o  kid: is a key identifier - this MUST be a base64url encoded value
      that is presented to the server in the HOBA client result (see

   o  challenge: MUST be a base64url encoded challenge value that the
      server chose to send to the client

   The HOBA-TBS string is the input to the client's signing process, but
   is not itself sent over the network since some fields are already
   inherent in the HTTP exchange.  The challenge however is sent over

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   the network so as to make it simpler for a server to be stateless.
   (One form of stateless challenge might be a ciphertext that the
   server decrypts and checks, but that is an implementation detail.)
   The value that is sent over the network is the HOBA "client result"
   which we now define.

   The HOBA "client result" is a dot-separated string that includes the
   signature and is sent in the HTTP Authorized header field value using
   the value syntax defined in Figure 2.  The "sig" value is the
   base64url encoded version of the binary output of the signing
   process.  The kid, challenge and nonce are as defined above and are
   also base64url encoded.

              HOBA-RES = kid "." challenge "." nonce "." sig
              sig = unreserved

                    Figure 2: HOBA Client Result value

   The HOBA scheme is far from new, for example, the basic idea is
   pretty much identical to the first two messages from "Mechanism R" on
   page 6 of [MI93] which predates HOBA by 20 years.

3.  Introduction to the HOBA-http 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.  Note that the HOBA auth-scheme
   might not be the only one that the server includes in a WWW-

   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 the
      string made up of the base64url encoded octets 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.

   o  An "expires" attribute MUST be included that specifies the number
      of seconds from the time the HTTP response is emitted for which
      responses to this challenge can be accepted.

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   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 UA encodes the HOBA
   client-result (a string matching the HOBA-RES production in Figure 2
   as an auth-param with the name "result" and returns that in the
   Authorization header.

   The server MUST check the cryptographic correctness of the signature
   based on a public key it knows for the kid in the signatures, and if
   the server cannot do that, or if the signature fails cryptographic
   checks, then validation has failed.  The server can use any
   additional mechanisms to validate the signature.  If the validation
   fails, or if the server chooses reject the signature for any reason
   whatsoever, the server aborts the transaction via a 403 Forbidden
   HTTP response. [[ This maybe should be 401 Unauthorized. ]]

   Note that a HOBA signature is good for however long the expires
   attribute allows.  This means that replay is possible within the time
   window specified by the "expires" value chosen by the server.
   Servers can attempt to detect any such replay and MAY react to such
   replays by responding with a second (or subsequent) 401-status HTTP
   response containing a new challenge.  [[ How could such detection be
   done in face of cut-and-paste attacks? ]]

   UAs MAY optimise their use of challenges by pre-fetching a challenge
   value, for example after expires/2 seconds have elapsed, using the
   ".well-known/hoba/getChal" scheme described later in this document.
   This also allows for pre-calculation of HOBA signatures, if that is
   required in order to produce a responsive user interface.

4.  Introduction to the HOBA-js Mechanism

   Web sites using JavaScript can also perform origin-bound
   authentication without needing to involve the HTTP layer, and by
   inference not needing HOBA-http support in browsers.  HOBA-js is not
   an on-the-wire protocol like HOBA-http is: instead, it is a design
   pattern that can be realized completely in JavaScript served in
   normal HTML pages.

   One element is required for HOBA-js: localStorage (see http:// from HTML5.  is used for persistent key
   storage.  For example, an implementation would store a dictionary
   account identifier, public key, private key tuples in the origin's
   localStorage for subsequent authentication requests.  How this
   information is actually stored in localStorage is an implementation

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   detail.  This type of key storage relies on the security properties
   of the same-origin policy that localStorage enforces.  See the
   security considerations for discussion about attacks on localStorage.

   Because of JavaScript's same-origin policy, scripts from subdomains
   do not have access to the same localStorage that scripts in their
   parent domains do.  For larger or more complex sites, this could be
   an issue that requires enrollment into subdomains, which could be a
   hassle for users.  One way to get around this is to use session
   cookies because they can be used across subdomains.  That is, with
   HOBA-js, the user might log in using a single well-known domain, and
   then the server uses session cookies to navigate around a site.

   Another element will be highly desirable for HOBA-js when it becomes
   available: WebCrypto (see  In
   lieu of WebCrypto, JavaScript crypto libraries can be employed with
   the known deficiencies of PRNG, and the general immaturity of those

5.  HOBA's Authentication Process

   This section describes how clients and servers use HOBA for
   authentication.  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.  This section also
   covers the actions that give HOBA similar user features as today's
   passwords have.

5.1.  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 client 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.

5.2.  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.

5.3.  Authentication Phase

   The authentication phase is completely dependent on the policies and
   practices of the server.  That is, this phase involves no
   standardized protocol in HOBA-http; in HOBA-js, there is no suggested
   interaction template.

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   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

   If this stage of the process involves additional information for
   authentication, such as asking the user which account she wants to
   use (in the case where a UA is used for multiple accounts on a site),
   the server can prompt the user for account identifying information or
   the user could choose based on HTML offered by the server before the
   401 is triggered.  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.

   During the authentication phase, if the server does not recognize the
   CPK, it could use HTML and JavaScript 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.  Thus, logging in on a new user agent is identical to
   logging in with an existing account.

   If the server does not recognize the CPK the server might send the
   client through a either a join or login-new-UA (see below) process.
   This process is completely up to the server, and probably entails
   using HTML and JavaScript 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 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 UA even for unprotected resources
   and set 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.

6.  Other Parts of the HOBA Process

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   The authentication process is more than just the act of
   authentication.  In password-based authentication and HOBA, there are
   other processes that are needed both before and after an
   authentication step.  This section covers those processes.  Where
   possible, it combines practices of HOBA-http and HOBA-js; where that
   is not possible, the differences are called out.

   All additional services MUST be performed in TLS-protected sessions
   ([RFC5246]).  If the current HTTP traffic is not running under TLS, a
   new session is started before any of the actions described here are

   HOBA-http uses a well-known URL [RFC5785] "hoba" as a base URI for
   performing many tasks: "".
   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-http 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. ]]

6.1.  Registration

   Normally, a registration (also called "joining") is expected to
   happen after a UA receives a WWW-Authenticate for a web-origin and
   realm (for HOBA-http) or on demand (for HOBA-js) for which it has no
   associated CPK.  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 in question.

   Note that 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 to use the same
   or no realm.

   The registration message for HOBA-http is sent as a POST message to
   the URL ".well-known/hoba/register" with an HTML form (x-www-form-
   encoded) described below; The registration message for HOBA-js can be
   in any format specified by the server, but it could be the same as
   the one described here for HOBA-http.  It is up to the server to
   decide what kind of user interaction is required before the account
   is finally set up.

   The registration message sent to server has one mandatory field (pub)
   and some optional fields that allow the UA to specify the type and
   value of key and device identifiers that the UA wishes to use.

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   o  pub: is a mandatory field containing the PEM formatted public key
      of the client.  See Appendix C of [RFC6376] for an example of how
      to generate this key format.

   o  kidtype: contains the type of key identifier, this is a numeric
      value intended to contain one of the values from Section 9.4.  If
      this is not present then the mandatory-to-implement hashed public
      key option MUST be used.

   o  kid: contains the key identifier as a base64url encoded string
      that is of the type indicated in the kidtype.  If the kid is a
      hash of a public key then the correct (base64url encoded) hash
      value MUST be provided and the server SHOULD check that and refuse
      the registration if an incorrect value was supplied.

   o  didtype: specifies a kind of device identifier intended to contain
      one of the values from Section 9.5, if absent then the "string"
      form of device identifier MUST be used.

   o  did: a UTF8 string that specifies the device identifier.  This can
      be used to help a user be confident that authentication has
      worked, e.g., following authentication some web content might say
      "You last logged in from device 'did' at time T."

   o  curtime: the number of milliseconds since the Unix epoch,
      formatted as [[ not yet specified ]].  This is based on the
      expectation that the synchronization between the browser's and
      server's clocks is sufficiently reliable.  This is used to guard
      against the replay of a legitimate registration message.  The
      server needs to have a replay cache covering a signature timeout
      window.  This might 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, the server will reject it.
      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
   use a nonce method.  This is worthy of more discussion. ]].

   [[ A comment raised on the list was: how does the UA know that the
   registration process has completed successfully or badly?  That's not
   yet handled.  Also need to specify what happens if more data is
   needed. ]]

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6.2.  Associating Additional Keys to an Exiting Account

6.2.1.  Moving private keys

   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.
   If this approach is taken, then there is no impact on the HOBA-http
   or HOBA-js so this is a pure UA implementation issue and not
   discussed further.

6.2.2.  One time password

   To enroll a new UA using an existing UA, the user requests a one-time
   password that will be entered in the new UA.  On HOBA-http, this is
   done with the URL ".well-known/hoba/associate-start" using a POST
   message [[ more detail is clearly needed here ]].  The server
   displays a one-time password that expires in a short amount of time
   (such as 30 minutes) on the currently enrolled UA.  The new UA
   generates a new CPK and sends it to the server.  In HOBA-http, this
   is done with a POST to ".well-known/hoba/associate-finish" [[ more
   detail needed here ]].

   The one-time password should be easy for a user to type into the new
   UA, but at the same time needs to have enough randomness to prevent
   an attacker from succeeding if they try to use the password during
   its short lifetime.  After the server gets the password in the new
   UA, it verifies the signature and verifies that the password supplied
   is in a list of unexpired one-time passwords.  If so, the server
   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 needs make the outcome clear to the user.

   Alternatively, if an already-enrolled UA is not available to the user
   when they want to associate a new UA with an existing account, and
   the site has an out-of-band communication mechanism such as email or
   SMS associated with that account, the user can request that a one-
   time password be sent to the user out of band.  The user receives the
   one-time password and, using the new UA, tells it to the server.  On
   HOBA-http, this is done with the URL ".well-known/hoba/associate-
   from-oob" using a POST message [[ more detail is clearly needed here

   [[ Note: the implementation doesn't do this - I omitted a
   human memorable OTP for now.  The reason is I think the probability

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   that spammers would try those is too high.  So maybe we should delete
   this entirely? ]]

6.2.3.  Out of band

   There are various other ways in which a server can provide ways to
   allow a new UA to be registered to the same account.  For example,
   simply providing a web page where, using the first UA, the user can
   generate a URL (containing some cryptographic value) that the user
   then later de-references on the second UA would suffice.  The user
   could have e-mail that URL to herself for example, of the web server
   accessed at the first UA could automatically do that.

   [[ Need to think about how much detail to put in here.  Its all
   fairly obvious, but we probably should talk about the security
   considerations.  The site btw has a few of these methods
   implemented. ]]

6.3.  Logging Out

   The user can tell the server it wishes to log out.  With HOBA-http,
   this is done by going to the URL ".well-known/hoba/logout".  The UA
   SHOULD also delete session cookies associated with the session so
   that the user's state is no longer "logged in."

   The server MUST NOT allow TLS session resumption for any logged out
   session.  [[ Need to determine if this is needed or even a good idea.

   The server SHOULD also revoke or delete any cookies associated with
   the session.

6.4.  Getting a Fresh Challenge

   The UA can get a "fresh" challenge from the server.  In HOBA-http, it
   sends a POST message to ".well-known/hoba/getchal".  If successful,
   the response response MUST include a fresh (base64url encoded) HOBA
   challenge for this origin in the body of the response.

7.  Mandatory-to-Implement Algorithms

   RSA-SHA256 MUST be supported.  RSA-SHA1 MAY be used.  RSA modulus
   lengths of at least 2048 bits SHOULD be used.

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

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   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.  Any authentication
   mechanism allows tracking within account. ]]

   [[ 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 UAs under the control of an
   attacker.  But XSS attacks are in reality across the board
   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

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   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/

8.2.  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 a server is potentially enrolling
   the device for a long period and the user may not want to have to be
   responsible for the credential for that long.  To alleviate this
   problem, the user can request that the credential be erased from the
   browser.  Similarly, during 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 beyond the current browser session.

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. [[ Still need to figure out what it means to do
   this through proxies. ]]

9.2.  .well-known URLs

   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.

9.3.  Algorithm Names

   TBD, hopefully re-use and existing registry

   "0" means RSA-SHA256

   "1" means RSA-SHA1

   [[ Will most likely add an elliptic curve signature mechanism. ]]

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9.4.  Key Identifier Types

   "0" means a hashed public key, as done in DANE.  [RFC6698]

   "1" means a URI, such as a mailto: or acct: URI, but anything
   conforming to [RFC3986] is ok.i

   "2" means an unformatted string, at the user's/UA's whim

9.5.  Device Identifier Types

   "0" means an unformatted nickname, at the user's/UA's whim

10.  Implementation Status

   [[ Note to RFC editor - please delete this section before
   publication. ]]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to [RFC6982] "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, by considering the running code as evidence of valuable
   experimentation and feedback that has made the implemented protocols
   more mature.  It is up to the individual working groups to use this
   information as they see fit".

   At the time of writing there are two known implementations.  One done
   by Stephen Farrell of HOBA-HTTP and a HOBA-JS variant implements the
   -01 version of HOBA and is available from which site
   also includes a demonstration of HOBA.

   There is another implementation by Michael Thomas of an HOBA-JS

11.  Acknowledgements

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   Thanks to the following for good comments received during the
   preparation of this specification: Julian Reschke, James Manger,
   Michael Sweet, Ilari Liusvaara, [[ and many more to be added ]].  All
   errors and stupidities are of course the editors' fault.

12.  References

12.1.  Normative References

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [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

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

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

12.2.  Informative References

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-22
              (work in progress), February 2013.

   [MI93]     Mitchell, and Thomas, "Standardising Authentication
              Protocols Based on Public-Key Techniques.", Journal of
              Computer Security 2 (1993): 23-36. , 1993.

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

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   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", STD 76, RFC 6376, July

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982, July

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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Appendix B.  Examples

   [[ Will add more later and probably use  Note these
   still use the -01 version of the "origin" abnf production. ]]

   The following values show an example of HOBA-HTTP authentication to
   the origin  Carriage-returns have been added
   and need to be removed to validate the example.

   -----BEGIN PRIVATE KEY-----
   -----END PRIVATE KEY-----

   Key Identifier: Zhh5vD5ovE0NqTDOufSUFRu5dzZMe-KOrMX2cN3vqWw

   Challenge: zzrYL7BaOQtlzOsl4fMY+EYcG4eT2h+JXi+jEGzozQ0=

   Nonce: xXSFdZ-7ahM

   Tbsorigin: httpshoba-local.ie443

   The resulting signature is:

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   The final HTTP header field sent with a request is then:

   Authorization: result="Zhh5vD5ovE0NqTDOufSUFRu5dzZMe-KOrMX2cN3vq

Authors' Addresses

   Stephen Farrell
   Trinity College Dublin
   Dublin  2

   Phone: +353-1-896-2354

   Paul Hoffman
   VPN Consortium


   Michael Thomas


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