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Internet Engineering Task Force (IETF) S. Farrell
Request for Comments: 7486 Trinity College Dublin
Category: Experimental P. Hoffman
ISSN: 2070-1721 VPN Consortium
M. Thomas
Phresheez
March 2015
HTTP Origin-Bound Authentication (HOBA)
Abstract
HTTP Origin-Bound Authentication (HOBA) is a digital-signature-based
design for an HTTP authentication method. The design can also be
used in JavaScript-based authentication embedded in HTML. HOBA is an
alternative to HTTP authentication schemes that require passwords and
therefore avoids all problems related to passwords, such as leakage
of server-side password databases.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7486.
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Copyright Notice
Copyright (c) 2015 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
(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. 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Interfacing to Applications (Cookies) . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Step-by-Step Overview of HOBA-http . . . . . . . . . . . 6
2. The HOBA Authentication Scheme . . . . . . . . . . . . . . . 6
3. Introduction to the HOBA-http Mechanism . . . . . . . . . . . 9
4. Introduction to the HOBA-js Mechanism . . . . . . . . . . . . 10
5. HOBA's Authentication Process . . . . . . . . . . . . . . . . 11
5.1. CPK Preparation Phase . . . . . . . . . . . . . . . . . . 11
5.2. Signing Phase . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Authentication Phase . . . . . . . . . . . . . . . . . . 11
6. Other Parts of the HOBA Process . . . . . . . . . . . . . . . 12
6.1. Registration . . . . . . . . . . . . . . . . . . . . . . 13
6.1.1. Hobareg Definition . . . . . . . . . . . . . . . . . 14
6.2. Associating Additional Keys to an Existing Account . . . 16
6.2.1. Moving Private Keys . . . . . . . . . . . . . . . . . 16
6.2.2. Human-Memorable One-Time Password (Don't Do This One) 16
6.2.3. Out-of-Band URL . . . . . . . . . . . . . . . . . . . 17
6.3. Logging Out . . . . . . . . . . . . . . . . . . . . . . . 17
6.4. Getting a Fresh Challenge . . . . . . . . . . . . . . . . 17
7. Mandatory-to-Implement Algorithms . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8.1. Privacy Considerations . . . . . . . . . . . . . . . . . 18
8.2. localStorage Security for JavaScript . . . . . . . . . . 19
8.3. Multiple Accounts on One User Agent . . . . . . . . . . . 20
8.4. Injective Mapping for HOBA-TBS . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9.1. HOBA Authentication Scheme . . . . . . . . . . . . . . . 21
9.2. .well-known URI . . . . . . . . . . . . . . . . . . . . . 21
9.3. Algorithm Names . . . . . . . . . . . . . . . . . . . . . 21
9.4. Key Identifier Types . . . . . . . . . . . . . . . . . . 22
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9.5. Device Identifier Types . . . . . . . . . . . . . . . . . 22
9.6. Hobareg HTTP Header Field . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Problems with Passwords . . . . . . . . . . . . . . 26
Appendix B. Example . . . . . . . . . . . . . . . . . . . . . . 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
HTTP Origin-Bound Authentication (HOBA) is an authentication design
that can be used as an HTTP authentication scheme [RFC7235] 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 [RFC7235]. 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 username/password authentication methods such as HTTP Basic,
HTTP Digest, and web forms have been in use for many years but are
susceptible to theft of server-side password databases. Instead of
passwords, HOBA uses digital signatures in a challenge-response
scheme as its 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.
HOBA session management is identical to username/password session
management, with a server-side session management tool or script
inserting a session cookie [RFC6265] into the output to the browser.
Use of Transport Layer Security (TLS) for the HTTP session is still
necessary to prevent session cookie hijacking.
HOBA keys are "bare keys", so there is no need for the semantic
overhead of X.509 public key certificates, particularly with respect
to naming and trust anchors. The Client Public Key (CPK) structures
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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 needed for modern HTTP
authentication:
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.
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 without having to register
for each separately.
o Logout features can be useful for UAs, so HOBA defines a way to
close a current HTTP "session".
o Digital signatures can be expensive to compute, so HOBA defines a
way for HTTP servers to indicate how long a given challenge value
is valid, and a way for UAs to fetch a fresh challenge at any
time.
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 does not define a mechanism for deleting the association between
an existing CPK and an account. Such a mechanism can be implemented
at the application layer.
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.
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 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.
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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 the 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
Account: 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 at least one CPK and a web origin; it
will also optionally include an HTTP "realm" as defined in the HTTP
authentication specification [RFC7235]. 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.
Client public key (CPK): A CPK is the public key and associated
cryptographic parameters needed for a server to validate a signature.
HOBA-http: We use this term when describing something that is
specific to HOBA as an HTTP authentication mechanism.
HOBA-js: We use this term when describing something that is unrelated
to HOBA-http but is relevant for HOBA as a design pattern that can be
implemented in a browser in JavaScript.
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".
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).
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1.3. Step-by-Step Overview of HOBA-http
Step-by-step, a typical HOBA-http registration and authentication
flow might look like this:
1. The client connects to the server and makes a request, and the
server's response includes a WWW-Authenticate header field that
contains the "HOBA" auth-scheme, along with associated parameters
(see Section 3).
2. If the client was not already registered with the web origin and
realm it is trying to access, the "joining" process is invoked
(see Section 6.1). This creates a key pair and makes the CPK
known to the server so that the server can carry out the account
creation processes required.
3. The client uses the challenge from the HOBA auth-scheme
parameters, along with other information it knows about the web
origin and realm, to create and sign a HOBA to-be-signed (HOBA-
TBS) string (see Section 2).
4. The client creates a HOBA client-result (HOBA-RES), using the
signed HOBA-TBS for the "sig" value (see Section 2).
5. The client includes the Authorization header field in its next
request, using the "HOBA" auth-scheme and putting the HOBA
client-result in an auth-param named "result" (see Section 3).
6. The server authenticates the HOBA client-result (see
Section 5.1).
7. Typically, the server's response includes a session cookie that
allows the client to indicate its authentication state in future
requests (see Section 1.1).
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 a HOBA-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 blob with the private key
corresponding to the CPK for that web origin. The formatting chosen
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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
defined and allows the characters specified in Section 2.3 of
[RFC3986].
HOBA-TBS = len ":" nonce
len ":" alg
len ":" origin
len ":" [ realm ]
len ":" kid
len ":" challenge
len = 1*DIGIT
nonce = 1*base64urlchars
alg = 1*2DIGIT
origin = scheme "://" authority ":" port
; scheme, etc., are from RFC 3986
realm = unreserved
; realm is to be treated as in Section 2.2 of RFC 7235
kid = 1*base64urlchars
challenge = 1*base64urlchars
; Characters for Base64URL encoding from Table 2 of RFC 4648
; all of which are US-ASCII (see RFC 20)
base64urlchars = %x30-39 ; Digits
/ %x41-5A ; Uppercase letters
/ %x61-7A ; Lowercase letters
/ "-" / "_" / "=" ; Special characters
Figure 1: To-Be-Signed Data for HOBA
The fields above contain the following:
o len: Each field is preceded by the number of octets of the
following field, expressed as a decimal number in ASCII [RFC20].
Lengths are separated from field values by a colon character. So
if a nonce with the value "ABCD" were used, then that would be
preceeded by "4:" (see the example in Appendix B for details).
o nonce: 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]; guidelines for randomness are
given in [RFC4086].) 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.
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o alg: specifies the signature algorithm being used. See Section 7
for details of algorithm support requirements. The IANA-
registered algorithm values (see Section 9.3) are encoded as one-
or two-digit ASCII numbers. For example, RSA-SHA256 (number 0) is
encoded as the ASCII character "0" (0x30), while a future
algorithm registered as number 17 would be encoded as the ASCII
characters "17" (0x3137).
o origin: 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://www.example.com:8080/foo", then the bytes input to the
signature process will be "https://www.example.com:8080". There
is no default for the port number, and the port number MUST be
present.
o realm: a string with the syntactic restrictions defined in
[RFC7235]. If no realm is specified for this authentication, then
this is absent but is preceeded by a length of zero ("0:").
Recall that both sides know when this needs to be there,
independent of the encoding via a zero length.
o kid: a key identifier. This MUST be a base64url-encoded value
that is presented to the server in the HOBA client result (see
below).
o challenge: MUST be a base64url-encoded challenge value that the
server chose to send to the client. The challenge MUST be chosen
so that it is infeasible to guess and SHOULD be indistinguishable
from (the base64url encoding of) a random string that is at least
128 bits long.
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
the network so as to reduce the amount of state that needs to be
maintained by servers. (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 by
the UA 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 Authorization 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.
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HOBA-RES = kid "." challenge "." nonce "." sig
sig = 1*base64urlchars
Figure 2: HOBA Client Result Value
If a malformed message of any kind is received by a server, the
server MUST fail authentication. If a malformed message of any kind
is received by a client, the client MUST abandon that authentication
attempt. (The client is, of course, free to start another
authentication attempt if it desires.)
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-
Authenticate header.
The HOBA scheme has two REQUIRED attributes (challenge and max-age)
and one OPTIONAL attribute (realm):
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 MUST be
unique for every 401 HTTP response in order to prevent replay
attacks from passive observers.
o A "max-age" attribute MUST be included. It specifies the number
of seconds from the time the HTTP response is emitted for which
responses to this challenge can be accepted; for example, "max-
age: 10" would indicate ten seconds. If max-age is set to zero,
then that means that only one signature will be accepted for this
challenge.
o A "realm" attribute MAY be included to indicate the scope of
protection in the manner described in HTTP/1.1, Authentication
[RFC7235]. The "realm" attribute MUST NOT appear more than once.
When the "client response" is created, the UA encodes the HOBA
client-result and returns that in the Authorization header. The
client-result is a string matching the HOBA-RES production in
Figure 2 as an auth-param with the name "result".
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
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additional mechanisms to validate the signature. If the validation
fails, or if the server chooses to reject the signature for any
reason whatsoever, the server fails the request with a 401
Unauthorized HTTP response.
The server MUST check that the same web origin is used in all of the
server's TLS server certificates, the URL being accessed, and the
HOBA signature. If any of those checks fail, the server treats the
signature as being cryptographically incorrect.
Note that a HOBA signature is good for however long a non-zero max-
age parameter allows. This means that replay is possible within the
time window specified by the "max-age" value chosen by the server.
Servers can attempt to detect any such replay (via caching if they so
choose) and MAY react to such replays by responding with a second (or
subsequent) 401 HTTP response containing a new challenge.
To optimize their use of challenges, UAs MAY prefetch a challenge
value, for example, after (max-age)/2 seconds have elapsed, using the
".well-known/hoba/getchal" scheme described later in this document.
This also allows for precalculation 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 thing that is highly desirable for HOBA-js is WebCrypto (see
<http://www.w3.org/TR/WebCryptoAPI>), which is (at the time of
writing) starting to see deployment. In lieu of WebCrypto,
JavaScript crypto libraries can be employed with the known
deficiencies of their pseudo-random number generators and the general
immaturity of those libraries.
Without Webcrypto, one element is required for HOBA-js; localStorage
(see <http://www.w3.org/TR/webstorage/>) from HTML5 can be used for
persistent key storage. For example, an implementation would store a
dictionary account identifier as well as public key and private key
tuples in the origin's localStorage for subsequent authentication
requests. How this information is actually stored in localStorage is
an implementation 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
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attacks on localStorage. Note that IndexedDB (see
<http://www.w3.org/TR/IndexedDB/>) is an alternative to localStorage
that can also be used here and that is used by WebCrypto.
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
difficult 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 session cookies are used whilst the user navigates around the
site.
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 features similar to today's
password-based schemes.
5.1. CPK Preparation Phase
In the CPK preparation phase, the client determines if it already has
a CPK for the web origin with which it needs to authenticate. 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.
In the authentication phase, the server uses the key identifier (kid)
to determine 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.
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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 response 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 cannot determine the
correct 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 another
CPK. The server can then use some out-of-band method (such as a
confirmation email round trip, SMS, or a 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 UA is identical to logging in with an
existing account.
If the server does not recognize the CPK, the server might send the
client through 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
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.
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All HOBA interactions other than those defined in Section 5 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 performed.
HOBA-http uses a well-known URI [RFC5785] "hoba" as a base URI for
performing many tasks: "https://www.example.com/.well-known/hoba".
These URIs 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 have to be able to handle redirection of these
requests. However, as that would potentially cause security issues
when a re-direct brings the client to a different web origin, servers
implementing HOBA-http SHOULD NOT redirect to a different web origin
from below ".well-known/hoba" URLs. The above is considered
sufficient to allow experimentation with HOBA, but if at some point
HOBA is placed on the Standards Track, then a full analysis of off-
origin redirections would need to be documented.
6.1. Registration
Normally, a registration (also called "joining") is expected to
happen after a UA receives a 401 response 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 lightweight. 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, see <http://www.w3.org/TR/2014/REC-html5-20141028/
forms.html#url-encoded-form-data>), 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. When
the server's chosen registration flow is completed successfully, the
server MUST add a Hobareg HTTP header (see Section 6.1.1) to the HTTP
response message that completes the registration flow.
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The registration message sent to the 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.
o pub: a mandatory field containing the Privacy Enhanced Mail (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 defined in Section 9.5 MUST be used.
o did: a UTF-8 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."
Note that replay of registration (and other HOBA) messages is quite
possible. That, however, can be counteracted if challenge freshness
is ensured. See Section 2 for details. Note also that with HOBA-
http, the HOBA signature does not cover the POST message body. If
that is required, then HOBA-JS may be a better fit for registration
and other account management actions.
6.1.1. Hobareg Definition
Since registration can often be a multi-step process, e.g., requiring
a user to fill in contact details, the initial response to the HTTP
POST message defined above may not be the end of the registration
process even though the HTTP response has a 200 OK status. This
creates an issue for the UA since, during the registration process
(e.g., while dealing with interstitial pages), the UA doesn't yet
know whether the CPK is good for that web origin or not.
For this reason, the server MUST add a header field to the response
message when the registration has succeeded in order to indicate the
new state. The header to be used is "Hobareg", and the value when
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registration has succeeded is to be "regok". When registration is in
an intermediate state (e.g., on an HTTP response for an interstitial
page), the server MAY add this header with a value of "reginwork".
See Section 9.6 for the relevant IANA registration of this header
field.
For interstitial pages, the client MAY include a HOBA Authorization
header. This is not considered a "MUST", as that might needlessly
complicate client implementations, but is noted here in case a server
implementer assumes that all registration messages contain a HOBA
Authorization header.
Hobareg-val = "regok" / "reginwork"
Figure 3: Hobareg Header Field Definition
Figure 3 provides an ABNF definition for the values allowed in the
Hobareg header field. Note that these (and the header field name)
are case insensitive. Section 8.3.1 of [RFC7231] calls for
documenting the following details for this new header field:
o Only one single value is allowed in a Hobareg header field.
Should more than one (a list) be encountered, or any other ABNF-
invalid value, that SHOULD be interpreted as being the same as
"reginwork".
o The Hobareg header field can only be used in HTTP responses.
o Since Hobareg is only meant for responses, it ought not appear in
requests.
o The HTTP response code does affect the interpretation of Hobareg.
Registration is only considered to have succeeded if the regok
value is seen in a 2xx response. 4xx and other errors mean that
registration has failed regardless of the value of Hobareg seen.
The request method has no influence on the interpretation of
Hobareg.
o Intermediaries never insert, delete, or modify a Hobareg header
field.
o As a response-only header field, it is not appropriate to list a
Hobareg in a Vary response header field.
o Hobareg is allowed in trailers.
o As a response-only header field, Hobareg will not be preserved
across re-directs.
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o Hobareg itself discloses little security- or privacy-sensitive
information. If an attacker can somehow detect that a Hobareg
header field is being added, then that attacker would know that
the UA is in the process of registration, which could be
significant. However, it is likely that the set of messages
between the UA and server would expose this information in many
cases, regardless of whether or not TLS is used. Using TLS is
still, however, a good plan.
6.2. Associating Additional Keys to an Existing Account
From the user perspective, the UA having a CPK for a web origin will
often appear to be the same as having a way to sign in to an account
at that web site. Since users often have more than one UA, and since
the CPKs are, in general, UA specific, that raises the question of
how the user can sign in to that account from different UAs. And
from the server perspective, that turns into the question of how to
safely bind different CPKs to one account. In this section, we
describe some ways in which this can be done, as well as one way in
which this ought not be done.
Note that the context here is usually that the user has succeeded in
registering with one or more UAs (for the purposes of this section,
we call this "the first UA" below) and can use HOBA with those, and
the user is now adding another UA. The newest UA might or might not
have a CPK for the site in question. Since it is in fact trivial, we
assume that the site is able to put in place some appropriate,
quicker, easier registration for a CPK for the newest UA. The issue
then becomes one of binding the CPK from the newest UA with those of
other UAs bound to the 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. Human-Memorable One-Time Password (Don't Do This One)
It will be tempting for implementers to use a human-memorable One-
Time Password (OTP) in order to "authenticate" binding CPKs to the
same account. The workflow here would likely be something along the
lines of some server administrative utility generating a human-
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memorable OTP such as "1234" and sending that to the user out of band
for the user to enter at two web pages, each authenticated via the
relevant CPK. While this seems obvious enough and could even be
secure enough in some limited cases, we consider that this is too
risky to use in the Internet, and so servers SHOULD NOT provide such
a mechanism. The reason this is so dangerous is that it would be
trivial for an automated client to guess such tokens and "steal" the
binding intended for some other user. At any scale, there would
always be some in-process bindings so that even with only a trickle
of guesses (and hence not being detectable via message volume), an
attacker would have a high probability of succeeding in registering a
binding with the attacker's CPK.
This method of binding CPKs together is therefore NOT RECOMMENDED.
6.2.3. Out-of-Band URL
One easy binding method is to simply provide a web page where, using
the first UA, the user can generate a URL (containing some
"unguessable" cryptographically generated value) that the user then
later dereferences on the newest UA. The user could email that URL
to herself, for example, or the web server accessed at the first UA
could automatically do that.
Such a URL SHOULD contain at least the equivalent of 128 bits of
randomness.
6.3. Logging Out
The user can tell the server it wishes to log out. With HOBA-http,
this is done by sending a HOBA-authenticated POST message to the URL
".well-known/hoba/logout" on the site in question. 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.
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 MUST contain a fresh (base64url-encoded) HOBA challenge
for this origin in the body of the response. Whitespace in the
response MUST be ignored.
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7. Mandatory-to-Implement Algorithms
RSA-SHA256 MUST be supported. HOBA implementations MUST use RSA-
SHA256 if it is provided by the underlying cryptographic libraries.
RSA-SHA1 MAY be used. RSA modulus lengths of at least 2048 bits
SHOULD be used. RSA indicates the RSASSA-PKCS1-v1_5 algorithm
defined in Section 8.2 of [RFC3447], and SHA-1 and SHA-256 are
defined in [SHS]. Keys with moduli shorter than 2048 bits SHOULD
only be used in cases where generating 2048-bit (or longer) keys is
impractical, e.g., on very constrained or old devices.
8. Security Considerations
Binding my CPK with someone else's account would be fun and
profitable so SHOULD be appropriately hard. In particular, URLs or
other values generated by the server as part of any CPK binding
process 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.
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.
When the max-age parameter is not zero, then a HOBA signature has a
property that is like a bearer token for the relevant number of
seconds: it can be replayed for a server-selected duration.
Similarly, for HOBA-js, signatures might be replayable depending on
the specific implementation. The security considerations of
[RFC6750] therefore apply in any case where the HOBA signature can be
replayed. Server administrators can set the max-age to the minimum
acceptable value in such cases, which would often be expected to be
just a few seconds. There seems to be no reason to ever set the max-
age more than a few minutes; the value ought also decrease over time
as device capabilities improve. The administrator will most likely
want to set the max-age to something that is not too short for the
slowest signing device that is significant for that site.
8.1. Privacy Considerations
HOBA does, to some extent, impact privacy and could be considered to
represent a super-cookie to the server or to any entity on the path
from UA to HTTP server that can see the HOBA signature. This is
because we need to send a key identifier as part of the signature and
that will not vary for a given key. For this reason, and others, it
is strongly RECOMMENDED to only use HOBA over server-authenticated
TLS and to migrate web sites using HOBA to only use "https" URLs.
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UAs SHOULD provide users a way to manage their CPKs. Ideally, there
would be a way for a user to maintain their HOBA details for a site
while at the same time deleting other site information such as
cookies or non-HOBA HTML5 localStorage. However, as this is likely
to be complex, and appropriate user interfaces counterintuitive, we
expect that UAs that implement HOBA will likely treat HOBA
information as just some more site data that would disappear should
the user choose to "forget" that site.
Device identifiers are intended to specify classes of device in a way
that can assist with registration and with presentation to the user
of information about previous sessions, e.g., last login time.
Device identifier types MUST NOT be privacy sensitive, with values
that would allow tracking a user in unexpected ways. In particular,
using a device identifier type that is analogous to the International
Mobile Equipment Identifier (IMEI) would be a really bad idea and is
the reason for the "MUST NOT" above. In that case, "mobile phone"
could be an acceptable choice.
If possible, implementations ought to encourage the use of device
identifier values that are not personally identifying except for the
user concerned; for example, "Alice's mobile" is likely to be chosen
and is somewhat identifying, but "Alice's phone: UUID 1234-5567-
89abc-def0" would be a very bad choice.
8.2. localStorage Security for JavaScript
The use of localStorage (likely with a non-WebCrypto implementation
of HOBA-js) will undoubtedly be a cause for concern. localStorage
uses the same-origin model that 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 Cross-Site Scripting (XSS) attacks, which could conceivably take
the keying material and use it to create UAs under the control of an
attacker. XSS attacks are, in reality, devastating across the board
since they can and do steal credit card information, passwords,
perform illicit acts, etc. It's not evident that we are introducing
unique threats from which cleartext 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 that the user supplies. This may sound
counterintuitive, but the object here is to keep passwords off of
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servers to mitigate the multiplier effect of a large-scale compromise
(e.g., [ThreatReport]) 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 also to all of the sites on which a user used
the same password. 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/UA/user.
8.3. 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 could 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.
However, all such features really ought to be part of the operating
system or platform and not part of a HOBA implementation, so those
are not discussed further.
8.4. Injective Mapping for HOBA-TBS
The repeated length fields in the HOBA-TBS structure are present in
order to ensure that there is no possibility that the catenation of
different input values can cause confusion that might lead to an
attack, either against HOBA as specified here, or else an attack
against some other protocol that reused this to-be-signed structure.
Those fields ensure that the mapping from input fields to the HOBA-
TBS string is an injective mapping.
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9. IANA Considerations
IANA has made registrations and created new registries as described
below.
All new registries have been placed beneath a new "HTTP Origin-Bound
Authentication (HOBA) Parameters" category.
9.1. HOBA Authentication Scheme
A new scheme has been registered in the HTTP Authentication Scheme
Registry as follows:
Authentication Scheme Name: HOBA
Reference: Section 3 of RFC 7486
Notes (optional): The HOBA scheme can be used with either HTTP
servers or proxies. When used in response to a 407 Proxy
Authentication Required indication, the appropriate proxy
authentication header fields are used instead, as with any other HTTP
authentication scheme.
9.2. .well-known URI
A new .well-known URI has been registered in the Well-Known URIs
registry as described below.
URI Suffix: hoba
Change Controller: IETF
Reference: Section 6 of RFC 7486
Related Information: N/A
9.3. Algorithm Names
A new HOBA signature algorithms registry has been created as follows,
with Specification Required as the registration procedure. New HOBA
signature algorithms SHOULD be in use with other IETF Standards Track
protocols before being added to this registry.
Number Meaning Reference
----------- ------------------------------ ------------
0 RSA-SHA256 RFC 7486
1 RSA-SHA1 RFC 7486
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RSA is defined in Section 8.2 of [RFC3447], and SHA-1 and SHA-256 are
defined in [SHS].
For this registry, the number column should contain a small positive
integer. Following the ABNF in Figure 1, the maximum value for this
is decimal 99.
9.4. Key Identifier Types
A new HOBA Key Identifier Types registry has been created as follows,
with Specification Required as the registration procedure.
Number Meaning Reference
----------- ------------------------------ ------------
0 a hashed public key [RFC6698]
1 a URI [RFC3986]
2 an unformatted string, at the RFC 7486
user's/UA's whim
For the number 0, hashed public keys are as done in DNS-Based
Authentication of Named Entities (DANE) [RFC6698].
For this registry, the number column should contain a small positive
integer.
9.5. Device Identifier Types
A new HOBA Device Identifier Types registry has been created as
follows, with Specification Required as the registration procedure.
The designated expert for this registry is to carefully pay attention
to the notes on this field in Section 8.1, in particular, the "MUST
NOT" stated therein.
Number Meaning Reference
----------- ------------------------------ -----------
0 an unformatted string, at the RFC 7486
user's/UA's whim
For this registry, the number column should contain a small positive
integer.
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9.6. Hobareg HTTP Header Field
A new identifier has been registered in the Permanent Message Header
Field Names registry as described below.
Header Field Name: Hobareg
Protocol: http (RFC 7230)
Status: experimental
Author/Change controller: IETF
Reference: Section 6.1.1 of RFC 7486
Related information: N/A
10. References
10.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, October 1969,
<http://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003,
<http://www.rfc-editor.org/info/rfc3447>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
Farrell, et al. Experimental [Page 23]
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April
2010, <http://www.rfc-editor.org/info/rfc5785>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011, <http://www.rfc-editor.org/info/rfc6454>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012,
<http://www.rfc-editor.org/info/rfc6698>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012,
<http://www.rfc-editor.org/info/rfc6750>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
June 2014, <http://www.rfc-editor.org/info/rfc7231>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014,
<http://www.rfc-editor.org/info/rfc7235>.
[SHS] NIST, "Secure Hash Standard (SHS)", FIPS PUB 180-4, March
2012.
10.2. Informative References
[Bonneau] Bonneau, J., "The Science of Guessing: Analyzing an
Anonymized Corpus of 70 Million Passwords", IEEE Symposium
on Security and Privacy 538-552, 2012.
[MI93] Mitchell, C. and A. Thomas, "Standardising authentication
protocols based on public key techniques", Journal of
Computer Security Volume 2, 23-36, 1993.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005, <http://www.rfc-editor.org/info/rfc4086>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011, <http://www.rfc-editor.org/info/rfc6265>.
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RFC 7486 HTTP Origin-Bound Auth (HOBA) March 2015
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, September 2011,
<http://www.rfc-editor.org/info/rfc6376>.
[ThreatReport]
Sophos, "Security Threat Report 2013", January 2013,
<http://www.sophos.com/en-us/medialibrary/pdfs/other/
sophossecuritythreatreport2013.pdf>.
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Appendix A. Problems with Passwords
By far, the most common mechanism for web authentication is passwords
that can be remembered by the user, called "human-memorable
passwords". There is plenty of good research on how users typically
use human-memorable passwords (e.g., see [Bonneau]), 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 identifiers that go 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 Basic
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.
Note that passwords that are not human-memorable are still subject to
database attack, though they are of course unlikely to be reused
across many systems. Similarly, database attacks of some form or
other will work against any password-based authentication scheme,
regardless of the cryptographic protocol used. So for example, zero-
knowledge or Password-Authenticated Key Exchange (PAKE) schemes,
though making use of elegant cryptographic protocols, remain as
vulnerable to what is clearly the most common exploit seen when it
comes to passwords. HOBA is, however, not vulnerable to database
theft.
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Appendix B. Example
The following values show an example of HOBA-http authentication to
the origin "https://example.com:443". Carriage returns have been
added and need to be removed to validate the example.
Public Key:
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAviE8fMrGIPZN9up94M28
6o38B99fsz5cUqYHXXJlnHIi6gGKjqLgn3P7n4snUSQswLExrkhSr0TPhRDuPH_t
fXLKLBbh17ofB7t7shnPKxmyZ69hCLbe7pB1HvaBzTxPC2KOqskDiDBOQ6-JLHQ8
egXB14W-641RQt0CsC5nXzo92kPCdV4NZ45MW0ws3twCIUDCH0nibIG9SorrBbCl
DPHQZS5Dk5pgS7P5hrAr634Zn4bzXhUnm7cON2x4rv83oqB3lRqjF4T9exEMyZBS
L26m5KbK860uSOKywI0xp4ymnHMc6Led5qfEMnJC9PEI90tIMcgdHrmdHC_vpldG
DQIDAQAB
-----END PUBLIC KEY-----
Origin: https://example.com:443
Key Identifier: vesscamS2Kze4FFOg3e2UyCJPhuQ6_3_gzN-k_L6t3w
Challenge: pUE77w0LylHypHKhBqAiQHuGC751GiOVv4/7pSlo9jc=
Signature algorithm: RSA-SHA256 ("0")
Nonce: Pm3yUW-sW5Q
Signature:
VD-0LGVBVEVjfq4xEd35FjnOrIqzJ2OQMx5w8E52dgVvxFD6R0ryEsHcD31ykh0i
4YIzIHXirx7bE4x9yP-9fMBCEwnHJsYwYQhfRpmScwAz-Ih1Hn4yORTb-U66miUz
q04ZgTHm4jAj45afU20wYpGXY2r3W-FRKc6J6Glv_zI_ROghERalxgXG-QVGZrKP
tG0V593Yf9IPnFSpLyW6fnxscCMWUA9T-4NjMdypI-Ze4HsC9J06tRTOunQdofr9
6ZJ2i9LE6uKSUDLCD2oeEeSEvUR--4OGtrgjzYysHZkdVSxAi7OoQBK34EUWg9kI
S13qQA43m4IMExkbApqrSg
Authorization Header:
Authorization: HOBA result="vesscamS2Kze4FFOg3e2UyCJPhuQ6_3_gzN-
k_L6t3w.pUE77w0LylHypHKhBqAiQHuGC751GiOVv4/7pSlo9jc=.Pm3yUW-sW5Q
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Farrell, et al. Experimental [Page 27]
RFC 7486 HTTP Origin-Bound Auth (HOBA) March 2015
Acknowledgements
Thanks to the following for good comments received during the
preparation of this specification: Richard Barnes, David Black,
Alissa Cooper, Donald Eastlake, Amos Jeffries, Benjamin Kaduk, Watson
Ladd, Barry Leiba, Matt Lepinski, Ilari Liusvaara, James Manger,
Alexey Melnikov, Kathleen Moriarty, Yoav Nir, Mark Nottingham, Julian
Reschke, Pete Resnick, Michael Richardson, Yaron Sheffer, and Michael
Sweet. All errors and stupidities are of course the editors' fault.
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
Farrell, et al. Experimental [Page 28]