Network Working Group N. Williams
Internet-Draft Cryptonector
Intended status: Informational August 14, 2013
Expires: February 15, 2014
RESTful Authentication Pattern for the Hypertext Transport Protocol
(HTTP)
draft-ietf-httpauth-rest-auth-01
Abstract
This document proposes a "RESTful" pattern of authentication for
HTTP/1.0, 1.1, and 2.0. The goal is to make it easy to add
authentication mechanisms to HTTP applications and to make it easy to
implement them even without much help from the HTTP stack (though it
is best to integrate authentication into the stack, of course).
Another goal is to make it easy to reuse existing authentication
mechanisms by allowing the user (that is, the server's operators) to
choose what concrete authentication mechanism(s) to use.
Among other benefits of RESTauth: it is orthogonal to "HTTP routers"
and proxies, it results in session Uniform Resource Identifiers
(URIs) that can be DELETEd to logout, naturally supports multi-legged
authentication schemes, naturally supports clustering, and can be
universally implemented on the server side with such
server<->application interfaces as the Common Gateway Interface (CGI)
and FastCGI, among others.
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 February 15, 2014.
Copyright Notice
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Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1. Authentication Infrastructure and Credentials Reuse . . . 5
1.2. Protocol Outline . . . . . . . . . . . . . . . . . . . . . 6
1.3. API-Imposed Constraints . . . . . . . . . . . . . . . . . 7
1.4. Conventions used in this document . . . . . . . . . . . . 7
2. Alternatives . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. In-band HTTP Authentication . . . . . . . . . . . . . . . 8
2.2. Out-of-Band Bearer Token Mechanisms . . . . . . . . . . . 9
2.3. Authentication in TLS . . . . . . . . . . . . . . . . . . 9
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Negotiable Parameters . . . . . . . . . . . . . . . . . . 10
3.1.1. Strong Binding to TLS . . . . . . . . . . . . . . . . . . 11
3.1.2. WWW-Authenticate Header Value Prefix Syntax . . . . . . . 11
3.1.3. WWW-ChannelBinding-Types Header . . . . . . . . . . . . . 12
3.1.4. WWW-ChannelBinding-Type Header . . . . . . . . . . . . . . 12
3.1.5. WWW-SessionBinding-Type Header . . . . . . . . . . . . . . 12
3.1.6. WWW-ReplayProtection Header . . . . . . . . . . . . . . . 12
3.2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1. One Round Trip Optimization: Challenges Born in
WWW-Authenticate Headers . . . . . . . . . . . . . . . . . 13
3.3. Session Binding Types: Cookie, Channel Bound Session
URI, and MAC . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.1. The New WWW-Session-URI Header . . . . . . . . . . . . . . 14
3.3.2. The New WWW-Session-MAC Header . . . . . . . . . . . . . . 14
3.3.3. To MAC or not to MAC; A MAC Trailer?? . . . . . . . . . . 15
4. Representation of Authenticated Session Resources . . . . 16
5. Session URI Origin and Scope . . . . . . . . . . . . . . . 17
6. HTTP "Routing" and Authentication . . . . . . . . . . . . 18
7. Actual Authentication Mechanisms . . . . . . . . . . . . . 19
7.1. OAuth via RESTauth . . . . . . . . . . . . . . . . . . . . 19
7.1.1. OAuth 1.0 . . . . . . . . . . . . . . . . . . . . . . . . 19
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7.1.2. OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Adapting SSHv2 Authentication Mechanisms to RESTauth . . . 19
7.2.1. RESTauth Mechanism Names for SSHv2 Userauth Methods . . . 20
7.2.2. Nonces . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2.3. "Session ID" . . . . . . . . . . . . . . . . . . . . . . . 20
7.3. Adapting IKEv2 Authentication Mechanisms to RESTauth . . . 20
7.3.1. Adapting IKEv2 Password Authenticated Connection
Establishment (PACE) to RESTauth . . . . . . . . . . . . . 20
7.4. Using SASL Authentication Mechanisms with RESTauth . . . . 21
7.4.1. Using SCRAM in RESTauth . . . . . . . . . . . . . . . . . 21
7.4.2. Using SCRAM with Round Trip Optimization in RESTauth . . . 22
7.5. Using GSS-API Authentication Mechanisms with RESTauth . . 23
8. Implementation Advice . . . . . . . . . . . . . . . . . . 25
8.1. Server-Side Implementation Advice . . . . . . . . . . . . 25
8.1.1. Channel Binding on the Server Side . . . . . . . . . . . . 25
8.1.2. Server Cluster / Routing Support . . . . . . . . . . . . . 25
8.2. Client-Side Implementation Advice . . . . . . . . . . . . 26
8.2.1. Channel Binding on the Client Side . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . 28
10. Security Considerations . . . . . . . . . . . . . . . . . 29
11. TODO . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
There is a great need for improved authentication options in HTTP
applications, both web browser and non-browser applications. At this
time there are a number of proposals being made in the HTTPauth
Working Group (WG). This proposal is just one of many.
Some of the goals of RESTauth are:
o an authentication protocol that reuses existing authentication
mechanisms -- in principle any and all mechanisms that we have in
the Internet protocols, including: SASL [RFC4422], GSS-API
[RFC2743], SSHv2 [RFC4251], IKEv2 [RFC5996], and other frameworks,
as well as ad-hoc mechanisms and/or standard mechanisms used
outside such frameworks;
o an authentication protocol that layers above HTTP so that
applications may implement it with little or no help from (read:
modifications to) the existing HTTP stacks that they use, all the
while not precluding native support for RESTauth by any HTTP
stack;
o an authentication protocol that supports a notion of sessions
without depending on having a single HTTP/TLS/TCP connection, and
which can be logged out explicitly, with explicit scoping of
sessions;
o an authentication protocol that naturally supports HTTP server-
side routing and clustering.
We propose a pattern for HTTP [RFC2616] [TODO: add reference to
HTTP/2.0 as well?] authentication mechanisms that, by being
"RESTful", obtains these goals naturally.
1.1. Motivation
Existing HTTP authentication mechanisms leave much to be desired.
Existing "in-band" mechanisms:
o Basic [RFC2617];
o Digest [RFC2617];
o Negotiate [RFC4559].
Other existing mechanisms:
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o ad-hoc username-and-password authentication using HTML forms
POSTed over HTTP, plus web cookies;
o various bearer token mechanisms [TODO: add examples, such as
OAuth];
o various non-bearer token mechanisms which are not generalized or
generalizable to a larger universe of authentication mechanisms
[TODO: add examples, such as BrowserID].
In enterprise and educational settings it is common to need support
for Kerberos [RFC4120]. Of the above only Negotiate supports
Kerberos meaningfully or at all, and does so badly, requiring re-
authentication for every request in many implementations (among other
problems; see Section 2.1 for more).
Enterprises and educational settings also often have RADIUS
[RFC2865], often used via EAP [RFC3748]. These could be supported by
the generic security mechanism based on EAP, RADIUS, and SAML being
developed by the ABFAB WG [TODO: add references!]. These mechanisms
in particular will be difficult to use via Negotiate because they
involve multiple round trips, which Negotiate supports half-
heartedly; see Section 2.1.
1.1.1. Authentication Infrastructure and Credentials Reuse
All too commonly the community invents new authentication mechanisms
that require their own authentication infrastructures or bridges to
other mechanisms' existing infrastructures. This is a costly habit:
costly for those who must deploy these. A pattern of authentication
mechanism reuse would greatly improve this situation.
Most, if not all [classical, not quantum mechanical] authentication
mechanisms involve an exchange of one or more authentication
messages, accept as input the names of the peers being authenticated,
and on success output some information such as the names of the
authenticated peers, trust transit paths, session keys, and so on.
Actual mechanisms differ in minor details, such as which party sends
the first authentication mechanism (but there is always an initiator,
even if initiation is implied by connecting to a network, for example
as in EAP, which can be thought of as sending an empty initial
authentication message). These differences can be abstracted.
Indeed, we have at least _five_ authentication mechanism frameworks
in Internet protocols:
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o Generic Security Services (GSS-API) [RFC2743];
o Simple Authentication and Security Layers (SASL) [RFC4422];
o Secure Shell version 2 (SSHv2) [RFC4251];
o Internet Key Exchange Protocol versions 1 and 2 (IKEv2) [RFC5996];
o Extensible Authentication protocol (EAP) [RFC3748].
Five authentication frameworks is an embarrassment of riches. And
yet we continually implement new ad-hoc authentication mechanisms --
this is not just embarrassing (it isn't really embarrassing, as it is
a part of the human condition that we continually re-invent things),
but wasteful, both because it means we fail to reuse existing code
and specifications, and because it means users are faced with costly
deployment headaches.
There are also many authentication mechanisms that support (or could
easily be extended to) federation.
The desire to re-invent authentication mechanisms (and frameworks) to
avoid technologies of the past (e.g., ASN.1 and its encoding rules)
or specific instantiations of them (e.g., GSS-API) is understandable,
but at the very least it should be made easy for developers to add
application support for arbitrary authentication mechanisms of any
given users choice. The only way to enable use of the user's choice
of authentication mechanism is through a common protocol that embeds
that authentication mechanism's messages, and that is _exactly_ what
RESTauth aims to be for HTTP.
1.2. Protocol Outline
1. initial authentication messages are POSTed to an agreed-upon or
indicated "login" resource...
2. ....which then results in a new resource being created with the
authentication reply message as the new resource's
representation.
3. Thereafter any additional authentication message exchanges needed
(for multi-legged mechanisms) are POSTed to the new resource
without creating additional resources.
4. The resource created by the POSTing of the initial authentication
mechanism identifies the resulting session, and its URI is known
as the session URI.
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5. Session URIs can be used to multiplex multiple sessions over the
same TCP/TLS connections, implement logout, and share sessions
across multiple related servers.
Authentication using mechanisms that require that the server send the
first authentication message is also possible, in either of two ways:
the initial authentication message is sent in headers in a 401
response, or the client POSTs an empty first message to the login
resource.
1.3. API-Imposed Constraints
To the extent that existing Application Programming Interfaces (APIs)
assume specific styles of HTTP authentication message flows, if we
want those APIs to support RESTauth backwards-compatibly, then those
APIs may impose constraints on RESTauth.
For example, the Android Account Manager API assumes a single round
trip for authentication [TODO: add reference!]. But the Android
Account Manager could perform all but the last round trip on behalf
of the application, then let the application perform the last round
trip. In order for that to work we need the authentication message
exchange to be orthogonal to TCP/TLS connections -- that is, we need
it to be possible to use multiple TCP/TLS connections for completing
a single authentication exchange. This is because the application
and the account manager will likely be using different TCP/TLS
connections.
A typical constraining characteristic might be that an API assumes
the use of GET with tokens encoded into the URI or into a header, or
that the API makes no room for the use of headers in authentication
message exchanges.
One way to work around such constraints may be to provide various
options in RESTauth. Another might be to use OAuth 1.0 [RFC5849] or
2.0 [RFC6749] as a bridge: the API would use this framework under the
covers then obtain OAuth credentials from the server that the
application can then use in any way that the API's form allows for.
[[anchor1: TODO: Add a table/list of various known APIs and their
characteristics that might constrain this and/or other frameworks.]]
1.4. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Alternatives
2.1. In-band HTTP Authentication
RESTauth is "out-of-band" in the sense that the authentication
messages are exchanged independently of the application's requests
for normal resources, with authentication tokens sent as message
bodies rather than as header values. Of course, RESTauth exchanges
may well (and often will) happen in the same TCP/TLS connection as
normal application requests, so RESTauth is not out-of-band in the
sense of using distinct transport connections. We use "out-of-band"
and "in-band" very loosely in this section.
There exist several "in-band" HTTP authentication alternatives where
the authentication message exchanges happen in the context of
application resources. Here the HTTP verb and resource are
application-specific and have nothing to do with authentication, and
the authentication messages are exchanged via HTTP request and
response headers with the server responding with a 401 status code
until authentication is complete.
The extant "Basic" and "DIGEST-MD5" [RFC2617] HTTP authentication
methods, as well as HTTP/Negotiate [RFC4559] are "in-band" HTTP
authentication methods.
In so far as an in-band authentication method results in a cookie or
session URI/ID the distinction between in-band and out-of-band is
almost trivial, as described above: authentication messages in
headers vs. bodies, and HTTP verb and URL. However, if in-line
authentication methods are strongly tied to the TCP/TLS connections
over which they were utilized then that is a big disadvantage over
RESTauth: each connection requires re-authenticating, and support for
HTTP routing schemes is not clear. Indeed, in common implementations
of HTTP Basic, Digest, and Negotiate, cases every request requires
authentication, which can be particularly costly for the Negotiate
case.
Additionally, Negotiate can require multiple round trips but provides
no mechanism for explicitly associating a given GSS-API security
context token with a given GSS-API security context: it has to be
assumed that all messages are serialized and only one multi-round
trip security context establishment can be ongoing in any given HTTP
connection.
Even if the only difference between in-band and out-of-band is a
trivial one, using the REST pattern means that authentication can be
implemented using with no help from the HTTP stack (even though it's
desirable to have it implemented within/by the HTTP stack), whereas
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there may not be a way to implement in-band authentication without
help from the HTTP stack for some stacks.
2.2. Out-of-Band Bearer Token Mechanisms
Some HTTP/web authentication mechanisms used on the web involve out-
of-band (that is, outside the application's HTTP connections to a
server) communications to get bearer tokens, which the application
then includes in its HTTP requests (or perhaps it POSTs them to the
target server).
The main problem with bearer token systems is that they depend
utterly on the ability of TLS to authenticate services. Other
mechanisms, ones that depend on proof-of-secret/
private-key-possession, can often provide better security in the face
of weak TLS server authentication.
2.3. Authentication in TLS
A number of proposals use TLS [RFC5246] for authentication, some
adding extensions for sending user credentials encrypted to avoid the
overhead of renegotiation for privacy protection of user credentials.
There are several problems with user authentication in TLS:
o authentication mechanisms requiring multiple round trips are not
supported, and though there may not be any reason to not permit
them, in practice any extension of TLS handshakes to multiple
round trips is likely to cause much trouble in adapting existing
TLS implementations;
o new interfaces are required, affecting HTTP stacks and
applications both in addition to TLS itself (compare to RESTauth,
which requires only changes to applications, and which welcomes,
but does not require, support by HTTP stacks), and we know from
experience that it takes a long time to deploy solutions that
require modifying implementations of multiple network layers.
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3. Protocol
The are few normative protocol elements here besides the outline
given in Section 1. The normative protocol elements are:
o the form of the WWW-Authenticate header values -in 401 responses-
for RESTauth mechanisms;
o several new headers for advertising negotiable parameters that are
orthogonal to WWW-Authenticate;
o the POSTing of authentication messages from the client, with the
initial client authentication message going to either a pre-agreed
URI or to a URI named in the WWW-Authenticate headers;
o the creation of a session URI as a result of the initial POST, and
the subsequent POSTing of any additional authentication messages
to the session URI;
o the new session URI resource representation resulting from POSTs
being the server's response authentication message, if any;
o the DELETEion of session URIs as signaling logout;
o a new header for referencing session URIs in normal HTTP requests;
o the use of channel binding [RFC5056] to TLS [RFC5246] for session
protection;
And for applications that may not use TLS/HTTPS:
o the use of session keys as an option for integrity protection when
TLS is not used (a light-weight security mode); see
[I-D.williams-websec-session-continue-proto].
3.1. Negotiable Parameters
As can be seen in the ABNF in the preceding section, the server can
offer some negotiable parameters. These are:
o Authentication mechanism names;
o Channel binding types;
o Session binding types;
o Replay protection;
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Each WWW-Authenticate [RFC2617] header value offers a single
mechanism and negotiable parameters for it; because headers can have
multiple values, WWW-Authenticate provides a method for negotiating
authentication mechanisms. The WWW-ChannelBinding-Types header
(added here) allows the server to list channel binding types
supported by it.
3.1.1. Strong Binding to TLS
Strong binding to TLS is provided via channel binding [RFC5056].
When a RESTauth mechanism provides strong authentication of the
service to the user, the combination of RESTauth and channel binding
results in strong authentication of the server to the user even
though TLS is used for session transport protection.
3.1.2. WWW-Authenticate Header Value Prefix Syntax
The ABNF for RESTauth WWW-Authenticate header values is as follows:
challenge = ( "RA-" mechname SP restauth-challenge )
mechname = 1*( ALPHA / DIGIT / "-" )
restauth-challenge = ( login-uri SP session-types SP
replay-prot SP *1(mech-challenge) )
login-uri = absoluteURI
session-types = "s=" session-type /
(session-type ":" session-types)
session-type = "cookie" / "session-ID" /
"channel-bound-session-ID" /
"MAC"
; new session-types may be added
replay-prot = "r=" ("yes" / "no")
mech-challenge = <base64 encoded mech-specific data>
Figure 1: RESTauth WWW-Authenticate ABNF
For a DIGEST-like mechanism it might look like "WWW-Authenticate: RA-
Digest-SHA-256 tls-server-end-point session-ID no HE4SgWGrd/
3+O7t16HqusA==". For example, the mechname for the Kerberos V5 GSS-
API mechanism might be "gss-krb5", and a WWW-Authenticate header
value for it might look like "WWW-Authenticate: RA-gss-krb5
http://foo.example/restauth-login tls-server-end-point channel-bound-
session-ID r=no".
Note that mechanisms that may be used include: GSS mechanisms, SASL
mechanisms, ad-hoc mechanisms, and so on.
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3.1.3. WWW-ChannelBinding-Types Header
A new header is added by which servers MUST indicate which channel
binding [RFC5056] types -if any- they support for RESTauth
authentication; if the server does not support channel binding then
this header MUST be absent. The header is named WWW-ChannelBinding-
Types. Its values are channel binding types from the channel binding
type registry, such as the TLS channel binding types [RFC5929].
3.1.4. WWW-ChannelBinding-Type Header
A new header is added by which clients MUST indicate what channel
binding type they used when POSTing RESTauth authentication messages,
if any; if the client did not use channel binding then this header
MUST be absent. If the mechanism used has its own method for
indicating the use of channel binding, then this header MAY be
ommitted. The header is named WWW-ChannelBinding-Type. Its value is
a channel binding type from the channel binding type registry
[RFC5929].
3.1.5. WWW-SessionBinding-Type Header
A new header is added by which clients MUST indicate what session
binding type they choose when POSTing RESTauth authentication
messages. The header is named WWW-SessionBinding-Type. Its value is
a session binding type as shown in Figure 1. This header SHOULD be
present in RESTauth authentication HTTP requests, but may be ommitted
when the selected mechanism provides its own session binding facility
that is distinct from RESTauth's (this helps adapt OAuth to RESTauth
with minimal or no changes).
3.1.6. WWW-ReplayProtection Header
A new header is added by which clients MUST indicate whether they
desire replay protection when POSTing RESTauth authentication
messages. The header is named WWW-SessionBinding-Type. Its value is
"yes" or "no" (defaults to "no" if absent) as shown in Figure 1.
Replay protection is to be used only when TLS [RFC5246] is not used,
and only if a session binding type of "MAC" is also requested.
3.2. Protocol Flow
RESTauth can be initiated by a client that knows a priori that it
needs to or wants to use RESTauth. Servers can also tell clients
that access to certain resources require authentication, possibly
including RESTauth mechanisms. When the server tells the client that
it must authenticate (using a 401 response, as usual), the server may
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also give the client an initial authentication message for one or
more mechanisms.
When the client knows a priori that it must authenticate then the
client MUST know the RESTauth login URI a priori as well, as well as
negotiable parameters, all of which the client might know from either
an application protocol specification, or from caching this
information from earlier RESTauth exchanges.
The server MUST use a 401 HTTP status code and WWW-Authenticate
headers to inform the client of the need to authenticate in order to
access a given resource. For RESTauth mechanisms the WWW-
Authenticate header values MUST conform to the ABNF given in
Section 3.1.2.
To proceed the client chooses a suitable authentication mechanism
(for which, presumably, it has credentials for a desired client
identity), possibly a channel binding type, possibly a session type,
and whether to use replay protection.
3.2.1. One Round Trip Optimization: Challenges Born in WWW-Authenticate
Headers
Some mechanisms may optimize the protocol flow by allowing the server
to include challenges in the 401 response's WWW-Authenticate header
values. DIGEST-MD5 works this way, for example, sending a challenge
nonce to be fed into the digest function (along with other client-
side inputs).
RESTauth allows this, but this feature is OPTIONAL: it must always be
possible for a client to initiate RESTauth without first obtaining a
challenge in a WWW-Authenticate header value, in which case the
client may incur an extra protocol leg by obtaining the challenge (if
it is at all necessary) in the server's reply to the client's first
authentication message. There are two reasons for making this
optional:
1. to allow client applications that know a priori that they must
authenticate (and how to), requiring no further negotiation;
2. to support authentication mechanisms that require that the client
initiate authentication message exchanges.
A challenge may consist of a nonce, some encrypted or MACed nonce, a
time-stamp, certificates and digital signatures, etcetera. The
server may include a login URI in challenge-laden WWW-Authenticate
headers where the login URI encodes secure state regarding the
challenge (e.g., the challenge encrypted in a symmetric key known
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only to the server).
3.3. Session Binding Types: Cookie, Channel Bound Session URI, and MAC
A notion of session binding type is added for binding HTTP requests
to specific RESTauth login sessions. Three types are provided:
Cookies The traditional HTTP cookie approach to session binding;
Session URI HTTP requests carry a WWW-Session-URI header identifying
the session(s) (similar to cookies, but without all the associated
baggage);
Channel Bound Session URI Like Session URI, but may only be used in
HTTPS connections with the same channel bindings. (This implies
use of the 'tls-server-end-point' channel binding type.)
MAC HTTP requests carry a WWW-Session-URI header identifying the
session(s) and a WWW-Session-MAC header that carries a MAC or MACs
binding the session URI(s) to the request.
3.3.1. The New WWW-Session-URI Header
A new HTTP header is added called WWW-Session-URI whose values
consist of session URIs. At least one session URI MUST be included.
Each session URI is an absoluteURI. Session URIs MUST NOT have
unescaped commas (',') embedded in them. Servers MAY fail to
implement support for multiple session URIs being referenced by a
single request, in which case they MUST answer with error code <TBD>.
Servers MUST validate the session URI before processing the request;
if the session URI is invalid the server MUST respond with a 401 (or
TBD?) status code.
Note that referencing multiple session URIs is permitted, but this
may not be meaningful for the application, thus the server MAY reject
this (TODO: specify a status code for this?).
[[anchor2: I can imagine a webmail application where a client can be
logged in as multiple users and get a unified view of the users'
mailboxes. This seems unlikely, but why rule out such use cases?]]
3.3.2. The New WWW-Session-MAC Header
[[anchor3: Describe the header, its values, algorithm agility, and
what the MAC is to be taken over. Note too that this cannot apply to
request contents as we have to consider chunking, and besides, a MAC
of contents really has to go as a trailer, not a header.]]
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[[anchor4: We may want to remove this anyways and leave it for a
session continuation spec. Or we may want to require the use of
HTTPS.]]
3.3.3. To MAC or not to MAC; A MAC Trailer??
[[anchor5: ... This is only needed for RESTauth *without* TLS, which
will probably not be the common mode of use for RESTauth... unless we
can produce a MAC trailer extension for HTTP/2.0, in which case this
may well become a common mode of RESTauth usage.]]
[[anchor6: We may want to remove this anyways and leave it for a
session continuation spec. Or we may want to require the use of
HTTPS.]]
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4. Representation of Authenticated Session Resources
It will generally be useful to be able to GET a session resource to
obtain information about the authenticated user. A GET on a session
resource which is not fully established SHOULD return an empty body.
[[anchor7: TODO: Add a media type for session resource
representation.]]
[[anchor8: Use JSON instead of ABNF? A schema language would be
nice.]]
session = 1*( session-param )
session-param = session-param-name '=' session-param-value
session-param-name = 1*( ALPHA / DIGIT / '-' / '_' )
session-param-value = <quoted or base64>
Figure 2: Session resource representation ABNF
Session parameters include:
established "true" or "false"
expiration_time Datetime when the session expires.
session_key_MAC_req Session key for MACs in requests.
session_key_MAC_resp Session key for MACs in responses.
authorization_data Information about the authenticated user.
user_id The authenticated user identity.
The server MAY exclude any part of this when the entity requesting a
session resource is the session's user. The server MUST exclude (or
respond with 401) all of the session resource's representation when
the entity requesting it is not authenticated or authorized to see
it. The server SHOULD exclude locally-determined authorization_data
and/or user_id information when the entity requesting the resource is
the session's user.
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5. Session URI Origin and Scope
[[anchor9: TODO: Add a notion of session origin and scope. The
origin can probably be determined naturally from the session URI.
The scope should be set by the server. Perhaps we can also have the
scope reflected in the session URI's representation, which would
allow the scope of a session to change over time.]]
[[anchor10: Clearly, using a session from one origin at another will
require a channel binding verification operation. This will have to
be added.]]
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6. HTTP "Routing" and Authentication
It is common to deploy HTTP services with load-balanced servers
behind a load balancer and TLS concentrator. Other techniques may
also result in a multiplicity of servers acting on behalf of a single
service. The load balancers may even behave like routers and route
HTTP requests to the same server for all requests in a single
connection, or even route HTTP requests according to the verb and
resource. It helps to be able to have a notion of authenticated
sessions that can be referenced by all servers responding to a given
service name.
The server end of a RESTauth authentication message exchange may be
terminated by one server, by many servers sharing session state (via
the resources named by session URIs), or by a server-side HTTP
router. Once a RESTauth session is established we assume that all
servers responding to the same service name will be able to access
the session resource, validate session URIs, and obtain keys for
computing and validating session binding MACs. Alternatively, the
router may take responsibility for session binding and signal
authorization information from the established session to the HTTP
servers behind the router (however, we do not here specify any
methods for such signaling).
By using REST for the authentication message exchange we allow this
disconnection between "session" and "connection", which therefore
facilitates "routing" of HTTP requests and even off-loading of
authentication and/or session binding to HTTP "routers".
This approach should be flexible enough for all existing
architectures for deploying HTTP services.
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7. Actual Authentication Mechanisms
Here we describe (INFORMATIVELY for the time being) how to use or
adapt a variety of authentication mechanisms, from SSHv2, IKEv2,
SASL, GSS-API, and other frameworks, so as to quickly gain a set of
usable mechanisms, both, specification- and implementation-wise.
This section is also intended to show that adding RESTauth mechanisms
is easy.
Reuse of existing authentication mechanisms is a key goal of
RESTauth: let us stop inventing wheels that require costly deployment
of new authentication infrastructures (and credentials) and/or
bridges to other authentication infrastructures.
7.1. OAuth via RESTauth
OAuth 1.0 RFC5849 and OAuth 2.0 [RFC6749] are commonly deployed.
Being able to use OAuth via RESTauth would be useful. We attempt to
make RESTauth such that at least for OAuth 1.0 there is a standard
way to use OAuth such that it conforms to RESTauth.
7.1.1. OAuth 1.0
For OAuth 1.0 [RFC5849] the "form-encoded body" form (see section
3.5.2 of [RFC5849]) of OAuth 1.0 conforms to RESTauth without further
changes.
7.1.2. OAuth 2.0
[It looks like OAuth 2.0 [RFC6749] also uses POST to send tokens to
the server, and it looks like it too effectively conforms to
RESTauth.]
7.2. Adapting SSHv2 Authentication Mechanisms to RESTauth
SSHv2 "userauth" mechanisms [RFC4252] typically involve a digital
signature (or similar) of an SSHv2 session ID. There is no such
thing as an SSHv2 session ID in HTTP. A session URI cannot serve as
a stand-in for an SSHv2 session ID because a) the session URI is an
outcome of authentication in RESTauth, b) to prevent cut-n-paste and
replay attacks the client and the server both must contribute to the
entropy of the session ID that is signed by the client.
In order to adapt SSHv2 userauth methods properly (i.e., securely),
we have replace the SSHv2 session ID in the to-be-signed data with a
hash of the channel binding and nonces contributed by the client and
the server. As an optimization the server nonce can be sent as a
challenge (this saves a round trip).
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7.2.1. RESTauth Mechanism Names for SSHv2 Userauth Methods
For hash agility reasons the hash function name is part of the SSHv2
RESTauth mechanism name. To avoid "multi-level negotiation" the
SSHv2 userauth method name is also part of the RESTauth mechanism
name.
The RESTauth mechanism name form for SSHv2 userauth methods, then,
is: ssh-<SSHv2-userauth-method-name>-<hash-function-name>.
The following RESTauth mechanisms are defined here:
o ssh-publickey-SHA-256
o ssh-hostbased-SHA-256
7.2.2. Nonces
The client and the server must each contribute 128-bit nonces.
7.2.3. "Session ID"
The ssh-publickey-SHA-256 and ssh-hostbased-SHA-256 mechanisms use
the following instead of a traditional SSHv2 session ID:
o SHA-256(channel_binding || server_nonce || client_nonce)
Here the <channel_binding> is as per-[RFC5056]: the channel binding
type name, followed by the channel binding data (e.g., 'tls-server-
end-point' followed by the server EE certificate as sent in the
server's TLS Certificate message).
Note that use of channel binding when using SSHv2 mechanisms is
REQUIRED so as to defeat cut-n-paste attacks by weakly-authenticated
servers.
7.3. Adapting IKEv2 Authentication Mechanisms to RESTauth
[[anchor11: TBD.]]
7.3.1. Adapting IKEv2 Password Authenticated Connection Establishment
(PACE) to RESTauth
[[anchor12: TBD.]]
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7.4. Using SASL Authentication Mechanisms with RESTauth
Simple Authentication and Security Layers (SASL) [RFC4422] is a
simple, pluggable framework for authentication mechanisms.
To use a SASL mechanism in RESTauth just prefix "SA-" to the SASL
mechanism name and use that as the RESTauth mechanism name. If the
SASL mechanism is server-initiated then the server's challenge is
sent in the server's WWW-Authenticate header value as described
above. All other SASL authentication messages are exchanged as
described above (i.e., via POSTs, first to the login URI, then to the
session URI, with response messages as the new representation of the
session resource).
The HTTP status code functions as the application's outcome of
authentication message. If SASL succeeds but authorization fails
then the server should respond with a 401 status code to the POST of
the final SASL authentication message from the client.
The server's WWW-Authenticate header values function as the mechanism
listing operation. SASL security considerations [RFC4422] [RFC5801]
apply (particularly regarding the negotiation of channel binding
support).
7.4.1. Using SCRAM in RESTauth
The Salted Challenge Response Authentication Mechanism (SCRAM)
[RFC5802] is a DIGEST-like mechanism for SASL. Nothing special is
needed to use SCRAM versus any other SASL mechanism, except for a
round trip optimized form of SCRAM, if we decide to pursue that (see
Section 7.4.2).
The following figure shows what SCRAM in RESTauth looks like. Note
that the resource representations are taken verbatim from [RFC5802].
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C->S: GET /some-resources HTTP/1.1
Host: A.example
S->C: HTTP/1.1 401 Unauthorized
WWW-Authenticate: RA-SA-SCRAM-SHA-1 \
http://A.example/rest-sa-scram \
s=session-ID,MIC r=no
WWW-ChannelBinding-Types: tls-server-end-point
C->S: POST /rest-sa-scram HTTP/1.1
Host: A.example
WWW-ChannelBinding-Type: tls-server-end-point
WWW-SessionBinding-Type: session-ID
Content-Type: application/octet-stream
Content-Length: nnn
n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
S->C: HTTP/1.1 201
Location http://A.example/restauth-9d0af5f680d4ff46
Content-Type: application/octet-stream
Content-Length: nnn
r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
s=QSXCR+Q6sek8bf92,i=4096
C->S: POST /restauth-9d0af5f680d4ff46 HTTP/1.1
Host: A.example
Content-Type: application/octet-stream
Content-Length: nnn
c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
S->C: HTTP/1.1 200
Content-Type: application/octet-stream
Content-Length: nnn
v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
Figure 3: RESTauth w/ SCRAM
7.4.2. Using SCRAM with Round Trip Optimization in RESTauth
[[anchor13: This might work by having the authentication ID function
as the salt and the server offering a challenge nonce and iteration
count in its optimistic challenge. However, it's not clear that a
round trip optimized form of SCRAM is desirable.]]
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The following figure shows what a round trip optimized RESTauth w/
SCRAM exchange might look like.
[[anchor14: NOTE: SCRAM was not intended to be used this way. In
particular this approach forces the use of an algorithmic salt, to be
derived only from either the username or the username and the
server's name (or else to be remembered by the user, but that's not
likely).]]
C->S: GET /some-resources HTTP/1.1
Host: A.example
S->C: HTTP/1.1 401 Unauthorized
WWW-Authenticate: RA-SA-SCRAM-SHA-1 \
http://A.example/rest-sa-scram \
s=session-ID,MIC r=no \
r=fyko+d2l...JY1ZVvWVs7j,i=4096
WWW-ChannelBinding-Types: tls-server-end-point
C->S: POST /rest-sa-scram HTTP/1.1
Host: A.example
WWW-ChannelBinding-Type: tls-server-end-point
WWW-SessionBinding-Type: session-ID
Content-Type: application/octet-stream
Content-Length: nnn
n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL,
c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
S->C: HTTP/1.1 200
Content-Type: application/octet-stream
Content-Length: nnn
v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
Figure 4: RESTauth w/ round trip optimized SCRAM
7.5. Using GSS-API Authentication Mechanisms with RESTauth
The Generic Security Services Application Programming Interface (GSS-
API) [RFC2743] is another pluggable mechanism framework. Any GSS-API
mechanism that supports channel binding [RFC5056] can be used as SASL
mechanisms via the "SASL/GS2" bridge [RFC5801]. This includes the
Kerberos V5 GSS-API mechanism [RFC4121].
GSS-API security mechanisms could also be used without SASL/GS2, but
SASL/GS2 barely adds any overhead or complexity (a SASL
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implementation is not required in order to use SASL/GS2, just a GSS
implementation): a simple header is to be prefixed to the initial
security context token and to the channel binding data, with both
peers always providing channel binding data.
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8. Implementation Advice
RESTauth can be implemented without having to modify the HTTP nor TLS
stacks.
The simplest thing to do is to implement a small API to produce and
consume HTTP messages.
8.1. Server-Side Implementation Advice
On the server side the simplest thing to do is to implement POST
handlers for the login and session resource URI namespace. The
aspects of implementation that will be stack-specific are:
o WWW-Authenticate header value generation;
o Routing of GET and POST requests on the login and session
resources to the RESTauth handlers;
o The interface between the web server and the handlers.
The FCGI interface is widely supported, allowing RESTauth to be
implemented in a nearly universal way on the server side.
8.1.1. Channel Binding on the Server Side
The simplest way to implement channel binding on the server side is
to use 'tls-server-end-point' channel bindings, using a table of
server certificates indexed by fully-qualified server hostname
values, with the Host: header value used to index this table. The
server's end entity (EE) certificate is the channel binding data.
8.1.2. Server Cluster / Routing Support
Because each RESTauth session is a first-class resource named by a
URI (natch), multiple servers behind a given origin may recognize and
handle all the sessions that they should be able to: all the sessions
created at that origin, and all the sessions scoped to include that
origin. This is handled by the simple expedient of doing an HTTP GET
of the session resource claimed by a client. Note that the
representation of a session resource can be cached easily, and
updates can be checked with HEAD or conditional GETs, as one would
expect of any RESTful HTTP API.
It is important that server MUST NOT attempt to GET (or HEAD) session
resources for origins that the server does not respond to or which
the server does not expect to share sessions with the server's
origin(s).
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8.2. Client-Side Implementation Advice
There are many HTTPS client stacks, too many, each with its own APIs,
to make it possible to implement RESTauth universally. The
application will have to bridge any generic RESTauth APIs with the
HTTP stack.
A reasonable implementation strategy is to build a generic RESTauth
interface that
o consumes 401 responses (or just their WWW-Authenticate header
values)
o consumes channel binding data (see below)
o generates HTTP requests to send to login or session URIs
o consumes HTTP responses to requests sent to login or session URIs
The application would have to extract relevant responses and channel
binding data from the HTTP stack to feed to the RESTauth interface,
and it would have to extract requests from the RESTauth interface and
feed them to the HTTP stack.
The interfaces, abstractly, should be:
restauth_new() Create a RESTauth context handle;
restauth_401() Consume a 401 and update or create a RESTauth context
handle;
restauth_login() Produce a POST to a login resource URI (either
provided explicitly or obtained from a 401);
restauth_login_continue() Consume a response to a POST to a login
resource and output a) status (complete or continue), b) possibly
a POST to a session URI;
restauth_logout() Produce a DELETE of the given session URI or the
given RESTauth context's session URI.
Channel binding type and data would be given to restauth_login().
Specific programming language bindings of this API are easy enough to
produce. Whether the API outputs or consumes complete messages or
decomposed messages (start-line, headers, body, trailers) will depend
on the APIs of the HTTP stack being targeted. A utility API that
composes/decomposes HTTP messages will help make a RESTauth
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implementation widely reusable.
8.2.1. Channel Binding on the Client Side
Most HTTPS stacks provide a way for the client application to obtain
the server's EE certificate; this is sufficient to implement 'tls-
server-end-point' channel binding.
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9. IANA Considerations
TBD (header registrations, ...)
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10. Security Considerations
This entire document deals with security considerations. [Add more,
like about channel binding, same-origin-like constraints on the login
and session absolute URIs', ...]
Note that though servers can GET/HEAD session resources, they MUST
only do it for session resources for recognized origins. See
Section 8.1.2.
[[anchor15: ...]]
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11. TODO
[[anchor16: Add references (to HTTP/2.0, CGI/fCGI, ...).]]
[[anchor17: Decide whether to support a MAC type of session
continuation, or only channel bound sessions.]]
[[anchor18: Describe or remove the MAC session binding option and
replay protection in detail -- or remove it altogether. Describe how
to extract keys for MAC keying from SASL/GSS/PACE.]]
[[anchor19: Figure out how to adapt IKEv2 password-based methods to
RESTauth. This may not be worthwhile (since each method tends to
depend heavily on the entire IKEv2 framework in ways that add
messaging that we'd not need in RESTauth).]]
[[anchor20: Normatively specify bindings to SASL and/or GSS-API
security mechanisms. Include support for BrowserID.]]
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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.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
[I-D.williams-websec-session-continue-prob]
Williams, N., "Hypertext Transport Protocol (HTTP) Session
Continuation: Problem Statement",
draft-williams-websec-session-continue-prob-00 (work in
progress), January 2013.
[I-D.williams-websec-session-continue-proto]
Williams, N., "Hypertext Transport Protocol (HTTP) Session
Continuation Protocol",
draft-williams-websec-session-continue-proto-00 (work in
progress), January 2013.
12.2. Informative References
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
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RFC 3748, June 2004.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, June 2006.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
[RFC6631] Kuegler, D. and Y. Sheffer, "Password Authenticated
Connection Establishment with the Internet Key Exchange
Protocol version 2 (IKEv2)", RFC 6631, June 2012.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework",
RFC 6749, October 2012.
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Author's Address
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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