Network Working Group N. Williams
Internet-Draft Cryptonector
Intended status: Informational June 11, 2012
Expires: December 13, 2012
A Proposals for Classification and Analysis of HTTPbis Authentication
Proposals
draft-williams-httpbis-auth-classification-01
Abstract
This document proposes a classification scheme for HTTPbis
authentication proposals, to help with analysis and selection.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Threat Models . . . . . . . . . . . . . . . . . . . . . . 9
2.2. On Trust . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. On the TLS Server PKI . . . . . . . . . . . . . . . . . . 10
2.4. On Mutual Authentication and URI Schemes . . . . . . . . . 11
2.5. On Authentication Mechanism Message Counts . . . . . . . . 11
2.5.1. On One-Message Authentication Mechanisms . . . . . . . . . 12
2.6. Logon Sessions . . . . . . . . . . . . . . . . . . . . . . 12
2.7. Web Cookies, a Form of Bearer Tokens . . . . . . . . . . . 12
2.8. User Interface Issues . . . . . . . . . . . . . . . . . . 13
3. Classification Axes . . . . . . . . . . . . . . . . . . . 14
3.1. Dependence on TLS Server PKI . . . . . . . . . . . . . . . 15
3.2. Bearer Tokens vs. Proof of Possession . . . . . . . . . . 15
3.3. Layer at which Authentication Protocol Operates . . . . . 15
3.3.1. HTTP- vs. Application-Layer Authentication in the
Network Stack . . . . . . . . . . . . . . . . . . . . . . 17
3.3.2. HTTP- vs. Application-Layer Authentication in the API
Stack . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3.3. Choice of Layer . . . . . . . . . . . . . . . . . . . . . 21
3.3.4. User Authentication in the TLS Layer . . . . . . . . . . . 22
3.4. Party Responsible for Infrastructure Messaging . . . . . . 23
3.5. Number of Messages . . . . . . . . . . . . . . . . . . . . 24
3.6. Trust Establishment . . . . . . . . . . . . . . . . . . . 26
3.7. Threat Modeling . . . . . . . . . . . . . . . . . . . . . 28
3.8. Explicit versus Implicit Session Management . . . . . . . 28
3.9. In-Band versus Out-of-Band Authentication . . . . . . . . 28
4. Analysis of Some Possible Authentication Proposals . . . . 29
5. Author's Recommendations . . . . . . . . . . . . . . . . . 30
6. References . . . . . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
The HTTPbis WG is accepting proposals for new authentication systems
for HTTPbis, the successor to Hypertext Transport Protocol (HTTP)
version 1.1[RFC2616]. This document proposes a classification system
for these proposals. Several axes of classification are proposed,
and several simplified imagined or likely authentication systems are
used to illustrate the classification system.
The author assumes that the WG is interested primarily in new user
authentication proposals, with ones that provide mutual
authentication (of users and servers to each other) being in scope.
The author also assumes that Transport Layer Security (TLS) [RFC5246]
will continue to be used by HTTPbis for cryptographic session
protection.
Some familiarity with authentication systems is assumed. A glossary
is provided.
1.1. 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].
1.2. Scope
This document considers user authentication only in the context of
HTTP applications, whether they be web applications or otherwise.
Authentication of the service is also in scope, but authentication
methods that authenticate only the user to the service (with the
service authenticated by Transport Layer Security (TLS)) are in
scope.
There are at least two entities involved in authentication in this
context: the user (on the client side), one or more of the web server
host or the web server application/service, and any trusted third
parties that an authentication mechanism might involve.
1.3. Glossary
This section defines terms as they are used in _this_ document.
Readers are strongly encouraged to read this section before reading
any subsequent section.
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API Application Programming Interface. These are interfaces between
an application and a feature that is abstracted into a "library" -
a service provided by the platform's operating system.
API Layer A complex Internet application might require a large
number of APIs, such as, for example, one for every network layer.
In practice it is more common to have a single API that
encompasses all network layers below it, with the component
providing that API likely invoking other APIs itself. which in
turn invoke other APIs. For example, a web application might use
a library that presents a single API to all of the HTTP network
stack from HTTP all the way down to IP. Note that there need not
be a direct correspondence of network and API layers.
Authentication The process of establishing the veracity or origin of
some statement (e.g., of an entity's identity), usually by proxy
(e.g., with key-pairs to an asymmetric key cryptographic system
"speaking for" the authenticated entities). In this document, and
unless otherwise stated, "authentication" will refer to
authentication of identity of entities such as "users", "hosts",
and "services".
Authentication Mechanism A cryptographic protocol for authenticating
entity identities. Note that this does not cover POSTing
usernames and passwords in forms, but it does cover bearer token
mechanisms (if just barely).
Authentication Method A scheme for authenticating entity identities.
An authentication method can be non-cryptographic, covering HTTP
Basic authentication and usernames&passwords POSTed from HTML
forms.
Authentication Framework A protocol into which other authentication
mechanisms may be plugged in. For example: SASL[RFC4422], GSS-
API[RFC2743], EAP[RFC3748], among others.
Bearer Token A technique for authentication that involves a message
that can be presented by the authenticating entity to another. No
proof of possession is required for using bearer tokens, which
means that the token can be presented by any entity possessing the
token, which in turn means that bearer tokens must be sent with
confidentiality protection, as otherwise eavesdroppers can steal
them and use them to impersonate the subject.
Channel Binding A security protocol composition and analysis tool.
The purpose of channel binding[RFC5056] is to "bind" a secure
channel (at one layer in the network stack) into an authentication
protocol running at a higher layer in the stack, thereby ensuring
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that the channel is end-to-end and "speaks for" its end-points.
Confidentiality protection Cryptographic encryption of data.
Confidentiality protection is/must always be used with integrity
protection as well.
Data authentication Data origin authentication, a.k.a., integrity
protection.
Hardware Security Module (HSM) A hardware component of security-
critical trusted third parties. An HSM is intended to be
reasonably secure against physical and software attacks against it
-much more than traditional servers-, thus making it ideal for
storing non-extractable secret/private key material.
Integrity protection Cryptographic protection against modification
of data. See also "data authentication", above.
Mechanism Shorthand for "authentication mechanism", a protocol
defining messages to be exchanged in order to authenticate one
party to another (or two parties to each other).
Mutual Authentication Authentication of a user and a server/service
to each other.
Mutual Authentication (key confirmation sense) In some protocols key
exchange is bound to authentication of the service to the user
such that the service is finally authenticated when it sends a
proof-of-possession of the exchanged session key back to the user.
Protocols that use RSA key transport (e.g., TLS in common usage),
Diffie-Hellman with a persistent public key for the server, or
Needham-Schroeder protocols (such as Kerberos[RFC4120]), perform
server authentication in this way. A client may not always care
to receive key confirmation. For example, a Kerberos client for a
lossy logging application might not care that confidentiality
protected data ends up at the wrong server, as long as unintended
servers can't decrypt the data. Some clients may send application
data optimistically ahead of key confirmation from the server.
Such data should generally be confidentiality protected, and the
protocol should not be subject to MITM attacks where the MITM can
somehow modify what optimistic data is sent, nor should an active
attacker be able to replay such optimistic data.
Network Layer A layer in the OSI or Internet network model.
Examples of layers that are relevant to HTTP applications: IP,
TCP/UDP, TLS, HTTP, and the application layer.
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Proof of Possession A technique for authentication that involves
using a cryptographic operation to "prove" (not necessarily in a
rigorous sense) that the entity that creates the proof has access
to a private/secret key to a cryptosystem (e.g., a private RSA
key, a secret AES key, etcetera).
Public Key Infrastructure (PKI) An authentication system based on
public key cryptography and supporting hierarchical transitive
trust via trusted third parties known as Certificate Authorities
(CAs).
Relying Party An entity that authenticates another. For example, in
PKI the entity that validates another's certificate as part of the
process of authenticating that other entity, is a relying party.
SCRAM Salted Challenge Response Authentication Mechanism
(SCRAM)[RFC5802], a SASL[RFC4422] and GSS mechanism based on
password-derived pre-shared keys and challenge/response. SCRAM is
intended as the successor to SASL's DIGEST-MD5, and possibly to
HTTP's DIGEST-MD5.
Server A system with one or more IP addresses, serving HTTP on one
more TCP ports on those IP addresses. [A general definition would
not be constrained to HTTP only, but for the purposes of this
document this is good enough.]
Service An entity providing a service or services for an
application. Typically -but not always!- a service is closely
related to a host server, which may provide several services.
Usually we need to distinguish between the various services that a
single host provides, thus we often need to authenticate the
_service_ rather than the host server. For HTTP applications a
service may be a collection of resources available on one (or
more) ports on a given server.
Trust (in authentication) This word, "trust", is a terrible word: it
means too many things to too many people. But it's also a very
convenient word when everyone understands the meaning to be
accorded to it in any given context. For the time being this
document will use this word, "trust", as follows: to trust an
entity is to accept as fact assertions -relating to other
entities- made by the trusted entity. Alternative phrasing: to
trust an entity is to rely on it to make assertions relating to
other entities the truth of which cannot otherwise be ascertained.
For example, in a PKI a relying party relies on the certification
authorities (and related infrastructure) to make statements of
facts of the form "the public key <key> belongs to <subject name>"
(details elided). We only use "trust" in connection to "trusted
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third parties" - when an authenticated entity makes assertions
about itself we do not speak of trusting them to do so.
Trust (in user interfaces) One of the many alternative meanings of
"trust", and the only alternative one used in this document,
relates to user interfaces, namely: a trusted user interface is
one that the user can somehow ascertain that it is presented by
the operating system or browser platform and _not_ by some
possibly malicious peer.
Trust Path Continuing with the horrible word "trust", we use "trust
path" to the note the list of trusted third parties involved in
authenticating an entity to a relying party. This list is
ordered, though it could conceivably be set of lists when multiple
trust paths are possible.
Trusted Third Party An entity that can be relied up -by those
relying parties that trust it- to make assertions relating to
other entities, typically assertions about how to authenticate
those entities and/or of facts relevant to authorization at the
relying party.
[[anchor1: Fill out! Add some entries for OAuth, Kerberos, Basic,
DIGEST-MD5, EAP, GSS, SASL, ...]]
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2. Background
Web applications today use a variety of user authentication methods,
many of which are somewhat or deeply unsatisfying. Almost all of
these methods involve the user-agent being mostly dumb - not
participating in any cryptographic protocols other than TLS.
The most common user authentication methods used in web applications
today include:
o Username and password POSTed to the server from an HTML form.
Usually the URL to post to is an HTTPS URL. Not as often the URL
of the HTML page containing the form is also an HTTPS URL.
o HTTP Basic or DIGEST-MD5 authentication.
o Out-of-band methods:
* PINs sent to user devices via SMS (POSTed along with passwords)
* OTP tokens (POSTed along with passwords)
* login URLs e-mailed to the user
* passwords e-mailed to the user
Not much use is made of TLS user certificates, though that is
available as well.
These methods are somewhat-to-highly unsatisfactory for a variety of
reasons:
o Users have to remember/carry too many passwords, even when they
have many fewer "identities" (typically in the form of e-mail
addresses).
* Credential sharing becomes a problem: compromise of one site
can result in compromise of user accounts at unrelated sites.
Also, a malicious site posing as a friendly site can do the
same.
o The service is generally not authenticated to the user. TLS does
authenticate the server, but not necessarily the service, and
anyways only to the best of the TLS server PKI's ability.
* This problem derives in part from the nature of the HTTP URI
scheme: by identifying server hosts rather than services the
HTTP URI scheme fails to provide the user and user-agent with
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enough information by which to identify, and thence
authenticate, a service. New URI schemes may be required.
o The TLS server PKI is fundamentally weak.
o User credentials are too easy to "phish".
o OTP and out-of-band methods do not protect against MITMs, and thus
depend on the integrity of TLS and the TLS server PKI.
o HTTP/Negotiate[RFC4559], which effectively uses GSS-API[RFC2743]
mechanisms, usually NTLM [XXX Add reference] or Kerberos[RFC4120],
[RFC4121].
Additionally, there is no strong concept of "sessions" in web
applications. Sessions, such as they are, consist of HTTP requests
and responses united into a session by the web cookies they bear.
Not all web cookies are used for identifying sessions, and there is
no simple "logout" functionality. The biggest problem with web
cookies is that they are too easy to misuse or steal (e.g., given the
occasional TLS vulnerability, such as BEAST [XXX Add references!]).
Furthermore, there are uncomfortable user interface (UI) problems.
In particular it is difficult to convey to the user information about
the server's/service's identity and how it is authenticated (if at
all).
HTTP applications that are not web application have similar issues,
though some of them can also use SASL[RFC4422]. Non-web HTTP
applications also may not need cookies, instead using a single
HTTP/1.1 persistent connection over which to issue all requests that
make up a session - such applications have a stronger sense of
session than web applications do.
[[anchor2: XXX Finish this section.]]
2.1. Threat Models
[[anchor3: Talk about threat models and which are appropriate for
HTTPbis. Discuss the Internet threat model and its flaws (namely/
primarily, the local security assumption).]]
2.2. On Trust
[[anchor4: Describe issues w.r.t. "trust", such as transitivity,
introductions, and so on. This is important for evaluating
proposals. A proposal that replaces the TLS server PKI's primacy
with... another system with similar transitive trust issues may not
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be a useful proposal. On the other hand, it seems impossible to
avoid transitive trust when scaling to Internet scale. Understanding
this may help, for example, give impetus to improvements to the TLS
server PKI, or it may guide replacements, understand scalability, and
so on.]]
2.3. On the TLS Server PKI
The TLS server PKI, and, truly, any hierarchical or flat PKI intended
for authenticating servers or services to users has a fundamental
problem: the number of Names for which to issue certificates is too
large to expect the PKI administrators to do a good job of keeping
out the bad guys. Bad guys use any number of phishing techniques
such that the names of their services need not even match those of
the services that they wish to steal credentials for. The goal
should be to keep the bad guys out altogether, but this is also quite
difficult, if not impossible for many reasons including political
ones.
The TLS server PKI suffers from a number of other non-fundamental
problems, mostly due to legacy deployment:
o x.500-style naming, which utterly fails to match Internet naming
(domainnames, e-mail addresses, etcetera);
* The addition of subjectAlternativeName (SAN) does not
successfully address this problem because a) the
TBSCertificate's primary Name is still limited to being an
x.500 name, and b) too much of the deployed relying party base
simply lacks SAN support.
o incomplete implementations of the PKIX standards;
* For example, missing implementations of name constraints,
leading to the inability of CAs to safely issue intermediate CA
certificates to their customers as either such certificates
cannot contain critical name constraints or those are ignored
by some relying parties anyways, thus intermediate CAs have no
real constraints other than those enforceable by HSMs.
* In general it is not possible to make use of critical
certificate extensions in certificates that will be presented
to the Internet web's user-agents: they will either ignore such
extensions, fail soft (by prompting the user as to whether to
continue or fail), or fail hard. None of these relying party
behaviors are desirable on the Internet. This problem arises
from the nature of security protocols that use PKIs, which in
turn results from the off-line infrastructure nature of PKIs.
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By encouraging non-negotiation of security features, PKI pushes
future extensions into a critical/non-critical dichotomy, but
since critical extensions are difficult to deploy, the result
must either be additional negotiation in protocols using PKI
(e.g., TLS SNI), or non-use of critical extensions. Compare to
Kerberos, where there is a negotiation between the client and
the server _by proxy_ (i.e., mediated by the KDC).
The TLS server PKI also suffers from all the problems that trusted
third party systems suffer from, namely: the need to trust the third
parties. Fortunately there are a number of efforts under way to
improve the trustworthiness of TLS server PKI CAs by, for example,
making them auditable by the public [XXX Add references to CT,
Convergence, HSTS/TACK/other pinning schemes, and others!]
And yet the TLS server PKI is here to stay. It will not go away. We
can only minimize the dependence of the web's security on the TLS
server PKI. To do so requires authentication mechanisms that can
provide authentication of the server to the user in some manner such
that none of the above problems apply. The hardest PKI problem to
address is the fundamental problem described above: this requires
accepting a smaller scale of server/service authentication to the
user - a balkanization of sorts of the web, but see the discussion of
trust islands in Section 3.6.
2.4. On Mutual Authentication and URI Schemes
[[anchor5: Describe the limitations imposed by the Internet threat
model when there is no mutual authentication. Describe the two
types/senses of mutual authentication: authenticating the server (in
addition to the client) and key confirmation. Describe the
limitations, imposed by the HTTP URI scheme, on service
identification and authentication.]]
2.5. On Authentication Mechanism Message Counts
All authentication mechanism require some number of messages in order
to authenticate an entity. For example, TLS generally requires two
round-trips, while OAuth requires a single message from the client to
the server. Here we count only messages from the HTTP client to the
HTTP server; additional message exchanges may be required involving
trusted third parties.
The number of authentication messages that must be exchanged for a
given authentication mechanism is important. The API of at least one
important credential management facility is premised on
authentication mechanisms having exchanges of just one message -
adding new API is possible, but it would take a long time for
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applications to begin using it. Thus mechanisms that require just
one message are at a premium (but see the next section).
The number of authentication messages is also important for latency
reasons: since authentication message exchanges are synchronous, each
round trip time is added to the latency observed by the user.
The number of messages that an authentication mechanism needs to
exchange with infrastructure (e.g., trusted third parties) also
affects latency, but at least applications need never be aware of
messages exchanged with infrastructure - these can be abstracted away
by the APIs. Some authentication mechanisms have fast re-
authentication facilities such that the latency cost of
infrastructure messaging need not be incurred as frequently as the
entity authenticates to others.
[[anchor6: ...]]
2.5.1. On One-Message Authentication Mechanisms
Half round trip mechanisms depend utterly on some other system for
authentication of the server - in webauth this means the TLS server
PKI. To understand why imagine that an application sends the one
authentication message to a service, but it turns out that it is
speaking to an impersonator for that service. The impersonator can
at the very least obtain any sensitive data that the application is
willing to send immediately. Additionally, if there's no channel
bindings between the authentication mechanism and the service
impersonator then the one message can be sent by the impersonator to
the real service, letting the service impersonator impersonate the
user to the real service as well (thus being a proper MITM).
There exist a number of one-message webauth authentication mechanisms
that are widely deployed; we cannot forbid their use, we can only
document their security considerations, namely: that they depend
entirely on the TLS server PKI for their security.
2.6. Logon Sessions
[[anchor7: Discuss the binding of HTTP requests (and responses) to
logon sessions. Discuss logout.]]
2.7. Web Cookies, a Form of Bearer Tokens
[[anchor8: Discuss cookies as a form of bearer token and how the
situation is not as dire as with bearer tokens for user
authentication. Discuss alternatives based on MACing portions (or
all) of the HTTP requests (and responses) or the channel bindings
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data for the TLS channel.]]
2.8. User Interface Issues
[Discuss phishing issues, in particular the difficulty of creating
user interfaces in web apps that cannot be spoofed by either server
impersonators or MITMs. Reference Sam Hartman's anti-phishing I-D
[I-D.hartman-webauth-phishing].]
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3. Classification Axes
Several orthogonal classification axes are proposed:
1. Dependence on/independence of the TLS server PKI;
2. Solutions based on bearer tokens vs. ones based on proof of
possession;
3. Layer at which user authentication takes place: TLS, HTTPbis, or
the application layer (note: distinguishing network layer from
API layer);
4. Whether the client, the server, or both, engage in infrastructure
messaging;
5. Number of messages exchanged / "round trips";
6. Trust establishment: pair/group-wise non-transitive, federated or
otherwise transitive, hierarchical vs. mesh;
7. Threat modeling;
8. Explicit versus implicit session management;
9. In-band / out-of-band.
[[anchor9: Maybe add something about separation of password verifier
access, to limit the attack surface area for password recovery?]]
[[anchor10: Note: The author assumes that all acceptable proposals
will have HTTPbis continue to depend on TLS for transport security -
for confidentiality (encryption) and integrity (authentication)
protection of data exchanged by the HTTPbis client and server. If
this assumption is incorrect then we can add one more axis of
classification: dependence on / independence of TLS.]]
These nine classification axes are largely orthogonal to each other.
Other classification criteria are also possible and may be added in
future versions of this Internet-Draft. Some such possible
additional criteria are subjective, such as, for example: ease of
deployment, ease of implementation, etcetera. Perhaps the WG can
come to consensus regarding desirable properties based on objective
classification to narrow the set of proposals to consider. Or
perhaps the WG can consider a large number of proposals and use
objective classification to guide any applicability statements for
the proposals accepted. Ideally the WG can apply objective
classification first, then for each "bucket" of similar proposals the
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WG could consider more subjective classification criteria.
3.1. Dependence on TLS Server PKI
The web today depends utterly on the "TLS server PKI" for security.
This would be just fine were it not for the systemic weaknesses in
the TLS server PKI: the lack of name constraints, the large number of
trust anchors, the large number of certificate authority (CA)
compromises, and so on. Building on the TLS server PKI and thus
assuming its being sufficiently secure, is quite tempting, as it may
simplify various aspects of user authentication (not least by
providing server authentication a priori, thus saving the designers
the need to provide server authentication themselves).
This classification axis is very simple: either a proposed solution
depends on the TLS server PKI or it doesn't. Some shades of black
are imaginable in this case (if not likely).
3.2. Bearer Tokens vs. Proof of Possession
A bearer token is a message the presentation of which is sufficient
to authenticate the presenter. Stolen bearer tokens may be used to
trivially impersonate the subject, thus bearer tokens generally
require confidentiality protection in any protocols over which they
might be exchanged, and generally depend on authenticating the
relying party first.
Proof of possession systems consist of some secret/private key(s), an
authenticator message the "proves" possession of the secret or
private key(s) used in the construction of the authenticator, and a
token not unlike a bearer token but which securely indicates to the
relying party(ies) what keys the user must have used in the
construction of the authenticator. The relying party then validates
the authenticator to establish that the user did indeed possess the
necessary secret/private key(s) to the best of the cryptographic
capabilities of the authentication system used.
3.3. Layer at which Authentication Protocol Operates
It is possible to design user (and mutual) authentication mechanisms
that can work at any end-to-end layer between the HTTPbis client and
server. The relevant layers are:
o TLS,
o HTTPbis,
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o and the application layer.
We dismiss out of hand the possibility of that layer being TCP or
IPsec, though admittedly they are also end-to-end layers where user
authentication could theoretically be done.
We distinguish between network layers and API layers (see glossary).
A solution at the application _network_ layer might nonetheless be
implemented at the HTTP _API_ layer (and vice-versa).
User authentication is generally something that a transport layer
cannot know to initiate on its own: the application must be in
control of when (server- and client-side) to authenticate, how
(server- and/or client-side), with what credentials / as whom
(client-side). This means that authentication in the transport layer
requires APIs that give the application a measure of control. HTTP
API capabilities will vary, but HTTPbis is a good opportunity to
standardize an abstract API outlining capabilities and semantics to
be exposed to applications by an HTTP stack.
Note that on the user-agent side the platform may provide user
interaction facilities for authentication, thus simplifying user
authentication APIs. The application, on the server side, remains in
control over when to initiate authentication.
End-to-end session cryptographic protection is best done in the
lowest possible transport layer. For HTTP applications, historically
this means TLS; though it'd be technically feasible to provide
protection at lower layers it does not appear to be a realistic
option at this time.
User authentication is best "bound" into transport security layers,
in this case TLS. When user authentication is moved to higher layers
a "channel binding" problem arises: we would like to ensure that no
man-in-the-middle exists in the transport layer, with the MITM
terminating two TLS connections. For more information about channel
binding see [RFC5056].
UI and API issues are quite different for web applications versus
non-web applications. The former have rich UI elements (all of
HTML's) and programming models (scripting, particularly through
JavaScript). One problem that is particularly severe for web
applications, is the ability of server impersonators to emulate all
imaginable graphical user interfaces that the native user-agent might
wish to use to distinguish itself from the applications it runs.
Regardless of what layer implements authentication this problem will
arise in web applications.
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3.3.1. HTTP- vs. Application-Layer Authentication in the Network Stack
It's important to note that there need not be much difference between
HTTP-layer and application-layer user authentication, at least if we
assume a standard application-layer user authentication convention.
For argument's sake let's assume an application-layer user
authentication convention like the one in [I-D.williams-rest-gss],
and let's assume two possible HTTPbis HTTP-layer authentication
solutions: one that is most similar to HTTP/1.1's and one that uses a
new verb for authentication. Then let's look at what each of these
three solutions look like on the wire using the SCRAM mechanism for
cases where the client already knows it has to authenticate. For
brevity we elide any HTTP request and response where the server
indicates that the client must authenticate, as well as any requests/
responses involving negotiation of mechanism to use.
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C->S: HTTP/1.1 POST /rest-gss-login
Host: A.example
Content-Type: application/rest-gss-login
Content-Length: nnn
SCRAM-SHA-1,,MIC
n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
S->C: HTTP/1.1 201
Location http://A.example/rest-gss-session-9d0af5f680d4ff46
Content-Type: application/rest-gss-login
Content-Length: nnn
C
r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
s=QSXCR+Q6sek8bf92,i=4096
C->S: HTTP/1.1 POST /rest-gss-session-9d0af5f680d4ff46
Host: A.example
Content-Type: application/rest-gss-login
Content-Length: nnn
c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
S->C: HTTP/1.1 200
Content-Type: application/rest-gss-login
Content-Length: nnn
A
v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
Figure 1: REST-GSS Login w/ SCRAM Example
Figure 1
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C->S: HTTP/1.1 LOGIN
Host: A.example
Content-Type: application/SASL
Content-Length: nnn
SCRAM-SHA-1,,MIC
n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
S->C: HTTP/1.1 201
Location http://A.example/login-session-9d0af5f680d4ff46
Content-Type: application/SASL
Content-Length: nnn
C
r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
s=QSXCR+Q6sek8bf92,i=4096
C->S: HTTP/1.1 LOGINCONTINUE /login-session-9d0af5f680d4ff46
Host: A.example
Content-Type: application/SASL
Content-Length: nnn
c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
S->C: HTTP/1.1 200
Content-Type: application/SASL
Content-Length: nnn
A
v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
Figure 2: HTTPbis w/ New Verb Login w/ SCRAM Example
Figure 2
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C->S: HTTP/1.1 GET /location/of/interest/to/app
Host: A.example
S->C: HTTP/1.1/401 Unauthorized
Server: HTTPd/0.9
Date: Sun, 10 Apr 2005 20:26:47 GMT
WWW-Authenticate: <list of mechanisms>
Content-Type: text/html
Content-Length: nnn
<error document>
C->S: HTTP/1.1 GET /location/of/interest/to/app
Host: A.example
Authorization: SCRAM-SHA-1,,MIC
n,,n=user,r=fyko+d2lbbFgONRv9qkxdaw
S->C: HTTP/1.1 4xx
WWW-Authenticate: C
r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
s=QSXCR+Q6sek8bf92,i=4096
WWW-Authenticate-Session: 9d0af5f680d4ff46
C->S: HTTP/1.1 GET /location/of/interest/to/app
Host: A.example
Authorization-Session: 9d0af5f680d4ff46
Authorization: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
S->C: HTTP/1.1 200
WWW-Authenticate: A
v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
Content-Type: ...
Content-Length: nnn
<content>
Figure 3: Extended HTTP/1.1 Style Login w/ SCRAM Example
Figure 3
There's not much difference between the first two examples. The
third example has several important differences relative to the first
two examples:
o The URL is sent to the server before any chance to have completed
mutual authentication, should the selected mechanism provide
mutual authentication. If the client knows a priori to
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authenticate and the URL contains sensitive information then the
client has no choice but to leak this information prior to
completing mutual authentication, thus the client becomes
dependent on TLS for authenticating the server even when the
client could authenticate the server more strongly via the
selected HTTP authentication mechanism. This is an important
weakness.
o The whole sequence involves multiple requests/responses, which
goes against the stateless nature of HTTP. State is needed in all
three examples, but the first example is RESTful, while the second
employs a would-be new verb that provides for stateful
authentication. The third example simply cannot be thought of as
remotely RESTful. Perhaps this is not a problem.
* Alternatively mechanisms requiring multiple round trips can be
ruled out of scope. This would rule out quite a few desirable
mechanisms!
The main difference on the wire between a generic HTTP-layer user
authentication framework (like the one in the second example) and an
application-layer equivalent (as in the first example) can be so
minimal as to make the choice of layer seem like splitting hairs.
3.3.2. HTTP- vs. Application-Layer Authentication in the API Stack
There are HTTP stacks that make it possible to implement HTTP
authentication methods in the application (e.g., FCGI in web
servers), and nothing would prevent HTTP stacks from implementing a
_standard_ application-layer user authentication protocol either.
The APIs offered by an HTTP stack should look remarkably similar
regardless of which layer the user authentication protocol is
technically at. Once again, the difference between HTTP-layer and
standard application-layer user authentication is minimal.
Note however that if the HTTP stack does not implement
authentication, leaving it to the application to do so, then the
application developer runs the risk of making mistakes in the
implementation, such as failing to implement channel binding where
possible. Thus it is generally best if the HTTP stack implements
authentication - even if TLS is used for user authentication, the
HTTP stack should provide a singular API for authentication.
3.3.3. Choice of Layer
The choice of layer is clearly more important for APIs than on the
wire. On the wire the choice of layer is minimal, trivial even, when
the choice is between HTTP and the application layer.
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If the WG agrees that the distinction between HTTP-layer and
application-layer user authentication is or should be minimal then
how should the WG pick one of those two layers, if it decides not to
pursue TLS-layer user authentication?
A standard application-layer authentication scheme implies no changes
to HTTP itself, and may not rely on any particular features of
HTTP/1.1 or HTTPbis, thus it may be usable even with HTTP/1.0. This
is true of the REST-GSS proposal[I-D.williams-rest-gss], which is
also RESTful. This must be of some value.
An HTTP-layer authentication solution must either: a) not support
multi-round trip mechanisms, b) add verbs, or c) not be RESTful. (a)
works with HTTP/1.0, (b) would not work with HTTP/1.0. [The author
believes that RESTfulness is desirable.]
3.3.4. User Authentication in the TLS Layer
Issues:
o The transport cannot know when to require user authentication (on
the server side) or when to initiate it (on the client side).
Simply always initiating user authentication creates privacy
problems: the user may not want to disclose their identity all the
time!
o To address the problem of when to require or initiate user
authentication the TLS implementation must provide suitable APIs
to the application. And since the application will generally
decide that authentication is required only after (possibly well
after) a TLS connection is setup, the user generally must be
authenticated by renegotiating TLS, which in turn means that two
round trips will be needed just for that, at minimum, even if the
user authentication mechanism selected requires fewer round trips.
This is inefficient, though not fatal.
o The TLS community has resisted proposals for user authentication
mechanisms with arbitrary round trip counts before [references?
this is in reference to Stefan's TLS-GSS proposal...]. This may
no longer be true (or perhaps the author is misunderstanding or
misremembering the events in question), but if it is still the
case then the range of choices for user authentication in TLS is
significantly curtailed.
o Several major TLS implementations defer certificate validation
until the peer's Finished message is received. This means that
unless one is using TLS renegotiation (with the inner connection's
server certificate being the same as in the outer connection's)
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the user's identity and the payloads related to user
authentication will be revealed to the server before the server is
authenticated.
o User Interface issues:
* A user authentication framework and future mechanisms will
likely need to interact with the user. In some cases this may
be best done through a platform component, such as a credential
management facility. In other cases this may best be done by
the application. Driving user interaction from within the TLS
layer presents a slight complication: any interaction has to be
effected through application- or platform-provided code paths.
Adding interaction to existing TLS implementations may not be
trivial.
* ...
Benefits:
o Where the platform can provide credential management and user
interaction then user authentication in TLS can greatly simplify
HTTP applications: no user authentication APIs or UIs are then
needed in the application.
* Note however that the user may have a hard time identifying the
context in which they are being prompted by the system for
credentials or credential selection. This is usually not a
problem in smart-phone and other such small devices, where it
is generally clear what application is in the foreground, and
therefore the context of a prompt. But this is not necessarily
so on other platforms.
o Non-web applications typically know a priori when they wish to
authenticate. Typical non-web applications that use HTTP/1.1 over
a single TLS connection, with an application session consisting of
all the HTTP requests performed over that one connection. For
such applications having user authentication in the TLS layer may
be the simplest way to get user authentication into the
application.
3.4. Party Responsible for Infrastructure Messaging
[[anchor11: XXX Add references for OCSP, AAA, ...]]
"Infrastructure" consists, for the purposes of this document, of
services such as Identity Providers (IdPs), Certificate Revocation
Lists (CRLs) and their servers, Online Certificate Status Protocol
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(OCSP) responders, Kerberos Key Distribution Centers (KDCs), RADIUS/
DIAMETER servers, etcetera. These are services that run on parties
other than a client (e.g., a web browser / user agent) and an
application server. In some cases infrastructure services may be
physically co-located with the client or server, but by and large
they are physically separated; infrastructure services are always
logically separate from the client and server. [XXX Move this to
glossary.]
Some protocols require that the client do all or most of the message
exchanges with infrastructure, some require that the server do this
messaging, some require both to do some messaging. In some cases a
server might proxy a client's messages to infrastructure. There are
advantages to the client doing this messaging: namely a simpler
server, less subject to denial of service / resource consumption
attacks. [Are there advantages to the server doing this messaging?]
Consider a protocol like Kerberos. Kerberos relies on Key
Distribution Center (KDC) infrastructure, and it relies on the client
doing all the messaging needed to ultimately authenticate it to a
server. Kerberos can be used in a way such that the relying party
proxies this messaging for the client (see IAKERB), but even so the
client had to communicate with the KDCs in order to ultimately
authenticate to the relying party - IAKERB is simply a proxy
mechanism.
Now consider an authentication mechanism based on PKI. The only
online infrastructure in a PKI are the CRLs and OCSP responders. Of
course, a Certificate Authority (CA) can also be online, as in kca
[add reference], a CA that authenticates clients via Kerberos and
which issues fresh, short-lived certificates. Private keys for
certificates can also be served by online services such as SACRED and
browserid. The method of validating certificates currently
considered ideal is for the possessor of certificate's private key to
send both, the certificate and a current/fresh OCSP response for it
(or, rather, responses, for the entire certificate chain), thus the
PKI relying party should ideally not have to contact infrastructure;
in practice CRL checking is still the more commonly used method,
requiring infrastructure messaging on the relying party side.
The responsibility for infrastructure messaging varies widely.
3.5. Number of Messages
The number of messages that must be exchanged in order to
authenticate a peer varies a lot by authentication mechanism. Some
require just one message from the client to the server. Others
require a reply message from the server. Others require some larger
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number of messages (typically three or four). Yet others require a
variable number of messages.
Typically key exchange is also required in order to provide
confidentiality and integrity protection to the transport. Key
exchange protocols also vary in number of messages required. Key
exchange and authentication may be combined, either directly in a
single network layer, or across layers via channel binding.
One-message authentication protocols:
o OAuth
o Kerberos (w/o key confirmation)
o Public key signature schemes when authenticating only the client
o Diffie-Hellman (when the client knows the server's DH public key a
priori, and w/o key confirmation)
o RSA key transport (w/o key confirmation)
o all bearer token protocols (but see [ref to on channel bindings
section])
Two-message authentication protocols:
o Kerberos
o Diffie-Hellman with fixed public keys
o RSA key transport
Authentication protocols with three or more messages, or with
arbitrary numbers of messages:
o Most/all zero-knowledge password proof protocols (e.g., SRP)
(usually three or four messages)
o SCRAM, and other challenge-response protocols (usually three or
four messages)
o IAKERB (usually four messages)
o Pluggable frameworks (SASL, GSS, EAP) (arbitrary message counts,
usually dependent on what mechanism is selected)
It's worth pointing out that TLS is a three- to four-message
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protocol, but when providing confidentiality protection for the
client identity it becomes a six- to eight-message protocol (though
there is a proposal to improve this, getting back to three to four
messages [add reference to Marsh's I-D]).
Some authentication protocols can provide key exchange, others
cannot. Similarly, not all mechanisms can provide channel binding.
The total number of messages required is important. These message
exchanges are always ordered and synchronous; no progress can be made
by the application until they are completed. Over long distances the
time to complete each round trip add up to noticeable latency, and
there is much pressure to get this latency down to an absolute
minimum.
Integrating user authentication into TLS has the clear allure of
potentially cutting down the number of round trips necessary, but
it's not clear that this can be achieved in every case. In
particular it may not be clear that a client has to authenticate
until after a TLS connection is established over which the client may
request access to some resource that requires authenticated clients.
3.6. Trust Establishment
Pair-wise pre-shared keying systems require careful initial key
exchange, but otherwise have no transitive trust issues: every pair
of entities that has shared keying can communicate without the aid of
any other entity. However, pair-wise pre-shared keying does not
scale to the Internet as it is O(n^2), and it requires either "leap
of faith" (a.k.a., trust on first use, or TOFU) or physical proximity
for the key pre-sharing. Physical proximity
Authentication mechanisms that scale to the Internet of necessity
require some degree of trust transitivity. That is, there must be
many cases where Alice and Bob can communicate with each other only
because they can authenticate each other by way of one or more third
parties (e.g., Trent) that each of them trust a priori.
There are a number of issues with trust transitivity:
o Trusted third parties can mount MITM attacks on the parties that
rely on them
* Compromise of trusted third parties, therefore, has far
reaching, negative effects
* The longer a trust path, the less trustworthy -so to speak- it
is
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o Policy for determining acceptable trust paths is difficult to
express
o Mechanisms for establishing trust paths are often manual and prone
to error or abuse
There are several ways to use transitive trust. In hierarchical
transitive trust we organize the trusted third parties in such a way
that there should be a trust path for every pair of entities of
interest (e.g., every user to every server, every user to every user,
...) - think of PKI. In mesh systems trust transits through every
entity's "friends" - think of PGP.
There may be other models of transitive trust, such as one with
islands of trust. An islands of trust model would consist of
federations of transitive trust (using hierarchical or mesh models)
that are much smaller than the entire Internet, but large enough to
be of use to large numbers of users. For example, an online merchant
might provide for authentication of all users to a set of
participating vendors [XXX expand on this].
Given the need for transitive trust and the serious drawbacks of
transitive trust, some workarounds may be necessary, such as:
o Policy language for choosing suitable trust paths
o Facilities for limiting the length of, or otherwise shortening
trust paths
* By, for example, providing for bootstrapping of shorter trust
paths when a given trust path involves an "introducer" trusted
third party.
o "Pinning" facilities to force changes in the infrastructure to
proceed in ways which make some MITM attacks harder to mount
o Auditing -and compromise detection- facilities by which to show
that trusted third parties are not mounting MITM attacks
o Revocation facilities that actually work
o Root keys that are rarely used and live in HSMs
o Fast re-keying as a method for dealing with trusted third party
compromise
For an example of pinning, consider a TLS extension where self-
signed, persistent user certificates are used, possibly one per-
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origin for pseudonymity purposes. The user agent can enroll the user
certificates at their corresponding origin servers such that
thereafter no MITMs are possible that can impersonate the user to the
server. Of course, such a scheme suffers from needing a fall-back
authentication method when the user's device(s) that store the
relevant private keys are lost. Users would need to be able to fall-
back on an alternative authentication method for re-enrollment,
likely one that is susceptible to attack or else is inconvenient. In
this cases the pinning is on the server side; keep in mind that
pinning need not only be used on clients, but may be used even in the
distributed trust infrastructure (e.g., to shorten trust paths).
Ideally an authentication facility for HTTP/2.0 should support a
variety of trust establishment models, as it is not clear that one
mode is superior to the others. (Though certainly the hierarchical
model is likely the scheme that can have the most universal reach,
and therefore most minimize user credentials needed. However, users
may not mind having a small number of logon credentials for a trust
island model.)
3.7. Threat Modeling
[[anchor12: Cover the Internet threat model. Discuss the end-to-end
model and the hop-by-hop semantics of transitive trust.]]
3.8. Explicit versus Implicit Session Management
[[anchor13: Discuss lack of / weakness of application session concept
on the web. Discuss the historically limited application of TLS
sessions to HTTP apps. Discuss desirability of a real concept of
session and logout.]]
3.9. In-Band versus Out-of-Band Authentication
[[anchor14: Discuss out-of-band user authentication systems such as
ones where "tokens" are sent to users' mobile phones via SMS, as well
as systems where a "login URL" is sent to the user via e-mail.]]
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4. Analysis of Some Possible Authentication Proposals
[Cover:
o Authentication mechanisms:
* Bearer token systems
* Other half round trip systems, including Kerberos, OAuth
* PK w/ SACRED, browserid, smartcards
* ZKPPs
* Challenge/response password-based mechanisms (DIGEST-MD5,
SCRAM)
o Generic auth frameworks
* GSS, SASL, EAP (anything else? IKEv2? SSHv2?)
o Authentication in TLS, HTTP, and above HTTP
o OTP and out-of-band (SMS, e-mail) auth, both as part of
authentication mechanisms and as port of traditional webauth.
o Traditional webauth (passwords posted in forms), possibly with
password wallets (stateful and stateless)
]
[[anchor15: What else to cover?]]
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5. Author's Recommendations
It seems likely that no single user authentication method will
satisfy the needs of all web applications. Nor can we predict the
future. Moreover, some weak authentication approaches are perfectly
safe for accessing low-value resources, or in contexts where the
Internet threat model is overkill. This argues for a multitude of
solutions, and possibly a pluggable system.
The author proposes the following:
1. For all authentication mechanisms (i.e., cryptographic
authentication methods) use the GSS-API, possibly through the
thin shim of SASL/GS2[RFC5801].
1. do this above HTTP in the network stack, but...
2. ...recommend that this be implemented by HTTP stacks, rather
than by applications. I.e., authentication above HTTP on the
wire, but within HTTP as far as APIs are concerned.
2. Encourage the adoption of islands of trust / federation for
service authentication, rather than one single, world-wide PKI
for service authentication.
3. Encourage development of authentication mechanisms that fit the
chosen authentication framework and which have the following
features:
1. federation (even though it implies trusted third parties)
2. strong initial user authentication (e.g., with ZKPPs)
3. minimized password verifier attack surface area (e.g.,
minimize the number of servers that have access to password
verifiers)
4. trust path bootstrapping
5. short trust paths
6. auditable trusted third parties
7. [preferably] mutual authentication
4. Standardize weak authentication mechanisms (e.g., passwords
POSTed in forms) to facilitate the development of effective
password managers. [This is primarily for low-value sites.]
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5. Specify HTML and JavaScript interfaces for initiating
authentication, including the name of the service to authenticate
to. This will allow login pages to have a customized look, yet
allow for login operations to be performed by the browser
platform using a strong authentication mechanism. Specifically
there must be a method for kick-starting authentication such that
the user and/or device identity and credential input does not
happen through HTML forms but through browser/platform trusted
user interfaces.
6. Specify a new URI scheme that identifies services rather than
hosts. For example: svc:<service>@<domainname>/<local-part>. An
option to embed service authentication information (possibly a
digital signature, or a URL referring to a digital signature) may
prove useful.
1. Also specify a service location protocol.
7. Specify an abstract API for interfacing HTTPbis applications to
HTTPbis.
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6. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[I-D.williams-rest-gss]
Williams, N., "RESTful Hypertext Transfer Protocol
Application-Layer Authentication Using Generic Security
Services", draft-williams-rest-gss-01 (work in progress),
June 2012.
[I-D.hartman-webauth-phishing]
Hartman, S., "Requirements for Web Authentication
Resistant to Phishing", draft-hartman-webauth-phishing-09
(work in progress), August 2008.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[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.
[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.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
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[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, June 2006.
[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.
[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.
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Author's Address
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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