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302       content: "Security Requirements for HTTP";
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305       content: "Hoffman & Melnikov";
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326</style><link rel="Author" href="#rfc.authors">
327      <link rel="Copyright" href="#rfc.copyright">
328      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
329      <link rel="Chapter" title="2 Existing HTTP Security Mechanisms" href="#rfc.section.2">
330      <link rel="Chapter" title="3 Revisions To HTTP" href="#rfc.section.3">
331      <link rel="Chapter" title="4 Security Considerations" href="#rfc.section.4">
332      <link rel="Chapter" href="#rfc.section.5" title="5 Normative References">
333      <link rel="Appendix" title="A Acknowledgements" href="#rfc.section.A">
334      <link rel="Appendix" title="B Document History" href="#rfc.section.B">
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336      <link rel="schema.DC" href="">
337      <meta name="DC.Creator" content="Hoffman, P.">
338      <meta name="DC.Creator" content="Melnikov, A.">
339      <meta name="DC.Identifier" content="urn:ietf:id:draft-ietf-httpbis-security-properties-02">
340      <meta name="DC.Date.Issued" scheme="ISO8601" content="2008-07">
341      <meta name="DC.Description.Abstract" content="Recent IESG practice dictates that IETF protocols must specify mandatory-to-implement security mechanisms, so that all conformant implementations share a common baseline. This document examines all widely deployed HTTP security technologies, and analyzes the trade-offs of each.">
342   </head>
343   <body>
344      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
345         <tr>
346            <td class="header left">Network Working Group</td>
347            <td class="header right">P. Hoffman</td>
348         </tr>
349         <tr>
350            <td class="header left">Internet Draft</td>
351            <td class="header right">VPN Consortium</td>
352         </tr>
353         <tr>
354            <td class="header left">
355               &lt;draft-ietf-httpbis-security-properties-02&gt;
357            </td>
358            <td class="header right">A. Melnikov</td>
359         </tr>
360         <tr>
361            <td class="header left">Intended status: Informational</td>
362            <td class="header right">Isode Ltd.</td>
363         </tr>
364         <tr>
365            <td class="header left">Expires: January 2009</td>
366            <td class="header right">July 14, 2008</td>
367         </tr>
368      </table>
369      <p class="title">Security Requirements for HTTP<br><span class="filename">draft-ietf-httpbis-security-properties-02</span></p>
370      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
371      <p>By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she
372         is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section
373         6 of BCP 79.
374      </p>
375      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
376         that other groups may also distribute working documents as Internet-Drafts.
377      </p>
378      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
379         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
380         in progress”.
381      </p>
382      <p>The list of current Internet-Drafts can be accessed at &lt;<a href=""></a>&gt;.
383      </p>
384      <p>The list of Internet-Draft Shadow Directories can be accessed at &lt;<a href=""></a>&gt;.
385      </p>
386      <p>This Internet-Draft will expire in January 2009.</p>
387      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
388      <p>Recent IESG practice dictates that IETF protocols must specify mandatory-to-implement security mechanisms, so that all conformant
389         implementations share a common baseline. This document examines all widely deployed HTTP security technologies, and analyzes
390         the trade-offs of each.
391      </p>
392      <hr class="noprint">
393      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction
394      </h1>
395      <p id="rfc.section.1.p.1">Recent IESG practice dictates that IETF protocols are required to specify mandatory to implement security mechanisms. "The
396         IETF Standards Process" <a href="#RFC2026"><cite title="The Internet Standards Process -- Revision 3">[RFC2026]</cite></a> does not require that protocols specify mandatory security mechanisms. "Strong Security Requirements for IETF Standard Protocols" <a href="#RFC3365"><cite title="Strong Security Requirements for Internet Engineering Task Force Standard Protocols">[RFC3365]</cite></a> requires that all IETF protocols provide a mechanism for implementers to provide strong security. RFC 3365 does not define
397         the term "strong security".
398      </p>
399      <p id="rfc.section.1.p.2">"Security Mechanisms for the Internet" <a href="#RFC3631"><cite title="Security Mechanisms for the Internet">[RFC3631]</cite></a> is not an IETF procedural RFC, but it is perhaps most relevant. Section 2.2 states:
400      </p>
401      <div id="rfc.figure.u.1"></div><pre>
402  We have evolved in the IETF the notion of "mandatory to implement"
403  mechanisms.  This philosophy evolves from our primary desire to
404  ensure interoperability between different implementations of a
405  protocol.  If a protocol offers many options for how to perform a
406  particular task, but fails to provide for at least one that all
407  must implement, it may be possible that multiple, non-interoperable
408  implementations may result.  This is the consequence of the
409  selection of non-overlapping mechanisms being deployed in the
410  different implementations.
411</pre><p id="rfc.section.1.p.4">This document examines the effects of applying security constraints to Web applications, documents the properties that result
412         from each method, and will make Best Current Practice recommendations for HTTP security in a later document version. At the
413         moment, it is mostly a laundry list of security technologies and tradeoffs.
414      </p>
415      <hr class="noprint">
416      <h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;Existing HTTP Security Mechanisms
417      </h1>
418      <p id="rfc.section.2.p.1">For HTTP, the IETF generally defines "security mechanisms" as some combination of access authentication and/or a secure transport.</p>
419      <p id="rfc.section.2.p.2">[[ There is a suggestion that this section be split into "browser-like" and "automation-like" subsections. ]]</p>
420      <p id="rfc.section.2.p.3">[[ NTLM (shudder) was brought up in the WG a few times in the discussion of the -00 draft. Should we add a section on it?
421         ]]
422      </p>
423      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;Forms And Cookies
424      </h2>
425      <p id="rfc.section.2.1.p.1">Almost all HTTP authentication that involves a human using a web browser is accomplished through HTML forms, with session
426         identifiers stored in cookies. For cookies, most implementations rely on the "Netscape specification", which is described
427         loosely in section 10 of "HTTP State Management Mechanism" <a href="#RFC2109"><cite title="HTTP State Management Mechanism">[RFC2109]</cite></a>. The protocol in RFC 2109 is relatively widely implemented, but most clients don't advertise support for it. RFC 2109 was
428         later updated <a href="#RFC2965"><cite title="HTTP State Management Mechanism">[RFC2965]</cite></a>, but the newer version is not widely implemented.
429      </p>
430      <p id="rfc.section.2.1.p.2">Forms and cookies have many properties that make them an excellent solution for some implementers. However, many of those
431         properties introduce serious security trade-offs.
432      </p>
433      <p id="rfc.section.2.1.p.3">HTML forms provide a large degree of control over presentation, which is an imperative for many websites. However, this increases
434         user reliance on the appearance of the interface. Many users do not understand the construction of URIs <a href="#RFC3986"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, or their presentation in common clients <a href="#PhishingHOWTO"><cite title="Phishing Tips and Techniques">[PhishingHOWTO]</cite></a>. As a result, forms are extremely vulnerable to spoofing.
435      </p>
436      <p id="rfc.section.2.1.p.4">HTML forms provide acceptable internationalization if used carefully, at the cost of being transmitted as normal HTTP content
437         in all cases (credentials are not differentiated in the protocol).
438      </p>
439      <p id="rfc.section.2.1.p.5">Many Web browsers have an auto-complete feature that stores a user's information and pre-populates fields in forms. This is
440         considered to be a convenience mechanism, and convenience mechanisms often have negative security properties. The security
441         concerns with auto-completion are particularly poignant for web browsers that reside on computers with multiple users. HTML
442         forms provide a facility for sites to indicate that a field, such as a password, should never be pre-populated. However, it
443         is clear that some form creators do not use this facility when they should.
444      </p>
445      <p id="rfc.section.2.1.p.6">The cookies that result from a successful form submission make it unnecessary to validate credentials with each HTTP request;
446         this makes cookies an excellent property for scalability. Cookies are susceptible to a large variety of XSS (cross-site scripting)
447         attacks, and measures to prevent such attacks will never be as stringent as necessary for authentication credentials because
448         cookies are used for many purposes. Cookies are also susceptible to a wide variety of attacks from malicious intermediaries
449         and observers. The possible attacks depend on the contents of the cookie data. There is no standard format for most of the
450         data.
451      </p>
452      <p id="rfc.section.2.1.p.7">HTML forms and cookies provide flexible ways of ending a session from the client.</p>
453      <p id="rfc.section.2.1.p.8">HTML forms require an HTML rendering engine for which many protocols have no use.</p>
454      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;HTTP Access Authentication
455      </h2>
456      <p id="rfc.section.2.2.p.1">HTTP 1.1 provides a simple authentication framework, "HTTP Authentication: Basic and Digest Access Authentication" <a href="#RFC2617"><cite title="HTTP Authentication: Basic and Digest Access Authentication">[RFC2617]</cite></a>, which defines two optional mechanisms. Both of these mechanisms are extremely rarely used in comparison to forms and cookies,
457         but some degree of support for one or both is available in many implementations. Neither scheme provides presentation control,
458         logout capabilities, or interoperable internationalization.
459      </p>
460      <h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;Basic Authentication
461      </h3>
462      <p id="rfc.section.2.2.1.p.1">Basic Authentication (normally called just "Basic") transmits usernames and passwords in the clear. It is very easy to implement,
463         but not at all secure unless used over a secure transport.
464      </p>
465      <p id="rfc.section.2.2.1.p.2">Basic has very poor scalability properties because credentials must be revalidated with every request, and because secure
466         transports negate many of HTTP's caching mechanisms. Some implementations use cookies in combination with Basic credentials,
467         but there is no standard method of doing so.
468      </p>
469      <p id="rfc.section.2.2.1.p.3">Since Basic credentials are clear text, they are reusable by any party. This makes them compatible with any authentication
470         database, at the cost of making the user vulnerable to mismanaged or malicious servers, even over a secure channel.
471      </p>
472      <p id="rfc.section.2.2.1.p.4">Basic is not interoperable when used with credentials that contain characters outside of the ISO 8859-1 repertoire.</p>
473      <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;Digest Authentication
474      </h3>
475      <p id="rfc.section.2.2.2.p.1">In Digest Authentication, the client transmits the results of hashing user credentials with properties of the request and
476         values from the server challenge. Digest is susceptible to man-in-the-middle attacks when not used over a secure transport.
477      </p>
478      <p id="rfc.section.2.2.2.p.2">Digest has some properties that are preferable to Basic and Cookies. Credentials are not immediately reusable by parties that
479         observe or receive them, and session data can be transmitted along side credentials with each request, allowing servers to
480         validate credentials only when absolutely necessary. Authentication data session keys are distinct from other protocol traffic.
481      </p>
482      <p id="rfc.section.2.2.2.p.3">Digest includes many modes of operation, but only the simplest modes enjoy any degree of interoperability. For example, most
483         implementations do not implement the mode that provides full message integrity. Perhaps one reason is that implementation
484         experience has shown that in some cases, especially those involving large requests or responses such as streams, the message
485         integrity mode is impractical because it requires servers to analyze the full request before determining whether the client
486         knows the shared secret or whether message-body integrity has been violated and hence whether the request can be processed.
487      </p>
488      <p id="rfc.section.2.2.2.p.4">Digest is extremely susceptible to offline dictionary attacks, making it practical for attackers to perform a namespace walk
489         consisting of a few million passwords for most users.
490      </p>
491      <p id="rfc.section.2.2.2.p.5">Many of the most widely-deployed HTTP/1.1 clients are not compliant when GET requests include a query string <a href="#Apache_Digest"><cite title="Apache HTTP Server - mod_auth_digest">[Apache_Digest]</cite></a>.
492      </p>
493      <p id="rfc.section.2.2.2.p.6">Digest either requires that authentication databases be expressly designed to accommodate it, or requires access to cleartext
494         passwords. As a result, many authentication databases that chose to do the former are incompatible, including the most common
495         method of storing passwords for use with Forms and Cookies.
496      </p>
497      <p id="rfc.section.2.2.2.p.7">Many Digest capabilities included to prevent replay attacks expose the server to Denial of Service attacks.</p>
498      <p id="rfc.section.2.2.2.p.8">Digest is not interoperable when used with credentials that contain characters outside of the ISO 8859-1 repertoire.</p>
499      <h3 id="rfc.section.2.2.3"><a href="#rfc.section.2.2.3">2.2.3</a>&nbsp;Authentication Using Certificates in TLS
500      </h3>
501      <p id="rfc.section.2.2.3.p.1">Running HTTP over TLS provides authentication of the HTTP server to the client. HTTP over TLS can also provides authentication
502         of the client to the server using certificates. Although forms are a much more common way to authenticate users to HTTP servers,
503         TLS client certificates are widely used in some environments. The public key infrastructure (PKI) used to validate certificates
504         in TLS can be rooted in public trust anchors or can be based on local trust anchors.
505      </p>
506      <h3 id="rfc.section.2.2.4"><a href="#rfc.section.2.2.4">2.2.4</a>&nbsp;Other Access Authentication Schemes
507      </h3>
508      <p id="rfc.section.2.2.4.p.1">There are many niche schemes that make use of the HTTP Authentication framework, but very few are well documented. Some are
509         bound to transport layer connections.
510      </p>
511      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Negotiate (GSS-API) Authentication
512      </h4>
513      <p id="rfc.section.">Microsoft has designed an HTTP authentication mechanism that utilizes SPNEGO <a href="#RFC4178"><cite title="The Simple and Protected Generic Security Service Application Program Interface (GSS-API) Negotiation Mechanism">[RFC4178]</cite></a> GSSAPI <a href="#RFC4559"><cite title="SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows">[RFC4559]</cite></a>. In Microsoft's implementation, SPNEGO allows selection between Kerberos and NTLM (Microsoft NT Lan Manager protocols).
514      </p>
515      <p id="rfc.section.">In Kerberos, clients and servers rely on a trusted third-party authentication service which maintains its own authentication
516         database. Kerberos is typically used with shared secret key cryptography, but extensions for use of other authentication mechnanisms
517         such as PKIX certificates and two-factor tokens are also common. Kerberos was designed to work under the assumption that packets
518         traveling along the network can be read, modified, and inserted at will.
519      </p>
520      <p id="rfc.section.">Unlike Digest, Negotiate authentication can take multiple round trips (client sending authentication data in Authorization,
521         server sending authentication data in WWW-Authenticate) to complete.
522      </p>
523      <p id="rfc.section.">Kerberos authentication is generally more secure than Digest. However the requirement for having a separate network authentication
524         service might be a barrier to deployment.
525      </p>
526      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;Centrally-Issued Tickets
527      </h2>
528      <p id="rfc.section.2.3.p.1">Many large Internet services rely on authentication schemes that center on clients consulting a single service for a time-limited
529         ticket that is validated with undocumented heuristics. Centralized ticket issuing has the advantage that users may employ
530         one set of credentials for many services, and clients don't send credentials to many servers. This approach is often no more
531         than a sophisticated application of forms and cookies.
532      </p>
533      <p id="rfc.section.2.3.p.2">All of the schemes in wide use are proprietary and non-standard, and usually are undocumented. There are many standardization
534         efforts in progress, as usual.
535      </p>
536      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;Web Services
537      </h2>
538      <p id="rfc.section.2.4.p.1">Many security properties mentioned in this document have been recast in XML-based protocols, using HTTP as a substitute for
539         TCP. Like the amalgam of HTTP technologies mentioned above, the XML-based protocols are defined by an ever-changing combination
540         of standard and vendor-produced specifications, some of which may be obsoleted at any time <a href="#WS-Pagecount"><cite title="WS-Pagecount">[WS-Pagecount]</cite></a> without any documented change control procedures. These protocols usually don't have much in common with the Architecture
541         of the World Wide Web. It's not clear why the term "Web" is used to group them, but they are obviously out of scope for HTTP-based
542         application protocols.
543      </p>
544      <p id="rfc.section.2.4.p.2">[[ This section could really use a good definition of "Web Services" to differentiate it from REST. ]]</p>
545      <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;Transport Layer Security
546      </h2>
547      <p id="rfc.section.2.5.p.1">In addition to using TLS for client and/or server authentication, it is also very commonly used to protect the confidentiality
548         and integrity of the HTTP session. For instance, both HTTP Basic authentication and Cookies are often protected against snooping
549         by TLS.
550      </p>
551      <p id="rfc.section.2.5.p.2">It should be noted that, in that case, TLS does not protect against a breach of the credential store at the server or against
552         a keylogger or phishing interface at the client. TLS does not change the fact that Basic Authentication passwords are reusable
553         and does not address that weakness.
554      </p>
555      <hr class="noprint">
556      <h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;Revisions To HTTP
557      </h1>
558      <p id="rfc.section.3.p.1">Is is possible that HTTP will be revised in the future. "HTTP/1.1" <a href="#RFC2616"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a> and "Use and Interpretation of HTTP Version Numbers" <a href="#RFC2145"><cite title="Use and Interpretation of HTTP Version Numbers">[RFC2145]</cite></a> define conformance requirements in relation to version numbers. In HTTP 1.1, all authentication mechanisms are optional, and
559         no single transport substrate is specified. Any HTTP revision that adds a mandatory security mechanism or transport substrate
560         will have to increment the HTTP version number appropriately. All widely used schemes are non-standard and/or proprietary.
561      </p>
562      <hr class="noprint">
563      <h1 id="rfc.section.4" class="np"><a href="#rfc.section.4">4.</a>&nbsp;Security Considerations
564      </h1>
565      <p id="rfc.section.4.p.1">This entire document is about security considerations.</p>
566      <h1 class="np" id="rfc.references"><a href="#rfc.section.5" id="rfc.section.5">5.</a> Normative References
567      </h1>
568      <table summary="Normative References">
569         <tr>
570            <td class="reference"><b id="RFC2026">[RFC2026]</b></td>
571            <td class="top"><a href="" title="Harvard University">Bradner, S.</a>, “<a href="">The Internet Standards Process -- Revision 3</a>”, BCP&nbsp;9, RFC&nbsp;2026, October&nbsp;1996.
572            </td>
573         </tr> 
574         <tr>
575            <td class="reference"><b id="RFC2109">[RFC2109]</b></td>
576            <td class="top"><a href="" title="Bell Laboratories, Lucent Technologies">Kristol, D.M.</a> and <a href="" title="Netscape Communications Corp.">L. Montulli</a>, “<a href="">HTTP State Management Mechanism</a>”, RFC&nbsp;2109, February&nbsp;1997.
577            </td>
578         </tr> 
579         <tr>
580            <td class="reference"><b id="RFC2145">[RFC2145]</b></td>
581            <td class="top"><a href="" title="Western Research Laboratory">Mogul, J.C.</a>, <a href="" title="Department of Information and Computer Science">Fielding, R.T.</a>, <a href="" title="MIT Laboratory for Computer Science">Gettys, J.</a>, and <a href="" title="W3 Consortium">H.F. Nielsen</a>, “<a href="">Use and Interpretation of HTTP Version Numbers</a>”, RFC&nbsp;2145, May&nbsp;1997.
582            </td>
583         </tr> 
584         <tr>
585            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
586            <td class="top"><a href="" title="Department of Information and Computer Science">Fielding, R.</a>, <a href="" title="World Wide Web Consortium">Gettys, J.</a>, <a href="" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="" title="World Wide Web Consortium">Frystyk, H.</a>, <a href="" title="Xerox Corporation">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, and <a href="" title="World Wide Web Consortium">T. Berners-Lee</a>, “<a href="">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
587            </td>
588         </tr> 
589         <tr>
590            <td class="reference"><b id="RFC2617">[RFC2617]</b></td>
591            <td class="top"><a href="" title="Northwestern University, Department of Mathematics">Franks, J.</a>, <a href="" title="Verisign Inc.">Hallam-Baker, P.M.</a>, <a href="" title="AbiSource, Inc.">Hostetler, J.L.</a>, <a href="" title="Agranat Systems, Inc.">Lawrence, S.D.</a>, <a href="" title="Microsoft Corporation">Leach, P.J.</a>, Luotonen, A., and <a href="" title="Open Market, Inc.">L. Stewart</a>, “<a href="">HTTP Authentication: Basic and Digest Access Authentication</a>”, RFC&nbsp;2617, June&nbsp;1999.
592            </td>
593         </tr> 
594         <tr>
595            <td class="reference"><b id="RFC2965">[RFC2965]</b></td>
596            <td class="top"><a href="" title="Bell Laboratories, Lucent Technologies">Kristol, D. M.</a> and <a href="" title=", Inc.">L. Montulli</a>, “<a href="">HTTP State Management Mechanism</a>”, RFC&nbsp;2965, October&nbsp;2000.
597            </td>
598         </tr> 
599         <tr>
600            <td class="reference"><b id="RFC3365">[RFC3365]</b></td>
601            <td class="top">Schiller, J., “<a href="">Strong Security Requirements for Internet Engineering Task Force Standard Protocols</a>”, BCP&nbsp;61, RFC&nbsp;3365, August&nbsp;2002.
602            </td>
603         </tr> 
604         <tr>
605            <td class="reference"><b id="RFC3631">[RFC3631]</b></td>
606            <td class="top">Bellovin, S., Schiller, J., and C. Kaufman, “<a href="">Security Mechanisms for the Internet</a>”, RFC&nbsp;3631, December&nbsp;2003.
607            </td>
608         </tr> 
609         <tr>
610            <td class="reference"><b id="RFC3986">[RFC3986]</b></td>
611            <td class="top"><a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="Day Software">Fielding, R.</a>, and <a href="" title="Adobe Systems Incorporated">L. Masinter</a>, “<a href="">Uniform Resource Identifier (URI): Generic Syntax</a>”, STD&nbsp;66, RFC&nbsp;3986, January&nbsp;2005.
612            </td>
613         </tr> 
614         <tr>
615            <td class="reference"><b id="RFC4178">[RFC4178]</b></td>
616            <td class="top">Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, “<a href="">The Simple and Protected Generic Security Service Application Program Interface (GSS-API) Negotiation Mechanism</a>”, RFC&nbsp;4178, October&nbsp;2005.
617            </td>
618         </tr> 
619         <tr>
620            <td class="reference"><b id="RFC4559">[RFC4559]</b></td>
621            <td class="top">Jaganathan, K., Zhu, L., and J. Brezak, “<a href="">SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</a>”, RFC&nbsp;4559, June&nbsp;2006.
622            </td>
623         </tr> 
624         <tr>
625            <td class="reference"><b id="Apache_Digest">[Apache_Digest]</b></td>
626            <td class="top">Apache Software Foundation, , “<a href="">Apache HTTP Server - mod_auth_digest</a>”, &lt;<a href=""></a>&gt;.
627            </td>
628         </tr> 
629         <tr>
630            <td class="reference"><b id="PhishingHOWTO">[PhishingHOWTO]</b></td>
631            <td class="top">Gutmann, P., “<a href="">Phishing Tips and Techniques</a>”, February&nbsp;2008, &lt;<a href=""></a>&gt;.
632            </td>
633         </tr> 
634         <tr>
635            <td class="reference"><b id="WS-Pagecount">[WS-Pagecount]</b></td>
636            <td class="top">Bray, T., “<a href="">WS-Pagecount</a>”, September&nbsp;2004, &lt;<a href=""></a>&gt;.
637            </td>
638         </tr>
639      </table>
640      <hr class="noprint">
641      <h1 id="rfc.authors" class="np"><a href="#rfc.authors">Authors' Addresses</a></h1>
642      <address class="vcard"><span class="vcardline"><span class="fn">Paul Hoffman</span><span class="n hidden"><span class="family-name">Hoffman</span><span class="given-name">Paul</span></span></span><span class="org vcardline">VPN Consortium</span><span class="vcardline">EMail: <a href=""><span class="email"></span></a></span></address>
643      <address class="vcard"><span class="vcardline"><span class="fn">Alexey Melnikov</span><span class="n hidden"><span class="family-name">Melnikov</span><span class="given-name">Alexey</span></span></span><span class="org vcardline">Isode Ltd.</span><span class="vcardline">EMail: <a href=""><span class="email"></span></a></span></address>
644      <hr class="noprint">
645      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;Acknowledgements
646      </h1>
647      <p id="rfc.section.A.p.1">Much of the material in this document was written by Rob Sayre, who first promoted the topic. Many others on the HTTPbis Working
648         Group have contributed to this document in the discussion.
649      </p>
650      <hr class="noprint">
651      <h1 id="rfc.section.B" class="np"><a href="#rfc.section.B">B.</a>&nbsp;Document History
652      </h1>
653      <p id="rfc.section.B.p.1">[This entire section is to be removed when published as an RFC.]</p>
654      <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Changes between draft-sayre-http-security-variance-00 and   draft-ietf-httpbis-security-properties-00
655      </h2>
656      <p id="rfc.section.B.1.p.1">Changed the authors to Paul Hoffman and Alexey Melnikov, with permission of Rob Sayre.</p>
657      <p id="rfc.section.B.1.p.2">Made lots of minor editorial changes.</p>
658      <p id="rfc.section.B.1.p.3">Removed what was section 2 (Requirements Notation), the reference to RFC 2119, and any use of 2119ish all-caps words.</p>
659      <p id="rfc.section.B.1.p.4">In 3.2.1 and 3.2.2, changed "Latin-1 range" to "ISO 8859-1 repertoire" to match the definition of "TEXT" in RFC 2616.</p>
660      <p id="rfc.section.B.1.p.5">Added minor text to the Security Considerations section.</p>
661      <p id="rfc.section.B.1.p.6">Added URLs to the two non-RFC references.</p>
662      <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Changes between -00 and -01
663      </h2>
664      <p id="rfc.section.B.2.p.1">Fixed some editorial nits reported by Iain Calder.</p>
665      <p id="rfc.section.B.2.p.2">Added the suggestions about splitting for browsers and automation, and about adding NTLM, to be beginning of 2.</p>
666      <p id="rfc.section.B.2.p.3">In 2.1, added "that involves a human using a web browser" in the first sentence.</p>
667      <p id="rfc.section.B.2.p.4">In 2.1, changed "session key" to "session identifier".</p>
668      <p id="rfc.section.B.2.p.5">In 2.2.2, changed </p>
669      <div id="rfc.figure.u.2"></div><pre>
670Digest includes many modes of operation, but only the simplest modes
671enjoy any degree of interoperability.  For example, most
672implementations do not implement the mode that provides full message
673integrity.  Additionally, implementation experience has shown that
674the message integrity mode is impractical because it requires servers
675to analyze the full request before determining whether the client
676knows the shared secret.
677</pre><p> to </p>
678      <div id="rfc.figure.u.3"></div><pre>
679Digest includes many modes of operation, but only the simplest
680modes enjoy any degree of interoperability.  For example, most
681implementations do not implement the mode that provides full message
682integrity.  Perhaps one reason is that implementation experience has
683shown that in some cases, especially those involving large requests
684or responses such as streams, the message integrity mode is
685impractical because it requires servers to analyze the full request
686before determining whether the client knows the shared secret or
687whether message-body integrity has been violated and hence whether
688the request can be processed.
689</pre><p id="rfc.section.B.2.p.6">In 2.4, asked for a definition of "Web Services".</p>
690      <p id="rfc.section.B.2.p.7">In A, added the WG.</p>
691      <h2 id="rfc.section.B.3"><a href="#rfc.section.B.3">B.3</a>&nbsp;Changes between -01 and -02
692      </h2>
693      <p id="rfc.section.B.3.p.1">In section 2.1, added more to the paragraph on auto-completion of HTML forms.</p>
694      <p id="rfc.section.B.3.p.2">Added the section on TLS for authentication.</p>
695      <p id="rfc.section.B.3.p.3">Filled in section 2.5.</p>
696      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
697      <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the
698         authors retain all their rights.
699      </p>
700      <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION
704      </p>
705      <hr class="noprint">
706      <h1 class="np"><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
707      <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might
708         be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any
709         license under such rights might or might not be available; nor does it represent that it has made any independent effort to
710         identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and
711         BCP 79.
712      </p>
713      <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result
714         of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users
715         of this specification can be obtained from the IETF on-line IPR repository at &lt;<a href=""></a>&gt;.
716      </p>
717      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
718         rights that may cover technology that may be required to implement this standard. Please address the information to the IETF
719         at <a href=""></a>.
720      </p>
721   </body>
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