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INTERNET-DRAFT                                             Rohit Khare
<draft-ietf-tls-http-upgrade-00.txt>                         UC Irvine
Expires May 1999                                     November 16, 1998

                      Upgrading to TLS Within HTTP/1.1

  Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference material
   or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet Drafts Shadow
   Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
   ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

  Abstract

   This memo proposes a mechanism to upgrade HTTP/1.1 connections to use
   Transport Layer Security (TLS). Using an Upgrade: TLS/x.y request
   header would allow unsecured and secured traffic to share the same
   port (in this case, 80). A companion document describes the current
   practice of using a separate port for HTTP over TLS,
   <draft-ietf-tls-https-01.txt>.

  0. Motivation

   At the Washington DC IETF meeting in December 1997, the Applications
   Area Directors indicated they would like to see a mechanism for
   applying Transport Layer Security [TLS] within an HTTP connection, at
   the same port, instead of only being able to recommend a distinct port
   (443) and scheme (https). IANA has already issued ten new ports for
   application X over TLS/SSL to date.

   The TLS working group has moved forward with an extensive draft on
   properly implementing https (draft-ietf-tls-https-00), but there is
   alternate precedent for "securing" a regularly opened connection for
   SMTP and other applications (draft-hoffman-smtp-ssl,
   draft-newman-tls-imappop-03, murray-auth-ftp-ssl-00,
   draft-ietf-ldap-ext-ldapv3-TLS-00.txt ).

   There has already been extensive debate on the http-wg , ietf-tls and
   ietf-apps-tls mailing lists about the advisability of permitting
   optional 'upgrades' to secure connections within the same channel,
   primarily focusing on the thread of man-in-the-middle attacks. Our
   intent here is not to engage in this debate, but merely to document a
   proposed mechanism for doing either with HTTP. Several applications
   being built upon HTTP might use this mechanism, such as the Internet
   Printing Protocol; we look to them for implementation guidance.

  1. Introduction

   TLS, a/k/a SSL (Secure Sockets Layer) establishes a private end-to-end
   connection, optionally including strong mutual authentication, using a
   variety of cryptosystems. Initially, a handshake phase uses three
   subprotocols to set up a record layer, authenticate endpoints, set
   parameters, as well as report errors. Then, there is an ongoing
   layered record protocol that handles encryption, compression, and
   reassembly for the remainder of the connection. The latter is intended
   to be completely transparent. For example, there is no dependency
   between TLS's record markers and or certificates and HTTP/1.1's
   chunked encoding or authentication.

   The need to 'secure' running connections is not merely 'running SSL
   over port 80', an early challenge for firewall developers answered by
   Ari Luotonen's ssl-tunneling-02 draft in 1995 -- that scheme still
   requires a distinct port number to activate TLS.

   The HTTP/1.1 spec reserves CONNECT for future use, deferring to the
   more recent draft-luotonen-web-proxy-tunneling-00 proposal. This
   technique perpetuates the concept that security is indicated by a
   magic port number -- CONNECT establishes a generic TCP tunnel, so port
   number is the only way to specify the layering of TLS with HTTP
   (https) or with NTTP (snews).

   Instead, the preferred mechanism to initiate and insert TLS in an
   HTTP/1.1 session should be the Upgrade: header, as defined in section
   14.42 of rev-03. Ideally, TLS-capable clients should add "Upgrade:
   TLS/1.0" to their initial request, and TLS-capable servers may reply
   with "101 Switching Protocol", complete the handshake, and continue
   with the "normal" response to the original request. However, the
   specification quoth:

     "The Upgrade header field only applies to switching
     application-layer protocols upon the existing transport-layer
     connection."

   Aside from this minor semantic difference -- invoking TLS indeed
   changes the existing transport-layer connection -- this is an ideal
   application of Upgrade. This technique overlays the TLS-request on an
   HTTP method; requires client-initiation, and allows servers to choose
   whether or not to make the switch. Like the other examples of
   TLS-enabled application protocols, the original session is preserved
   across the TLS handshake; secured communications resumes with a
   servers' reply.

   The potential for a man-in-the-middle attack (wherein the "TLS/1.0"
   upgrade token is stripped out) is precisely the same as for mixed
   http/https use:

    1. Removing the token is similar to rewriting web pages to change
       https:// links to http:// links.
    2. The risk is only present if the server is willing to vend that
       information over an insecure channel in the first place
    3. If the client knows for a fact that a server is TLS-compliant, it
       can insist on it by only connecting as https:// or by only sending
       an upgrade request on a no-op method like OPTIONS.

   Furthermore, for clients which do not actively try to invoke TLS,
   servers can use Upgrade: to advertise TLS compliance, too. Since
   TLS-compliance should be considered a feature of the server and not
   the resource at hand, it should be sufficient to send it once, and let
   clients cache that fact.

  2. Potential Solution

   Define "TLS/x.y" as a reference to the TLS specification
   (draft-ietf-tls-protocol-03), with x and y bound to its major and
   minor version numbers. Section 6.2.1 of the current draft explains why
   the TLS version would currently be defined as 1.0, not the actual
   parameters on the wire (which is "3.1" for backwards compatibility
   with SSL3).

   An HTTP client may initiate an upgrade by sending "TLS/x.y" as one of
   the field-values of the Upgrade: header. The origin-server MAY respond
   with "101 Switching Protocols"; if so it MUST include the header
   "Upgrade: TLS/x.y" to indicate what it is switching to.

   Servers which can upgrade to TLS MAY include the header "TLS/x.y" in
   an Upgrade response header to inform the client; servers SHOULD
   include such indication in response to any OPTIONS request.

   Similarly, servers MAY require clients to switch to TLS first by
   responding with a new error code "418: Upgrade Required", which MUST
   specify the protocol to be supported. @@ This is a change to 'core'
   HTTP; if, processwise, it's too difficult to slip in a general-purpose
   error code, we may have to fall-back to "418: TLS Required".

   Upgrade is a hop-by-hop header (Section 13.5.1), so each intervening
   proxy which supports TLS MUST also request the same version of TLS/x.y
   on its subsequent request. Furthermore, any caching proxy which
   supports TLS MUST NOT reply from its cache when TLS/x.y has been
   requested (although clients are still recommended to explicitly
   include "Cache-control: no-cache").

   Note: proxy servers may be able to request or initiate a TLS-secured
   connection, e.g. the outgoing or incoming firewall of a trusted
   subnetwork.

  3. Next Steps

   While there is formal interest in promulgating a scheme for HTTP/TLS
   without allocating a new port number, implementations have been
   scarce. We cannot predict what might trigger adoption of this
   proposal.

   Note: The Mandatory extension scheme for HTTP is another mechanism,
   though arguably less aprropriate, since TLS does not modify the
   semantics of HTTP itself. TLS would be using Upgrade for its stated
   purpose -- to switch to an entirely different protocol.

   This document is available at http://www.ics.uci.edu/~rohit/http-tls.

    3.1 Open Issues

   There have been some questions about how to continue to resolve https:
   URLs with the scheme postulated here. There is a default assumption in
   many products that https and http:443 are equivalent.

   Similarly, when resolving a mixture of secured and unsecured URLs from
   the same site, some might postulate the need to "downgrade" the
   connection. We suggest simply reopening the HTTP connection without
   TLS.

  4. Acknowledgments

   Thanks to Paul Hoffman for his work on the STARTTLS command extension
   for ESMTP. Thanks to Roy Fielding for assistance with the rationale
   behind Upgrade: and OPTIONS.

  5. References

  1. http://www.ics.uci.edu/pub/ietf/http/hypermail/1997q4/0495.html
   2. http://www.w3.org/Protocols/HTTP/1.1/draft-ietf-http-v11-spec-rev-03.txt
   3. http://www.ietf.org/internet-drafts/draft-ietf-tls-https-00.txt
   4. http://www.imc.org/ietf-apps-tls/draft-hoffman-smtp-ssl
   5. http://www.ietf.org/internet-drafts/draft-newman-tls-imappop-03.txt
   6. http://www.consensus.com/ietf-tls/murray-auth-ftp-ssl-00.txt
   7. http://www.ics.uci.edu/pub/ietf/http/
   8. http://www.consensus.com/ietf-tls/
   9. http://www.imc.org/ietf-apps-tls/
  10. http://www.pwg.org/ipp/index.html
  11. http://www.consensus.com/ietf-tls/ssl-tunneling-02.txt
  12. http://www.ietf.org/internet-drafts/draft-luotonen-web-proxy-tunneling-00
.txt
  13. http://www.consensus.com/ietf-tls/tls-protocol-03.txt
  14. http://www.ics.uci.edu/~rohit/http-tls


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