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Repost: Upgrading to TLS within HTTP

From: Rohit Khare <rohit@bordeaux.ics.uci.edu>
Date: Tue, 17 Mar 1998 00:36:40 GMT
To: http-wg@cuckoo.hpl.hp.com
Cc: manchala@cp10.es.xerox.com, fielding@bordeaux.ics.uci.edu, ejw@bordeaux.ics.uci.edu
Message-Id: <9803161550.aa26611@paris.ics.uci.edu>

Upgrading to TLS Within HTTP

   _Rohit Khare_, UC Irvine, March 16, 1998
   
     _________________________________________________________________
                                      
  0. Motivation
  
   At the [1]Washington DC IETF meeting last year, the Applications Area
   Directors indicated they would like to see a mechanism for applying
   Transport Layer Security [TLS] within an [2]HTTP connection, at the
   same port, instead of only being able to recommend a distinct port
   (443) and scheme (https). The TLS working group has moved forward with
   an extensive draft on properly implementing https
   ([3]draft-ietf-tls-https-00), but there is alternate precedent in SMTP
   and other applications of TLS ([4]draft-hoffman-smtp-ssl,
   [5]draft-newman-tls-imappop-03, [6]murray-auth-ftp-ssl-00).
   
   There has already been extensive debate on the [7]http-wg ,
   [8]ietf-tls and [9]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 [10]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 [11]ssl-tunneling-02 draft in 1995. The HTTP/1.1 spec
   reserves CONNECT for future use, deferring to the more recent
   [12]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
   ([13]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
  
   I could proceed by formalizing Section 2 as an Internet-Draft, but
   under the jurisdiction of which IETF working group? Furthermore, I do
   not have access nor personal interest in a TLS-capable client/server
   pair to experiment with.
   
   N.B. I believe this work is completely separate from HTTP-extension
   work proceeding in the web evolution / http-extension working group.
   This uses Upgrade for its stated purpose -- to switch to an entirely
   different protocol -- not to define or modify HTTP methods and
   semantics.
   
   Please watch [14]http://www.ics.uci.edu/~rohit/http-tls for updates of
   this document and any Internet-Drafts relating to this proposal.
   
  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

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://ds.internic.net/internet-drafts/draft-ietf-tls-https-00.txt
   4. http://www.imc.org/ietf-apps-tls/draft-hoffman-smtp-ssl
   5. http://ds.internic.net/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://ds.internic.net/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
Received on Tuesday, 17 March 1998 03:07:36 EST

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