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Versions: 00 01 02 03 04 05 06 07 08 09 rfc4642                         
Network Working Group                                         J. Vinocur
INTERNET DRAFT                                        Cornell University
Document: draft-ietf-nntpext-tls-nntp-00.txt                   C. Newman
                                                        Sun Microsystems
                                                           February 2003


                          Using TLS with NNTP


Status of this memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC 2026.

     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."

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.html.

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.

Copyright Notice

     Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

     This memo defines an extension to the Network News Transport
     Protocol [NNTP] to provide connection-based encryption (via
     Transport Layer Security [TLS]).  The primary goal is to provide
     encryption for single-link confidentiality purposes, but data
     integrity and (optional) certificate-based peer entity
     authentication are also described.







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Table of Contents

     1. Introduction .............................................  2
        1.1. Conventions Used in this Document ...................  2
     2. Advertising Capabilities with the Extensions Mechanism ...  3
     3. Authentication Response Codes ............................  3
     4. STARTTLS Command .........................................  4
        4.1. STARTTLS Responses ..................................  5
        4.2. Processing After the STARTTLS Command ...............  5
        4.3. Result of the STARTTLS Command ......................  6
        4.4. STARTTLS Formal Syntax ..............................  6
     5. MULTIDOMAIN Extension ....................................  7
     6. Security Considerations ..................................  8
     7. Acknowledgements .........................................  9
     8. Normative References .....................................  9
     9. Informative References ................................... 10
     10. Authors' Addresses ...................................... 10

1. Introduction

     Historically, unencrypted NNTP [NNTP] connections were satisfactory
     for most purposes.  However, sending passwords unencrypted over the
     network is no longer appropriate, and sometimes strong encryption
     is desired for the entire connection.

     The STARTTLS extension provides a way to use the popular TLS [TLS]
     service with the existing NNTP protocol.  The current
     (unstandardized) use of TLS for NNTP is most commonly on a
     dedicated TCP port; this practice is discouraged for the reasons
     documented in section 7 of "Using TLS with IMAP, POP3 and ACAP"
     [TLS-IMAPPOP].  Therefore, this specification formalizes and
     extends the STARTTLS command already in occasional use by the
     installed base.

1.1. Conventions Used in this Document

     The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
     NOT", "MAY", and "OPTIONAL" in this document are to be interpreted
     as described in "Key words for use in RFCs to Indicate Requirement
     Levels" [KEYWORDS].

     Terms related to authentication are defined in "On Internet
     Authentication" [AUTH].

     This document assumes you are familiar with NNTP [NNTP] and TLS
     [TLS].

     In the examples, commands from the client are indicated with [C],



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     and responses from the server are indicated with [S].

2. Advertising Capabilities with the Extensions Mechanism

     The "LIST EXTENSIONS" command, documented in section 8 of [NNTP],
     provides a mechanism for clients to discover what extensions are
     available.

     A server supporting the STARTTLS command as defined in section 4
     will advertise the "STARTTLS" capability in response to the "LIST
     EXTENSIONS" command issued when no TLS layer is active (see section
     4.3).

     A server supporting multiple domains as defined in section 5 will
     advertise the "MULTIDOMAIN" capability in response to the "LIST
     EXTENSIONS" command.

     Example:
        [C] LIST EXTENSIONS
        [S] 202 Extensions supported:
        [S]  OVER
        [S]  PAT
        [S]  LISTGROUP
        [S]  STARTTLS
        [S]  MULTIDOMAIN
        [S] .

     Note that the STARTTLS command constitutes a mode changes and thus
     clients MUST wait for completion prior to sending additional
     commands.

3. Authentication Response Codes

     An NNTP server MAY respond to any client command with a 483
     response indicating that a strong encryption layer is required; in
     general this response will be given to commands which may send
     authentication data as plaintext.  A client MAY react to a 483
     response by establishing an encryption layer (for example, via
     STARTTLS), though a 483 response is not required prior to
     initiating encryption.  The client also MAY try a different command
     (for example, a type of authentication that would not risk password
     compromise, QUIT, or any other command).

     A server SHOULD only respond to a particular command as indicated
     in this document, however a client MUST support unexpected response
     codes by handling them based on the first digit as specified in
     [NNTP].




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4. STARTTLS Command

        STARTTLS [domain]

     A client issues the STARTTLS command to request negotiation of TLS.
     The client MUST NOT send any additional commands on the socket
     until after it has received the server response to the command.
     The STARTTLS command is usually used to request session encryption,
     although it can be used for client certificate authentication.  The
     optional argument to STARTTLS is only permitted if the MULTIDOMAIN
     extension is implemented as described in section 5.

     If the client receives a failure response to STARTTLS, the client
     must decide whether or not to continue the NNTP session.  Such a
     decision is based on local policy.  For instance, if TLS was being
     used for client authentication, the client might try to continue
     the session, in case the server allows it to do so even with no
     authentication.  However, if TLS was being negotiated for
     encryption, a client that gets a failure response needs to decide
     whether to continue without TLS encryption, to wait and try again
     later, or to give up and notify the user of the error.

     An NNTP server MAY require the client to perform a TLS negotiation
     before accepting any commands.  In this case, the server SHOULD
     return the 483 encryption-required response code to every command
     other than HELP, LIST EXTENSIONS, QUIT, and any commands that
     establish encryption, such as STARTTLS; the server MUST NOT return
     483 in response to those commands.

     After receiving a 382 response to a STARTTLS command, the client
     MUST start the TLS negotiation before giving any other NNTP
     commands.  If, after having issued the STARTTLS command, the client
     finds out that some failure prevents it from actually starting a
     TLS handshake, then it SHOULD immediately close the connection.

     Servers MUST be able to understand backwards-compatible TLS Client
     Hello messages (provided that client_version is TLS 1.0 or later),
     and clients MAY use backwards-compatible Client Hello messages.
     Neither clients or servers are required to actually support Client
     Hello messages for anything other than TLS 1.0.

     Although current use of TLS most often involves the dedication of
     port 563 for NNTP over TLS, the continued use of TLS on a separate
     port is discouraged for the reasons documented in section 7 of
     "Using TLS with IMAP, POP3 and ACAP" [TLS-IMAPPOP].






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4.1. STARTTLS Responses

        382 Continue with TLS negotiation
        403 TLS temporarily not available
        501 Command not supported or command syntax error
        580 Security layer already active

     Clients MUST support other response codes by processing them based
     on the first digit.

4.2. Processing After the STARTTLS Command

     After the TLS handshake has been completed, both parties MUST
     immediately decide whether or not to continue based on the
     authentication and privacy achieved. The NNTP client and server may
     decide to move ahead even if the TLS negotiation ended with no
     authentication and/or no privacy because NNTP services are often
     performed without authentication or privacy, but some NNTP clients
     or servers may want to continue only if a particular level of
     authentication and/or privacy was achieved.

     If the NNTP client decides that the level of authentication or
     privacy is not high enough for it to continue, it SHOULD issue a
     QUIT command immediately after the TLS negotiation is complete.  If
     the NNTP server decides that the level of authentication or privacy
     is not high enough for it to continue, it SHOULD do at least one of
     (1) close the connection, being aware that the client may interpret
     this behavior as a network problem and immediately reconnect and
     issue the same command sequence, or (2) keep the connection open
     and reply to NNTP commands from the client with the 483 response
     code (with a possible text string such as "Command refused due to
     lack of security"), however this behavior may tie up resources
     unacceptably.

     The decision of whether or not to believe the authenticity of the
     other party in a TLS negotiation is a local matter.  However, some
     general rules for the decisions are:

     o  The client MAY check that the identity presented in the server's
        certificate matches the intended server hostname or domain.
        This check is not required (and is probably unwise unless the
        MULTIDOMAIN extension defined in section 5 has been used), but
        if it is implemented and the match fails, the client SHOULD
        either request explicit user confirmation, or terminate the
        connection but allow the user to disable the check in the
        future.
     o  Generally an NNTP server would want to accept any verifiable
        certificate from a client, however authentication can be done



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        using the client certificate (perhaps in combination with the
        SASL EXTERNAL mechanism [SASL-NNTP], although an implementation
        supporting STARTTLS is not required to support that mechanism).
        The server MAY use information about the client certificate for
        identification of connections or posted articles (either in its
        logs or directly in posted articles).

4.3. Result of the STARTTLS Command

     Upon completion of the TLS handshake, the NNTP protocol is reset to
     the initial state (the state in NNTP directly after the connection
     is established).  The server MUST discard any knowledge obtained
     from the client, such as the result of a previous authentication,
     which was not obtained from the TLS negotiation itself; immediately
     after the TLS handshake, the server MUST issue a welcome banner
     (response code 200 or 201) without the client issuing any further
     command.  The client MUST discard any knowledge obtained from the
     server, such as the list of NNTP service extensions, which was not
     obtained from the TLS negotiation itself.

     The extensions returned in response to a LIST EXTENSIONS command
     received after the TLS handshake MAY be different than the list
     returned before the TLS handshake.  For example, an NNTP server
     supporting SASL [SASL-NNTP] might not want to advertise support for
     a particular mechanism unless a client has sent an appropriate
     client certificate during a TLS handshake.

     Both the client and the server MUST know if there is a TLS session
     active.  A client MUST NOT attempt to start a TLS session if a TLS
     session is already active. A server MUST NOT return the STARTTLS
     extension in response to a LIST EXTENSIONS command received after a
     TLS handshake has completed, and a server MUST respond with a 580
     response code if a STARTTLS command is received while a TLS session
     is already active.

4.4. STARTTLS Formal Syntax

     This amends the formal syntax for NNTP [NNTP] to add the STARTTLS
     command.  The syntax is defined using ABNF [ABNF], including the
     core rules from section 6 of [ABNF].

     An optional domain argument is available.  The MULTIDOMAIN
     extension defined in section 5 describes when this argument may and
     may not be sent.  The syntax for the domain element is as defined
     in section 4.1.2 of the revised SMTP specification [SMTP].

     command              /= starttls-command
     starttls-command      = "STARTTLS" [1*WSP domain] *WSP CRLF



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                    ; domain is defined in sec. 4.1.2 of [SMTP]
                    ; WSP and CRLF are defined in sec. 13 of [NNTP]

5. MULTIDOMAIN Extension

     Note to implementors of this draft specification:
            A facility analagous to the one described below may be
            provided in a future extension to the TLS specification
            [TLS-EXT].  Standardization of that facility would obsolete
            the extension described below, meaning that MULTIDOMAIN may
            be entirely removed in a future revision of this draft in
            favor of the protocol-independent implementation.

     Many modern Internet servers host several domain names using the
     same IP address, and each domain might have its own TLS server
     certificate.  Unless the client communicates the domain name it is
     using to the server, the server can only use the IP address to
     which the client connected to determine the appropriate server
     certificate to present; this interferes with the client's ability
     to compare the domain name it expects to the one listed in the
     certificate.  (The HTTP protocol [HTTP] added the "Host" request-
     header in order to resolve this issue.)

     If the MULTIDOMAIN extension is advertised, then clients SHOULD
     send the domain name used to connect to the server as the argument
     to the STARTTLS command.  Clients MAY send the domain argument
     without checking the result of LIST EXTENSIONS, but if the server
     does not implement this extension it may respond with a 501 error
     code (even if a STARTTLS command without argument would have been
     accepted).  The server MAY decline to enter TLS negotation if it
     supports this extension and the domain argument is not given.  The
     client SHOULD send a fully qualified domain whenever that
     information is available.

     The server MAY use the domain argument to select an appropriate
     server certificate to present to the client during TLS, though the
     method by which the server selects a certificate is beyond the
     scope of this document.  However, the server should be prepared to
     receive STARTTLS commands that lack the domain argument.

     The domain argument MUST be discarded following successful
     negotiation as discussed in section 4.3 and therefore cannot be
     used later to determine how to authenticate a client.  However,
     usernames of the form user@host provide a viable alternative for
     this functionality.






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6. Security Considerations

     In general, the security considerations of the TLS protocol [TLS]
     are applicable here; only the most important are highlighted
     specifically below.  Also, this extension is not intended to cure
     the security considerations described in section 14 of [NNTP];
     those considerations remain relevant to any NNTP implementation.

     Use of STARTTLS cannot protect protocol exchanges conducted prior
     to authentication.  For this reason, the LIST EXTENSIONS command
     SHOULD be re-issued after successful negotiation of a security
     layer, and other protocol state SHOULD be re-negotiated as well.

     It should be noted that NNTP is not an end-to-end mechanism. Thus,
     if an NNTP client/server pair decide to add TLS privacy, they are
     securing the transport only for that link.  Further, because
     delivery of a single piece of news may go between more than two
     NNTP servers, adding TLS privacy to one pair of servers does not
     mean that the entire NNTP chain has been made private.  Further,
     just because an NNTP server can authenticate an NNTP client, it
     does not mean that the articles from the NNTP client were
     authenticated by the NNTP client when the client received them.

     Both the NNTP client and server must check the result of the TLS
     negotiation to see whether an acceptable degree of authentication
     and privacy was achieved.  Ignoring this step completely
     invalidates using TLS for security.  The decision about whether
     acceptable authentication or privacy was achieved is made locally,
     is implementation-dependent, and is beyond the scope of this
     document.

     The NNTP client and server should note carefully the result of the
     TLS negotiation.  If the negotiation results in no privacy, or if
     it results in privacy using algorithms or key lengths that are
     deemed not strong enough, or if the authentication is not good
     enough for either party, the client may choose to end the NNTP
     session with an immediate QUIT command, or the server may choose
     not to accept any more NNTP commands.

     The client and server should also be aware that the TLS protocol
     permits privacy and security capabilities to be renegotiated mid-
     connection (see section 7.4.1 of [TLS]).  For example, one of the
     parties may desire minimal encryption after any authentication
     steps have been performed.  This underscores the fact that security
     is not present simply because TLS has been negotiated; the nature
     of the established security layer must be considered.

     A man-in-the-middle attack can be launched by deleting the 382



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     response from the server. This would cause the client not to try to
     start a TLS session.  Another man-in-the-middle attack is to allow
     the server to announce its STARTTLS capability, but to alter the
     client's request to start TLS and the server's response.  An NNTP
     client can partially protect against these attacks by recording the
     fact that a particular NNTP server offers TLS during one session
     and generating an alarm if it does not appear in the LIST
     EXTENSIONS response for a later session (of course, the STARTTLS
     extension would not be listed after a security layer is in place).

     If the TLS negotiation fails or if the client receives a 483
     response, the client has to decide what to do next.  The client has
     to choose among three main options:  to go ahead with the rest of
     the NNTP session, to retry TLS at a later time, or to give up and
     postpone newsreading activity.  If a failure or error occurs, the
     client can assume that the server may be able to negotiate TLS in
     the future, and should try to negotiate TLS in a later session.
     However, if the client and server were only using TLS for
     authentication and no previous 480 response was received, the
     client may want to proceed with the NNTP session, in case some of
     the operations the client wanted to perform are accepted by the
     server even if the client is unauthenticated.

     Before the TLS handshake has begun, any protocol interactions are
     performed in the clear and may be modified by an active attacker.
     For this reason, clients and servers MUST discard any sensitive
     knowledge obtained prior to the start of the TLS handshake upon
     completion of the TLS handshake.

7. Acknowledgements

     A significant amount of the STARTTLS text was lifted from RFC 3207
     by Paul Hoffman.

     Special acknowledgement goes also to the people who commented
     privately on intermediate revisions of this document, as well as
     the members of the IETF NNTP Working Group for continual insight in
     discussion.

8. Normative References

     [ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax
     Specifications:  ABNF", RFC 2234, November 1997.

     [AUTH] Haller, N., Atkinson, R., "On Internet Authentication", RFC 1704,
     Bell Communications Research, October 1994.





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     [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, Harvard University, March 1997.

     [NNTP] Barber, S., "Network News Transport Protocol"
     (draft-ietf-nntpext-base-16.txt).

     [SMTP] Klensin, J., "Simple Mail Transport Protocol", RFC 2821, AT&T
     Laboratories, April 2001.

     [TLS] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246,
     Certicom, January 1999.

     [TLS-EXT] Blake-Wilson, S., Nystrom, M., "Transport Layer Security (TLS)
     Extensions" (draft-ietf-tls-extensions-06.txt).

     [TLS-IMAPPOP] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
     2595, Innosoft, June 1999.

9. Informative References

     [HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
     L., Leach, P., Berners-Lee, T., "Hypertext Transfer Protocol --
     HTTP/1.1", RFC 2616, June 1999.

     [SASL-NNTP] Vinocur, J., Newman, C., "Using SASL with NNTP", Work
     in Progress.

10. Authors' Addresses

     Jeffrey M. Vinocur
     Department of Computer Science
     Upson Hall
     Cornell University
     Ithaca, NY  14853

     EMail: vinocur@cs.cornell.edu


     Chris Newman
     Sun Microsystems
     1050 Lakes Drive, Suite 250
     West Covina, CA  91790

     EMail: cnewman@iplanet.com







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Full Copyright Statement

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