Network Working Group                                         J. Vinocur
INTERNET DRAFT                                        Cornell University
Document: draft-ietf-nntpext-tls-nntp-02.txt                   C. Newman
                                                        Sun Microsystems
                                                            K. Murchison
                                                      Oceana Matrix Ltd.
                                                          September 2004

                          Using TLS with NNTP

Status of this memo

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     with RFC 3668.

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Copyright Notice

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


     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 possible.

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Table of Contents

     0. Changes from Previous Version ............................  2
     1. Introduction .............................................  3
        1.1. Conventions Used in this Document ...................  3
     2. Advertising Capabilities with the Extensions Mechanism ...  3
     3. STARTTLS Command .........................................  4
        3.1. Usage ...............................................  4
        3.2. Description .........................................  4
           3.2.1. Processing After the STARTTLS Command ..........  5
           3.2.2. Result of the STARTTLS Command .................  6
        3.3. Examples ............................................  7
     4. Augmented BNF Syntax for the STARTTLS Extension ..........  8
        4.1. Commands ............................................  8
        4.2. LIST EXTENSIONS responses ...........................  8
     5. Summary of Response Codes ................................  8
     6. Security Considerations ..................................  9
     7. IANA Considerations ...................................... 10
     8. References ............................................... 11
        8.1. Normative References ................................ 11
        8.2. Informative References .............................. 12
     9. Authors' Addresses ....................................... 12
     10. Acknowledgements ........................................ 12
     11. Intellectual Property Rights ............................ 13
     12. Copyright ............................................... 13

0. Changes from Previous Version

     o  Example of 580 response
     o  Summary of Response Codes section
     o  IANA Considerations section

     o  Augmented BNF section follows base document
     o  Removed unused references

     o  STARTTLS has initial responses and one subsequent response
     o  Client should just go ahead and send a command after STARTTLS,
        since there is no greeting
     o  Addressed why STARTTLS should not be advertised after authentication

     o  Assorted updates of phrasing and typographical varieties
     o  No spaces before output of LIST EXTENSIONS

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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].

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

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

     A server supporting the STARTTLS command as defined in section 3
     will advertise the "STARTTLS" capability in response to the LIST
     EXTENSIONS command.  However, this capability is not advertised
     after successful authentication [NNTP-AUTH], nor is it advertised
     once a TLS layer is active (see section 3.2.2).  This capability
     may be advertised both before and after any use of MODE READER,
     with the same semantics.

     As the STARTTLS command is related to security, cached results of
     LIST EXTENSIONS from a previous session MUST NOT be used, as per
     section 11.6 of [NNTP].


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        [S] 202 Extensions supported:
        [S] OVER
        [S] HDR
        [S] LISTGROUP
        [S] STARTTLS
        [S] .

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

3. STARTTLS Command

3.1. Usage

     This command MUST NOT be pipelined.



        Initial responses
           382 Continue with TLS negotiation
           502 Command unavailable [1]

        Subsequent response
           580 TLS negotiation failed

    [1] If a TLS layer is already active, or authentication has
        occurred, STARTTLS is not a valid command (see sections 3.2 and

     Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT return
     483 in response to STARTTLS.

3.2. Description

     A client issues the STARTTLS command to request negotiation of TLS.
     This command MUST NOT be pipelined as per section 3.3 of [NNTP].
     The STARTTLS command is usually used to request session encryption,
     although it can be used for client certificate authentication.

     An NNTP server returns the 483 response to indicate that a secure
     or encrypted connection is required for the command sent by the
     client.  Use of the STARTTLS command as described below is one way
     to establish a connection with these properties.  The client MAY

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     therefore send STARTTLS after receiving a 483 response; the client
     also MAY decide to send STARTTLS without previously receiving a 483
     response.  Additionally, the server MUST NOT return 483 in response
     to the STARTTLS command.

     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.

     After receiving a 382 response to a STARTTLS command, the client
     MUST start the TLS negotiation before giving any other NNTP
     commands.  The TLS negotiation begins with the first octet
     following the CRLF of the 382 response.  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.  However, the TLS
     extension for Server Name Indication [TLS-EXT] SHOULD be
     implemented by all clients; it also SHOULD be implemented by any
     server implementing STARTTLS that is known by multiple names
     (otherwise it is not possible for a server with several hostnames
     to present the correct certificate to the client).

     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].

3.2.1. Processing After the STARTTLS Command

     After the TLS handshake has been completed successfully, 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

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     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

     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 may fail in the absence of the
        TLS server_name extension [TLS-EXT], as described above), 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
        using the client certificate (perhaps in combination with the
        SASL EXTERNAL mechanism [NNTP-AUTH], although an implementation
        supporting STARTTLS is not required to support SASL in general
        or that mechanism in particular).  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).

3.2.2. Result of the STARTTLS Command

     If the TLS handshake fails in such a way that it is possible to
     recover the underlying NNTP session, the server will send a 580
     response (without encryption), beginning with the first post-
     handshake octet.

     Upon successful completion of the TLS handshake, the NNTP protocol
     is reset to the state immediately after the initial greeting
     response (see 5.1 of [NNTP]) has been sent.  In this case, as no
     greeting is sent, the next step is for the client to send a
     command.  The server MUST discard any knowledge obtained from the

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     client, such as the current newsgroup and article number, that was
     not obtained from the TLS negotiation itself.  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 [NNTP-AUTH] 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 502
     response code if a STARTTLS command is received while a TLS session
     is already active.

3.3. Examples

     Example of a client being prompted to use encryption and
     negotiating it successfully (showing the removal of STARTTLS from
     the extensions list once a TLS layer is active), followed by an
     (inappropriate) attempt by the client to initiate another TLS

        [S] 202 Extensions supported:
        [S] STARTTLS
        [S] OVER
        [S] .
        [C] GROUP local.confidential
        [S] 483 Encryption or stronger authentication required
        [C] STARTTLS
        [S] 382 Continue with TLS negotiation
        [TLS negotiation occurs here]
        [Following successful negotiation, traffic is via the TLS layer]
        [S] 202 Extensions supported:
        [S] OVER
        [S] .
        [C] STARTTLS
        [S] 502 STARTTLS not allowed with active TLS layer

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     Example of a request to begin TLS negotiation declined by the

        [C] STARTTLS
        [S] 403 TLS temporarily not available

     Example of a failed attempt to negotiate TLS, with recovery of the
     underlying NNTP session:

        [C] GROUP local.confidential
        [S] 483 Encryption or stronger authentication required
        [C] STARTTLS
        [S] 382 Continue with TLS negotiation
        [TLS negotiation is attempted here]
        [Following failed negotiation, traffic resumes without TLS]
        [S] 580 TLS negotiation failed
        [C] GROUP local.public
        [S] 211 1234 3000234 3002322 local.public

4. Augmented BNF Syntax for the STARTTLS Extension

     This section describes the syntax of the STARTTLS extension.  It
     extends the syntax in [NNTP], and non-terminals not defined in this
     document are defined there.

4.1. Commands

     This syntax extends the non-terminal "command", which represents an
     NNTP command.

     command =/ starttls-command

     starttls-command = "STARTTLS"

4.2. LIST EXTENSIONS responses

     This syntax defines the specific LIST EXTENSIONS responses for the
     STARTTLS extension.

     extension-descriptor =/ starttls-extension
     starttls-extension = %x53.  ; "STARTTLS"

5. Summary of Response Codes

     This section contains a list of every new response code defined in
     this document, whether it is multi-line, which commands can
     generate it, what arguments it has, and what its meaning is.

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     Response code 382
        Generated by: STARTTLS
        Meaning: continue with TLS negotiation

     Response code 580
        Generated by: STARTTLS
        Meaning: TLS negotiation failed

6. Security Considerations

     In general, the security considerations of the TLS protocol [TLS]
     and any implemented extensions [TLS-EXT] 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 11 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

     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.

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     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
     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. IANA Considerations

     This section gives a formal definition of the STARTTLS extension as
     required by Section 8 of [NNTP] for the IANA registry.

     o  The STARTTLS extension provides connection-based encryption via
        Transport Layer Security (TLS).

     o  The extension-label is "STARTTLS".

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     o  The extension-label has no arguments.

     o  The extension defines one new command, STARTTLS, whose
        behaviour, arguments, and responses are defined in Section 3.

     o  The extension does not associate any new responses with pre-
        existing NNTP commands.

     o  The extension does affect the overall behaviour of both server
        and client, in that after successful use of the new command, all
        communication is transmitted with the TLS layer as an

     o  The extension does not affect the maximum length of commands and
        initial response lines.

     o  The extension does not alter pipelining, but the STARTTLS
        command cannot be pipelined.

     o  Use of this extension does alter the output from LIST
        EXTENSIONS; once the new command has been used successfully,
        this extension can no longer be advertised by LIST EXTENSIONS.

     o  The extension does not cause any pre-existing command to produce
        a 401, 480, or 483 response.

     o  The STARTTLS command can be used before or after the MODE READER
        command, with the same semantics.

     o  Published Specification: This document.

     o  Author, Change Controller, and Contact for Further Information:
        Author of this document.

8. References

8.1. Normative References

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

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

     [NNTP] Feather, C., "Network News Transport Protocol",
     draft-ietf-nntpext-base-*.txt, Work in Progress.

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

     [TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
     Wright, T., "Transport Layer Security (TLS) Extensions", RFC 3546, June

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

8.2. Informative References

     [NNTP-AUTH] Vinocur, J., Newman, C., Murchison, K., "NNTP Extension
     for Authentication", draft-ietf-nntpext-auth-*.txt, Work in

9. Authors' Addresses

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


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


     Kenneth Murchison
     Oceana Matrix Ltd.
     21 Princeton Place
     Orchard Park, NY 14127 USA


10. 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

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     privately on intermediate revisions of this document, as well as
     the members of the IETF NNTP Working Group for continual insight in

11. Intellectual Property Rights

     The IETF takes no position regarding the validity or scope of any
     intellectual property or other rights that might be claimed to
     pertain to the implementation or use of the technology described in
     this document or the extent to which any license under such rights
     might or might not be available; neither does it represent that it
     has made any effort to identify any such rights.  Information on
     the IETF's procedures with respect to rights in standards-track and
     standards-related documentation can be found in BCP-11.  Copies of
     claims of rights made available for publication and any assurances
     of licenses to be made available, or the result of an attempt made
     to obtain a general license or permission for the use of such
     proprietary rights by implementers or users of this specification
     can be obtained from the IETF Secretariat.

     The IETF invites any interested party to bring to its attention any
     copyrights, patents or patent applications, or other proprietary
     rights which may cover technology that may be required to practice
     this standard.  Please address the information to the IETF
     Executive Director.

12. Copyright

     Copyright (C) The Internet Society (2004).  This document is
     subject to the rights, licenses and restrictions contained in BCP
     78, and except as set forth therein, the authors retain all their

     This document and the information contained herein is provided on

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