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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 4642

NNTP Extensions Working Group                               K. Murchison
Internet Draft                                        Oceana Matrix Ltd.
Expires: March 2006                                           J. Vinocur
                                                      Cornell University
                                                               C. Newman
                                                        Sun Microsystems
                                                          September 2005


                          Using TLS with NNTP
                     draft-ietf-nntpext-tls-nntp-09


Status of this memo

     By submitting this Internet-Draft, each author represents that any
     applicable patent or other IPR claims of which he or she is aware
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     Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

     Copyright (C) The Internet Society (2005).

Abstract

     This memo defines an extension to the Network News Transport
     Protocol (NNTP) to allow an NNTP client and server to use Transport
     Layer Security (TLS).  The primary goal is to provide encryption
     for single-link confidentiality purposes, but data integrity,
     (optional) certificate-based peer entity authentication, and



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     (optional) data compression are also possible.

Note to the RFC Editor

     The normative references to RFC 2234, RFC 2246, and RFC 3546,
     and the informative reference to RFC 2222 may be replaced by
     draft-crocker-abnf-rfc2234bis, draft-ietf-tls-rfc2246-bis
     draft-ietf-tls-rfc3526bis, and draft-ietf-sasl-rfc2222bis
     respectively should any or all of those documents reach RFC status
     before this one.

     The normative reference to [NNTP] and the informative reference to
     [NNTP-AUTH] are documents which are expected to be published
     simultaneously with this one and so can be replaced by references
     to the resulting RFCs.

Table of Contents

     1. Introduction .............................................  2
        1.1. Conventions Used in this Document ...................  3
     2. The STARTTLS Extension ...................................  3
        2.1. Advertising the STARTTLS Extension ..................  3
        2.2. STARTTLS Command ....................................  4
           2.2.1. Usage ..........................................  4
           2.2.2. Description ....................................  4
           2.2.3. Examples .......................................  7
     3. Augmented BNF Syntax for the STARTTLS Extension ..........  8
        3.1. Commands ............................................  8
        3.2. Capability entries ..................................  8
     4. Summary of Response Codes ................................  8
     5. Security Considerations ..................................  9
     6. IANA Considerations ...................................... 11
     7. References ............................................... 12
        7.1. Normative References ................................ 12
        7.2. Informative References .............................. 12
     8. Authors' Addresses ....................................... 13
     9. Acknowledgments .......................................... 13
     10. Intellectual Property Rights ............................ 13
     11. Copyright ............................................... 14

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 integrity and/or
     confidentiality protection is desired for the entire connection.

     The TLS protocol (formerly known as SSL) provides a way to secure



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     an application protocol from tampering and eavesdropping.  Although
     advanced SASL authentication mechanisms [NNTP-AUTH] can provide a
     lightweight version of this service, TLS is complimentary to both
     simple authentication-only SASL mechanisms and deployed clear-text
     password login commands.

     In some existing implementations, TCP port 563 has been dedicated
     to NNTP over TLS.  These implementations begin the TLS negotiation
     immediately upon connection, and then continue with the initial
     steps of an NNTP session.  This 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].

     This specification formalizes the STARTTLS command already in
     occasional use by the installed base.  The STARTTLS command
     rectifies a number of the problems with using a separate port for a
     "secure" protocol variant, and is the preferred way of using TLS
     with NNTP.

1.1. Conventions Used in this Document

     The notational conventions used in this document are the same as
     those in [NNTP] and any term not defined in this document has the
     same meaning as in that one.

     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].

     In the examples, commands from the client are indicated with [C],
     and responses from the server are indicated with [S].

2. The STARTTLS Extension

     This extension provides a new STARTTLS command and has the
     capability label STARTTLS.

2.1. Advertising the STARTTLS Extension

     A server supporting the STARTTLS command as defined in this
     document will advertise the "STARTTLS" capability label in response
     to the CAPABILITIES command.  However, this capability MUST NOT be
     advertised once a TLS layer is active (see section 2.2.2.2), or
     after successful authentication [NNTP-AUTH].  This capability MAY
     be advertised both before and after any use of MODE READER, with
     the same semantics.




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     As the STARTTLS command is related to security, cached results of
     CAPABILITIES from a previous session MUST NOT be relied on, as per
     section 12.6 of [NNTP].

     Example:

        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] IHAVE
        [S] STARTTLS
        [S] LIST ACTIVE NEWSGROUPS
        [S] .

2.2. STARTTLS Command

2.2.1. Usage

     This command MUST NOT be pipelined.

     Syntax
        STARTTLS

     Responses

        382 Continue with TLS negotiation
        502 Command unavailable [1]
        580 Can not initiate TLS negotiation

     [1] If a TLS layer is already active, or authentication has
     occurred, STARTTLS is not a valid command (see section 2.2.2.2).

     NOTE: Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT
     return either 480 or 483 in response to STARTTLS.

2.2.2. Description

     A client issues the STARTTLS command to request negotiation of TLS.
     The STARTTLS command is usually used to initiate session security,
     although it can also be used for client and/or server certificate
     authentication and/or data compression.

     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
     therefore use the STARTTLS command after receiving a 483 response.



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     If a server advertises the STARTTLS capability, a client MAY
     attempt to use the STARTTLS command at any time during a session to
     negotiate TLS without having received a 483 response.  Servers
     SHOULD accept such unsolicited TLS negotiation requests.

     If the server is unable to initiate the TLS negotiation for any
     reason (e.g. a server configuration or resource problem), the
     server MUST reject the STARTTLS command with a 580 response, and
     SHOULD either reject subsequent restricted NNTP commands from the
     client with a 483 response code (possibly with a text string such
     as "Command refused due to lack of security"), or reject a
     subsequent restricted command with a 400 response code (possibly
     with a text string such as "Connection closing due to lack of
     security") and close the connection.  Otherwise, the server issues
     a 382 response and TLS negotiation begins.  A server MUST NOT under
     any circumstances reply to a STARTTLS command with either a 480 or
     483 response.

     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.

     Upon 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 for both the client and
     server 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 nor servers are required to actually support Client
     Hello messages for anything other than TLS 1.0.  However, the TLS
     extension for Server Name Indication ("server_name") [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).

     If the TLS negotiation fails, both client and server SHOULD



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     immediately close the connection.  Note that while continuing the
     NNTP session is theoretically possible, in practice a TLS
     negotiation failure often leaves the session in an indeterminate
     state, therefore interoperability can not be guaranteed.

     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, with the exception that
     if a MODE READER command has been issued, the effects of it (if
     any) are not reversed.  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 client, such as the current
     newsgroup and article number, that was not obtained from the TLS
     negotiation itself.  Likewise, the client SHOULD discard and MUST
     NOT rely on any knowledge obtained from the server, such as the
     capability list, which was not obtained from the TLS negotiation
     itself.

     The server remains in the non-authenticated state, even if client
     credentials are supplied during the TLS negotiation.  The AUTHINFO
     SASL command [NNTP-AUTH] with the EXTERNAL mechanism [SASL] MAY be
     used to authenticate once TLS client credentials are successfully
     exchanged, but servers supporting the STARTTLS command are not
     required to support AUTHINFO in general or that mechanism in
     particular.  The server MAY use information from the client
     certificate for identification of connections or posted articles
     (either in its logs or directly in posted articles).

     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
     capability label in response to a CAPABILITIES 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.  Additionally, the client MUST NOT issue
     a MODE READER command while a TLS session is active and a server
     MUST NOT advertise the MODE-READER capability.

     The capability list returned in response to a CAPABILITIES command
     received after a successful 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.







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2.2.3. Examples

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

        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] STARTTLS
        [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
        [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 protected by TLS]
        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
        [S] OVER
        [S] .
        [C] GROUP local.confidential
        [S] 211 1234 3000234 3002322 local.confidential
        [C] STARTTLS
        [S] 502 STARTTLS not allowed with active TLS layer

     Example of a request to begin TLS negotiation declined by the
     server:

        [C] STARTTLS
        [S] 580 Can not initiate TLS negotiation

     Example of a failed attempt to negotiate TLS, followed by two
     attempts at selecting groups only available under a security layer
     (in the first case the server allows the session to continue, in
     the second it closes the connection).  Note that unrestricted
     commands such as CAPABILITIES are unaffected by the failure:

        [C] STARTTLS
        [S] 382 Continue with TLS negotiation



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        [TLS negotiation is attempted here]
        [Following failed negotiation, traffic resumes without TLS]
        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] STARTTLS
        [S] LIST ACTIVE NEWSGROUPS OVERVIEW.FMT
        [S] OVER
        [S] .
        [C] GROUP local.confidential
        [S] 483 Encryption or stronger authentication required
        [C] GROUP local.private
        [S] 400 Closing connection due to lack of security

3. Augmented BNF Syntax for the STARTTLS Extension

     This section describes the formal syntax of the STARTTLS extension
     using ABNF [ABNF].  It extends the syntax in section 9 of [NNTP],
     and non-terminals not defined in this document are defined there.
     The [NNTP] ABNF should be imported first before attempting to
     validate these rules.

3.1. Commands

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

     command =/ starttls-command

     starttls-command = "STARTTLS"

3.2. Capability entries

     This syntax extends the non-terminal "capability-entry", which
     represents a capability that may be advertised by the server.

     capability-entry =/ starttls-capability

     starttls-capability = "STARTTLS"

4. 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: can not initiate TLS negotiation

5. Security Considerations

     Security issues are discussed throughout this memo.

     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 12 of [NNTP]; those considerations remain
     relevant to any NNTP implementation.

     NNTP client and server implementations MUST implement the
     TLS_RSA_WITH_RC4_128_MD5 [TLS] cipher suite, and SHOULD implement
     the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [TLS] cipher suite.  This is
     important as it assures that any two compliant implementations can
     be configured to interoperate.  All other cipher suites are
     OPTIONAL.

     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 the
     establishment of a security layer.  Furthermore, the CAPABILITIES
     command SHOULD be re-issued upon the establishment 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 confidentiality,
     they are securing the transport only for that link.  Similarly,
     because delivery of a single piece of news may go between more than
     two NNTP servers, adding TLS confidentiality to one pair of servers
     does not mean that the entire NNTP chain has been made private.
     Furthermore, 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 itself
     received them (prior to forwarding them to the server).

     During the TLS negotiation, the client MUST check its understanding
     of the server hostname against the server's identity as presented
     in the server Certificate message, in order to prevent man-in-the-



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     middle attacks.  Matching is performed according to these rules:

     -  The client MUST use the server hostname it used to open the
        connection (or the hostname specified in TLS "server_name"
        extension [TLS-EXT]) as the value to compare against the server
        name as expressed in the server certificate.  The client MUST
        NOT use any form of the server hostname derived from an insecure
        remote source (e.g., insecure DNS lookup).  CNAME
        canonicalization is not done.

     -  If a subjectAltName extension of type dNSName is present in the
        certificate, it SHOULD be used as the source of the server's
        identity.

     -  Matching is case-insensitive.

     -  A "*" wildcard character MAY be used as the left-most name
        component in the certificate.  For example, *.example.com would
        match a.example.com, foo.example.com, etc. but would not match
        example.com.

     -  If the certificate contains multiple names (e.g. more than one
        dNSName field), then a match with any one of the fields is
        considered acceptable.


     If the match fails, the client SHOULD either ask for explicit user
     confirmation, or terminate the connection with a QUIT command and
     indicate the server's identity is suspect.

     Additionally, clients MUST verify the binding between the identity
     of the servers to which they connect and the public keys presented
     by those servers.  Clients SHOULD implement the algorithm in
     section 6 of [PKI-CERT] for general certificate validation, but MAY
     supplement that algorithm with other validation methods that
     achieve equivalent levels of verification (such as comparing the
     server certificate against a local store of already-verified
     certificates and identity bindings).

     A man-in-the-middle attack can be launched by deleting the STARTTLS
     capability label in the CAPABILITIES 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 CAPABILITIES response for a



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     later session (of course, the STARTTLS capability would not be
     listed after a security layer is in place).

     If the client receives a 483 or 580 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 (re)try
     TLS later in the session, or to give up and postpone
     newsreading/transport activity.  If an 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.

6. IANA Considerations

     This section gives a formal definition of the STARTTLS extension as
     required by Section 3.3.3 of [NNTP] for the IANA registry.

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

     o  The capability label for this extension is "STARTTLS".

     o  The capability label has no arguments.

     o  This extension defines one new command, STARTTLS, whose
        behavior, arguments, and responses are defined in Section 2.2.

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

     o  This extension does affect the overall behavior of both server
        and client, in that after successful use of the STARTTLS
        command, all communication is transmitted with the TLS layer as
        an intermediary.

     o  This extension does not affect the maximum length of commands or
        initial response lines.

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

     o  Use of this extension does alter the capabilities list; once the
        STARTTLS command has been used successfully, the STARTTLS
        capability can no longer be advertised by CAPABILITIES.



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        Additionally, the MODE-READER capability MUST NOT be advertised
        after a successful TLS negotiation.

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

     o  This extension is unaffected by any use of the MODE READER
        command, however the MODE READER command MUST NOT be used in the
        same session following a successful TLS negotiation.

     o  Published Specification: This document.

     o  Contact for Further Information: Authors of this document.

     o  Change Controller: IESG <iesg@ietf.org>.

7. References

7.1. 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", BCP 14, RFC 2119, March 1997.

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

     [PKI-CERT] Housley, R., Polk, W., Ford, W., Solo, D.,
     "Internet X.509 Public Key Infrastructure Certificate
     and Certificate Revocation List (CRL) Profile",
     RFC 3280, April 2002.

     [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 2003.

7.2. Informative References

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





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     [SASL] Myers, J., "Simple Authentication and Security Layer
     (SASL)", RFC 2222, October 1997.

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

8. Authors' Addresses

     Kenneth Murchison
     Oceana Matrix Ltd.
     2495 Main St, Suite 401
     Buffalo, NY  14214

     Email: ken@oceana.com


     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: Chris.Newman@sun.com

9. Acknowledgments

     A significant amount of the text in this document was lifted from
     RFC 2595 by Chris Newman and RFC 3207 by Paul Hoffman.

     Special acknowledgment 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.

10. Intellectual Property Rights

     The IETF takes no position regarding the validity or scope of any
     Intellectual Property Rights 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



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     rights might or might not be available; nor does it represent that
     it has made any independent effort to identify any such rights.
     Information on the procedures with respect to rights in RFC
     documents can be found in BCP 78 and BCP 79.

     Copies of IPR disclosures made to the IETF Secretariat 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 on-line IPR repository
     at http://www.ietf.org/ipr.

     The IETF invites any interested party to bring to its attention any
     copyrights, patents or patent applications, or other proprietary
     rights that may cover technology that may be required to implement
     this standard.  Please address the information to the IETF at
     ietf-ipr@ietf.org.

11. Copyright

     Copyright (C) The Internet Society (2005).

     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 rights.

     This document and the information contained herein are provided on
     an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
     REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
     THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
     EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
     THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
     ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
     PARTICULAR PURPOSE.

















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