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204 lines
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HTML
204 lines
11 KiB
HTML
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<meta name="generator" content="LibreOffice 5.1.6.2 (Linux)"/>
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<meta name="changed" content="2018-06-21T14:25:04.182337599"/>
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td p { margin-left: 0.79in; margin-right: 0.79in; color: #000000; font-size: 12pt }
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<body lang="en-US" text="#000000" dir="ltr">
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<p><?xml version="1.0" encoding="UTF-8"?><!--This file was converted to xhtml by LibreOffice - see http://cgit.freedesktop.org/libreoffice/core/tree/filter/source/xslt for the code.-->
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</p>
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<h1><a name="a__Replacement_of_use_of_SHA-1_in_the_SRP_Client_Proof_with_a_SHA-2_Message_Digest"></a>
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Replacement of use of SHA-1 in the SRP Client Proof with a SHA-2
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Message Digest</h1>
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<p class="p1">The Firebird implementation of the Secure Remote
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Protocol (SRP) for password based user authentication has been
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updated following a security review of the original Firebird SRP-6a
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implementation taking into account current NIST guidance on the use
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of SHA-1 – see NIST Special Publication 800-131A, Revision 1,
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Transitions: Recommendation for Transitioning the Use of
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Cryptographic Algorithms and Key Lengths
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(<a href="http://dx.doi.org/10.6028/NIST.SP.800-131Ar1">http://dx.doi.org/10.6028/NIST.SP.800-131Ar1</a>)
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chapter 9. This guidance disallows the general use of SHA-1 for
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“Digital Signature Generation” whilst permitting continued use
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for “Digital Signature Verification”. The background to making
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this change is given below.</p>
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<p class="p4">By default, the SHA-256 message digest is now used
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instead of SHA-1 for generating the Client Proof. Alternatively,
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SHA-1 (deprecated and for legacy use only) may be used for the Client
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Proof. Separate AuthServer and AuthClient plugins are available for
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each supported message digest, with the following names:</p>
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<table width="703" cellpadding="0" cellspacing="0">
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<col width="174">
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<col width="528">
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<tr>
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<td width="174" style="border: none; padding: 0in">
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<ul>
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<li/>
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<p class="p6" align="left" style="margin-right: 0in">•Srp </p>
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</ul>
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</td>
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<td width="528" style="border: none; padding: 0in">
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<p class="p4" align="left">SHA-1 Client Proof</p>
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</td>
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</tr>
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<tr>
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<td width="174" style="border: none; padding: 0in">
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<ul>
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<li/>
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<p class="p6" align="left" style="margin-right: 0in">•Srp256 </p>
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</ul>
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</td>
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<td width="528" style="border: none; padding: 0in">
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<p class="p4" align="left">SHA-256 Client Proof</p>
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</td>
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</tr>
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</table>
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<p class="p5">Both client and server must have an SRP authentication
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plugin in common in order to enable successfully authentication of a
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user's password.
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</p>
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<p class="p5">There is no change to the SRP User Manager. This is
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still called (“Srp”) and the User Manager and the security
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database are not affected by the choice of message digest used to
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compute the client proof.</p>
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<p class="p4">The “firebird.conf” default configuration file
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entries for AuthServer and AuthClient are now:</p>
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<p class="p4">AuthServer = Srp256<br/>
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AuthClient = Srp256, Srp,
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Legacy_Auth (Non -windows clients)<br/>
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AuthClient = Srp256, Srp,
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Win_Sspi, Legacy_Auth (windows clients)</p>
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<p class="p4">With these settings, a Firebird Server is using Srp256
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to authenticate a client using SHA-256 to compute the client proof
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and is thus compatible with Firebird 3.0.4 or newer clients. On the
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other hand, a Firebird client will authenticate the user with any
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server version down to at least 2.5.</p>
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<p class="p7">A deployment where both client and servers support the
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legacy Srp (using SHA-1) and one or more of the SHA-2 authentication
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plugins (e.g. Srp256) should be avoided. This is because an attacker
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might be able to disrupt the Srp256 authentication thereby forcing
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Firebird to use the weaker Srp SHA-1 client proof without the user
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being aware.</p>
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<h2 class="western"><a name="a__REASON_FOR_CHANGE"></a>REASON FOR
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CHANGE</h2>
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<p class="p1">Review of the Firebird SRP implementation appears to
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indicate that most uses of SHA-1 continue to be permitted under NIST
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guidance except for its use in generating the client proof. The SRP
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client proof may be characterised as a “Poor Man's Digital
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Signature” in that it provides a two party proof of identity rather
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than the third party proof normally expected from a Digital Signature
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i.e. it is not a non-repudiable proof. Nevertheless, it is believed
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that generation of the client proof falls under the heading of
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“Digital Signature Generation” when considering the NIST
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Guidance.</p>
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<p class="p2">Continued use of SHA-1 in order to generate the client
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proof appears to risk leakage of the encryption key used to encrypt
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“over-the-wire” encryption and which hence also provides peer
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entity authentication during the lifetime of the connection. This may
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result in an attacker being able to monitor confidential
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communication either during the connection or at some later date and
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this could include leakage of an encryption key used to encrypt the
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user database, if this is passed from client to server during the
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connection.</p>
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<p class="p2">Such an attack is viable if weaknesses in SHA-1 can be
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exploited to allow a brute force attack on the client proof to be
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computationally feasible. All parts of the message on which the
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client proof is based may be known to an attacker with the exception
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of the shared session key and such an attack would concentrate on
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revealing this key. If it were possible to reveal the shared session
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key in real time then additionally a man-in-the-middle attack would
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be feasible.</p>
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<p class="p2">The severity of this issue is viewed as Important but
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not Critical. Users that rely on SRP (using SHA-1)/over the wire
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encryption to protect confidential communication have a long term
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risk that the confidentiality of their data may be compromised.
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The attack may also be mitigated through the use of other procedures
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to protect communications (e.g. a secure VPN).</p>
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<p class="p9">The update adds a new directory to the source code tree
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(src/common/sha2) containing an implementation of the SHA-2 family of
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message digests derived from the implementation published by Olivier
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Gay <<a href="mailto:olivier.gay@a3.epfl.ch">olivier.gay@a3.epfl.ch</a>>
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(see https://github.com/ouah/sha2). The following copyright notice is
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included at the request of the original author and applies to the
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files in src/common/sha2:</p>
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<p class="p10" style="margin-bottom: 0.2in">FIPS 180-2
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SHA-224/256/384/512 implementation</p>
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<p class="p10" style="margin-bottom: 0.2in">Last update: 02/02/2007</p>
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<p class="p10" style="margin-bottom: 0.2in">Issue date: 04/30/2005</p>
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<p class="p10" style="margin-bottom: 0.2in">https://github.com/ouah/sha2</p>
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<p class="p10" style="margin-bottom: 0.2in"> </p>
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<p class="p10" style="margin-bottom: 0.2in">Copyright (C) 2005, 2007
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Olivier Gay <olivier.gay@a3.epfl.ch></p>
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<p class="p10" style="margin-bottom: 0.2in">All rights reserved.</p>
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<p class="p10" style="margin-bottom: 0.2in"> </p>
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<p class="p10" style="margin-bottom: 0.2in">Redistribution and use in
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source and binary forms, with or without</p>
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<p class="p10" style="margin-bottom: 0.2in">modification, are
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permitted provided that the following conditions</p>
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<p class="p10" style="margin-bottom: 0.2in">are met:</p>
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<p class="p10" style="margin-bottom: 0.2in">1. Redistributions of
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source code must retain the above copyright</p>
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<p class="p10" style="margin-bottom: 0.2in"> notice, this
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list of conditions and the following disclaimer.</p>
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<p class="p10" style="margin-bottom: 0.2in">2. Redistributions in
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binary form must reproduce the above copyright</p>
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<p class="p10" style="margin-bottom: 0.2in"> notice, this
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list of conditions and the following disclaimer in the</p>
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<p class="p10" style="margin-bottom: 0.2in"> documentation
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and/or other materials provided with the distribution.</p>
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<p class="p10" style="margin-bottom: 0.2in">3. Neither the name of
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the project nor the names of its contributors</p>
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<p class="p10" style="margin-bottom: 0.2in"> may be used
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to endorse or promote products derived from this software</p>
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<p class="p10" style="margin-bottom: 0.2in"> without
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specific prior written permission.</p>
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<p class="p10" style="margin-bottom: 0.2in"> </p>
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<p class="p10" style="margin-bottom: 0.2in">THIS SOFTWARE IS PROVIDED
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BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND</p>
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<p class="p10" style="margin-bottom: 0.2in">ANY EXPRESS OR IMPLIED
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WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE</p>
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<p class="p10" style="margin-bottom: 0.2in">IMPLIED WARRANTIES OF
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MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE</p>
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<p class="p10" style="margin-bottom: 0.2in">ARE DISCLAIMED. IN
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NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE</p>
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<p class="p10" style="margin-bottom: 0.2in">FOR ANY DIRECT, INDIRECT,
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INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL</p>
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<p class="p10" style="margin-bottom: 0.2in">DAMAGES (INCLUDING, BUT
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NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS</p>
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<p class="p10" style="margin-bottom: 0.2in">OR SERVICES; LOSS OF USE,
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DATA, OR PROFITS; OR BUSINESS INTERRUPTION)</p>
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<p class="p10" style="margin-bottom: 0.2in">HOWEVER CAUSED AND ON ANY
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THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT</p>
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<p class="p10" style="margin-bottom: 0.2in">LIABILITY, OR TORT
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(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY</p>
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<p class="p10" style="margin-bottom: 0.2in">OUT OF THE USE OF THIS
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SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF</p>
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<p class="p10" style="margin-bottom: 0.2in">SUCH DAMAGE.</p>
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</body>
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