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8 cardoe 1.1 <title>GLEP 59 -- Manifest2 hash policies and security implications</title>
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25     <tbody valign="top">
26     <tr class="field"><th class="field-name">GLEP:</th><td class="field-body">59</td>
27     </tr>
28     <tr class="field"><th class="field-name">Title:</th><td class="field-body">Manifest2 hash policies and security implications</td>
29     </tr>
30 robbat2 1.10 <tr class="field"><th class="field-name">Version:</th><td class="field-body">1.7</td>
31 cardoe 1.1 </tr>
32 robbat2 1.10 <tr class="field"><th class="field-name">Last-Modified:</th><td class="field-body"><a class="reference external" href="http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/proj/en/glep/glep-0059.txt?cvsroot=gentoo">2010/02/02 05:49:27</a></td>
33 cardoe 1.1 </tr>
34     <tr class="field"><th class="field-name">Author:</th><td class="field-body">Robin Hugh Johnson &lt;robbat2&#32;&#97;t&#32;gentoo.org&gt;,</td>
35     </tr>
36     <tr class="field"><th class="field-name">Status:</th><td class="field-body">Draft</td>
37     </tr>
38     <tr class="field"><th class="field-name">Type:</th><td class="field-body">Standards Track</td>
39     </tr>
40     <tr class="field"><th class="field-name">Content-Type:</th><td class="field-body"><a class="reference external" href="glep-0002.html">text/x-rst</a></td>
41     </tr>
42     <tr class="field"><th class="field-name">Requires:</th><td class="field-body"><a class="reference external" href="http://www.gentoo.org/proj/en/glepglep-0044.html">44</a></td>
43     </tr>
44     <tr class="field"><th class="field-name">Created:</th><td class="field-body">October 2006</td>
45     </tr>
46 robbat2 1.4 <tr class="field"><th class="field-name">Updated:</th><td class="field-body">November 2007, June 2008, July 2008, October 2008, January 2010</td>
47 cardoe 1.1 </tr>
48     <tr class="field"><th class="field-name">Updates:</th><td class="field-body">44</td>
49     </tr>
50 robbat2 1.5 <tr class="field"><th class="field-name">Post-History:</th><td class="field-body">December 2009, January 2010</td>
51 robbat2 1.3 </tr>
52 cardoe 1.1 </tbody>
53     </table>
54     <hr />
55     <div class="contents topic" id="contents">
56     <p class="topic-title first">Contents</p>
57     <ul class="simple">
58     <li><a class="reference internal" href="#abstract" id="id1">Abstract</a></li>
59     <li><a class="reference internal" href="#motivation" id="id2">Motivation</a></li>
60     <li><a class="reference internal" href="#specification" id="id3">Specification</a><ul>
61     <li><a class="reference internal" href="#the-bad-news" id="id4">The bad news</a></li>
62     <li><a class="reference internal" href="#how-fast-can-md5-be-broken" id="id5">How fast can MD5 be broken?</a></li>
63     <li><a class="reference internal" href="#the-good-news" id="id6">The good news</a></li>
64 robbat2 1.5 <li><a class="reference internal" href="#what-should-be-done" id="id7">What should be done</a></li>
65 robbat2 1.10 <li><a class="reference internal" href="#checksum-depreciation-timing" id="id8">Checksum depreciation timing</a><ul>
66     <li><a class="reference internal" href="#general-principles" id="id9">General principles:</a></li>
67     <li><a class="reference internal" href="#immediate-plans" id="id10">Immediate plans:</a></li>
68 cardoe 1.1 </ul>
69     </li>
70 robbat2 1.10 </ul>
71     </li>
72     <li><a class="reference internal" href="#backwards-compatibility" id="id11">Backwards Compatibility</a></li>
73     <li><a class="reference internal" href="#references" id="id12">References</a></li>
74     <li><a class="reference internal" href="#thanks-to" id="id13">Thanks to</a></li>
75     <li><a class="reference internal" href="#copyright" id="id14">Copyright</a></li>
76 cardoe 1.1 </ul>
77     </div>
78     <div class="section" id="abstract">
79     <h1><a class="toc-backref" href="#id1">Abstract</a></h1>
80     <p>While Manifest2 format allows multiple hashes, the question of which
81     checksums should be present, why, and the security implications of such
82     have never been resolved. This GLEP covers all of these issues, and
83     makes recommendations as to how to handle checksums both now, and in
84     future.</p>
85     </div>
86     <div class="section" id="motivation">
87     <h1><a class="toc-backref" href="#id2">Motivation</a></h1>
88     <p>This GLEP is being written as part of the work on signing the Portage
89     tree, but is only tangentially related to the actual signing of
90     Manifests. Checksums present one possible weak point in the overall
91     security of the tree - and a comprehensive security plan is needed.</p>
92 robbat2 1.8 <p>This GLEP is not mandatory for the tree-signing specification, but
93     instead aims to improve the security of the hashes used in Manifest2.
94     As such, it is also able to stand on it's own.</p>
95 cardoe 1.1 </div>
96     <div class="section" id="specification">
97     <h1><a class="toc-backref" href="#id3">Specification</a></h1>
98     <div class="section" id="the-bad-news">
99     <h2><a class="toc-backref" href="#id4">The bad news</a></h2>
100     <p>First of all, I'd like to cover the bad news in checksum security.
101     A much discussed point, as been the simple question: What is the
102     security of multiple independent checksums on the same data?
103     The most common position (and indeed the one previously held by myself),
104     is that multiple checksums would be an increase in security, but we
105     could not provably quantify the amount of security this added.
106     The really bad news, is that this position is completely and utterly
107     wrong. Many of you will be aghast at this. There is extremely little
108 robbat2 1.5 added security in multiple checksums as noted by Joux [J04]. For any set
109     of checksums, the actual strength lies in that of the strongest
110     checksum.</p>
111     <p>Wang et al [W04] extended Joux's [J04] work on SHA-0 to cover MD4, MD5,
112     HAVAL-128 and RIPEMD families of hashes.</p>
113 cardoe 1.1 </div>
114     <div class="section" id="how-fast-can-md5-be-broken">
115     <h2><a class="toc-backref" href="#id5">How fast can MD5 be broken?</a></h2>
116 robbat2 1.5 <p>For a general collision, not a pre-image attack, since the announcement
117     by Wang et al [W04], the time required to break MD5 has been massively
118     reduced. Originally at 1 hour on a near-supercomputer (IBM P690) and
119     estimated at 64 hours with a Pentium-3 1.7Ghz. This has gone down to
120     less than in two years, to 17 seconds [K06a].</p>
121 robbat2 1.10 <ul class="simple">
122     <li>08/2004 - 1 hour, IBM pSeries 690 (32x 1.7Ghz POWER4+) = 54.4 GHz-Hours</li>
123     <li>03/2005 - 8 hours, Pentium-M 1.6Ghz = 12.8 Ghz-Hours</li>
124     <li>11/2005 - 5 hours, Pentium-4 1.7Ghz = 8.5 Ghz-Hours</li>
125     <li>03/2006 - 1 minute, Pentium-4 3.2Ghz = .05 Ghz-Hours</li>
126     <li>04/2006 - 17 seconds, Pentium-4 3.2Ghz = .01 Ghz-Hours</li>
127     </ul>
128 cardoe 1.1 <p>If we accept a factor of 800x as a sample of how much faster a checksum
129     may be broken over the course of 2 years (MD5 using the above data is
130     &gt;2000x), then existing checksums do not stand a significant chance of
131     survival in the future. We should thus accept that whatever checksums we
132     are using today, will be broken in the near future, and plan as best as
133 robbat2 1.5 possible. (A brief review [H04] of the SHA1 attacks indicates an
134 cardoe 1.1 improvement of ~600x in the same timespan).</p>
135     <p>And for those that claim implementation of these procedures is not yet
136     feasible, see [K06b] for an application that can produce two
137 robbat2 1.5 self-extracting EXE files, with identical MD5s, and whatever payload you
138     want.</p>
139 cardoe 1.1 </div>
140     <div class="section" id="the-good-news">
141     <h2><a class="toc-backref" href="#id6">The good news</a></h2>
142 robbat2 1.5 <p>Of the checksums presently used by Manifest2 (SHA1, SHA256, RIPEMD160),
143     one stands close to being completely broken: SHA1; and another is
144     significantly weakened: RIPEMD160. The SHA2 series has suffered some
145     attacks, but still remains reasonably solid [G07],[K08].</p>
146 cardoe 1.1 <p>To reduce the potential for future problems and any single checksum
147     break leading to a rapid decrease in security, we should incorporate the
148     strongest hash available from each family of checksums, and be prepared
149     to retire old checksums actively, unless there is a overriding reason to
150 robbat2 1.5 keep a specific checksum, such as part of a migration plan.</p>
151 cardoe 1.1 </div>
152     <div class="section" id="what-should-be-done">
153     <h2><a class="toc-backref" href="#id7">What should be done</a></h2>
154     <p>Portage should always try to verify all supported hashes that are
155     available in a Manifest2, starting with the strongest ones as maintained
156     by a preference list. Over time, the weaker checksums should be removed
157     from Manifest2 files, once all old Portage installations have had
158 robbat2 1.10 sufficient time to upgrade. Stronger checksums shall be added as soon as
159     an implementation is available in Portage. Weak checksums may be removed
160     as long as the depreciation process is followed (see below).</p>
161 robbat2 1.5 <p>As soon as feasible, we should add the SHA512 and WHIRLPOOL algorithms.
162     In future, as stream-based checksums are developed (in response to the
163     development by NIST [AHS]), they should be considered and used.</p>
164     <p>The SHA512 algorithm is available in Python 2.5, which has been a
165 robbat2 1.9 dependency of Portage since approximately Portage 2.1.6.13.</p>
166 robbat2 1.5 <p>The WHIRLPOOL checksum is not available within the PyCrypto library or
167     hashlib that is part of Python 2.5, but there are multiple alternative
168     Python implementations available, ranging from pure Python to C-based
169     (python-mhash).</p>
170     <p>The existence unsupported hash is not considered to be a failure unless
171     no supported hashes are available for a given Manifest entry.</p>
172     </div>
173     <div class="section" id="checksum-depreciation-timing">
174     <h2><a class="toc-backref" href="#id8">Checksum depreciation timing</a></h2>
175 robbat2 1.10 <div class="section" id="general-principles">
176     <h3><a class="toc-backref" href="#id9">General principles:</a></h3>
177     <p>A minimum set of depreciated checksums shall be maintained only to
178     support old package manager versions where needed by historically used
179     trees:</p>
180     <ul class="simple">
181     <li>New package manager versions should NOT use depreciated checksums in</li>
182     <li>New trees with that have never used the depreciated checksums may omit
183     them for reasons of size, but are still strongly suggested to include
184     them.</li>
185     <li>Removal of depreciated checksums shall happen after no less than 18
186     months or one major Portage version cycle, whichever is greater.</li>
187     </ul>
188     </div>
189     <div class="section" id="immediate-plans">
190     <h3><a class="toc-backref" href="#id10">Immediate plans:</a></h3>
191 robbat2 1.5 <p>For the current Portage, both SHA1 and RIPEMD160 should be immediately
192     removed, as they present no advantages over the already present SHA256.
193     SHA256 cannot be replaced immediately with SHA512, as existing Portage
194     versions need at least one supported algorithm present (SHA256 support
195     was added in June 2006), so it must be retained for some while.</p>
196 robbat2 1.10 <p>Immediately:</p>
197     <ul class="simple">
198     <li>Add WHIRLPOOL and SHA512.</li>
199     <li>Remove SHA1 and RIPEMD160.</li>
200     </ul>
201     <p>After the majority of Portage installations include SHA512 support:</p>
202     <ul class="simple">
203     <li>Remove SHA256.</li>
204     </ul>
205     </div>
206 cardoe 1.1 </div>
207     </div>
208     <div class="section" id="backwards-compatibility">
209 robbat2 1.10 <h1><a class="toc-backref" href="#id11">Backwards Compatibility</a></h1>
210 cardoe 1.1 <p>Old versions of Portage may support and expect only specific checksums.
211     This is accounted for in the checksum depreciation discussion.</p>
212 robbat2 1.9 <p>For maximum compatiability, we should only have to include each of the
213     old algorithms that we are officially still supporting, as well as the
214     new ones that we prefer.</p>
215 cardoe 1.1 </div>
216     <div class="section" id="references">
217 robbat2 1.10 <h1><a class="toc-backref" href="#id12">References</a></h1>
218 cardoe 1.1 <dl class="docutils">
219     <dt>[AHS] NIST (2007). &quot;NIST's Plan for New Cryptographic Hash Functions&quot;,</dt>
220     <dd>(Advanced Hash Standard). <a class="reference external" href="http://csrc.nist.gov/pki/HashWorkshop/">http://csrc.nist.gov/pki/HashWorkshop/</a></dd>
221     <dt>[BOBO06] Boneh, D. and Boyen, X. (2006). &quot;On the Impossibility of</dt>
222     <dd>Efficiently Combining Collision Resistant Hash Functions&quot;; Proceedings
223     of CRYPTO 2006, Dwork, C. (Ed.); Lecture Notes in Computer Science
224     4117, pp. 570-583. Available online from:
225     <a class="reference external" href="http://crypto.stanford.edu/~dabo/abstracts/hashing.html">http://crypto.stanford.edu/~dabo/abstracts/hashing.html</a></dd>
226     <dt>[H04] Hawkes, P. and Paddon, M. and Rose, G. (2004). &quot;On Corrective</dt>
227     <dd>Patterns for the SHA-2 Family&quot;. CRYPTO 2004 Cryptology ePrint Archive,
228     Report 2004/204. Available online from:
229     <a class="reference external" href="http://eprint.iacr.org/2004/207.pdf">http://eprint.iacr.org/2004/207.pdf</a></dd>
230 robbat2 1.2 <dt>[J04] Joux, Antoie. (2004). &quot;Multicollisions in Iterated Hash</dt>
231     <dd>Functions - Application to Cascaded Constructions;&quot; Proceedings of
232     CRYPTO 2004, Franklin, M. (Ed); Lecture Notes in Computer Science
233     3152, pp. 306-316. Available online from:
234     <a class="reference external" href="http://web.cecs.pdx.edu/~teshrim/spring06/papers/general-attacks/multi-joux.pdf">http://web.cecs.pdx.edu/~teshrim/spring06/papers/general-attacks/multi-joux.pdf</a></dd>
235 cardoe 1.1 <dt>[K06a] Klima, V. (2006). &quot;Tunnels in Hash Functions: MD5 Collisions</dt>
236     <dd>Within a Minute&quot;. Cryptology ePrint Archive, Report 2006/105.
237     Available online from: <a class="reference external" href="http://eprint.iacr.org/2006/105.pdf">http://eprint.iacr.org/2006/105.pdf</a></dd>
238     <dt>[K06b] Klima, V. (2006). &quot;Note and links to high-speed MD5 collision</dt>
239     <dd>proof of concept tools&quot;. Available online from:
240     <a class="reference external" href="http://cryptography.hyperlink.cz/2006/trick.txt">http://cryptography.hyperlink.cz/2006/trick.txt</a></dd>
241     <dt>[K08] Klima, V. (2008). &quot;On Collisions of Hash Functions Turbo SHA-2&quot;.</dt>
242     <dd>Cryptology ePrint Archive, Report 2008/003. Available online from:
243     <a class="reference external" href="http://eprint.iacr.org/2008/003.pdf">http://eprint.iacr.org/2008/003.pdf</a></dd>
244     <dt>[G07] Gligoroski, D. and Knapskog, S.J. (2007). &quot;Turbo SHA-2&quot;.</dt>
245     <dd>Cryptology ePrint Archive, Report 2007/403. Available online from:
246     <a class="reference external" href="http://eprint.iacr.org/2007/403.pdf">http://eprint.iacr.org/2007/403.pdf</a></dd>
247     <dt>[W04] Wang, X. et al: &quot;Collisions for Hash Functions MD4, MD5,</dt>
248     <dd>HAVAL-128 and RIPEMD&quot;, rump session, CRYPTO 2004, Cryptology ePrint
249     Archive, Report 2004/199, first version (August 16, 2004), second
250     version (August 17, 2004). Available online from:
251     <a class="reference external" href="http://eprint.iacr.org/2004/199.pdf">http://eprint.iacr.org/2004/199.pdf</a></dd>
252     </dl>
253     </div>
254     <div class="section" id="thanks-to">
255 robbat2 1.10 <h1><a class="toc-backref" href="#id13">Thanks to</a></h1>
256 cardoe 1.1 <dl class="docutils">
257     <dt>I'd like to thank the following folks, in no specific order:</dt>
258     <dd><ul class="first last simple">
259     <li>Ciaran McCreesh (ciaranm) - for pointing out the Joux (2004) paper,
260     and also being stubborn enough in not accepting a partial solution.</li>
261     <li>Marius Mauch (genone), Zac Medico (zmedico) and Brian Harring
262     (ferringb): for being knowledgeable about the Portage Manifest2
263     codebase.</li>
264     </ul>
265     </dd>
266     </dl>
267     </div>
268     <div class="section" id="copyright">
269 robbat2 1.10 <h1><a class="toc-backref" href="#id14">Copyright</a></h1>
270 robbat2 1.4 <p>Copyright (c) 2006-2010 by Robin Hugh Johnson. This material may be
271 cardoe 1.1 distributed only subject to the terms and conditions set forth in the
272     Open Publication License, v1.0.</p>
273     <p>vim: tw=72 ts=2 expandtab:</p>
274     </div>
275    
276     </div>
277     <div class="footer">
278     <hr class="footer" />
279     <a class="reference external" href="glep-0059.txt">View document source</a>.
280 robbat2 1.10 Generated on: 2010-02-07 10:37 UTC.
281 cardoe 1.1 Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.
282    
283     </div>
284     </body>
285     </html>

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