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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 <tr class="field"><th class="field-name">Version:</th><td class="field-body">1.6</td>
31 </tr>
32 <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">2010/01/31 09:55:43</a></td>
33 </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 <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 </tr>
48 <tr class="field"><th class="field-name">Updates:</th><td class="field-body">44</td>
49 </tr>
50 <tr class="field"><th class="field-name">Post-History:</th><td class="field-body">December 2009, January 2010</td>
51 </tr>
52 </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 <li><a class="reference internal" href="#what-should-be-done" id="id7">What should be done</a></li>
65 <li><a class="reference internal" href="#checksum-depreciation-timing" id="id8">Checksum depreciation timing</a></li>
66 </ul>
67 </li>
68 <li><a class="reference internal" href="#backwards-compatibility" id="id9">Backwards Compatibility</a></li>
69 <li><a class="reference internal" href="#references" id="id10">References</a></li>
70 <li><a class="reference internal" href="#thanks-to" id="id11">Thanks to</a></li>
71 <li><a class="reference internal" href="#copyright" id="id12">Copyright</a></li>
72 </ul>
73 </div>
74 <div class="section" id="abstract">
75 <h1><a class="toc-backref" href="#id1">Abstract</a></h1>
76 <p>While Manifest2 format allows multiple hashes, the question of which
77 checksums should be present, why, and the security implications of such
78 have never been resolved. This GLEP covers all of these issues, and
79 makes recommendations as to how to handle checksums both now, and in
80 future.</p>
81 </div>
82 <div class="section" id="motivation">
83 <h1><a class="toc-backref" href="#id2">Motivation</a></h1>
84 <p>This GLEP is being written as part of the work on signing the Portage
85 tree, but is only tangentially related to the actual signing of
86 Manifests. Checksums present one possible weak point in the overall
87 security of the tree - and a comprehensive security plan is needed.</p>
88 <p>This GLEP is not mandatory for the tree-signing specification, but
89 instead aims to improve the security of the hashes used in Manifest2.
90 As such, it is also able to stand on it's own.</p>
91 </div>
92 <div class="section" id="specification">
93 <h1><a class="toc-backref" href="#id3">Specification</a></h1>
94 <div class="section" id="the-bad-news">
95 <h2><a class="toc-backref" href="#id4">The bad news</a></h2>
96 <p>First of all, I'd like to cover the bad news in checksum security.
97 A much discussed point, as been the simple question: What is the
98 security of multiple independent checksums on the same data?
99 The most common position (and indeed the one previously held by myself),
100 is that multiple checksums would be an increase in security, but we
101 could not provably quantify the amount of security this added.
102 The really bad news, is that this position is completely and utterly
103 wrong. Many of you will be aghast at this. There is extremely little
104 added security in multiple checksums as noted by Joux [J04]. For any set
105 of checksums, the actual strength lies in that of the strongest
106 checksum.</p>
107 <p>Wang et al [W04] extended Joux's [J04] work on SHA-0 to cover MD4, MD5,
108 HAVAL-128 and RIPEMD families of hashes.</p>
109 </div>
110 <div class="section" id="how-fast-can-md5-be-broken">
111 <h2><a class="toc-backref" href="#id5">How fast can MD5 be broken?</a></h2>
112 <p>For a general collision, not a pre-image attack, since the announcement
113 by Wang et al [W04], the time required to break MD5 has been massively
114 reduced. Originally at 1 hour on a near-supercomputer (IBM P690) and
115 estimated at 64 hours with a Pentium-3 1.7Ghz. This has gone down to
116 less than in two years, to 17 seconds [K06a].</p>
117 <p>08/2004 - 1 hour, IBM pSeries 690 (32x 1.7Ghz POWER4+) = 54.4 GHz-Hours
118 03/2005 - 8 hours, Pentium-M 1.6Ghz = 12.8 Ghz-Hours
119 11/2005 - 5 hours, Pentium-4 1.7Ghz = 8.5 Ghz-Hours
120 03/2006 - 1 minute, Pentium-4 3.2Ghz = .05 Ghz-Hours
121 04/2006 - 17 seconds, Pentium-4 3.2Ghz = .01 Ghz-Hours</p>
122 <p>If we accept a factor of 800x as a sample of how much faster a checksum
123 may be broken over the course of 2 years (MD5 using the above data is
124 &gt;2000x), then existing checksums do not stand a significant chance of
125 survival in the future. We should thus accept that whatever checksums we
126 are using today, will be broken in the near future, and plan as best as
127 possible. (A brief review [H04] of the SHA1 attacks indicates an
128 improvement of ~600x in the same timespan).</p>
129 <p>And for those that claim implementation of these procedures is not yet
130 feasible, see [K06b] for an application that can produce two
131 self-extracting EXE files, with identical MD5s, and whatever payload you
132 want.</p>
133 </div>
134 <div class="section" id="the-good-news">
135 <h2><a class="toc-backref" href="#id6">The good news</a></h2>
136 <p>Of the checksums presently used by Manifest2 (SHA1, SHA256, RIPEMD160),
137 one stands close to being completely broken: SHA1; and another is
138 significantly weakened: RIPEMD160. The SHA2 series has suffered some
139 attacks, but still remains reasonably solid [G07],[K08].</p>
140 <p>To reduce the potential for future problems and any single checksum
141 break leading to a rapid decrease in security, we should incorporate the
142 strongest hash available from each family of checksums, and be prepared
143 to retire old checksums actively, unless there is a overriding reason to
144 keep a specific checksum, such as part of a migration plan.</p>
145 </div>
146 <div class="section" id="what-should-be-done">
147 <h2><a class="toc-backref" href="#id7">What should be done</a></h2>
148 <p>Portage should always try to verify all supported hashes that are
149 available in a Manifest2, starting with the strongest ones as maintained
150 by a preference list. Over time, the weaker checksums should be removed
151 from Manifest2 files, once all old Portage installations have had
152 sufficient time to upgrade. We should be prepared to add stronger
153 checksums wherever possible, and to remove those that have been
154 defeated.</p>
155 <p>As soon as feasible, we should add the SHA512 and WHIRLPOOL algorithms.
156 In future, as stream-based checksums are developed (in response to the
157 development by NIST [AHS]), they should be considered and used.</p>
158 <p>The SHA512 algorithm is available in Python 2.5, which has been a
159 dependency of Portage since approximately Portage 2.1.6.13.</p>
160 <p>The WHIRLPOOL checksum is not available within the PyCrypto library or
161 hashlib that is part of Python 2.5, but there are multiple alternative
162 Python implementations available, ranging from pure Python to C-based
163 (python-mhash).</p>
164 <p>The existence unsupported hash is not considered to be a failure unless
165 no supported hashes are available for a given Manifest entry.</p>
166 </div>
167 <div class="section" id="checksum-depreciation-timing">
168 <h2><a class="toc-backref" href="#id8">Checksum depreciation timing</a></h2>
169 <p>For the current Portage, both SHA1 and RIPEMD160 should be immediately
170 removed, as they present no advantages over the already present SHA256.
171 SHA256 cannot be replaced immediately with SHA512, as existing Portage
172 versions need at least one supported algorithm present (SHA256 support
173 was added in June 2006), so it must be retained for some while.</p>
174 <p>Immediately:
175 - Add WHIRLPOOL and SHA512.
176 - Remove SHA1 and RIPEMD160.</p>
177 <p>After the majority of Portage installations include SHA512 support:
178 - Remove SHA256.</p>
179 </div>
180 </div>
181 <div class="section" id="backwards-compatibility">
182 <h1><a class="toc-backref" href="#id9">Backwards Compatibility</a></h1>
183 <p>Old versions of Portage may support and expect only specific checksums.
184 This is accounted for in the checksum depreciation discussion.</p>
185 <p>For maximum compatiability, we should only have to include each of the
186 old algorithms that we are officially still supporting, as well as the
187 new ones that we prefer.</p>
188 </div>
189 <div class="section" id="references">
190 <h1><a class="toc-backref" href="#id10">References</a></h1>
191 <dl class="docutils">
192 <dt>[AHS] NIST (2007). &quot;NIST's Plan for New Cryptographic Hash Functions&quot;,</dt>
193 <dd>(Advanced Hash Standard). <a class="reference external" href="http://csrc.nist.gov/pki/HashWorkshop/">http://csrc.nist.gov/pki/HashWorkshop/</a></dd>
194 <dt>[BOBO06] Boneh, D. and Boyen, X. (2006). &quot;On the Impossibility of</dt>
195 <dd>Efficiently Combining Collision Resistant Hash Functions&quot;; Proceedings
196 of CRYPTO 2006, Dwork, C. (Ed.); Lecture Notes in Computer Science
197 4117, pp. 570-583. Available online from:
198 <a class="reference external" href="http://crypto.stanford.edu/~dabo/abstracts/hashing.html">http://crypto.stanford.edu/~dabo/abstracts/hashing.html</a></dd>
199 <dt>[H04] Hawkes, P. and Paddon, M. and Rose, G. (2004). &quot;On Corrective</dt>
200 <dd>Patterns for the SHA-2 Family&quot;. CRYPTO 2004 Cryptology ePrint Archive,
201 Report 2004/204. Available online from:
202 <a class="reference external" href="http://eprint.iacr.org/2004/207.pdf">http://eprint.iacr.org/2004/207.pdf</a></dd>
203 <dt>[J04] Joux, Antoie. (2004). &quot;Multicollisions in Iterated Hash</dt>
204 <dd>Functions - Application to Cascaded Constructions;&quot; Proceedings of
205 CRYPTO 2004, Franklin, M. (Ed); Lecture Notes in Computer Science
206 3152, pp. 306-316. Available online from:
207 <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>
208 <dt>[K06a] Klima, V. (2006). &quot;Tunnels in Hash Functions: MD5 Collisions</dt>
209 <dd>Within a Minute&quot;. Cryptology ePrint Archive, Report 2006/105.
210 Available online from: <a class="reference external" href="http://eprint.iacr.org/2006/105.pdf">http://eprint.iacr.org/2006/105.pdf</a></dd>
211 <dt>[K06b] Klima, V. (2006). &quot;Note and links to high-speed MD5 collision</dt>
212 <dd>proof of concept tools&quot;. Available online from:
213 <a class="reference external" href="http://cryptography.hyperlink.cz/2006/trick.txt">http://cryptography.hyperlink.cz/2006/trick.txt</a></dd>
214 <dt>[K08] Klima, V. (2008). &quot;On Collisions of Hash Functions Turbo SHA-2&quot;.</dt>
215 <dd>Cryptology ePrint Archive, Report 2008/003. Available online from:
216 <a class="reference external" href="http://eprint.iacr.org/2008/003.pdf">http://eprint.iacr.org/2008/003.pdf</a></dd>
217 <dt>[G07] Gligoroski, D. and Knapskog, S.J. (2007). &quot;Turbo SHA-2&quot;.</dt>
218 <dd>Cryptology ePrint Archive, Report 2007/403. Available online from:
219 <a class="reference external" href="http://eprint.iacr.org/2007/403.pdf">http://eprint.iacr.org/2007/403.pdf</a></dd>
220 <dt>[W04] Wang, X. et al: &quot;Collisions for Hash Functions MD4, MD5,</dt>
221 <dd>HAVAL-128 and RIPEMD&quot;, rump session, CRYPTO 2004, Cryptology ePrint
222 Archive, Report 2004/199, first version (August 16, 2004), second
223 version (August 17, 2004). Available online from:
224 <a class="reference external" href="http://eprint.iacr.org/2004/199.pdf">http://eprint.iacr.org/2004/199.pdf</a></dd>
225 </dl>
226 </div>
227 <div class="section" id="thanks-to">
228 <h1><a class="toc-backref" href="#id11">Thanks to</a></h1>
229 <dl class="docutils">
230 <dt>I'd like to thank the following folks, in no specific order:</dt>
231 <dd><ul class="first last simple">
232 <li>Ciaran McCreesh (ciaranm) - for pointing out the Joux (2004) paper,
233 and also being stubborn enough in not accepting a partial solution.</li>
234 <li>Marius Mauch (genone), Zac Medico (zmedico) and Brian Harring
235 (ferringb): for being knowledgeable about the Portage Manifest2
236 codebase.</li>
237 </ul>
238 </dd>
239 </dl>
240 </div>
241 <div class="section" id="copyright">
242 <h1><a class="toc-backref" href="#id12">Copyright</a></h1>
243 <p>Copyright (c) 2006-2010 by Robin Hugh Johnson. This material may be
244 distributed only subject to the terms and conditions set forth in the
245 Open Publication License, v1.0.</p>
246 <p>vim: tw=72 ts=2 expandtab:</p>
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