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<tr class="field"><th class="field-name">GLEP:</th><td class="field-body">59</td> |
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</tr> |
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<tr class="field"><th class="field-name">Title:</th><td class="field-body">Manifest2 hash policies and security implications</td> |
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</tr> |
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<tr class="field"><th class="field-name">Version:</th><td class="field-body">1.7</td> |
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</tr> |
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<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> |
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</tr> |
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<tr class="field"><th class="field-name">Author:</th><td class="field-body">Robin Hugh Johnson <robbat2 at gentoo.org>,</td> |
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</tr> |
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<tr class="field"><th class="field-name">Status:</th><td class="field-body">Draft</td> |
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</tr> |
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<tr class="field"><th class="field-name">Type:</th><td class="field-body">Standards Track</td> |
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</tr> |
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<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> |
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</tr> |
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<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> |
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</tr> |
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<tr class="field"><th class="field-name">Created:</th><td class="field-body">October 2006</td> |
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</tr> |
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<tr class="field"><th class="field-name">Updated:</th><td class="field-body">November 2007, June 2008, July 2008, October 2008, January 2010</td> |
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</tr> |
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<tr class="field"><th class="field-name">Updates:</th><td class="field-body">44</td> |
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</tr> |
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<tr class="field"><th class="field-name">Post-History:</th><td class="field-body">December 2009, January 2010</td> |
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</tr> |
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</tbody> |
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</table> |
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<hr /> |
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<div class="contents topic" id="contents"> |
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<p class="topic-title first">Contents</p> |
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<ul class="simple"> |
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<li><a class="reference internal" href="#abstract" id="id1">Abstract</a></li> |
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<li><a class="reference internal" href="#motivation" id="id2">Motivation</a></li> |
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<li><a class="reference internal" href="#specification" id="id3">Specification</a><ul> |
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<li><a class="reference internal" href="#the-bad-news" id="id4">The bad news</a></li> |
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<li><a class="reference internal" href="#how-fast-can-md5-be-broken" id="id5">How fast can MD5 be broken?</a></li> |
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<li><a class="reference internal" href="#the-good-news" id="id6">The good news</a></li> |
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<li><a class="reference internal" href="#what-should-be-done" id="id7">What should be done</a></li> |
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<li><a class="reference internal" href="#checksum-depreciation-timing" id="id8">Checksum depreciation timing</a><ul> |
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<li><a class="reference internal" href="#general-principles" id="id9">General principles:</a></li> |
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<li><a class="reference internal" href="#immediate-plans" id="id10">Immediate plans:</a></li> |
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</ul> |
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</li> |
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</ul> |
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</li> |
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<li><a class="reference internal" href="#backwards-compatibility" id="id11">Backwards Compatibility</a></li> |
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<li><a class="reference internal" href="#references" id="id12">References</a></li> |
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<li><a class="reference internal" href="#thanks-to" id="id13">Thanks to</a></li> |
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<li><a class="reference internal" href="#copyright" id="id14">Copyright</a></li> |
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</ul> |
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</div> |
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<div class="section" id="abstract"> |
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<h1><a class="toc-backref" href="#id1">Abstract</a></h1> |
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<p>While Manifest2 format allows multiple hashes, the question of which |
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checksums should be present, why, and the security implications of such |
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have never been resolved. This GLEP covers all of these issues, and |
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makes recommendations as to how to handle checksums both now, and in |
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future.</p> |
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</div> |
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<div class="section" id="motivation"> |
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<h1><a class="toc-backref" href="#id2">Motivation</a></h1> |
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<p>This GLEP is being written as part of the work on signing the Portage |
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tree, but is only tangentially related to the actual signing of |
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Manifests. Checksums present one possible weak point in the overall |
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security of the tree - and a comprehensive security plan is needed.</p> |
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<p>This GLEP is not mandatory for the tree-signing specification, but |
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instead aims to improve the security of the hashes used in Manifest2. |
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As such, it is also able to stand on it's own.</p> |
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</div> |
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<div class="section" id="specification"> |
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<h1><a class="toc-backref" href="#id3">Specification</a></h1> |
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<div class="section" id="the-bad-news"> |
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<h2><a class="toc-backref" href="#id4">The bad news</a></h2> |
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<p>First of all, I'd like to cover the bad news in checksum security. |
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A much discussed point, as been the simple question: What is the |
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security of multiple independent checksums on the same data? |
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The most common position (and indeed the one previously held by myself), |
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is that multiple checksums would be an increase in security, but we |
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could not provably quantify the amount of security this added. |
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The really bad news, is that this position is completely and utterly |
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wrong. Many of you will be aghast at this. There is extremely little |
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added security in multiple checksums as noted by Joux [J04]. For any set |
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of checksums, the actual strength lies in that of the strongest |
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checksum.</p> |
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<p>Wang et al [W04] extended Joux's [J04] work on SHA-0 to cover MD4, MD5, |
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HAVAL-128 and RIPEMD families of hashes.</p> |
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</div> |
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<div class="section" id="how-fast-can-md5-be-broken"> |
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<h2><a class="toc-backref" href="#id5">How fast can MD5 be broken?</a></h2> |
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<p>For a general collision, not a pre-image attack, since the announcement |
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by Wang et al [W04], the time required to break MD5 has been massively |
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reduced. Originally at 1 hour on a near-supercomputer (IBM P690) and |
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estimated at 64 hours with a Pentium-3 1.7Ghz. This has gone down to |
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less than in two years, to 17 seconds [K06a].</p> |
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<ul class="simple"> |
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<li>08/2004 - 1 hour, IBM pSeries 690 (32x 1.7Ghz POWER4+) = 54.4 GHz-Hours</li> |
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<li>03/2005 - 8 hours, Pentium-M 1.6Ghz = 12.8 Ghz-Hours</li> |
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<li>11/2005 - 5 hours, Pentium-4 1.7Ghz = 8.5 Ghz-Hours</li> |
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<li>03/2006 - 1 minute, Pentium-4 3.2Ghz = .05 Ghz-Hours</li> |
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<li>04/2006 - 17 seconds, Pentium-4 3.2Ghz = .01 Ghz-Hours</li> |
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</ul> |
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<p>If we accept a factor of 800x as a sample of how much faster a checksum |
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may be broken over the course of 2 years (MD5 using the above data is |
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>2000x), then existing checksums do not stand a significant chance of |
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survival in the future. We should thus accept that whatever checksums we |
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are using today, will be broken in the near future, and plan as best as |
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possible. (A brief review [H04] of the SHA1 attacks indicates an |
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improvement of ~600x in the same timespan).</p> |
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<p>And for those that claim implementation of these procedures is not yet |
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feasible, see [K06b] for an application that can produce two |
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self-extracting EXE files, with identical MD5s, and whatever payload you |
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want.</p> |
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</div> |
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<div class="section" id="the-good-news"> |
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<h2><a class="toc-backref" href="#id6">The good news</a></h2> |
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<p>Of the checksums presently used by Manifest2 (SHA1, SHA256, RIPEMD160), |
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one stands close to being completely broken: SHA1; and another is |
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significantly weakened: RIPEMD160. The SHA2 series has suffered some |
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attacks, but still remains reasonably solid [G07],[K08].</p> |
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<p>To reduce the potential for future problems and any single checksum |
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break leading to a rapid decrease in security, we should incorporate the |
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strongest hash available from each family of checksums, and be prepared |
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to retire old checksums actively, unless there is a overriding reason to |
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keep a specific checksum, such as part of a migration plan.</p> |
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</div> |
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<div class="section" id="what-should-be-done"> |
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<h2><a class="toc-backref" href="#id7">What should be done</a></h2> |
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<p>Portage should always try to verify all supported hashes that are |
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available in a Manifest2, starting with the strongest ones as maintained |
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by a preference list. Over time, the weaker checksums should be removed |
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from Manifest2 files, once all old Portage installations have had |
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sufficient time to upgrade. Stronger checksums shall be added as soon as |
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an implementation is available in Portage. Weak checksums may be removed |
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as long as the depreciation process is followed (see below).</p> |
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<p>As soon as feasible, we should add the SHA512 and WHIRLPOOL algorithms. |
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In future, as stream-based checksums are developed (in response to the |
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development by NIST [AHS]), they should be considered and used.</p> |
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<p>The SHA512 algorithm is available in Python 2.5, which has been a |
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dependency of Portage since approximately Portage 2.1.6.13.</p> |
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<p>The WHIRLPOOL checksum is not available within the PyCrypto library or |
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hashlib that is part of Python 2.5, but there are multiple alternative |
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Python implementations available, ranging from pure Python to C-based |
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(python-mhash).</p> |
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<p>The existence unsupported hash is not considered to be a failure unless |
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no supported hashes are available for a given Manifest entry.</p> |
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</div> |
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<div class="section" id="checksum-depreciation-timing"> |
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<h2><a class="toc-backref" href="#id8">Checksum depreciation timing</a></h2> |
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<div class="section" id="general-principles"> |
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<h3><a class="toc-backref" href="#id9">General principles:</a></h3> |
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<p>A minimum set of depreciated checksums shall be maintained only to |
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support old package manager versions where needed by historically used |
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trees:</p> |
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<ul class="simple"> |
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<li>New package manager versions should NOT use depreciated checksums in</li> |
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<li>New trees with that have never used the depreciated checksums may omit |
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them for reasons of size, but are still strongly suggested to include |
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them.</li> |
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<li>Removal of depreciated checksums shall happen after no less than 18 |
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months or one major Portage version cycle, whichever is greater.</li> |
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</ul> |
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</div> |
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<div class="section" id="immediate-plans"> |
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<h3><a class="toc-backref" href="#id10">Immediate plans:</a></h3> |
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<p>For the current Portage, both SHA1 and RIPEMD160 should be immediately |
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removed, as they present no advantages over the already present SHA256. |
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SHA256 cannot be replaced immediately with SHA512, as existing Portage |
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versions need at least one supported algorithm present (SHA256 support |
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was added in June 2006), so it must be retained for some while.</p> |
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<p>Immediately:</p> |
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<ul class="simple"> |
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<li>Add WHIRLPOOL and SHA512.</li> |
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<li>Remove SHA1 and RIPEMD160.</li> |
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</ul> |
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<p>After the majority of Portage installations include SHA512 support:</p> |
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<ul class="simple"> |
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<li>Remove SHA256.</li> |
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</ul> |
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</div> |
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</div> |
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</div> |
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<div class="section" id="backwards-compatibility"> |
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<h1><a class="toc-backref" href="#id11">Backwards Compatibility</a></h1> |
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<p>Old versions of Portage may support and expect only specific checksums. |
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This is accounted for in the checksum depreciation discussion.</p> |
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<p>For maximum compatiability, we should only have to include each of the |
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old algorithms that we are officially still supporting, as well as the |
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new ones that we prefer.</p> |
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</div> |
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<div class="section" id="references"> |
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<h1><a class="toc-backref" href="#id12">References</a></h1> |
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<dl class="docutils"> |
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<dt>[AHS] NIST (2007). "NIST's Plan for New Cryptographic Hash Functions",</dt> |
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<dd>(Advanced Hash Standard). <a class="reference external" href="http://csrc.nist.gov/pki/HashWorkshop/">http://csrc.nist.gov/pki/HashWorkshop/</a></dd> |
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<dt>[BOBO06] Boneh, D. and Boyen, X. (2006). "On the Impossibility of</dt> |
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<dd>Efficiently Combining Collision Resistant Hash Functions"; Proceedings |
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of CRYPTO 2006, Dwork, C. (Ed.); Lecture Notes in Computer Science |
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4117, pp. 570-583. Available online from: |
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<a class="reference external" href="http://crypto.stanford.edu/~dabo/abstracts/hashing.html">http://crypto.stanford.edu/~dabo/abstracts/hashing.html</a></dd> |
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<dt>[H04] Hawkes, P. and Paddon, M. and Rose, G. (2004). "On Corrective</dt> |
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<dd>Patterns for the SHA-2 Family". CRYPTO 2004 Cryptology ePrint Archive, |
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Report 2004/204. Available online from: |
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<a class="reference external" href="http://eprint.iacr.org/2004/207.pdf">http://eprint.iacr.org/2004/207.pdf</a></dd> |
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<dt>[J04] Joux, Antoie. (2004). "Multicollisions in Iterated Hash</dt> |
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<dd>Functions - Application to Cascaded Constructions;" Proceedings of |
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CRYPTO 2004, Franklin, M. (Ed); Lecture Notes in Computer Science |
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3152, pp. 306-316. Available online from: |
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<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 |
<dt>[K06a] Klima, V. (2006). "Tunnels in Hash Functions: MD5 Collisions</dt> |
| 236 |
<dd>Within a Minute". Cryptology ePrint Archive, Report 2006/105. |
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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). "Note and links to high-speed MD5 collision</dt> |
| 239 |
<dd>proof of concept tools". Available online from: |
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<a class="reference external" href="http://cryptography.hyperlink.cz/2006/trick.txt">http://cryptography.hyperlink.cz/2006/trick.txt</a></dd> |
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<dt>[K08] Klima, V. (2008). "On Collisions of Hash Functions Turbo SHA-2".</dt> |
| 242 |
<dd>Cryptology ePrint Archive, Report 2008/003. Available online from: |
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<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). "Turbo SHA-2".</dt> |
| 245 |
<dd>Cryptology ePrint Archive, Report 2007/403. Available online from: |
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<a class="reference external" href="http://eprint.iacr.org/2007/403.pdf">http://eprint.iacr.org/2007/403.pdf</a></dd> |
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<dt>[W04] Wang, X. et al: "Collisions for Hash Functions MD4, MD5,</dt> |
| 248 |
<dd>HAVAL-128 and RIPEMD", rump session, CRYPTO 2004, Cryptology ePrint |
| 249 |
Archive, Report 2004/199, first version (August 16, 2004), second |
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version (August 17, 2004). Available online from: |
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<a class="reference external" href="http://eprint.iacr.org/2004/199.pdf">http://eprint.iacr.org/2004/199.pdf</a></dd> |
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</dl> |
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</div> |
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<div class="section" id="thanks-to"> |
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<h1><a class="toc-backref" href="#id13">Thanks to</a></h1> |
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<dl class="docutils"> |
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<dt>I'd like to thank the following folks, in no specific order:</dt> |
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<dd><ul class="first last simple"> |
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<li>Ciaran McCreesh (ciaranm) - for pointing out the Joux (2004) paper, |
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and also being stubborn enough in not accepting a partial solution.</li> |
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<li>Marius Mauch (genone), Zac Medico (zmedico) and Brian Harring |
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(ferringb): for being knowledgeable about the Portage Manifest2 |
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codebase.</li> |
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</ul> |
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</dd> |
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</dl> |
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</div> |
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<div class="section" id="copyright"> |
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<h1><a class="toc-backref" href="#id14">Copyright</a></h1> |
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<p>Copyright (c) 2006-2010 by Robin Hugh Johnson. This material may be |
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distributed only subject to the terms and conditions set forth in the |
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Open Publication License, v1.0.</p> |
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<p>vim: tw=72 ts=2 expandtab:</p> |
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</div> |
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