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Revise GLEP59 per Calchan questions: Python 2.5 is widely deployed now and provides SHA512. RIPEMD160 is broken. WHIRLPOOL added. Migration plan detail added.

1 GLEP: 59
2 Title: Manifest2 hash policies and security implications
3 Version: $Revision: 1.4 $
4 Last-Modified: $Date: 2010/01/13 03:26:53 $
5 Author: Robin Hugh Johnson <robbat2@gentoo.org>,
6 Status: Draft
7 Type: Standards Track
8 Content-Type: text/x-rst
9 Requires: 44
10 Created: October 2006
11 Updated: November 2007, June 2008, July 2008, October 2008, January 2010
12 Updates: 44
13 Post-History: December 2009, January 2010
14
15 Abstract
16 ========
17 While Manifest2 format allows multiple hashes, the question of which
18 checksums should be present, why, and the security implications of such
19 have never been resolved. This GLEP covers all of these issues, and
20 makes recommendations as to how to handle checksums both now, and in
21 future.
22
23 Motivation
24 ==========
25 This GLEP is being written as part of the work on signing the Portage
26 tree, but is only tangentially related to the actual signing of
27 Manifests. Checksums present one possible weak point in the overall
28 security of the tree - and a comprehensive security plan is needed.
29
30 Specification
31 =============
32 The bad news
33 ------------
34 First of all, I'd like to cover the bad news in checksum security.
35 A much discussed point, as been the simple question: What is the
36 security of multiple independent checksums on the same data?
37 The most common position (and indeed the one previously held by myself),
38 is that multiple checksums would be an increase in security, but we
39 could not provably quantify the amount of security this added.
40 The really bad news, is that this position is completely and utterly
41 wrong. Many of you will be aghast at this. There is extremely little
42 added security in multiple checksums as noted by Joux [J04]. For any set
43 of checksums, the actual strength lies in that of the strongest
44 checksum.
45
46 Wang et al [W04] extended Joux's [J04] work on SHA-0 to cover MD4, MD5,
47 HAVAL-128 and RIPEMD families of hashes.
48
49 How fast can MD5 be broken?
50 ---------------------------
51 For a general collision, not a pre-image attack, since the announcement
52 by Wang et al [W04], the time required to break MD5 has been massively
53 reduced. Originally at 1 hour on a near-supercomputer (IBM P690) and
54 estimated at 64 hours with a Pentium-3 1.7Ghz. This has gone down to
55 less than in two years, to 17 seconds [K06a].
56
57 08/2004 - 1 hour, IBM pSeries 690 (32x 1.7Ghz POWER4+) = 54.4 GHz-Hours
58 03/2005 - 8 hours, Pentium-M 1.6Ghz = 12.8 Ghz-Hours
59 11/2005 - 5 hours, Pentium-4 1.7Ghz = 8.5 Ghz-Hours
60 03/2006 - 1 minute, Pentium-4 3.2Ghz = .05 Ghz-Hours
61 04/2006 - 17 seconds, Pentium-4 3.2Ghz = .01 Ghz-Hours
62
63 If we accept a factor of 800x as a sample of how much faster a checksum
64 may be broken over the course of 2 years (MD5 using the above data is
65 >2000x), then existing checksums do not stand a significant chance of
66 survival in the future. We should thus accept that whatever checksums we
67 are using today, will be broken in the near future, and plan as best as
68 possible. (A brief review [H04] of the SHA1 attacks indicates an
69 improvement of ~600x in the same timespan).
70
71 And for those that claim implementation of these procedures is not yet
72 feasible, see [K06b] for an application that can produce two
73 self-extracting EXE files, with identical MD5s, and whatever payload you
74 want.
75
76 The good news
77 -------------
78 Of the checksums presently used by Manifest2 (SHA1, SHA256, RIPEMD160),
79 one stands close to being completely broken: SHA1; and another is
80 significantly weakened: RIPEMD160. The SHA2 series has suffered some
81 attacks, but still remains reasonably solid [G07],[K08].
82
83 To reduce the potential for future problems and any single checksum
84 break leading to a rapid decrease in security, we should incorporate the
85 strongest hash available from each family of checksums, and be prepared
86 to retire old checksums actively, unless there is a overriding reason to
87 keep a specific checksum, such as part of a migration plan.
88
89 What should be done
90 -------------------
91 Portage should always try to verify all supported hashes that are
92 available in a Manifest2, starting with the strongest ones as maintained
93 by a preference list. Over time, the weaker checksums should be removed
94 from Manifest2 files, once all old Portage installations have had
95 sufficient time to upgrade. We should be prepared to add stronger
96 checksums wherever possible, and to remove those that have been
97 defeated.
98
99 As soon as feasible, we should add the SHA512 and WHIRLPOOL algorithms.
100 In future, as stream-based checksums are developed (in response to the
101 development by NIST [AHS]), they should be considered and used.
102
103 The SHA512 algorithm is available in Python 2.5, which has been a
104 dependency of Portage since approximately Python 2.1.6.13.
105
106 The WHIRLPOOL checksum is not available within the PyCrypto library or
107 hashlib that is part of Python 2.5, but there are multiple alternative
108 Python implementations available, ranging from pure Python to C-based
109 (python-mhash).
110
111 The existence unsupported hash is not considered to be a failure unless
112 no supported hashes are available for a given Manifest entry.
113
114 Checksum depreciation timing
115 ----------------------------
116 For the current Portage, both SHA1 and RIPEMD160 should be immediately
117 removed, as they present no advantages over the already present SHA256.
118 SHA256 cannot be replaced immediately with SHA512, as existing Portage
119 versions need at least one supported algorithm present (SHA256 support
120 was added in June 2006), so it must be retained for some while.
121
122 Immediately:
123 - Add WHIRLPOOL and SHA512.
124 - Remove SHA1 and RIPEMD160.
125
126 After the majority of Portage installations include SHA512 support:
127 - Remove SHA256.
128
129 Backwards Compatibility
130 =======================
131 Old versions of Portage may support and expect only specific checksums.
132 This is accounted for in the checksum depreciation discussion.
133
134 References
135 ==========
136
137 [AHS] NIST (2007). "NIST's Plan for New Cryptographic Hash Functions",
138 (Advanced Hash Standard). http://csrc.nist.gov/pki/HashWorkshop/
139
140 [BOBO06] Boneh, D. and Boyen, X. (2006). "On the Impossibility of
141 Efficiently Combining Collision Resistant Hash Functions"; Proceedings
142 of CRYPTO 2006, Dwork, C. (Ed.); Lecture Notes in Computer Science
143 4117, pp. 570-583. Available online from:
144 http://crypto.stanford.edu/~dabo/abstracts/hashing.html
145
146 [H04] Hawkes, P. and Paddon, M. and Rose, G. (2004). "On Corrective
147 Patterns for the SHA-2 Family". CRYPTO 2004 Cryptology ePrint Archive,
148 Report 2004/204. Available online from:
149 http://eprint.iacr.org/2004/207.pdf
150
151 [J04] Joux, Antoie. (2004). "Multicollisions in Iterated Hash
152 Functions - Application to Cascaded Constructions;" Proceedings of
153 CRYPTO 2004, Franklin, M. (Ed); Lecture Notes in Computer Science
154 3152, pp. 306-316. Available online from:
155 http://web.cecs.pdx.edu/~teshrim/spring06/papers/general-attacks/multi-joux.pdf
156
157 [K06a] Klima, V. (2006). "Tunnels in Hash Functions: MD5 Collisions
158 Within a Minute". Cryptology ePrint Archive, Report 2006/105.
159 Available online from: http://eprint.iacr.org/2006/105.pdf
160
161 [K06b] Klima, V. (2006). "Note and links to high-speed MD5 collision
162 proof of concept tools". Available online from:
163 http://cryptography.hyperlink.cz/2006/trick.txt
164
165 [K08] Klima, V. (2008). "On Collisions of Hash Functions Turbo SHA-2".
166 Cryptology ePrint Archive, Report 2008/003. Available online from:
167 http://eprint.iacr.org/2008/003.pdf
168
169 [G07] Gligoroski, D. and Knapskog, S.J. (2007). "Turbo SHA-2".
170 Cryptology ePrint Archive, Report 2007/403. Available online from:
171 http://eprint.iacr.org/2007/403.pdf
172
173 [W04] Wang, X. et al: "Collisions for Hash Functions MD4, MD5,
174 HAVAL-128 and RIPEMD", rump session, CRYPTO 2004, Cryptology ePrint
175 Archive, Report 2004/199, first version (August 16, 2004), second
176 version (August 17, 2004). Available online from:
177 http://eprint.iacr.org/2004/199.pdf
178
179 Thanks to
180 =========
181 I'd like to thank the following folks, in no specific order:
182 - Ciaran McCreesh (ciaranm) - for pointing out the Joux (2004) paper,
183 and also being stubborn enough in not accepting a partial solution.
184 - Marius Mauch (genone), Zac Medico (zmedico) and Brian Harring
185 (ferringb): for being knowledgeable about the Portage Manifest2
186 codebase.
187
188 Copyright
189 =========
190 Copyright (c) 2006-2010 by Robin Hugh Johnson. This material may be
191 distributed only subject to the terms and conditions set forth in the
192 Open Publication License, v1.0.
193
194 vim: tw=72 ts=2 expandtab:

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