|GLEP 59: Manifest2 hash policies and security implications|
|Author||Robin Hugh Johnson <email@example.com>|
While Manifest2 format allows multiple hashes, the question of which checksums should be present, why, and the security implications of such have never been resolved. This GLEP covers all of these issues, and makes recommendations as to how to handle checksums both now, and in future.
This GLEP is being written as part of the work on signing the Portage tree, but is only tangentially related to the actual signing of Manifests. Checksums present one possible weak point in the overall security of the tree - and a comprehensive security plan is needed.
This GLEP is not mandatory for the tree-signing specification, but instead aims to improve the security of the hashes used in Manifest2 . As such, it is also able to stand on its own.
The bad news
First of all, I'd like to cover the bad news in checksum security. A much discussed point, as been the simple question: What is the security of multiple independent checksums on the same data? The most common position (and indeed the one previously held by myself), is that multiple checksums would be an increase in security, but we could not provably quantify the amount of security this added. The really bad news, is that this position is completely and utterly wrong. Many of you will be aghast at this. There is extremely little added security in multiple checksums as noted by Joux . For any set of checksums, the actual strength lies in that of the strongest checksum.
How fast can MD5 be broken?
For a general collision, not a pre-image attack, since the announcement by Wang et al , the time required to break MD5 has been massively reduced. Originally at 1 hour on a near-supercomputer (IBM P690) and estimated at 64 hours with a Pentium-3 1.7Ghz. This has gone down to less than in two years, to 17 seconds .
- 08/2004 - 1 hour, IBM pSeries 690 (32x 1.7Ghz POWER4+) = 54.4 GHz-Hours
- 03/2005 - 8 hours, Pentium-M 1.6Ghz = 12.8 Ghz-Hours
- 11/2005 - 5 hours, Pentium-4 1.7Ghz = 8.5 Ghz-Hours
- 03/2006 - 1 minute, Pentium-4 3.2Ghz = .05 Ghz-Hours
- 04/2006 - 17 seconds, Pentium-4 3.2Ghz = .01 Ghz-Hours
If we accept a factor of 800x as a sample of how much faster a checksum may be broken over the course of 2 years (MD5 using the above data is >2000x), then existing checksums do not stand a significant chance of survival in the future. We should thus accept that whatever checksums we are using today, will be broken in the near future, and plan as best as possible. (A brief review  of the SHA1 attacks indicates an improvement of ~600x in the same timespan).
And for those that claim implementation of these procedures is not yet feasible, see  for an application that can produce two self-extracting EXE files, with identical MD5s, and whatever payload you want.
The good news
Of the checksums presently used by Manifest2 (SHA1, SHA256, RIPEMD160), one stands close to being completely broken: SHA1; and another is significantly weakened: RIPEMD160. The SHA2 series has suffered some attacks, but still remains reasonably solid  
To reduce the potential for future problems and any single checksum break leading to a rapid decrease in security, we should incorporate the strongest hash available from each family of checksums, and be prepared to retire old checksums actively, unless there is a overriding reason to keep a specific checksum, such as part of a migration plan.
What should be done
Portage should always try to verify all supported hashes that are available in a Manifest2, starting with the strongest ones as maintained by a preference list. Over time, the weaker checksums should be removed from Manifest2 files, once all old Portage installations have had sufficient time to upgrade. Stronger checksums shall be added as soon as an implementation is available in Portage. Weak checksums may be removed as long as the depreciation process is followed (see below).
As soon as feasible, we should add the SHA512 and WHIRLPOOL algorithms. In future, as stream-based checksums are developed (in response to the development by NIST ), they should be considered and used.
The SHA512 algorithm is available in Python 2.5, which has been a dependency of Portage since approximately Portage 220.127.116.11.
The WHIRLPOOL checksum is not available within the PyCrypto library or hashlib that is part of Python 2.5, but there are multiple alternative Python implementations available, ranging from pure Python to C-based (python-mhash).
The existence unsupported hash is not considered to be a failure unless no supported hashes are available for a given Manifest entry.
Checksum depreciation timing
A minimum set of depreciated checksums shall be maintained only to support old package manager versions where needed by historically used trees:
- New package manager versions should NOT use depreciated checksums in
- New trees with that have never used the depreciated checksums may omit them for reasons of size, but are still strongly suggested to include them.
- Removal of depreciated checksums shall happen after no less than 18 months or one major Portage version cycle, whichever is greater.
For the current Portage, both SHA1 and RIPEMD160 should be immediately removed, as they present no advantages over the already present SHA256. SHA256 cannot be replaced immediately with SHA512, as existing Portage versions need at least one supported algorithm present (SHA256 support was added in June 2006), so it must be retained for some while.
- Add WHIRLPOOL and SHA512.
- Remove SHA1 and RIPEMD160.
After the majority of Portage installations include SHA512 support:
- Remove SHA256.
Old versions of Portage may support and expect only specific checksums. This is accounted for in the checksum depreciation discussion.
For maximum compatibility, we should only have to include each of the old algorithms that we are officially still supporting, as well as the new ones that we prefer.
I'd like to thank the following folks, in no specific order:
- Ciaran McCreesh (ciaranm) - for pointing out the Joux (2004) paper, and also being stubborn enough in not accepting a partial solution.
- Marius Mauch (genone), Zac Medico (zmedico) and Brian Harring (ferringb): for being knowledgeable about the Portage Manifest2 codebase.
- Mauch, M. (2005) GLEP44 - Manifest2 format. GLEP 44
- Joux, Antoie. (2004). "Multicollisions in Iterated Hash Functions - Application to Cascaded Constructions;" Proceedings of CRYPTO 2004, Franklin, M. (Ed); Lecture Notes in Computer Science 3152, pp. 306-316. Available online from: http://web.cecs.pdx.edu/~teshrim/spring06/papers/general-attacks/multi-joux.pdf
- Wang, X. et al: "Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD", rump session, CRYPTO 2004, Cryptology ePrint Archive, Report 2004/199, first version (August 16, 2004), second version (August 17, 2004). Available online from: http://eprint.iacr.org/2004/199.pdf
- Klima, V. (2006). "Tunnels in Hash Functions: MD5 Collisions Within a Minute". Cryptology ePrint Archive, Report 2006/105. Available online from: http://eprint.iacr.org/2006/105.pdf
- Hawkes, P. and Paddon, M. and Rose, G. (2004). "On Corrective Patterns for the SHA-2 Family". CRYPTO 2004 Cryptology ePrint Archive, Report 2004/204. Available online from: http://eprint.iacr.org/2004/207.pdf
- Klima, V. (2006). "Note and links to high-speed MD5 collision proof of concept tools". Available online from: http://cryptography.hyperlink.cz/2006/trick.txt
- Klima, V. (2008). "On Collisions of Hash Functions Turbo SHA-2". Cryptology ePrint Archive, Report 2008/003. Available online from: http://eprint.iacr.org/2008/003.pdf
- G07] Gligoroski, D. and Knapskog, S.J. (2007). "Turbo SHA-2". Cryptology ePrint Archive, Report 2007/403. Available online from: http://eprint.iacr.org/2007/403.pdf
- NIST (2007). "NIST's Plan for New Cryptographic Hash Functions", (Advanced Hash Standard). http://csrc.nist.gov/pki/HashWorkshop/
- Boneh, D. and Boyen, X. (2006). "On the Impossibility of Efficiently Combining Collision Resistant Hash Functions"; Proceedings of CRYPTO 2006, Dwork, C. (Ed.); Lecture Notes in Computer Science 4117, pp. 570-583. Available online from: http://crypto.stanford.edu/~dabo/abstracts/hashing.html
Copyright (c) 2006-2010 by Robin Hugh Johnson. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0.