path: root/glep-0059.rst

GLEP: 59
Title: Manifest2 hash policies and security implications
Version: $Revision$
Last-Modified: $Date$
Author: Robin Hugh Johnson <robbat2@gentoo.org>, 
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Requires: 44
Created: October 2006
Post-History: 1-Dec-2009, 31-Jan-2010

Abstract
========
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.

Motivation
==========
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
[GLEP44]. As such, it is also able to stand on its own.

Specification
=============
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 [J04]. For any set
of checksums, the actual strength lies in that of the strongest
checksum.

Wang et al [W04] extended Joux's [J04] work on SHA-0 to cover MD4, MD5,
HAVAL-128 and RIPEMD families of hashes.

How fast can MD5 be broken?
---------------------------
For a general collision, not a pre-image attack, since the announcement
by Wang et al [W04], 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 [K06a].

- 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 [H04] 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 [K06b] 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 [G07],[K08]. 

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 [AHS]), 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 2.1.6.13.

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
----------------------------
General principles:
~~~~~~~~~~~~~~~~~~~
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.

Immediate plans:
~~~~~~~~~~~~~~~~
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.

Immediately:

- Add WHIRLPOOL and SHA512.

- Remove SHA1 and RIPEMD160.

After the majority of Portage installations include SHA512 support:

- Remove SHA256.

Backwards Compatibility
=======================
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.

References
==========

[AHS] NIST (2007). "NIST's Plan for New Cryptographic Hash Functions",
  (Advanced Hash Standard). http://csrc.nist.gov/pki/HashWorkshop/

[BOBO06] 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

[H04] 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

[J04] 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

[K06a] 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

[K06b] 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

[K08] 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

[W04] 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

Thanks to
=========
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.

References
==========
.. [GLEP44] Mauch, M. (2005) GLEP44 - Manifest2 format.
   http://www.gentoo.org/proj/en/glep/glep-0044.html	

Copyright
=========
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.

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