On Mon, Jan 28, 2008 at 10:39:03AM -0700, Neal McBurnett wrote: > On Mon, Jan 28, 2008 at 05:20:52PM +0000, Matt Zimmerman wrote: > > On Mon, Jan 28, 2008 at 09:28:48AM -0700, Neal McBurnett wrote: > > > > (I'm all in favor of moving to SHA256 or whatever is considered best > > > > practice these days. I've just not heard that MD5 is really as broken as > > > > I think Chris suggests here.) > > > > > > One easy thing to do is to also publish sha256 sums of the CD > > > images, so if MD5 preimage attacks are developed, that would help. > > > > > > I think we should do that now, and consider a hash function in a > > > different class also (whirlpool?). > > > > > > Shipping more hash functions in the base install would help a lot in a > > > crisis, so users have what they need to validate software updates. > > > I guess coreutils has the md5 and sha families well covered, but > > > again, something different like whirlpool could help a lot some day. > > > > Perhaps we should publish detached signatures for each ISO rather than > > signing MD5SUMS? > > From what I've heard, the main principle for dealing with hash issues > is "algorithm agility" - i.e. making it easy for folks to use multiple > algorithms. > > Publishing detached signatures is a way to make the user interface > easier (perhaps) for folks that want to validate the gpg signature. > But I would think many (especially those without a good way to trust > the gpg key, as noted previously) would want to just be able to > validate hashes. > > I would still argue for the use of multiple hash algorithms, and I > guess for gpg that means multiple detached signatures, one per hash > algorithm. And some are not supported by all versions of gpg.... > > I'd suggest we publish a "CHECKSUMS" file with a good assortment of > hashes in text format, and also sign that.
There are two reasons for checking the hashes: Authentication - the downloaded image is in fact the official one provided by the Ubuntu project, unaltered Integrity - the downloaded image hasn't been randomly corrupted in transit (it happens that verifying authenticity ensures integrity as a side effect) Authentication, I believe, would be better served by signing the image directly. This both avoids an attack on the intervening checksums in MD5SUMS and provides a cryptographically stronger check. I believe the .gpg format already supports multiple signatures with different algorithms, so this would be reasonably future-proof. Integrity is served well enough by the existing MD5 hashes, which are still extremely robust against unintentional corruption. The above is based on only a very basic understanding of cryptography, however, so corrections are welcome from folks with more experience in this area. -- - mdz -- Ubuntu-devel-discuss mailing list [email protected] Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-devel-discuss
