I wondered if one could make a logical argument that says, one could use a
combination of hashes that have different collision resistances (e.g.
SHA1⊕MD5) for each file, extending to any number of hashes to satisfy that
the combination is unique for all files...

So I did a little research, and the short answer is NO!  It turns out that
the combined hash would be no better than the best of the hash functions in
the combo:


The Internet is a wonderful thing.

On Mon, Feb 27, 2017 at 9:29 AM Bakul Shah <ba...@bitblocks.com> wrote:

> My argument is that an archival system that can't store some files, no
> matter how they were generated, is not good enough.  A hash collision
> researcher may have a legitimate reason to store such files.
> On Feb 27, 2017, at 9:07 AM, Charles Forsyth <charles.fors...@gmail.com>
> wrote:
> On 27 February 2017 at 16:47, Charles Forsyth <charles.fors...@gmail.com>
> wrote:
> On 27 February 2017 at 15:46, Dave MacFarlane <driu...@gmail.com> wrote:
> Why not skip sha-256 and go directly to Sha3?
> blake2 has also been suggested
> also, it's not clear it's urgent for venti. the scam is to make a new
> value that produces the same hash as an earlier important value where the
> hash plays a part in certifying the value,
> or where software uses the shorthand of comparing hashes to compare values
> and acts on that without comparing the values.
> with venti, the hash is produced as a side-effect of storing a value, and
> it also records the value itself.
> when the hash is presented, the stored block is returned. the hash itself
> is a compact address and doesn't certify the value (ie, nothing that uses
> venti assumes that it also certifies the value).
> any attempt to store a different value with the same hash will be
> detected. using any hash function has a chance of collision (newer, longer
> hashes reduce that, but it's rare as it is).
> because venti is write-once, no-one can change your venti contents subtly
> without access to the storage device, but if they've got access to the
> storage they don't need to be subtle.
> with the collision-maker and access to the storage device, they can make a
> previously certain vac: mean something different, but it still needs raw
> access to the device, it can't be done through
> the venti protocol.

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