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Re: Fast alternative hash

From: Branko Čibej <brane_at_apache.org>
Date: Wed, 1 Mar 2017 14:15:03 +0100

On 01.03.2017 13:52, Stefan Fuhrmann wrote:
> On 01.03.2017 05:17, Greg Stein wrote:
>> I really like this idea.
>>
>> And we could take a copy of APR's sha1 code, and rejigger it to
>> perform *both* hashes during the same scan of the raw bytes. I would
>> expect the time taken to extend by (say) 1.1X rather than a full 2X.
>> The inner loop might cost a bit more, but we'd only scan the bytes
>> once. Very handy, when you're talking about megabytes in a stream-y
>> environment.
>>
>> (and medium-term, push this dual-sha1 computation back into APR)
>>
>>
>> On Sun, Feb 26, 2017 at 10:08 AM, Garance A Drosehn <drosih_at_rpi.edu
>> <mailto:drosih_at_rpi.edu>> wrote:
>>
>> On 24 Feb 2017, at 15:46, Stefan Sperling wrote:
>> >
>> > I believe we should prepare a new working format for 1.10.0
>> > which addresses this problem. I don't see a good way of fixing
>> > it without a format bump. The bright side of this is that it
>> > gives us a good reason to get 1.10.0 ready ASAP.
>> >
>> > We can switch to a better hash algorithm with a WC format
>> > bump.
>>
>> One of the previous messages mentioned that better hash
>> algorithms are more expensive. So let me mention a tactic
>> that I used many years ago, when MD5 was the best digest
>> algorithm that I knew of, and I didn't trust it for the
>> larger files I was working with at the time:
>>
>> Instead of going with a completely different hash algorithm,
>> just double-down on the one you're using. What I did was to
>> calculate one digest the standard way, and then a second one
>> which summed up every-other-byte (or every 3rd byte, or ...).
>> So to get a collision, not only do two files have to get the
>> same digest-result for all their data, but they have to also
>> get the same digest-result when exactly half the data is
>> skipped over.
>>
>> (I did this a long time ago, and forget the details. What
>> I may have done for performance reasons was every-other-word,
>> not every-other-byte)
>>
>> My thinking was that *any* single algorithm which processes
>> all the data is going to get collisions, eventually. But it
>> will be much harder for someone to generate a duplicate file
>> where there will also be a collision when summing up only
>> half of the data.
>>
>> I'm not claiming this is great cure-all solution, but just
>> that it's an alternate tactic which might be interesting.
>> People could create repositories with just the one digest,
>> or upgrade it to use multiple digests if they have the need.
>>
>> I found a few benchmarks which suggest that sha-256 is maybe
>> twice as expensive as sha-1, so calculating two sha-1 digests
>> might be a reasonable alternative.
>>
>
> That is also known as bit-slicing. The neat thing is that
> you create N (e.g. 4) interleaved sub-streams who's
> checksum can be calculated *concurrently*, e.g using
> SIMD instructions. so, you end up being ~3 times *faster*
> than calculating the normal checksum.
>
> Because the interleaved streams may look quite similar
> (think bitmaps), you probably want to "salt" them. A simple
> rotate or XOR might do - but I'm not an expert on this.
> the goal is to end up with 4 reasonably independent
> streams, hence sub-hashes.
>
> So, the full sequence would look something like this:
>
> * Split text T into interleaved sub-streams T1,..,T4
> * Salt them S1 = salt(T1, 1), ..., S4 = salt(T4, 4)
> * Calculate sub-stream hashes using bit-sliced code
> D1, ..., D4 = sha1_4x(S1, ..., S4) = sha1(S1), ..., sha1(S4)
> * Calculate the final checksum D = sha2(D1|...|D4)
>
> Not only would that solve the current sha1 issue but
> neatly address the fact that nowadays we can read
> data faster from disk that we could checksum it.

For some definition of "solve" — i.e., until a more generic attack
method is invented. :)

-- Brane
Received on 2017-03-01 14:15:15 CET

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