[pacman-dev] [PATCH] Replace MD5 with SHA-256 as a default file integrity check in PKGBUILDs

[Filipe Laíns]

On Thu, 2020-01-23 at 02:25 +0100, Artur Juraszek wrote:
> Hi all,
> 
> While poking through Arch's package system, I noticed that despite its
> bad reputation, MD5 remains a default, and even some kind of a "recommendation", due
> to its presence in the example PKBUILDs, hashing algorithm for file integrity verification.
> 
> Is there a reason to not have it changed to a more future-proof one? I mean, at least for now,
> it seems good enough to protect before a so-called "2nd preimage attack", which is the primary
> concern in the classic file verification scenario, BUT:
> 
> a) given the huge size of AUR and its rather chaotic nature, it is not that hard to imagine
> _a_ malicious upstream which could try to sneak some nasty changes in its own files,
> with AUR maintainer not noticing anything - leveraging flaws which do exist and are quite
> well-explored even today.
> 
> b) it's already shown its weaknesses and it is not going to be any better - the only research direction
> is to found more (practical) attacks against MD5, so faster the change, fewer the people possibly
> affected in the future
> 
> Attaching a patch which, I think, replaces MD5 with SHA256 as a default completely - it's my first
> change in ABS-related code, though, so please do not hesitate to criticize if something's wrong ;]
> 
> --
> Artur Juraszek

I think we should change it to sha512 instead. sha256 and sha512 are
pretty similar but sha512 is faster on 64-bit machine. Since 64-bit is
the new standard for high-power computing, and the only architecture we
support, it would be more beneficial to chose sha512.

A quick benchmark on my machine confirms this:

$ dd if=/dev/zero of=example.img bs=4096 count=512000
512000+0 records in
512000+0 records out
2097152000 bytes (2.1 GB, 2.0 GiB) copied, 2.77283 s, 756 MB/s

$ time sha256sum example.img
274fbb979251bcaceab594dd89d5adfec310e8851e320b5b5f90fd5f18d76149  examp
le.img
real 4.79
user 4.47
sys 0.30

$ time sha512sum example.img
241497cb61e24fcdaf33a13f5635951ff7c21cb27904e6f3de7b221031b0216800cbce1
a667a66aafbdb7ffbfe2a39564b4cb48efea1d3721093fa7663e7a8c9  example.img
real 3.33
user 3.09
sys 0.21

sha512 is ~1.5s than sha256 when calculating the checksum of a 2GiB
zero-ed file.

Thank you,
Filipe Laíns
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