BreakingChanges: document upcoming change from "sha1" to "sha256"

Starting with 8e42eb0 (doc: sha256 is no longer experimental,
2023-07-31), the "sha256" object format is no longer considered to be
experimental. Furthermore, the SHA-1 hash function is actively
recommended against by for example NIST and FIPS 140-2, and attacks
against it are becoming more practical both due to new weaknesses
(SHAppening, SHAttered, Shambles) and due to the ever-increasing
computing power. It is only a matter of time before it can be considered
to be broken completely.

Let's plan for this event by being active instead of waiting for it to
happend and announce that the default object format is going to change
from "sha1" to "sha256" with Git 3.0.

All major Git implementations (libgit2, JGit, go-git) support the
"sha256" object format and are thus prepared for this change. The most
important missing piece in the puzzle is support in forges. But while
GitLab recently gained experimental support for the "sha256" object
format though, to the best of my knowledge GitHub doesn't support it
yet. Ideally, announcing this upcoming change will encourage forges to
start building that support.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

Documentation/BreakingChanges.txt

@@ -70,6 +70,36 @@ be changed to or replaced in case the alternative was implemented already.
 
 === Changes
 
+* The default hash function for new repositories will be changed from "sha1"
+  to "sha256". SHA-1 has been deprecated by NIST in 2011 and is nowadays
+  recommended against in FIPS 140-2 and similar certifications. Furthermore,
+  there are practical attacks on SHA-1 that weaken its cryptographic properties:
++
+  ** The SHAppening (2015). The first demonstration of a practical attack
+     against SHA-1 with 2^57 operations.
+  ** SHAttered (2017). Generation of two valid PDF files with 2^63 operations.
+  ** Birthday-Near-Collision (2019). This attack allows for chosen prefix
+     attacks with 2^68 operations.
+  ** Shambles (2020). This attack allows for chosen prefix attacks with 2^63
+     operations.
++
+While we have protections in place against known attacks, it is expected
+that more attacks against SHA-1 will be found by future research. Paired
+with the ever-growing capability of hardware, it is only a matter of time
+before SHA-1 will be considered broken completely. We want to be prepared
+and will thus change the default hash algorithm to "sha256" for newly
+initialized repositories.
++
+An important requirement for this change is that the ecosystem is ready to
+support the "sha256" object format. This includes popular Git libraries,
+applications and forges.
++
+There is no plan to deprecate the "sha1" object format at this point in time.
++
+Cf. <[email protected]>,
+<[email protected]>,
+<CA+EOSBncr=4a4d8n9xS4FNehyebpmX8JiUwCsXD47EQDE+DiUQ@mail.gmail.com>.
+
 === Removals
 
 == Superseded features that will not be deprecated