[PATCH v2 0/6] doc: improvements for hash-function-transition

From: Thomas Ackermann via GitGitGadget <gitgitgadget@gmail.com>
Date: Feb 2, 2021, 16:19 UTC
Message-ID: <pull.858.v2.git.1612282749.gitgitgadget@gmail.com>
In-Reply-To: <pull.858.git.1612093734.gitgitgadget@gmail.com>

Some asciidoc formatting errors and some minor formatting inconsistencies in hash-function-transition.txt were fixed.

Content-wise the rationale for choosing SHA-256 was shortened and moved to the beginning of the document and an incomplete sentence was corrected.

Changes since v1:

 * Better commit messages.
 * Details on SHA-1 weaknesses were removed from the rationale.
 * All http links to lore.kernel.org in the tree were changed to https
   links.

Thanks to Ævar for his suggestions and help.

Signed-off-by: Thomas Ackermann th.acker@arcor.de

Thomas Ackermann (6):
  doc hash-function-transition: fix asciidoc output
  doc hash-function-transition: use SHA-1 and SHA-256 consistently
  doc hash-function-transition: use upper case consistently
  doc hash-function-transition: fix incomplete sentence
  doc hash-function-transition: move rationale upwards
  doc: use https links

 .../technical/hash-function-transition.txt    | 279 ++++++++----------
 t/t0021-conversion.sh                         |   4 +-
 2 files changed, 132 insertions(+), 151 deletions(-)

base-commit: e6362826a0409539642a5738db61827e5978e2e4
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-858%2Ftacker66%2Fdoc_hash_function_transition-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-858/tacker66/doc_hash_function_transition-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/858

Range-diff vs v1:

1: 3efe3392e9d ! 1: f36c5dd4c1e doc hash-function-transition: fix asciidoc output
@@ Metadata
## Commit message ##
doc hash-function-transition: fix asciidoc output

- fix asciidoc output for lists, special characters and verbatim text while retaining the readabilty of the original text file
+ Asciidoc requires lists to start with an empty line and uses
+ different characters for indentation levels ("-", "*", "**", ...).
+ For special symbols like a dash "--" has to be used and there is
+ no double arrow "<->", so a left and right arrow "<-->" has to be
+ combined for that. Lastly for verbatim output a newline followed
+ by an indentation has to be used.
+
+ Fix asciidoc output for lists, special characters and verbatim
+ text while retaining the readabilty of the original text file.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>

2: 62ca087d4eb ! 2: 681ce4129dc doc hash-function-transition: use SHA-1 and SHA-256 consistently
@@ Metadata
## Commit message ##
doc hash-function-transition: use SHA-1 and SHA-256 consistently

- use SHA-1 and SHA-256 instead of sha1 and sha256 when referring to the hash type
+ Use SHA-1 and SHA-256 instead of sha1 and sha256 when referring
+ to the hash type.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>

3: 37e3fd6aaa0 ! 3: 4f622fffcc5 doc hash-function-transition: use upper case consistently
@@ Metadata
## Commit message ##
doc hash-function-transition: use upper case consistently

- use upper case consistently in Document History
+ Use upper case consistently in Document History.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>

6: 302c7b8dce0 = 4: 58295cadffe doc hash-function-transition: fix incomplete sentence
5: 2cdb0f8e2ed ! 5: 711a37969b6 doc hash-function-transition: move rationale upwards
@@ Metadata
## Commit message ##
doc hash-function-transition: move rationale upwards

- move rationale for new hash function to beginning of document
+ Move rationale for new hash function to beginning of document
+ so that it appears before the concrete move to SHA-256 is described.

- rationale now appears before the concrete move to SHA-256 is described
+ Remove details about SHA-1 weaknesses. Instead add references
+ to the details of how the new hash function was chosen.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>

@@ Documentation/technical/hash-function-transition.txt: advantages:

-Over time some flaws in SHA-1 have been discovered by security
-researchers. On 23 February 2017 the SHAttered attack
+-(https://shattered.io) demonstrated a practical SHA-1 hash collision.
+Over time some flaws in SHA-1 have been discovered by security researchers.
-+In early 2005, around the time that Git was written, Xiaoyun Wang,
-+Yiqun Lisa Yin, and Hongbo Yu announced an attack finding SHA-1
-+collisions in 2^69 operations. In August they published details.
-+Luckily, no practical demonstrations of a collision in full SHA-1 were
-+published until 10 years later: on 23 February 2017 the SHAttered attack
- (https://shattered.io) demonstrated a practical SHA-1 hash collision.

Git v2.13.0 and later subsequently moved to a hardened SHA-1
-implementation by default, which isn't vulnerable to the SHAttered
-attack.
-+implementation by default that mitigates the SHAttered attack, but
-+SHA-1 is still believed to be weak.
++implementation by default, but SHA-1 is still believed to be weak.

- Thus Git has in effect already migrated to a new hash that isn't SHA-1
- and doesn't share its vulnerabilities, its new hash function just
+-Thus Git has in effect already migrated to a new hash that isn't SHA-1
+-and doesn't share its vulnerabilities, its new hash function just
+-happens to produce exactly the same output for all known inputs,
+-except two PDFs published by the SHAttered researchers, and the new
+-implementation (written by those researchers) claims to detect future
+-cryptanalytic collision attacks.
+-
+-Regardless, it's considered prudent to move past any variant of SHA-1
++Thus it's considered prudent to move past any variant of SHA-1
+ to a new hash. There's no guarantee that future attacks on SHA-1 won't
+ be published in the future, and those attacks may not have viable
+ mitigations.
@@ Documentation/technical/hash-function-transition.txt: SHA-1 still possesses the other properties such as fast object lookup
and safe error checking, but other hash functions are equally suitable
that are believed to be cryptographically secure.

+Choice of Hash
+--------------
-+The hash to replace the hardened SHA-1 should be stronger than SHA-1
-+was: we would like it to be trustworthy and useful in practice for at
-+least 10 years.
-+
-+Some other relevant properties:
-+
-+1. A 256-bit hash (long enough to match common security practice; not
-+ excessively long to hurt performance and disk usage).
-+
-+2. High quality implementations should be widely available (e.g., in
-+ OpenSSL and Apple CommonCrypto).
-+
-+3. The hash function's properties should match Git's needs (e.g. Git
-+ requires collision and 2nd preimage resistance and does not require
-+ length extension resistance).
++There were several contenders for a successor hash to SHA-1, including
++SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.
+
-+4. As a tiebreaker, the hash should be fast to compute (fortunately
-+ many contenders are faster than SHA-1).
++In late 2018 the project picked SHA-256 as its successor hash.
+
-+We choose SHA-256.
++See 0ed8d8da374 (doc hash-function-transition: pick SHA-256 as
++NewHash, 2018-08-04) and numerous mailing list threads at the time,
++particularly the one starting at
++https://lore.kernel.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/
++for more information.
+
Goals
-----
4: d4abf1cf78e ! 6: d6041b7e9e8 doc hash-function-transition: use https links consistently
@@ Metadata
Author: Thomas Ackermann <th.acker@arcor.de>

## Commit message ##
- doc hash-function-transition: use https links consistently
+ doc: use https links

- use only https links in References
+ Use only https links for lore.kernel.org.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>

## Documentation/technical/hash-function-transition.txt ##
+@@ Documentation/technical/hash-function-transition.txt: Document History
+ bmwill@google.com, jonathantanmy@google.com, jrnieder@gmail.com,
+ sbeller@google.com
+
+-* Initial version sent to http://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com
++* Initial version sent to https://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com
+
+ 2017-03-03 jrnieder@gmail.com
+ Incorporated suggestions from jonathantanmy and sbeller:
@@ Documentation/technical/hash-function-transition.txt: Later history:

References:
@@ Documentation/technical/hash-function-transition.txt: Later history:
+ [3] https://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
+ [4] https://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
[5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/
+
+ ## t/t0021-conversion.sh ##
+@@ t/t0021-conversion.sh: filter_git () {
+ # Compare two files and ensure that `clean` and `smudge` respectively are
+ # called at least once if specified in the `expect` file. The actual
+ # invocation count is not relevant because their number can vary.
+-# c.f. http://lore.kernel.org/git/xmqqshv18i8i.fsf@gitster.mtv.corp.google.com/
++# c.f. https://lore.kernel.org/git/xmqqshv18i8i.fsf@gitster.mtv.corp.google.com/
+ test_cmp_count () {
+ expect=$1
+ actual=$2
+@@ t/t0021-conversion.sh: test_cmp_count () {
+
+ # Compare two files but exclude all `clean` invocations because Git can
+ # call `clean` zero or more times.
+-# c.f. http://lore.kernel.org/git/xmqqshv18i8i.fsf@gitster.mtv.corp.google.com/
++# c.f. https://lore.kernel.org/git/xmqqshv18i8i.fsf@gitster.mtv.corp.google.com/
+ test_cmp_exclude_clean () {
+ expect=$1
+ actual=$2

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