Subject: [PATCH v3 0/6] doc: improvements for hash-function-transition

From: "Thomas Ackermann via GitGitGadget" <[email-masked]>
Date: Fri, 05 Feb 2021 18:22:23 +0000

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 a little bit
and moved to the beginning of the document. Also an incomplete sentence was
corrected.

Changes since v2:

 * Move a stray change from 1/6 back to 2/6; fix an incomplete conversion in
   2/6.
 * Rework rationale based on the comments from Junio and Brian.
 * Rebased on current master.

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, Junio and Brian for their suggestions and help.

Signed-off-by: Thomas Ackermann [email-masked]

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    | 293 +++++++++---------
 t/t0021-conversion.sh                         |   4 +-
 2 files changed, 150 insertions(+), 147 deletions(-)

base-commit: 30b29f044a2b30f0667eb21559959e03eb1bd04f
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-858%2Ftacker66%2Fdoc_hash_function_transition-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-858/tacker66/doc_hash_function_transition-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/858

Range-diff vs v2:

 1:  f36c5dd4c1e3 ! 1:  7c78d0c1c30a doc hash-function-transition: fix asciidoc output
     @@ Documentation/technical/hash-function-transition.txt: network byte order):
       Loose object index
       ~~~~~~~~~~~~~~~~~~
      @@ Documentation/technical/hash-function-transition.txt: the following steps:
     -    they will be discarded.)
       3. convert to sha256: open a new (sha256) packfile. Read the topologically
          sorted list just generated. For each object, inflate its
     --   sha1-content, convert to sha256-content, and write it to the sha256
     +    sha1-content, convert to sha256-content, and write it to the sha256
      -   pack. Record the new sha1<->sha256 mapping entry for use in the idx.
     -+   SHA-1 content, convert to SHA-256 content, and write it to the SHA-256
     -+   pack. Record the new SHA-1<-->SHA-256 mapping entry for use in the idx.
     ++   pack. Record the new sha1<-->sha256 mapping entry for use in the idx.
       4. sort: reorder entries in the new pack to match the order of objects
          in the pack the server generated and include blobs. Write a sha256 idx
          file
 2:  681ce4129dc3 ! 2:  69ebc9a8f19a doc hash-function-transition: use SHA-1 and SHA-256 consistently
     @@ Documentation/technical/hash-function-transition.txt: repository extensions.
      -that objects referenced by the object are named using their sha256-names
      -instead of sha1-names. Because a blob object does not refer to any
      -other object, its sha1-content and sha256-content are the same.
     -+The SHA-256-content of an object is the same as its SHA-1 content, except
     ++The SHA-256 content of an object is the same as its SHA-1 content, except
      +that objects referenced by the object are named using their SHA-256 names
      +instead of SHA-1 names. Because a blob object does not refer to any
      +other object, its SHA-1 content and SHA-256 content are the same.
     @@ Documentation/technical/hash-function-transition.txt: the following steps:
      -3. convert to sha256: open a new (sha256) packfile. Read the topologically
      +3. convert to SHA-256: open a new SHA-256 packfile. Read the topologically
          sorted list just generated. For each object, inflate its
     -    SHA-1 content, convert to SHA-256 content, and write it to the SHA-256
     -    pack. Record the new SHA-1<-->SHA-256 mapping entry for use in the idx.
     +-   sha1-content, convert to sha256-content, and write it to the sha256
     +-   pack. Record the new sha1<-->sha256 mapping entry for use in the idx.
     ++   SHA-1 content, convert to SHA-256 content, and write it to the SHA-256
     ++   pack. Record the new SHA-1<-->SHA-256 mapping entry for use in the idx.
       4. sort: reorder entries in the new pack to match the order of objects
      -   in the pack the server generated and include blobs. Write a sha256 idx
      +   in the pack the server generated and include blobs. Write a SHA-256 idx
 3:  4f622fffcc5d = 3:  06b781206e4c doc hash-function-transition: use upper case consistently
 4:  58295cadffe5 = 4:  7a29f06c3f25 doc hash-function-transition: fix incomplete sentence
 5:  711a37969b6f ! 5:  ee0fa2ec1d0f doc hash-function-transition: move rationale upwards
     @@ Commit message
          Move rationale for new hash function to beginning of document
          so that it 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.
     +    Remove some of the details about SHA-1 weaknesses and add references
     +    to the details on how the new hash function was chosen instead.
      
          Signed-off-by: Thomas Ackermann <[email-masked]>
      
       ## Documentation/technical/hash-function-transition.txt ##
     -@@ Documentation/technical/hash-function-transition.txt: advantages:
     -   methods have a short reliable string that can be used to reliably
     -   address stored content.
     - 
     --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.
     +@@ Documentation/technical/hash-function-transition.txt: researchers. On 23 February 2017 the SHAttered attack
       
       Git v2.13.0 and later subsequently moved to a hardened SHA-1
     --implementation by default, which isn't vulnerable to the SHAttered
     + implementation by default, which isn't vulnerable to the SHAttered
      -attack.
     -+implementation by default, but SHA-1 is still believed to be weak.
     ++attack, but SHA-1 is still 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
     @@ Documentation/technical/hash-function-transition.txt: SHA-1 still possesses the
       
      +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).
     ++
     ++4. As a tiebreaker, the hash should be fast to compute (fortunately
     ++   many contenders are faster than SHA-1).
     ++
      +There were several contenders for a successor hash to SHA-1, including
      +SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.
      +
 6:  d6041b7e9e87 = 6:  c31d6e258fd0 doc: use https links

-- 
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