* [PATCH v2 1/6] doc hash-function-transition: fix asciidoc output
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
@ 2021-02-02 16:19 ` Thomas Ackermann via GitGitGadget
2021-02-02 16:19 ` [PATCH v2 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently Thomas Ackermann via GitGitGadget
` (6 subsequent siblings)
7 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-02 16:19 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
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>
---
.../technical/hash-function-transition.txt | 81 +++++++++++--------
1 file changed, 46 insertions(+), 35 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 6fd20ebbc25..4b04829d537 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -94,7 +94,7 @@ Overview
--------
We introduce a new repository format extension. Repositories with this
extension enabled use SHA-256 instead of SHA-1 to name their objects.
-This affects both object names and object content --- both the names
+This affects both object names and object content -- both the names
of objects and all references to other objects within an object are
switched to the new hash function.
@@ -191,21 +191,21 @@ hash functions. They have the following format (all integers are in
network byte order):
- A header appears at the beginning and consists of the following:
- - The 4-byte pack index signature: '\377t0c'
- - 4-byte version number: 3
- - 4-byte length of the header section, including the signature and
+ * The 4-byte pack index signature: '\377t0c'
+ * 4-byte version number: 3
+ * 4-byte length of the header section, including the signature and
version number
- - 4-byte number of objects contained in the pack
- - 4-byte number of object formats in this pack index: 2
- - For each object format:
- - 4-byte format identifier (e.g., 'sha1' for SHA-1)
- - 4-byte length in bytes of shortened object names. This is the
+ * 4-byte number of objects contained in the pack
+ * 4-byte number of object formats in this pack index: 2
+ * For each object format:
+ ** 4-byte format identifier (e.g., 'sha1' for SHA-1)
+ ** 4-byte length in bytes of shortened object names. This is the
shortest possible length needed to make names in the shortened
object name table unambiguous.
- - 4-byte integer, recording where tables relating to this format
+ ** 4-byte integer, recording where tables relating to this format
are stored in this index file, as an offset from the beginning.
- - 4-byte offset to the trailer from the beginning of this file.
- - Zero or more additional key/value pairs (4-byte key, 4-byte
+ * 4-byte offset to the trailer from the beginning of this file.
+ * Zero or more additional key/value pairs (4-byte key, 4-byte
value). Only one key is supported: 'PSRC'. See the "Loose objects
and unreachable objects" section for supported values and how this
is used. All other keys are reserved. Readers must ignore
@@ -213,37 +213,36 @@ network byte order):
- Zero or more NUL bytes. This can optionally be used to improve the
alignment of the full object name table below.
- Tables for the first object format:
- - A sorted table of shortened object names. These are prefixes of
+ * A sorted table of shortened object names. These are prefixes of
the names of all objects in this pack file, packed together
without offset values to reduce the cache footprint of the binary
search for a specific object name.
- - A table of full object names in pack order. This allows resolving
+ * A table of full object names in pack order. This allows resolving
a reference to "the nth object in the pack file" (from a
reachability bitmap or from the next table of another object
format) to its object name.
- - A table of 4-byte values mapping object name order to pack order.
+ * A table of 4-byte values mapping object name order to pack order.
For an object in the table of sorted shortened object names, the
value at the corresponding index in this table is the index in the
previous table for that same object.
-
This can be used to look up the object in reachability bitmaps or
to look up its name in another object format.
- - A table of 4-byte CRC32 values of the packed object data, in the
+ * A table of 4-byte CRC32 values of the packed object data, in the
order that the objects appear in the pack file. This is to allow
compressed data to be copied directly from pack to pack during
repacking without undetected data corruption.
- - A table of 4-byte offset values. For an object in the table of
+ * A table of 4-byte offset values. For an object in the table of
sorted shortened object names, the value at the corresponding
index in this table indicates where that object can be found in
the pack file. These are usually 31-bit pack file offsets, but
large offsets are encoded as an index into the next table with the
most significant bit set.
- - A table of 8-byte offset entries (empty for pack files less than
+ * A table of 8-byte offset entries (empty for pack files less than
2 GiB). Pack files are organized with heavily used objects toward
the front, so most object references should not need to refer to
this table.
@@ -252,10 +251,10 @@ network byte order):
up to and not including the table of CRC32 values.
- Zero or more NUL bytes.
- The trailer consists of the following:
- - A copy of the 20-byte SHA-256 checksum at the end of the
+ * A copy of the 20-byte SHA-256 checksum at the end of the
corresponding packfile.
- - 20-byte SHA-256 checksum of all of the above.
+ * 20-byte SHA-256 checksum of all of the above.
Loose object index
~~~~~~~~~~~~~~~~~~
@@ -350,8 +349,8 @@ 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
- 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
file
@@ -391,6 +390,7 @@ existing "gpgsig" field. Its signed payload is the sha256-content of the
commit object with any "gpgsig" and "gpgsig-sha256" fields removed.
This means commits can be signed
+
1. using SHA-1 only, as in existing signed commit objects
2. using both SHA-1 and SHA-256, by using both gpgsig-sha256 and gpgsig
fields.
@@ -408,6 +408,7 @@ sha256-content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
SIGNATURE-----" delimited in-body signature removed.
This means tags can be signed
+
1. using SHA-1 only, as in existing signed tag objects
2. using both SHA-1 and SHA-256, by using gpgsig-sha256 and an in-body
signature.
@@ -598,7 +599,7 @@ The user can also explicitly specify which format to use for a
particular revision specifier and for output, overriding the mode. For
example:
-git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}
+ git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}
Choice of Hash
--------------
@@ -636,6 +637,7 @@ We choose SHA-256.
Transition plan
---------------
Some initial steps can be implemented independently of one another:
+
- adding a hash function API (vtable)
- teaching fsck to tolerate the gpgsig-sha256 field
- excluding gpgsig-* from the fields copied by "git commit --amend"
@@ -647,9 +649,9 @@ Some initial steps can be implemented independently of one another:
- introducing index v3
- adding support for the PSRC field and safer object pruning
-
The first user-visible change is the introduction of the objectFormat
extension (without compatObjectFormat). This requires:
+
- teaching fsck about this mode of operation
- using the hash function API (vtable) when computing object names
- signing objects and verifying signatures
@@ -657,6 +659,7 @@ extension (without compatObjectFormat). This requires:
repository
Next comes introduction of compatObjectFormat:
+
- implementing the loose-object-idx
- translating object names between object formats
- translating object content between object formats
@@ -669,6 +672,7 @@ Next comes introduction of compatObjectFormat:
"Object names on the command line" above)
The next step is supporting fetches and pushes to SHA-1 repositories:
+
- allow pushes to a repository using the compat format
- generate a topologically sorted list of the SHA-1 names of fetched
objects
@@ -734,6 +738,7 @@ Using hash functions in parallel
Objects newly created would be addressed by the new hash, but inside
such an object (e.g. commit) it is still possible to address objects
using the old hash function.
+
* You cannot trust its history (needed for bisectability) in the
future without further work
* Maintenance burden as the number of supported hash functions grows
@@ -749,6 +754,7 @@ sha1-content based signatures.
In other words, a single signature was used to attest to the object
content using both hash functions. This had some advantages:
+
* Using one signature instead of two speeds up the signing process.
* Having one signed payload with both hashes allows the signer to
attest to the sha1-name and sha256-name referring to the same object.
@@ -756,6 +762,7 @@ content using both hash functions. This had some advantages:
to be detected quickly using current versions of git.
However, it also came with disadvantages:
+
* Verifying a signed object requires access to the sha1-names of all
objects it references, even after the transition is complete and
translation table is no longer needed for anything else. To support
@@ -782,16 +789,17 @@ 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 http://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com
2017-03-03 jrnieder@gmail.com
Incorporated suggestions from jonathantanmy and sbeller:
+
* describe purpose of signed objects with each hash type
* redefine signed object verification using object content under the
first hash function
2017-03-06 jrnieder@gmail.com
+
* Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2]
* Make sha3-based signatures a separate field, avoiding the need for
"hash" and "nohash" fields (thanks to peff[3]).
@@ -805,6 +813,7 @@ Incorporated suggestions from jonathantanmy and sbeller:
especially Junio).
2017-09-27 jrnieder@gmail.com, sbeller@google.com
+
* use placeholder NewHash instead of SHA3-256
* describe criteria for picking a hash function.
* include a transition plan (thanks especially to Brandon Williams
@@ -816,12 +825,14 @@ Incorporated suggestions from jonathantanmy and sbeller:
Later history:
- See the history of this file in git.git for the history of subsequent
- edits. This document history is no longer being maintained as it
- would now be superfluous to the commit log
+* See the history of this file in git.git for the history of subsequent
+ edits. This document history is no longer being maintained as it
+ would now be superfluous to the commit log
+
+References:
-[1] http://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
-[2] http://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
-[3] http://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
-[4] http://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
-[5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/
+ [1] http://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
+ [2] http://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
+ [3] http://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
+ [4] http://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
+ [5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v2 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
2021-02-02 16:19 ` [PATCH v2 1/6] doc hash-function-transition: fix asciidoc output Thomas Ackermann via GitGitGadget
@ 2021-02-02 16:19 ` Thomas Ackermann via GitGitGadget
2021-02-02 19:39 ` Junio C Hamano
2021-02-02 16:19 ` [PATCH v2 3/6] doc hash-function-transition: use upper case consistently Thomas Ackermann via GitGitGadget
` (5 subsequent siblings)
7 siblings, 1 reply; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-02 16:19 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
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>
---
.../technical/hash-function-transition.txt | 122 +++++++++---------
1 file changed, 61 insertions(+), 61 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 4b04829d537..51acf2c10b7 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -107,7 +107,7 @@ mapping to allow naming objects using either their SHA-1 and SHA-256 names
interchangeably.
"git cat-file" and "git hash-object" gain options to display an object
-in its sha1 form and write an object given its sha1 form. This
+in its SHA-1 form and write an object given its SHA-1 form. This
requires all objects referenced by that object to be present in the
object database so that they can be named using the appropriate name
(using the bidirectional hash mapping).
@@ -115,7 +115,7 @@ object database so that they can be named using the appropriate name
Fetches from a SHA-1 based server convert the fetched objects into
SHA-256 form and record the mapping in the bidirectional mapping table
(see below for details). Pushes to a SHA-1 based server convert the
-objects being pushed into sha1 form so the server does not have to be
+objects being pushed into SHA-1 form so the server does not have to be
aware of the hash function the client is using.
Detailed Design
@@ -151,38 +151,38 @@ repository extensions.
Object names
~~~~~~~~~~~~
-Objects can be named by their 40 hexadecimal digit sha1-name or 64
-hexadecimal digit sha256-name, plus names derived from those (see
+Objects can be named by their 40 hexadecimal digit SHA-1 name or 64
+hexadecimal digit SHA-256 name, plus names derived from those (see
gitrevisions(7)).
-The sha1-name of an object is the SHA-1 of the concatenation of its
-type, length, a nul byte, and the object's sha1-content. This is the
+The SHA-1 name of an object is the SHA-1 of the concatenation of its
+type, length, a nul byte, and the object's SHA-1 content. This is the
traditional <sha1> used in Git to name objects.
-The sha256-name of an object is the SHA-256 of the concatenation of its
-type, length, a nul byte, and the object's sha256-content.
+The SHA-256 name of an object is the SHA-256 of the concatenation of its
+type, length, a nul byte, and the object's SHA-256 content.
Object format
~~~~~~~~~~~~~
The content as a byte sequence of a tag, commit, or tree object named
-by sha1 and sha256 differ because an object named by sha256-name refers to
-other objects by their sha256-names and an object named by sha1-name
-refers to other objects by their sha1-names.
+by SHA-1 and SHA-256 differ because an object named by SHA-256 name refers to
+other objects by their SHA-256 names and an object named by SHA-1 name
+refers to other objects by their SHA-1 names.
-The sha256-content of an object is the same as its sha1-content, except
-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
+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.
-The format allows round-trip conversion between sha256-content and
-sha1-content.
+The format allows round-trip conversion between SHA-256 content and
+SHA-1 content.
Object storage
~~~~~~~~~~~~~~
Loose objects use zlib compression and packed objects use the packed
format described in Documentation/technical/pack-format.txt, just like
-today. The content that is compressed and stored uses sha256-content
-instead of sha1-content.
+today. The content that is compressed and stored uses SHA-256 content
+instead of SHA-1 content.
Pack index
~~~~~~~~~~
@@ -287,18 +287,18 @@ To remove entries (e.g. in "git pack-refs" or "git-prune"):
Translation table
~~~~~~~~~~~~~~~~~
-The index files support a bidirectional mapping between sha1-names
-and sha256-names. The lookup proceeds similarly to ordinary object
-lookups. For example, to convert a sha1-name to a sha256-name:
+The index files support a bidirectional mapping between SHA-1 names
+and SHA-256 names. The lookup proceeds similarly to ordinary object
+lookups. For example, to convert a SHA-1 name to a SHA-256 name:
1. Look for the object in idx files. If a match is present in the
- idx's sorted list of truncated sha1-names, then:
- a. Read the corresponding entry in the sha1-name order to pack
+ idx's sorted list of truncated SHA-1 names, then:
+ a. Read the corresponding entry in the SHA-1 name order to pack
name order mapping.
- b. Read the corresponding entry in the full sha1-name table to
+ b. Read the corresponding entry in the full SHA-1 name table to
verify we found the right object. If it is, then
- c. Read the corresponding entry in the full sha256-name table.
- That is the object's sha256-name.
+ c. Read the corresponding entry in the full SHA-256 name table.
+ That is the object's SHA-256 name.
2. Check for a loose object. Read lines from loose-object-idx until
we find a match.
@@ -312,10 +312,10 @@ Since all operations that make new objects (e.g., "git commit") add
the new objects to the corresponding index, this mapping is possible
for all objects in the object store.
-Reading an object's sha1-content
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The sha1-content of an object can be read by converting all sha256-names
-its sha256-content references to sha1-names using the translation table.
+Reading an object's SHA-1 content
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The SHA-1 content of an object can be read by converting all SHA-256 names
+its SHA-256 content references to SHA-1 names using the translation table.
Fetch
~~~~~
@@ -338,7 +338,7 @@ the following steps:
1. index-pack: inflate each object in the packfile and compute its
SHA-1. Objects can contain deltas in OBJ_REF_DELTA format against
objects the client has locally. These objects can be looked up
- using the translation table and their sha1-content read as
+ using the translation table and their SHA-1 content read as
described above to resolve the deltas.
2. topological sort: starting at the "want"s from the negotiation
phase, walk through objects in the pack and emit a list of them,
@@ -347,12 +347,12 @@ the following steps:
(This list only contains objects reachable from the "wants". If the
pack from the server contained additional extraneous objects, then
they will be discarded.)
-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.
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
file
5. clean up: remove the SHA-1 based pack file, index, and
topologically sorted list obtained from the server in steps 1
@@ -377,16 +377,16 @@ experimenting to get this to perform well.
Push
~~~~
Push is simpler than fetch because the objects referenced by the
-pushed objects are already in the translation table. The sha1-content
+pushed objects are already in the translation table. The SHA-1 content
of each object being pushed can be read as described in the "Reading
-an object's sha1-content" section to generate the pack written by git
+an object's SHA-1 content" section to generate the pack written by git
send-pack.
Signed Commits
~~~~~~~~~~~~~~
We add a new field "gpgsig-sha256" to the commit object format to allow
signing commits without relying on SHA-1. It is similar to the
-existing "gpgsig" field. Its signed payload is the sha256-content of the
+existing "gpgsig" field. Its signed payload is the SHA-256 content of the
commit object with any "gpgsig" and "gpgsig-sha256" fields removed.
This means commits can be signed
@@ -404,7 +404,7 @@ Signed Tags
~~~~~~~~~~~
We add a new field "gpgsig-sha256" to the tag object format to allow
signing tags without relying on SHA-1. Its signed payload is the
-sha256-content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
+SHA-256 content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
SIGNATURE-----" delimited in-body signature removed.
This means tags can be signed
@@ -416,11 +416,11 @@ This means tags can be signed
Mergetag embedding
~~~~~~~~~~~~~~~~~~
-The mergetag field in the sha1-content of a commit contains the
-sha1-content of a tag that was merged by that commit.
+The mergetag field in the SHA-1 content of a commit contains the
+SHA-1 content of a tag that was merged by that commit.
-The mergetag field in the sha256-content of the same commit contains the
-sha256-content of the same tag.
+The mergetag field in the SHA-256 content of the same commit contains the
+SHA-256 content of the same tag.
Submodules
~~~~~~~~~~
@@ -495,7 +495,7 @@ Caveats
-------
Invalid objects
~~~~~~~~~~~~~~~
-The conversion from sha1-content to sha256-content retains any
+The conversion from SHA-1 content to SHA-256 content retains any
brokenness in the original object (e.g., tree entry modes encoded with
leading 0, tree objects whose paths are not sorted correctly, and
commit objects without an author or committer). This is a deliberate
@@ -514,15 +514,15 @@ allow lifting this restriction.
Alternates
~~~~~~~~~~
-For the same reason, a sha256 repository cannot borrow objects from a
-sha1 repository using objects/info/alternates or
+For the same reason, a SHA-256 repository cannot borrow objects from a
+SHA-1 repository using objects/info/alternates or
$GIT_ALTERNATE_OBJECT_REPOSITORIES.
git notes
~~~~~~~~~
-The "git notes" tool annotates objects using their sha1-name as key.
+The "git notes" tool annotates objects using their SHA-1 name as key.
This design does not describe a way to migrate notes trees to use
-sha256-names. That migration is expected to happen separately (for
+SHA-256 names. That migration is expected to happen separately (for
example using a file at the root of the notes tree to describe which
hash it uses).
@@ -556,7 +556,7 @@ unclear:
Git 2.12
-Does this mean Git v2.12.0 is the commit with sha1-name
+Does this mean Git v2.12.0 is the commit with SHA-1 name
e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with
new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7?
@@ -676,7 +676,7 @@ The next step is supporting fetches and pushes to SHA-1 repositories:
- allow pushes to a repository using the compat format
- generate a topologically sorted list of the SHA-1 names of fetched
objects
-- convert the fetched packfile to sha256 format and generate an idx
+- convert the fetched packfile to SHA-256 format and generate an idx
file
- re-sort to match the order of objects in the fetched packfile
@@ -748,38 +748,38 @@ using the old hash function.
Signed objects with multiple hashes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Instead of introducing the gpgsig-sha256 field in commit and tag objects
-for sha256-content based signatures, an earlier version of this design
-added "hash sha256 <sha256-name>" fields to strengthen the existing
-sha1-content based signatures.
+for SHA-256 content based signatures, an earlier version of this design
+added "hash sha256 <SHA-256 name>" fields to strengthen the existing
+SHA-1 content based signatures.
In other words, a single signature was used to attest to the object
content using both hash functions. This had some advantages:
* Using one signature instead of two speeds up the signing process.
* Having one signed payload with both hashes allows the signer to
- attest to the sha1-name and sha256-name referring to the same object.
+ attest to the SHA-1 name and SHA-256 name referring to the same object.
* All users consume the same signature. Broken signatures are likely
to be detected quickly using current versions of git.
However, it also came with disadvantages:
-* Verifying a signed object requires access to the sha1-names of all
+* Verifying a signed object requires access to the SHA-1 names of all
objects it references, even after the transition is complete and
translation table is no longer needed for anything else. To support
- this, the design added fields such as "hash sha1 tree <sha1-name>"
- and "hash sha1 parent <sha1-name>" to the sha256-content of a signed
+ this, the design added fields such as "hash sha1 tree <SHA-1 name>"
+ and "hash sha1 parent <SHA-1 name>" to the SHA-256 content of a signed
commit, complicating the conversion process.
-* Allowing signed objects without a sha1 (for after the transition is
+* Allowing signed objects without a SHA-1 (for after the transition is
complete) complicated the design further, requiring a "nohash sha1"
- field to suppress including "hash sha1" fields in the sha256-content
+ field to suppress including "hash sha1" fields in the SHA-256 content
and signed payload.
Lazily populated translation table
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some of the work of building the translation table could be deferred to
push time, but that would significantly complicate and slow down pushes.
-Calculating the sha1-name at object creation time at the same time it is
-being streamed to disk and having its sha256-name calculated should be
+Calculating the SHA-1 name at object creation time at the same time it is
+being streamed to disk and having its SHA-256 name calculated should be
an acceptable cost.
Document History
@@ -801,7 +801,7 @@ Incorporated suggestions from jonathantanmy and sbeller:
2017-03-06 jrnieder@gmail.com
* Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2]
-* Make sha3-based signatures a separate field, avoiding the need for
+* Make SHA3-based signatures a separate field, avoiding the need for
"hash" and "nohash" fields (thanks to peff[3]).
* Add a sorting phase to fetch (thanks to Junio for noticing the need
for this).
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* Re: [PATCH v2 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently
2021-02-02 16:19 ` [PATCH v2 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently Thomas Ackermann via GitGitGadget
@ 2021-02-02 19:39 ` Junio C Hamano
2021-02-02 23:19 ` Junio C Hamano
0 siblings, 1 reply; 25+ messages in thread
From: Junio C Hamano @ 2021-02-02 19:39 UTC (permalink / raw)
To: Thomas Ackermann via GitGitGadget
Cc: git, Ævar Arnfjörð Bjarmason, Thomas Ackermann
"Thomas Ackermann via GitGitGadget" <gitgitgadget@gmail.com> writes:
> From: Thomas Ackermann <th.acker@arcor.de>
>
> Use SHA-1 and SHA-256 instead of sha1 and sha256 when referring
> to the hash type.
Ahh. [1/6] was supposed to be only about formatting, and I found it
a bit irritating that it had some of these changes mixed in, as it
was not entirely clear to me that [1/6] covered all those lowercase
sha1 and sha256 instances, or just some of them.
Moving them from [1/6] to this step would help future readers by
reducing such irritation (I do not know if it is worth it until
I read through the series to the end).
> Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
> ---
> .../technical/hash-function-transition.txt | 122 +++++++++---------
> 1 file changed, 61 insertions(+), 61 deletions(-)
Thanks.
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v2 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently
2021-02-02 19:39 ` Junio C Hamano
@ 2021-02-02 23:19 ` Junio C Hamano
0 siblings, 0 replies; 25+ messages in thread
From: Junio C Hamano @ 2021-02-02 23:19 UTC (permalink / raw)
To: Thomas Ackermann via GitGitGadget
Cc: git, Ævar Arnfjörð Bjarmason, Thomas Ackermann
Junio C Hamano <gitster@pobox.com> writes:
> "Thomas Ackermann via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
>> From: Thomas Ackermann <th.acker@arcor.de>
>>
>> Use SHA-1 and SHA-256 instead of sha1 and sha256 when referring
>> to the hash type.
>
> Ahh. [1/6] was supposed to be only about formatting, and I found it
> a bit irritating that it had some of these changes mixed in, as it
> was not entirely clear to me that [1/6] covered all those lowercase
> sha1 and sha256 instances, or just some of them.
>
> Moving them from [1/6] to this step would help future readers by
> reducing such irritation (I do not know if it is worth it until
> I read through the series to the end).
It seems that only one hunk in 1/6 had premature conversion.
I've tried to locally move it around before queuing.
Thanks.
1: eea107fb0e ! 1: 5df3cc249d 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: 58934c8b43 ! 2: 8488d4f5f1 doc hash-function-transition: use SHA-1 and SHA-256 consistently
@@ 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: 4a710c8715 = 3: 454a9437cf doc hash-function-transition: use upper case consistently
4: 7e690524ac = 4: b2c881b66b doc hash-function-transition: fix incomplete sentence
5: 80089fe818 = 5: 4ee4775ca3 doc hash-function-transition: move rationale upwards
6: b221eae801 = 6: c27c52ca0c doc: use https links
^ permalink raw reply [flat|nested] 25+ messages in thread
* [PATCH v2 3/6] doc hash-function-transition: use upper case consistently
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
2021-02-02 16:19 ` [PATCH v2 1/6] doc hash-function-transition: fix asciidoc output Thomas Ackermann via GitGitGadget
2021-02-02 16:19 ` [PATCH v2 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently Thomas Ackermann via GitGitGadget
@ 2021-02-02 16:19 ` Thomas Ackermann via GitGitGadget
2021-02-02 16:19 ` [PATCH v2 4/6] doc hash-function-transition: fix incomplete sentence Thomas Ackermann via GitGitGadget
` (4 subsequent siblings)
7 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-02 16:19 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Use upper case consistently in Document History.
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
.../technical/hash-function-transition.txt | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 51acf2c10b7..2eba25cf87c 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -794,8 +794,8 @@ sbeller@google.com
2017-03-03 jrnieder@gmail.com
Incorporated suggestions from jonathantanmy and sbeller:
-* describe purpose of signed objects with each hash type
-* redefine signed object verification using object content under the
+* Describe purpose of signed objects with each hash type
+* Redefine signed object verification using object content under the
first hash function
2017-03-06 jrnieder@gmail.com
@@ -814,13 +814,13 @@ Incorporated suggestions from jonathantanmy and sbeller:
2017-09-27 jrnieder@gmail.com, sbeller@google.com
-* use placeholder NewHash instead of SHA3-256
-* describe criteria for picking a hash function.
-* include a transition plan (thanks especially to Brandon Williams
+* Use placeholder NewHash instead of SHA3-256
+* Describe criteria for picking a hash function.
+* Include a transition plan (thanks especially to Brandon Williams
for fleshing these ideas out)
-* define the translation table (thanks, Shawn Pearce[5], Jonathan
+* Define the translation table (thanks, Shawn Pearce[5], Jonathan
Tan, and Masaya Suzuki)
-* avoid loose object overhead by packing more aggressively in
+* Avoid loose object overhead by packing more aggressively in
"git gc --auto"
Later history:
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v2 4/6] doc hash-function-transition: fix incomplete sentence
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
` (2 preceding siblings ...)
2021-02-02 16:19 ` [PATCH v2 3/6] doc hash-function-transition: use upper case consistently Thomas Ackermann via GitGitGadget
@ 2021-02-02 16:19 ` Thomas Ackermann via GitGitGadget
2021-02-02 16:19 ` [PATCH v2 5/6] doc hash-function-transition: move rationale upwards Thomas Ackermann via GitGitGadget
` (3 subsequent siblings)
7 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-02 16:19 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
Documentation/technical/hash-function-transition.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 2eba25cf87c..86b09ea0f21 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -315,7 +315,7 @@ for all objects in the object store.
Reading an object's SHA-1 content
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The SHA-1 content of an object can be read by converting all SHA-256 names
-its SHA-256 content references to SHA-1 names using the translation table.
+of its SHA-256 content references to SHA-1 names using the translation table.
Fetch
~~~~~
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v2 5/6] doc hash-function-transition: move rationale upwards
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
` (3 preceding siblings ...)
2021-02-02 16:19 ` [PATCH v2 4/6] doc hash-function-transition: fix incomplete sentence Thomas Ackermann via GitGitGadget
@ 2021-02-02 16:19 ` Thomas Ackermann via GitGitGadget
2021-02-02 19:54 ` Junio C Hamano
2021-02-02 16:19 ` [PATCH v2 6/6] doc: use https links Thomas Ackermann via GitGitGadget
` (2 subsequent siblings)
7 siblings, 1 reply; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-02 16:19 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
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.
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
.../technical/hash-function-transition.txt | 62 +++++--------------
1 file changed, 16 insertions(+), 46 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 86b09ea0f21..475f2f501a6 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -27,22 +27,12 @@ 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.
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, 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
-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.
@@ -57,6 +47,19 @@ 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
+--------------
+There were several contenders for a successor hash to SHA-1, including
+SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.
+
+In late 2018 the project picked SHA-256 as its successor hash.
+
+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
-----
1. The transition to SHA-256 can be done one local repository at a time.
@@ -601,39 +604,6 @@ example:
git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}
-Choice of Hash
---------------
-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, in 2017.
-
-Git v2.13.0 and later subsequently moved to a hardened SHA-1
-implementation by default that mitigates the SHAttered attack, but
-SHA-1 is still believed to be weak.
-
-The hash to replace this 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).
-
-We choose SHA-256.
-
Transition plan
---------------
Some initial steps can be implemented independently of one another:
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* Re: [PATCH v2 5/6] doc hash-function-transition: move rationale upwards
2021-02-02 16:19 ` [PATCH v2 5/6] doc hash-function-transition: move rationale upwards Thomas Ackermann via GitGitGadget
@ 2021-02-02 19:54 ` Junio C Hamano
2021-02-02 23:23 ` brian m. carlson
0 siblings, 1 reply; 25+ messages in thread
From: Junio C Hamano @ 2021-02-02 19:54 UTC (permalink / raw)
To: Thomas Ackermann via GitGitGadget
Cc: brian m. carlson, Jonathan Nieder, git,
Ævar Arnfjörð Bjarmason, Thomas Ackermann
"Thomas Ackermann via GitGitGadget" <gitgitgadget@gmail.com> writes:
> From: Thomas Ackermann <th.acker@arcor.de>
>
> 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.
>
> Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
> ---
> .../technical/hash-function-transition.txt | 62 +++++--------------
> 1 file changed, 16 insertions(+), 46 deletions(-)
Hmph, this might turn out to be a bit more controversial than its
worth. I'd summon/cc a few people from the original discussion.
> -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.
>
> 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, but SHA-1 is still believed to be weak.
Even if we've hardended against one particular form of attack, we
still have incentive to switch away from SHA-1. It is unclear why
we just do not add ", but ..." to the original and instead remove
the half-sentence about sha1dc.
> @@ -57,6 +47,19 @@ 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
> +--------------
> +There were several contenders for a successor hash to SHA-1, including
> +SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.
> +
> +In late 2018 the project picked SHA-256 as its successor hash.
> +
> +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.
I personally think this is referring too much to external document
for typical readers, and lost too much relative to the original. I
do not mind losing the history of how we reached the conclusion that
SHA-1 is no longer viable at all, but I am not sure if we want to
lose the list of criteria we used when choosing (i.e. stronger than
SHA-1, 256-bit, quality implementations, etc.) from this section.
> -The hash to replace this 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).
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v2 5/6] doc hash-function-transition: move rationale upwards
2021-02-02 19:54 ` Junio C Hamano
@ 2021-02-02 23:23 ` brian m. carlson
0 siblings, 0 replies; 25+ messages in thread
From: brian m. carlson @ 2021-02-02 23:23 UTC (permalink / raw)
To: Junio C Hamano
Cc: Thomas Ackermann via GitGitGadget, Jonathan Nieder, git,
Ævar Arnfjörð Bjarmason, Thomas Ackermann
[-- Attachment #1: Type: text/plain, Size: 4212 bytes --]
On 2021-02-02 at 19:54:20, Junio C Hamano wrote:
> "Thomas Ackermann via GitGitGadget" <gitgitgadget@gmail.com> writes:
> > -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.
> >
> > 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, but SHA-1 is still believed to be weak.
>
> Even if we've hardended against one particular form of attack, we
> still have incentive to switch away from SHA-1. It is unclear why
> we just do not add ", but ..." to the original and instead remove
> the half-sentence about sha1dc.
I think we should keep the original statement about the attack since
it's relevant to why we want to change. I also think we should say,
"but SHA-1 is still weak". Saying "is still believed to be" implies
doubt or uncertainty, and the fact that multiple collisions have been
performed and can be trivially verified should remove any doubt.
Even if SHA-1 were still perfectly secure (which it is not), it can only
by design provide an 80-bit security level, which is inadequate by
today's standards.
> > @@ -57,6 +47,19 @@ 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
> > +--------------
> > +There were several contenders for a successor hash to SHA-1, including
> > +SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.
> > +
> > +In late 2018 the project picked SHA-256 as its successor hash.
> > +
> > +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.
>
> I personally think this is referring too much to external document
> for typical readers, and lost too much relative to the original. I
> do not mind losing the history of how we reached the conclusion that
> SHA-1 is no longer viable at all, but I am not sure if we want to
> lose the list of criteria we used when choosing (i.e. stronger than
> SHA-1, 256-bit, quality implementations, etc.) from this section.
I don't have a problem including this and in fact I think it might be
valuable, but I think we should keep the below data as well.
> > -The hash to replace this 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).
I'd prefer to keep the original criteria here, because I think it's
useful to document what they were for why we'd want to change. For
example, we occasionally get random users asking why we didn't pick a
hash with length extension resistance or why we didn't pick SHA-3.
The fact that we don't need length extension attack resistance and that
most non-Linux crypto libraries provide an extremely limited set of
crypto primitives are essential to our decision. I think if SHA-3-256
had been more widely available, it would have been the winning candidate.
--
brian m. carlson (he/him or they/them)
Houston, Texas, US
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 263 bytes --]
^ permalink raw reply [flat|nested] 25+ messages in thread
* [PATCH v2 6/6] doc: use https links
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
` (4 preceding siblings ...)
2021-02-02 16:19 ` [PATCH v2 5/6] doc hash-function-transition: move rationale upwards Thomas Ackermann via GitGitGadget
@ 2021-02-02 16:19 ` Thomas Ackermann via GitGitGadget
2021-02-02 19:57 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Junio C Hamano
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
7 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-02 16:19 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Use only https links for lore.kernel.org.
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
Documentation/technical/hash-function-transition.txt | 10 +++++-----
t/t0021-conversion.sh | 4 ++--
2 files changed, 7 insertions(+), 7 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 475f2f501a6..e7e6bd95ff9 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -759,7 +759,7 @@ 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:
@@ -801,8 +801,8 @@ Later history:
References:
- [1] http://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
- [2] http://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
- [3] http://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
- [4] http://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
+ [1] https://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
+ [2] https://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
+ [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/
diff --git a/t/t0021-conversion.sh b/t/t0021-conversion.sh
index e4c4de5c745..e828ee964c1 100755
--- a/t/t0021-conversion.sh
+++ b/t/t0021-conversion.sh
@@ -34,7 +34,7 @@ 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
@@ -49,7 +49,7 @@ 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
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* Re: [PATCH v2 0/6] doc: improvements for hash-function-transition
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
` (5 preceding siblings ...)
2021-02-02 16:19 ` [PATCH v2 6/6] doc: use https links Thomas Ackermann via GitGitGadget
@ 2021-02-02 19:57 ` Junio C Hamano
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
7 siblings, 0 replies; 25+ messages in thread
From: Junio C Hamano @ 2021-02-02 19:57 UTC (permalink / raw)
To: Thomas Ackermann via GitGitGadget
Cc: git, Ævar Arnfjörð Bjarmason, Thomas Ackermann
"Thomas Ackermann via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 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.
The series was more-or-less a pleasant read, except for a minor nit
about sha1 => SHA-1 sha256 => SHA-256 changes in 1/6 that should
have been in 2/6, and some changes in 5/6 that I found questionable.
Thanks.
^ permalink raw reply [flat|nested] 25+ messages in thread* [PATCH v3 0/6] doc: improvements for hash-function-transition
2021-02-02 16:19 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Thomas Ackermann via GitGitGadget
` (6 preceding siblings ...)
2021-02-02 19:57 ` [PATCH v2 0/6] doc: improvements for hash-function-transition Junio C Hamano
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 1/6] doc hash-function-transition: fix asciidoc output Thomas Ackermann via GitGitGadget
` (5 more replies)
7 siblings, 6 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann
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 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 | 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 <th.acker@arcor.de>
## 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
--
gitgitgadget
^ permalink raw reply [flat|nested] 25+ messages in thread* [PATCH v3 1/6] doc hash-function-transition: fix asciidoc output
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently Thomas Ackermann via GitGitGadget
` (4 subsequent siblings)
5 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
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>
---
.../technical/hash-function-transition.txt | 79 +++++++++++--------
1 file changed, 45 insertions(+), 34 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 6fd20ebbc254..b23d23151a57 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -94,7 +94,7 @@ Overview
--------
We introduce a new repository format extension. Repositories with this
extension enabled use SHA-256 instead of SHA-1 to name their objects.
-This affects both object names and object content --- both the names
+This affects both object names and object content -- both the names
of objects and all references to other objects within an object are
switched to the new hash function.
@@ -191,21 +191,21 @@ hash functions. They have the following format (all integers are in
network byte order):
- A header appears at the beginning and consists of the following:
- - The 4-byte pack index signature: '\377t0c'
- - 4-byte version number: 3
- - 4-byte length of the header section, including the signature and
+ * The 4-byte pack index signature: '\377t0c'
+ * 4-byte version number: 3
+ * 4-byte length of the header section, including the signature and
version number
- - 4-byte number of objects contained in the pack
- - 4-byte number of object formats in this pack index: 2
- - For each object format:
- - 4-byte format identifier (e.g., 'sha1' for SHA-1)
- - 4-byte length in bytes of shortened object names. This is the
+ * 4-byte number of objects contained in the pack
+ * 4-byte number of object formats in this pack index: 2
+ * For each object format:
+ ** 4-byte format identifier (e.g., 'sha1' for SHA-1)
+ ** 4-byte length in bytes of shortened object names. This is the
shortest possible length needed to make names in the shortened
object name table unambiguous.
- - 4-byte integer, recording where tables relating to this format
+ ** 4-byte integer, recording where tables relating to this format
are stored in this index file, as an offset from the beginning.
- - 4-byte offset to the trailer from the beginning of this file.
- - Zero or more additional key/value pairs (4-byte key, 4-byte
+ * 4-byte offset to the trailer from the beginning of this file.
+ * Zero or more additional key/value pairs (4-byte key, 4-byte
value). Only one key is supported: 'PSRC'. See the "Loose objects
and unreachable objects" section for supported values and how this
is used. All other keys are reserved. Readers must ignore
@@ -213,37 +213,36 @@ network byte order):
- Zero or more NUL bytes. This can optionally be used to improve the
alignment of the full object name table below.
- Tables for the first object format:
- - A sorted table of shortened object names. These are prefixes of
+ * A sorted table of shortened object names. These are prefixes of
the names of all objects in this pack file, packed together
without offset values to reduce the cache footprint of the binary
search for a specific object name.
- - A table of full object names in pack order. This allows resolving
+ * A table of full object names in pack order. This allows resolving
a reference to "the nth object in the pack file" (from a
reachability bitmap or from the next table of another object
format) to its object name.
- - A table of 4-byte values mapping object name order to pack order.
+ * A table of 4-byte values mapping object name order to pack order.
For an object in the table of sorted shortened object names, the
value at the corresponding index in this table is the index in the
previous table for that same object.
-
This can be used to look up the object in reachability bitmaps or
to look up its name in another object format.
- - A table of 4-byte CRC32 values of the packed object data, in the
+ * A table of 4-byte CRC32 values of the packed object data, in the
order that the objects appear in the pack file. This is to allow
compressed data to be copied directly from pack to pack during
repacking without undetected data corruption.
- - A table of 4-byte offset values. For an object in the table of
+ * A table of 4-byte offset values. For an object in the table of
sorted shortened object names, the value at the corresponding
index in this table indicates where that object can be found in
the pack file. These are usually 31-bit pack file offsets, but
large offsets are encoded as an index into the next table with the
most significant bit set.
- - A table of 8-byte offset entries (empty for pack files less than
+ * A table of 8-byte offset entries (empty for pack files less than
2 GiB). Pack files are organized with heavily used objects toward
the front, so most object references should not need to refer to
this table.
@@ -252,10 +251,10 @@ network byte order):
up to and not including the table of CRC32 values.
- Zero or more NUL bytes.
- The trailer consists of the following:
- - A copy of the 20-byte SHA-256 checksum at the end of the
+ * A copy of the 20-byte SHA-256 checksum at the end of the
corresponding packfile.
- - 20-byte SHA-256 checksum of all of the above.
+ * 20-byte SHA-256 checksum of all of the above.
Loose object index
~~~~~~~~~~~~~~~~~~
@@ -351,7 +350,7 @@ the following steps:
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
- pack. Record the new sha1<->sha256 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
@@ -391,6 +390,7 @@ existing "gpgsig" field. Its signed payload is the sha256-content of the
commit object with any "gpgsig" and "gpgsig-sha256" fields removed.
This means commits can be signed
+
1. using SHA-1 only, as in existing signed commit objects
2. using both SHA-1 and SHA-256, by using both gpgsig-sha256 and gpgsig
fields.
@@ -408,6 +408,7 @@ sha256-content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
SIGNATURE-----" delimited in-body signature removed.
This means tags can be signed
+
1. using SHA-1 only, as in existing signed tag objects
2. using both SHA-1 and SHA-256, by using gpgsig-sha256 and an in-body
signature.
@@ -598,7 +599,7 @@ The user can also explicitly specify which format to use for a
particular revision specifier and for output, overriding the mode. For
example:
-git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}
+ git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}
Choice of Hash
--------------
@@ -636,6 +637,7 @@ We choose SHA-256.
Transition plan
---------------
Some initial steps can be implemented independently of one another:
+
- adding a hash function API (vtable)
- teaching fsck to tolerate the gpgsig-sha256 field
- excluding gpgsig-* from the fields copied by "git commit --amend"
@@ -647,9 +649,9 @@ Some initial steps can be implemented independently of one another:
- introducing index v3
- adding support for the PSRC field and safer object pruning
-
The first user-visible change is the introduction of the objectFormat
extension (without compatObjectFormat). This requires:
+
- teaching fsck about this mode of operation
- using the hash function API (vtable) when computing object names
- signing objects and verifying signatures
@@ -657,6 +659,7 @@ extension (without compatObjectFormat). This requires:
repository
Next comes introduction of compatObjectFormat:
+
- implementing the loose-object-idx
- translating object names between object formats
- translating object content between object formats
@@ -669,6 +672,7 @@ Next comes introduction of compatObjectFormat:
"Object names on the command line" above)
The next step is supporting fetches and pushes to SHA-1 repositories:
+
- allow pushes to a repository using the compat format
- generate a topologically sorted list of the SHA-1 names of fetched
objects
@@ -734,6 +738,7 @@ Using hash functions in parallel
Objects newly created would be addressed by the new hash, but inside
such an object (e.g. commit) it is still possible to address objects
using the old hash function.
+
* You cannot trust its history (needed for bisectability) in the
future without further work
* Maintenance burden as the number of supported hash functions grows
@@ -749,6 +754,7 @@ sha1-content based signatures.
In other words, a single signature was used to attest to the object
content using both hash functions. This had some advantages:
+
* Using one signature instead of two speeds up the signing process.
* Having one signed payload with both hashes allows the signer to
attest to the sha1-name and sha256-name referring to the same object.
@@ -756,6 +762,7 @@ content using both hash functions. This had some advantages:
to be detected quickly using current versions of git.
However, it also came with disadvantages:
+
* Verifying a signed object requires access to the sha1-names of all
objects it references, even after the transition is complete and
translation table is no longer needed for anything else. To support
@@ -782,16 +789,17 @@ 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 http://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com
2017-03-03 jrnieder@gmail.com
Incorporated suggestions from jonathantanmy and sbeller:
+
* describe purpose of signed objects with each hash type
* redefine signed object verification using object content under the
first hash function
2017-03-06 jrnieder@gmail.com
+
* Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2]
* Make sha3-based signatures a separate field, avoiding the need for
"hash" and "nohash" fields (thanks to peff[3]).
@@ -805,6 +813,7 @@ Incorporated suggestions from jonathantanmy and sbeller:
especially Junio).
2017-09-27 jrnieder@gmail.com, sbeller@google.com
+
* use placeholder NewHash instead of SHA3-256
* describe criteria for picking a hash function.
* include a transition plan (thanks especially to Brandon Williams
@@ -816,12 +825,14 @@ Incorporated suggestions from jonathantanmy and sbeller:
Later history:
- See the history of this file in git.git for the history of subsequent
- edits. This document history is no longer being maintained as it
- would now be superfluous to the commit log
+* See the history of this file in git.git for the history of subsequent
+ edits. This document history is no longer being maintained as it
+ would now be superfluous to the commit log
+
+References:
-[1] http://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
-[2] http://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
-[3] http://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
-[4] http://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
-[5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/
+ [1] http://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
+ [2] http://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
+ [3] http://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
+ [4] http://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
+ [5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v3 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 1/6] doc hash-function-transition: fix asciidoc output Thomas Ackermann via GitGitGadget
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 3/6] doc hash-function-transition: use upper case consistently Thomas Ackermann via GitGitGadget
` (3 subsequent siblings)
5 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
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>
---
.../technical/hash-function-transition.txt | 126 +++++++++---------
1 file changed, 63 insertions(+), 63 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index b23d23151a57..8c01608cbfa0 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -107,7 +107,7 @@ mapping to allow naming objects using either their SHA-1 and SHA-256 names
interchangeably.
"git cat-file" and "git hash-object" gain options to display an object
-in its sha1 form and write an object given its sha1 form. This
+in its SHA-1 form and write an object given its SHA-1 form. This
requires all objects referenced by that object to be present in the
object database so that they can be named using the appropriate name
(using the bidirectional hash mapping).
@@ -115,7 +115,7 @@ object database so that they can be named using the appropriate name
Fetches from a SHA-1 based server convert the fetched objects into
SHA-256 form and record the mapping in the bidirectional mapping table
(see below for details). Pushes to a SHA-1 based server convert the
-objects being pushed into sha1 form so the server does not have to be
+objects being pushed into SHA-1 form so the server does not have to be
aware of the hash function the client is using.
Detailed Design
@@ -151,38 +151,38 @@ repository extensions.
Object names
~~~~~~~~~~~~
-Objects can be named by their 40 hexadecimal digit sha1-name or 64
-hexadecimal digit sha256-name, plus names derived from those (see
+Objects can be named by their 40 hexadecimal digit SHA-1 name or 64
+hexadecimal digit SHA-256 name, plus names derived from those (see
gitrevisions(7)).
-The sha1-name of an object is the SHA-1 of the concatenation of its
-type, length, a nul byte, and the object's sha1-content. This is the
+The SHA-1 name of an object is the SHA-1 of the concatenation of its
+type, length, a nul byte, and the object's SHA-1 content. This is the
traditional <sha1> used in Git to name objects.
-The sha256-name of an object is the SHA-256 of the concatenation of its
-type, length, a nul byte, and the object's sha256-content.
+The SHA-256 name of an object is the SHA-256 of the concatenation of its
+type, length, a nul byte, and the object's SHA-256 content.
Object format
~~~~~~~~~~~~~
The content as a byte sequence of a tag, commit, or tree object named
-by sha1 and sha256 differ because an object named by sha256-name refers to
-other objects by their sha256-names and an object named by sha1-name
-refers to other objects by their sha1-names.
+by SHA-1 and SHA-256 differ because an object named by SHA-256 name refers to
+other objects by their SHA-256 names and an object named by SHA-1 name
+refers to other objects by their SHA-1 names.
-The sha256-content of an object is the same as its sha1-content, except
-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
+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.
-The format allows round-trip conversion between sha256-content and
-sha1-content.
+The format allows round-trip conversion between SHA-256 content and
+SHA-1 content.
Object storage
~~~~~~~~~~~~~~
Loose objects use zlib compression and packed objects use the packed
format described in Documentation/technical/pack-format.txt, just like
-today. The content that is compressed and stored uses sha256-content
-instead of sha1-content.
+today. The content that is compressed and stored uses SHA-256 content
+instead of SHA-1 content.
Pack index
~~~~~~~~~~
@@ -287,18 +287,18 @@ To remove entries (e.g. in "git pack-refs" or "git-prune"):
Translation table
~~~~~~~~~~~~~~~~~
-The index files support a bidirectional mapping between sha1-names
-and sha256-names. The lookup proceeds similarly to ordinary object
-lookups. For example, to convert a sha1-name to a sha256-name:
+The index files support a bidirectional mapping between SHA-1 names
+and SHA-256 names. The lookup proceeds similarly to ordinary object
+lookups. For example, to convert a SHA-1 name to a SHA-256 name:
1. Look for the object in idx files. If a match is present in the
- idx's sorted list of truncated sha1-names, then:
- a. Read the corresponding entry in the sha1-name order to pack
+ idx's sorted list of truncated SHA-1 names, then:
+ a. Read the corresponding entry in the SHA-1 name order to pack
name order mapping.
- b. Read the corresponding entry in the full sha1-name table to
+ b. Read the corresponding entry in the full SHA-1 name table to
verify we found the right object. If it is, then
- c. Read the corresponding entry in the full sha256-name table.
- That is the object's sha256-name.
+ c. Read the corresponding entry in the full SHA-256 name table.
+ That is the object's SHA-256 name.
2. Check for a loose object. Read lines from loose-object-idx until
we find a match.
@@ -312,10 +312,10 @@ Since all operations that make new objects (e.g., "git commit") add
the new objects to the corresponding index, this mapping is possible
for all objects in the object store.
-Reading an object's sha1-content
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The sha1-content of an object can be read by converting all sha256-names
-its sha256-content references to sha1-names using the translation table.
+Reading an object's SHA-1 content
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The SHA-1 content of an object can be read by converting all SHA-256 names
+its SHA-256 content references to SHA-1 names using the translation table.
Fetch
~~~~~
@@ -338,7 +338,7 @@ the following steps:
1. index-pack: inflate each object in the packfile and compute its
SHA-1. Objects can contain deltas in OBJ_REF_DELTA format against
objects the client has locally. These objects can be looked up
- using the translation table and their sha1-content read as
+ using the translation table and their SHA-1 content read as
described above to resolve the deltas.
2. topological sort: starting at the "want"s from the negotiation
phase, walk through objects in the pack and emit a list of them,
@@ -347,12 +347,12 @@ the following steps:
(This list only contains objects reachable from the "wants". If the
pack from the server contained additional extraneous objects, then
they will be discarded.)
-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
- 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
file
5. clean up: remove the SHA-1 based pack file, index, and
topologically sorted list obtained from the server in steps 1
@@ -377,16 +377,16 @@ experimenting to get this to perform well.
Push
~~~~
Push is simpler than fetch because the objects referenced by the
-pushed objects are already in the translation table. The sha1-content
+pushed objects are already in the translation table. The SHA-1 content
of each object being pushed can be read as described in the "Reading
-an object's sha1-content" section to generate the pack written by git
+an object's SHA-1 content" section to generate the pack written by git
send-pack.
Signed Commits
~~~~~~~~~~~~~~
We add a new field "gpgsig-sha256" to the commit object format to allow
signing commits without relying on SHA-1. It is similar to the
-existing "gpgsig" field. Its signed payload is the sha256-content of the
+existing "gpgsig" field. Its signed payload is the SHA-256 content of the
commit object with any "gpgsig" and "gpgsig-sha256" fields removed.
This means commits can be signed
@@ -404,7 +404,7 @@ Signed Tags
~~~~~~~~~~~
We add a new field "gpgsig-sha256" to the tag object format to allow
signing tags without relying on SHA-1. Its signed payload is the
-sha256-content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
+SHA-256 content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
SIGNATURE-----" delimited in-body signature removed.
This means tags can be signed
@@ -416,11 +416,11 @@ This means tags can be signed
Mergetag embedding
~~~~~~~~~~~~~~~~~~
-The mergetag field in the sha1-content of a commit contains the
-sha1-content of a tag that was merged by that commit.
+The mergetag field in the SHA-1 content of a commit contains the
+SHA-1 content of a tag that was merged by that commit.
-The mergetag field in the sha256-content of the same commit contains the
-sha256-content of the same tag.
+The mergetag field in the SHA-256 content of the same commit contains the
+SHA-256 content of the same tag.
Submodules
~~~~~~~~~~
@@ -495,7 +495,7 @@ Caveats
-------
Invalid objects
~~~~~~~~~~~~~~~
-The conversion from sha1-content to sha256-content retains any
+The conversion from SHA-1 content to SHA-256 content retains any
brokenness in the original object (e.g., tree entry modes encoded with
leading 0, tree objects whose paths are not sorted correctly, and
commit objects without an author or committer). This is a deliberate
@@ -514,15 +514,15 @@ allow lifting this restriction.
Alternates
~~~~~~~~~~
-For the same reason, a sha256 repository cannot borrow objects from a
-sha1 repository using objects/info/alternates or
+For the same reason, a SHA-256 repository cannot borrow objects from a
+SHA-1 repository using objects/info/alternates or
$GIT_ALTERNATE_OBJECT_REPOSITORIES.
git notes
~~~~~~~~~
-The "git notes" tool annotates objects using their sha1-name as key.
+The "git notes" tool annotates objects using their SHA-1 name as key.
This design does not describe a way to migrate notes trees to use
-sha256-names. That migration is expected to happen separately (for
+SHA-256 names. That migration is expected to happen separately (for
example using a file at the root of the notes tree to describe which
hash it uses).
@@ -556,7 +556,7 @@ unclear:
Git 2.12
-Does this mean Git v2.12.0 is the commit with sha1-name
+Does this mean Git v2.12.0 is the commit with SHA-1 name
e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with
new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7?
@@ -676,7 +676,7 @@ The next step is supporting fetches and pushes to SHA-1 repositories:
- allow pushes to a repository using the compat format
- generate a topologically sorted list of the SHA-1 names of fetched
objects
-- convert the fetched packfile to sha256 format and generate an idx
+- convert the fetched packfile to SHA-256 format and generate an idx
file
- re-sort to match the order of objects in the fetched packfile
@@ -748,38 +748,38 @@ using the old hash function.
Signed objects with multiple hashes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Instead of introducing the gpgsig-sha256 field in commit and tag objects
-for sha256-content based signatures, an earlier version of this design
-added "hash sha256 <sha256-name>" fields to strengthen the existing
-sha1-content based signatures.
+for SHA-256 content based signatures, an earlier version of this design
+added "hash sha256 <SHA-256 name>" fields to strengthen the existing
+SHA-1 content based signatures.
In other words, a single signature was used to attest to the object
content using both hash functions. This had some advantages:
* Using one signature instead of two speeds up the signing process.
* Having one signed payload with both hashes allows the signer to
- attest to the sha1-name and sha256-name referring to the same object.
+ attest to the SHA-1 name and SHA-256 name referring to the same object.
* All users consume the same signature. Broken signatures are likely
to be detected quickly using current versions of git.
However, it also came with disadvantages:
-* Verifying a signed object requires access to the sha1-names of all
+* Verifying a signed object requires access to the SHA-1 names of all
objects it references, even after the transition is complete and
translation table is no longer needed for anything else. To support
- this, the design added fields such as "hash sha1 tree <sha1-name>"
- and "hash sha1 parent <sha1-name>" to the sha256-content of a signed
+ this, the design added fields such as "hash sha1 tree <SHA-1 name>"
+ and "hash sha1 parent <SHA-1 name>" to the SHA-256 content of a signed
commit, complicating the conversion process.
-* Allowing signed objects without a sha1 (for after the transition is
+* Allowing signed objects without a SHA-1 (for after the transition is
complete) complicated the design further, requiring a "nohash sha1"
- field to suppress including "hash sha1" fields in the sha256-content
+ field to suppress including "hash sha1" fields in the SHA-256 content
and signed payload.
Lazily populated translation table
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some of the work of building the translation table could be deferred to
push time, but that would significantly complicate and slow down pushes.
-Calculating the sha1-name at object creation time at the same time it is
-being streamed to disk and having its sha256-name calculated should be
+Calculating the SHA-1 name at object creation time at the same time it is
+being streamed to disk and having its SHA-256 name calculated should be
an acceptable cost.
Document History
@@ -801,7 +801,7 @@ Incorporated suggestions from jonathantanmy and sbeller:
2017-03-06 jrnieder@gmail.com
* Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2]
-* Make sha3-based signatures a separate field, avoiding the need for
+* Make SHA3-based signatures a separate field, avoiding the need for
"hash" and "nohash" fields (thanks to peff[3]).
* Add a sorting phase to fetch (thanks to Junio for noticing the need
for this).
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v3 3/6] doc hash-function-transition: use upper case consistently
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 1/6] doc hash-function-transition: fix asciidoc output Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 2/6] doc hash-function-transition: use SHA-1 and SHA-256 consistently Thomas Ackermann via GitGitGadget
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 4/6] doc hash-function-transition: fix incomplete sentence Thomas Ackermann via GitGitGadget
` (2 subsequent siblings)
5 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Use upper case consistently in Document History.
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
.../technical/hash-function-transition.txt | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 8c01608cbfa0..9e13919a0e5b 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -794,8 +794,8 @@ sbeller@google.com
2017-03-03 jrnieder@gmail.com
Incorporated suggestions from jonathantanmy and sbeller:
-* describe purpose of signed objects with each hash type
-* redefine signed object verification using object content under the
+* Describe purpose of signed objects with each hash type
+* Redefine signed object verification using object content under the
first hash function
2017-03-06 jrnieder@gmail.com
@@ -814,13 +814,13 @@ Incorporated suggestions from jonathantanmy and sbeller:
2017-09-27 jrnieder@gmail.com, sbeller@google.com
-* use placeholder NewHash instead of SHA3-256
-* describe criteria for picking a hash function.
-* include a transition plan (thanks especially to Brandon Williams
+* Use placeholder NewHash instead of SHA3-256
+* Describe criteria for picking a hash function.
+* Include a transition plan (thanks especially to Brandon Williams
for fleshing these ideas out)
-* define the translation table (thanks, Shawn Pearce[5], Jonathan
+* Define the translation table (thanks, Shawn Pearce[5], Jonathan
Tan, and Masaya Suzuki)
-* avoid loose object overhead by packing more aggressively in
+* Avoid loose object overhead by packing more aggressively in
"git gc --auto"
Later history:
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v3 4/6] doc hash-function-transition: fix incomplete sentence
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
` (2 preceding siblings ...)
2021-02-05 18:22 ` [PATCH v3 3/6] doc hash-function-transition: use upper case consistently Thomas Ackermann via GitGitGadget
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 5/6] doc hash-function-transition: move rationale upwards Thomas Ackermann via GitGitGadget
2021-02-05 18:22 ` [PATCH v3 6/6] doc: use https links Thomas Ackermann via GitGitGadget
5 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
Documentation/technical/hash-function-transition.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 9e13919a0e5b..5ff9ee027cff 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -315,7 +315,7 @@ for all objects in the object store.
Reading an object's SHA-1 content
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The SHA-1 content of an object can be read by converting all SHA-256 names
-its SHA-256 content references to SHA-1 names using the translation table.
+of its SHA-256 content references to SHA-1 names using the translation table.
Fetch
~~~~~
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* [PATCH v3 5/6] doc hash-function-transition: move rationale upwards
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
` (3 preceding siblings ...)
2021-02-05 18:22 ` [PATCH v3 4/6] doc hash-function-transition: fix incomplete sentence Thomas Ackermann via GitGitGadget
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
2021-02-05 20:48 ` Ævar Arnfjörð Bjarmason
2021-02-05 18:22 ` [PATCH v3 6/6] doc: use https links Thomas Ackermann via GitGitGadget
5 siblings, 1 reply; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Move rationale for new hash function to beginning of document
so that it appears before the concrete move to SHA-256 is described.
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 <th.acker@arcor.de>
---
.../technical/hash-function-transition.txt | 76 +++++++++----------
1 file changed, 34 insertions(+), 42 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 5ff9ee027cff..0c4cb98cd4e9 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -33,16 +33,9 @@ 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
-attack.
+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
-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.
@@ -57,6 +50,38 @@ 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).
+
+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.
+
+In late 2018 the project picked SHA-256 as its successor hash.
+
+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
-----
1. The transition to SHA-256 can be done one local repository at a time.
@@ -601,39 +626,6 @@ example:
git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}
-Choice of Hash
---------------
-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, in 2017.
-
-Git v2.13.0 and later subsequently moved to a hardened SHA-1
-implementation by default that mitigates the SHAttered attack, but
-SHA-1 is still believed to be weak.
-
-The hash to replace this 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).
-
-We choose SHA-256.
-
Transition plan
---------------
Some initial steps can be implemented independently of one another:
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread* Re: [PATCH v3 5/6] doc hash-function-transition: move rationale upwards
2021-02-05 18:22 ` [PATCH v3 5/6] doc hash-function-transition: move rationale upwards Thomas Ackermann via GitGitGadget
@ 2021-02-05 20:48 ` Ævar Arnfjörð Bjarmason
2021-02-05 21:49 ` Junio C Hamano
0 siblings, 1 reply; 25+ messages in thread
From: Ævar Arnfjörð Bjarmason @ 2021-02-05 20:48 UTC (permalink / raw)
To: Thomas Ackermann via GitGitGadget
Cc: git, Junio C Hamano, brian m. carlson, Thomas Ackermann
On Fri, Feb 05 2021, Thomas Ackermann via GitGitGadget wrote:
> diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
> index 5ff9ee027cff..0c4cb98cd4e9 100644
> --- a/Documentation/technical/hash-function-transition.txt
> +++ b/Documentation/technical/hash-function-transition.txt
> @@ -33,16 +33,9 @@ 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
> -attack.
> +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
> -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.
> @@ -57,6 +50,38 @@ 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.
I missed version 2 of this. I don't think it's an improvement to
completely remove the description of us using sha1collisiondetection by
default, i.e. effectively revert 5988eb631a3 (doc
hash-function-transition: clarify what SHAttered means, 2018-03-26)
I can see how my comment on v1 could have been read like that. FWIW I
didn't mean remove the whole thing, but that I don't think it adds much
value to our description of how we use SHA-1 to go into the level of
detail of mentioning several researchers by name, there's Wikipedia for
that.
I think what we should instead do is have some brief summary of the
vulnerabilities and how they're impacting git.
Maybe I'm barking up the wrong tree here, and what I'm describing should
be in a "man 5 gitsecurity" or something.
But anyway, I think it adds a lot of value to somewhere have not just
what amounts to "sha-1 sucks, see research papers", but to have some
brief human-readable summary of what the practical impact is on users.
In 2018 it was true that sha1collisiondetection was mitigating the known
attack in practice, and that's also true about this new attack[1] (maybe
there's others I missed ...).
Then there's the fact that we don't *just* rely on SHA-1, but e.g. the
"don't re-write objects we have already". So as a practical attack on
someone using git ...
Oh, and the attacks currently all seem to require file formats like JPEG
or PDF for anything practical, i.e. being able to spew in lots of
arbitrary data into some data segment, as opposed to e.g. creating a
program that compiles.
None of this is meant as some overall defense of SHA-1, just that most
of our users aren't security researchers, and will be helped by a
summary of how this system they're using using SHA-1, and having read
that it's "broken" or "believed to be weak" translates to a threat to
them in practice.
1. https://eprint.iacr.org/2020/014.pdf
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v3 5/6] doc hash-function-transition: move rationale upwards
2021-02-05 20:48 ` Ævar Arnfjörð Bjarmason
@ 2021-02-05 21:49 ` Junio C Hamano
0 siblings, 0 replies; 25+ messages in thread
From: Junio C Hamano @ 2021-02-05 21:49 UTC (permalink / raw)
To: Ævar Arnfjörð Bjarmason
Cc: Thomas Ackermann via GitGitGadget, git, brian m. carlson,
Thomas Ackermann
Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
> I missed version 2 of this. I don't think it's an improvement to
> completely remove the description of us using sha1collisiondetection by
> default, i.e. effectively revert 5988eb631a3 (doc
> hash-function-transition: clarify what SHAttered means, 2018-03-26)
> ...
> I can see how my comment on v1 could have been read like that. FWIW I
> didn't mean remove the whole thing, but that I don't think it adds much
> value to our description of how we use SHA-1 to go into the level of
> detail of mentioning several researchers by name, there's Wikipedia for
> that.
True.
> I think what we should instead do is have some brief summary of the
> vulnerabilities and how they're impacting git.
I am not sure.
> Maybe I'm barking up the wrong tree here, and what I'm describing should
> be in a "man 5 gitsecurity" or something.
I agree with your that it belongs to some other document, but not
here, where the primary thing is to outline how the migration will
go, and the part we are seeing is merely giving a background story.
At this point in time, readers would not have to learn the details
from this document. People already know that we are not happy with
SHA-1 and is on our way to migrate to SHA-256.
> But anyway, I think it adds a lot of value to somewhere have not just
> what amounts to "sha-1 sucks, see research papers", but to have some
> brief human-readable summary of what the practical impact is on users.
Yeah. I think Thomas has in [v3 5/6] gives our readers about the
right level of details. If I were to change anything, I'd do "but
SHA-1 is {+considered+} still weak."
> In 2018 it was true that sha1collisiondetection was mitigating the known
> attack in practice, and that's also true about this new attack[1] (maybe
> there's others I missed ...).
>
> Then there's the fact that we don't *just* rely on SHA-1, but e.g. the
> "don't re-write objects we have already". So as a practical attack on
> someone using git ...
>
> Oh, and the attacks currently all seem to require file formats like JPEG
> or PDF for anything practical, i.e. being able to spew in lots of
> arbitrary data into some data segment, as opposed to e.g. creating a
> program that compiles.
>
> None of this is meant as some overall defense of SHA-1, just that most
> of our users aren't security researchers, and will be helped by a
> summary of how this system they're using using SHA-1, and having read
> that it's "broken" or "believed to be weak" translates to a threat to
> them in practice.
All of the above are good thing for somebody to write about, but I
am not sure it fits well in the context of this document. This is
primarily about how the migration should go, and the target audience
is those of us who are already committed to the plan. The backstory
on how the plan came about makes a nice introductory reading but it
would not be productive to spend too much bits for the purpose of the
document and its target audience, I would think.
Thanks.
^ permalink raw reply [flat|nested] 25+ messages in thread
* [PATCH v3 6/6] doc: use https links
2021-02-05 18:22 ` [PATCH v3 " Thomas Ackermann via GitGitGadget
` (4 preceding siblings ...)
2021-02-05 18:22 ` [PATCH v3 5/6] doc hash-function-transition: move rationale upwards Thomas Ackermann via GitGitGadget
@ 2021-02-05 18:22 ` Thomas Ackermann via GitGitGadget
5 siblings, 0 replies; 25+ messages in thread
From: Thomas Ackermann via GitGitGadget @ 2021-02-05 18:22 UTC (permalink / raw)
To: git
Cc: Junio C Hamano, Ævar Arnfjörð Bjarmason,
brian m. carlson, Thomas Ackermann, Thomas Ackermann
From: Thomas Ackermann <th.acker@arcor.de>
Use only https links for lore.kernel.org.
Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
---
Documentation/technical/hash-function-transition.txt | 10 +++++-----
t/t0021-conversion.sh | 4 ++--
2 files changed, 7 insertions(+), 7 deletions(-)
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
index 0c4cb98cd4e9..7c1630bf8324 100644
--- a/Documentation/technical/hash-function-transition.txt
+++ b/Documentation/technical/hash-function-transition.txt
@@ -781,7 +781,7 @@ 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:
@@ -823,8 +823,8 @@ Later history:
References:
- [1] http://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
- [2] http://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
- [3] http://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
- [4] http://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
+ [1] https://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
+ [2] https://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
+ [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/
diff --git a/t/t0021-conversion.sh b/t/t0021-conversion.sh
index e4c4de5c7456..e828ee964c1b 100755
--- a/t/t0021-conversion.sh
+++ b/t/t0021-conversion.sh
@@ -34,7 +34,7 @@ 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
@@ -49,7 +49,7 @@ 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
--
gitgitgadget
^ permalink raw reply related [flat|nested] 25+ messages in thread