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From: Jonathan Nieder <jrnieder@gmail.com>
To: Shawn Pearce <spearce@spearce.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Git Mailing List <git@vger.kernel.org>, Stefan Beller <sbeller@google.com>, bmwill@google.com, Jonathan Tan <jonathantanmy@google.com>, Jeff King <peff@peff.net>, David Lang <david@lang.hm>, "brian m. carlson" <sandals@crustytoothpaste.net>, Masaya Suzuki <masayasuzuki@google.com>, demerphq@gmail.com, The Keccak Team <keccak@noekeon.org>, Johannes Schindelin <Johannes.Schindelin@gmx.de>
Subject: [PATCH v4] technical doc: add a design doc for hash function transition
Date: Wed, 27 Sep 2017 21:43:21 -0700
Message-ID: <20170928044320.GA84719@aiede.mtv.corp.google.com> (raw)
In-Reply-To: <CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com>

This document describes what a transition to a new hash function for
Git would look like.  Add it to Documentation/technical/ as the plan
of record so that future changes can be recorded as patches.

Also-by: Brandon Williams <bmwill@google.com>
Also-by: Jonathan Tan <jonathantanmy@google.com>
Also-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
On Thu, Mar 09, 2017 at 11:14 AM, Shawn Pearce wrote:
> On Mon, Mar 6, 2017 at 4:17 PM, Jonathan Nieder <jrnieder@gmail.com> wrote:

>> Thanks for the kind words on what had quite a few flaws still.  Here's
>> a new draft.  I think the next version will be a patch against
>> Documentation/technical/.
>
> FWIW, I like this approach.

Okay, here goes.

Instead of sharding the loose object translation tables by first byte,
we went for a single table.  It simplifies the design and we need to
keep the number of loose objects under control anyway.

We also included a description of the transition plan and tried to
include a summary of what has been agreed upon so far about the choice
of hash function.

Thanks to Junio for reviving the discussion and in particular to Dscho
for pushing this forward and making the missing pieces clearer.

Thoughts of all kinds welcome, as always.

 Documentation/Makefile                             |   1 +
 .../technical/hash-function-transition.txt         | 797 +++++++++++++++++++++
 2 files changed, 798 insertions(+)
 create mode 100644 Documentation/technical/hash-function-transition.txt

diff --git a/Documentation/Makefile b/Documentation/Makefile
index 2415e0d657..471bb29725 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -67,6 +67,7 @@ SP_ARTICLES += howto/maintain-git
 API_DOCS = $(patsubst %.txt,%,$(filter-out technical/api-index-skel.txt technical/api-index.txt, $(wildcard technical/api-*.txt)))
 SP_ARTICLES += $(API_DOCS)
 
+TECH_DOCS += technical/hash-function-transition
 TECH_DOCS += technical/http-protocol
 TECH_DOCS += technical/index-format
 TECH_DOCS += technical/pack-format
diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt
new file mode 100644
index 0000000000..417ba491d0
--- /dev/null
+++ b/Documentation/technical/hash-function-transition.txt
@@ -0,0 +1,797 @@
+Git hash function transition
+============================
+
+Objective
+---------
+Migrate Git from SHA-1 to a stronger hash function.
+
+Background
+----------
+At its core, the Git version control system is a content addressable
+filesystem. It uses the SHA-1 hash function to name content. For
+example, files, directories, and revisions are referred to by hash
+values unlike in other traditional version control systems where files
+or versions are referred to via sequential numbers. The use of a hash
+function to address its content delivers a few advantages:
+
+* Integrity checking is easy. Bit flips, for example, are easily
+  detected, as the hash of corrupted content does not match its name.
+* Lookup of objects is fast.
+
+Using a cryptographically secure hash function brings additional
+advantages:
+
+* Object names can be signed and third parties can trust the hash to
+  address the signed object and all objects it references.
+* Communication using Git protocol and out of band communication
+  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. https://shattered.io demonstrated a practical SHA-1 hash
+collision. As a result, SHA-1 cannot be considered cryptographically
+secure any more. This impacts the communication of hash values because
+we cannot trust that a given hash value represents the known good
+version of content that the speaker intended.
+
+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.
+
+Goals
+-----
+Where NewHash is a strong 256-bit hash function to replace SHA-1 (see
+"Selection of a New Hash", below):
+
+1. The transition to NewHash can be done one local repository at a time.
+   a. Requiring no action by any other party.
+   b. A NewHash repository can communicate with SHA-1 Git servers
+      (push/fetch).
+   c. Users can use SHA-1 and NewHash identifiers for objects
+      interchangeably (see "Object names on the command line", below).
+   d. New signed objects make use of a stronger hash function than
+      SHA-1 for their security guarantees.
+2. Allow a complete transition away from SHA-1.
+   a. Local metadata for SHA-1 compatibility can be removed from a
+      repository if compatibility with SHA-1 is no longer needed.
+3. Maintainability throughout the process.
+   a. The object format is kept simple and consistent.
+   b. Creation of a generalized repository conversion tool.
+
+Non-Goals
+---------
+1. Add NewHash support to Git protocol. This is valuable and the
+   logical next step but it is out of scope for this initial design.
+2. Transparently improving the security of existing SHA-1 signed
+   objects.
+3. Intermixing objects using multiple hash functions in a single
+   repository.
+4. Taking the opportunity to fix other bugs in Git's formats and
+   protocols.
+5. Shallow clones and fetches into a NewHash repository. (This will
+   change when we add NewHash support to Git protocol.)
+6. Skip fetching some submodules of a project into a NewHash
+   repository. (This also depends on NewHash support in Git
+   protocol.)
+
+Overview
+--------
+We introduce a new repository format extension. Repositories with this
+extension enabled use NewHash instead of SHA-1 to name their objects.
+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.
+
+NewHash repositories cannot be read by older versions of Git.
+
+Alongside the packfile, a NewHash repository stores a bidirectional
+mapping between NewHash and SHA-1 object names. The mapping is generated
+locally and can be verified using "git fsck". Object lookups use this
+mapping to allow naming objects using either their SHA-1 and NewHash 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
+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).
+
+Fetches from a SHA-1 based server convert the fetched objects into
+NewHash 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
+aware of the hash function the client is using.
+
+Detailed Design
+---------------
+Repository format extension
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A NewHash repository uses repository format version `1` (see
+Documentation/technical/repository-version.txt) with extensions
+`objectFormat` and `compatObjectFormat`:
+
+	[core]
+		repositoryFormatVersion = 1
+	[extensions]
+		objectFormat = newhash
+		compatObjectFormat = sha1
+
+Specifying a repository format extension ensures that versions of Git
+not aware of NewHash do not try to operate on these repositories,
+instead producing an error message:
+
+	$ git status
+	fatal: unknown repository extensions found:
+		objectformat
+		compatobjectformat
+
+See the "Transition plan" section below for more details on these
+repository extensions.
+
+Object names
+~~~~~~~~~~~~
+Objects can be named by their 40 hexadecimal digit sha1-name or 64
+hexadecimal digit newhash-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
+traditional <sha1> used in Git to name objects.
+
+The newhash-name of an object is the NewHash of the concatenation of its
+type, length, a nul byte, and the object's newhash-content.
+
+Object format
+~~~~~~~~~~~~~
+The content as a byte sequence of a tag, commit, or tree object named
+by sha1 and newhash differ because an object named by newhash-name refers to
+other objects by their newhash-names and an object named by sha1-name
+refers to other objects by their sha1-names.
+
+The newhash-content of an object is the same as its sha1-content, except
+that objects referenced by the object are named using their newhash-names
+instead of sha1-names. Because a blob object does not refer to any
+other object, its sha1-content and newhash-content are the same.
+
+The format allows round-trip conversion between newhash-content and
+sha1-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 newhash-content
+instead of sha1-content.
+
+Pack index
+~~~~~~~~~~
+Pack index (.idx) files use a new v3 format that supports multiple
+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
+    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
+      shortest possible length needed to make names in the shortened
+      object name table unambiguous.
+    - 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
+    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
+    unrecognized keys.
+- 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
+    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 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.
+    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
+    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
+    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
+    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.
+- Zero or more NUL bytes.
+- Tables for the second object format, with the same layout as above,
+  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 NewHash checksum at the end of the
+    corresponding packfile.
+
+  - 20-byte NewHash checksum of all of the above.
+
+Loose object index
+~~~~~~~~~~~~~~~~~~
+A new file $GIT_OBJECT_DIR/loose-object-idx contains information about
+all loose objects. Its format is
+
+  # loose-object-idx
+  (newhash-name SP sha1-name LF)*
+
+where the object names are in hexadecimal format. The file is not
+sorted.
+
+The loose object index is protected against concurrent writes by a
+lock file $GIT_OBJECT_DIR/loose-object-idx.lock. To add a new loose
+object:
+
+1. Write the loose object to a temporary file, like today.
+2. Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the lock.
+3. Rename the loose object into place.
+4. Open loose-object-idx with O_APPEND and write the new object
+5. Unlink loose-object-idx.lock to release the lock.
+
+To remove entries (e.g. in "git pack-refs" or "git-prune"):
+
+1. Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the
+   lock.
+2. Write the new content to loose-object-idx.lock.
+3. Unlink any loose objects being removed.
+4. Rename to replace loose-object-idx, releasing the lock.
+
+Translation table
+~~~~~~~~~~~~~~~~~
+The index files support a bidirectional mapping between sha1-names
+and newhash-names. The lookup proceeds similarly to ordinary object
+lookups. For example, to convert a sha1-name to a newhash-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
+       name order mapping.
+    b. Read the corresponding entry in the full sha1-name table to
+       verify we found the right object. If it is, then
+    c. Read the corresponding entry in the full newhash-name table.
+       That is the object's newhash-name.
+ 2. Check for a loose object. Read lines from loose-object-idx until
+    we find a match.
+
+Step (1) takes the same amount of time as an ordinary object lookup:
+O(number of packs * log(objects per pack)). Step (2) takes O(number of
+loose objects) time. To maintain good performance it will be necessary
+to keep the number of loose objects low. See the "Loose objects and
+unreachable objects" section below for more details.
+
+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 newhash-names
+its newhash-content references to sha1-names using the translation table.
+
+Fetch
+~~~~~
+Fetching from a SHA-1 based server requires translating between SHA-1
+and NewHash based representations on the fly.
+
+SHA-1s named in the ref advertisement that are present on the client
+can be translated to NewHash and looked up as local objects using the
+translation table.
+
+Negotiation proceeds as today. Any "have"s generated locally are
+converted to SHA-1 before being sent to the server, and SHA-1s
+mentioned by the server are converted to NewHash when looking them up
+locally.
+
+After negotiation, the server sends a packfile containing the
+requested objects. We convert the packfile to NewHash format using
+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
+   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,
+   excluding blobs, in reverse topologically sorted order, with each
+   object coming later in the list than all objects it references.
+   (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 newhash: open a new (newhash) packfile. Read the topologically
+   sorted list just generated. For each object, inflate its
+   sha1-content, convert to newhash-content, and write it to the newhash
+   pack. Record the new sha1<->newhash 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 newhash idx
+   file
+5. clean up: remove the SHA-1 based pack file, index, and
+   topologically sorted list obtained from the server in steps 1
+   and 2.
+
+Step 3 requires every object referenced by the new object to be in the
+translation table. This is why the topological sort step is necessary.
+
+As an optimization, step 1 could write a file describing what non-blob
+objects each object it has inflated from the packfile references. This
+makes the topological sort in step 2 possible without inflating the
+objects in the packfile for a second time. The objects need to be
+inflated again in step 3, for a total of two inflations.
+
+Step 4 is probably necessary for good read-time performance. "git
+pack-objects" on the server optimizes the pack file for good data
+locality (see Documentation/technical/pack-heuristics.txt).
+
+Details of this process are likely to change. It will take some
+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
+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
+send-pack.
+
+Signed Commits
+~~~~~~~~~~~~~~
+We add a new field "gpgsig-newhash" 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 newhash-content of the
+commit object with any "gpgsig" and "gpgsig-newhash" 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 NewHash, by using both gpgsig-newhash and gpgsig
+   fields.
+3. using only NewHash, by only using the gpgsig-newhash field.
+
+Old versions of "git verify-commit" can verify the gpgsig signature in
+cases (1) and (2) without modifications and view case (3) as an
+ordinary unsigned commit.
+
+Signed Tags
+~~~~~~~~~~~
+We add a new field "gpgsig-newhash" to the tag object format to allow
+signing tags without relying on SHA-1. Its signed payload is the
+newhash-content of the tag with its gpgsig-newhash 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 NewHash, by using gpgsig-newhash and an in-body
+   signature.
+3. using only NewHash, by only using the gpgsig-newhash field.
+
+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 newhash-content of the same commit contains the
+newhash-content of the same tag.
+
+Submodules
+~~~~~~~~~~
+To convert recorded submodule pointers, you need to have the converted
+submodule repository in place. The translation table of the submodule
+can be used to look up the new hash.
+
+Loose objects and unreachable objects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Fast lookups in the loose-object-idx require that the number of loose
+objects not grow too high.
+
+"git gc --auto" currently waits for there to be 6700 loose objects
+present before consolidating them into a packfile. We will need to
+measure to find a more appropriate threshold for it to use.
+
+"git gc --auto" currently waits for there to be 50 packs present
+before combining packfiles. Packing loose objects more aggressively
+may cause the number of pack files to grow too quickly. This can be
+mitigated by using a strategy similar to Martin Fick's exponential
+rolling garbage collection script:
+https://gerrit-review.googlesource.com/c/gerrit/+/35215
+
+"git gc" currently expels any unreachable objects it encounters in
+pack files to loose objects in an attempt to prevent a race when
+pruning them (in case another process is simultaneously writing a new
+object that refers to the about-to-be-deleted object). This leads to
+an explosion in the number of loose objects present and disk space
+usage due to the objects in delta form being replaced with independent
+loose objects.  Worse, the race is still present for loose objects.
+
+Instead, "git gc" will need to move unreachable objects to a new
+packfile marked as UNREACHABLE_GARBAGE (using the PSRC field; see
+below). To avoid the race when writing new objects referring to an
+about-to-be-deleted object, code paths that write new objects will
+need to copy any objects from UNREACHABLE_GARBAGE packs that they
+refer to to new, non-UNREACHABLE_GARBAGE packs (or loose objects).
+UNREACHABLE_GARBAGE are then safe to delete if their creation time (as
+indicated by the file's mtime) is long enough ago.
+
+To avoid a proliferation of UNREACHABLE_GARBAGE packs, they can be
+combined under certain circumstances. If "gc.garbageTtl" is set to
+greater than one day, then packs created within a single calendar day,
+UTC, can be coalesced together. The resulting packfile would have an
+mtime before midnight on that day, so this makes the effective maximum
+ttl the garbageTtl + 1 day. If "gc.garbageTtl" is less than one day,
+then we divide the calendar day into intervals one-third of that ttl
+in duration. Packs created within the same interval can be coalesced
+together. The resulting packfile would have an mtime before the end of
+the interval, so this makes the effective maximum ttl equal to the
+garbageTtl * 4/3.
+
+This rule comes from Thirumala Reddy Mutchukota's JGit change
+https://git.eclipse.org/r/90465.
+
+The UNREACHABLE_GARBAGE setting goes in the PSRC field of the pack
+index. More generally, that field indicates where a pack came from:
+
+ - 1 (PACK_SOURCE_RECEIVE) for a pack received over the network
+ - 2 (PACK_SOURCE_AUTO) for a pack created by a lightweight
+   "gc --auto" operation
+ - 3 (PACK_SOURCE_GC) for a pack created by a full gc
+ - 4 (PACK_SOURCE_UNREACHABLE_GARBAGE) for potential garbage
+   discovered by gc
+ - 5 (PACK_SOURCE_INSERT) for locally created objects that were
+   written directly to a pack file, e.g. from "git add ."
+
+This information can be useful for debugging and for "gc --auto" to
+make appropriate choices about which packs to coalesce.
+
+Caveats
+-------
+Invalid objects
+~~~~~~~~~~~~~~~
+The conversion from sha1-content to newhash-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
+feature of the design to allow the conversion to round-trip.
+
+More profoundly broken objects (e.g., a commit with a truncated "tree"
+header line) cannot be converted but were not usable by current Git
+anyway.
+
+Shallow clone and submodules
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Because it requires all referenced objects to be available in the
+locally generated translation table, this design does not support
+shallow clone or unfetched submodules. Protocol improvements might
+allow lifting this restriction.
+
+Alternates
+~~~~~~~~~~
+For the same reason, a newhash repository cannot borrow objects from a
+sha1 repository using objects/info/alternates or
+$GIT_ALTERNATE_OBJECT_REPOSITORIES.
+
+git notes
+~~~~~~~~~
+The "git notes" tool annotates objects using their sha1-name as key.
+This design does not describe a way to migrate notes trees to use
+newhash-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).
+
+Server-side cost
+~~~~~~~~~~~~~~~~
+Until Git protocol gains NewHash support, using NewHash based storage
+on public-facing Git servers is strongly discouraged. Once Git
+protocol gains NewHash support, NewHash based servers are likely not
+to support SHA-1 compatibility, to avoid what may be a very expensive
+hash reencode during clone and to encourage peers to modernize.
+
+The design described here allows fetches by SHA-1 clients of a
+personal NewHash repository because it's not much more difficult than
+allowing pushes from that repository. This support needs to be guarded
+by a configuration option --- servers like git.kernel.org that serve a
+large number of clients would not be expected to bear that cost.
+
+Meaning of signatures
+~~~~~~~~~~~~~~~~~~~~~
+The signed payload for signed commits and tags does not explicitly
+name the hash used to identify objects. If some day Git adopts a new
+hash function with the same length as the current SHA-1 (40
+hexadecimal digit) or NewHash (64 hexadecimal digit) objects then the
+intent behind the PGP signed payload in an object signature is
+unclear:
+
+	object e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7
+	type commit
+	tag v2.12.0
+	tagger Junio C Hamano <gitster@pobox.com> 1487962205 -0800
+
+	Git 2.12
+
+Does this mean Git v2.12.0 is the commit with sha1-name
+e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with
+new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7?
+
+Fortunately NewHash and SHA-1 have different lengths. If Git starts
+using another hash with the same length to name objects, then it will
+need to change the format of signed payloads using that hash to
+address this issue.
+
+Object names on the command line
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+To support the transition (see Transition plan below), this design
+supports four different modes of operation:
+
+ 1. ("dark launch") Treat object names input by the user as SHA-1 and
+    convert any object names written to output to SHA-1, but store
+    objects using NewHash.  This allows users to test the code with no
+    visible behavior change except for performance.  This allows
+    allows running even tests that assume the SHA-1 hash function, to
+    sanity-check the behavior of the new mode.
+
+ 2. ("early transition") Allow both SHA-1 and NewHash object names in
+    input. Any object names written to output use SHA-1. This allows
+    users to continue to make use of SHA-1 to communicate with peers
+    (e.g. by email) that have not migrated yet and prepares for mode 3.
+
+ 3. ("late transition") Allow both SHA-1 and NewHash object names in
+    input. Any object names written to output use NewHash. In this
+    mode, users are using a more secure object naming method by
+    default.  The disruption is minimal as long as most of their peers
+    are in mode 2 or mode 3.
+
+ 4. ("post-transition") Treat object names input by the user as
+    NewHash and write output using NewHash. This is safer than mode 3
+    because there is less risk that input is incorrectly interpreted
+    using the wrong hash function.
+
+The mode is specified in configuration.
+
+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^{newhash}
+
+Selection of a New 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.
+
+The hash function NewHash to replace 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).
+
+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).
+
+Some hashes under consideration are SHA-256, SHA-512/256, SHA-256x16,
+K12, and BLAKE2bp-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-newhash field
+- excluding gpgsig-* from the fields copied by "git commit --amend"
+- annotating tests that depend on SHA-1 values with a SHA1 test
+  prerequisite
+- using "struct object_id", GIT_MAX_RAWSZ, and GIT_MAX_HEXSZ
+  consistently instead of "unsigned char *" and the hardcoded
+  constants 20 and 40.
+- 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:
+- implementing the loose-object-idx
+- teaching fsck about this mode of operation
+- using the hash function API (vtable) when computing object names
+- signing objects and verifying signatures
+- rejecting attempts to fetch from or push to an incompatible
+  repository
+
+Next comes introduction of compatObjectFormat:
+- translating object names between object formats
+- translating object content between object formats
+- generating and verifying signatures in the compat format
+- adding appropriate index entries when adding a new object to the
+  object store
+- --output-format option
+- ^{sha1} and ^{newhash} revision notation
+- configuration to specify default input and output format (see
+  "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
+- convert the fetched packfile to newhash format and generate an idx
+  file
+- re-sort to match the order of objects in the fetched packfile
+
+The infrastructure supporting fetch also allows converting an existing
+repository. In converted repositories and new clones, end users can
+gain support for the new hash function without any visible change in
+behavior (see "dark launch" in the "Object names on the command line"
+section). In particular this allows users to verify NewHash signatures
+on objects in the repository, and it should ensure the transition code
+is stable in production in preparation for using it more widely.
+
+Over time projects would encourage their users to adopt the "early
+transition" and then "late transition" modes to take advantage of the
+new, more futureproof NewHash object names.
+
+When objectFormat and compatObjectFormat are both set, commands
+generating signatures would generate both SHA-1 and NewHash signatures
+by default to support both new and old users.
+
+In projects using NewHash heavily, users could be encouraged to adopt
+the "post-transition" mode to avoid accidentally making implicit use
+of SHA-1 object names.
+
+Once a critical mass of users have upgraded to a version of Git that
+can verify NewHash signatures and have converted their existing
+repositories to support verifying them, we can add support for a
+setting to generate only NewHash signatures. This is expected to be at
+least a year later.
+
+That is also a good moment to advertise the ability to convert
+repositories to use NewHash only, stripping out all SHA-1 related
+metadata. This improves performance by eliminating translation
+overhead and security by avoiding the possibility of accidentally
+relying on the safety of SHA-1.
+
+Updating Git's protocols to allow a server to specify which hash
+functions it supports is also an important part of this transition. It
+is not discussed in detail in this document but this transition plan
+assumes it happens. :)
+
+Alternatives considered
+-----------------------
+Upgrading everyone working on a particular project on a flag day
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Projects like the Linux kernel are large and complex enough that
+flipping the switch for all projects based on the repository at once
+is infeasible.
+
+Not only would all developers and server operators supporting
+developers have to switch on the same flag day, but supporting tooling
+(continuous integration, code review, bug trackers, etc) would have to
+be adapted as well. This also makes it difficult to get early feedback
+from some project participants testing before it is time for mass
+adoption.
+
+Using hash functions in parallel
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+(e.g. https://public-inbox.org/git/22708.8913.864049.452252@chiark.greenend.org.uk/ )
+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
+  (they will never go away, so they accumulate). In this proposal, by
+  comparison, converted objects lose all references to SHA-1.
+
+Signed objects with multiple hashes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Instead of introducing the gpgsig-newhash field in commit and tag objects
+for newhash-content based signatures, an earlier version of this design
+added "hash newhash <newhash-name>" fields to strengthen the existing
+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 newhash-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
+  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 newhash-content of a signed
+  commit, complicating the conversion process.
+* Allowing signed objects without a sha1 (for after the transition is
+  complete) complicated the design further, requiring a "nohash sha1"
+  field to suppress including "hash sha1" fields in the newhash-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 newhash-name calculated should be
+an acceptable cost.
+
+Document History
+----------------
+
+2017-03-03
+bmwill@google.com, jonathantanmy@google.com, jrnieder@gmail.com,
+sbeller@google.com
+
+Initial version sent to
+http://public-inbox.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]).
+* Add a sorting phase to fetch (thanks to Junio for noticing the need
+  for this).
+* Omit blobs from the topological sort during fetch (thanks to peff).
+* Discuss alternates, git notes, and git servers in the caveats
+  section (thanks to Junio Hamano, brian m. carlson[4], and Shawn
+  Pearce).
+* Clarify language throughout (thanks to various commenters,
+  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
+  for fleshing these ideas out)
+* define the translation table (thanks, Shawn Pearce[5], Jonathan
+  Tan, and Masaya Suzuki)
+* avoid loose object overhead by packing more aggressively in
+  "git gc --auto"
+
+[1] http://public-inbox.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
+[2] http://public-inbox.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
+[3] http://public-inbox.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
+[4] http://public-inbox.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
+[5] https://public-inbox.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/
-- 
2.14.2.822.g60be5d43e6-goog


  parent reply index

Thread overview: 113+ messages in thread (expand / mbox.gz / Atom feed / [top])
2017-03-04  1:12 RFC: Another proposed hash function transition plan Jonathan Nieder
2017-03-05  2:35 ` Linus Torvalds
2017-03-06  0:26   ` brian m. carlson
2017-03-06 18:24     ` Brandon Williams
2017-06-15 10:30       ` Which hash function to use, was " Johannes Schindelin
2017-06-15 11:05         ` Mike Hommey
2017-06-15 13:01           ` Jeff King
2017-06-15 16:30             ` Ævar Arnfjörð Bjarmason
2017-06-15 19:34               ` Johannes Schindelin
2017-06-15 21:59                 ` Adam Langley
2017-06-15 22:41                   ` brian m. carlson
2017-06-15 23:36                     ` Ævar Arnfjörð Bjarmason
2017-06-16  0:17                       ` brian m. carlson
2017-06-16  6:25                         ` Ævar Arnfjörð Bjarmason
2017-06-16 13:24                           ` Johannes Schindelin
2017-06-16 17:38                             ` Adam Langley
2017-06-16 20:52                               ` Junio C Hamano
2017-06-16 21:12                                 ` Junio C Hamano
2017-06-16 21:24                                   ` Jonathan Nieder
2017-06-16 21:39                                     ` Ævar Arnfjörð Bjarmason
2017-06-16 20:42                             ` Jeff King
2017-06-19  9:26                               ` Johannes Schindelin
2017-06-15 21:10             ` Mike Hommey
2017-06-16  4:30               ` Jeff King
2017-06-15 17:36         ` Brandon Williams
2017-06-15 19:20           ` Junio C Hamano
2017-06-15 19:13         ` Jonathan Nieder
2017-03-07  0:17   ` RFC v3: " Jonathan Nieder
2017-03-09 19:14     ` Shawn Pearce
2017-03-09 20:24       ` Jonathan Nieder
2017-03-10 19:38         ` Jeff King
2017-03-10 19:55           ` Jonathan Nieder
2017-09-28  4:43       ` Jonathan Nieder [this message]
2017-09-29  6:06         ` [PATCH v4] technical doc: add a design doc for hash function transition Junio C Hamano
2017-09-29  8:09           ` Junio C Hamano
2017-09-29 17:34           ` Jonathan Nieder
2017-10-02  8:25             ` Junio C Hamano
2017-10-02 19:41             ` Jason Cooper
2017-10-02  9:02         ` Junio C Hamano
2017-10-02 19:23         ` Jason Cooper
2017-10-03  5:40         ` Junio C Hamano
2017-10-03 13:08           ` Jason Cooper
2017-10-04  1:44         ` Junio C Hamano
2017-09-06  6:28     ` RFC v3: Another proposed hash function transition plan Junio C Hamano
2017-09-08  2:40       ` Junio C Hamano
2017-09-08  3:34         ` Jeff King
2017-09-11 18:59         ` Brandon Williams
2017-09-13 12:05           ` Johannes Schindelin
2017-09-13 13:43             ` demerphq
2017-09-13 22:51               ` Jonathan Nieder
2017-09-14 18:26                 ` Johannes Schindelin
2017-09-14 18:40                   ` Jonathan Nieder
2017-09-14 22:09                     ` Johannes Schindelin
2017-09-13 23:30               ` Linus Torvalds
2017-09-14 18:45                 ` Johannes Schindelin
2017-09-18 12:17                   ` Gilles Van Assche
2017-09-18 22:16                     ` Johannes Schindelin
2017-09-19 16:45                       ` Gilles Van Assche
2017-09-29 13:17                         ` Johannes Schindelin
2017-09-29 14:54                           ` Joan Daemen
2017-09-29 22:33                             ` Johannes Schindelin
2017-09-30 22:02                               ` Joan Daemen
2017-10-02 14:26                                 ` Johannes Schindelin
2017-09-18 22:25                     ` Jonathan Nieder
2017-09-26 17:05                   ` Jason Cooper
2017-09-26 22:11                     ` Johannes Schindelin
2017-09-26 22:25                       ` [PATCH] technical doc: add a design doc for hash function transition Stefan Beller
2017-09-26 23:38                         ` Jonathan Nieder
2017-09-26 23:51                       ` RFC v3: Another proposed hash function transition plan Jonathan Nieder
2017-10-02 14:54                         ` Jason Cooper
2017-10-02 16:50                           ` Brandon Williams
2017-10-02 14:00                       ` Jason Cooper
2017-10-02 17:18                         ` Linus Torvalds
2017-10-02 19:37                           ` Jeff King
2017-09-13 16:30             ` Jonathan Nieder
2017-09-13 21:52               ` Junio C Hamano
2017-09-13 22:07                 ` Stefan Beller
2017-09-13 22:18                   ` Jonathan Nieder
2017-09-14  2:13                     ` Junio C Hamano
2017-09-14 15:23                       ` Johannes Schindelin
2017-09-14 15:45                         ` demerphq
2017-09-14 22:06                           ` Johannes Schindelin
2017-09-13 22:15                 ` Junio C Hamano
2017-09-13 22:27                   ` Jonathan Nieder
2017-09-14  2:10                     ` Junio C Hamano
2017-09-14 12:39               ` Johannes Schindelin
2017-09-14 16:36                 ` Brandon Williams
2017-09-14 18:49                 ` Jonathan Nieder
2017-09-15 20:42                   ` Philip Oakley
2017-03-05 11:02 ` RFC: " David Lang
     [not found]   ` <CA+dhYEXHbQfJ6KUB1tWS9u1MLEOJL81fTYkbxu4XO-i+379LPw@mail.gmail.com>
2017-03-06  9:43     ` Jeff King
2017-03-06 23:40   ` Jonathan Nieder
2017-03-07  0:03     ` Mike Hommey
2017-03-06  8:43 ` Jeff King
2017-03-06 18:39   ` Jonathan Tan
2017-03-06 19:22     ` Linus Torvalds
2017-03-06 19:59       ` Brandon Williams
2017-03-06 21:53       ` Junio C Hamano
2017-03-07  8:59     ` Jeff King
2017-03-06 18:43   ` Junio C Hamano
2017-03-07 18:57 ` Ian Jackson
2017-03-07 19:15   ` Linus Torvalds
2017-03-08 11:20     ` Ian Jackson
2017-03-08 15:37       ` Johannes Schindelin
2017-03-08 15:40       ` Johannes Schindelin
2017-03-20  5:21         ` Use base32? Jason Hennessey
2017-03-20  5:58           ` Michael Steuer
2017-03-20  8:05             ` Jacob Keller
2017-03-21  3:07               ` Michael Steuer
2017-03-13  9:24 ` RFC: Another proposed hash function transition plan The Keccak Team
2017-03-13 17:48   ` Jonathan Nieder
2017-03-13 18:34     ` ankostis
2017-03-17 11:07       ` Johannes Schindelin

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