technical doc: add a design doc for hash function transition

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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

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/