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diff --git a/Documentation/technical/hash-function-transition.txt b/Documentation/technical/hash-function-transition.txt new file mode 100644 index 0000000..ed57481 --- /dev/null +++ b/Documentation/technical/hash-function-transition.txt @@ -0,0 +1,830 @@ +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. On 23 February 2017 the SHAttered attack +(https://shattered.io) demonstrated a practical SHA-1 hash collision. + +Git v2.13.0 and later subsequently moved to a hardened SHA-1 +implementation by default, which isn't vulnerable to the SHAttered +attack, but SHA-1 is still weak. + +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. + +If SHA-1 and its variants were to be truly broken, Git's hash function +could not be considered cryptographically secure any more. This would +impact the communication of hash values because we could not trust +that a given hash value represented 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. + +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. + a. Requiring no action by any other party. + b. A SHA-256 repository can communicate with SHA-1 Git servers + (push/fetch). + c. Users can use SHA-1 and SHA-256 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 SHA-256 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 SHA-256 repository. (This will + change when we add SHA-256 support to Git protocol.) +6. Skip fetching some submodules of a project into a SHA-256 + repository. (This also depends on SHA-256 support in Git + protocol.) + +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 +of objects and all references to other objects within an object are +switched to the new hash function. + +SHA-256 repositories cannot be read by older versions of Git. + +Alongside the packfile, a SHA-256 repository stores a bidirectional +mapping between SHA-256 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 SHA-256 names +interchangeably. + +"git cat-file" and "git hash-object" gain options to display an object +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). + +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 SHA-1 form so the server does not have to be +aware of the hash function the client is using. + +Detailed Design +--------------- +Repository format extension +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +A SHA-256 repository uses repository format version `1` (see +Documentation/technical/repository-version.txt) with extensions +`objectFormat` and `compatObjectFormat`: + + [core] + repositoryFormatVersion = 1 + [extensions] + objectFormat = sha256 + compatObjectFormat = sha1 + +The combination of setting `core.repositoryFormatVersion=1` and +populating `extensions.*` ensures that all versions of Git later than +`v0.99.9l` will die instead of trying to operate on the SHA-256 +repository, instead producing an error message. + + # Between v0.99.9l and v2.7.0 + $ git status + fatal: Expected git repo version <= 0, found 1 + # After v2.7.0 + $ 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 SHA-1 name or 64 +hexadecimal digit SHA-256 name, plus names derived from those (see +gitrevisions(7)). + +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 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 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 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 SHA-256 content and +SHA-1 content. + +Object storage +~~~~~~~~~~~~~~ +Loose objects use zlib compression and packed objects use the packed +format described in linkgit:gitformat-pack[5], just like +today. The content that is compressed and stored uses SHA-256 content +instead of SHA-1 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 SHA-256 checksum at the end of the + corresponding packfile. + + * 20-byte SHA-256 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 + (sha256-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 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 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 SHA-1 name table to + verify we found the right object. If it is, then + 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. + +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 SHA-1 content +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The SHA-1 content of an object can be read by converting all SHA-256 names +of its SHA-256 content references to SHA-1 names using the translation table. + +Fetch +~~~~~ +Fetching from a SHA-1 based server requires translating between SHA-1 +and SHA-256 based representations on the fly. + +SHA-1s named in the ref advertisement that are present on the client +can be translated to SHA-256 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 SHA-256 when looking them up +locally. + +After negotiation, the server sends a packfile containing the +requested objects. We convert the packfile to SHA-256 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 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, + 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 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 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 + 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 SHA-1 content +of each object being pushed can be read as described in the "Reading +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 SHA-256 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. +3. using only SHA-256, by only using the gpgsig-sha256 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-sha256" to the tag object format to allow +signing tags without relying on SHA-1. Its signed payload is the +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 + +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. +3. using only SHA-256, by only using the gpgsig-sha256 field. + +Mergetag embedding +~~~~~~~~~~~~~~~~~~ +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 SHA-256 content of the same commit contains the +SHA-256 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 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 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 +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 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 SHA-1 name as key. +This design does not describe a way to migrate notes trees to use +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). + +Server-side cost +~~~~~~~~~~~~~~~~ +Until Git protocol gains SHA-256 support, using SHA-256 based storage +on public-facing Git servers is strongly discouraged. Once Git +protocol gains SHA-256 support, SHA-256 based servers are likely not +to support SHA-1 compatibility, to avoid what may be a very expensive +hash re-encode during clone and to encourage peers to modernize. + +The design described here allows fetches by SHA-1 clients of a +personal SHA-256 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 SHA-256 (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 SHA-1 name +e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with +new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7? + +Fortunately SHA-256 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 SHA-256. This allows users to test the code with no + visible behavior change except for performance. This 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 SHA-256 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 SHA-256 object names in + input. Any object names written to output use SHA-256. 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 + SHA-256 and write output using SHA-256. 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^{sha256} + +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" +- 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: + +- 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: + +- implementing the loose-object-idx +- 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 ^{sha256} 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 SHA-256 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 SHA-256 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 SHA-256 object names. + +When objectFormat and compatObjectFormat are both set, commands +generating signatures would generate both SHA-1 and SHA-256 signatures +by default to support both new and old users. + +In projects using SHA-256 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 SHA-256 signatures and have converted their existing +repositories to support verifying them, we can add support for a +setting to generate only SHA-256 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 SHA-256 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://lore.kernel.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-sha256 field in commit and tag objects +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 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 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 <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 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 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 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 +---------------- + +2017-03-03 +bmwill@google.com, jonathantanmy@google.com, jrnieder@gmail.com, +sbeller@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: + +* 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" + +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 + +References: + + [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/ |