Linus Torvalds wrote: > On Fri, Mar 3, 2017 at 5:12 PM, Jonathan Nieder <jrnieder@xxxxxxxxx> wrote: >> This document is still in flux but I thought it best to send it out >> early to start getting feedback. > > This actually looks very reasonable if you can implement it cleanly > enough. 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/. As before, comments welcome, both here and inline at https://goo.gl/gh2Mzc Changes since v2: 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). Sincerely, Jonathan Git hash function transition ============================ Status: Draft Last Updated: 2017-03-06 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 ----- 1. The transition to SHA3-256 can be done one local repository at a time. a. Requiring no action by any other party. b. A SHA3-256 repository can communicate with SHA-1 Git servers (push/fetch). c. Users can use SHA-1 and SHA3-256 identifiers for objects interchangeably. 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 SHA3-256 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 SHA3-256 repository. (This will change when we add SHA3-256 support to Git protocol.) 6. Skip fetching some submodules of a project into a SHA3-256 repository. (This also depends on SHA3-256 support in Git protocol.) Overview -------- We introduce a new repository format extension `sha3`. Repositories with this extension enabled use SHA3-256 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. sha3 repositories cannot be read by older versions of Git. Alongside the packfile, a sha3 repository stores a bidirectional mapping between sha3 and sha1 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 sha1 and sha3 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 sha3 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 --------------- Object names ~~~~~~~~~~~~ Objects can be named by their 40 hexadecimal digit sha1-name or 64 hexadecimal digit sha3-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 sha3-name of an object is the SHA3-256 of the concatenation of its type, length, a nul byte, and the object's sha3-content. Object format ~~~~~~~~~~~~~ The content as a byte sequence of a tag, commit, or tree object named by sha1 and sha3 differ because an object named by sha3-name refers to other objects by their sha3-names and an object named by sha1-name refers to other objects by their sha1-names. The sha3-content of an object is the same as its sha1-content, except that objects referenced by the object are named using their sha3-names instead of sha1-names. Because a blob object does not refer to any other object, its sha1-content and sha3-content are the same. The format allows round-trip conversion between sha3-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 sha3-content instead of sha1-content. Translation table ~~~~~~~~~~~~~~~~~ A fast bidirectional mapping between sha1-names and sha3-names of all local objects in the repository is kept on disk. The exact format of that mapping is to be determined. All operations that make new objects (e.g., "git commit") add the new objects to the translation table. (This work could have been 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 sha3-name calculated should be an acceptable cost.) Reading an object's sha1-content ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The sha1-content of an object can be read by converting all sha3-names its sha3-content references to sha1-names using the translation table. Fetch ~~~~~ Fetching from a SHA-1 based server requires translating between SHA-1 and SHA3-256 based representations on the fly. SHA-1s named in the ref advertisement that are present on the client can be translated to SHA3-256 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 SHA3-256 when looking them up locally. After negotiation, the server sends a packfile containing the requested objects. We convert the packfile to SHA3-256 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 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 sha3: open a new (sha3) packfile. Read the topologically sorted list just generated. For each object, inflate its sha1-content, convert to sha3-content, and write it to the sha3 pack. Include the new sha1<->sha3 mapping entry in the translation table. 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 sha3 idx file. 5. clean up: remove the SHA-1 based pack file, index, and topologically sorted list obtained from the server and 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-sha3" 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 sha3-content of the commit object with any "gpgsig" and "gpgsig-sha3" 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 SHA3-256, by using both gpgsig-sha3 and gpgsig fields. 3. using only SHA3-256, by only using the gpgsig-sha3 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-sha3" to the tag object format to allow signing tags without relying on SHA-1. Its signed payload is the sha3-content of the tag with its gpgsig-sha3 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 SHA3-256, by using gpgsig-sha3 and an in-body signature. 3. using only SHA3-256, by only using the gpgsig-sha3 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 sha3-content of the same commit contains the sha3-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. Caveats ------- Invalid objects ~~~~~~~~~~~~~~~ The conversion from sha1-content to sha3-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 sha3 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 sha3-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 SHA3-256 support, using sha3 based storage on public-facing Git servers is strongly discouraged. Once Git protocol gains SHA3-256 support, sha3 based servers are likely not to support sha1 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 SHA256 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 SHA2-256 (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@xxxxxxxxx> 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 SHA3-256 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. 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@xxxxxxxxxxxxxxxxxxxxxx/ ) 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-sha3 field in commit and tag objects for sha3-content based signatures, an earlier version of this design added "hash sha3 <sha3-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 sha3-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 sha3-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 sha3-content and signed payload. Document History ---------------- 2017-03-03 bmwill@xxxxxxxxxx, jonathantanmy@xxxxxxxxxx, jrnieder@xxxxxxxxx, sbeller@xxxxxxxxxx Initial version sent to http://public-inbox.org/git/20170304011251.GA26789@xxxxxxxxxxxxxxxxxxxxxxxxx 2017-03-03 jrnieder@xxxxxxxxx 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@xxxxxxxxx * 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). [1] http://public-inbox.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@xxxxxxxxxxxxxx/ [2] http://public-inbox.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@xxxxxxxxxxxxxx/ [3] http://public-inbox.org/git/20170306084353.nrns455dvkdsfgo5@xxxxxxxxxxxxxxxxxxxxx/ [4] http://public-inbox.org/git/20170304224936.rqqtkdvfjgyezsht@xxxxxxxxxxxxxxxxxxxxxxxxxx
