Or: How To Avoid Reading This Article
There was much angst over the SHAttered attack against SHA1 when it was announced in early 2017. If you are concerned about this and its implications for Fossil, simply upgrade to Fossil 2.1 or later, and the problem will go away. Everything will continue to work as before.
But if you are curious and want a deeper understanding of what is going on, read on...
The first snapshot-based distributed version control system was Monotone. Many of the ideas behind the design of Fossil were copied from Monotone, including the use of a SHA1 hash to assign names to artifacts. Git and Mercurial did the same thing.
The SHA1 hash algorithm is used only to create names for artifacts in Fossil (and in Git, Mercurial, and Monotone). It is not used for security. Nevertheless, when the SHAttered attack found two different PDF files with the same SHA1 hash, many users learned that "SHA1 is broken". They see that Fossil (and Git, Mercurial, and Monotone) use SHA1 and they therefore conclude that "Fossil is broken". This is not true, but it is a public relations problem. So the decision was made to migrate Fossil away from SHA1.
This article describes how that migration is occurring.
The Hardened SHA1 algorithm automatically detects when the artifact being hashed is specifically designed to exploit the known weaknesses in the SHA1 algorithm, and when it detects such an attack it changes the hash algorithm (by increasing the number of rounds in the compression function) to make the algorithm secure again. If the attack detection gets a false-positive, that means that Hardened SHA1 will get a different answer than the standard FIPS PUB 180-4 SHA1, but the creators of Hardened SHA1 (see the second paper ) report that the probability of a false-positive is vanishingly small - less than 1 false positive out of 1027 hashes.
Hardened SHA1 is slower (and a lot bigger) but Fossil does not do that much hashing, so performance is not really an issue.
All versions of Fossil moving forward will use Hardened SHA1. So if someone says "SHA1 is broken, and Fossil uses SHA1, therefore Fossil is broken," you can rebut the argument by pointing out that Fossil uses Hardened SHA1, not generic SHA1, and Hardened SHA1 is not broken.
Prior to Fossil version 2.0 (2017-03-03), all artifacts in all Fossil repositories were named by only a SHA1 hash. Version 2.0 extended the Fossil file format to allow artifacts to be named by either SHA1 or SHA3-256 hashes. (SHA3-256 is the only variant of SHA3 that Fossil uses for artifact naming, so for the remainder of this article it will be called simply "SHA3". Similarly, "Hardened SHA1" will shortened to "SHA1" in the remaining text.)
To be clear: Fossil (version 2.0 and later) allows the SHA1 and SHA3 hashes to be mixed within the same repository. Older check-ins, created years ago, continue to be named using their legacy SHA1 hashes while newer check-ins are named using modern SHA3 hashes. There is no need to "convert" a repository from SHA1 over to SHA3. You can see this in Fossil itself. The recent [9d9ef82234f63758] check-in uses a SHA3 hash whereas the older [1669115ab9d05c18] check-in uses a SHA1 hash.
Other than permitting the use of SHA3 in addition to SHA1, there were no file format changes in Fossil version 2.0 relative to the previous version 1.37. Both Fossil 2.0 and Fossil 1.37 read and write all the same repositories and sync with one another, as long as none of the repositories contain artifacts named using SHA3. If a repository does contain artifacts named using SHA3, Fossil 1.37 will not know how to interpret those artifacts and will generate various warnings and errors.
If newer versions of Fossil are able to use either SHA1 or SHA3 to name artifacts, which hash algorithm is actually used? That question is answered by the "hash policy". These are the supported hash policies:
|Name all new artifacts using the (Hardened) SHA1 hash algorithm.|
|auto||Name new artifacts using the SHA1 hash algorithm, but if any artifacts are encountered which are already named using SHA3, then automatically switch the hash policy to "sha3"|
|sha3||Name new artifacts using the SHA3 hash algorithm if the artifact does not already have a SHA1 name. If the artifact already has a SHA1 name, then continue to use the older SHA1 name. Use SHA3 for new artifacts that have never before been encountered.|
|sha3-only||Name new artifacts using the SHA3 hash algorithm even if the artifact already has a SHA1 name. In other words, force the use of SHA3. This can cause some artifacts to be added to the repository twice, once under their SHA1 name and again under their SHA3 name, but delta compression will prevent that from causing repository size problems.|
|shun-sha1||Like "sha3-only" but at this level do not accept a push of SHA1-named artifacts. If another Fossil instance tries to push a SHA1-named artifact, that artifact is discarded and ignored.|
For Fossil 2.0, and obviously also for Fossil 1.37 and before, the only hash policy supported was the one now called "sha1", meaning that all new artifacts were named using a SHA1 hash. Even though Fossil 2.0 added the capability of understanding SHA3 hashes, it never actually generates any SHA3 hashes.
From Fossil 2.1 through 2.9, the default hash policy for legacy repositories changed to "auto", meaning that Fossil continued to generate only SHA1 hashes until it encountered one artifact with a SHA3 hash. Once those older versions of Fossil saw a single SHA3 hash, they automatically switched to "sha3" mode and thereafter generated only SHA3 hashes.
When a new repository is created by cloning, the hash policy is copied from the parent.
For new repositories created using the fossil new command the default hash policy is "sha3". That means new repositories will normally hold nothing except SHA3 hashes. The hash policy for new repositories can be overridden using the "--sha1" option to the "fossil new" command.
If you are still on Fossil 2.1 through 2.9 but you want Fossil to go ahead and start using SHA3 hashes, change the hash policy to "sha3" using a command like this:
fossil hash-policy sha3
The next check-in will use a SHA3 hash, so that when that check-in is pushed to colleagues, their clones will include the new SHA3-named artifact, so their local Fossil instances will automatically convert their clones to "sha3" mode as well.
Of course, if some members of your team stubbornly refuse to upgrade past Fossil 1.37, you should avoid changing the hash policy and creating artifacts with SHA3 names, because once you do that your recalcitrant coworkers will no longer be able to collaborate.
Fossil 2.10 changed the default hash policy to "sha3" mode even for legacy repositories, so if you upgrade to the latest version of Fossil, all of your new artifacts will use a SHA3 hash. Legacy SHA1 artifacts continue to use their original names, but new artifacts will use SHA3 names. You might not even notice this automatic change over to stronger hashes.
We decided to make the change to pure SHA3 since the last known distributor of Fossil 1.x binaries — Debian 9 — was finally replaced in June 2019 by Debian 10, which included Fossil 2.8. All other known sources of Fossil 1.x binaries upgraded well before that point.