Hash Based Package Downloads - part 2 of 2.

Posted by Michael Snoyman - 31 January, 2018

In our previous post, we define a common problem around reproducible build plans. The solution we desired was some form of cryptographic hash based configuration and download system for packages, package metadata, and snapshot definitions. This blog post will describe a potential concrete implementation.

Federated download service

Who hosts these files? Well, anyone. The level of trust needed in a service providing these files once again reduces to trusting cryptographic hashes. For example, if the protocol was simply to download the hash deadbeef from http://totally-not-a-hacker.com/sha256/deadbeef, that would be safe. It may not be reliable, because that website could go down, but I wouldn't worry about building with the wrong version of the package.

This opens up a world where the complete federation of package hosting is possible. We could have a discovery mechanism for finding new hosts, a mechanism to detect broken hosts or hosts that regularly return incorrect data, and anything else we can dream up. Also, these hosts would not need to be tied to any specific language ecosystem. The same host could happily provide downloads for a wide range of content, since it will all be referenced via cryptographic hash.

As a simple beginning, we can define a wire protocol (discussed below), provide a config value for how to contact the server, and run a single, centralized host. Expanding in the future is certainly possible.

More efficient file content sharing

This is totally optional, and I hesitate to even include it in this blog post. But I'm going to do so anyway, at the risk of encouraging a bunch of bikeshedding on this one section. For the record: I'd rather focus the discussion on the rest of this document instead of this section, which only represents an optimization for disk space and bandwidth usage.

In the Haskell package ecosystem, and in many others as well, the primary mechanism for package storage is the tarball. This has a number of downsides, but the most important is that it replicates contents. Many versions of a package end up sharing identical content for many files. We end up storing and transferring that duplicated content unnecessarily.

Instead, we can have our file store additionally support tree storage. I'm unapologetically ripping off Git right now (see prior art below for why this isn't just Git). We would end up with some kind of a tree manifest that looks something like:

null terminated file path\NUL
sha256 of the file contents
is this executable?

Repeated many times. We could ensure that the same input always generates the same output with requirements around the ordering of the file paths. And there may be some extra metadata on files in a tree that should be stored, such as modification date. However, in my experience, these are the only three things you actually want for reproducible builds.

With a system like this in place, downloading a package would look something like this:

  1. Look up the hash of the tree in the snapshot file
  2. Download the manifest from the server
  3. Check which file contents we don't have locally already
  4. Download all of the remaining file contents from the server

This is strictly not necessary, downloading tarballs instead will work just fine. But this is a nice optimization we can easily grab if desired.

Wire protocol

I'd recommend that we start off with an HTTPS-based wire protocol for this. This isn't because HTTPS is best suited for this case, but because it has the best success for traversing various firewalls.

Note that this may be an appropriate usecase for insecure HTTP instead of HTTPS, since the hashes themselves provide the security we're looking for. On the other hand, using insecure HTTP will still break privacy, in that an eavesdropper could determine which content you were downloading.

Anyway, in the future other protocols can be implemented as well, such as on raw TCP. Some kind of streaming protocol for supporting large files better may make sense. But at its most basic, I'm imagining something along the lines of:

  • GET /file/sha256/#hash returns the file contents as a response body
  • GET /tree/sha256/#hash returns the tree manifest as a response body
  • POST /file/sha256 allows you to upload a request body consisting of multiple SHA256s. The response body would be the file size, in decimal, followed by a colon, followed by the contents, followed by a comma. Or, in other words, netstrings. This alternative to the GET request will allow more efficient downloading of many files.
  • POST /tree/sha256 would be the same thing but for trees

Prior art

As I've discussed this over the past few weeks with coworkers and friends, the most recurring reference I've heard is to IPFS. I unfortunately have to admit some ignorance on IPFS right now, as I've only been able to briefly review it. My biggest concern from that initial review is speed: it may not be well optimized enough for a use case involving bulk downloading of many files at once. But this certainly requires more research. If anyone is familiar with IPFS and is interested in the problem described here, please drop a comment.

But the most blatant prior art is really Git. Anyone familiar with Git's blob/tree objects will see what a striking similarity the proposal above has. There are some simplifications above due to our alternate use case (totally immutable file and package storage):

  • There are no commits at all
  • There's no way to modify files or trees
  • As an optimization, instead of trees-of-trees like Git uses (which is great for optimizing a mutable tree case), we use one deep tree
  • Importantly: instead of SHA1, we use the more secure SHA256.
    • Side note: I haven't scoped it out fully above, but proper implementation of this proposal should ensure the easy ability to extend to other hash functions in the future

Additionally, I have some prior experience with using Git for a file storage mechanism. In previous versions of Stack, we used a Git-based approach for storing cabal files. We've since moved over to Hackage Security, the package index mechanism utilized by Hackage (which is upstream of the Stackage project). There are some downsides with depending on Git for this kind of package index storage:

  • It can be quite difficult to rely on an external command line tool for core functionality like this. Ensuring the tool is available, dealing with multiple versions of the tool, and so on are annoyances. Having a clear wire protocol and local storage mechanism can be much simpler.
  • Git is not well set up for downloading a subset of blobs and trees, something we explicitly want to support here.
  • We've had reports of very slow git clone speeds for people in different regions. Unfortunately, we've also recently started receiving these same complaints about Hackage Security updates. It may simply be that the Hackage package index is large enough to cause trouble. However, two aspects of this proposal will hopefully improve the situation:
    • It's designed from the ground up for efficient transfer of lots of small files
    • Due to its nature, we can download only the set of metadata files necessary for a specific snapshot, not the entire package index

The biggest thing that prevented me from writing this blog post was how much overlap exists with Git, and a desire to avoid a Not Invented Here solution. But the differences with Git are large enough that I think a new implementation is, in this case, warranted.


This is all just a design right now. And the design will hopefully help others in a similar situation. But concretely for the Haskell, Stackage and Stack use case, the current rollout plan looks something like this:

  • Implement the storage solution described above
  • Begin mirroring all of Hackage into this storage solution
  • Begin including the relevant SHA256s for this storage solution in future Stackage snapshots (in addition to all current metadata)
  • Add support to Stack to use the SHA256 hashes instead of the current download mechanisms

This plan is designed to be implemented in reasonable chunks, allow thorough testing of each component on its own, respect complete backwards compatibility, and seamlessly upgrade users to a more secure experience when complete.

If anyone's interested in collaborating on this, please reach out, there is plenty of work (and a lot of it quite fun stuff!) to be done. And for that matter, if anyone outside of the Haskell community is interesting in creating an implementation of this, having cross language compatibility would be awesome.

Topics: Haskell, haskell stack, hash based package

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