We're happy to announce that all users of Haskell packages can
now securely download packages. As a tl;dr, here are the changes
you need to make:
- Add the
relevant GPG key by
following the instructions
- Install stackage-update
cabal update && cabal install stackage
- From now on, replace usage of
cabal update with
stk update --verify --hashes
- From now on, replace usage of
cabal install ...
stk install ...
This takes advantage of the all-cabal-hashes
repository, which contains cabal files that are modified to contain
package hashes and sizes. The way we generate the all-cabal-hashes
is interesting in its own right, but I won't shoehorn that
discussion into this blog post. Wait for a separate blog post soon
for a description of our lightweight architecture for this.
Note that this is an implementation of
Mathieu's secure distribution proposal, with some details
modified to work with the current state of our tooling (i.e., lack
of package hash information from Hackage).
How it works
The all-cabal-hashes repository contains all of the cabal files
Hackage knows about. These cabal files are tweaked to have a few
extra metadata fields, including cryptographic hashes of the
package tarball and the size of the package, in bytes. (It also
contains the same data in a JSON file, which is what we currently
use due to cabal issue
#2585.) There is also a tag on the repo,
current-hackage, which always points at the latest
commit and is GPG signed. (If you're wondering, we use a tag
instead of just commit signing since it's easier to verify a tag
When you run
stk update --verify --hashes, it
fetches the latest content from that repository, verifies the GPG
signature, generates a
00-index.tar file, and places
it in the same location that
cabal update would place
it. At this point, you have a verified package index on your
location machine, which contains cryptographic signatures and sizes
for each package tarball.
Now, when you run
stk install ..., the
stackage-install tool handles all downloads for you (subject to
caveats, like cabal issue
#2566). stackage-install will look up all of the hashes and
sizes that are present in your package index, and verify them
during download. In particular:
- If the server tries to send more data than expected, the
download stops immediately and an exception is thrown.
- If the server sends less data than expected, an exception is
- If the hash does not match what was expected, an exception is
Only when the hash and size match does the file get written. In
this way, tarballs are only made available to the rest of your
build tools after they have been verified.
What about Windows?
In mailing list discussions, some people were concerned about
supporting Windows, in particular that Git and GPG may be difficult
to install and configure on Windows. But as I
shared on Google+ last week, MinGHC will now be shipping
with both of those tools. I've tested things myself on Windows with
the new versions of MinGHC, stackage-update, and stackage-install,
and the instructions above worked without a hitch.
Of course, if others discover problems- either on Windows or
elsewhere- please report them so they can be fixed.
Speed and reliability
In addition to the security benefits of this tool chain, there
are also two other obvious benefits. By downloading the package
index updates via Git, we are able to download only the differences
since the last time we downloaded. This leads to less bandwidth
usage and a quicker download.
This toolchain also replaces connections to Hackage with two
high reliability services: Amazon S3 (which holds the package
contents) and Github. Using off the shelf, widely used services in
place of hosting everything ourself reduces our community burden
and increases our ecosystem's reliability.
There are unfortunately still some caveats with this.
- The biggest hole in the fence is that we have no way of
securing distribution of packages from Hackage itself. While
all-cabal-hashes downloads the package index from Hackage via HTTPS
(avoiding MITM attacks), there are still other attack vectors to be
concerned about (such as breaching the Hackage server itself). The
improved Hackage security page documents many of these
concerns. Ideally, Hackage would be modified to perform package
index signing itself.
- Due to cabal issue
#2566, it's still possible that cabal-install may download
packages for you instead of stackage-install, though these
situations should be rare. Hopefully integrating this download code
directly with a build tool will eliminate that weakness.
- There is still no verification of package author signatures, so
that if someone's Hackage credentials are compromised (which is
unfortunately very probable), a corrupted package could be
present. This is something Chris Done and Tim Dysinger
are working on. We're looking for others in the community to
work with us on pushing forward on this. If you're interested,
please contact us.
What's great about this toolchain is how shallow it is. All of
the heavy lifting is handled by Git, GPG, Amazon S3, Github, and
(as you'll see in a later blog post) Travis CI. We mostly just wrap
around these high quality tools and services. Not only was this a
practical decision (reduce development time and code burden), but
also a security decision. Instead of creating a Haskell-only
security and distribution framework, we're reusing the same
components that are being tried and tested on a daily basis by the
greater software community. While this doesn't guarantee the
tooling we use is bug free, it does mean that the "many eyeballs"
Using preexisting tools also means that we open up the
possibility of use cases never before considered. For example,
someone contacted me (anonymity preserved) about a use case where
he wanted to be able to identify which version of Hackage was being
used. Until now, such a concept didn't exist. With a Git-based
package index, the Hackage version can be identified by its
I'm sure others will come up with new and innovative tricks to
pull off, and I look forward to hearing about them.
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