This server observes and records checksums of publicly downloadable artifacts. It offers verifiable proof that the claimed observations are immutable, i.e. this server cannot serve a different claim in the future.

It can be used to detect URLs whose content breaks immutability guarantees (e.g. usually versioned software releases).

$ GO111MODULE=on go get getsum.pub/getsum@latest

$ getsum https://some.com/url/to/a/file

getsum returns the hash of a file. The hash is fetched by getsum.pub by reading https://some.com/url/to/a/file.sha256 and falling back to https://some.com/url/to/a/SHA256SUMS (we'll add more ways to seeding the hash; although we're likeley keep avoiding downloading large files from the internet)

However, getsum.pub doesn't directly serve the hash to the getsum.pub client. Instead it generates a fake Go module getsum.pub/https/some.com/ovzgy/orxq/me/mzuwyzi (all path components are base32-encoded to ensure any URL is a valid Go import path) which contains the original URL and the sha256 checksum of it.

The getsum client then fetches this module using the https://proxy.golang.org and verifies the checksum using https://sum.golang.org. (read more about it here)

Thus we leverage an existing large scale transparent log to ensure that files are indeed immutable the original publisher of that URL never changes the file (and the published .sha file).

getsum doesn't download the file, you need to use a tool like curl or wget, but it can verify whether the file you just downloaded matches the published hash and that the hash hasn't been modified.

$ wget https://some.com/url/to/a/file \
  && getsum -c file https://some.com/url/to/a/file \
  && echo "good file, continue"