sigsum-log-go

Sigsum logging brings transparency to signed checksums. What a checksum represents is up to you. For example, it could be the cryptographic hash of a provenance file, a Firefox binary, or a text document.

Sigsum logging can be used to: 1. Discover which checksum signatures were produced by what secret signing keys. 2. Be sure that everyone observes the same signed checksums.

How it works

Suppose that you develop software and publish binaries. You sign those binaries and make them available to users in a package repository and on your website. You are committed to distribute the same signed binaries to every user. That is an easy claim to make. However, word is cheap and sometimes things go wrong. How would you even know if your signing infrastructure got compromised? A few select users might already receive maliciously signed binaries that include a backdoor. This is where we can help by adding transparency.

For each binary you can log a signed checksum that corresponds to that binary. If such a sigsum appears in the log that you did not expect: excellent, now you know that your signing infrastructure was compromised at some point. Similarly, you can also detect if a binary from your website or package repository misses a corresponding log entry by inspecting the log. The claim that the same binaries are published for everyone can be verified.

Starting to apply the pattern of transparent logging is already an improvement without any end-user enforcement. It becomes easier to detect honest mistakes and attacks against your website or package repository.

To make the most out of a sigsum log, end-users should start to enforce public logging in the future. This means that a binary in the above example would be rejected unless a corresponding sigsum is publicly logged.

Design considerations

We had several design considerations in mind while developing sigsum logging. A short preview is listed below. Refer to our design document and API specification for additional details. Feedback is welcomed and encouraged! - Preserved data flows: an end-user can enforce transparent logging without making additional outbound network connections. Proofs of public logging should be provided using the same distribution mechanism as the data. In the above example the software publisher would put these proofs into their package repository. - Sharding to simplify log life cycles: starting to operate a log is easier than closing it down in a reliable way. We have a predefined sharding interval that determines the time during which the log will be active. - Defenses against log spam and poisoning: to maximize a log's utility it should be open for anyone to use. However, accepting logging requests from anyone at arbitrary rates can lead to abusive usage patterns. We store as little metadata as possible to combat log poisoning. We piggyback on DNS to combat log spam. - Built-in mechanisms that ensure a globally consistent log: transparency logs rely on gossip protocols to detect forks. We built a proactive gossip protocol directly into the log. It is based on witness cosigning. - No cryptographic agility: the only supported signature scheme is Ed25519. The only supported hash function is SHA256. Not having any cryptographic agility makes the protocol and the data formats simpler and more secure. - Few and simple (de)serialization parsers: complex (de)serialization parsers increase attack surfaces and make the system more difficult to use in constrained environments. End-users need a small subset of Trunnel to work with signed and logged data. The log's network clients also need to parse ASCII key-value pairs.

Public prototype

We implemented sigsum logging as a Trillian personality. A public prototype is up and running with zero promises of uptime, stability, etc. The log's base URL is http://tlog-poc.system-transparency.org:4780/st/v0. The log's public verification key is bc9308dab23781b8a13d59a9e67bc1b8c1585550e72956525a20e479b1f74404.

An experimental witness is also up and running with zero promises of uptime, stability, etc. The public verification key is 777528f5fd96f95713b8c2bb48bce2c83628e39ad3bfbd95bc0045b143fe5c34.

You can talk to the log by passing ASCII key-value pairs. For example, fetch a tree head and a log entry:

$ echo "TODO: update to sigsum links"
$ curl http://tlog-poc.system-transparency.org:4780/st/v0/get-tree-head-latest
timestamp=1623053394
tree_size=1
root_hash=f337c7045b3233a921acc64688b729816a10f95f8be00910418aaa3c71245d5d
signature=50e88b935f6010dedb61314685371d16bf180be99bbd3463a0b6934be78c11ebf8cc81688e7d11b0dc593f2ea0453f6be8ed60abb825b5a08535a68cc007e20e
key_hash=2c27a6bafcbe210753c64666ca108025c68f28ded8933ebb2c4ef0987d7a6302
$
$ printf "start_size=0\nend_size=0\n" | curl --data-binary @- http://tlog-poc.system-transparency.org:4780/st/v0/get-leaves
shard_hint=0
checksum=0000000000000000000000000000000000000000000000000000000000000000
signature_over_message=0e0424c7288dc8ebec6b2ebd45e14e7d7f86dd7b0abc03861976a1c0ad8ca6120d4efd58aeab167e5e84fcffd0fab5861ceae85dec7f4e244e7465e41c5d5207
key_hash=9d6c91319b27ff58043ff6e6e654438a4ca15ee11dd2780b63211058b274f1f6

We are currently working on tooling that makes it easier to interact with the log.