# Signature Transparency Logging
Signature Transparency Logging allows you to add signed checksums into an
append-only and tamper-evident log.  What a checksum represents is up to the
submitter.  For example, it could be a cryptographic hash of a Firefox update, a
Debian package, or a document.  You can use Signature Transparency Logging to:
1. Discover which signatures were produced by what secret signing keys.
2. Be sure that everyone observes the same signed checksums.

We abbreviate Signature Transparency Logging as _siglog_.

## 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 siglog can help by adding transparency.

For each binary you can log a signed checksum that corresponds to that binary.
If a signed checksum 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.  TODO: fixme.

For example, binaries (maliciously signed or not) that have yet to be logged can
be detected by a monitor.  To make the most out of siglog, end-users should
enforce public logging sometime in the future.  This means that a binary in the
above example would be _rejected_ unless a corresponding signed checksum is
logged.  Such enforcement will require a gradual roll-out to be realistic.

## Design considerations
We had several design considerations in mind while developing siglog.  A short
preview is listed below.  Please refer to our [design document](https://github.com/system-transparency/stfe/blob/main/doc/design.md)
and [API specification](https://github.com/system-transparency/stfe/blob/main/doc/api.md)
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 siglog as a [Trillian](https://transparency.dev/#trillian)
[personality](https://github.com/google/trillian/blob/master/docs/Personalities.md).
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`.

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