From ccdd98a67a483e61a8557e08d5c54739f7028b80 Mon Sep 17 00:00:00 2001
From: Rasmus Dahlberg <rasmus.dahlberg@kau.se>
Date: Wed, 10 Feb 2021 18:59:09 +0100
Subject: added witness cosigning protocol documentation

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+# Witness cosigning
+Transparency logs were designed to be cryptographically verifiable in the
+presence of a gossip-audit model that ensures everyone observes _the same
+cryptographically verifiable log_.  The gossip-audit model is largely undefined
+in today's existing transparency logging ecosystems, which means that the logs
+must be trusted to play by the rules.   We wanted to avoid that outcome in our
+ecosystem.  Therefore, a gossip-audit model is built into the log.
+
+The basic idea is that an STH should only be considered valid if it is cosigned
+by a number of witnesses that verify the append-only property.  Which witnesses
+to trust and under what circumstances is defined by a client-side _witness
+cosigning policy_.  For example,
+	"require no witness cosigning",
+	"must have at least `k` signatures from witnesses A...J", and
+	"must have at least `k` signatures from witnesses A...J where one is from
+		witness B".
+
+Witness cosigning policies are beyond the scope of this specification.
+
+The log is configured with a list of witness namespaces. The only supported
+witness namespace format is `ed25519_v1`.  The signature must span a serialized
+`signed_tree_head_v1` item.
+
+## Public endpoints
+Witnesses are expected to poll the `next-cosi` endpoint.  If a new STH is
+obtained:
+1. Verify that the log is append-only by fetching a consistency proof from the
+latest STH that this witness co-signed.  Stop if no valid proof is available.
+2. Sign the STH and submit using the `add-cosi` API.
+
+### add-cosi
+```
+POST https://<base url>/st/v1/add-cosi
+```
+
+Input:
+- sth: an `StItem` of type `signed_tree_head_v1`.
+- namespace: a `Namespace` item of type `ed25519_v1`.
+- signature: covers the specified STH.
+
+Output:
+- None
+
+### get-cosi
+```
+GET https://<base url>/st/v1/get-cosi
+```
+
+Input:
+- None
+
+Output:
+- A map with keys "sth" and "signatures".  The former is an `StItem` of type
+`signed_tree_head_v1`.  The latter an array where each entry is a map with a
+"witness" (`ed25519_v1` namespace) and a "signature".
+
+### next-cosi
+```
+GET https://<base url>/st/v1/next-cosi
+```
+
+Input
+- None
+
+Output:
+- an `StItem` of type `signed_tree_head_v1`, which corresponds to the STH
+that is currently being cosigned. Stable for a period of time, e.g., 10 minutes.
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