Merge branch 'api-refactor-take-two' into main

1 files changed, 186 insertions, 178 deletions

diff --git a/doc/api.md b/doc/api.md
index eb35df4..ea5a2a0 100644
--- a/doc/api.md
+++ b/doc/api.md
@@ -1,12 +1,12 @@
 # Sigsum Logging API v0
 This document outlines the sigsum logging API, version 0.  The broader picture
 is not explained here.  We assume that you are already familiar with the sigsum
-logging [design document](https://github.com/sigsum/sigsum/blob/main/doc/design.md).
+logging [design document](https://git.sigsum.org/sigsum/tree/doc/design.md).
 
 **Warning.**
 This is a work-in-progress document that may be moved or modified.
 
-## Overview
+## 1 - Overview
 A log implements an HTTP(S) API for accepting requests and sending responses.
 
 - Requests without any input data use the HTTP GET method.
@@ -18,46 +18,41 @@ ASCII-encoded key/value pairs.
 The motivation for using a text based key/value format for request and
 response data is that it is simple to parse.  Note that this format is
 not being used for the serialization of signed or logged data, where a
-more well defined and storage efficient format is desirable.  A
-submitter should distribute log responses to their end-users in any
-format that suits them.  The (de)serialization required for
-_end-users_ is a small subset of Trunnel.  Trunnel is an "idiot-proof"
-wire-format in use by the Tor project.
-
-## Primitives
-### Cryptography
+more well defined and storage efficient format is desirable.
+
+A claimant should distribute log responses to their believers in any format that
+suits them.  The (de)serialization required for _believers_ is a small subset of
+Trunnel.  Trunnel is an "idiot-proof" wire-format in use by the Tor project.
+
+## 2 - Primitives
+### 2.1 - Cryptography
 Logs use the same Merkle tree hash strategy as
 [RFC 6962,§2](https://tools.ietf.org/html/rfc6962#section-2).
 The hash functions must be
 [SHA256](https://csrc.nist.gov/csrc/media/publications/fips/180/4/final/documents/fips180-4-draft-aug2014.pdf).
-Logs must sign tree heads using
-[Ed25519](https://tools.ietf.org/html/rfc8032).  Log witnesses
-must cosign signed tree heads using Ed25519.
+Logs and witnesses must sign tree heads using
+[Ed25519](https://tools.ietf.org/html/rfc8032).
 
 All other parts that are not Merkle tree related should use SHA256 as
 the hash function.  Using more than one hash function would increases
 the overall attack surface: two hash functions must be collision
 resistant instead of one.
 
-### Serialization
+### 2.2 - Serialization
 Log requests and responses are transmitted as ASCII-encoded key/value
 pairs, for a smaller dependency than an alternative parser like JSON.
 Some input and output data is binary: cryptographic hashes and
-signatures.  Binary data must be Base16-encoded, also known as hex
+signatures.  Binary data must be base16-encoded, also known as hex
 encoding.  Using hex as opposed to base64 is motivated by it being
-simpler, favoring ease of decoding and encoding over efficiency on the
-wire.
+simpler, favoring ease of decoding and encoding over efficiency on the wire.
 
-We use the
-[Trunnel](https://gitweb.torproject.org/trunnel.git) [description language](https://www.seul.org/~nickm/trunnel-manual.html)
+We use the [Trunnel](https://gitweb.torproject.org/trunnel.git)
+[description language](https://www.seul.org/~nickm/trunnel-manual.html)
 to define (de)serialization of data structures that need to be signed or
-inserted into the Merkle tree.  Trunnel is more expressive than the
-[SSH wire format](https://tools.ietf.org/html/rfc4251#section-5).
-It is about as expressive as the
+inserted into the Merkle tree.  It is about as expressive as the
 [TLS presentation language](https://tools.ietf.org/html/rfc8446#section-3).
-A notable difference is that Trunnel supports integer constraints.
-The Trunnel language is also readable by humans _and_ machines.
-"Obviously correct code" can be generated in C and Go.
+However, it is readable by humans _and_ machines.
+"Obviously correct code" can be generated in C, Go, etc.
 
 A fair summary of our Trunnel usage is as follows.
 
@@ -69,26 +64,34 @@ collection.  You should not need a general-purpose Trunnel
 you may use it though.  The main point of using Trunnel is that it
 makes a simple format explicit and unambiguous.
 
-#### Merkle tree head
-A tree head contains a timestamp, a tree size, and a root hash.  The timestamp
-is included so that monitors can ensure _liveliness_.  It is the time since the
-UNIX epoch (January 1, 1970 00:00 UTC) in seconds.  The tree size specifies the
-current number of leaves.  The root hash fixes the structure and content of the
-Merkle tree.
+### 2.3 - Merkle tree
+#### 2.3.1 - Tree head
+A tree head contains a timestamp, a tree size, a root hash, and a key hash.
 
 ```
 struct tree_head {
 	u64 timestamp;
 	u64 tree_size;
 	u8 root_hash[32];
+	u8 key_hash[32];
 };
 ```
+`timestamp` is the time since the UNIX epoch (January 1, 1970 00:00 UTC) in
+seconds.  It is included so that monitors can be convinced of _freshness_ if
+enough witnesses added their cosignatures, see below.
+
+`tree_size` is the number of leaves in a log.
 
-#### (Co)signed Merkle tree head
-A log signs the serialized tree head using Ed25519.  A witness cosigns the
-serialized _signed tree head_ using Ed25519.  This means that a witness
-signature can not be mistaken for a log signature and vice versa.
+`root_hash` is a Merkle tree root hash that fixes a log's structure and content.
 
+`key_hash` is a log's hashed public key.  The key is encoded as defined in
+[RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2)
+before hashing it.  The result is used as a unique log identifier that prevents
+an [attack](https://git.sigsum.org/sigsum/tree/archive/2021-08-10-witnessing-broader-discuss#n95)
+in multi-log ecosystems.
+
+#### 2.3.2 - (Co)signed tree head
+A signed tree head contains a tree head and a signature.
 ```
 struct signed_tree_head {
 	struct tree_head tree_head;
@@ -96,87 +99,96 @@ struct signed_tree_head {
 };
 ```
 
-A witness must not cosign a signed tree head if it is inconsistent with prior
-history or if the timestamp is backdated or future-dated more than 12 hours.
+`tree_head` describes a log's state, see Section 2.3.1.
+
+`signature` is a log's signature over `tree_head`.
+
+A witness cosigns the same serialized _tree head_.  Note that tree heads are
+scoped to a specific log to ensure that a witness signature for log X cannot be
+confused with a witness signature for log Y.
 
-#### Merkle tree leaf
-Logs support a single leaf type.  It contains a shard hint, a
-checksum, a signature, and a key hash.
+A witness must not cosign a tree head if it is inconsistent with prior history
+or if the timestamp is backdated more than 5 minutes.  A witness can be viewed
+as playing two roles: Verifier(append-only) and Verifier(freshness)
+[\[ts\]](https://git.sigsum.org/sigsum/tree/archive/2021-08-31-checkpoint-timestamp-continued#n84).
+
+#### 2.3.3 - Tree leaf
+Logs support a single leaf type.  It contains a claimant's statement,
+signature, and key hash.
 
 ```
-struct tree_leaf {
+struct statement {
     u64 shard_hint;
     u8 checksum[32];
+}
+
+struct tree_leaf {
+    struct statement statement;
     u8 signature[64];
     u8 key_hash[32];
 }
 ```
 
-`shard_hint` is chosen by the submitter to match the log's shard interval, see
-design document.
+`shard_hint` must match a log's shard interval and is determined by a claimant.
 
-`checksum` is computed by the submitter and represents some opaque data.
+`checksum` represents some opaque data and is computed by a claimant.
 
-`signature` is a signature over the serialized `shard_hint` and `checksum`.
-It must be possible to verify the signature using the submitter's public
-verification key.
+`signature` is a signature over a serialized `statement`.  It must be possible
+to verify this signature using the claimant's public verification key.
 
-`key_hash` is a hash of the submitter's public verification key.  It is included
-in `tree_leaf` so that the leaf can be attributed to the submitter.  A hash,
+`key_hash` is a hash of the claimant's public verification key.  It is included
+in `tree_leaf` so that each leaf can be attributed to a claimant.  A hash,
 rather than the full public key, is used to motivate verifiers to locate the
 appropriate key and make an explicit trust decision.
 
-## Public endpoints
-Every log has a base URL that identifies it uniquely.  The only
-constraint is that it must be a valid HTTP(S) URL that can have the
-`/sigsum/v0/<endpoint>` suffix appended.  For example, a complete endpoint
-URL could be `https://log.example.com/2021/sigsum/v0/get-tree-head-cosigned`.
-
-Input data (in requests) is POST:ed in the HTTP message body as ASCII
-key/value pairs.
+## 3 - Public endpoints
+Every log has a fixed and unique base URL.  It must be a valid HTTP(S) URL that
+can have the `/sigsum/v0/<endpoint>` suffix appended.  Example of a base URL:
+`https://log.example.com/2021`.
+
+Input data (in requests) is POST:ed in the HTTP message body as line-terminated
+ASCII key/value pairs.  In more detail, the key-value format is `Key=Value\n`.
+Everything before the first equal-sign is considered a key.
+Everything after the first equal sign and before the next new line character is
+considered a value.  Different keys may appear in any order.  A key may be
+repeated, in which case the relative order must be preserved.  Example:
+```
+blue=first value for blue key
+red=some value for red key
+blue=second value for blue key
+```
 
-Output data (in replies) is sent in the HTTP message body in the same
-format as the input data, i.e. as ASCII key/value pairs on the format
-`Key=Value`
+Output data (in replies) is sent in the HTTP message body using the same
+key-value format as input data.
 
 The HTTP status code is 200 OK to indicate success.  A different HTTP
-status code is used to indicate failure, in which case a log should
-respond with a human-readable string describing what went wrong using
-the key `error`. Example: `error=Invalid signature`.
+status code is used to indicate failure.  A log must respond with a
+human-readable string describing what went wrong using the key `error`.
+Example:
+```
+error=Invalid signature
+```
 
-### get-tree-head-cosigned
-Returns the latest cosigned tree head. Used together with
-`get-proof-by-hash` and `get-consistency-proof` for verifying the tree.
+### 3.1 - get-tree-head-latest
+Returns the latest signed tree head.  Used for debugging purposes.
 
 ```
-GET <base url>/sigsum/v0/get-tree-head-cosigned
+GET <base url>/sigsum/v0/get-tree-head-latest
 ```
 
 Input:
 - None
 
-Output on success:
-- `timestamp`: `tree_head.timestamp` ASCII-encoded decimal number,
-  seconds since the UNIX epoch.
-- `tree_size`: `tree_head.tree_size` ASCII-encoded decimal number.
-- `root_hash`: `tree_head.root_hash` hex-encoded.
-- `signature`: hex-encoded Ed25519 log signature over `timestamp`,
-  `tree_size` and `root_hash` serialized into a `tree_head` as
-  described in section `Merkle tree head`.
-- `cosignature`: hex-encoded Ed25519 witness signature over `timestamp`,
-  `tree_size`, `root_hash`, and `signature` serialized into a `signed_tree_head`
-  as described in section `(Co)signed Merkle tree head`.
-- `key_hash`: a hash of the witness verification key that can be used to
-  verify the above `cosignature`.  The key is encoded as defined
-  in [RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2), 
-  and then hashed using SHA256.  The hash value is hex-encoded.
+Output:
+- `timestamp`: `tree_head.timestamp`, ASCII-encoded decimal number.
+- `tree_size`: `tree_head.tree_size`, ASCII-encoded decimal number.
+- `root_hash`: `tree_head.root_hash`, hex-encoded.
+- `signature`: log signature over a serialized `tree_head`, hex-encoded.
 
-The `cosignature` and `key_hash` fields may repeat. The first witness signature
-corresponds to the first key hash, the second witness signature corresponds to
-the second key hash, etc.  At least one witness signature must be returned on
-success.  The number of witness signatures and key hashes must match.
+Note that `tree_head.key_hash` is known by the querying party.  Therefore, it is
+not returned in Sections 3.1-3.3.
 
-### get-tree-head-to-sign
+### 3.2 - get-tree-head-to-sign
 Returns the latest signed tree head to be cosigned.  Used by witnesses.
 
 ```
@@ -187,93 +199,97 @@ Input:
 - None
 
 Output on success:
-- `timestamp`: `tree_head.timestamp` ASCII-encoded decimal number,
-  seconds since the UNIX epoch.
-- `tree_size`: `tree_head.tree_size` ASCII-encoded decimal number.
-- `root_hash`: `tree_head.root_hash` hex-encoded.
-- `signature`: hex-encoded Ed25519 log signature over `timestamp`,
-  `tree_size` and `root_hash` serialized into a `tree_head` as
-  described in section `Merkle tree head`.
-
-### get-tree-head-latest
-Returns the latest signed tree head.  Used for debugging purposes.
+- `timestamp`: `tree_head.timestamp`, ASCII-encoded decimal number.
+- `tree_size`: `tree_head.tree_size`, ASCII-encoded decimal number.
+- `root_hash`: `tree_head.root_hash`, hex-encoded.
+- `signature`: log signature over a serialized `tree_head`, hex-encoded.
+
+### 3.3 - get-tree-head-cosigned
+Returns the latest cosigned tree head. Used together with `get-inclusion-proof`
+and `get-consistency-proof`.  Ensures that verifiers see the same statements as
+believers.
 
 ```
-GET <base url>/sigsum/v0/get-tree-head-latest
+GET <base url>/sigsum/v0/get-tree-head-cosigned
 ```
 
-Input an output follows the same formatting as `get-tree-head-to-sign`.
+Input:
+- None
+
+Output on success:
+- `timestamp`: `tree_head.timestamp`, ASCII-encoded decimal number.
+- `tree_size`: `tree_head.tree_size`, ASCII-encoded decimal number.
+- `root_hash`: `tree_head.root_hash`, hex-encoded.
+- `signature`: log signature over a serialized `tree_head`, hex-encoded.
+- `cosignature`: witness signature over the same serialized `tree_head`,
+  hex-encoded.
+- `key_hash`: hashed witness verification key that can be used to verify the
+  above cosignature.  The key is encoded as defined in [RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2)
+  before hashing.  The resulting hash value is hex-encoded.
+
+The `cosignature` and `key_hash` fields may repeat. The first witness signature
+corresponds to the first key hash, the second witness signature corresponds to
+the second key hash, etc.  At least one witness signature must be returned on
+success.  The number of witness signatures and key hashes must match.
 
-### get-proof-by-hash
+### 3.4 - get-inclusion-proof
 ```
-POST <base url>/sigsum/v0/get-proof-by-hash
+POST <base url>/sigsum/v0/get-inclusion-proof
 ```
 
 Input:
 - `leaf_hash`: leaf identifying which `tree_leaf` the log should prove
   inclusion of, hex-encoded.
 - `tree_size`: tree size of the tree head that the proof should be
-  based on, as an ASCII-encoded decimal number.
+  based on, ASCII-encoded decimal number.
 
 Output on success:
-- `tree_size`: tree size that the proof is based on, as an
+- `leaf_index`: zero-based index of the leaf that the proof is based on,
   ASCII-encoded decimal number.
-- `leaf_index`: zero-based index of the leaf that the proof is based
-  on, as an ASCII-encoded decimal number.
 - `inclusion_path`: node hash, hex-encoded.
 
 The leaf hash is computed using the RFC 6962 hashing strategy.  In
-other words, `SHA256(0x00 | tree_leaf)`.
+other words, `H(0x00 | tree_leaf)`.
 
-`inclusion_path` may be omitted or repeated to represent an inclusion
-proof of zero or more node hashes.  The order of node hashes follow
-from the hash strategy, see RFC 6962.
+`inclusion_path` must contain one or more hashes.  The order of node hashes
+follow from the hash strategy, see RFC 6962.
 
 Example: `echo "leaf_hash=241fd4538d0a35c2d0394e4710ea9e6916854d08f62602fb03b55221dcdac90f
-tree_size=4711" | curl --data-binary @- localhost/sigsum/v0/get-proof-by-hash`
+tree_size=4711" | curl --data-binary @- localhost/sigsum/v0/get-inclusion-proof`
 
-### get-consistency-proof
+### 3.5 - get-consistency-proof
 ```
 POST <base url>/sigsum/v0/get-consistency-proof
 ```
 
 Input:
-- `new_size`: tree size of a newer tree head, as an ASCII-encoded
-  decimal number.
-- `old_size`: tree size of an older tree head that the log should
-  prove is consistent with the newer tree head, as an ASCII-encoded
-  decimal number.
+- `new_size`: tree size of a newer tree head, ASCII-encoded decimal number.
+- `old_size`: tree size of an older tree head that the log should prove is
+  consistent with the newer tree head, ASCII-encoded decimal number.
 
 Output on success:
-- `new_size`: tree size of the newer tree head that the proof is based
-  on, as an ASCII-encoded decimal number.
-- `old_size`: tree size of the older tree head that the proof is based
-  on, as an ASCII-encoded decimal number.
 - `consistency_path`: node hash, hex-encoded.
 
-`consistency_path` may be omitted or repeated to represent a
-consistency proof of zero or more node hashes.  The order of node
+`consistency_path` must contain one or more hashes.  The order of node
 hashes follow from the hash strategy, see RFC 6962.
 
 Example: `echo "new_size=4711
 old_size=42" | curl --data-binary @- localhost/sigsum/v0/get-consistency-proof`
 
-### get-leaves
+### 3.6 - get-leaves
 ```
 POST <base url>/sigsum/v0/get-leaves
 ```
 
 Input:
-- `start_size`: index of the first leaf to retrieve, as an
-  ASCII-encoded decimal number.
-- `end_size`: index of the last leaf to retrieve, as an ASCII-encoded
-  decimal number.
+- `start_size`: index of the first leaf to retrieve, ASCII-encoded decimal
+  number.
+- `end_size`: index of the last leaf to retrieve, ASCII-encoded decimal number.
 
 Output on success:
-- `shard_hint`: `tree_leaf.message.shard_hint` as an ASCII-encoded
-  decimal number.
-- `checksum`: `tree_leaf.message.checksum`, hex-encoded.
-- `signature`: `tree_leaf.signature_over_message`, hex-encoded.
+- `shard_hint`: `tree_leaf.statement.shard_hint`, ASCII-encoded decimal number.
+- `checksum`: `tree_leaf.statement.checksum`, hex-encoded.
+- `signature`: `tree_leaf.signature`, hex-encoded.
 - `key_hash`: `tree_leaf.key_hash`, hex-encoded.
 
 All fields may be repeated to return more than one leaf.  The first
@@ -282,51 +298,36 @@ list refers to the second leaf, etc.  The size of each list must
 match.
 
 A log may return fewer leaves than requested.  At least one leaf
-must be returned on HTTP status code 200 OK.
+must be returned on success.
 
 Example: `echo "start_size=42
 end_size=4711" | curl --data-binary @- localhost/sigsum/v0/get-leaves`
 
-### add-leaf
+### 3.7 - add-leaf
 ```
 POST <base url>/sigsum/v0/add-leaf
 ```
 
 Input:
-- `shard_hint`: number within the log's shard interval as an
-  ASCII-encoded decimal number.
-- `checksum`: the cryptographic checksum that the submitter wants to
-  log, hex-encoded.
-- `signature`: the submitter's signature over `tree_leaf.shard_hint` and
-  `tree_leaf.checksum`, see section `Merkle tree leaf`.  The resulting signature
-  is hex-encoded.
-- `verification_key`: the submitter's public verification key.  The
-  key is encoded as defined in
-  [RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2)
-  and then hex-encoded.
-- `domain_hint`: domain name indicating where `tree_leaf.key_hash`
-  can be found as a DNS TXT resource record with hex-encoding.
+- `shard_hint`: `tree_leaf.statement.shard_hint`, ASCII-encoded decimal number.
+- `checksum`: `tree_leaf.statement.checksum`, hex-encoded.
+- `signature`: `tree_leaf.signature`, hex-encoded.
+- `verification_key`: public verification key that can be used to verify the
+  above signature.  The key is encoded as defined in [RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2),
+  then hex-encoded.
+- `domain_hint`: domain name indicating where `tree_leaf.key_hash` can be found
+  as a DNS TXT resource record with hex-encoding.
 
 Output on success:
 - None
 
-The submission will not be accepted if `signature` is
-invalid or if the key hash retrieved using `domain_hint` does not
-match a hash over `verification_key`.
+A submission will not be accepted if `signature` is invalid or if the retrieved
+key hash does not match the specified verification key.  A submission may also
+not be accepted if the second-level domain name exceeded its rate limit.
 
-The submission may also not be accepted if the second-level domain
-name exceeded its rate limit.  By coupling every add-leaf request to
-a second-level domain, it becomes more difficult to spam logs.  You
-would need an excessive number of domain names.  This becomes costly
-if free domain names are rejected.
-
-Logs don't publish domain-name to key bindings because key
-management is more complex than that.
-
-Public logging should not be assumed to have happened until an
-inclusion proof is available.  An inclusion proof should not be relied
-upon unless it leads up to a trustworthy signed tree head.  Witness
-cosigning can make a tree head trustworthy.
+Public logging must not be assumed to have happened until an inclusion proof is
+available.  An inclusion proof should not be relied upon unless it leads up to a
+trustworthy tree head.  Witness cosigning makes a tree head trustworthy.
 
 Example: `echo "shard_hint=1640995200
 checksum=cfa2d8e78bf273ab85d3cef7bde62716261d1e42626d776f9b4e6aae7b6ff953
@@ -334,35 +335,42 @@ signature=c026687411dea494539516ee0c4e790c24450f1a4440c2eb74df311ca9a7adf2847b99
 verification_key=c9a674888e905db1761ba3f10f3ad09586dddfe8581964b55787b44f318cbcdf
 domain_hint=example.com" | curl --data-binary @- localhost/sigsum/v0/add-leaf`
 
-### add-cosignature
+### 3.8 - add-cosignature
 ```
 POST <base url>/sigsum/v0/add-cosignature
 ```
 
 Input:
-- `cosignature`: Ed25519 witness signature over `signed_tree_head`, hex-encoded.
-- `key_hash`: hash of the witness' public verification key that can be
-  used to verify `cosignature`.  The key is encoded as defined in
-  [RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2),
-  and then hashed using SHA256. The hash value is hex-encoded.
+- `cosignature`: witness signature over `tree_head`, hex-encoded.
+- `key_hash`: hashed witness verification key that can be used to verify the
+  above cosignature.  The key is encoded as defined in [RFC 8032, section 5.1.2](https://tools.ietf.org/html/rfc8032#section-5.1.2)
+  prior to hashing.  The resulting hash value is hex-encoded.
 
 Output on success:
 - None
 
-`key_hash` can be used to identify which witness cosigned a signed tree
-head.  A key-hash, rather than the full verification key, is used to
-motivate verifiers to locate the appropriate key and make an explicit
-trust decision.
+`key_hash` can be used to identify which witness cosigned a tree head.  A
+key-hash, rather than the full verification key, is used to motivate verifiers
+to locate the appropriate key and make an explicit trust decision.
+
+Note that logs must be configured with relevant witness public keys.
 
 Example: `echo "cosignature=d1b15061d0f287847d066630339beaa0915a6bbb77332c3e839a32f66f1831b69c678e8ca63afd24e436525554dbc6daa3b1201cc0c93721de24b778027d41af
 key_hash=662ce093682280f8fbea9939abe02fdba1f0dc39594c832b411ddafcffb75b1d" | curl --data-binary @- localhost/sigsum/v0/add-cosignature`
 
-## Summary of log parameters
-- **Public key**: The Ed25519 verification key to be used for
-  verifying tree head signatures.
-- **Shard interval start**: The earliest time at which logging
+## 4 - Parameter summary
+Ed25519 as signature scheme. SHA256 as hash function.
+
+### 4.1 - Log
+- **Public key**: public verification key that is used to verify tree head
+  signatures.
+- **Base URL**: Where the log can be reached over HTTP(S).  It is the
+  prefix to be used to construct a version 0 specific endpoint.
+- **Shard interval start**: the earliest time at which logging
   requests are accepted as the number of seconds since the UNIX epoch.
 - **Shard interval end**: The latest time at which logging
   requests are accepted as the number of seconds since the UNIX epoch.
-- **Base URL**: Where the log can be reached over HTTP(S).  It is the
-  prefix to be used to construct a version 0 specific endpoint.
+
+### 4.2 - Witness
+- **Public key**: public verification key that is used to verify tree head
+  cosignatures.