path: root/doc/api.md

# System Transparency Logging: API v0
This document describes details of the System Transparency logging
API, version 0.  The broader picture is not explained here.  We assume
that you have read the System Transparency Logging design document.
It can be found
[here](https://github.com/system-transparency/stfe/blob/design/doc/design.md).

**Warning.**
This is a work-in-progress document that may be moved or modified.

## Overview
The log implements an HTTP(S) API:

- Requests to the log use the HTTP GET method.
- Input data (in requests) and output data (in responses) are
  expressed as ASCII-encoded key/value pairs.
- Requests use HTTP entity headers for input data while responses use
  the HTTP message body for output data.
- Binary data is hex-encoded before being transmitted.

The motivation for using a text based key/value format for request and
response data is that it's 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 may 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
The log uses 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).
The log must sign tree heads using
[Ed25519](https://tools.ietf.org/html/rfc8032).  The log's witnesses
must also sign tree heads using Ed25519.

All other parts that are not Merkle tree related also 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
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
encoding.  Using hex as opposed to base64 is motivated by it being
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)
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
[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.

A fair summary of our Trunnel usage is as follows.

All integers are 64-bit, unsigned, and in network byte order.
Fixed-size byte arrays are put into the serialization buffer in-order,
starting from the first byte.  Variable length byte arrays first
declare their length as an integer, which is then followed by that
number of bytes.  These basic types are concatenated to form a
collection.  You should not need a general-purpose Trunnel
(de)serialization parser to work with this format.  If you have one,
you may use it though.  The main point of using Trunnel is that it
makes a simple format explicit and unambiguous.

#### Merkle tree head
Tree heads are signed by the log and its witnesses.  It 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.

```
struct tree_head {
	u64 timestamp;
	u64 tree_size;
	u8 root_hash[32];
};
```

The serialized tree head must be signed using Ed25519.  A witness must
not cosign a tree head if it is inconsistent with prior history or if
the timestamp is backdated or future-dated more than 12 hours.

#### Merkle tree leaf
The log supports a single leaf type.  It contains a shard hint, a
checksum over whatever the submitter wants to log a checksum for, a
signature that the submitter computed over the shard hint and the
checksum, and a hash of the submitter's public verification key, that
can be used to verify the signature.

```
struct message {
    u64 shard_hint;
    u8 checksum[32];
};

struct tree_leaf {
    struct message;
    u8 signature_over_message[32];
    u8 key_hash[32];
}
```

`message` is composed of the `shard_hint`, chosen by the submitter to
match the shard interval for the log, and the submitter's `checksum`
to be logged.

`signature_over_message` is a signature over `message`, using the
submitter's verification key. It must be possible to verify the
signature using the submitter's public verification key, as indicated
by `key_hash`.

`key_hash` is a hash of the submitter's verification key used for
signing `message`. It is included in `tree_leaf` so that the leaf can
be attributed to the submitter.  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
`/st/v0/<endpoint>` suffix appended.  For example, a complete endpoint
URL could be
`https://log.example.com/2021/st/v0/get-signed-tree-head`.

Input data (in requests) is sent as ASCII key/value pairs as HTTP
entity headers, with their keys prefixed with the string
`stlog-`. Example: For sending `treee_size=4711` as input a client
would send the HTTP header `stlog-tree_size: 4711`.

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`. Example: For sending `tree_size=4711` as output a log
would send an HTTP message body consisting of `stlog-tree_size: 4711`.

The HTTP status code is 200 OK to indicate success.  A different HTTP
status code is used to indicate failure.  The log should set the value
value for the key `error` to a human-readable string describing what
went wrong.  For example, `error: invalid signature`, `error: rate
limit exceeded`, or `error: unknown leaf hash`.

### get-tree-head-cosigned
Returns the latest cosigned tree head. Used together with
`get-proof-by-hash` and `get-consistency-proof` for verifying the log.

```
GET <base url>/st/v0/get-tree-head-cosigned
```

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 signature over `tree_head`
  serialzed as described in section `Merkle tree head`.
- "key_hash": a hash of the public verification key (belonging to
  either the log or to one of its witnesses), which can be used to
  verify the most recent `signature`.  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.

The "signature" and "key_hash" fields may repeat. The first signature
corresponds to the first key hash, the second signature corresponds to
the second key hash, etc.  The number of signatures and key hashes
must match.

### get-tree-head-to-sign
Returns the latest tree head to be signed by log witnesses. Used by
witnesses.

```
GET <base url>/st/v0/get-tree-head-to-sign
```

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 signature over `tree_head`
  serialzed as described in section `Merkle tree head`.
- "key_hash": a hash of the log's public verification key, which can
  be used to verify `signature`.  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.

There is exactly one `signature` and one `key_hash` field. The
`key_hash` refers to the log's public verification key.


### get-tree-head-latest
Returns the latest tree head, signed only by the log. Used for
debugging purposes.

```
GET <base url>/st/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 signature over `tree_head`
  serialzed as described in section `Merkle tree head`.
- "key_hash": a hash of the log's public verification key that can be
  used to verify `signature`.  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.

There is exactly one `signature` and one `key_hash` field. The
`key_hash` refers to the log's public verification key.


### get-proof-by-hash
```
GET <base url>/st/v0/get-proof-by-hash
```

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.

Output on success:
- "tree_size": tree size that the proof is based on, as an
  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)`.

`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.

### get-consistency-proof
```
GET <base url>/st/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.

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
hashes follow from the hash strategy, see RFC 6962.

### get-leaves
```
GET <base url>/st/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.

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.
- "key_hash": `tree_leaf.key_hash`, hex-encoded.

All fields may be repeated to return more than one leaf.  The first
value in each list refers to the first leaf, the second value in each
list refers to the second leaf, etc.  The size of each list must
match.

The log may return fewer leaves than requested.  At least one leaf
must be returned on HTTP status code 200 OK.

### add-leaf
```
GET <base url>/st/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_over_message": the submitter's signature over
  `tree_leaf.message`, 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.

Output on success:
- None

The submission will not be accepted if `signature_over_message` is
invalid or if the key hash retrieved using `domain_hint` does not
match a hash over `verification_key`.

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 the log.  You
would need an excessive number of domain names.  This becomes costly
if free domain names are rejected.

The log does not 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.

### add-cosignature
```
GET <base url>/st/v0/add-cosignature
```

Input:
- "signature": Ed25519 signature over `tree_head`, hex-encoded.
- "key_hash": hash of the witness' public verification key that can be
  used to verify `signature`.  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 on success:
- None

`key_hash` can be used to identify which witness signed the log's 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.

## Summary of log parameters
- **Public key**: an Ed25519 verification key that can be used to
  verify the log's tree head signatures.
- **Log identifier**: the hashed public verification key using SHA256.
- **Shard interval**: the time during which the log accepts logging
  requests.  The shard interval's start and end are inclusive and
  expressed as the number of seconds since the UNIX epoch.
- **Base URL**: where the log can be reached over HTTP(S).  It is the
  prefix before a version-0 specific endpoint.