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-# 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
-Logs implement an HTTP(S) API for accepting requests and sending
-responses.
-
-- Input data in requests and output data in responses are expressed as
-  ASCII-encoded key/value pairs.
-- Requests with input data use HTTP POST to send the data to a log.
-- 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
-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 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 both by a 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
-Logs support 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[64];
-    u8 key_hash[32];
-}
-```
-
-`message` is composed of the `shard_hint`, chosen by the submitter to
-match the shard interval for the log it's submitting to, 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-tree-head-cosigned`.
-
-Input data (in requests) is POST:ed in the HTTP message body as ASCII
-key/value pairs.
-
-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`
-
-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.`.
-
-### 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.
-
-```
-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 `timestamp`,
-  `tree_size` and `root_hash` serialized into a `tree_head` 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 `timestamp`,
-  `tree_size` and `root_hash` serialized into a `tree_head` 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 `timestamp`,
-  `tree_size` and `root_hash` serialized into a `tree_head` 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
-```
-POST <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.
-
-Example: `echo "leaf_hash=241fd4538d0a35c2d0394e4710ea9e6916854d08f62602fb03b55221dcdac90f
-tree_size=4711" | curl --data-binary @- localhost/st/v0/get-proof-by-hash`
-
-### get-consistency-proof
-```
-POST <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.
-
-Example: `echo "new_size=4711
-old_size=42" | curl --data-binary @- localhost/st/v0/get-consistency-proof`
-
-### get-leaves
-```
-POST <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.
-
-A log may return fewer leaves than requested.  At least one leaf
-must be returned on HTTP status code 200 OK.
-
-Example: `echo "start_size=42
-end_size=4711" | curl --data-binary @- localhost/st/v0/get-leaves`
-
-### add-leaf
-```
-POST <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 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.
-
-Example: `echo "shard_hint=1640995200
-checksum=cfa2d8e78bf273ab85d3cef7bde62716261d1e42626d776f9b4e6aae7b6ff953
-signature_over_message=c026687411dea494539516ee0c4e790c24450f1a4440c2eb74df311ca9a7adf2847b99273af78b0bda65dfe9c4f7d23a5d319b596a8881d3bc2964749ae9ece3
-verification_key=c9a674888e905db1761ba3f10f3ad09586dddfe8581964b55787b44f318cbcdf
-domain_hint=example.com" | curl --data-binary @- localhost/st/v0/add-leaf`
-
-### add-cosignature
-```
-POST <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 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.
-
-Example: `echo "signature=d1b15061d0f287847d066630339beaa0915a6bbb77332c3e839a32f66f1831b69c678e8ca63afd24e436525554dbc6daa3b1201cc0c93721de24b778027d41af
-key_hash=662ce093682280f8fbea9939abe02fdba1f0dc39594c832b411ddafcffb75b1d" | curl --data-binary @- localhost/st/v0/add-cosignature`
-
-## Summary of log parameters
-- **Public key**: The Ed25519 verification key to be used for
-  verifying tree head signatures.
-- **Log identifier**: The public verification key `Public key` hashed
-  using SHA256.
-- **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.