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diff --git a/doc/api.md b/doc/api.md new file mode 100644 index 0000000..32e1bf6 --- /dev/null +++ b/doc/api.md @@ -0,0 +1,371 @@ +# 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,ยง2](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 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 +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. 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 +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. + +``` +struct tree_head { + u64 timestamp; + u64 tree_size; + u8 root_hash[32]; +}; +``` + +#### (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. + +``` +struct signed_tree_head { + struct tree_head tree_head; + u8 signature[64]; +}; +``` + +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. + +#### Merkle tree leaf +Logs support a single leaf type. It contains a shard hint, a +checksum, a signature, and a key hash. + +``` +struct tree_leaf { + u64 shard_hint; + u8 checksum[32]; + u8 signature[64]; + u8 key_hash[32]; +} +``` + +`shard_hint` is chosen by the submitter to match the log's shard interval, see +design document. + +`checksum` is computed by the submitter and represents some opaque data. + +`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. + +`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, +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 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. + +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-tree-head-to-sign +Returns the latest signed tree head to be cosigned. 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 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. + +``` +GET <base url>/st/v0/get-tree-head-latest +``` + +Input an output follows the same formatting as `get-tree-head-to-sign`. + +### 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`: 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. + +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`. + +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=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: +- `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. + +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. + +Example: `echo "cosignature=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. +- **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. diff --git a/doc/claimant.md b/doc/claimant.md new file mode 100644 index 0000000..6728fef --- /dev/null +++ b/doc/claimant.md @@ -0,0 +1,71 @@ +# Claimant model +## **System<sup>CHECKSUM</sup>** +System<sup>CHECKSUM</sup> is about the claims made by a data publisher. +* **Claim<sup>CHECKSUM</sup>**: + _I, data publisher, claim that the data_: + 1. has cryptographic hash X + 2. is produced by no-one but myself +* **Statement<sup>CHECKSUM</sup>**: signed checksum<br> +* **Claimant<sup>CHECKSUM</sup>**: data publisher<br> + The data publisher is a party that wants to publish some data. +* **Believer<sup>CHECKSUM</sup>**: end-user<br> + The end-user is a party that wants to use some published data. +* **Verifier<sup>CHECKSUM</sup>**: data publisher<br> + Only the data publisher can verify the above claims. +* **Arbiter<sup>CHECKSUM</sup>**:<br> + There's no official body. Invalidated claims would affect reputation. + +System<sup>CHECKSUM\*</sup> can be defined to make more specific claims. Below +is a reproducible builds example. + +### **System<sup>CHECKSUM-RB</sup>**: +System<sup>CHECKSUM-RB</sup> is about the claims made by a _software publisher_ +that makes reproducible builds available. +* **Claim<sup>CHECKSUM-RB</sup>**: + _I, software publisher, claim that the data_: + 1. has cryptographic hash X + 2. is the output of a reproducible build for which the source can be located + using X as an identifier +* **Statement<sup>CHECKSUM-RB</sup>**: Statement<sup>CHECKSUM</sup> +* **Claimant<sup>CHECKSUM-RB</sup>**: software publisher<br> + The software publisher is a party that wants to publish the output of a + reproducible build. +* **Believer<sup>CHECKSUM-RB</sup>**: end-user<br> + The end-user is a party that wants to run an executable binary that built + reproducibly. +* **Verifier<sup>CHECKSUM-RB</sup>**: any interested party<br> + These parties try to verify the above claims. For example: + * the software publisher itself (_"has my identity been compromised?"_) + * rebuilders that check for locatability and reproducibility +* **Arbiter<sup>CHECKSUM-RB</sup>**:<br> + There's no official body. Invalidated claims would affect reputation. + +## **System<sup>CHECKSUM-LOG</sup>**: +System<sup>CHECKSUM-LOG</sup> is about the claims made by a _log operator_. +It adds _discoverability_ into System<sup>CHECKSUM\*</sup>. Discoverability +means that Verifier<sup>CHECKSUM\*</sup> can see all +Statement<sup>CHECKSUM</sup> that Believer<sup>CHECKSUM\*</sup> accept. + +* **Claim<sup>CHECKSUM-LOG</sup>**: + _I, log operator, make available:_ + 1. a globally consistent append-only log of Statement<sup>CHECKSUM</sup> +* **Statement<sup>CHECKSUM-LOG</sup>**: signed tree head +* **Claimant<sup>CHECKSUM-LOG</sup>**: log operator<br> + Possible operators might be: + * a small subset of data publishers + * members of relevant consortia +* **Believer<sup>CHECKSUM-LOG</sup>**: + * Believer<sup>CHECKSUM\*</sup> + * Verifier<sup>CHECKSUM\*</sup><br> +* **Verifier<sup>CHECKSUM-LOG</sup>**: third parties<br> + These parties verify the above claims. Examples include: + * members of relevant consortia + * non-profits and other reputable organizations + * security enthusiasts and researchers + * log operators (cross-ecosystem) + * monitors (cross-ecosystem) + * a small subset of data publishers (cross-ecosystem) +* **Arbiter<sup>CHECKSUM-LOG</sup>**:<br> + There is no official body. The ecosystem at large should stop using an + instance of System<sup>CHECKSUM-LOG</sup> if cryptographic proofs of log + misbehavior are preseneted by some Verifier<sup>CHECKSUM-LOG</sup>. diff --git a/doc/design.md b/doc/design.md new file mode 100644 index 0000000..2e01a34 --- /dev/null +++ b/doc/design.md @@ -0,0 +1,251 @@ +# System Transparency Logging: Design v0 +We propose System Transparency logging. It is similar to Certificate +Transparency, except that cryptographically signed checksums are logged as +opposed to X.509 certificates. Publicly logging signed checksums allow anyone +to discover which keys produced what signatures. As such, malicious and +unintended key-usage can be _detected_. We present our design and conclude by +providing two use-cases: binary transparency and reproducible builds. + +**Target audience.** +You are most likely interested in transparency logs or supply-chain security. + +**Preliminaries.** +You have basic understanding of cryptographic primitives like digital +signatures, hash functions, and Merkle trees. You roughly know what problem +Certificate Transparency solves and how. + +**Warning.** +This is a work-in-progress document that may be moved or modified. A future +revision of this document will bump the version number to v1. Please let us +know if you have any feedback. + +## Introduction +Transparency logs make it possible to detect unwanted events. For example, + are there any (mis-)issued TLS certificates [\[CT\]](https://tools.ietf.org/html/rfc6962), + did you get a different Go module than everyone else [\[ChecksumDB\]](https://go.googlesource.com/proposal/+/master/design/25530-sumdb.md), + or is someone running unexpected commands on your server [\[AuditLog\]](https://transparency.dev/application/reliably-log-all-actions-performed-on-your-servers/). +A System Transparency log makes signed checksums transparent. The overall goal +is to facilitate detection of unwanted key-usage. + +## Threat model and (non-)goals +We consider a powerful attacker that gained control of a target's signing and +release infrastructure. This covers a weaker form of attacker that is able to +sign data and distribute it to a subset of isolated users. For example, this is +essentially what the FBI requested from Apple in the San Bernardino case [\[FBI-Apple\]](https://www.eff.org/cases/apple-challenges-fbi-all-writs-act-order). +The fact that signing keys and related infrastructure components get +compromised should not be controversial these days [\[SolarWinds\]](https://www.zdnet.com/article/third-malware-strain-discovered-in-solarwinds-supply-chain-attack/). + +The attacker can also gain control of the transparency log's signing key and +infrastructure. This covers a weaker form of attacker that is able to sign log +data and distribute it to a subset of isolated users. For example, this could +have been the case when a remote code execution was found for a Certificate +Transparency Log [\[DigiCert\]](https://groups.google.com/a/chromium.org/g/ct-policy/c/aKNbZuJzwfM). + +Any attacker that is able to position itself to control these components will +likely be _risk-averse_. This is at minimum due to two factors. First, +detection would result in a significant loss of capability that is by no means +trivial to come by. Second, detection means that some part of the attacker's +malicious behavior will be disclosed publicly. + +Our goal is to facilitate _detection_ of compromised signing keys. We consider +a signing key compromised if an end-user accepts an unwanted signature as valid. +The solution that we propose is that signed checksums are transparency logged. +For security we need a collision resistant hash function and an unforgeable +signature scheme. We also assume that at most a threshold of seemingly +independent parties are adversarial. + +It is a non-goal to disclose the data that a checksum represents. For example, +the log cannot distinguish between a checksum that represents a tax declaration, +an ISO image, or a Debian package. This means that the type of detection we +support is more _coarse-grained_ when compared to Certificate Transparency. + +## Design +We consider a data publisher that wants to digitally sign their data. The data +is of opaque type. We assume that end-users have a mechanism to locate the +relevant public verification keys. Data and signatures can also be retrieved +(in)directly from the data publisher. We make little assumptions about the +signature tooling. The ecosystem at large can continue to use `gpg`, `openssl`, +`ssh-keygen -Y`, `signify`, or something else. + +We _have to assume_ that additional tooling can be installed by end-users that +wish to enforce transparency logging. For example, none of the existing +signature tooling supports verification of Merkle tree proofs. A side-effect of +our design is that this additional tooling makes no outbound connections. The +above data flows are thus preserved. + +### A bird's view +A central part of any transparency log is the data stored by the log. The data is stored by the +leaves of an append-only Merkle tree. Our leaf structure contains four fields: +- **shard_hint**: a number that binds the leaf to a particular _shard interval_. +Sharding means that the log has a predefined time during which logging requests +are accepted. Once elapsed, the log can be shut down. +- **checksum**: a cryptographic hash of some opaque data. The log never +sees the opaque data; just the hash made by the data publisher. +- **signature**: a digital signature that is computed by the data publisher over +the leaf's shard hint and checksum. +- **key_hash**: a cryptographic hash of the data publisher's public verification key that can be +used to verify the signature. + +#### Step 1 - preparing a logging request +The data publisher selects a shard hint and a checksum that should be logged. +For example, the shard hint could be "logs that are active during 2021". The +checksum might be the hash of a release file. + +The data publisher signs the selected shard hint and checksum using a secret +signing key. Both the signed message and the signature is stored +in the leaf for anyone to verify. Including a shard hint in the signed message +ensures that a good Samaritan cannot change it to log all leaves from an +earlier shard into a newer one. + +A hash of the public verification key is also stored in the leaf. This makes it +possible to attribute the leaf to the data publisher. For example, a data publisher +that monitors the log can look for leaves that match their own key hash(es). + +A hash, rather than the full public verification key, is used to motivate the +verifier to locate the key and make an explicit trust decision. Not disclosing the public +verification key in the leaf makes it more unlikely that someone would use an untrusted key _by +mistake_. + +#### Step 2 - submitting a logging request +The log implements an HTTP(S) API. Input and output is human-readable and uses +a simple key-value format. A more complex parser like JSON is not needed +because the exchanged data structures are primitive enough. + +The data publisher submits their shard hint, checksum, signature, and public +verification key as key-value pairs. The log will use the public verification +key to check that the signature is valid, then hash it to construct the `key_hash` part of the leaf. + +The data publisher also submits a _domain hint_. The log will download a DNS +TXT resource record based on the provided domain name. The downloaded result +must match the public verification key hash. By verifying that the submitter +controls a domain that is aware of the public verification key, rate limits can +be applied per second-level domain. As a result, you would need a large number +of domain names to spam the log in any significant way. + +Using DNS to combat spam is convenient because many data publishers already have +a domain name. A single domain name is also relatively cheap. Another +benefit is that the same anti-spam mechanism can be used across several +independent logs without coordination. This is important because a healthy log +ecosystem needs more than one log in order to be reliable. DNS also has built-in +caching which data publishers can influence by setting TTLs accordingly. + +The submitter's domain hint is not part of the leaf because key management is +more complex than that. A separate project should focus on transparent key +management. The scope of our work is transparent _key-usage_. + +The log will _try_ to incorporate a leaf into the Merkle tree if a logging +request is accepted. There are no _promises of public logging_ as in +Certificate Transparency. Therefore, the submitter needs to wait for an +inclusion proof to appear before concluding that the logging request succeeded. Not having +inclusion promises makes the log less complex. + +#### Step 3 - distributing proofs of public logging +The data publisher is responsible for collecting all cryptographic proofs that +their end-users will need to enforce public logging. The collection below +should be downloadable from the same place that published data is normally hosted. +1. **Opaque data**: the data publisher's opaque data. +2. **Shard hint**: the data publisher's selected shard hint. +3. **Signature**: the data publisher's leaf signature. +4. **Cosigned tree head**: the log's tree head and a _list of signatures_ that +state it is consistent with prior history. +5. **Inclusion proof**: a proof of inclusion based on the logged leaf and tree +head in question. + +The data publisher's public verification key is known. Therefore, the first three fields are +sufficient to reconstruct the logged leaf. The leaf's signature can be +verified. The final two fields then prove that the leaf is in the log. If the +leaf is included in the log, any monitor can detect that there is a new +signature made by a given data publisher, 's public verification key. + +The catch is that the proof of logging is only as convincing as the tree head +that the inclusion proof leads up to. To bypass public logging, the attacker +needs to control a threshold of independent _witnesses_ that cosign the log. A +benign witness will only sign the log's tree head if it is consistent with prior +history. + +#### Summary +The log is sharded and will shut down at a predefined time. The log can shut +down _safely_ because end-user verification is not interactive. The difficulty +of bypassing public logging is based on the difficulty of controlling a +threshold of independent witnesses. Witnesses cosign tree heads to make them +trustworthy. + +Submitters, monitors, and witnesses interact with the log using an HTTP(S) API. +Submitters must prove that they own a domain name as an anti-spam mechanism. +End-users interact with the log _indirectly_ via a data publisher. It is the +data publisher's job to log signed checksums, distribute necessary proofs of +logging, and monitor the log. + +### A peek into the details +Our bird's view introduction skipped many details that matter in practise. Some +of these details are presented here using a question-answer format. A +question-answer format is helpful because it is easily modified and extended. + +#### What cryptographic primitives are supported? +The only supported hash algorithm is SHA256. The only supported signature +scheme is Ed25519. Not having any cryptographic agility makes the protocol less +complex and more secure. + +We can be cryptographically opinionated because of a key insight. Existing +signature tools like `gpg`, `ssh-keygen -Y`, and `signify` cannot verify proofs +of public logging. Therefore, _additional tooling must already be installed by +end-users_. That tooling should verify hashes using the log's hash function. +That tooling should also verify signatures using the log's signature scheme. +Both tree heads and tree leaves are being signed. + +#### Why not let the data publisher pick their own signature scheme and format? +Agility introduces complexity and difficult policy questions. For example, +which algorithms and formats should (not) be supported and why? Picking Ed25519 +is a current best practise that should be encouraged if possible. + +There is not much we can do if a data publisher _refuses_ to rely on the log's +hash function or signature scheme. + +#### What if the data publisher must use a specific signature scheme or format? +They may _cross-sign_ the data as follows. +1. Sign the data as they're used to. +2. Hash the data and use the result as the leaf's checksum to be logged. +3. Sign the leaf using the log's signature scheme. + +For verification, the end-user first verifies that the usual signature from step 1 is valid. Then the +end-user uses the additional tooling (which is already required) to verify the rest. +Cross-signing should be a relatively comfortable upgrade path that is backwards +compatible. The downside is that the data publisher may need to manage an +additional key-pair. + +#### What (de)serialization parsers are needed? +#### What policy should be used? +#### Why witness cosigning? +#### Why sharding? +Unlike X.509 certificates which already have validity ranges, a +checksum does not carry any such information. Therefore, we require +that the submitter selects a _shard hint_. The selected shard hint +must be in the log's _shard interval_. A shard interval is defined by +a start time and an end time. Both ends of the shard interval are +inclusive and expressed as the number of seconds since the UNIX epoch +(January 1, 1970 00:00 UTC). + +Sharding simplifies log operations because it becomes explicit when a +log can be shutdown. A log must only accept logging requests that +have valid shard hints. A log should only accept logging requests +during the predefined shard interval. Note that _the submitter's +shard hint is not a verified timestamp_. The submitter should set the +shard hint as large as possible. If a roughly verified timestamp is +needed, a cosigned tree head can be used. + +Without a shard hint, the good Samaritan could log all leaves from an +earlier shard into a newer one. Not only would that defeat the +purpose of sharding, but it would also become a potential +denial-of-service vector. + +#### TODO +Add more key questions and answers. +- Log spamming +- Log poisoning +- Why we removed identifier field from the leaf +- Explain `latest`, `stable` and `cosigned` tree head. +- Privacy aspects +- How does this whole thing work with more than one log? + +## Concluding remarks +Example of binary transparency and reproducible builds. |