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@@ -1,2 +1,353 @@ -# STFE -Work in progress +# System Transparency Logging +This document provides a sketch of System Transparency (ST) logging. The basic +idea is to insert hashes of system artifacts into a public, append-only, and +tamper-evident transparency log, such that any enforcing client can be sure that +they see the same system artifacts as everyone else. A system artifact could +be a browser update, an operating system image, a Debian package, or more +generally something that is opaque. + +We take inspiration from the Certificate Transparency Front-End +([CTFE](https://github.com/google/certificate-transparency-go/tree/master/trillian/ctfe)) +that implements [RFC 6962](https://tools.ietf.org/html/rfc6962) for +[Trillian](https://transparency.dev). + +## Log parameters and overview +An ST log is defined by the following parameters: +- `log_identifier`: namespace of type `ed25519_v1` that defines the +log's signing algorithm and public verification key. +- `supported_namespaces`: a list of namespace types the log supports. +Submitters must use a supported namespace type when adding new entries. +- `base_url`: prefix used by clients that contact the log, e.g., +example.com:1234/log. + +ST logs use the same hash strategy as described in RFC 6962: SHA256 with `0x00` +as leaf node prefix and `0x01` as interior node prefix. + +In contrast to Certificate Transparency (CT) **there is no Maximum Merge Delay +(MMD)**. New entries are merged into the log as soon as possible, and no client +should trust that something is logged until an inclusion proof can be provided +that references a trustworthy STH. Therefore, **there are no "promises" of +public logging** as in CT. + +To produce trustworthy STHs a simple form of [witness +cosigning](https://arxiv.org/pdf/1503.08768.pdf) is built into the log. + +## Minimum acceptance criteria and scope +A log should accept a submission if it is: +- Well-formed, see data structure definitions below. +- Digitally signed by a registered namespace. + +Rate limits may be applied per namespace to combat spam. Namespaces may also be +used by clients to determine which entries belong to who. It is up to the +submitters to communicate trusted namespaces to their own clients. In other +words, there are no mappings from namespaces to identities built into the log. +There is also no revocation of namespaces: **we facilitate _detection_ of +compromised signing keys by making artifact hashes public, which is not to be +confused with _prevention_ or even _recovery_ after detection**. + +## Data structure definitions +Data structures are defined and serialized using the presentation language in +[RFC 5246, §4](https://tools.ietf.org/html/rfc5246). A definition of the log's +Merkle tree can be found in [RFC 6962, +§2](https://tools.ietf.org/html/rfc6962#section-2). + +### Namespace +A _namespace_ is a versioned data structure that contains a public verification +key (or fingerprint), as well as enough information to determine its format, +signing, and verification operations. Namespaces are used as identifiers, both +for the log itself and the parties that submit artifact hashes and cosignatures. + +``` +enum { + reserved(0), + ed25519_v1(1), + (65535) +} NamespaceFormat; + +struct { + NamespaceFormat format; + select (format) { + case ed25519_v1: Ed25519V1; + } message; +} Namespace; +``` + +Our namespace format is inspired by Keybase's +[key-id](https://keybase.io/docs/api/1.0/kid). + +#### Ed25519V1 +At this time the only supported namespace type is based on Ed25519. The +namespace field contains the full verification key. Signing operations and +serialized formats are defined by [RFC +8032](https://tools.ietf.org/html/rfc8032). +``` +struct { + opaque namespace<32>; // public verification key +} Ed25519V1; +``` + +### `StItem` +A general-purpose `TransItem` is defined in [RFC 6962/bis, +§4.5](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.5). +We define our own `TransItem`, but name it `StItem` to emphasize that they are +not the same. + +``` +enum { + reserved(0), + signed_tree_head_v1(1), + cosigned_tree_head_v1(2), + consistency_proof_v1(3), + inclusion_proof_v1(4), + checksum_v1(5), // leaf type + (65535) +} StFormat; + +struct { + StFormat format; + select (format) { + case signed_tree_head_v1: SignedTreeHeadV1; + case cosigned_tree_head_v1: CosignedTreeHeadV1; + case consistency_proof_v1: ConsistencyProofV1; + case inclusion_proof_v1: InclusionProofV1; + case checksum_v1: ChecksumV1; + } message; +} StItem; +``` + +#### `signed_tree_head_v1` +For the most part we use the same signed tree head definition as in [RFC +6962/bis, +§4.9](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.10). +There is one modification: our log identifier is a namespace rather than an +[OID](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.4). +``` +opaque NodeHash<32..2^8-1>; + +struct { + uint64 timestamp; + uint64 tree_size; + NodeHash root_hash; + Extension sth_extensions<0..2^16-1>; +} TreeHeadV1; + +struct { + Namespace log_id; + TreeHeadV1 tree_head; + opaque signature<0..2^16-1>; +} SignedTreeHeadV1; +``` + +#### `cosigned_tree_head_v1` +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. + +A cosigned STH is composed of an STH and a list of cosignatures. A cosignature +must cover the serialized STH as an `StItem`, and be produced with a witness +namespace of type `ed25519_v1`. + +``` +struct { + SignedTreeHeadV1 sth; + CosignatureV1 <0..4294967295>; // vector of cosignatures +} CosignedTreeHeadV1; + +struct { + Namespace witness_id; + opaque signature<1..65535>; +} CosignatureV1; +``` + +#### `consistency_proof_v1` +For the most part we use the same consistency proof definition as in [RFC +6962/bis, +§4.11](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.11). +There are two modifications: our log identifier is a namespace rather than an +[OID](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.4), +and a consistency proof may be empty. + +``` +struct { + Namespace log_id; + uint64 tree_size_1; + uint64 tree_size_2; + NodeHash consistency_path<0..2^16-1>; +} ConsistencyProofV1; +``` + +#### `inclusion_proof_v1` +For the most part we use the same inclusion proof definition as in [RFC +6962/bis, +§4.12](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.12). +There are two modifications: our log identifier is a namespace rather than an +[OID](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.4), +and an inclusion proof may be empty. +``` +struct { + Namespace log_id; + uint64 tree_size; + uint64 leaf_index; + NodeHash inclusion_path<0..2^16-1>; +} InclusionProofV1; +``` + +#### `checksum_v1` +A checksum entry contains a package identifier such as `foobar-1.2.3` and an +artifact hash. It also contains a namespace that allows clients to distinguish +artifact hashes from two different software publishers A and B. For example, +the `checksum_v1` can help [enforce public binary logging before accepting a new +software update](https://wiki.mozilla.org/Security/Binary_Transparency). + +``` +struct { + opaque package<1..2^8-1>; // package identifier + opaque checksum<1..64>; // hash of some artifact + Namespace namespace; +} ChecksumV1; +``` + +It is assumed that clients know how to find the real artifact source (if not +already at hand), such that the logged hash can be recomputed and compared for +equality. The log is not aware of how artifact hashes are computed, which means +that it is up to the submitters to define hash functions, data formats, and +such. + +## Public endpoints +Clients talk to the log using HTTP(S). Parameters for HTTP GET requests are +URL-encoded with `Content-Type: application/x-www-form-urlencoded`. In +contrast, HTTP POST requests post a single base64-encoded serialized `StItem` +with `Content-Type: text/plain`. The log returns HTTP Status `200 OK` to +signal success, and the response (if any) is serialized and then base64-encoded +with `Content-Type: text/plain`. + +### add-entry +``` +POST https://<base url>/st/v1/add-entry +``` + +Input: +- An `StItem` of type `checksum_v1`. + +No output. + +### add-cosignature +``` +POST https://<base url>/st/v1/add-cosignature +``` + +Input: +- An `StItem` of type `cosigned_tree_head_v1`. The list of cosignatures must +be of length one, the witness signature must cover the item's STH, and that STH +must additionally match the log's stable STH that is currently being cosigned. + +No output. + +### get-latest-sth +``` +GET https://<base url>/st/v1/get-latest-sth +``` + +No input. + +Output: +- An `StItem` of type `signed_tree_head_v1` that corresponds to the most +recent STH. + +### get-stable-sth +``` +GET https://<base url>/st/v1/get-stable-sth +``` + +No input. + +Output: +- An `StItem` of type `signed_tree_head_v1` that corresponds to a stable STH +that witnesses should cosign. The same STH is returned for a period of time. + +### get-cosigned-sth +``` +GET https://<base url>/st/v1/get-cosigned-sth +``` + +No input. + +Output: +- An `StItem` of type `cosigned_tree_head_v1` that corresponds to the most +recent cosigned STH. + +### get-proof-by-hash +``` +GET https://<base url>/st/v1/get-proof-by-hash +``` + +Input: +- `hash`: a base-64 encoded leaf hash using the log's hash function. +- `tree_size`: the tree size that the proof should be based on in decimal. + +Output: +- An `StItem` of type `inclusion_proof_v1`. + +### get-consistency-proof +``` +GET https://<base url>/st/v1/get-consistency-proof +``` + +Input: +- first: the `tree_size` of the older tree in decimal. +- second: the `tree_size` of the newer tree in decimal. + +Output: +- An `StItem` of type `consistency_proof_v1`. + +### get-entries +``` +GET https://<base url>/st/v1/get-entries +``` + +Input: +- `start`: 0-based index of first entry to retrieve in decimal. +- `end`: 0-based index of last entry to retrieve in decimal. + +Output: +- An `StItem` list where each entry is of type `checksum_v1`. The first +`StItem` corresponds to the start index, the second one to `start+1`, etc. The +log may return fewer entries than requested. + +## Witness behavior +TODO: docdoc expected witness behavior somewhere + +# Appendix A +In the future other namespace types might be supported. For example, we could +add [RSASSA-PKCS1-v1_5](https://tools.ietf.org/html/rfc3447#section-8.2) as +follows: +1. Add `rsa_v1` format and RSAV1 namespace. This is what we would register on +the server-side such that the server knows the namespace and complete key. +``` +struct { + opaque namespace<32>; // key fingerprint + // + some encoding of public key +} RSAV1; +``` +2. Add `rsassa_pkcs1_5_v1` format and `RSASSAPKCS1_5_v1`. This is what the +submitter would use to communicate namespace and RSA signature mode. +``` +struct { + opaque namespace<32>; // key fingerprint + // + necessary parameters, e.g., SHA256 as hash function +} RSASSAPKCS1_5V1; +``` |