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@@ -1,372 +1,2 @@ # 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 -An ST log is defined by the following parameters: -- `log_identifier`: a `Namespace` of type `ed25519_v1` that defines the log's -signing algorithm and public verification key. -- `supported_namespaces`: a list of namespace types that the log supports. -Entities must use a supported namespace type when posting signed data to the -log. -- `base_url`: prefix used by clients that contact the log, e.g., -example.com:1234/log. -- `final_cosigned_tree_head`: an `StItem` of type `cosigned_tree_head_v*`. Not -set until the log is turned into read-only mode in preparation of a shutdown. - -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. -Witnesses poll the log for the next stable STH, and verify that it is consistent -before posting a cosignature that can then be served by the log. - -## Acceptance criteria and scope -A log should accept a leaf 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), - (2^16-1) -} 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), - signed_checksum_v1(5), // leaf type - (2^16-1) -} 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 signed_checksum_v1: SignedChecksumV1; - } message; -} StItem; - -struct { - StItem items<0..2^32-1>; -} StItemList; -``` - -#### `signed_tree_head_v1` -We use the same tree head definition as in [RFC 6962/bis, -§4.9](https://tools.ietf.org/html/draft-ietf-trans-rfc6962-bis-34#section-4.9). -The resulting _signed_ tree head is packaged differently: a namespace is used as -log identifier, and it is communicated in a `SignatureV1` structure. -``` -struct { - TreeHeadV1 tree_head; - SignatureV1 signature; -} SignedTreeHeadV1; - -struct { - uint64 timestamp; - uint64 tree_size; - NodeHash root_hash; - Extension extensions<0..2^16-1>; -} TreeHeadV1; -opaque NodeHash<32..2^8-1>; - -struct { - Namespace namespace; - opaque signature<1..2^16-1>; -} SignatureV1; -``` - -#### `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 signed_tree_head; - SignatureV1 cosignatures<0..2^32-1>; // vector of cosignatures -} CosignedTreeHeadV1; -``` - -#### `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; -``` - -#### `signed_checksum_v1` -A checksum entry contains a package identifier like `foobar-1.2.3` and an -artifact hash. It is then signed so that clients can distinguish artifact -hashes from two different software publishers A and B. For example, the -`signed_checksum_v1` type can help [enforce public binary logging before -accepting a new software -update](https://wiki.mozilla.org/Security/Binary_Transparency). - -``` -struct { - ChecksumV1 data; - SignatureV1 signature; -} SignedChecksumV1; - -struct { - opaque identifier<1..128>; - opaque checksum<1..64>; -} 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). Successfully processed requests are -responded to with HTTP status code `200 OK`, and any returned data is -serialized. Endpoints without input parameters use HTTP GET requests. -Endpoints that have input parameters HTTP POST a TLS-serialized data structure. -The HTTP content type `application/octet-stream` is used when sending data. - -### add-entry -``` -POST https://<base url>/st/v1/add-entry -``` - -Input: -- An `StItem` of type `signed_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 -``` -POST https://<base url>/st/v1/get-proof-by-hash -``` - -Input: -``` -struct { - opaque hash[32]; // leaf hash - uint64 tree_size; // tree size that the proof should be based on -} GetProofByHashV1; -``` - -Output: -- An `StItem` of type `inclusion_proof_v1`. - -### get-consistency-proof -``` -POST https://<base url>/st/v1/get-consistency-proof -``` - -Input: -``` -struct { - uint64 first; // first tree size that the proof should be based on - uint64 second; // second tree size that the proof should be based on -} GetConsistencyProofV1; -``` - -Output: -- An `StItem` of type `consistency_proof_v1`. - -### get-entries -``` -POST https://<base url>/st/v1/get-entries -``` - -Input: -``` -struct { - uint64 start; // 0-based index of first entry to retrieve - uint64 end; // 0-based index of last entry to retrieve in decimal. -} GetEntriesV1; -``` - -Output: -- An `StItem` list where each entry is of type `signed_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. - -# 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; -``` +TODO: fixme. |