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authorRasmus Dahlberg <rasmus.dahlberg@kau.se>2021-02-23 12:57:09 +0100
committerRasmus Dahlberg <rasmus.dahlberg@kau.se>2021-02-23 12:57:09 +0100
commit15102a44b646f300ac8b35ce18fe3d1fded7ba1d (patch)
tree9b021acaedd4477ee94ad9dcb534590bce8e747b /markup
parent0a1cc19a3cf51495fd12570940c877c85e4d6ac4 (diff)
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-# System Transparency Log
-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 an operating system image, a Debian package, or a browser update (to mention
-a few examples).
-
-An ST log can be implemented on-top of
-[Trillian](https://trillian.transparency.dev) using a custom STFE personality.
-For reference you may look at Certificate Transparency (CT) logging and
-[CTFE](https://github.com/google/certificate-transparency-go/tree/master/trillian/ctfe),
-which implements [RFC 6962](https://tools.ietf.org/html/rfc6962).
-
-We take inspiration from from RFC 6962 and its follow-up specification [RFC
-6962/bis](https://datatracker.ietf.org/doc/draft-ietf-trans-rfc6962-bis/).
-
-## Log parameters
-A log is defined by the following immutable parameters:
-- Log identifier: serialized 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: where can this log be reached? E.g., example.com:1234/log
-
-Note that **there is no Maximum Merge Delay (MMD)**, which is in contrast to RFC
-6962. 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.
-
-We use the same hash strategy as described in RFC 6962: SHA256 with `0x00` as
-leaf node prefix and `0x01` as interior node prefix.
-
-## Minimum acceptance criteria
-A log should accept a submission if it is:
-- Well-formed, see below.
-- Digitally signed
- - Verification key must be registered in the log as a namespace
- - Proves which namespace submitted an entry for logging
-
-## 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).
-
-### Repurposing `TransItem` as `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. Some definitions are re-used while others are added.
-
-```
-enum {
- reserved(0),
- signed_tree_head_v1(1), // defined in RFC 6962/bis, §4.10
- signed_debug_info_v1(2), // defined below, think "almost SCT"
- consistency_proof_v1(3), // defined in RFC 6962/bis, §4.11
- inclusion_proof_v1(4), // defined in RFC 6962/bis, §4.12
- checksum_v1(5), // defined below, think "leaf data"
- (65535)
-} StFormat;
-
-struct {
- StFormat format;
- select (format) {
- case signed_tree_head_v1: SignedTreeHeadV1;
- case signed_debug_info_v1: SignedDebugInfoV1;
- case consistency_proof_v1: ConsistencyProofV1;
- case inclusion_proof_v1: InclusionProofV1;
- case checksum_v1: ChecksumV1;
- } message;
-} StItem;
-```
-
-### 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.
-```
-enum {
- reserved(0),
- ed25519_v1(1),
- (65535)
-} NamespaceFormat;
-
-struct {
- NamespaceFormat format;
- select (format) {
- case ed25519_v1: Ed25519V1;
- } message;
-} Namespace;
-```
-
-A log may reject submissions that correspond to an unknown namespace, or because
-a trusted namespace exceeded a configured rate limit. As such, it is possible
-to use namespaces as an anti-spam feature.
-
-Namespaces also allow us to separate different ecosystems. For example,
-software publisher _A_ can be sure that publicly logged artifact hashes are only
-considered valid if signed by their own namespace(s).
-
-(Credit: 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, see [RFC
-8032](https://tools.ietf.org/html/rfc8032). The namespace field contains the
-full verification key.
-```
-struct {
- opaque namespace<32>; // public verification key
-} Ed25519V1;
-```
-
-#### Other
-In the future we will support other key types, such as RSA. 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;
-```
-
-This is just one of several options. To be decided later on.
-
-### Merkle tree leaf types
-In the future there might be several types of leaves. Say, one for operating
-system packages, another one for Debian packages, and a third one for
-general-purpose checksums. For now we only define the latter.
-
-TODO: scope of this spec should only be checksum
-
-#### Checksum
-
-```
-struct {
- opaque package<1..2^8-1>; // package identifier
- opaque checksum<1..64>; // hash of some artifact
- Namespace namespace;
-} ChecksumV1;
-```
-
-A checksum entry contains a package identifier such as `foobar-1.2.3` and an
-artifact hash. For example, the checksum type could be used by Firefox to
-[enforce public binary logging before accepting a new software
-update](https://wiki.mozilla.org/Security/Binary_Transparency). It is assumed
-that the entities relying on the checksum type know how to find the artifact
-source (if not already at hand) and then reproduce the logged hash from it.
-
-Namespace is used to determine which ecosystem an artifact belongs to. However,
-note that we do not connect namespaces to real-world identities: it is up to the
-respective ecosystems to communicate their own namespaces, e.g., by establishing
-namespaces based on trusted verification keys that already sign their artifacts.
-
-### Signed Debug Info
-RFC 6962 uses Signed Certificate Timestamps (SCTs) as promises of public
-logging within a time known as the Maximum Merge Delay (MMD). We provide no
-such promise: a Signed Debug Info (SDI) is an intent to log because the
-submitter is authorized to do so and the entry appears to be valid. It will be
-merged into the log's Merkle tree as soon as possible on a best-effort basis.
-If an unexpected delay is encountered, the submitter can present the issued SDI
-to the log operator (who can then investigate the underlying reason further).
-```
-struct {
- Namespace log_id;
- opaque message<1..2^16-1> // debug string that is only meant for the log
- opaque signature <1..2^16-1; // computed over a leaf-type StItem
-} SignedDebugInfoV1;
-```
-
-Signature formatting and verification operations follow from the log's
-namespace identifier.
-
-TODO: remove SDI?
-
-## Public endpoints
-Clients talk to the log with HTTPS GET/POST requests. POST parameters
-are JSON objects, GET parameters are URL encoded, and serialized data is
-expressed as base-64. See details in as in RFC 6962,
-[§4](https://tools.ietf.org/html/rfc6962#section-4).
-
-TODO: remove json
-TODO: and b64?
-
-Unless specified otherwise, the data in question is serialized.
-
-### add-entry
-```
-POST https://<base url>/st/v1/add-entry
-```
-
-Input:
-- item: an `StItem` that corresponds to a valid leaf type. Only
-`checksum_v1` at this time.
-- signature: covers the submitted item.
-
-Output:
-- an `StItem` structure of type `signed_debug_info_v1` that covers the added
-item.
-
-### 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 array of objects, each consisting of
- - leaf: `StItem` that corresponds to the leaf's type.
- - signature: signature that covers the retrieved item using the below
- signature scheme.
-
-### get-namespaces
-```
-GET https://<base url>/st/v1/get-namespaces
-```
-
-No input.
-
-Output:
-- an array of base-64 encoded namespaces that the log accept.
-
-### get-proof-by-hash
-```
-GET https://<base url>/st/v1/get-proof-by-hash
-```
-
-Input:
-- hash: a base-64 encoded leaf hash.
-- tree_size: the tree size that the proof should be based on in decimal.
-
-The leaf hash value is computed as in RFC 6962/bis,
-[§4.7](https://datatracker.ietf.org/doc/html/draft-ietf-trans-rfc6962-bis-34#section-4.7).
-
-Output:
-- an `StItem` of type `inclusion_proof_v1`. Note that this structure includes
-both the leaf index and an audit path for the tree size.
-
-### 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` that corresponds to
-the requested tree sizes.
-
-### get-sth
-```
-GET https://<base url>/st/v1/get-sth
-```
-
-No input.
-
-Output:
-- an `StItem` of type `signed_tree_head_v1`, which corresponds to the most
-recently known STH.
diff --git a/markup/witness.md b/markup/witness.md
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-# Witness cosigning
-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.
-
-The log is configured with a list of witness namespaces. The only supported
-witness namespace format is `ed25519_v1`. The signature must span a serialized
-`signed_tree_head_v1` item.
-
-## Public endpoints
-Witnesses are expected to poll the `next-cosi` endpoint. If a new STH is
-obtained:
-1. Verify that the log is append-only by fetching a consistency proof from the
-latest STH that this witness co-signed. Stop if no valid proof is available.
-2. Sign the STH and submit using the `add-cosi` API.
-
-### add-cosi
-```
-POST https://<base url>/st/v1/add-cosi
-```
-
-Input:
-- sth: an `StItem` of type `signed_tree_head_v1`.
-- namespace: a `Namespace` item of type `ed25519_v1`.
-- signature: covers the specified STH.
-
-Output:
-- None
-
-### get-cosi
-```
-GET https://<base url>/st/v1/get-cosi
-```
-
-Input:
-- None
-
-Output:
-- A map with keys "sth" and "signatures". The former is an `StItem` of type
-`signed_tree_head_v1`. The latter an array where each entry is a map with a
-"witness" (`ed25519_v1` namespace) and a "signature".
-
-### next-cosi
-```
-GET https://<base url>/st/v1/next-cosi
-```
-
-Input
-- None
-
-Output:
-- an `StItem` of type `signed_tree_head_v1`, which corresponds to the STH
-that is currently being cosigned. Stable for a period of time, e.g., 10 minutes.