Metadata for offline signers

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+# RFC-0000: Metadata for offline signers
+
+|                 |                                                                                             |
+| --------------- | ------------------------------------------------------------------------------------------- |
+| **Start Date**  | 2023-10-31                                                                                  |
+| **Description** | Add SignedExtension to check Metadata Root Hash                                             |
+| **Authors**     | Alzymologist Oy, Zondax LLC, Parity Technologies                                                    |
+
+## Summary
+
+Add a metadata digest value to signed data to supplement signer party with proof of correct extrinsic interpretation. This would ensure that hardware wallets always use correct metadata to decode the information for the user.
+
+The digest value is generated once before release and is well-known and deterministic. The digest mechanism is designed to be modular and flexible. It also supports partial metadata transfer as needed by the signing party's extrinsic decoding mechanism. This considers signing devices potentially limited communication bandwidth and/or memory capacity.
+
+## Motivation
+
+### Background
+
+While all blockchain systems support (at least in some sense) offline signing used in air-gapped wallets and lightweight embedded devices, only few allow simultaneously complex upgradeable logic and full message decoding on the cold off-line signer side; Substrate is one of these heartening few, and therefore - we should build on this feature to greatly improve transaction security, and thus in general, network resilience.
+
+As a starting point, it is important to recognise that prudence and due care are naturally required. As we build further reliance on this feature we should be very careful to make sure it works correctly every time so as not to create false sense of security.
+
+In order to enable decoding that is small and optimized for chain storage transactions, a metadata entity is used, which is not at all small in itself (on the order of half-MB for most networks). This is a dynamic data chunk which completely describes chain interfaces and properties that could be made into a portable scale-encoded string for any given network version and passed along into an off-chain device to familiarize it with latest network updates. Of course, compromising this metadata anywhere in the path could result in differences between what user sees and signs, thus it is essential that we protect it.
+
+Therefore, we have 2 problems to be solved:
+
+1. Metadata is large, takes long time to be passed into a cold storage device with memory insufficient for its storage; metadata SHOULD be shortened and transmission SHOULD be optimized.
+2. Metadata authenticity SHOULD be ensured.
+
+As of now, there is no working solution for (1), as the whole metadata has to be passed to the device. On top of this, the solution for (2) heavily relies on a trusted party managing keys and ensuring metadata is indeed authentic: creating poorly decentralized points of potential failure.
+
+### Solution requirements
+
+#### Include metadata digest into signature
+
+Some cryptographically strong digest of metadata MAY be included into signable blob. There SHALL NOT be storage overhead for this blob, nor computational overhead; thus MUST be a constant within a given runtime, deterministically defined by metadata.
+
+ - Metadata information that could be used in signable extrinsic decoding MAY be included in digest, its inclusion MUST be indicated in signed extensions;
+ - Digest MUST be deterministic with respect to metadata;
+ - Digest MUST be cryptographically strong against pre-image, both first (finding an input that results in given digest) and second (finding an input that results in same digest as some other input given);
+ - Extra-metadata information necessary for extrinsic decoding and constant within runtime version MUST be included in digest;
+ - It SHOULD be possible to quickly withdraw offline signing mechanism without access to cold signing devices;
+ - Digest format SHOULD be versioned.
+ - Work necessary for proving metadata authenticity MAY be omitted at discretion of signer device design (to support automation tools).
+
+#### Reduce metadata size
+
+Metadata should be stripped from parts that are not necessary to parse a signable extrinsic, then it should be separated into a finite set of self-descriptive chunks. Thus, a subset of chunks necessary for signable extrinsic decoding and rendering could be sent, possibly in small portions (ultimately - one at a time), to cold device together with proof.
+
+ - Single chunk with proof payload size SHOULD fit within few kB;
+ - Chunks handling mechanism SHOULD support chunks being sent in any order without memory utilization overhead;
+ - Unused enum variants MUST be stripped (this has great impact on transmitted metadata size; examples: era enum, enum with all calls for call batching).
+
+## Stakeholders
+
+All chain teams are stakeholders, as implementing this feature would require timely effort on their side and would impact compatibility with older tools.
+
+This feature is essential for **all** offline signer tools; many regular signing tools might make use of it. In general, this RFC greatly improves security of any network implementing it, as many governing keys are used with offline signers.
+
+Implementing this RFC would remove requirement to maintain metadata portals manually, as task of metadata verification would be effectively moved to consensus mechanism of the chain.
+
+## Explanation
+
+Detailed description of metadata shortening and digest process is provided in [metadata-shortener](https://github.com/Alzymologist/metadata-shortener) crate (see `cargo doc --open` and examples). Below are presented algorithms of the process.
+
+### Definitions
+
+
+#### Metadata descriptor
+
+Values for:
+
+1. `u8` metadata shortening protocol version, 
+2. SCALE-encoded `ExtrinsicMetadata`,
+3. SCALE-encoded `spec_version` `String`,
+4. SCALE-encoded `spec_name` `String`,
+5. `u16` ss58 prefix,
+6. `u8` decimals value or `0u8` if no units are defined,
+7. SCALE-encoded `tokenSymbol` `String` defined on chain to identify the name of currency (available for example through `system.properties()` RPC call) or empty string if no base units are defined,
+
+```
+enum MetadataDescriptor {
+  V0,
+  V1(MetadataDescriptorV1),
+}
+
+struct MetadataDescriptorV1 {
+  extrinsic_metadata: Vec<u8>, // SCALE from `ExtrinsicMetadata`
+  spec_version: Vec<u8>, // SCALE form `String`
+  spec_name: Vec<u8>, // SCALE from `String`
+  ss58_prefix: u16,
+  decimals: u8,
+  token_symbol: Vec<u8>, // SCALE from `String`
+}
+```
+
+constitute metadata descriptor. This is minimal information that is, together with (shortened) types registry, sufficient to decode any signable transaction.
+
+#### Merkle tree
+
+A **Complete Binary Merkle Tree** (**CBMT**) is proposed as digest structure.
+
+Every node of the proposed tree has a 32-bit value.
+
+A terminal node of the tree we call **leaf**. Its value is input for digest.
+
+The top node of the tree we call **root**.
+
+All node values for non-leave nodes are not terminal are computed through non-commutative **merge** procedure of child nodes.
+
+In CBMT, all layers must be populated, except for the last one, that must have complete filling from the left.
+
+Nodes are numbered top-down and left-to-right starting with 0 at the top of tree.
+
+```
+Example 8-node tree
+
+        0
+     /     \
+    1       2
+   / \     / \
+  3   4   5   6
+ / \
+7   8
+
+Nodes 4, 5, 6, 7, 8 are leaves
+Node 0 is root
+
+```
+
+### General flow
+
+1. The metadata is converted into lean modular form (vector of chunks)
+2. A Merkle tree is constructed from the metadata chunks
+3. A root of tree is merged with the hash of the `MetadataDescriptor`
+4. Resulting value is a constant to be included in `additionalSigned` to prove that the metadata seen by cold device is genuine
+
+### Metadata modularization
+
+Structure of types in shortened metadata exactly matches structure of types in `scale-info` at MetadataV14 state, but `doc` field is always empty
+
+```
+struct Type {
+  path: Path, // vector of strings
+  type_params: Vec<TypeParams>,
+  type_def: TypeDef, // enum of various types
+  doc: Vec<String>,
+}
+
+struct TypeParams {
+  name: String,
+  ty: Option<Type>,
+}
+```
+
+1. Types registry is stripped from `docs` fields.
+2. Types records are separated into chunks, with enum variants being individual chunks differing by variant index; each chunk consisting of `id` (same as in full metadata registry) and SCALE-encoded 'Type' description (reduced to 1-variant enum for enum variants). Enums with 0 variants are treated as regular types.
+3. Chunks are sorted by `id` in ascending order; chunks with same `id` are sorted by enum variant index in ascending order.
+
+```
+types_registry = metadataV14.types
+modularized_registry = EmptyVector<id, type>
+for (id, type) in types.registry.iterate_enumerate {
+  type.doc = empty_vector
+  if (type is ReduceableEnum) { // false for 0-variant enums
+    for variant in type.variants.iterate {
+      variant_type = Type {
+        path: type.path
+        type_params: empty_vector
+        type_def: TypeDef::Variant(variants: [variant])
+      }
+      modularized_registry.push(id, variant_type)
+    }
+  } else {
+    modularized_registry.push(id, type)
+  }
+}
+
+modularized_registry.sort(|a, b| {
+    if a.id == b.id { //only possible for variants
+      a.variant_index > b.variant_index
+    } else { a.id > b.id }
+  }
+)
+
+```
+
+### Merging protocol
+
+`blake3` transformation of concatenated child nodes (`blake3(left + right)`) as merge procedure;
+
+### Complete Binary Merkle Tree construction protocol
+
+1. Leaves are numbered in ascending order. Leaf index is associated with corresponding chunk.
+2. Merge is performed using the leaf with highest index as right and leaf with second to highest index as left children; result is pushed to the end of nodes queue and leaves are discarded.
+3. Step (2) is repeated until no leaves or just one leaf remains; in latter case, the last leaf is pushed to the front of the nodes queue.
+4. Right node and then left node is popped from the front of the nodes queue and merged; the result is sent to the end of the queue.
+5. Step (4) is repeated until only one node remains; this is tree root.
+
+
+```
+queue = empty_queue
+
+while (leaves.length>1) {
+  right = leaves.pop_last
+  left = leaves.pop_last
+  queue.push_back(merge(left, right))
+}
+
+if leaves.length == 1 {
+  queue.push_front(leaves.last)
+}
+
+while queue.len() > 1 {
+  right = queue.pop_front
+  left = queue.pop_front
+  queue.push_back(merge(left, right))
+}
+
+return queue.pop
+```
+
+```
+Resulting tree for metadata consisting of 5 nodes (numbered from 0 to 4):
+
+       root
+     /     \
+    *       *
+   / \     / \
+  *   0   1   2
+ / \
+3   4
+```
+
+### Digest
+
+1. Blake3 hash is computed for each chunk of modular short metadata registry.
+3. Complete Binary Merkle Tree is constructed as described above.
+4. Root hash of this tree (left) is merged with metadata descriptor blake3 hash (right); this is metadata digest.
+
+Version number and corresponding resulting metadata digest MUST be included into Signed Extensions as specified in Chain Verification section below.
+
+### Chain verification
+
+The root of metadata computed by cold device MAY be included into Signed Extensions; if it is included, the transaction will pass as valid iff hash of metadata as seen by cold storage device is identical to consensus hash of metadata, ensuring fair signing protocol.
+
+The Signed Extension representing metadata digest is a single byte representing both digest vaule inclusion and shortening protocol version; this MUST be included in Signed Extensions set. Depending on its value, a digest value is included as `additionalSigned` to signature computation according to following specification:
+
+| signed extension value | digest value   | comment                            |
+|------------------------|----------------|------------------------------------|
+| `0x00`                 |                | digest is not included             |
+| `0x01`                 | 32-byte digest | this represents protocol version 1 |
+| `0x02` - `0xFF`        | *reserved*     | reserved for future use            |
+
+## Drawbacks
+
+### Increased transaction size
+
+A 1-byte increase in transaction size due to signed extension value. Digest is not included in transferred transaction, only in signing process.
+
+### Transition overhead
+
+Some slightly out of spec systems might experience breaking changes as new content of signed extensions is added - tools that delay their transition instead of preparing ahead of time would break for the duration of delay. It is important to note, that there is no real overhead in processing time nor complexity, as the metadata checking mechanism is voluntary.
+
+## Testing, Security, and Privacy
+
+The metadata shortening protocol should be extensively tested on all available examples of metadata before releasing changes to either metadata or shortener. Careful code review should be performed on shortener implementation code to ensure security. The main metadata tree would inevitably be constructed on runtime build which would also ensure correctness.
+
+To be able to recall shortener protocol in case of vulnerability issues, a version byte is included.
+
+## Performance, Ergonomics, and Compatibility
+
+### Performance
+
+This is negligibly short pessimization during build time on the chain side. Cold wallets performance would improve mostly as metadata validity mechanism that was taking most of effort in cold wallet support would become trivial.
+
+### Ergonomics
+
+The proposal was optimized for cold storage wallets usage with minimal impact on all other parts of the ecosystem
+
+### Compatibility
+
+Proposal in this form is not compatible with older tools that do not implement proper MetadataV14 self-descriptive features; those would have to be upgraded to include a new signed extensions field.
+
+## Prior Art and References
+
+This project was developed upon a Polkadot Treasury grant; relevant development links are located in [metadata-offline-project](https://github.com/Alzymologist/metadata-offline-project) repository.
+
+## Unresolved Questions
+
+
+## Future Directions and Related Material
+
+Changes to code of all cold signers to implement this mechanism SHOULD be done when this is enabled; non-cold signers may perform extra metadata check for better security. Ultimately, signing anything without decoding it with verifiable metadata should become discouraged in all situations where a decision-making mechanism is involved (that is, outside of fully automated blind signers like trade bots or staking rewards payout tools).
+