Response Verification

Introduction

The Internet Computer Protocol features two primary types of calls that can be used to interact with canisters: query calls and update calls.

Query Calls	Update Calls
Readonly, cannot change canister state.	Can change canister state.
Response does not go through consensus.	Response goes through consensus.
Answered by a single replica on the canister's subnet.	Answered by every replica on the canister's subnet.

The different properties of query and update calls present a trade off in terms of performance and security. Since query call responses do not go through consensus and are fulfilled by only a single replica, they are much faster than update calls. In contrast, the lack of consensus poses a security concern and these responses cannot be trusted since any individual replica can arbitrarily change the response.

Canister certification provides a solution to this problem by pre-calculating responses and putting those pre-calculated responses through consensus. Consensus will produce a certificate for the pre-calculated responses that can be returned with the responses by any individual replica. Any clients requesting these responses can then verify this certificate to validate that consensus has agreed on this response prior to the request. This approach will give the performance characteristics of a query call, with the security profile of an update call, providing the best of both types of calls.

Canister certification is enabled by the Certified data feature of the Internet Computer. This feature allows canisters to specify a 32 byte blob that will be included in consensus. How this 32 byte blob is determined is entirely up to canister developers to decide for themselves. This repo provides solutions that rely on merkle trees and won't discuss any other methods, but there are many other possibilities for imaginative developers that would like to roll their own solution.

In the next consensus round following this 32 byte blob being set (via ic0.certified_data_set), the Internet Computer will prepare a certificate for the canister. This certificate, and the process for verifying it, is described in detail in the interface spec.

When responding to query calls, a canister can request the certificate (via ic0.certified_data_size and ic0.certified_data_copy) and then include this certificate in the response, to allow for clients to validate the response's authenticity.

Standard Certification

The standard, or default, method of certification is the simplest form of certification. The high-level procedure for certification is as follows:

1. Pre-calculate any query call responses that need to be certified.
2. Arrange query call response hashes into a merkle tree.
3. Calculate the root hash of the merkle tree.
4. Set the root hash as the canister's certified data.
5. If an update call is received that changes state, return to step 1.
6. If a query call is received, return the pre-calculated response along with the certified data certificate.

There are a number of questions that a developer needs to answer on a per-project basis when using this style of certification that should be agreed upon with clients beforehand and done consistently by both client and canister:

• How are responses hashed?
  • What properties are included/excluded? (e.g., response_timestamp is excluded but id is included).
  • What format of each property is hashed? (e.g., leb, big/little endian encoding for numbers).
  • In what order are properties hashed? (e.g., sort properties alphabetically).
  • The representation independent hash from the Internet Computer Protocol spec can be used, or serve as inspiration for developers.
• How are response hashes arranged in the tree?
  • Each exposed canister method should have a pre-determined path where its response hash will be presented in the merkle tree (e.g., the get_count method returns a response and the hash of this response is present at the ["count"] path of the merkle tree).
  • This pre-determined path may use parameters of the request or calling principal to arrange more dynamic data in the tree (e.g., the get_account method returns a response and the hash of this response is present at the ["account", caller.to_text()] path of the merkle tree).

Process Flow

flowchart TB;
  subgraph lifecycle_hooks[Lifecycle hooks]
    init;
    post_upgrade;
  end
  init-->certification;
  post_upgrade-->certification;

  subgraph update_calls
    update_call;
    update_state;
    update_call-->update_state;
  end
  update_state-->certification;

  subgraph query_calls
    direction TB;

    query_call;
    serve_response[Serve pre-calculated response];
    query_call-->serve_response;
  end

  subgraph certification[Certification]
    direction TB;

    certification_step_one[pre-calculate response];
    certification_step_two[insert response hash into merkle tree];
    certification_step_three[calculate root hash of merkle tree];
    certification_step_four[set canister certified data];

    certification_step_one-->certification_step_two-->certification_step_three-->certification_step_four;
  end

Resources

• ic-certification Cargo crate.
• ic-certification docs.
• Example with a certified counter.
• ic-certification source code.
• ic-representation-independant-hash Cargo crate.
• ic-representation-independant-hash docs.
• ic-representation-independent-hash source code.

Certificate verification

Certifications of a response need to be verified by the caller. The process for this is as follows:

1. Extract the canister's certificate from the response and verify it:
   • If the certificate has a delegation:
     1. Verify the delegation's signature using the Internet Comptuer Protocol's public key.
     2. Assert that the delegation is valid for the canister's canister ID.
     3. Extract the subnet's public key from the delegation.
     4. Verify the certificate's signature using the subnet's public key.
   • Otherwise:
     1. Verify the certificate's signature using the Internet Computer Protocol's public key.
2. Verify that the certificate's time is not too far in the past or future.
3. Extract the canister's merkle tree from the response and calculate it's root hash.
4. Ensure that the merkle tree's root hash matches the canister's certified data in the certificate.
5. Calculate the hash of the canister's response.
6. Ensure that the response hash is in the merkle tree at the expected path.

Steps 1 through 4 (inclusive) is encapsulated by the @dfinity/certificate-verification (JavaScript) and the ic-certificate-verification (Rust) packages.

For testing code that verifies responses, the @dfinity/certification-testing (JavaScript) and ic-certification-testing (Rust, unpublished) packages can be used to create certifications.

Resources

• @dfinity/certificate-verification source code.
• @dfinity/certificate-verification NPM package.
• ic-certificate-verification Cargo crate.
• ic-certificate-verification docs.
• ic-certificate-verification source code.
• @dfinity/certification-testing NPM package.
• @dfinity/certification-testing source code.
• ic-certification-testing source code (unpublished).

HTTP Certification

Canisters may leverage the HTTP Gateway Protocol to serve HTTP-compatible responses to HTTP-aware clients, such as a web browser. This is a much more involved process than standard certification. With standard certification canister developers are free to make a lot of decisions on their own because they will likely also control the clients that will be performing certification. This is in contrast to HTTP certification where a generic client (the HTTP Gateway) will verify all canisters implementing this protocol.

Process Flow

flowchart TB;
  subgraph lifecycle_hooks[Lifecycle hooks]
    init;
    post_upgrade;
  end
  init-->certification;
  post_upgrade-->certification;

  subgraph query_calls[Query calls]
    http_request;
    should_upgrade{State change?};
    serve_response[Serve pre-calculated response];
    upgrade[Upgrade to update call];

    http_request-->should_upgrade;
    should_upgrade -- No-->serve_response;
    should_upgrade -- Yes-->upgrade;
  end
  upgrade-->update_calls;

  subgraph update_calls[Update calls]
    http_request_update;
    update_state;

    http_request_update-->update_state;
  end
  update_state-->certification;

  subgraph certification[Certification]
    direction TB;

    certification_step_one[pre-calculate response];
    certification_step_two[pre-calculate CEL expression];
    certification_step_three[pre-calculate certification];

    certification_step_one[pre-calculate response];
    certification_step_two[pre-calculate CEL expression];
    certification_step_three[insert response hash into merkle tree];
    certification_step_four[calculate root hash of merkle tree];
    certification_step_five[set canister certified data];

    certification_step_one-->certification_step_two-->certification_step_three-->certification_step_four-->certification_step_five;
  end

Resources

• ic-http-certification Cargo crate.
• ic-http-certification docs.
• Example project serving a REST API.
• Example project with a custom implementation serving static assets.
• ic-http-certification source code.

Response Verification

Response verification on the Internet Computer is the process of verifying that an HTTP-compatible canister response from a replica has gone through consensus with other replicas hosting the same canister. It is the counterpart to HTTP Certification.

The ic-response-verification and @dfinity/response-verification packages encapsulate this verification protocol. It is used by ICX Proxy and the local HTTP Proxy and may be used by other implementations of the HTTP Gateway Protocol in the future.

Resources

• ic-response-verificaiton source code
• @dfinity/response-verification source code
• Example project that verifies responses in NodeJS
• Example project that verifies responses in Rust
• Example project that verifies responses in the browser

Miscellaneous Projects

Representation Independent Hash

This is a utility crate to implement representation independent hashing of data.

• ic-representation-independant-hash Cargo crate.
• ic-representation-independant-hash docs.
• ic-representation-independent-hash source code.

CBOR

This is a utilty crate to implement decoding of CBOR-encoded data.

• ic-cbor Cargo crate.
• ic-cbor docs.
• ic-cbor source code.

Contributing

Check out the contribution guidelines.

Command Reference

Make sure to follow the system setup instructions first.

Command	Description
cargo build	Build all Cargo crates
cargo test	Test all Cargo crates
cargo fmt	Format all Cargo crates
pnpm build	Build all NPM packages
pnpm test	Test all NPM packages
pnpm format	Format all NPM packages

Certification

Command	Description
cargo build -p ic-certification	Build Cargo crate
cargo test -p ic-certification	Test Cargo crate
cargo doc -p ic-certification --no-deps --open	Build Cargo crate docs

Certificate Verification

Command	Description
pnpm run -F @dfinity/certificate-verification build	Build NPM package
pnpm run -F @dfinity/certificate-verification test	Test NPM package

Certification Testing

Command	Description
cargo build -p ic-certification-testing	Build Cargo crate
cargo doc -p ic-certification-testing --no-deps --open	Build Cargo crate docs
pnpm run -F @dfinity/certification-testing build	Build NPM package

HTTP Certification

Command	Description
cargo build -p ic-http-certification	Build Cargo crate
cargo test -p ic-http-certification	Test Cargo crate
cargo doc -p ic-http-certification --no-deps --open	Build Cargo crate docs

Response Verification

Command	Description
cargo build -p ic-response-verification	Build Cargo crate
cargo test -p ic-response-verification	Test Cargo crate
wasm-pack test --node packages/ic-response-verification --features=js	Test Cargo crate WASM
cargo doc -p ic-response-verification --no-deps --open	Build Cargo crate docs
pnpm run -F @dfinity/response-verification build	Build NPM package
pnpm run -F @dfinity/response-verification test	Test NPM package
./scripts/e2e.sh	Run e2e tests

Representation Independent Hash

Command	Description
cargo build -p ic-representation-independent-hash	Build Cargo crate
cargo test -p ic-representation-independent-hash	Test Cargo crate
cargo doc -p ic-representation-independent-hash --no-deps --open	Build Cargo crate docs

CBOR

Command	Description
cargo build -p ic-cbor	Build Cargo crate
cargo test -p ic-cbor	Test Cargo crate
cargo doc -p ic-cbor --no-deps --open	Build Cargo crate docs

System Setup

• Install pre-commit
• Install commitizen
• Install Rust
• Install wasm-pack
• Install NVM
• Install DFX

Install the correct version of NodeJS:

nvm install

Activate the correct version of NodeJS:

nvm use

Install and activate the correct version of PNPM:

corepack enable

Working on WASM crates

Until Cargo supports per package targets, the WASM crates are excluded from the default_members array of the Cargo workspace. Commands such as cargo build and cargo check will not include these crates, so they must be built separately with the corresponding pnpm command listed under projects.

Since rust-analyzer will also apply the same target to all crates, these crates will show errors in the IDE. To workaround this, create a .cargo/config.toml file:

[build]
target = "wasm32-unknown-unknown"

While this file exists, some of the non-WASM crates will show errors instead. Delete the file to work on the non-WASM crates.

Making a Commit

cz commit

See Conventional commits for more information on the commit message formats.

Adding a new package

• Follow the Package naming conventions when naming the package.
• Add the package's package manager file to the version_files field in .cz.yaml.
  • package.json for NPM packages
  • Nothing for for Cargo crates, it is already covered by the root Cargo.toml
• Set the initial version of the package in its package manager file to match the current version in the version field in .cz.yaml
  • For package.json, set the version manually
  • For Cargo.toml, use version.workspace = true
• Add the package's package manager file(s) to the add-paths property in the Create Pull Request job of the Create Release PR workflow in .github/workflows/create-release-pr.yml
  • package.json for NPM packages
  • No files need to be added for Cargo crates
• If the package is a Rust crate:
  • Add the package to the members section in Cargo.toml and the default-members section
    • If the package must be compiled to WASM then do not add it to the default-members section
  • Add a Release ic-<package-name> Cargo crate job to the Release workflow in .github/workflows/release.yml
  • Add target/package/ic-<package-name>-${{ github.ref_name }}.crate to the artifacts property in the Create Github release job of the Release workflow in .github/workflows/release.yml
    • Make sure every entry except the last is comma delimited
  • If the crate has dependenencies in this repository, make sure it is published after the dependencies
  • If the crate has a dependent in this repository, make sure it is published before the dependents
• If the package is an NPM package:
  • Add the package to pnpm-workspace.yaml
  • Add a Pack @dfinity/<package-name> NPM package job to the Release workflow in .github/workflows/release.yml
  • Add a Release @dfinity/<package-name> NPM package job to the Release workflow in .github/workflows/release.yml
  • Add dfinity-<package-name>-${{ github.ref_name }}.tgz to the artifacts property in the Create Github release job of the Create Release PR workflow in .github/workflows/create-release-pr.yml
    • Make sure every entry except the last is comma delimited

Package naming conventions

Cargo crates are named ic-<package-name>, likewise for the folder name. If the Cargo crate will be compiled to WASM then the folder name is ic-<package-name>-wasm.

NPM packages are named @dfinity/<package-name>. If the NPM package is a pure JS package then the folder name is <package-name>-js. If the NPM package is built from a Rust crate then the folder name is ic-<package-name>-wasm.

Referencing a Cargo crate

A Cargo crate can be referenced using a relative path in Cargo.toml:

[dependencies]
ic-response-verification-test-utils = { path = "../ic-response-verification-test-utils" }

If the referencing Cargo crate is published to crates.io then the current version must be included and the referenced crate must also be published:

[dependencies]
ic-response-verification-test-utils = { path = "../ic-response-verification-test-utils", version = "1.0.0" }

If a version is included in a dev dependency then the referenced dev dependency must also be published, but the version can be omitted for dev dependencies to avoid this.

Referencing an NPM package

An NPM package can be referenced using the package name and PNPM workspace protocol in package.json:

{
  "dependencies": {
    "@dfinity/certificate-verification": "workspace:*"
  }
}