certified-variables

name	certified-variables
description	Serve cryptographically verified responses from query calls using Merkle trees and subnet BLS signatures. Covers certified data API, RbTree/CertTree construction, witness generation, and frontend certificate validation. Use when query responses need verification, certified data, or response authenticity proofs.
license	Apache-2.0
compatibility	icp-cli >= 0.2.2
metadata	{"title":"Certified Variables","category":"Security"}

Certified Variables & Certified Assets

What This Is

Query responses on the Internet Computer come from a single replica and are NOT verified by consensus. A malicious or faulty replica could return fabricated data. Certification solves this: the canister stores a hash in the subnet's certified state tree during update calls, and then query responses include a certificate signed by the subnet's threshold BLS key proving the data is authentic. The result is responses that are both fast (no consensus delay) AND cryptographically verified.

Prerequisites

• Rust: ic-certified-map crate (for Merkle tree), ic-cdk (for certified_data_set / data_certificate)
• Motoko: CertifiedData module (included in mo:core/mo:base), ic-certification package (mops add ic-certification) for Merkle tree with witness support
• Frontend: @icp-sdk/core (>= 5.0.0) (agent, principal), @dfinity/certificate-verification (>= 3.1.0)

Canister IDs

No external canister IDs required. Certification uses the IC system API exposed through CDK wrappers:

• ic_cdk::api::certified_data_set (Rust) / CertifiedData.set (Motoko) -- called during update calls to set the certified hash (max 32 bytes)
• ic_cdk::api::data_certificate (Rust) / CertifiedData.getCertificate (Motoko) -- called during query calls to retrieve the subnet certificate

The IC root public key (needed for client-side verification):

• Mainnet: 308182301d060d2b0601040182dc7c0503010201060c2b0601040182dc7c05030201036100814c0e6ec71fab583b08bd81373c255c3c371b2e84863c98a4f1e08b74235d14fb5d9c0cd546d9685f913a0c0b2cc5341583bf4b4392e467db96d65b9bb4cb717112f8472e0d5a4d14505ffd7484b01291091c5f87b98883463f98091a0baaae
• Local: available from icp (agent handles this automatically)

Mistakes That Break Your Build

1. Trying to store more than 32 bytes of certified data. The certified_data_set API accepts exactly one blob of at most 32 bytes. You cannot certify arbitrary data directly. Instead, build a Merkle tree over your data and certify only the root hash (32 bytes). The tree structure provides proofs for individual values.

2. Calling certified_data_set in a query call. Certification can ONLY be set during update calls (which go through consensus). Calling it in a query traps. Pattern: set the hash during writes, read the certificate during queries.

3. Forgetting to include the certificate in query responses. The certificate is obtained via data_certificate() during query calls. If you return data without the certificate, clients cannot verify anything. Always return a tuple of (data, certificate, witness).

4. Not updating the certified hash after data changes. If you modify the data but forget to call certified_data_set with the new root hash, query responses will fail verification because the certificate proves a stale hash.

5. Building the witness for the wrong key. The witness (Merkle proof) must correspond to the exact key being queried. A witness for key "users/alice" will not verify key "users/bob".

6. Assuming data_certificate() returns a value in update calls. It returns null/None during update calls. Certificates are only available during query calls.

7. Certifying data at canister init but not on upgrades. After a canister upgrade, the certified data is cleared. You must call certified_data_set in both #[init] and #[post_upgrade] (Rust) or system func postupgrade (Motoko) to re-establish certification.

8. Not validating certificate freshness on the client. The certificate's state tree contains a /time field with the timestamp when the subnet produced it. Clients MUST check that this timestamp is recent (recommended: within 5 minutes of current time). Without this check, an attacker could replay a stale certificate with outdated data. Always verify certificate_time is within an acceptable delta before trusting the response.

How Certification Works

UPDATE CALL (goes through consensus):
  1. Canister modifies data
  2. Canister builds/updates Merkle tree
  3. Canister calls certified_data_set(root_hash)  -- 32 bytes
  4. Subnet includes root_hash in its certified state tree

QUERY CALL (single replica, no consensus):
  1. Client sends query
  2. Canister calls data_certificate() -- gets subnet BLS signature
  3. Canister builds witness (Merkle proof) for the requested key
  4. Canister returns: { data, certificate, witness }

CLIENT VERIFICATION:
  1. Verify certificate signature against IC root public key
  2. Extract root_hash from certificate's state tree
  3. Verify witness: root_hash + witness proves data is in the tree
  4. Trust the data

Implementation

Rust

Cargo.toml:

[package]
name = "certified_vars_backend"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]

[dependencies]
candid = "0.10"
ic-cdk = "0.19"
ic-certified-map = "0.4"
serde = { version = "1", features = ["derive"] }
serde_bytes = "0.11"
ciborium = "0.2"

Complete certified key-value store:

use candid::{CandidType, Deserialize};
use ic_cdk::{init, post_upgrade, query, update};
use ic_certified_map::{AsHashTree, RbTree};
use serde_bytes::ByteBuf;
use std::cell::RefCell;

thread_local! {
    // RbTree is a Merkle-tree-backed map: keys and values are byte slices
    static TREE: RefCell<RbTree<Vec<u8>, Vec<u8>>> = RefCell::new(RbTree::new());
}

// Update the certified data hash after any modification
fn update_certified_data() {
    TREE.with(|tree| {
        let tree = tree.borrow();
        // root_hash() returns a 32-byte SHA-256 hash of the entire tree
        ic_cdk::api::certified_data_set(&tree.root_hash());
    });
}

#[init]
fn init() {
    update_certified_data();
}

#[post_upgrade]
fn post_upgrade() {
    // Assumes data has already been deserialized from stable memory into the TREE.
    // CRITICAL: re-establish certification after upgrade — certified_data is cleared on upgrade.
    update_certified_data();
}

#[update]
fn set(key: String, value: String) {
    TREE.with(|tree| {
        let mut tree = tree.borrow_mut();
        tree.insert(key.as_bytes().to_vec(), value.as_bytes().to_vec());
    });
    // Must update certified hash after every data change
    update_certified_data();
}

#[update]
fn delete(key: String) {
    TREE.with(|tree| {
        let mut tree = tree.borrow_mut();
        tree.delete(key.as_bytes());
    });
    update_certified_data();
}

#[derive(CandidType, Deserialize)]
struct CertifiedResponse {
    value: Option<String>,
    certificate: ByteBuf,      // subnet BLS signature
    witness: ByteBuf,          // Merkle proof for this key
}

#[query]
fn get(key: String) -> CertifiedResponse {
    // data_certificate() is only available in query calls
    let certificate = ic_cdk::api::data_certificate()
        .expect("data_certificate only available in query calls");

    TREE.with(|tree| {
        let tree = tree.borrow();

        // Look up the value
        let value = tree.get(key.as_bytes())
            .map(|v| String::from_utf8(v.clone()).unwrap());

        // Build a witness (Merkle proof) for this specific key
        let witness = tree.witness(key.as_bytes());

        // Serialize the witness as CBOR
        let mut witness_buf = vec![];
        ciborium::into_writer(&witness, &mut witness_buf)
            .expect("Failed to serialize witness as CBOR");

        CertifiedResponse {
            value,
            certificate: ByteBuf::from(certificate),
            witness: ByteBuf::from(witness_buf),
        }
    })
}

// Batch set multiple values in one update call (more efficient)
#[update]
fn set_many(entries: Vec<(String, String)>) {
    TREE.with(|tree| {
        let mut tree = tree.borrow_mut();
        for (key, value) in entries {
            tree.insert(key.as_bytes().to_vec(), value.as_bytes().to_vec());
        }
    });
    // Single certification update for all changes
    update_certified_data();
}

HTTP Certification (v2) for Custom HTTP Canisters

For canisters serving HTTP responses directly (not through the asset canister), responses must be certified so the HTTP gateway can verify them.

Additional Cargo.toml dependency:

[package]
name = "http_certified_backend"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]

[dependencies]
ic-http-certification = "3.1"

Certifying HTTP responses:

Note: The HTTP certification API is evolving rapidly. Verify these examples against the latest ic-http-certification docs before use.

use ic_http_certification::{
    HttpCertification, HttpCertificationPath, HttpCertificationTree,
    HttpCertificationTreeEntry, HttpRequest, HttpResponse,
    DefaultCelBuilder, DefaultResponseCertification,
};
use std::cell::RefCell;

thread_local! {
    static HTTP_TREE: RefCell<HttpCertificationTree> = RefCell::new(
        HttpCertificationTree::default()
    );
}

// Define what gets certified using CEL (Common Expression Language)
fn certify_response(path: &str, request: &HttpRequest, response: &HttpResponse) {
    // Full certification: certify both request path and response body
    let cel = DefaultCelBuilder::full_certification()
        .with_response_certification(DefaultResponseCertification::certified_response_headers(
            vec!["Content-Type", "Content-Length"],
        ))
        .build();

    // Create the certification from the CEL expression, request, and response
    let certification = HttpCertification::full(&cel, request, response, None)
        .expect("Failed to create HTTP certification");

    let http_path = HttpCertificationPath::exact(path);

    HTTP_TREE.with(|tree| {
        let mut tree = tree.borrow_mut();
        let entry = HttpCertificationTreeEntry::new(http_path, certification);
        tree.insert(&entry);

        // Update canister certified data with tree root hash
        ic_cdk::api::certified_data_set(&tree.root_hash());
    });
}

Motoko

Using CertifiedData module:

import CertifiedData "mo:core/CertifiedData";
import Blob "mo:core/Blob";
import Nat8 "mo:core/Nat8";
import Text "mo:core/Text";
import Map "mo:core/Map";
import Array "mo:core/Array";
import Iter "mo:core/Iter";
// Requires: mops add sha2
import Sha256 "mo:sha2/Sha256";

persistent actor {

  // Simple certified single-value example:
  var certifiedValue : Text = "";

  // Set a certified value (update call only)
  public func setCertifiedValue(value : Text) : async () {
    certifiedValue := value;
    // Hash the value and set as certified data (max 32 bytes)
    let hash = Sha256.fromBlob(#sha256, Text.encodeUtf8(value));
    CertifiedData.set(hash);
  };

  // Get the certified value with its certificate (query call)
  public query func getCertifiedValue() : async {
    value : Text;
    certificate : ?Blob;
  } {
    {
      value = certifiedValue;
      certificate = CertifiedData.getCertificate();
    }
  };
};

Certified key-value store with Merkle tree (advanced):

For certifying multiple values with per-key witnesses, use the ic-certification mops package (mops add ic-certification). It provides a real Merkle tree (CertTree) that can generate proofs for individual keys:

import CertifiedData "mo:core/CertifiedData";
import Blob "mo:core/Blob";
import Text "mo:core/Text";
// Requires: mops add ic-certification
import CertTree "mo:ic-certification/CertTree";

persistent actor {

  // CertTree.Store is stable -- persists across upgrades
  let certStore : CertTree.Store = CertTree.newStore();
  let ct = CertTree.Ops(certStore);

  // Set certified data on init
  ct.setCertifiedData();

  // Set a key-value pair and update certification
  public func set(key : Text, value : Text) : async () {
    ct.put([Text.encodeUtf8(key)], Text.encodeUtf8(value));
    // CRITICAL: call after every mutation to update the subnet-certified root hash
    ct.setCertifiedData();
  };

  // Delete a key and update certification
  public func remove(key : Text) : async () {
    ct.delete([Text.encodeUtf8(key)]);
    ct.setCertifiedData();
  };

  // Query with certificate and Merkle witness for the requested key
  public query func get(key : Text) : async {
    value : ?Blob;
    certificate : ?Blob;
    witness : Blob;
  } {
    let path = [Text.encodeUtf8(key)];
    // reveal() generates a Merkle proof for this specific path
    let witness = ct.reveal(path);
    {
      value = ct.lookup(path);
      certificate = CertifiedData.getCertificate();
      witness = ct.encodeWitness(witness);
    }
  };

  // Re-establish certification after upgrade
  // (CertTree.Store is stable, so the tree data survives, but certified_data is cleared)
  system func postupgrade() {
    ct.setCertifiedData();
  };
};

Frontend Verification (TypeScript)

Uses @dfinity/certificate-verification which handles the full 6-step verification:

1. Verify certificate BLS signature against IC root key
2. Validate certificate freshness (/time within maxCertificateTimeOffsetMs)
3. CBOR-decode the witness into a HashTree
4. Reconstruct the witness root hash
5. Compare reconstructed root hash with certified_data from the certificate
6. Return the verified HashTree for value lookup

import { verifyCertification } from "@dfinity/certificate-verification";
import { lookup_path, HashTree } from "@icp-sdk/core/agent";
import { Principal } from "@icp-sdk/core/principal";

const MAX_CERT_TIME_OFFSET_MS = 5 * 60 * 1000; // 5 minutes

async function getVerifiedValue(
  rootKey: ArrayBuffer,
  canisterId: string,
  key: string,
  response: { value: string | null; certificate: ArrayBuffer; witness: ArrayBuffer }
): Promise<string | null> {
  // verifyCertification performs steps 1-5:
  //  - verifies BLS signature on the certificate
  //  - checks certificate /time is within maxCertificateTimeOffsetMs
  //  - CBOR-decodes the witness into a HashTree
  //  - reconstructs root hash from the witness tree
  //  - compares it against certified_data in the certificate
  // Throws CertificateTimeError or CertificateVerificationError on failure.
  const tree: HashTree = await verifyCertification({
    canisterId: Principal.fromText(canisterId),
    encodedCertificate: response.certificate,
    encodedTree: response.witness,
    rootKey,
    maxCertificateTimeOffsetMs: MAX_CERT_TIME_OFFSET_MS,
  });

  // Step 6: Look up the specific key in the verified witness tree.