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Derive a Lit Action Public Key Locally

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Question

I want to call Lit.Actions.signAsAction. I know the action identity is derived from the Action’s IPFS CID, but I cannot find a way to obtain the public key outside of the Action runtime. Lit.Actions.getActionPublicKey works within the Action, while executeJs only exposes signatures.<name>.publicKey after a signing operation. Is there a way to deterministically derive the Action’s public key locally without running the Action?

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Answer

Yes. Inside the Lit Action you can deterministically derive the Action identity (and therefore its public key) from the same inputs the nodes use: the Action’s IPFS CID and the signing scheme. The snippet below shows the complete flow:
  1. Produce the 32-byte message hash the Lit nodes expect.
  2. Call Lit.Actions.signAsAction to sign that message with the Action identity.
  3. Derive the Action public key via Lit.Actions.getActionPublicKey, passing the Action CID and signing scheme.
  4. Optionally verify the signature with Lit.Actions.verifyActionSignature.
const { sigName, toSign } = jsParams; // 'publicKey' not required; derive it from the Action IPFS CID
const { keccak256, arrayify } = ethers.utils;

(async () => {
  // 1) Produce a 32-byte hash of the input (Lit Actions expect a 32-byte message for ECDSA schemes)
  const msgBytes = new TextEncoder().encode(toSign);
  const msgHashHex = keccak256(msgBytes);       // 0x-prefixed hex string
  const msgHashBytes = arrayify(msgHashHex);    // Uint8Array

  // 2) Sign as the current Lit Action (deterministic Action identity, not a PKP)
  //    Supported schemes include 'EcdsaK256Sha256' (secp256k1) among others.
  const signingScheme = 'EcdsaK256Sha256';
  const signature = await Lit.Actions.signAsAction({
    toSign: msgHashBytes,
    sigName,
    signingScheme,
  });

  // 3) Derive this Action's public key deterministically from its IPFS CID + scheme
  //    This does not require a PKP and is always the same for a given (CID, scheme).
  const actionIpfsCid = Lit.Auth.actionIpfsIdStack[0];
  const actionPublicKey = await Lit.Actions.getActionPublicKey({
    signingScheme,
    actionIpfsCid,
  });

  // 4) (Optional) Verify that the signature was produced by this Action identity
  const verified = await Lit.Actions.verifyActionSignature({
    signingScheme,
    actionIpfsCid,
    toSign: msgHashBytes,
    signOutput: signature,
  });

  // 5) Return a structured response for clients to consume
  Lit.Actions.setResponse({
    response: JSON.stringify({
      sigName,
      signingScheme,
      message: toSign,
      messageHash: msgHashHex,
      signature,          // string; format depends on scheme
      actionPublicKey,    // string; hex or JSON depending on scheme
      verified,           // boolean
    }),
  });
})();
This approach keeps the derivation entirely within the Lit Action context. Because the public key depends only on the Action CID and signing scheme, you can rely on Lit.Actions.getActionPublicKey for a deterministic identity without needing to execute the Action externally first.

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Derive the Same Public Key from Client Code

If you prefer to resolve the Lit Action public key outside of the Action runtime - e.g., inside tests or other tooling—the SDK now exposes a helper that calls the on-chain PubkeyRouter contract. 
import { createLitClient } from "@lit-protocol/lit-client";
import { nagaDev } from "@lit-protocol/networks";
import { keccak256, stringToBytes } from "viem";

const litClient = await createLitClient({ network: nagaDev });
const derivedKeyId = keccak256(stringToBytes(`lit_action_${actionIpfsCid}`));
const actionPublicKey = await litClient.utils.getDerivedKeyId(derivedKeyId);

console.log("Derived Lit Action pubkey:", actionPublicKey);
Under the hood, getDerivedKeyId routes through PubkeyRouter.getDerivedPubkey, passing the staking contract address and the default Naga key set (naga-keyset1) (will be dynamic in the future). This mirrors the same key derivation the nodes perform, letting you confirm identities or signatures without re-running the Lit Action.