Invalid key object type public, expected private.

The two nouns swap with the operation: verify() with a private-only input reports "expected private or public", sign() with a secret reports the secret variant. The structure of the message is stable.

Node.js crypto — TypeError, code ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE

What it means

You passed a KeyObject of the wrong class for the operation: crypto.sign() needs a private key but received a public one (or verify() received a secret key, etc.). Node's key objects know their own type — public, private or secret — and each API accepts specific types only.

Why it happens

• Swapped variables at the call site

  common

  publicKey and privateKey exchanged in a destructuring, or a function parameter named key receiving whichever the caller had. Signing with the public key is impossible by construction, and Node says so instead of failing mysteriously later.

• Loading produced a different type than assumed

  common

  createPublicKey(privatePem) intentionally derives a public key — code that then treats the result as a private key for signing fails here. Similarly, a PEM file that actually contains a public key loads as type "public" no matter what the filename says.

• Secret (symmetric) key in an asymmetric API

  occasional

  createSecretKey() output (an HMAC/AES key) handed to sign/verify for ECDSA. Symmetric material cannot participate in public-key signatures.

How to fix it

1. 1.

   Assert key types at the boundary

   KeyObject.type is "public" | "private" | "secret". A one-line assertion converts a confusing crypto error into an obvious one at load time.

   import { createPrivateKey } from "node:crypto";
   
   const signingKey = createPrivateKey(pem);
   if (signingKey.type !== "private") {
     throw new Error(`expected private key, got ${signingKey.type}`);
   }

2. 2.

   Use each half of the pair for its own operation

   Private signs, public verifies. If you only hold a private key, derive the public half for verification instead of reusing the private object everywhere.

   import { createPublicKey, sign, verify } from "node:crypto";
   
   const signature = sign("sha256", data, privateKey);          // private → sign
   const publicKey = createPublicKey(privateKey);               // derive public half
   const ok = verify("sha256", data, publicKey, signature);     // public → verify

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