Fixing "secretOrPrivateKey Must Be Asymmetric Key for RS256"

When working with JWT authentication using the RS256 algorithm, Node.js may throw the following error:

Error: secretOrPrivateKey must be an asymmetric key for RS256

This happens when the key passed to jsonwebtoken.sign() or jsonwebtoken.verify() is not a valid RSA key. RS256 is an asymmetric algorithm and requires:

• a private RSA key for signing
• a public RSA key for verification

This article explains the causes of the error and offers several reliable solutions.

1. RS256 Requires an Asymmetric Key Pair

RS256 uses RSA encryption. This means you must use a real RSA private key, not a string or HMAC secret.

Correct RSA private key format:

-----BEGIN PRIVATE KEY-----
YOUR_KEY_CONTENT
-----END PRIVATE KEY-----

If your key does not look like this, RS256 will fail.

2. Generate a Valid RSA Private Key

If you need to generate a valid RSA key pair, use OpenSSL:

openssl genpkey -algorithm RSA -out private_key.pem -pkeyopt rsa_keygen_bits:2048

Then extract the public key:

openssl rsa -in private_key.pem -pubout -out public_key.pem

This ensures both keys are properly formatted and compatible with RS256.

3. Correctly Sign JWT Using RS256

A common mistake is passing a simple string instead of a real private key.

Incorrect:

jwt.sign(payload, "my-secret", { algorithm: "RS256" });

Correct:

import jwt from "jsonwebtoken";
import fs from "fs";

const privateKey = fs.readFileSync("./private_key.pem", "utf8");

const token = jwt.sign(
  { userId: 1 },
  privateKey,
  { algorithm: "RS256", expiresIn: "1h" }
);

Ensure the key is loaded exactly as it exists in the .pem file.

4. Correctly Verify JWT Using the Public Key

import jwt from "jsonwebtoken";
import fs from "fs";

const publicKey = fs.readFileSync("./public_key.pem", "utf8");

jwt.verify(token, publicKey, { algorithms: ["RS256"] }, (err, decoded) => {
  if (err) {
    console.error("Verification failed:", err);
    return;
  }
  console.log("Decoded token:", decoded);
});

The algorithm must be explicitly defined for security.

5. Keys Stored in Environment Variables

When storing PEM keys in .env files, newline characters must be preserved.

Write them like this in .env:

PRIVATE_KEY="-----BEGIN PRIVATE KEY-----
abc123...
-----END PRIVATE KEY-----"

Or escape newlines:

PRIVATE_KEY="-----BEGIN PRIVATE KEY-----\nabc123...\n-----END PRIVATE KEY-----"

Then load:

const privateKey = process.env.PRIVATE_KEY.replace(/\n/g, "
");

Missing or malformed newline characters often cause RS256 failures.

6. Using JWK Instead of PEM (Advanced)

Some identity providers (Auth0, AWS Cognito, Okta) use JWK (JSON Web Key) format.
To use JWK, convert it using a library like jwk-to-pem:

npm install jwk-to-pem

import jwkToPem from "jwk-to-pem";

const pem = jwkToPem(jwkObject);

jwt.verify(token, pem, { algorithms: ["RS256"] });

This avoids issues when working with public keys from external auth systems.

7. Validate That the Key Is RSA and Not HMAC

A frequent hidden cause: the key is actually an HMAC secret, not RSA.

Diagnostic:

console.log(privateKey.includes("BEGIN RSA"));

If it does not contain a PEM header, it’s not an RSA key.

8. Common Mistakes to Avoid

Mistake: Using HS256 secret with RS256.
Fix: Ensure algorithm and key type match.

Mistake: Storing key in JSON config without newline preservation.
Fix: Use .replace(/\n/g, "\n") normalization.

Mistake: Reading file incorrectly.
Fix: Always specify encoding in readFileSync.

Summary

The error "secretOrPrivateKey must be an asymmetric key when using RS256" almost always means the key is incorrect or improperly formatted.

To fix it:

1. Use a valid PEM RSA private key.
2. Use a corresponding public key for verification.
3. Ensure environment variables preserve newlines.
4. Verify algorithm matches the key type.
5. Consider JWK conversion for cloud providers.

With the correct setup, RS256 provides a secure and standards-compliant way to sign and verify JWTs.