RSA Keys for JWT Signing (RS256)

RSA Keys for JWT Signing (RS256)

JSON Web Tokens (JWT) can be signed using RSA keys with the RS256 algorithm (RSA Signature with SHA-256). This enables asymmetric token verification where the signing authority and verifying parties use different keys.

RS256 vs HS256

Feature	HS256 (HMAC)	RS256 (RSA)
Key type	Shared secret	Public/private key pair
Signing	Secret key	Private key
Verification	Same secret key	Public key
Key distribution	Must share secret securely	Only share public key
Use case	Single service	Microservices, third-party

How RS256 Signing Works

JWT = Header.Payload.Signature

Header:  {"alg": "RS256", "typ": "JWT"}
Payload: {"sub": "1234567890", "name": "John", "iat": 1516239022}

Signature = RSA_Sign(
  SHA256(base64url(Header) + "." + base64url(Payload)),
  privateKey
)

Implementation Example

// Signing (auth server — has private key)
const privateKey = await crypto.subtle.importKey(
  "pkcs8", privateKeyDer,
  { name: "RSASSA-PKCS1-v1_5", hash: "SHA-256" },
  false, ["sign"]
);
const signature = await crypto.subtle.sign(
  "RSASSA-PKCS1-v1_5", privateKey, dataToSign
);

// Verification (any service — only needs public key)
const publicKey = await crypto.subtle.importKey(
  "spki", publicKeyDer,
  { name: "RSASSA-PKCS1-v1_5", hash: "SHA-256" },
  false, ["verify"]
);
const isValid = await crypto.subtle.verify(
  "RSASSA-PKCS1-v1_5", publicKey, signature, dataToSign
);

JWKS (JSON Web Key Set)

Public keys for JWT verification are often published as a JWKS endpoint:

{
  "keys": [{
    "kty": "RSA",
    "kid": "key-id-1",
    "use": "sig",
    "n": "0vx7agoebGcQ...",
    "e": "AQAB"
  }]
}

Why RS256 for Microservices

In a microservices architecture, the authentication service signs tokens with its private key. All other services verify tokens using the public key — no shared secret distribution needed. Key rotation is simpler because only the JWKS endpoint needs updating.