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To find out whether an authentication provider signs a particular JSON Web Token (JWT) with HS256 or RS256, inspect the token header’s alg field, then confirm it against the provider’s configuration and validation documentation for that exact token type. The header is a clue, not proof that the token is authentic or a trusted instruction for your verifier.
What HS256 and RS256 mean
Both names identify a signing algorithm that uses SHA-256, but they use different cryptographic constructions and key arrangements. The IETF’s 2015 RFC 7518 defines HS256 as HMAC with SHA-256 and RS256 as RSASSA-PKCS1-v1_5 with SHA-256.
| Algorithm | Signing and verification | Practical implication |
|---|---|---|
| HS256 | The issuer and verifier use the same shared secret to create and check the message authentication code. | Any verifier that holds the secret can also create a valid MAC. RFC 7518 requires an HS256 key at least as long as SHA-256’s 256-bit output. |
| RS256 | The issuer signs with a private key; verifiers check the signature with the corresponding public key. | A verifier with only the public key can validate signatures but cannot sign as the issuer. RFC 7518 requires RSA keys of at least 2048 bits for the RSA algorithms covered in its section. |
These requirements come from RFC 7518, Section 3.2 for HS256 key length and its RSA algorithm section for RSA key length. The standard lists HS256 as Required for JOSE implementations and RS256 as Recommended; those implementation labels do not determine what a specific provider or token uses.
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- Identify the exact token and issuer. Note whether you are examining an ID token, an API access token, or another JWT, and identify the tenant or user pool that issued it. A provider can make choices in a particular product feature or API configuration, so a general provider name is not enough.
- Read the JWT header’s
algvalue. It indicates which algorithm the token claims was used. Treat it only as a diagnostic clue: the header is part of the token and is not proof of a valid signature. - Check the matching provider configuration and official validation guidance. Confirm the expected issuer, the accepted algorithm for that token type, and the trusted verification-key source.
- For RS256, obtain the public key from the provider’s trusted JWKS. Match the token’s
kid(key ID) to the corresponding entry in that key set. Do not assume a single key will remain valid indefinitely if the provider rotates signing keys. - Reject or investigate a mismatch. If the token header conflicts with the algorithm or key source expected for that issuer and token type, do not expand the verifier’s accepted algorithms based on the header. Investigate the issuer, configuration, or token instead.
Signature verification is only part of JWT validation. Configure the verifier using the expected issuer context, trusted keys, and allowed algorithm; then validate the signature and relevant claims. The provider’s documentation should define the appropriate validation requirements for your application.
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What provider documentation establishes
Auth0
Auth0’s support article, last updated September 10, 2025, says RS256 is the default for JWTs and explains its public/private-key model; it also points to the tenant’s JWKS for RS256 public keys. Its API setup documentation describes selecting an API signing algorithm, including HS256 and RS256. Read these statements in their respective contexts: the default statement does not establish the live setting for every Auth0 token or API.
Amazon Cognito User Pools
AWS documents that Cognito User Pool tokens use RS256. For verification, its JWT verification guidance directs consumers to the public JWKS and to match the token’s kid to a key. AWS also warns that signing keys can rotate, so a verifier should not pin one key permanently.
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Other providers and token types
Do not infer another provider’s behavior from Auth0 or Cognito. Confirm the configuration and current official validation documentation for the exact issuer and token type you use; provider defaults and available settings can vary and change.
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RS256 is not automatically safer in every deployment. The operational question is who must be trusted with signing power and how verification keys are distributed and maintained.
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- Consider the key-distribution model. With HS256, every verifier needs a secret that can also mint valid tokens. With RS256, verifiers can receive a public key while the issuer retains the private signing key.
- Account for compromise and rotation. If an HS256 shared secret is exposed to a verifier, that verifier can generate valid MACs. Auth0’s setup guidance notes that after a compromised HS256 secret, the API must be redeployed with a new secret. Auth0 says RS256 supports key rotation without redeploying the API with a new secret; Cognito documents rotating signing keys and publishing public keys through JWKS.
- Follow the provider’s supported configuration. The available algorithm can depend on provider, product, API, and token type. Use the provider’s current documented requirements rather than choosing an algorithm solely by its name.
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