Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteYour authentication provider’s JWT signing algorithm depends on the provider, token type, and current configuration. Inspect the token header’s alg value for a clue, then confirm it against the provider’s configuration and validation documentation. The header is not proof that the token is authentic and must not dictate which algorithms your verifier accepts.
How to identify the algorithm for your JWT
- Identify the token and issuer. Establish whether you are checking an ID token, an API access token, or another JWT, and record its issuer or tenant. Algorithm settings may differ by product feature or API.
- Read the header’s
algfield. A JWT header may declareHS256orRS256. Treat that value as a diagnostic clue, not a trusted configuration instruction. - Confirm the expected setup with the provider. Check the configuration and official validation documentation for that exact token type. Confirm the expected issuer, allowed algorithm, and trusted verification key source.
- Validate using that trusted context. For RS256, use the provider’s trusted JWKS and match the token’s
kidto the corresponding key. If the declared algorithm conflicts with the expected configuration, reject the token or investigate the mismatch; do not expand accepted algorithms based on the header.
What HS256 and RS256 mean
These names specify different signature constructions. HS256 is HMAC with SHA-256; RS256 is RSASSA-PKCS1-v1_5 with SHA-256. Both use SHA-256, but their key-distribution models differ.
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| Decision point | HS256 | RS256 |
|---|---|---|
| Signing and verification | The same shared secret creates and verifies the MAC. | The issuer signs with a private key; consumers verify with a public key. |
| Who can mint a valid token | Any verifier with the shared secret can also create a valid MAC. | A consumer with only the public key can verify but cannot sign as the issuer. |
| Key exposure | A verifier compromise can expose the secret used for signing. | The public verification key can be distributed while the issuer retains the private signing key. |
| Rotation and operations | Replacing a compromised secret may require coordinated deployment. Auth0’s API setup documentation says a compromised HS256 secret requires redeploying the API with a new secret. | Verifiers retrieve public keys from the provider’s JWKS. Auth0 describes RS256 key rotation without redeploying the API with a new secret; AWS warns Cognito signing keys can rotate. |
The IETF’s RFC 7518 (2015) lists HS256 as required and RS256 as recommended for JOSE implementations. It requires an HS256 key at least as large as the hash output—256 bits—and RSA keys of at least 2048 bits for the RSA algorithms covered by the relevant section. These standards facts do not reveal which algorithm a particular provider has configured.
What the provider examples establish
Auth0
Auth0’s support article, last updated September 10, 2025, says RS256 is the default for JWTs and points to tenant JWKS for RS256 public keys. Its API setup documentation describes choosing a signing algorithm for an API and includes HS256 and RS256 options. Read those statements in their documented contexts: they do not establish that every Auth0 token type or configuration uses the same algorithm. Auth0: Sign Tokens · Auth0: Call Your API Using the Authorization Code Flow
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Amazon Cognito User Pools
AWS documents RS256 for User Pool tokens. For verification, its guidance directs consumers to the pool’s public JWKS and to find the key whose kid matches the token header. AWS also notes that signing keys may rotate, so a verifier should not rely on one permanently pinned key. AWS: Verifying a JWT from an Amazon Cognito User Pool
Other providers and token types
These examples do not establish behavior for every identity provider, tenant, or token type. Check the live configuration and validation instructions for the issuer you actually use rather than generalizing from a provider-wide label or another product’s default.
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Why the header cannot choose your verification policy
A JWT’s alg field says what algorithm the token claims was used; it does not establish that the signature is valid or that the token came from the expected issuer. The verifier must already know which issuer it trusts, which algorithms it accepts, and where the relevant keys come from. Verify the signature and the claims required by your application using that configured context. If the token’s header and expected configuration disagree, reject it and investigate rather than silently accepting a broader set of algorithms.
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RS256 is not automatically safer in every deployment. The practical question is who must hold signing capability and how your provider manages verification keys. With HS256, each verifier that checks tokens holds a secret that can also mint them. With RS256, verifiers can hold public keys while the issuer keeps the private signing key. Auth0 recommends RS256 in its API setup context; for other deployments, follow the provider’s current requirements for the specific token type. RFC 7518 defines the algorithms and key requirements; provider configuration determines what is actually in use. IETF RFC 7518
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