Convert JWT Signing Algorithm
Switch your JWT's signing algorithm seamlessly. Convert tokens from HS256 to RS256, or between other algorithms, by re-signing with a new key—ideal for auth system migration.
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⚠️ Important Notes:
- Algorithm conversion requires re-signing the token
- You need the appropriate key for the target algorithm
- HS* algorithms use symmetric keys (shared secret)
- RS* and ES* algorithms require private keys
HMAC (HS*)
HS256, HS384, HS512 - Symmetric encryption using shared secrets
RSA (RS*)
RS256, RS384, RS512 - Asymmetric encryption using key pairs
ECDSA (ES*)
ES256, ES384, ES512 - Elliptic curve digital signatures
Converting JWT Algorithms Explained
The algorithm converter takes an existing JWT and changes the alg field in the header from one algorithm to another (e.g., HS256 to RS256). It re-encodes the header with the new algorithm and regenerates the signature using the appropriate method.
For HMAC algorithms (HS256, HS384, HS512), the tool uses the provided secret key to create a new signature. For asymmetric algorithms (RS256, ES256), it marks where a private key would be required—actual signing needs cryptographic libraries that handle RSA or EC keys.
Algorithm families:
- HS256/384/512 - HMAC with SHA-256/384/512. Symmetric (shared secret). Fast, simple.
- RS256/384/512 - RSA with SHA-256/384/512. Asymmetric (public/private key pair). Better for distributed verification.
- ES256/384/512 - ECDSA with P-256/P-384/P-521 curves. Asymmetric. Smaller signatures than RSA.
- none - No signature. Only for testing. Never use in production.
The converter preserves the payload exactly—only the header and signature change. This lets you test how different algorithms affect token size and compatibility.
When You'd Actually Use This
Migrating from symmetric to asymmetric keys
Your app uses HS256 but you need to distribute verification to multiple services. Convert to RS256 to see what the new token structure looks like before implementing key management.
Testing algorithm compatibility
Your API gateway supports multiple algorithms. Generate tokens with HS256, RS256, and ES256 to verify your verification code handles all three correctly.
Understanding algorithm impact on token size
RS256 signatures are 256 bytes; HS256 is 32 bytes. Convert the same payload with different algorithms to see how signature size affects your HTTP header budget.
Security training demonstrations
Show developers why the alg: none attack works. Convert a signed token to "none" to demonstrate why servers must validate the algorithm.
Debugging algorithm mismatch errors
Your verifier expects RS256 but receives HS256 tokens. Convert to see the exact header difference and understand why verification fails.
Preparing for compliance requirements
Your security audit requires SHA-512 instead of SHA-256. Convert from HS256 to HS512 to test compatibility before updating production systems.
What to Know Before Using
Converted tokens aren't cryptographically valid.The tool simulates signatures but doesn't perform real cryptographic operations. Use converted tokens for testing structure only, not actual authentication.
Algorithm conversion requires new keys.Switching from HS256 to RS256 isn't just a header change—you need an RSA key pair. The converter shows the format but can't generate valid signatures without proper keys.
The "none" algorithm is dangerous.Some vulnerable JWT libraries accept alg: none tokens as unsigned. Never allow this in production—it bypasses all security.
Payload stays identical.Algorithm conversion only changes the header and signature. The payload claims (exp, sub, etc.) remain unchanged. You're not modifying token content.
Critical security note: Never accept algorithm changes from untrusted sources. Always configure your verifier to expect specific algorithms. The "algorithm confusion" attack exploits servers that accept whatever algorithm the token claims.
Common Questions
Why would I convert from HS256 to RS256?
HS256 requires sharing a secret with every service that verifies tokens. RS256 uses a private key for signing and public keys for verification—you can distribute public keys freely without compromising security.
Does algorithm conversion affect token expiry?
No. The exp claim is in the payload, which doesn't change during conversion. A token expiring in 1 hour before conversion still expires in 1 hour after.
Which algorithm should I use?
Single service? HS256 is fine. Multiple verifiers or microservices? RS256 or ES256. Need smallest tokens? ES256 has shorter signatures than RS256. Avoid "none" entirely.
Can I convert back to the original algorithm?
Yes, conversion is reversible in theory. However, each conversion creates a new signature—you need the appropriate key for the target algorithm each time.
How does algorithm choice affect performance?
HS256 is fastest (simple HMAC). RS256 is slower (RSA operations). ES256 is in between. For most apps, verification time is negligible compared to network latency.
What happens if my verifier doesn't support the new algorithm?
Verification fails with "unsupported algorithm" or similar error. Always check your JWT library's supported algorithms before converting production tokens.
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