ECDSA sign
Sign a message with your ECDSA private key — NIST P-256 or P-384, DER or raw r||s — and get a base64/hex signature. Deterministic (RFC-6979). Runs in your browser; the key never leaves your device.
About this tool
ECDSA Sign signs a message with an elliptic-curve private key and returns the signature in base64 and hex. Paste your PEM key, pick the curve and output format, and sign — entirely in your browser via WebAssembly. Your key and message are never uploaded.
Options
- Curve — P-256 (NIST secp256r1, hashed with SHA-256) or P-384 (NIST secp384r1, SHA-384). The curve must match the key you paste.
- Signature format —
- DER: ASN.1
SEQUENCE { r, s }, the encoding OpenSSL produces and most X.509 / TLS tooling expects. - raw: fixed-length
r || s(IEEE-P1363), the form used by JOSE/JWT (ES256,ES384) and WebCrypto.
- DER: ASN.1
Deterministic signatures
Signing uses RFC-6979 deterministic nonces, so signing the same message with the same key always yields the same signature — no randomness required, and no risk of a repeated-nonce key leak.
Key format
The key must be PEM-encoded PKCS#8 (-----BEGIN PRIVATE KEY-----). To
generate one:
openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256
If you have an old-style SEC1 key (-----BEGIN EC PRIVATE KEY-----), convert it:
openssl pkcs8 -topk8 -nocrypt -in sec1.pem
Verify a signature with the matching public key using your standard ECDSA tooling (OpenSSL, WebCrypto, JOSE libraries).
FAQ
Why do I get the same signature every time I sign?
That's by design: the tool uses RFC-6979 deterministic nonces, so a given key + message always produces the same signature. It still verifies exactly like a randomized ECDSA signature — determinism just removes the repeated-nonce risk.
Which format do I need for a JWT (ES256/ES384)?
Choose raw. JOSE/JWT and WebCrypto expect the fixed-length r || s encoding — 64 bytes for P-256 (ES256), 96 bytes for P-384 (ES384). DER is the ASN.1 form OpenSSL and X.509/TLS tooling use; the two are not interchangeable.
It says "invalid EC private key" — what's wrong?
The key must be PEM PKCS#8 (-----BEGIN PRIVATE KEY-----) and its curve must match the one you selected — a P-384 key won't parse when the curve is set to P-256. If your file says BEGIN EC PRIVATE KEY (SEC1), convert it first with openssl pkcs8 -topk8 -nocrypt.
Is P-521 or Ed25519 supported?
No — this tool signs with NIST P-256 and P-384 only (the curve fixes the hash: SHA-256 or SHA-384). Ed25519 is a different signature scheme (EdDSA), not ECDSA.
Does my private key leave the browser?
No. Signing runs locally in WebAssembly; the key and message are never sent to a server. Still, treat any pasted production key with care and prefer test keys where possible.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool ecdsa-sign "The message to sign" 'private_key=-----BEGIN PRIVATE KEY-----
...
-----END PRIVATE KEY-----'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/ecdsa-sign/?message=The%20message%20to%20sign&private_key=-----BEGIN%20PRIVATE%20KEY-----%0A...%0A-----END%20PRIVATE%20KEY-----&curve=p256&format=derMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.