How to Use Google Tink for Secure JWT Signing with ECDSA

Read full article here: https://www.slashid.com/blog/tink-jwt-ecdsa/?utm_source=nhimg

Recent identity breaches and API key leaks have shown how fragile digital trust can be when cryptographic operations are handled poorly. In many incidents, attackers didn’t exploit a vulnerability in encryption itself — they exploited developers’ mistakes when managing keys, signing tokens, or verifying JWTs.

Google’s Tink cryptography library was built to prevent exactly that. It provides safe-by-default APIs for encryption, signing, and key management, helping developers build secure token systems without needing to become cryptography experts.

In this post, we explore how to use Tink to create, sign, and verify JSON Web Tokens (JWTs) using ECDSA (Elliptic Curve Digital Signature Algorithm) — one of the most secure and efficient algorithms in modern identity systems.

Understanding JWTs: The Digital Envelope of Identity

A JSON Web Token (JWT) is a compact and URL-safe way of transmitting claims between two parties. JWTs are used everywhere, from OAuth access tokens and OpenID Connect (OIDC) ID tokens to internal microservice authentication.

A JWT is composed of three parts:

1. Header — declares the token type (JWT) and the signing algorithm (e.g., ES256).
2. Payload — carries the claims, such as the user’s identity, role, or expiry time.
3. Signature — verifies the token’s authenticity and integrity.

For example, the token:

eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjMiLCJuYW1lIjoiSm9obiBEb2UiLCJleHAiOjE3MzAwNjQ5MDZ9.N4hU7dVd3a...

is essentially a digitally signed statement saying, “This user’s identity and claims are authentic.”

JWTs are signed, not encrypted — meaning their contents can be read by anyone, but only verified or created by someone holding the private key.

Why Signing JWTs Correctly Matters

JWT signatures are the backbone of modern identity systems. A weak implementation can lead to catastrophic failures — from allowing token forgery to key leakage.

Real-world incidents have shown what happens when JWT verification is misconfigured:

• Systems accepting "alg": "none" allowed unsigned tokens to be treated as valid.
• Libraries using deterministic signatures with poor randomness made private keys recoverable.
• Inconsistent key rotation practices left tokens valid long after the keys should have expired.

This is where Google Tink comes in — it enforces secure defaults, safe key management, and non-deterministic signing, making it far harder for developers to “shoot themselves in the foot.”

Why Use ECDSA (ES256) Instead of RSA?

Historically, RS256 (RSA with SHA-256) was the default JWT algorithm. While still secure, it has drawbacks:

• Large key sizes (3072 bits for RSA vs 256 bits for ECDSA for equivalent strength)
• Slower key generation and larger token signatures
• Deterministic output, meaning the same input always produces the same signature

By contrast, ECDSA (Elliptic Curve Digital Signature Algorithm) — particularly ES256 (ECDSA with SHA-256 on the P-256 curve) — offers:

• Stronger security with smaller keys
• Non-deterministic signatures (each signature is unique)
• Faster key generation and lower computational cost

However, ECDSA is sensitive to randomness — reusing or leaking nonce values (the per-signature random number) can expose your private key. This famously happened to Sony’s PlayStation 3 signing keys and even to some Bitcoin wallets on Android.

That’s why libraries like Tink are invaluable — they handle nonce generation correctly and safely by design.

Building a Secure JWT System with Google Tink

Let’s walk through how Tink helps developers create a secure JWT lifecycle — from signing to verification to key management.

1-Creating a Raw JWT

The first step is to create a RawJWT object containing user claims:

jwtID := uuid.New().String()

now := time.Now()

expiry := now.Add(15 * time.Minute)

opts := &jwt.RawJWTOptions{

  Subject: &userID,

  Issuer:  &issuer,

  JWTID:   &jwtID,

  IssuedAt: &now,

  ExpiresAt: &expiry,

  CustomClaims: claims,

}

rawJWT, _ := jwt.NewRawJWT(opts)

This defines the claims, expiration, and metadata for the token before it’s signed.

2-Signing the JWT (Private Key)

To sign the JWT, we first retrieve the keyset (private key) and use Tink’s secure Signer interface:

signingKeyset, _ := keysetsRepo.GetKeyset()

signer, _ := jwt.NewSigner(signingKeyset)

signedToken, _ := signer.SignAndEncode(rawJWT)

This generates a signed, encoded JWT string that can be safely shared with clients or other services.

3-Verifying the JWT (Public Key)

Verification uses only the public key — ensuring that even if the verifying service is compromised, the private signing key remains secure.

verificationKeyset, _ := keysetsRepo.GetPublicKeyset()

verifier, _ := jwt.NewVerifier(verificationKeyset)

validatorOpts := &jwt.ValidatorOpts{

  ExpectedIssuer: &issuer,

  ExpectIssuedInThePast: true,

}

validator, _ := jwt.NewValidator(validatorOpts)

verifiedJWT, err := verifier.VerifyAndDecode(signedToken, validator)

If verification passes, you can trust the token’s integrity and claims.

Safe Key Storage and Rotation

A critical aspect of JWT security is how you store your signing keys. Storing them unencrypted on disk is a common pitfall.

Tink integrates seamlessly with cloud Key Management Services (KMS) such as:

• Google Cloud KMS
• AWS KMS
• Azure Key Vault

By using a KMS-managed master key, you can encrypt your local Tink keysets and decrypt them securely during runtime.

Example using Google Cloud KMS:

gcpClient, _ := gcpkms.NewClientWithOptions(context.Background(), keyURIPrefix)

registry.RegisterKMSClient(gcpClient)

masterKey, _ := gcpClient.GetAEAD(masterKeyURI)

This ensures that your local system never directly handles plaintext keys, a must for regulated environments.

Real-World Benefits: Why Tink Outperforms Hand-Rolled Crypto

Tink embodies Google’s security philosophy: “easy to use, hard to misuse.”

Here’s what that means in practice:

Automatic key rotation — simplifying cryptographic hygiene
Algorithm agility — migrate from ES256 to ES512 or Ed25519 without rewriting logic
Interoperable keysets — consistent support across Go, Java, Python, and C++
Safe defaults — non-deterministic ECDSA signatures and mandatory AEAD encryption
KMS integration — production-ready encryption and decryption pipelines

For organizations building authentication systems, this dramatically reduces both development complexity and security risk.

Lessons from the Field

Many JWT-related breaches share one root cause: developers managing crypto manually — from key generation to signature validation.

Common mistakes include:

• Mixing up private/public keys
• Using incorrect JOSE header parameters
• Forgetting to rotate keys
• Storing signing keys in plaintext config files

Tink prevents all of these by abstracting away cryptographic details while enforcing correct workflows.

Conclusion: Secure Tokens, Simplified

Building secure identity systems shouldn’t require PhD-level cryptography expertise.

By combining ECDSA’s modern security with Tink’s safety-first design, developers can sign and verify JWTs confidently — without risking nonce leaks, key exposure, or weak configurations.

In today’s identity-driven threat landscape, safe-by-default cryptography is no longer optional — it’s essential.
With Google Tink, you can stop worrying about crypto mistakes and focus on what matters: building resilient, trustworthy systems.