Signing a JWT in Kotlin / Android: HS256 Example

Kotlin / Android JWT generator quickstart

Create a token with your public API key in the sub claim, a near-term Unix timestamp in exp, and an HS256 signature made with your private API key.

Sapling JWTs use HS256 with a sub claim containing your public API key and an exp claim containing a Unix timestamp. The examples use a one-hour lifetime; shorten it further when your application can refresh tokens easily.

Kotlin / Android HS256 · sub + exp

This example uses com.auth0:java-jwt.

import com.auth0.jwt.JWT
import com.auth0.jwt.algorithms.Algorithm
import java.time.Instant
import java.util.Date

fun main() {
    val publicKey = System.getenv("SAPLING_PUBLIC_KEY")
        ?: error("Set SAPLING_PUBLIC_KEY first.")
    val privateKey = System.getenv("SAPLING_PRIVATE_KEY")
        ?: error("Set SAPLING_PRIVATE_KEY first.")

    val token = JWT.create()
        .withSubject(publicKey)
        .withExpiresAt(Date.from(Instant.now().plusSeconds(3600)))
        .sign(Algorithm.HMAC256(privateKey))

    println(token)
}

Generated token text/plain

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.<base64url-claims>.<signature>

Return the compact token to the browser and pass it to Sapling.init as the key. Issue a fresh token after it expires; never send the private key.

How a JWT is structured

A JSON Web Token (JWT) is a compact, URL-safe way to pass signed claims between two parties, most often for authentication. It has three base64url-encoded parts separated by dots: header.payload.signature.

Header: The token type (JWT) and the signing algorithm, such as HS256 (HMAC with SHA-256) or RS256 (RSA).
Payload: The claims: statements about the subject plus metadata such as an expiry. Claims are registered (standard names below), public, or private names agreed between the two parties.
Signature: The encoded header and payload signed with a secret (or private key) using the header's algorithm. It proves who issued the token and that it hasn't been changed in transit.

Standard JWT claims

Sapling checks only sub and exp, but these are the registered claims you will see in Kotlin / Android JWT libraries:

iss (issuer): who issued the token.
sub (subject): who the token is about. For Sapling, your public API key.
aud (audience): the recipients the token is intended for.
exp (expiration time): seconds since the Unix epoch after which the token must be rejected.
nbf (not before): seconds since the epoch before which the token must be rejected.
iat (issued at): when the token was issued, in seconds since the epoch.
jti (JWT ID): a unique identifier that lets a server reject replayed tokens.

Use a vetted JWT library

Sign tokens with an established, maintained cryptography library rather than hand-rolling HMAC and base64url encoding. Those libraries are peer-reviewed, handle edge cases such as algorithm confusion and clock skew, and get patched when new vulnerabilities appear.

Reference Frontend key management and JWT documentation All languages Choose another quickstart

About Kotlin / Android

Kotlin is a programming language that interoperates with Java on the JVM. While it is cross-platform, widespread adoption of Kotlin stems from its first-class support on the Android platform for Android applications.