Example of JWK creation

JwtHelpers.kt

object JwtHelpers {

    data class KeyPair(val public: RSAPublicKey, val private: RSAPrivateKey)

    // we need static keyPairs for consistent testing
    val rsaKeyPair = run {
        // keySize = 512
        // Choose two distinct prime numbers p and q.
        val p = BigInteger("80015048226958274584861777197787136869318208410134237774308318008156105353397")
        val q = BigInteger("84597213199129215561490406209976328146625812489295431631787994073201979267081")

        // Compute n = pq (modulus)
        val modulus = p.multiply(q)
        // Compute φ(n) = φ(p)φ(q) = (p − 1)(q − 1) = n - (p + q -1), where φ is Euler's totient function.
        // and choose an integer e such that 1 < e < φ(n) and gcd(e, φ(n)) = 1; i.e., e and φ(n) are coprime.
        val m = p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE))

        // random (no) coprime of m
        val publicExponent = BigInteger("3558378466727931118468155250915611321631043711054806878764573684269336548927293476472136929952773308920033222349399256587177891621088267708150487725894837")
        // Determine d as d ≡ e−1 (mod φ(n)); i.e., d is the multiplicative inverse of e (modulo φ(n)).
        val privateExponent = publicExponent.modInverse(m)

        val spec = RSAPublicKeySpec(modulus, publicExponent)
        val privateSpec = RSAPrivateKeySpec(modulus, privateExponent)
        val factory = KeyFactory.getInstance("RSA")
        val pub = factory.generatePublic(spec) as RSAPublicKey
        val priv = factory.generatePrivate(privateSpec) as RSAPrivateKey
        KeyPair(pub, priv)
    }

    val jwkKeys = """
{
    "keys": [
        {
            "kty": "RSA",
            "use": "sig",
            "kid": "${rsaKeyPair.public.encoded.encodeBase64()}",
            "x5t": "${rsaKeyPair.public.encoded.encodeBase64()}",
            "n": "${rsaKeyPair.public.modulus.toByteArray().encodeBase64()}",
            "e": "${rsaKeyPair.public.publicExponent.toByteArray().encodeBase64()}"
        }
    ]
}
    """.trimIndent()
}