Method to get a JWT signed by a GCP service account for OIDC

GCPAuthenticationInterceptor.java

private String getSignedJwt() {
    long now = System.currentTimeMillis();
    RSAPrivateKey privateKey = (RSAPrivateKey) credentials.getPrivateKey();
    Algorithm algorithm = Algorithm.RSA256(null, privateKey);
    return JWT.create()
            .withKeyId(credentials.getPrivateKeyId())
            .withIssuer(credentials.getClientEmail())
            .withSubject(credentials.getClientEmail())
            .withAudience(OAUTH_TOKEN_URI)
            .withIssuedAt(new Date(now))
            .withExpiresAt(new Date(now + EXPIRATION_TIME_IN_MILLIS))
            .withClaim("target_audience", clientId)
            .sign(algorithm);
}