Question: i need some help to validate a jwt signature with a ECDSA public key. I'm reading the key from a .pem file with bouncy castle and using jjwt to do the validation. I'm getting an error while validating the signature.

```
Security.addProvider(new BouncyCastleProvider());
String jwt = "eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiJ9.eyJtc2kiOiI5NzE1NTA5ODc2NTUiLCJmZWEiOiJzaWdudXAtZGF0YSIsImlzcyI6IkNEUCIsImV4cCI6MTU1NDU2NjMzNiwiaWF0IjoxNTU0MzkzNTM2LCJzaWQiOiIwNDI0MDMwMDg5NzI4MTg3QG5haS5lcGMubW5jMTMwLm1jYzMxMC4zZ3BwbmV0d29yay5vcmcifQ.RwxoGmFd1_dQPeGN-0gnWIW79xXvGHoyJKBbCKajgO75UooceS6tskxwqViEuP1gZD66UE8Bd2L0FaeI2aS_IA";
PemReader pemReader = new PemReader(new FileReader("/publickey.pem"));
X509EncodedKeySpec spec = new X509EncodedKeySpec(pemReader.readPemObject().getContent());
KeyFactory kf = KeyFactory.getInstance("ECDSA","BC");
PublicKey publicKey = kf.generatePublic(spec);
Jws<Claims> claims = Jwts.parser().setSigningKey(publicKey).parseClaimsJws(jwt);

```

I'm getting a Signature Exception with: Unable to verify Elliptic Curve signature using configured ECPublicKey. error decoding signature bytes.

Answer: 
This will work as well without mentioning any algorithm.

```
public boolean isTokenValid(String token) {
    try {
        String certificate = "GET_YOUR_PUBLIC_CERTIFICATE_HERE"; //Either from REST call or reading from a cert file.
        getPublicKeyAndParseToken(token, certificate);
        return true;
    } catch (IOException e) {
        log.error("", e);
    } catch (Exception e) {
        log.error("", e);
        log.error("JWT Not-Verified");
    }
    return false;
}

private void getPublicKeyAndParseToken(String token, String certificate) throws IOException, CertificateException {
    log.debug("Certificate:: " + certificate); //Only for debugging purpose
    InputStream is = new ByteArrayInputStream(certificate.getBytes(StandardCharsets.UTF_8));
    CertificateFactory cf = CertificateFactory.getInstance("X.509");
    Certificate cert = cf.generateCertificate(is);
    PublicKey publicKey = cert.getPublicKey();

    Jws parsedClaimsJws = Jwts.parser().setSigningKey(publicKey).parseClaimsJws(token);
    log.debug("Header:: " + parsedClaimsJws.getHeader()); //Only for debugging purpose
    log.debug("Body:: " + parsedClaimsJws.getBody()); //Only for debugging purpose
}

```

Don't forget to use version '0.9.x' of jjwt library. I've below dependency in my build.gradle:

```
compile('io.jsonwebtoken:jjwt:0.9.1')

```
Problem found, i was using an old jjwt lib (0.6). Changed to 0.9 with the same code and it works as expected.

Thanks