用已有的JWT公钥在Java中创建签名

Question

现在，我不知道如何创建签名。我找到了一个创建Keypair的解决方案，但由于我已经有了这个，我不知道如何修改代码。

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.Signature;

import sun.misc.BASE64Encoder;

public class MainClass {
  public static void main(String[] args) throws Exception {
    KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
    kpg.initialize(1024);
    KeyPair keyPair = kpg.genKeyPair();

    byte[] data = "test".getBytes("UTF8");

    Signature sig = Signature.getInstance("MD5WithRSA");
    sig.initSign(keyPair.getPrivate());
    sig.update(data);
    byte[] signatureBytes = sig.sign();
    System.out.println("Singature:" + new  
           BASE64Encoder().encode(signatureBytes));

    sig.initVerify(keyPair.getPublic());
    sig.update(data);

    System.out.println(sig.verify(signatureBytes));
  }
}

Answer 1

Api框上写着

RSA keypair必须为PEM格式

因此，您需要将公钥导出到PEM。使用以下代码(java8)

 public static String exportPublicKeyAsPem(PublicKey publicKey) throws Exception {
    StringWriter sw = new StringWriter();

    sw.write("-----BEGIN PUBLIC KEY-----\n");
    sw.write(Base64.getEncoder().encodeToString(publicKey.getEncoded()));
    sw.write("\n-----END PUBLIC KEY-----\n");

    return sw.toString();
 }

public static String signJWT (String header, String payload, PrivateKey privateKey) throws Exception{
    String token = 
            Base64.getUrlEncoder().encodeToString(header.getBytes())
            + "."
            +Base64.getUrlEncoder().encodeToString(payload.getBytes());

    Signature sig = Signature.getInstance("SHA256WithRSA");
    sig.initSign(privateKey);
    sig.update(token.getBytes());
    byte[] signature = sig.sign();

    return token + "." + Base64.getUrlEncoder().encodeToString(signature);
}

public final static void main(String argv[]) throws Exception{

    String header = "{\"alg\": \"RS256\",\"typ\": \"JWT\"}";
    String payload = "{\"sub\": \"1234567890\",\"name\": \"John Doe\"}";

    KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
    kpg.initialize(1024);
    KeyPair keyPair = kpg.genKeyPair();

    String publicKeyPem = exportPublicKeyAsPem(keyPair.getPublic());
    String signedToken = signJWT (header, payload, keyPair.getPrivate());

    System.out.println(publicKeyPem);
    System.out.println(signedToken);
}