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import java.io.File;
import java.io.FileInputStream;
import java.io.FileWriter;
import java.security.Key;
import java.security.KeyPair;
import java.security.KeyStore;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.UnrecoverableKeyException;
import java.security.cert.Certificate;

import sun.misc.BASE64Encoder;

public class ExportPrivateKey {
        private File keystoreFile;
        private String keyStoreType;
        private char[] password;
        private String alias;
        private File exportedFile;

        public static KeyPair getPrivateKey(KeyStore keystore, String alias, char[] password) {
                try {
                        Key key=keystore.getKey(alias,password);
                        if(key instanceof PrivateKey) {
                                Certificate cert=keystore.getCertificate(alias);
                                PublicKey publicKey=cert.getPublicKey();
                                return new KeyPair(publicKey,(PrivateKey)key);
                        }
                } catch (UnrecoverableKeyException e) {
        } catch (NoSuchAlgorithmException e) {
        } catch (KeyStoreException e) {
        }
        return null;
        }

        public void export() throws Exception{
                KeyStore keystore=KeyStore.getInstance(keyStoreType);
                BASE64Encoder encoder=new BASE64Encoder();
                keystore.load(new FileInputStream(keystoreFile),password);
                KeyPair keyPair=getPrivateKey(keystore,alias,password);
                PrivateKey privateKey=keyPair.getPrivate();
                String encoded=encoder.encode(privateKey.getEncoded());
                FileWriter fw=new FileWriter(exportedFile);
                fw.write("-----BEGIN PRIVATE KEY-----n");
                fw.write(encoded);
                fw.write("n");
                fw.write("-----END PRIVATE KEY-----");
                fw.close();
        }

        public static void main(String args[]) throws Exception {
            if (args.length<5) {
              System.out.println("nnUse:njava ExportPrivateKey {keystore_path} JKS {keystore_password} {alias} {target_file}nn");
              return;
            }
                ExportPrivateKey export=new ExportPrivateKey();
                export.keystoreFile=new File(args[0]);
                export.keyStoreType=args[1];
                export.password=args[2].toCharArray();
                export.alias=args[3];
                export.exportedFile=new File(args[4]);
                export.export();
        }
}