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javacryptographyrsapemder

How to write private key to password protected DER format in Java?


I have an RSA key pair that I generated in Java and I need to programmatically write the private key to the same format that openssl does when I run this command (and enter the appropriate data for the prompts, namely a passphrase to protect the private key):

openssl req -out request.csr -newkey rsa:2048 -keyout privkeyfile

The Java code to generate the key pair is pretty standard:

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize(2048);
KeyPair keyPair = keyGen.genKeyPair();

A sample output of running that openssl command (on my Windows machine) is:

-----BEGIN ENCRYPTED PRIVATE KEY-----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-----END ENCRYPTED PRIVATE KEY-----

EDIT Changed the sample output from openssl

EDIT I tried to read the openssl generated private key file with Java using the code below to try and get some of the parameters but I ended up getting the following exception:

Exception in thread "main" java.io.IOException: ObjectIdentifier() -- data isn't an object ID (tag = 48)
    at sun.security.util.ObjectIdentifier.<init>(Unknown Source)
    at sun.security.util.DerInputStream.getOID(Unknown Source)
    at com.sun.crypto.provider.PBES2Parameters.engineInit(PBES2Parameters.java:267)
    at java.security.AlgorithmParameters.init(Unknown Source)
    at sun.security.x509.AlgorithmId.decodeParams(Unknown Source)
    at sun.security.x509.AlgorithmId.<init>(Unknown Source)
    at sun.security.x509.AlgorithmId.parse(Unknown Source)
    at javax.crypto.EncryptedPrivateKeyInfo.<init>(EncryptedPrivateKeyInfo.java:95)
    at crypto.ReadOpensslKey.main(ReadOpensslKey.java:35)

Java code to read file:

package crypto;

import org.bouncycastle.util.encoders.Base64;
import javax.crypto.EncryptedPrivateKeyInfo;  
import javax.crypto.SecretKeyFactory;  
import javax.crypto.spec.PBEKeySpec;  
import java.io.IOException;  
import java.nio.file.Files;  
import java.nio.file.Paths;  
import java.security.InvalidKeyException;  
import java.security.KeyFactory;  
import java.security.NoSuchAlgorithmException;  
import java.security.PrivateKey;  
import java.security.spec.InvalidKeySpecException;  
import java.security.spec.PKCS8EncodedKeySpec; 

public class ReadOpensslKey {

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

        String encrypted = new String(Files.readAllBytes(Paths.get("<insert path to openssl generated privkeyfile>")));  

        //Create object from encrypted private key  
        encrypted = encrypted.replace("-----BEGIN ENCRYPTED PRIVATE KEY-----", "");  
        encrypted = encrypted.replace("-----END ENCRYPTED PRIVATE KEY-----", "");  
        EncryptedPrivateKeyInfo pkInfo = new EncryptedPrivateKeyInfo(Base64.decode(encrypted));  // exception is thrown here
        System.out.println(pkInfo.getAlgName());
        PBEKeySpec keySpec = new PBEKeySpec("abcde".toCharArray()); // password  
        SecretKeyFactory pbeKeyFactory = SecretKeyFactory.getInstance(pkInfo.getAlgName());  
        PKCS8EncodedKeySpec encodedKeySpec = pkInfo.getKeySpec(pbeKeyFactory.generateSecret(keySpec));  
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");  
        PrivateKey encryptedPrivateKey = keyFactory.generatePrivate(encodedKeySpec);  
    }

}

Solution

  • I ended up getting it to work using the following code:

    import java.io.FileOutputStream;
    import java.io.StringWriter;
    import java.nio.charset.StandardCharsets;
    import java.security.KeyPair;
    import java.security.KeyPairGenerator;
    import java.security.SecureRandom;
    import java.security.Security;
    
    import org.bouncycastle.jce.provider.BouncyCastleProvider;
    import org.bouncycastle.openssl.PKCS8Generator;
    import org.bouncycastle.openssl.jcajce.JcaPEMWriter;
    import org.bouncycastle.openssl.jcajce.JcaPKCS8Generator;
    import org.bouncycastle.openssl.jcajce.JceOpenSSLPKCS8EncryptorBuilder;
    import org.bouncycastle.operator.OutputEncryptor;
    import org.bouncycastle.util.io.pem.PemObject; 
    
    public class WriteOpensslKey {
    
        public static void main(String[] args) throws Exception {
    
            // provider is needed for the encryptor builder
            Security.addProvider(new BouncyCastleProvider());
    
            // generate key pair
            KeyPairGenerator kpGen = KeyPairGenerator.getInstance("RSA");  
            kpGen.initialize(2048, new SecureRandom());  
            KeyPair keyPair = kpGen.generateKeyPair();  
    
            // construct encryptor builder to encrypt the private key
            JceOpenSSLPKCS8EncryptorBuilder encryptorBuilder = new JceOpenSSLPKCS8EncryptorBuilder(PKCS8Generator.AES_256_CBC);  
            encryptorBuilder.setRandom(new SecureRandom());  
            encryptorBuilder.setPasssword("password".toCharArray());
            OutputEncryptor encryptor = encryptorBuilder.build();  
    
            // construct object to create the PKCS8 object from the private key and encryptor
            JcaPKCS8Generator pkcsGenerator = new JcaPKCS8Generator(keyPair.getPrivate(), encryptor);  
            PemObject pemObj = pkcsGenerator.generate();  
            StringWriter stringWriter = new StringWriter();  
            try (JcaPEMWriter pemWriter = new JcaPEMWriter(stringWriter)) {  
                pemWriter.writeObject(pemObj);  
            }  
    
            // write PKCS8 to file
            String pkcs8Key = stringWriter.toString();  
            FileOutputStream fos = new FileOutputStream("<path to output file>");  
            fos.write(pkcs8Key.getBytes(StandardCharsets.UTF_8));  
            fos.flush();  
            fos.close();  
    
        }
    
    }
    

    I was then able to use this private key with openssl to sign a file as a quick test that it works.

    Huge thanks to @dave_thompson_085 for pointing me in the right direction!