Encrypting with RSA private key in Java

Question

I'm trying to encrypt some content with an RSA private key.

I'm following this example: http://www.junkheap.net/content/public_key_encryption_java

but converting it to use private keys rather than public. Following that example, I think what I need to do is:

• Read in a DER-format private key
• Generate a PCKS8EncodedKeySpec
• call generatePrivate() from KeyFactory to get a private key object
• Use that private key object with the Cipher object to do the encryption

So, the steps:

The key was generated from openssl with:

openssl genrsa -aes256 -out private.pem 2048

and then was converted to DER format with:

openssl rsa -in private.pem -outform DER -out private.der

I generate the PKCS8EncodedKeySpec with:

byte[] encodedKey = new byte[(int)inputKeyFile.length()];

try {
    new FileInputStream(inputKeyFile).read(encodedKey);
} catch (FileNotFoundException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(encodedKey);
return privateKeySpec;

And then generate the private key object with:

PrivateKey pk = null;

try {
    KeyFactory kf = KeyFactory.getInstance(RSA_METHOD);
    pk = kf.generatePrivate(privateKeySpec);
} catch (NoSuchAlgorithmException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
} catch (InvalidKeySpecException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}
return pk;

However, on the call to:

pk = kf.generatePrivate(privateKeySpec);

I get:

java.security.spec.InvalidKeySpecException: Unknown key spec.
at com.sun.net.ssl.internal.ssl.JS_KeyFactory.engineGeneratePrivate(DashoA12275)
at com.sun.net.ssl.internal.ssl.JSA_RSAKeyFactory.engineGeneratePrivate(DashoA12275)
at java.security.KeyFactory.generatePrivate(KeyFactory.java:237)

Questions:

• Is the general approach right?
• Is the PCKS8EncodedKeySpec the right keyspec to use?
• Any thoughts on the invalid key spec error?

Answer 1

First of all, I'm confused why you are planning to use a Cipher to encrypt with a private key, rather than signing with a Signature. I'm not sure that all RSA Cipher providers will use the correct block type for setup, but it's worth a try.

Setting that aside, though, I think that you are trying to load a non-standard OpenSSL-format key. Converting it to DER with rsa is essentially just a base-64 decode; the structure of the key is not PKCS #8.

Instead, after genrsa, use the openssl pkcs8 command to convert the generated key to unencrypted PKCS #8, DER format:

openssl pkcs8 -topk8 -nocrypt -in private.pem -outform der -out private.der

This will produce an unencrypted private key that can be loaded with a PKCS8EncodedKeySpec.