如何使用匹配的DER私钥解密公共PEM密钥加密的字节数组？

Question

So I am creating a basic socket program where I want to send an encrypted string in C to a Java program. My C program encrypts the string with a public PEM key. I have converted the matching private PEM key to a DER key and now want to decrypt the string that was sent to my Java program. How do I do this?

At the moment I am getting an IllegalBlockSizeException stating that "Data must not be longer than 256 bytes" when trying to run the code as it stands.

This is what I have at the moment:

C client program...

 //Get our public key from the publickey file created by server
    FILE *publicKeyFile = fopen("publicKey.pem", "rb");
    RSA *rsa = RSA_new();
    rsa = PEM_read_RSA_PUBKEY(publicKeyFile, &rsa, NULL, NULL);
    if(rsa == NULL) {
            printf("Error with public key...\n");
    }
    else {
            //if the public key is correct we will encrypt the message
            RSA_public_encrypt(2048, sigMessage, sigMessage, rsa, RSA_PKCS1_PADDING);
    }

Java decryption...

    public static String decrypt(byte[] encryptedMessage) {

    try {
        Cipher rsa;
        rsa = Cipher.getInstance("RSA");
        PrivateKey ourKey = getKey("resources/privateKey.der");
        rsa.init(Cipher.DECRYPT_MODE, ourKey);
        byte[] utf8 = rsa.doFinal(encryptedMessage);
        return new String(utf8, "UTF8");
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

public static PrivateKey getKey(String filePath) throws IOException, NoSuchAlgorithmException, InvalidKeySpecException {

        File f = new File(filePath);
        FileInputStream fis = new FileInputStream(f);
        DataInputStream dis = new DataInputStream(fis);
        byte[] keyBytes = new byte[(int) f.length()];
        dis.readFully(keyBytes);
        dis.close();

        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory kf = KeyFactory.getInstance("RSA");
        return kf.generatePrivate(spec);
    }

Answer 1

You can't encrypt 2,048 bytes with RSA unless you have an RSA key of over 16K. You probably have a 2048 bit RSA key, which limits you to under 256 bytes. Check out the openssl man page for RSA_public_encrypt :

flen must be less than RSA_size (rsa) - 11 for the PKCS #1 v1.5 based padding modes, less than RSA_size(rsa) - 41 for RSA_PKCS1_OAEP_PADDING and exactly RSA_size(rsa) for RSA_NO_PADDING. The random number generator must be seeded prior to calling RSA_public_encrypt().

And RSA_size :

RSA_size() returns the RSA modulus size in bytes . It can be used to determine how much memory must be allocated for an RSA encrypted value.

You should not encrypt a full 256 bytes with a 2048 bit key, though. You want to always use random padding with RSA encryption, and choose OAEP over v1.5.