Java - Encrypting a string with an existing public key file

Question

I have been studying this for the last 4-5 hours and cannot find an answer that really works, despite finding answers that used everything from several methods to a whole class of 100 lines. I can not imagine that there is no such simple function to do such a trivial thing: P

I have an already existing set of public / private keys (in fact, there are two sets - one generated by ssh-keygen, and the other openssl, so any format works great).

All I follow is a simple java equivalent of what I write in python, for example -

key_object = someModule.KeyObject(nameOfPublicKeyFile) def encrypt (SomePlainText) : return someOtherModule.encrypt(key_object, SomePlainText) 

Any help would be awesome!

Answer 1

These openssl shell commands create an RSA key pair and write the public and private keys to DER files.

Here the private key file is not password protected (-nocrypt) to make things simple.

 $ openssl genrsa -out keypair.pem 2048 Generating RSA private key, 2048 bit long modulus ............+++ ................................+++ e is 65537 (0x10001) $ openssl rsa -in keypair.pem -outform DER -pubout -out public.der writing RSA key $ openssl pkcs8 -topk8 -nocrypt -in keypair.pem -outform DER -out private.der 

Now that you have the DER files, you can read them in Java and use KeySpec and KeyFactory to create PublicKey and PrivateKey .

 public byte[] readFileBytes(String filename) throws IOException { Path path = Paths.get(filename); return Files.readAllBytes(path); } public PublicKey readPublicKey(String filename) throws IOException, NoSuchAlgorithmException, InvalidKeySpecException { X509EncodedKeySpec publicSpec = new X509EncodedKeySpec(readFileBytes(filename)); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); return keyFactory.generatePublic(publicSpec); } public PrivateKey readPrivateKey(String filename) throws IOException, NoSuchAlgorithmException, InvalidKeySpecException { PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(readFileBytes(filename)); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); return keyFactory.generatePrivate(keySpec); } 

Using public and private keys, you can encrypt and decrypt small amounts of data (which correspond to your RSA module). I recommend supplementing OAEP .

 public byte[] encrypt(PublicKey key, byte[] plaintext) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException { Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding"); cipher.init(Cipher.ENCRYPT_MODE, key); return cipher.doFinal(plaintext); } public byte[] decrypt(PrivateKey key, byte[] ciphertext) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException { Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding"); cipher.init(Cipher.DECRYPT_MODE, key); return cipher.doFinal(ciphertext); } 

Here it is associated with simple encryption and decryption:

 public void Hello() { try { PublicKey publicKey = readPublicKey("public.der"); PrivateKey privateKey = readPrivateKey("private.der"); byte[] message = "Hello World".getBytes("UTF8"); byte[] secret = encrypt(publicKey, message); byte[] recovered_message = decrypt(privateKey, secret); System.out.println(new String(recovered_message, "UTF8")); } catch (Exception e) { e.printStackTrace(); } } 

Answer 2

I would like to share a piece of code ... well, actually a whole class that can do what you need if you customize it to your needs. I used this in one of my applications, where I used to encrypt / decrypt the file with the generated public / private keys. The same can be applied to strings.

 import java.security.*; import java.security.spec.*; import javax.crypto.*; import javax.crypto.spec.*; import java.io.*; import java.util.*; /** * This class encrypts and decrypts a file using CipherStreams * and a 256-bit Rijndael key. The key is then encrypted using * a 1024-bit RSA key, which is password-encrypted. */ public class FileEncryptorRSA { /** * When files are encrypted, this will be appended to the end * of the filename. */ private static final String ENCRYPTED_FILENAME_SUFFIX=".encrypted"; /** * When files are decrypted, this will be appended to the end * of the filename. */ private static final String DECRYPTED_FILENAME_SUFFIX=".decrypted"; /** * Number of times the password will be hashed with MD5 * when transforming it into a TripleDES key. */ private static final int ITERATIONS = 1000; /** * FileEncryptor is started with one of three options: * * -c: create key pair and write it to 2 files * -e: encrypt a file, given as an argument * -d: decrypt a file, given as an argument */ public static void main (String[] args) throws Exception { if ((args.length < 1) || (args.length > 2)) { usage(); } else if ("-c".equals(args[0])) { createKey(); } else if ("-e".equals(args[0])) { encrypt(args[1]); } else if ("-d".equals(args[0])) { decrypt(args[1]); } else { usage(); } } private static void usage() { System.err.println("Usage: java FileEncryptor -c|-e|-d [filename]"); System.exit(1); } /** * Creates a 1024 bit RSA key and stores it to * the filesystem as two files. */ private static void createKey() throws Exception { BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Password to encrypt the private key: "); String password = in.readLine(); System.out.println("Generating an RSA keypair..."); // Create an RSA key KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA"); keyPairGenerator.initialize(1024); KeyPair keyPair = keyPairGenerator.genKeyPair(); System.out.println("Done generating the keypair.\n"); // Now we need to write the public key out to a file System.out.print("Public key filename: "); String publicKeyFilename = in.readLine(); // Get the encoded form of the public key so we can // use it again in the future. This is X.509 by default. byte[] publicKeyBytes = keyPair.getPublic().getEncoded(); // Write the encoded public key out to the filesystem FileOutputStream fos = new FileOutputStream(publicKeyFilename); fos.write(publicKeyBytes); fos.close(); // Now we need to do the same thing with the private key, // but we need to password encrypt it as well. System.out.print("Private key filename: "); String privateKeyFilename = in.readLine(); // Get the encoded form. This is PKCS#8 by default. byte[] privateKeyBytes = keyPair.getPrivate().getEncoded(); // Here we actually encrypt the private key byte[] encryptedPrivateKeyBytes = passwordEncrypt(password.toCharArray(),privateKeyBytes); fos = new FileOutputStream(privateKeyFilename); fos.write(encryptedPrivateKeyBytes); fos.close(); } /** * Encrypt the given file with a session key encrypted with an * RSA public key which will be read in from the filesystem. */ private static void encrypt(String fileInput) throws Exception { BufferedReader in = new BufferedReader (new InputStreamReader(System.in)); System.out.print("Public Key to encrypt with: "); String publicKeyFilename = in.readLine(); // Load the public key bytes FileInputStream fis = new FileInputStream(publicKeyFilename); ByteArrayOutputStream baos = new ByteArrayOutputStream(); int theByte = 0; while ((theByte = fis.read()) != -1) { baos.write(theByte); } fis.close(); byte[] keyBytes = baos.toByteArray(); baos.close(); // Turn the encoded key into a real RSA public key. // Public keys are encoded in X.509. X509EncodedKeySpec keySpec = new X509EncodedKeySpec(keyBytes); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); PublicKey publicKey = keyFactory.generatePublic(keySpec); // Open up an output file for the output of the encryption String fileOutput = fileInput + ENCRYPTED_FILENAME_SUFFIX; DataOutputStream output = new DataOutputStream (new FileOutputStream(fileOutput)); // Create a cipher using that key to initialize it Cipher rsaCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding"); rsaCipher.init(Cipher.ENCRYPT_MODE, publicKey); // Now create a new 256 bit Rijndael key to encrypt the file itself. // This will be the session key. KeyGenerator rijndaelKeyGenerator = KeyGenerator.getInstance("Rijndael"); rijndaelKeyGenerator.init(256); System.out.println("Generating session key..."); Key rijndaelKey = rijndaelKeyGenerator.generateKey(); System.out.println("Done generating key."); // Encrypt the Rijndael key with the RSA cipher // and write it to the beginning of the file. byte[] encodedKeyBytes= rsaCipher.doFinal(rijndaelKey.getEncoded()); output.writeInt(encodedKeyBytes.length); output.write(encodedKeyBytes); // Now we need an Initialization Vector for the symmetric cipher in CBC mode SecureRandom random = new SecureRandom(); byte[] iv = new byte[16]; random.nextBytes(iv); // Write the IV out to the file. output.write(iv); IvParameterSpec spec = new IvParameterSpec(iv); // Create the cipher for encrypting the file itself. Cipher symmetricCipher = Cipher.getInstance("Rijndael/CBC/PKCS5Padding"); symmetricCipher.init(Cipher.ENCRYPT_MODE, rijndaelKey, spec); CipherOutputStream cos = new CipherOutputStream(output, symmetricCipher); System.out.println("Encrypting the file..."); FileInputStream input = new FileInputStream(fileInput); theByte = 0; while ((theByte = input.read()) != -1) { cos.write(theByte); } input.close(); cos.close(); System.out.println("File encrypted."); return; } /** * Decrypt the given file. * Start by getting the RSA private key * and decrypting the session key embedded * in the file. Then decrypt the file with * that session key. */ private static void decrypt(String fileInput) throws Exception { BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); System.out.print("Private Key to decrypt with: "); String privateKeyFilename = in.readLine(); System.out.print("Password for the private key: "); String password = in.readLine(); // Load the private key bytes FileInputStream fis = new FileInputStream(privateKeyFilename); ByteArrayOutputStream baos = new ByteArrayOutputStream(); int theByte = 0; while ((theByte = fis.read()) != -1) { baos.write(theByte); } fis.close(); byte[] keyBytes = baos.toByteArray(); baos.close(); keyBytes = passwordDecrypt(password.toCharArray(), keyBytes); // Turn the encoded key into a real RSA private key. // Private keys are encoded in PKCS#8. PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(keyBytes); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); PrivateKey privateKey = keyFactory.generatePrivate(keySpec); // Create a cipher using that key to initialize it Cipher rsaCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding"); // Read in the encrypted bytes of the session key DataInputStream dis = new DataInputStream(new FileInputStream(fileInput)); byte[] encryptedKeyBytes = new byte[dis.readInt()]; dis.readFully(encryptedKeyBytes); // Decrypt the session key bytes. rsaCipher.init(Cipher.DECRYPT_MODE, privateKey); byte[] rijndaelKeyBytes = rsaCipher.doFinal(encryptedKeyBytes); // Transform the key bytes into an actual key. SecretKey rijndaelKey = new SecretKeySpec(rijndaelKeyBytes, "Rijndael"); // Read in the Initialization Vector from the file. byte[] iv = new byte[16]; dis.read(iv); IvParameterSpec spec = new IvParameterSpec(iv); Cipher cipher = Cipher.getInstance("Rijndael/CBC/PKCS5Padding"); cipher.init(Cipher.DECRYPT_MODE, rijndaelKey, spec); CipherInputStream cis = new CipherInputStream(dis, cipher); System.out.println("Decrypting the file..."); FileOutputStream fos = new FileOutputStream(fileInput + DECRYPTED_FILENAME_SUFFIX); // Read through the file, decrypting each byte. theByte = 0; while ((theByte = cis.read()) != -1) { fos.write(theByte); } cis.close(); fos.close(); System.out.println("Done."); return; } /** * Utility method to encrypt a byte array with a given password. * Salt will be the first 8 bytes of the byte array returned. */ private static byte[] passwordEncrypt(char[] password, byte[] plaintext) throws Exception { // Create the salt. byte[] salt = new byte[8]; Random random = new Random(); random.nextBytes(salt); // Create a PBE key and cipher. PBEKeySpec keySpec = new PBEKeySpec(password); SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("PBEWithSHAAndTwofish-CBC"); SecretKey key = keyFactory.generateSecret(keySpec); PBEParameterSpec paramSpec = new PBEParameterSpec(salt, ITERATIONS); Cipher cipher = Cipher.getInstance("PBEWithSHAAndTwofish-CBC"); cipher.init(Cipher.ENCRYPT_MODE, key, paramSpec); // Encrypt the array byte[] ciphertext = cipher.doFinal(plaintext); // Write out the salt, then the ciphertext and return it. ByteArrayOutputStream baos = new ByteArrayOutputStream(); baos.write(salt); baos.write(ciphertext); return baos.toByteArray(); } /** * Utility method to decrypt a byte array with a given password. * Salt will be the first 8 bytes in the array passed in. */ private static byte[] passwordDecrypt(char[] password, byte[] ciphertext) throws Exception { // Read in the salt. byte[] salt = new byte[8]; ByteArrayInputStream bais = new ByteArrayInputStream(ciphertext); bais.read(salt,0,8); // The remaining bytes are the actual ciphertext. byte[] remainingCiphertext = new byte[ciphertext.length-8]; bais.read(remainingCiphertext,0,ciphertext.length-8); // Create a PBE cipher to decrypt the byte array. PBEKeySpec keySpec = new PBEKeySpec(password); SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("PBEWithSHAAndTwofish-CBC"); SecretKey key = keyFactory.generateSecret(keySpec); PBEParameterSpec paramSpec = new PBEParameterSpec(salt, ITERATIONS); Cipher cipher = Cipher.getInstance("PBEWithSHAAndTwofish-CBC"); // Perform the actual decryption. cipher.init(Cipher.DECRYPT_MODE, key, paramSpec); return cipher.doFinal(remainingCiphertext); } } 

EDIT:

You will need to change the Java policy on the JVM to the Java Cryptography Extension (JCE). Unlimited jurisdiction to use this code. All related information about changes to JAVA policy can be found HERE

Answer 3

Here is a good example:

• http://javadigest.wordpress.com/2012/08/26/rsa-encryption-example/

and there are many more. (Google for the "java encrypt RSA example" if this link breaks.)

---

I can not find an answer that really works

Try related above. If this does not work, follow the edit or comment to say what is going wrong.

I can not imagine that there is no such simple function to do such a trivial thing .: P

Sorry, but your imagination must be broken :-)

This is actually not a trivial thing. And this is complicated by the fact that Java is trying to support a wide range of crypto-functional and cryptotechnological stacks using a single unified API.