1. MT311 Java Application Development and Programming Languages Li Tak Sing ( 李德成 )

2. Security For Java applications, there are two main areas of security issues: 1 system security 2 information security.

3. System security System security refers to the safety and stability of the computing environment. The safety and stability can be breached in a number of ways. When a malicious application (such as a virus) executes itself, it can cause damage to the system — for example, by deleting some critical files and rendering the computer inoperable.

4. Information security Or, the malicious application can intentionally or unintentionally consume too many resources, such as computing time, disk space, or network bandwidth, thereby causing the system to perform improperly.

5. Information security Information security, however, refers to the secrecy and integrity of data. For example, when you send an email, how do you ensure that only the targeted recipients can read the message? When you receive an email, how do you ensure that the message has not been tampered with and that it is from the supposed sender?

6. System security In the security policy model, resources can be granted or denied different types of access independently. For example, a file can be a resource, and the read action can be differentiated from the write action. So, you can easily grant read-only access to a particular file. You can do the same with objects, allowing you to create security policies for runtime objects as well.

7. System security Java makes things even more interesting by allowing different policies to apply to different applications, or to different invocations of the same application.

8. Security policy As you know by now, applets are small applications that can be embedded in webpages. When Java was first introduced, applets were sensational because they provided a cross-platform solution for making a webpage more interesting. To safeguard users from malicious applets, applets are run in a sandbox, which imposes rather stringent restrictions on what the applets can do.

9. Applet security If you run an applet through a browser and then the applet tries to read a local file, an error message would appear.

10. Allowing an applet to access a local file To remove the restriction, we need to specify a different policy. The format of the policy file is quite simple, and you can create one using a text editor. For example, below is a simple file that grants rights for applets from plbpc001.ouhk.edu.hk ‘.java.policy’:

11. Changing the policy grant codeBase "http://plbpc001.ouhk.edu.hk/-" { permission java.security.AllPermission; }; You should put this file in the "My Documents" directory. As this would grant permission to applets from the host to do everything, you should remove this file after testing it.

12. Cryptography Information security is save guarded by cryptography.

13. Cryptography Cryptography has four main objectives: – Confidentiality — the information cannot be understood by anyone for whom it was not intended. – Integrity — the information cannot be altered in storage or transit between sender and intended receiver without the alteration being detected.

14. Cryptography – Non-repudiation — the creator/sender of the information cannot deny at a later stage his or her intentions in the creation or transmission of the information. – Authentication — the sender and receiver can confirm each other’s identity and the origin/destination of the information.

15. Secret Key method In the secret key method, the sender and the receiver share the same secret key. Then, the sender first encrypts the message with the key and sends the encrypted message to the receiver who decrypts the message with the same key.

16. Secret key message encrypted message encrypted message key send over an unsafe channel

17. Java secret key APIs Java’s cryptographic APIs are defined in the java.security and javax.crypto packages. For a basic encryption, we need a secret key and a cryptographic algorithm. The Java classes for those are: – SecretKey — this class encapsulates the secret key for use in encryption and decryption – Cipher — this class provides cryptographic APIs for encryption and decryption.

18. Creating a secret key The following code would create a secret key: // the key itself as a byte array byte[] key = new byte[] {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'}; // create a KeySpec specifically for DES for our key DESKeySpec spec = new DESKeySpec(key); // retrieve a DES SecretKeyFactory SecretKeyFactory factory = SecretKeyFactory.getInstance("DES"); // generate the actual SecretKey object SecretKey secret = factory.generateSecret(spec);

19. Creating a cipher To create a DES cipher, you first need to find a DES algorithm provider using Cipher.getInstance: Cipher c = Cipher.getInstance("DES"); The same Cipher object can be used for either encryption or decryption, depending on how you initialize the object: – Encryption: c.init(Cipher.ENCRYPT_MODE, secret); – Decryption c.init(Cipher.DECRYPT_MODE, secret);

20. To encrypt or decrypt a message Now, with Cipher object, you can encrypt or decrypt any bytes easily with: c.update(byte[] buf); You can invoke update as many times as necessary to encrypt or decrypt the entire message. To retrieve the encrypted or decrypted result, you invoke the doFinal method: byte[] inEncrypted = c.doFinal();

21. Overall algorithm To create a secret key To create a cipher Invoke the update method continuously until the end of data Invoke the doFinal method to get the final result.

22. Conversion to or from byte[] You should notice that all the cryptography APIs work on byte[]. So no method what is the format of your original message, you must convert it to byte[].

23. Conversion to or from byte[] To convert a String to byte[], you can use the following method of String: public byte[] getBytes() To convert a byte[] to a String, you can use the following constructor of String: public String(byte[] bytes)

24. Encryption The following method would encrypt a message: static byte[] encrypt(String st, Cipher c) { c.update(st.getBytes()); return c.doFinal(); }

25. Decryption The following method would decrypt a message: public static String decrypt(byte[] message, Cipher c) { c.update(message); byte[] result=c.doFinal(); return new String(result); }

26. Message digest The above method would only be able to protect the confidentiality of the message. It cannot protect the integrity of the message because the receiver would not know whether the message has been altered in any way. To protect the integrity, we need the message digest.

27. Hash function A message digest, typically of fixed length, is generated using a special mathematical transformation called a hash function. A hash function is basically a transformation that takes any arbitrary input and produces an output in a finite space.

28. Hash function For message digests, we need a hash function that has two properties: – It must be extremely difficult to produce the same message digest from two different messages, i.e. the hash must be one-to-one. – It must be extremely difficult to produce the original message from a given message digest, i.e. the hash must be irreversible.

29. Message digest Message digest alone cannot be used to protect the integrity of message. This is because anyone can use the same hash function to protect a message digest of an altered message. So the secret key method must be used together with the message digest.

30. Secret key method with message digest message message digest encrypted message sent over unsafe channel compare produce key

31. Message digest The creation of a MessageDigest is similar to that of Cipher, using getInstance: MessageDigest md = MessageDigest.getInstance("MD5"); Once you have a MessageDigest object, you can feed data to it using the update method: md.update(inbuf); Finally, you can retrieve the final hash using the digest method: md.digest();

32. Message digest The following method generate the message digest of a String: public static byte[] md(String st) { MessageDigest md = MessageDigest.getInstance("MD5"); md.update(st.getBytes()); return md.digest(); }

33. Compare two byte[] Since message digests are in the format of byte[], we need to compare two byte[]'s. you should use a for loop to compare two byte[]'s.

34. Message Authentication Code (MAC) As we have mentioned earlier, we cannot use the message digest alone to guarantee the integrity of a message. We need to add a secret key protection so that the resulting string would also depends on the secret key. Such an encrypted message digest is called a message authentication code(MAC). MAC protects the authentication and integrity of a message.

35. MAC When a user want to send a message, it would use the secret key to produce a MAC of the message. Then he/she sends the message and the MAC to the receiver. The receiver would then use the same secret key and the message to generate another MAC. The two MACs would then be compared. If they are the same, then the message is really from the supposed sender and the message has not been messed with by others.

36. message MAC keyed hash message MAC send to the recipient keyed hash compare

37. MAC The followings are the steps to create an MAC of a message: – create a secret key – create an Mac object – initialize the Mac object with the key – update the Mac object with the contents of the message – get the MAC from the Mac object.

38. Create a secret key KeyGenerator kg = KeyGenerator.getInstance("HmacMD5"); SecretKey sk = kg.generateKey(); Note that the key is randomly generated. So different key would be generated if the generateKey is invoked for many times. So if you want to share this key, you need to save the key to a file and then share the file.

39. Create an Mac object Mac mac = Mac.getInstance("HmacMD5");

40. Initialize the Mac object with the key mac.init(sk);

41. update the Mac object with the contents of the message mac.update(buf); where buf is an array of bytes. You can call this method as many times as you wish.

42. get the MAC from the Mac object. byte[] result=mac.doFinal();

43. The MD5 program KeyGenerator kg = KeyGenerator.getInstance("HmacMD5"); SecretKey sk = kg.generateKey(); Mac mac = Mac.getInstance("HmacMD5"); mac.init(sk); mac.update(buf); byte[] result=mac.doFinal();

44. Public key method In the secret key method, there is a problem in distributing the key because both the sender and recipient need to have the same key. It is possible that the key is intercepted when it is transmitted from one person to another.

45. Public key method The public key method, you need a pair of keys to perform the encryption and decryption process. The two keys are called public key and private key. You cannot deduce the private key from the public key. When a message is encrypted with the public key, it must be decrypted with the private key. When a message is decrypted with the private key, it must be decrypted with the public key.

46. Public key methods A user first generates a key pair that consists of a public key and private key. He/she now informs others about the public key. Then anybody can now encrypt a message with the public key and send the encrypted message to the user. Now, the user can decrypt the message with the private key.

47. Public key methods Note that the public key does not have to be send over a secured channel because any one who knows the public key would still not be able to decrypt any message that has been encrypted with the public key. The method protects the confidentiality of the message.

48. Java classes for the public key methods Most java classes for the public key methods are in the package java.security. KeyPairGenerator – the static method static public KeyPairGenerator getInstance(String) would return a KeyPairGenerator with the specified method. The most popular method is RSA. So the following statement get a RSA public key generator:

49. original message encrypted with public key encrypted message encrypted message original message decrypted with private key original message encrypted with private key encrypted message encrypted message original message decrypted with public key

50. Java classes for the public key methods KeyPairGenerator generator=KeyPairGenerator("RSA"); The following method of KeyPairGenerator would initial the KeyPairGenerator to generate a key of the specified byte length: public void initialize(int length) For example, the following statement would initialize a KeyPairGenerator to generate a key of size 2048 bits long: generator.initialize(2048);

51. Java classes for the public key methods The following statement create a key pair: KeyPair key=generator.generateKeyPair(); The following statement finds the public key: PublicKey publicKey=key.getPublic(); The following statement finds the private key: Privatekey privatekey=key.getPrivate();

52. Java classes for the public key methods After obtaining the key pair, we can use Cipher to encode or decode a message like what we have done in the secret key algorithm. The following statement initializes a Cipher to be used to encrypt a message using the private key: Cipher cipher=Cipher.getInstance("RSA"); cipher.init(Cipher.ENCRYPT_MODE, key.getPrivate());

53. Java classes for the public key methods The following statement initializes a Cipher to be used to encrypt a message using the public key: Cipher cipher=Cipher.getInstance("RSA"); cipher.init(Cipher.ENCRYPT_MODE, key.getPublic());

54. Java classes for the public key methods The following statement initializes a Cipher to be used to decrypt a message using the private key: Cipher cipher=Cipher.getInstance("RSA"); cipher.init(Cipher.DECRYPT_MODE, key.getPivate());

55. Java classes for the public key methods The following statement initializes a Cipher to be used to decrypt a message using the public key: Cipher cipher=Cipher.getInstance("RSA"); cipher.init(Cipher.DECRYPT_MODE, key.getPublic());

56. Java classes for the public key methods Then, we can use the update and doFinal method Cipher to encrypt or decrypt the message. If the message is short, we can simply use the doFinal straight away to decrypt the message.

57. Digital signature When you receive a message from a person, how do you be sure that the message is really from that person? This can be done by using digital signature.

58. Digital signature A digital signature is like a MAC but the key used is the private key of the key pair. To produce a digital signature, we first create a message digest and then encrypt the message digest with the private key of the message. We would say that we sign the message with the private key.

59. Digital signature Then, the sender would send the message together with the digital signature. The receiver would then first use the public key of the sender to decrypt the digital signature and then get back the message digest. The receiver then calculate another message digest from the message.

60. Digital signature If the two digital digests match, then we can sure of two things: – integrity. That is the message has not been changed by another person. – authentication. The message is really from the supposed sender because only he/she has the private key.

61. message digest hash digital signature encrypt with private key message digital signature send to the recipient message digest hash message digest decrypt with public key compare

62. Java classes to create a digital signature First we create a key pair first like what we did for the public key method. However, we would specify that we want to use the DSA algorithm which is a digital signature algorithm: KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");

63. Java classes to create a digital signature Then, we initialize the key as before: kpg.initialize(1024); Then, we can use this key pair generator to generate a key pair: KeyPair keyPair=kpg.generateKeyPair();

64. Java classes to create a digital signature We can then get the public key and private key of the key pair: PublicKey publicKey=keyPair.getPublic(); PrivateKey privateKey=keyPair.getPrivate(); You should then save the two keys to a files and send the public key to your recipients.

65. Java classes to create a digital signature Now, assume that we want to create a digital signature of a message. Signature sig=Signature.getInstance("DSA"); sig.initSign(privateKey); Then, you can use the update method of Signature to check the contents of the message: sig.update(message);

66. Java classes to create a digital signature The digital signature is generated by invoked by the method sign: sig.sign(); This method returns an array of bytes which is the digital signature of the message. To verify a signature, we need the following statements: Signature sig=Signature.getInstance("DSA"); sig.initVerify(publicKey);

67. Java classes to create a digital signature Then, we use the update method of signature to put in the content of the message just like what we did when we generated the digitial message. sig.update(message);

68. Java classes to create a digital signature Then, we can check whether the message has been correctly signed by invoking the method verify of Signature: sig.verify(); This method would return true or false depending on whether the message has been correctly signed or not.