Cryptography is a field looking at techniques for encoding and verifying things securely. It tends to focus on the following issues:

  • encryption of data so that an unauthorized third party cannot read it without a key of some sort;
  • authentication and validation (or certification);
  • computer protocols for using the previous two techniques correctly and in a way that allows all parties to know how they’re being used (e.g. the TLS protocol allows a client to connect to a server over the Internet without the two machines previously knowing things such as session key or even preferred encryption method, maximum key length, etc).

Symmetric-key encryption in Java

This is the case where the same key is used to both encrypt and decrypt. It is sometimes called secret-key encryption, single-key encryption or simply symmetric encryption.

In Java, use an instance of javax.crypto.Cipher.

Encryption example

String passPhrase = "secret sequence of bytes";
byte[] dataToEncrypt = "data to encrypt".getBytes();

MessageDigest digest = MessageDigest.getInstance("SHA");
digest.update(passPhrase.getBytes());

Key key = new SecretKeySpec(digest.digest(), 0, 16, "AES");

Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, key);

byte[] encryptedData = cipher.doFinal(dataToEncrypt);

System.out.println(Arrays.toString(encryptedData));

Decryption example

String passPhrase = "secret sequence of bytes";
byte[] dataToDecrypt = new byte[] { -84,-66,-113,54,125,48,-28,-60,36,52,-57,109,105,-17,111,-12 };

MessageDigest digest = MessageDigest.getInstance("SHA");
digest.update(passPhrase.getBytes());

Key key = new SecretKeySpec(digest.digest(), 0, 16, "AES");

Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, key);

byte[] decryptedData = cipher.doFinal(dataToDecrypt);

System.out.println(new String(decryptedData));

Asymmetric (public key) encryption in Java

Asymmetric encryption, also known as public key encryption or public key cryptography, refers to a cryptographic algorithm which requires two separate keys, one of which is secret (or private) and one of which is public:

  • the public key is used to encrypt;
  • the private key is used to decrypt.

A common paradigm in a centralized client-server system is that a private key is held securely on the server, while the corresponding public key is distributed to all clients.

In Java, the keys will be instances of java.security.PublicKey and java.security.PrivateKey.

To generate a new pair:

KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
KeyPair keyPair = keyPairGenerator.generateKeyPair();
PublicKey publicKey = keyPair.getPublic();
PrivateKey privateKey = keyPair.getPrivate();

The keys can also be transformed into byte arrays for storage and transmission:

KeyFactory keyFactory = KeyFactory.getInstance("RSA");
byte[] publicKeyBytes = publicKey.getEncoded();
KeySpec keySpec = new X509EncodedKeySpec(publicKeyBytes);
PublicKey keyFromBytes = keyFactory.generatePublic(keySpec);

Encryption example

String passPhrase = "secret sequence of bytes";
byte[] dataToEncrypt = "data to encrypt".getBytes();

MessageDigest digest = MessageDigest.getInstance("SHA");
digest.update(passPhrase.getBytes());

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, publicKey);

byte[] encryptedData = cipher.doFinal(dataToEncrypt);

System.out.println(new String(encryptedData));

Decryption example

String passPhrase = "secret sequence of bytes";
byte[] dataToDecrypt = /* read from stream/file/etc */;

MessageDigest digest = MessageDigest.getInstance("SHA");
digest.update(passPhrase.getBytes());

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.DECRYPT_MODE, privateKey);

byte[] decryptedData = cipher.doFinal(dataToDecrypt);

System.out.println(new String(decryptedData));

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