Using the Security API to Generate and Verify a Signature

Step 2: Generate Public and Private Keys

The first thing we must do in order to be able to create a digital signature is generate a key pair: a private key and a corresponding public key. A key is a number generated by a random-number generator. A private key is required for signing data, and the corresponding public key is required for verifying the signature.

A key pair is generated using the KeyPairGenerator class.

In this example we will generate a public-private key pair for the algorithm named "DSA" (Digital Signature Algorithm). We will generate keys with a 1024-bit length.

Generating a key pair is done with the following steps:

Create a Key Pair Generator

The first step is to get a key pair generator object for generating keys for the DSA signature algorithm. Put the following statement after the

        try {

line in the file created in the previous step, Step 1:

            KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA");

Initialize the Key Pair Generator

The next step is to initialize the key pair generator. All key pair generators share the concepts of a "strength" and a source of randomness. The KeyPairGenerator class initialize method has these two types of arguments.

The "strength" for a DSA key generator is the key length (in bits), which we will set to 1024.

The source of randomness must be an instance of the SecureRandom class. For simplicity, we will use the empty constructor for SecureRandom. It will automatically generate a "seed" value required for the random number generation.

            keyGen.initialize(1024, new SecureRandom());

Note: the SecureRandom default seed generation algorithm has not yet been thoroughly studied or widely deployed. So if we had a specific seed value that we wanted used instead, we would call the SecureRandom constructor that takes a byte array argument. Thus, if userSeed was a byte array containing the desired seed, we'd call

            keyGen.initialize(1024, new SecureRandom(userSeed));

Generate the Pair of Keys

The final step is generating the key pair and storing the keys in an instance of the KeyPair class:

            KeyPair pair = keyGen.generateKeyPair();

Using the Security API to Generate and Verify a Signature