Generating a digital signature with DSA

Question

I want a digital signature like below code:Sign[publickey, a||b].

Integer a;
String b,c;
a=12; b= "i am fine";
c=a+b;  
Signature DSA = Signature.getInstance(c, "SUN"); 
DSA.initSign(pvKey);

Answer 1

"12iamfine" is not a valid parameter for Signature.getInstance() . You need to pass the name of the algorithm, not your data that you want to sign (I assume that's what c is...).

I've summarized the code from the following blog for generating the signature: https://compscipleslab.wordpress.com/2012/11/18/generating-verifying-digital-signatures/

//Create a KeyPairGenerator object for generating keys for the DSA signature algorithm.
KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA", "SUN");

//Instantiate SecureRandom Object, which will act as a source of random numbers. 
//Here, we use SHA1PRNG 
SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");

//Initialize the KeyPairGenerator with the key-length and the source of randomness
keyGen.initialize(1024, random);

//Generate the key-pair
KeyPair pair = keyGen.generateKeyPair();

//Collect the public & private key from the KeyPair into separate objects
PrivateKey privkey = pair.getPrivate();
PublicKey pubkey = pair.getPublic();

//Create a Signature object, you have to supply two arguments,first the algorithm 
//name & the provider. Here we use SHA1withDSA supplied by the SUN provider
Signature dsa = Signature.getInstance("SHA1withDSA", "SUN");

//Initialize it with the private key before using it for signing.
dsa.initSign(privkey);

//Supply the Signature Object the data to Be Signed
BufferedInputStream bufin = new BufferedInputStream(new FileInputStream(inputfile));
byte[] buffer = new byte[1024];
int len;

while ((len = bufin.read(buffer)) >=0) {
    dsa.update(buffer, 0, len);
}

bufin.close();

//Sign the data i.e. generate a signature for it
byte[] realSig = dsa.sign();