CODEWITHSUNDEEP
javaencryptionaes
Programmer0
Programmer0

Reputation: 21

AES encryption/decryption java bouncy castle explanation?

Can someone please explain what this program is doing pointing out some of the major points? I'm looking at the code and I'm completely lost. I just need explanation on the encryption/decryption phases. I think it generates an AES 192 key at one point but I'm not 100% sure. I'm not sure what the byte/ivBytes are used for either. 

import java.security.Key;
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.spec.IvParameterSpec;

public class RandomKey
{
    public static void main(String[] args) throws Exception
    {
    byte[] input = new byte[] { 
                        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 
                        0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
                        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07 };


    byte[] ivBytes = new byte[] { 
                        0x00, 0x00, 0x00, 0x01, 0x04, 0x05, 0x06, 0x07,
                        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01 };

    //initializing a new initialization vector  
    IvParameterSpec ivSpec  = new IvParameterSpec(ivBytes);
    //what does this actually do?
    Cipher cipher = Cipher.getInstance("AES/CTR/NoPadding", "BC");
    //what does this do?
    KeyGenerator generator = KeyGenerator.getInstance("AES","BC");
    //I assume this generates a key size of 192 bits
    generator.init(192);
    //does this generate a random key?
    Key encryptKey = generator.generateKey();

    System.out.println("input: " +toHex(input));

    //encryption phase
    cipher.init(Cipher.ENCRYPT_MODE, encryptKey, ivSpec);
    //what is this doing?
    byte[] cipherText = new byte[cipher.getOutputSize(input.length)];
    //what is this doing?
    int ctLength = cipher.update(input, 0, input.length, cipherText,0);

    //getting the cipher text length i assume?
    ctLength += cipher.doFinal (cipherText, ctLength );
    System.out.println ("Cipher: " +toHex(cipherText) + " bytes: " + ctLength);


    //decryption phase
    cipher.init(Cipher.DECRYPT_MODE, encryptKey, ivSpec);
    //storing the ciphertext in plaintext i'm assuming?
    byte[] plainText = new byte[cipher.getOutputSize(ctLength)];
    int ptLength = cipher.update(cipherText, 0, ctLength, plainText, 0);
    //getting plaintextLength i think?
    ptLength= cipher.doFinal (plainText, ptLength);
    System.out.println("plain: " + toHex(plainText, ptLength));  

    }

private static String digits = "0123456789abcdef";

public static String toHex(byte[] data, int length)
{
    StringBuffer buf = new StringBuffer();

    for (int i=0; i!= length; i++)
    {
        int v = data[i] & 0xff;

        buf.append(digits.charAt(v >>4));
        buf.append(digits.charAt(v & 0xf));
    }
    return buf.toString();

}

public static String toHex(byte[] data)
{
    return toHex(data, data.length);
}
}

Upvotes: 2

Views: 8073

Answers (2)

rossum
rossum

Reputation: 15685

 //what does this actually do?
 Cipher cipher = Cipher.getInstance("AES/CTR/NoPadding", "BC");

This uses the Bouncy Castle ("BC") provider to get an instance of the AES cypher, set up in Counter mode (CTR) with no padding (NoPadding). See Wikipedia for Block Cypher modes and Padding. The Javadoc will also help you.

//what does this do?
KeyGenerator generator = KeyGenerator.getInstance("AES","BC");

Again this uses the Bouncy Castle provider to set up a key generator for AES. You can read the Javadoc to learn more.

//what is this doing?
byte[] cipherText = new byte[cipher.getOutputSize(input.length)];

It sets up an array of bytes large enough to hold the encrypted output.

//what is this doing?
int ctLength = cipher.update(input, 0, input.length, cipherText,0);

It is actually doing the encyphering. Check the Javadoc for the update() method for a good explanation.

//getting the cipher text length i assume?
ctLength += cipher.doFinal (cipherText, ctLength );

No. Look at that += It is updating the cyphertext length. Again read the Javadoc for the differences between the update() and doFinal() methods.

Upvotes: 2

Fz3
Fz3

Reputation: 33

Did you try looking here?

http://docs.oracle.com/javase/6/docs/api/javax/crypto/Cipher.html

http://docs.oracle.com/javase/6/docs/api/javax/crypto/KeyGenerator.html

The code you have is a pretty straight forward code example of how to encrypt and decrypt byte data in Java.

Once you have read through the class documentation you can better articulate your questions about the behavior of this code.

Upvotes: 1

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