How to decrypt AES using a key

Question

The example here will Generate the secret key specs first.
http://java.sun.com/developer/technicalArticles/Security/AES/AES_v1.html
I already have a Decrypt Key used in my server application. How can I use that key to decrypt the msg sent from server?

Answer 1

Hi
I wrote this code to check Java encryption with AES and a key. This worked fine for me. Please have a look.
Encrypt and decrypt using the DES private key algorithm
import java.security.*;
import javax.crypto.*;
import javax.crypto.spec.SecretKeySpec;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
public class AESEncrypt {
    public static void main (String[] args) throws Exception {
        Security.addProvider(new BouncyCastleProvider());
        byte[] plainText = "LOGIN=2222=v2-0-b7=SMST=smst=ASI".getBytes("utf-8");
        // Get a DES private key
        System.out.println( "\nAES key" );
        String strKey = "75de8a33d3f18f1c29d86fa42b1894c7";
        byte[] keyBytes = hexToBytes(strKey);
        // skeyspec is the key to encrypt and decrypt
        SecretKeySpec skeySpec = new SecretKeySpec(keyBytes, "AES");
        System.out.println("Key: " + asHex(key.getEncoded()));
        System.out.println( "Finish generating AES key" );
        // Creates the DES Cipher object (specifying the algorithm, mode, and padding).
        Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding");
        // Print the provider information
        System.out.println( "\n" + cipher.getProvider().getInfo() );
        System.out.println( "\nStart encryption" );
        // Initializes the Cipher object.
        cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
        // Encrypt the plaintext using the public key
        byte[] cipherText = cipher.doFinal(plainText);
        System.out.println( "Finish encryption: cipherText: " + asHex(cipherText));
        System.out.println( "\nStart decryption" );
        // Initializes the Cipher object.
        cipher.init(Cipher.DECRYPT_MODE, skeySpec);
        // Decrypt the ciphertext using the same key
        byte[] newPlainText = cipher.doFinal(cipherText);
        System.out.println( "Finish decryption: " );
        System.out.print( asHex(newPlainText) );
    public static String asHex (byte buf[]) {
      StringBuffer strbuf = new StringBuffer(buf.length * 2);
      int i;
      for (i = 0; i < buf.length; i++) {
       if (((int) buf[i] & 0xff) < 0x10)
         strbuf.append("0");
       strbuf.append(Long.toString((int) buf[i] & 0xff, 16));
      return strbuf.toString();
    public static byte[] hexToBytes(char[] hex) {
        int length = hex.length / 2;
        byte[] raw = new byte[length];
        for (int i = 0; i < length; i++) {
            int high = Character.digit(hex[i * 2], 16);
            int low = Character.digit(hex[i * 2 + 1], 16);
            int value = (high << 4) | low;
            if (value > 127) value -= 256;
            raw[i] = (byte)value;
        return raw;
    public static byte[] hexToBytes(String hex) {
        return hexToBytes(hex.toCharArray());
}