How to encrypt and decrypt strings in android using AES?


Simple helper class to encrypt and decrypt strings using AES128. The result is Ascii-encoded (actually hex, no base64), so no byte[] has to be stored. A SEED value is used as a shared secret ("Master-Password"). Only with the same SEED the stored values can be decrypted. 


import java.security.SecureRandom;


import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;


/**
 * Usage:
 * <pre>
 * String crypto = SimpleCrypto.encrypt(masterpassword, cleartext)
 * ...
 * String cleartext = SimpleCrypto.decrypt(masterpassword, crypto)
 * </pre>
 * @author ferenc.hechler
 */
public class SimpleCrypto {


 public static String encrypt(String seed, String cleartext) throws Exception {
  byte[] rawKey = getRawKey(seed.getBytes());
  byte[] result = encrypt(rawKey, cleartext.getBytes());
  return toHex(result);
 }

 public static String decrypt(String seed, String encrypted) throws Exception {
  byte[] rawKey = getRawKey(seed.getBytes());
  byte[] enc = toByte(encrypted);
  byte[] result = decrypt(rawKey, enc);
  return new String(result);
 }


 private static byte[] getRawKey(byte[] seed) throws Exception {
  KeyGenerator kgen = KeyGenerator.getInstance("AES");
  SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
  sr.setSeed(seed);
     kgen.init(128, sr); // 192 and 256 bits may not be available
     SecretKey skey = kgen.generateKey();
     byte[] raw = skey.getEncoded();
     return raw;
 }



 private static byte[] encrypt(byte[] raw, byte[] clear) throws Exception {
     SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
  Cipher cipher = Cipher.getInstance("AES");
     cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
     byte[] encrypted = cipher.doFinal(clear);
  return encrypted;
 }


 private static byte[] decrypt(byte[] raw, byte[] encrypted) throws Exception {
     SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
  Cipher cipher = Cipher.getInstance("AES");
     cipher.init(Cipher.DECRYPT_MODE, skeySpec);
     byte[] decrypted = cipher.doFinal(encrypted);
  return decrypted;
 }


 public static String toHex(String txt) {
  return toHex(txt.getBytes());
 }
 public static String fromHex(String hex) {
  return new String(toByte(hex));
 }

 public static byte[] toByte(String hexString) {
  int len = hexString.length()/2;
  byte[] result = new byte[len];
  for (int i = 0; i < len; i++)
   result[i] = Integer.valueOf(hexString.substring(2*i, 2*i+2), 16).byteValue();
  return result;
 }


 public static String toHex(byte[] buf) {
  if (buf == null)
   return "";
  StringBuffer result = new StringBuffer(2*buf.length);
  for (int i = 0; i < buf.length; i++) {
   appendHex(result, buf[i]);
  }
  return result.toString();
 }
 private final static String HEX = "0123456789ABCDEF";
 private static void appendHex(StringBuffer sb, byte b) {
  sb.append(HEX.charAt((b>>4)&0x0f)).append(HEX.charAt(b&0x0f));
 }

}

Comments

  1. Every time I encrypt a raw string such as "aaa" it comes back with a different encrypted string. Is this the expected behavior?

    ReplyDelete
  2. Also for decryption, I am consistently getting "PAD BLOCK CORRUPTED" after encrypting 'a' and then trying to decrypt it immediately using the String returned.

    ReplyDelete

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