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    <p><strong>If you take user input for the password make sure to read <a href="https://stackoverflow.com/a/3452620/1188357">this answer</a>.</strong></p> <p>You should take a look at: <a href="http://developer.android.com/reference/javax/crypto/CipherInputStream.html" rel="noreferrer">CipherInputStream</a> and <a href="http://developer.android.com/reference/javax/crypto/CipherOutputStream.html" rel="noreferrer">CipherOutputStream</a>. They are used to encrypt and decrypt byte streams.</p> <p>I have a file named <code>cleartext</code>. The file contains:</p> <blockquote> <pre><code>Hi, I'm a clear text. How are you? That's awesome! </code></pre> </blockquote> <p>Now, you have an <code>encrypt()</code> function:</p> <pre><code>static void encrypt() throws IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException { // Here you read the cleartext. FileInputStream fis = new FileInputStream("data/cleartext"); // This stream write the encrypted text. This stream will be wrapped by another stream. FileOutputStream fos = new FileOutputStream("data/encrypted"); // Length is 16 byte // Careful when taking user input!!! https://stackoverflow.com/a/3452620/1188357 SecretKeySpec sks = new SecretKeySpec("MyDifficultPassw".getBytes(), "AES"); // Create cipher Cipher cipher = Cipher.getInstance("AES"); cipher.init(Cipher.ENCRYPT_MODE, sks); // Wrap the output stream CipherOutputStream cos = new CipherOutputStream(fos, cipher); // Write bytes int b; byte[] d = new byte[8]; while((b = fis.read(d)) != -1) { cos.write(d, 0, b); } // Flush and close streams. cos.flush(); cos.close(); fis.close(); } </code></pre> <p>After you execute this function, there should be a file names <code>encrypted</code>. The file contains the encrypted characters.</p> <p>For decryption you have the <code>decrypt</code> function:</p> <pre><code>static void decrypt() throws IOException, NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException { FileInputStream fis = new FileInputStream("data/encrypted"); FileOutputStream fos = new FileOutputStream("data/decrypted"); SecretKeySpec sks = new SecretKeySpec("MyDifficultPassw".getBytes(), "AES"); Cipher cipher = Cipher.getInstance("AES"); cipher.init(Cipher.DECRYPT_MODE, sks); CipherInputStream cis = new CipherInputStream(fis, cipher); int b; byte[] d = new byte[8]; while((b = cis.read(d)) != -1) { fos.write(d, 0, b); } fos.flush(); fos.close(); cis.close(); } </code></pre> <p>After the execution of decrypt, there should be a file named <code>decrypted</code>. This file contains the free text.</p> <p>You write you're a "noob" but depending on the use-case of encryption you could do a lot of harm if you're not doing it the right way. Know your tools!</p> <p>Usage of CipherOutputStream <a href="http://docs.oracle.com/javase/1.5.0/docs/guide/security/CryptoSpec.html#CipherOutput" rel="noreferrer">Oracle documentation</a>:</p> <pre><code>SecretKeySpec skeySpec = new SecretKeySpec(y.getBytes(), "AES"); FileInputStream fis; FileOutputStream fos; CipherOutputStream cos; // File you are reading from fis = new FileInputStream("/tmp/a.txt"); // File output fos = new FileOutputStream("/tmp/b.txt"); // Here the file is encrypted. The cipher1 has to be created. // Key Length should be 128, 192 or 256 bit =&gt; i.e. 16 byte SecretKeySpec skeySpec = new SecretKeySpec("MyDifficultPassw".getBytes(), "AES"); Cipher cipher1 = Cipher.getInstance("AES"); cipher1.init(Cipher.ENCRYPT_MODE, skeySpec); cos = new CipherOutputStream(fos, cipher1); // Here you read from the file in fis and write to cos. byte[] b = new byte[8]; int i = fis.read(b); while (i != -1) { cos.write(b, 0, i); i = fis.read(b); } cos.flush(); </code></pre> <p>Thus, the encryption should work. When you reverse the process, you should be able to read the decrypted bytes.</p>
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    1. COThank you for the answer Kiril. I still dont have an idea how to implement this using CipherInputStream. Could you give me a few example for working with that?
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    2. COi already edit the source, it contained no error but still cant encrypt the file. anything wrong with my code Kiril?
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    3. COHi, I'll provide a clean snippet! But check the length of your SecretKey. Should be i.e. 16 byte. => 16 String characters.
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