Question

I am working on an AES 256 bit File encryption tool. The way it works now is by:

reading an entire file into a String,
encrypting the string,
deleting the old file, then outputting the encrypted String into the old File object.

This works for small Files, but if you were to try to encrypt a Large file, such as one larger than the Heap Space java has access too, you receive errors. Long story short, how can i read the file one MB at a time, as a String, encrypt that string, then write that MB to a temp file? This would allow for large files to be encrypted.

Answer 1

No need for temp files. Just use File Streams (FileInputStream and FileOutputStream). Streaming I/O was exactly created for processing long files in chunks. You can use FileReader/Writer if you want to read/write strings. There is an example:

  public void decrypt(File in, File out) throws IOException, InvalidKeyException {
    aesCipher.init(Cipher.DECRYPT_MODE, aeskeySpec);
    CipherInputStream is = new CipherInputStream(new FileInputStream(in), aesCipher);
    FileOutputStream os = new FileOutputStream(out);

    int i;
    byte[] b = new byte[1024];
    while((i=is.read(b))!=-1) {
            os.write(b, 0, i);
    }