Question

I am implementing a Cipher Block Chaining for school work and the question asks for a method taking String and returning another String. At first, I thought it was odd and that byte[] variables would be much more adequate, but implemented a method still. Basically, here’s the code :

static public String encode(String message) {
   byte[] dataMessage = message.getBytes();
   ByteArrayOutputStream out = new ByteArrayOutputStream();

   byte last = (byte) (Math.random() * 256);
   byte cur;
   out.write(last);

   for (byte b : data) {
      cur = (byte) (b^last);
      System.out.println("Encode '" + (char) b + "' = " + b + "^" + last + " > " + cur );
      out.write( cur );
      last = cur;
   }

   System.out.println("**ENCODED BYTES = " + Arrays.toString(out.toByteArray()));
   System.out.println("**ENCODED STR   = " + Arrays.toString(out.toString().getBytes()));

   return out.toString();
}

The decode method works similarly. Some times, the method will spit results like

Encode 'H' = 72^109 > 37
Encode 'e' = 101^37 > 64
Encode 'l' = 108^64 > 44
Encode 'l' = 108^44 > 64
Encode 'o' = 111^64 > 47
**ENCODED BYTES = [109, 37, 64, 44, 64, 47]
**ENCODED STR   = [109, 37, 64, 44, 64, 47]

But sometimes will also spit things like

Encode 'H' = 72^-63 > -119
Encode 'e' = 101^-119 > -20
Encode 'l' = 108^-20 > -128
Encode 'l' = 108^-128 > -20
Encode 'o' = 111^-20 > -125
**ENCODED BYTES = [-63, -119, -20, -128, -20, -125]
**ENCODED STR   = [-17, -65, -67, -17, -65, -67, -17, -65, -67, -17, -65, -67]

I presume that this has something to do with UTF-8 (the system’s default encoding), but I’m not familiar enough to figure out exactly why such a string would be returned with the given bytes.

Answer 1

You can’t take an arbitrary sequence of bytes and assume it’s a valid UTF-8 encoded string. So, I suspect that the toString method, as documented, replaces malformed-input and unmappable-character sequences with the default replacement string for the platform’s default character set.

You should thus not transform purely binary data into a String like this. Use some encoding like Hex or Base64 to transform your bytes to a printable string, and vice-versa.

Apache commons-codec has a Base64 utility class.