php image steganography nearly same start

Question

i am working on a php-script which encodes given text and hides that in an Image using LSB. But the encoded text is a Byte Array (text encrypted using mcrypt with rijndael-256 and then unpacked with unpack("C*", $encryptedText);) i have tp add the array-size at the beginning of the Array. if i would not do this, reading the Bytes from the Image again would be terrible later on, because the script would not know where to stop reading. I added size Information at the beginning of the Array using These lines of code:

$size = count($byteArray); array_unshift($byteArray, $size >> 24, ($size & 0xff0000) >> 16, ($size & 0xff00) >> 8, $size & 0xff);

so the size is added in integer Format (4bytes), but now every Image created would have the characteristics that the first hidden Bytes start mostly with Zeros, besause $size is mostly in the range of 60000 or lower. is there any way i can encode size or Change other parts of the program so that it works and the beginning of the bytearry is not nearly the same every time?

Answer 1

Instead of always having the first 4 bytes encoding how long your message is, you can use the last two bits from the first byte to encode how many bytes you need to read for $size. Say, 00 = 1, 01 = 2, 10 = 3 and 11 = 4. For example, if $size is small enough to be expressed with just two bytes, the first few bytes will read as follow:

• First byte: xxxxxx01
• Second and third bytes: $size
• Fourth byte and onward: ByteArray...

You can spice things up further by using a randomised embedding method. You can use a pseudorandom number generator, or chaotic maps, such as the Logistic Map, or Tent Map. The seed or initial condition parameters will be required by the receipt to decipher in what order to read the bytes to extract the message. For example, consider 5 bytes to embed data and 5 numbers generated between 0 and 1.

(0.2843, 0.5643, 0.0904, 0.4308, 0.9866)

Sorting the numbers in ascending order gives you the following order, which you can use to embed your secret:

(3, 1, 4, 2, 5)