I'm creating a program that "hides" an encrypted file at the end of a JPEG. The problem is, when retrieving this encrypted file again, I need to be able to determine when the JPEG it was stored in ends. At first I thought this wouldn't be a problem because I can just run through the file checking for 0xFF and 0xD9, the bytes JPEG uses to end the image. However... I'm noticing that in quite a few JPEGs, this combination of bytes is not exclusive... So my program thinks the image has ended randomly half way through it.
I'm thinking there must be a set way of expressing that a JPEG has finished, otherwise me adding a load of bytes to the end of the file would obviously corrupt it... Is there a practical way to do this?
Well, there are always two places in the file that you can find with 100% reliability. The beginning and the end. So, when you add the hidden file, add another 4 bytes that stores the original length of the file and a special signature that's always distinct. When reading it back, first seek to the end - 8 and read that length and signature. Then just seek to that position.