Cryptography Reference
In-Depth Information
the simplest attacks in this case is to blend the files together,
either by averaging the individual elements of the file or by
creating a hybrid by taking different parts from each file.
Access to the File and Algorithm
An ideal steganographic algorithm
canwithstand scrutiny even if the attacker knows the algorithm
itself. Clearly, basic algorithms that hide and unveil informa-
tion can't resist this attack. Anyone who knows the algorithm
can use this it to extract the information.
But this can work if you keep some part of the algorithm se-
cret and use it as the “key” to unlock the information. Many
algorithms in this topic use a cryptographically secure random
number generator to control how the information is blended
into a file. The seed value to this random number stream acts
like a key. If you don't know it, you can't generate the random
number stream and you can't unblend the information.
Destroy Everything Attack
Some people argue that steganography
is not particularly useful because an attacker could simply de-
stroy the message by blurring a photo or adding noise to a
sound file. One common technique used against the kind of
block compression algorithms like JPEG is to rotate an image
45 degrees, blur the image, sharpen it again, and then rotate it
back. This mixes information from different blocks of the im-
age, effectively removing some schemes like the ones in Chap-
ter 14.
This technique is a problem, but it can be computationally pro-
hibitive for many users and it introduces its own side effects.
A site like Flickr.com might consider doing this to all incom-
ing images to deter communications, but it would require a fair
amount of computation.
It is also not an artful attack. Anyone can destroy messages.
Cryptography and many other protocols are also vulnerable to
it.
RandomTweaking Attacks
Some attackers may not try to deter-
mine the existence of a message with any certainty. An attacker
could just add small, random tweaks to all files in the hope of
destroying whatever message may be there. During World War
II, the government censors would add small changes to num-
bers in telegrams in the hopes of destroying covert communi-
cations. This approach is not very useful because it sacrifices
overall accuracy for the hope of squelching a message. Many
