Is it safe to initialize a random number generator with MD5 for pixel selection in image steganography?

Question

This question is related to "Is it safe to initialize a random number generator with MD5?".

But in the context of steganography we don't know which pixels have been selected to hide information. In addition, the content could be encrypted, so we do not know if the hidden information that we extract (with a guessed password) is the message or noise.

In this scenario, is it safe to initialize a PRNG with the MD5 hash of a password? and what interests me the most: if it is not, how can it be exploited?

Answer 1

Is it safe to use MD5 as a redundant step in an otherwise secure setup? yes.

Is using MD5 to garble a low entropy password safe? No, because one would iterate through the passwords and check if the selected pixel seem to be modified, and identify the correct password.

In fact if you use the same password for pixel selection and encryption, It may be possible to break an otherwise strong encryption.

One can use image processing techbiques to asses how likely a specific pixel is to have been modified, and compare different possible collectiona of pixels. Not sufficient if we have too many possible options but with a reasonable dictionary probably solveable.

If you have a strong independent encryption and the modification to the image is not detectable at all, and the original password ia very high entropy, you don't need MD5 at all in the first step.

What is the purpose of MD5 in your scheme? It doesn't seem to add anything.