Assume I assigned the letters A-Z to random numbers in the range of 0-100. The mapping of plaintext characters to ciphertext values constitutes my cipher alphabet(and key).
I encrypt a message using a normal substitution cipher. I then create a sequence of random values the same length as my initial ciphertext using numbers in the range of 0-100 that were not used in my cipher alphabet. I then randomly weave my initial cipher text and my random sequence together.
Specifically, I would start with the first ciphertext character and randomly decide wether or not to add noise. If I decide to add noise I will insert the first unused element from my random sequence either before or after the current ciphertext character (again chosen randomly). When I reach the end of the cipher text there may still be unused values from the random sequence. If there are I go back to the first character of ciphertext (which could be an encrypted value or noise added in a previous pass) and continue inserting values randomly until all of the values from the random sequence have been used.
The final result is the message that would be transmitted. If you know the key description is trivial since you can just ignore the "noise". But if you don't know the key how would you go about decrypting this? specifically what are the techniques for separating noise from meaningful data or is there some other attack scheme?
Note: I am asking because of hobbyist curiosity. I know modern encryption algorithms are secure and don't need to be meddled with. And I know the scenario I described results in a larger transmission, etc. I just want to know how you break it.