What effect does adding noise to a substitution cipher have?
I did some simulations and found that after adding noise, the common English letters (e, a, t, r, s, etc) still stuck out like sore thumbs. Knowing which characters represent these letters, and assuming the attacker knows we added noise, makes this only trivially harder than a straight substitution attack.

But that got me thinking, what if for each letter A-Z I choose some random amount of random numbers to represent that letter. Each random sequence must be uniquely identifiable when concatenated with any other value (I don't know an easy way to generate this kind of key but its possible to construct manually if your careful). For example I could choose the following values for A, B, and C:

A: 36 52 47
B: 36
C: 95 47


Given the string "36 52 47 95 47 95 47 36 52 46 36" we can tell that it represents the characters "ACCAB", but without knowing the key, frequency analysis wouldn't work (for example B shows up once in our plaintext, but its corresponding cipher text value 36 shows up three times).

I don't know if this type of cypheralphabet has a name, but I realized that on its own, it isn't that strong. We can look at the frequency of pairs of numbers in the ciphertext and determine what numbers are more likely part of the same character representation. If you can split the numbers up correctly, then this is the same as a standard substitution cipher.

But if we add noise to this weird random character length cipher alphabet, in the same way it was originally described, then there is no way I can see to determine which groups of numbers represent single characters, and basic frequency analysis doesn't work. What am I missing?