"a cipher (or cypher) is an algorithm for performing encryption or decryption"
So when you say "cipher", that means you want every possible algorithm. Since algorithms can produce output shorter than the input, you get the infinite amount of inputs.
With one-time pads, fourier analysis would only work if the key is not truly random.
How do you reverse a "mod26"? You can't. I give you the number 17. You know this number is produced using the equation: "SECRET mod 26 = 17". How do you know if 43 or 69 was the input?
So when you say "cipher", that means you want every possible algorithm. Since algorithms can produce output shorter than the input, you get the infinite amount of inputs.
With one-time pads, fourier analysis would only work if the key is not truly random.
How do you reverse a "mod26"? You can't. I give you the number 17. You know this number is produced using the equation: "SECRET mod 26 = 17". How do you know if 43 or 69 was the input?