Is this cipher secure?

Question

Encryption

I recently came up with a cipher based on a simple substitution using a function like D(x) = f(x) mod n on the segment [a,b] with increment h each time increases by h, while the value of the variable is within the specified interval), where f(x) is a random function that gives integer values ​​for any x; For example: 25x^2+2x-2 or (x^x)mod n etc.; n - number of characters in the alphabet. Help, please, determine its reliability.
Pseudocode (for function f(x)=x^3, n=17;a=0,b - tends to infinity;h = 1):
Input len;//Encryption
n = 17;
for i = 0 to len{
Input message;
NewMessage = ((message*message)+h)mod(n);
h++;
OutputNewMessage;
}
//Decryption
For c =0 to len{
message = (sqrt(NewMessage)-h)mod(n);
h++;
output message;
}

Answer 1

"Birthdays" attack works on every character in n increments ( (f(x)+hn) mod n = f(x) mod n), then stats and goodbye.

Answer 2

Never invent anything in the field of cryptography until you have at least a doctorate in mathematics. As a warm-up for the mind - you can, but God forbid you carry this gag into commercial products.
For the same reason that I stated above, no one here is likely to be able to analyze your algorithm for cryptographic strength.
I don’t know, maybe your algorithm is brilliant and no one in the entire history of computer cryptography has thought of it, despite the fact that the best mathematicians on the planet worked on the invention of ciphers, but I strongly doubt it.

Answer 3

How to determine the reliability of the algorithm, if the decryption algorithm is not written?

Answer 4

What is the meaning of this cipher? The disassembler will do everything.