Convert each letter in the ciphertext to a corresponding number according to the mapping A=1, B=2, ..., Z=26, resulting in C?...C?:

For the i-th letter, calculate the offset as f(i) = i^2 + 1.

Perform modulo 26 subtraction at each position to obtain the plaintext digit P?

P? ≡ C? - f(i)? (mod 26)

If the result is less than or equal to 0, then add 26.

Revert P?...P? to their corresponding letters, and you will……