Convert each letter in the ciphertext to a corresponding number according to the mapping A=1, B=2, ..., Z=26, resulting in C₁...C₈:
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 Pᵢ
Pᵢ ≡ Cᵢ - f(i) (mod 26)
If the result is less than or equal to 0, then add 26.
Revert P₁...P₈ to their corresponding letters, and you will……
