Function decrypt 

pub fn decrypt<const W: usize, const H: usize>(
    input: EncryptedData<W, H>,
) -> Result<DecryptedData, GridError>

Decrypts a WHY2-encrypted data into raw i64 values.

This function reverses the full WHY2 encryption pipeline using CTR mode:

$$ P_i = C_i \oplus E_K(\text{Nonce} + i) $$

where $E_K$ is the WHY2 block cipher and $i$ is the block counter.

• Generates round keys from the master key
• Applies CTR mode decryption (XOR with keystream blocks)

Parameters

• input: An EncryptedData struct containing the encrypted grids and key grid.

• Ok(DecryptedData) struct containing:

  • output: A vector of decrypted i64 values
  • key: The original key Grid flattened into a vector

• Err(String) if Grid area is 1