def plaintext_to_grid(k: int, plaintext: str) -> list[list[str]]:
    """Return the grid with k columns from the given plaintext.

    Preconditions:
        - k >= 1
        - len(plaintext) % k == 0
        - plaintext != ''
    """
    return [list(plaintext[i:i + k]) for i in range(0, len(plaintext), k)]