required in actual practice) and the letters written in from top to bottom. Removing the grille, the grid is filled with random letters and numbers. Then, one hopes, only the possessor of the grille or a copy can read out the hidden letters or numbers – which could, for example, be the key to a polyalphabetic cipher such as that proposed around the same time by Giambattista della Porta. The grille and the grid are kept separately. If there is only one copy of the grille and one of the grid, the loss of either results is the loss of both. Clearly, in the case of communication by grille cipher, both sender and recipient must possess an identical copy of the grille. The loss of a grille leads to the probable loss of all secret correspondence encrypted with that grille. Either the messages cannot be read (i.e., decrypted) or someone else (with the lost grille) may be reading them. A further use for such a grille has been suggested: it is a method of generating pseudo-random sequences from a pre-existing text. This view has been proposed in connection with the Voynich manuscript. It is an area of cryptography that David Kahn termed enigmatology and touches on the works of Dr John Dee and ciphers supposedly embedded in the works of Shakespeare proving that Francis Bacon wrote them, which William F. Friedman examined and discredited. Trellis ciphers The Elizabethan spymaster Sir Francis Walsingham (1530–1590) is reported to have used a "trellis" to conceal the letters of a plaintext in communication with his agents. However, he generally preferred the combined code-cipher method known as a nomenclator, which was the practical state-of-the-art in his day. The trellis was described as a device with spaces that was reversible. It appears to have been a transposition tool that produced something much like the Rail fence cipher and resembled a chess board. Cardano is not known to have proposed this variation, but he was a chess player who wrote a book on gaming, so the pattern would have been familiar to him. Whereas the ordinary Cardan grille has arbitrary perforations, if his method of cutting holes is applied to the white squares of a chess board a regular pattern results. The encipherer begins with the board in the wrong position for chess. Each successive letter of the message is written in a single square. If the