2. By cipher is also understood a mode of secret writing. Public ministers and other public agents frequently use ciphers in their correspondence, and it is sometimes very useful so to correspond in times of war. A key is given to each minister before his departure, namely, the cipher for writing ciphers, (chiffre chiffrant,) and the cipher for deciphering (chiffre dechiffrant.) Besides these, it is usual to give him a common cipher, (chiffre banal,) -which is known to all the ministers of the same power, who occasionally use it in their correspondence with each other.
3. When it is suspected that, a cipher becomes known to the cabinet where the minister is residing, recourse is had to a preconcerted sign in order to annul, entirely or in part, what has been written in cipher, or rather to indicate that the contents are to be understood in an inverted or contrary sense. A cipher of reserve is also employed in extraordinary cases.