Chapter XII

On the Oblique Transposition into Consonants Eight or Four in Number:
Whereto is attached a key.

We come now to the third Particular Mode (see the preceding chapter), wherein, in order to express the secret, we make use of four or eight consonants only, which are transposed one with another. Such a Mode, however, admits not of resolution, unless there has been used a Key, or in other words, a sentence, whereby the secret is hidden from view and clothed as in a wrapper; by means of which key the reader is afterwards enabled to arrive at the knowledge of the secret. For, since, in the first Mode wherein we write with four consonants, each consonant signifies four letters, and in the second Mode, wherein we use eight consonants, each consonant expresses two letters, we have need of a sort of index which shall enable us to know which letter is to be understood by the Transpositive consonant sued; and this fact the following explanation will show. In the figure below I give from de Vigenere, pp. 266a, b, 257a, and 222a, the tables of both Modes (the Mode, namely, of writing with four letters and that of writing with eight), together with the Transpositive alphabet.

The Transpositive alphabet, which is found at the bottom of the table, answers for either table, by reason of the method of concealment of the second secret. For be it known that in this case there are two secret texts, the one of which is involved by the unadorned and simple Transposition (on which see also above c.6) and is performed with reference to the second, which is the principal or more secret one, and which is hidden by four or eight letters only. The reason for the use of two secrets is not far to seek, for the less important secret cooperates, both to conceal and to reveal the key, with the principal one, which is the special object of our attention. Now it is the case here as elsewhere, that, if the sentence were left unconcealed, the key would alone suffice to lead us to the discovery of the secret; but since the matter would not be free from suspicion if, between the parts of unconcealed words, certain less common

F after letters

G after letters

3 4 5 6

a l h b

3 4 5 6

e r m c

Q after letters

X after letters

3 4 5 6

I s n d

3 4 5 6

n o t p

By the side of one of these eight unequal letters

After these eight equal letters

equals