2.1 SHANNON'S CLASSIFICATION OF SECRECY TRANSFORMATIONS
Two building-blocks were identified in Claude Shannon's [1949] formulation of the design principles for secrecy systems:
- Substitution. Ciphertext results when the letters in the plaintext x = (x0, x1, …, xn−1) are substituted by the letters in a ciphertext alphabet (x0, x1, …, xn−1) → (y0, y1, …, yn−1).
- Transposition. Ciphertext results when the positions of letters in the plaintext x = (x0, x1, …, xn−1) are rearranged (x0, x1, …, xn−1) → (xπ0, xπ1, …, xπn−1) according to a permutation π = (π0, π1, …, πn−1).
Shannon proposed that an effective encipherment system might be built by iterating the two operations substitution (confusion) and transposition (diffusion).
Giovanni Battista della Porta (1535–1615) was born into a wealthy Naples family. He made contributions to astrology, optics, meteorology, magic, and cryptography. Porta's four-volume work “Magia Naturalis” was first published in 1555 and later expanded to twenty volumes. His place in cryptography is due to his book “De Furtivis Literarum Notis,” published in 1563, which described digraphic substitution and transposition and is considered the first serious work in cryptography.
This chapter defines columnar transposition and illustrates two techniques for its cryptanalysis.
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