Computer Security and Cryptography

2.10 CODA

Although cribbing and Markov scoring often permit a successful attack on columnar transposition ciphertext, there are several possible modifications of the rules that may strengthen the encipherment method.

M1. The rectangular shape of X might be replaced by a triangle:

The plaintext Good morning. How are you today? Would be read into X by rows and read out according to a transposition τ. There are details to be supplied so that τ does not depend on the length of the plaintext.

Figure 2.5 A grille.

M2. The rectangular shape of X might be retained, but a grille as shown in Figure 2.5 would be used to construct X. The plaintext would read into X as before except that certain (perhaps keydependent) positions in X would be skipped.

M3. The rectangular shape of the array might be retained, but encipherment would involve two steps:

(a) The plaintext would be read into an array X₁ of width N₁ and read out according to the transposition τ = (τ₀, τ₁, …, τ_{N1 − 1});

(b) The resulting intermediate ciphertext would be read into an array X₂ of width N₂ and read out according to the transposition τ = (τ₀, τ₂, …, τ_{N2 − 1}).

We will not pursue the analysis of any of these modifications, leaving them to the interested reader.