want to make.
To encrypt with a double transposition cipher, we first write the plaintext into an array of a given size and then permute the rows and columns according to specified permutations. For example, suppose we write the plaintext attackatdawn into a
If we transpose (or permute) the rows according to
The ciphertext is read from the final array as
For this double transposition cipher, the key consists of the row and column permutations. Anyone who knows the key can simply put the ciphertext into the appropriate sized matrix and undo the permutations to recover the plaintext For example, to decrypt (2.3), the ciphertext is first put into a
and we see that we have recovered the plaintext, attackatdawn .
The bad news is that, unlike a simple substitution, the double transposition does nothing to disguise the letters that appear in the message. The good news is that the double transposition appears to thwart an attack that relies on the statistical information contained in the plaintext, since the plaintext statistics are dispersed throughout the ciphertext.
Even this simplified version of the double transposition is not entirely trivial to break. The idea of smearing plaintext information through the ciphertext is so useful that it is employed by modern block ciphers, as we will see in the next chapter.
2.3.5 One‐Time Pad
The one‐time pad, which is also known as the Vernam cipher, is a provably secure cryptosystem. Historically it has been used in various times and places, but it's not practical for most situations. However, it does nicely illustrate some important concepts that we'll see again later.
For simplicity, let's consider an alphabet with only eight letters. Our alphabet and the corresponding binary representation of letters appear in Table 2.1. It's important to note that the mapping between letters and bits is not secret. This mapping serves a similar purpose as, say, the ASCII code, which is not much of a secret either.
Table 2.1 Abbreviated alphabet
| Letter | e | h | i | k | l | r | s | t |
| Binary | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111 |
Suppose that Trudy, who is working as a Nazi spy in London during World War II, wants to use a one‐time pad to encrypt the plaintext message
She first consults Table 2.1 to convert the plaintext letters to the bit string
The one‐time pad key consists of a randomly selected string of bits that is the same length as the message. The key is then XORed with the plaintext to yield the ciphertext. For the mathematically
