One way to scramble the letters of a message is to separate the message into two groups of characters, where the first group is composed of the even-numbered characters and the second group is composed of the odd-numbered characters. If we create one string out of the even-numbered characters and another out of the odd-numbered characters, we can concatenate the two new strings together to form the ciphertext string. Because this results in a string with the characters shuffled to new positions, we call this a transposition cipher, also sometimes called the rail fence cipher. FIGURE 3.3 illustrates the idea behind this encryption algorithm.

Now that we have a scheme for scrambling the plaintext message, let’s write a Python function that takes the plaintext message as a parameter and returns the ciphertext message. The key to this algorithm lies in our ability to put the even characters from the plaintext into one string and the odd characters into another. One way to do this is to use a counter and a string iterator. A string iterator is a for loop in which the loop variable takes on the value of each character in the string.

The string iterator uses this form of the for loop:

With each iteration of the for loop, the loop variable ch takes on the value of the next character in the string stringVariable. So, for the first iteration, ch becomes the character at index 0 of stringVariable; for the second iteration, ch becomes the character at index 1 of stringVariable; and so on.

As we iterate over each character of the string, we increment our counter. If the counter is an even number, we concatenate the current character to our even-character string. If the counter is odd, we concatenate the character to our odd-character string. In this so-called rail fence cipher, each rail contains part of the original string. In this case, because we are dividing the string into two parts, we call it a two-rail cipher.

How can we tell if a number is even or odd? Recall that for any even number N, when you divide N by 2, there is no remainder. For any odd number N, N divided by 2 has a remainder of 1. In Python, we can obtain the remainder by using the modulo operator. For any even number N, N % 2 is 0. For any odd number N, N % 2 is 1. We can use the expression charCount % 2 == 0 as the condition in an if statement to test whether the character counter is even or odd. LISTING 3.2 shows the encryption algorithm using the strategy described here.

The scramble2Encrypt function makes use of the accumulator pattern in several different places. First, evenChars and oddChars start as empty strings and accumulate characters as the for loop executes. The accumulator pattern works just as well for concatenating strings as it does for adding numbers. When we apply the accumulator pattern to strings, we build up a string that starts from nothing (the empty string, ""), growing the string one character at a time. The next use of the accumulator pattern is a familiar numeric accumulator using the charCount variable.

On line 5 of Listing 3.2 we begin a loop that will iterate over each character in the plainText string. When we iterate over a string, the loop variable references each character in the string one after another. On line 6, we make use of the test we devised to decide whether charCount is even or odd. If charCount is even, we concatenate the current character referenced by the loop variable ch to the existing value of evenChars. Because concatenation creates a new string, evenChars becomes this newly constructed string when we perform the assignment. If charCount is odd, we concatenate oddChars and ch to create a new string referenced by oddChars.

Finally, after all characters in the plaintext string have been processed, on line 11 we create the cipherText string by concatenating oddChars and evenChars. The order of variables in the concatenation is not accidental. We put oddChars first for a reason: If our string has an odd number of characters, oddChars will be one character shorter than the number of even characters. Can you explain why? We will see in a moment that this point is important when we want to split the ciphertext back into two pieces.

SESSION 3.10 shows the output of the scramble2Encrypt function for five different test inputs. Notice that some of the test cases are just nonsense strings, but they are chosen carefully to make it easy to see if the function is working the way we think it should. Also notice that we test some boundary cases such as a string of length 1 and even an empty string.