Saturday, October 5, 2013
- The transposition cipher method is one of the cryptography methods used for securing the communication from eavesdroppers.
- This method of encryption shifts the positions of the units or letters of the plain text based up on some regular system so that a permutation of the plain text is generated.
- This permuted plain text is termed as the cipher text.
- Thus, the cipher text is generated by changing the order of the units.
- Mathematically the following functions are used:
Ø Bijective function: For encryption of the character’s position and
Ø Inverse function: For decrypting the message
Now we shall see about some of the implementations of the transposition cipher:
1. Rail fence cipher:
- This form of the transposition cipher has been named so because of the way that it follows for encoding.
- Here, the characters of the plain text are written on the successive rails in a downwards manner of some imagined fence.
- Then, we move upwards once getting to the bottom.
- For reading the message, it is taken in rows.
2. Route cipher:
- In this form of transposition cipher, a grid of given dimensions is taken on which the characters of the plain text are written out.
- Then, the message is read based up on the pattern mentioned in the key.
- For example, the pattern might be inwards spiral in clockwise direction starting from topmost right.
- The route ciphers may use many keys unlike the rail fence cipher.
- In fact, the number of keys used for enumerating the messages of reasonable length by modern machinery might be too great.
- Also, it is not necessary that all the keys might be good in equal terms.
- Excessive chunks of the plain text might be left if bad routes are chosen.
- Also, the plain text might be simply reversed, thus giving a clue to the crypt analysts about the routes.
- The union route cipher is a variation of the traditional route cipher.
- The difference between the two is that this one transposed the whole words unlike route cipher which transposed individual letters.
- But since transposing the whole words could expose them, they were first hidden by a code.
- The entire null words might be added for adding humor to the cipher text.
3. Columnar transposition:
- In this form of transposition cipher, a fixed length is determined for the rows in which the message is written.
- But for reading the message a column by column approach is followed where some scrambled order if followed for choosing the columns.
- A keyword is chosen which is used for defining the permutation of the columns as well as the width of the rows.
- The spare spaces might be filled with the null characters in case of the regular columnar transposition.
- On the other hand, in these spaces are left as such in the irregular columnar transposition cipher.
- The keyword specifies some order following which the message is read column - wise.
- The column lengths have to be worked out by the recipient for deciphering the message.
- This is done based up on division of the length of the message specified by the key length.
4. Double transposition:
- A single columnar transposition is vulnerable to attacks since the possible lengths of the column and anagrams can be guessed.
- Therefore, a stronger version of it i.e., the double transposition is followed.
- This is a two-time application of the columnar transposition.
- For both the transpositions, either the same key might be used or different keys.
- This was the most complicated cipher before the coming of the VIC cipher.
- It offered reliable operation under difficult conditions.