Understanding some Traditional Cipher techniques

In previous sessions we have learned modern cryptographic techniques which include Symmetric key encryption and asymmetric key encryption. Today we are going to learn about some traditional cipher technique which people used earlier to protect important information.

Revision from previous class

• Kerckhoff’s principle of cryptography
• Cryptographic attacks
• Active Attacks
• Passive Attacks
• Various types of cryptographic attacks

Session 1 Link- Cryptography: Basic Understanding

Session 2 Link- Cryptosystems and its types

Session 3 Link- Understanding major cryptographic attacks

Some of the Traditional Cipher techniques are:

Modern cipher techniques use digital binary data to protect the systems. There are mainly two types of traditional cipher techniques, which are discussed as follows:

1. Monoalphabetic techniques
2. Polyalphabetic techniques

Monoalphabetic Cipher techniques

Caesar Traditional Cipher techniques – type A

Cipher is a set of steps written in the form of algorithm to perform both encryption and decryption of the information. Whereas, Caesar traditional cipher techniques are the simplest among all traditional cipher techniques in which each and every letter in the information is substituted by another letter to make a pattern of secret information.

It is simple shift cipher. We shift position of alphabet based on the agreement between sender and receiver. Say for example:

I need to send a message- MY NAME IS AVINASH

If I have to send above message then I will left/right shift 2 (n) position of each alphabet in message on agreement with receiver. Thus, secure message will look like below based on letter shifting:

OA PCOG KU CXKPCUJ – MY NAME IS AVINASH [2 positions RIGHT shift]

KW LYKC GQ YTGLYQF – MY NAME IS AVINASH [2 positions LEFT shift]

This traditional cipher technique is highly vulnerable to security. Attacker can easily guess and break the sequence, thus, secret information can easily be revealed.

Caesar Traditional Cipher techniques – type B

The type B of Caesar traditional cipher techniques also called as Simple substitution cipher.

It is, however, more secure method to protect the information.

Permutation (arrangement) technique is used to secure the information. Sender and receiver agree on certain sets of permutation. After agreement sender include permutation and sends information. Receiver decodes the cipher text by detaching the permutation.

By doing so we have factorial 26 times way to protect the information that is highly secure. Even in modern times no brute force can decode the information. It only lacks in its design.

Polyalphabetic Cipher techniques

Playfair Cipher

Playfair cipher is the advanced form of type B of caesar traditional cipher techniques i.e., Simple substitution cipher.

In this technique, sender encrypts two pair of letters (alphabets) despite going with single alphabet as done in previous types. After pairing the letters they prepare a 5 x 5 grid to encrypt the information in a certain set of algorithm. Only 25 alphabets will be used and any one middle alphabet will be omitted, thus, we have 25 x 25 arrangements to encrypt the information.

Once sender transfer encrypted information receiver backtrack the algorithm to decrypt the information.

Example:

Suppose we have send information as- LOVE INDIA

We make pair of two letters here: LO VE IN DI AZ

Letter ‘Z’ can be added in case of odd letters as we did in above example.

Here are three steps to encrypt the information in this technique:

1. If two letters occur in same column then take adjacent letters as the substitute of main letters
2. If two letters occur in same row then take succeeding letters as the substitute of main letters
3. If two letters occur diagonally opposite then make a rectangle by connecting these letters and take diagonally opposite letters from another diagonal of same rectangle as the substitute of main letters

This technique is very secure as compared to previous traditional cipher technique. Here we have 625 arrangements to encrypt the information. This cipher technique is not used to send critical information.

Vigenere Cipher

It is highly robust and secure cryptographic technique. These days this cipher technique is used by military and political agencies. It is better than Caesar traditional cipher techniques.

In this cipher technique, sender and receiver agree on a certain keyword. Then they convert that keyword to its numeric place. Again they include converted numeric code into the plain text and they shift position of letters in plain text as per number acquired from keyword.

Example:

Let say keyword is: Cool

Now replace each letter to its numeric position in alphabet.

C = 3

O= 15

L= 12

Plain text is: Love India

Here is the encrypted message:

Receiver will back track the sequence to decrypt the information.