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It is a cryptography that was used but now has fallen. Classical cipher can be practically computed and solved by hand. And they are also very simple to break with modern technology.

- In substitution cipher, the letters of plain text are replaced by other letters.
- Caeser cipher is the well-known example of Substitution cipher.
- In this cipher, each letter of message is replaced by the letter 3 position later in the alphabet.
- Therefore A is replaced by D,B by E, C by F etc ,and X,Y and Z are replaced by A, B and C respectively.
- For example, “CIPHER” encrypts as “FLSKHU”.

In this cipher, plain text encrypts as given reference substitution. The simple substitution cipher offers very little communication security, and it can be easily broken even by hand.

Keys for the simple substitution cipher usually consist of 26 letter.

An example key is:

plain alphabet : abcdefghijklmnopqrstuvwxyz

cipher alphabet: phqgiumeaylnofdxjkrcvstzwb

An example of encryption using the above key:

plaintext : defend the east wall of the castle

ciphertext: giuifg cei iprc tpnn du cei qprcni

It is easy to see how each character in the plaintext is replaced with the corresponding letter in the cipher alphabet.

One Time Pad

- OTP is an encryption technique that cannot be cracked but required to use of a one-time pre shared key, the same size as, or longer than, the message being sent.
- In this technique, a plaintext is paired with a random secret key.
- Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition.

Example:

Plain text is HELLO.

Secret key is XMCKL.

**Decryption :**

The cipher text is to be sent as "EQNVZ". Receiver uses the key and the same process, but in reverse, to obtain the plaintext. Here the key is subtracted from the ciphertext, again using modular arithmetic:

Codebook cipher

- In cryptography, a codebook is a document used for implementing a code.
- A codebook contains a lookup table for coding and decoding; each word or phrase has one or more strings which replace it.
- To decipher messages written in code, corresponding copies of the codebook must be available at either end.
- A codebook is usually made in two parts, one part being for converting plaintext to ciphertext, the other for the opposite purpose.
- Both are usually organized similar to a standard dictionary, with plaintext words (in the first part) and ciphertext words (in the second part) presented like dictionary headwords.