Cryptography

Cryptography comes from Greek words for “secret writing”. In cryptography, the plain text messages are transformed to ciphertext messages, in-order to provide security. The process of transforming plaintext to ciphertext is known as encryption and process of transforming ciphertext to plaintext is known as decryption. The process of braking ciphers, known as cryptanalysis, and the art of devising them known as cryptology. Cryptography algorithms are divided into two categories: symmetric key (or secret key) algorithms and asymmetric key (or public key) algorithms.

Cryptography 

Symmetric key cryptography

In symmetric key cryptography, sender uses the key and encryption algorithm to produce ciphertext. Receiver uses the same key and decryption algorithm to decrypt ciphertext. The key used here is secret, it is only known to sender and receiver (not known to public).  

Asymmetric Key Cryptography

In asymmetric key cryptography, the sender uses public key, announced by the receiver to public, and encryption algorithm to produce ciphertext. Receiver uses private key and decryption algorithm to produce plaintext from ciphertext. Private keys are always secret and kept by receivers.

Now we discussed three keys:

• Secret Key: it is a shared key used in symmetric key cryptography.
• Public key and
• Private key

Symmetric key cryptography

Asymmetric key cryptography

Symmetric key cryptography algorithms are again divided into two categories:  traditional algorithms (character oriented) and modern algorithms (bit oriented). Traditional algorithms are again divided into substitution ciphers and transposition ciphers.

Traditional ciphers 

Substitution ciphers

In substitution cipher, each symbol is replaced by another symbol. For example, we can replace Z with V, and 6 with 7.  

In a monoalphabetic cipher, a symbol in the plaintext always replaced with the same symbol in the ciphertext. Monoalphabetic cipher does not consider the position of the symbol in the plane text.  

One of the simplest monoalphabetic cipher is shift cipher. In this cipher, the encryption algorithm shift key characters down, with key equal to some number. The decryption algorithm shift key characters up. For example, assume that the key is 5, then plaintext HAPPY replaced to CVKKT in ciphertext.  The shift ciphers also known as Caesar cipher, because Julius Caesar used shift cipher to communicate with officers.

In a Polyalphabetic cipher, each occurrence of a symbol has different substitution. For example, character Z in the first word of plaintext replaced to D, and character Z at the middle replaced to A. Here the algorithms use keys to determine which symbol is replace a particular symbol at a particular position.

Transposition Cipher

Transposition ciphers, just reorder the letters.  A character in the first position of plaintext may appear at 12^th position in ciphertext, and a character at 6^th position in the plaintext may appear at 18^th position in ciphertext.

For example;

Plaintext:     5 9 4 3

Ciphertext:  3 4 9 5

Next :  Modern Ciphers

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