The basic work of the cipher

The basic work of the cipher

First of all we have to know that a cipher is a concealed way of writing messages so to protect them from access by unauthorized persons.

The Caesar Cipher – in this cipher, every plain text letter is substituted with another letter that is an unchanging number spots down the alphabet.

In the example above, the cipher for B will be C, C will be D and D will be E.

Perfect substitution cipher – plaintext is replaced by ciphertext following a given defined system. A string of random bits which are of the same length as the plaintext are chosen, they are then used to find the corresponding ciphertext using XOR.

e.g. p1   p2….. pn  -plaintext

       b1   b2……bn

       c1  c2……cn  -ciphertext

The vernam cipher also referred to as the one time pad, it uses a character to character combination of plaintext to produce ciphertext. To convert the ciphertext to plaintext, one would use a character to character combination of the ciphertext to produce the plaintext.

   SENDING

                message:  0  0  1  0  1  1

                pad:            1   0  0  1  1 0

                XOR             ——————

                cipher:        1   0 1  1  0  0 1

  RECEIVING

                 cipher:  1  0  1  1  0  0  1

                 pad:      1  0  0  1  1   0 0

                 XOR      ——————-

            message: 0  1  1  0  0   1 1

A block cipher is whereby a cryptographic key operates on a group of bits as opposed to a single bit only. The block size may be 64 bits, 128 bits or sometimes variable in size.

In a stream cipher, there is change on individual plaintext digits as the time changes. Unlike the block cipher, the plaintext is encrypted one bit at a time in the stream cipher, hence the encryption of each digit is completely reliant on the current state.