Use the shift cipher with key in Software

Draw Code 39 Full ASCII in Software Use the shift cipher with key

= 15 to decrypt the message "WTAAD"

We decrypt one character at a time Each character is shifted 15 characters up Letter W is decrypted to H Letter T is decrypted to E The first A is decrypted to L The second A is decrypted to L And, finally, D is decrypted to O The plaintext is HELLO

In a transposition cipher, there is no substitution of characters; instead, their locations change A character in the first position of the plaintext may appear in the tenth position of the ciphertext A character in the eighth position may appear in the first position In other words, a transposition cipher reorders the symbols in a block of symbols

Key In a transposition cipher, the key is a mapping between the position of the symbols in the plaintext and cipher text For example, the following shows the key using a block of four characters:

Plaintext: Ciphertext:

2 4 1 3

123 4

In encryption, we move the character at position 2 to position 1, the character at position 4 to position 2, and so on In decryption, we do the reverse Note that, to be more effective, the key should be long, which means encryption and decryption of long blocks of data Figure 308 shows encryption and decryption for our four-character

Plaintext (block) Key Plaintext (block)

23 4

Ciphertext (block)

Reorder (upward direction)

block using the above key The figure shows that the encryption and decryption use the same key The encryption applies it from downward while decryption applies it upward

We first remove the spaces in the message We then divide the text into blocks of four characters We add a bogus character Z at the end of the third block The result is HELL OMYD EARZ We create a three-block ciphertext ELHLMDOYAZER

Using Example 305, decrypt the message "ELHLMDOYAZER"

The result is HELL OMYD EARZ After removing the bogus character and combining the characters, we get the original message "HELLO MY DEAR"

The traditional ciphers we have studied so far are character-oriented With the advent of the computer, ciphers need to be bit-oriented This is so because the information to be encrypted is not just text; it can also consist of numbers, graphics, audio, and video data It is convenient to convert these types of data into a stream of bits, encrypt the stream, and then send the encrypted stream In addition, when text is treated at the bit level, each character is replaced by 8 (or 16) bits, which means the number of symbols becomes 8 (or 16) Mingling and mangling bits provides more security than mingling and mangling characters Modem ciphers use a different strategy than the traditional ones A modern symmetric cipher is a combination of simple ciphers In other words, a modern cipher uses several simple ciphers to achieve its goal We first discuss these simple ciphers

Modern ciphers today are normally made of a set of simple ciphers, which are simple predefined functions in mathematics or computer science The first one discussed here is called the XOR cipher because it uses the exclusive-or operation as defined in computer science Figure 309 shows an XOR cipher Figure 309 XOR cipher

An XOR operation needs two data inputs plaintext, as the first and a key as the second In other words, one of the inputs is the block to be the encrypted, the other input is a key; the result is the encrypted block Note that in an XOR cipher, the size of the key, the plaintext, and the ciphertext are all the same XOR ciphers have a very interesting property: the encryption and decryption are the same