| In symmetric-key cryptography, the algorithm used for decryption is the inverse of the algorithm used for encryption. |
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| This means that if the encryption algorithm uses a combination of addition and multiplication, the decryption algorithm uses a combination of division and subtraction. |
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| Traditional Ciphers |
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| In the earliest and simplest ciphers, a character was the unit of data to be encrypted. These traditional ciphers involved either substitution or transposition. |
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| Substitution Cipher |
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| A cipher using the substitution method substitutes one symbol with another. If the symbols in the plaintext are alphabetic characters, we replace one character with another. |
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| For example, we can replace character A with D and character T with Z. If the symbols are digits (0 to 9), we can replace 3 with 7 and 2 with 6. |
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| We will concentrate on alphabetic characters. Substitution can be categorized as either monoalphabetic or polyalphabetic. |
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| Monoalphabetic Substitution |
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| In mono alphabetic substitution, a character in the plaintext is always changed to the same character in the ciphertext regardless of its position in the text. |
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| For example, if the algorithm says that character A in the plaintext must be changed to character D, every character A is changed to character D, regardless of its position in the text. |
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| The first recorded ciphertext was used by Julius Caesar and is still called the Caesar cipher. |
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| Example |
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| ATTACK becomes |
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| DWWDFN |
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| Polyalphabetic Substitution |
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| In polyalphabetic substitution, each occurrence of a character can have a different substitute. The relationship between a character in the plaintext to a character in the ciphertext is one-to-many. |
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| Character A can be changed to D in the beginning of the text, but it could be changed to N at the middle. We discuss a very simple one. |
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| It is obvious that if the relationship between plaintext characters and ciphertext characters is one-to-many, the key must tell us which of the many possible characters can be chosen for encryption. |
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| Let us define our key as “take the position of the character in the text, divide the number by 9, and let the remainder be the shift value.” |
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| With this scenario, the character at position 1 will be shifted one character, the character at position 2 will be shifted two characters, and the character in position 14 will be shifted four characters (14 mod 9 is 4).So now the word ATTACK becomes |
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| A T T A C K -plaintext |
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| B V W E H Q -encrypted |
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| Transpositional Cipher |
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| In a transpositional cipher, the characters retain their plaintext form but change their positions to create the ciphertext. |
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| The text is organized into a two-dimensional table, and the columns are interchanged according to a key. The key defines which columns should be swapped. |
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| As you have guessed, transpositional cryptography is not very secure either. |
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| The character frequencies are preserved, and the attacker can find the plaintext through trial and error. |
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| This method can be combined with other methods to provide more sophisticated ciphers. |
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| Example |
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| 7 4 5 1 2 9 3 6 KEY |
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| P L E A S E T R |
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| A N S F E R O N |
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| E M I L L I O N |
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| D O L L A R S T plain text |
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| O M Y S W I S S |
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| B A N K A C C O |
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| U N T S I X T W |
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| O T W O A B C D |
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| Plaintext |
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| PLEASE TRANSFER ONE MILLION DOLLARS IN MY SWISS BANK ACCOUNT SIX TWO TWO. |
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| Ciphertext |
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| AFLLSKSOSELAWAIATOOSSCTCLNMOMANTESILYNTWRNNTSOWDPAEDOBUOERIRICXB |
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| Block Cipher |
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| Traditional ciphers used a character or symbol as the unit of encryption/decryption. Modem ciphers, on the other hand, use a block of bits as the unit of encryption/decryption. |