THE VIGENERE ALGORITHM
Cryptography Symmetric algorithm
Dr. Imad Al Saeed
Transposition Cipher
Simple to understand, but if properly used, produces ciphertext that is difficult to decipher.
Also known as a permutation cipher; involves simply rearranging the values within a block based on an established pattern.
Can be done at the bit level or at the byte (character) level.
To make the encryption even stronger, the keys and block sizes can be increased to 128 bits or more.
Transposition Cipher Method
Simple to understand, but if properly used, produces ciphertext that is difficult to decipher
Rearranges values within a block to create ciphertext
Can be done at the bit level or at the byte (character) level
To make the encryption even stronger, the keys and block sizes can be increased to 128 bits or more
Uses block padding method to facilitate algorithm
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Transposition Cipher
A transposition Cipher is one in which the order of character is changes to obscure message.
An early version of a transposition Cipher was a Scytale, in which paper was wrapped around a stick and the message was written. Once unwrapped, the message would be unreadable until the message was wrapped around a same-size stick again.
One modern transportation cipher is done by writing the message in row, then forming the encrypted from the text in the columns.
Example: Transposition Cipher
Encrypt the message “Meet at three pm today at the usual location” using rows of 6 characters.
The encoded message would be:
MTMASC EHTTUA EROTAT TEDHLI AEAELO TPYUON
The space could be removed or repositioned to hide the size of the table used, since that the encryption key in this message.
Example: Transposition Cipher
Decrypt the message:
AES EAO OIT VUI NNN NEE RLC FTE LNV LER LS
If it was encrypted using a tabular transposition cipher with row of length 4 characters.
Since there are 10 x 3 + 2 = 32 characters and each row as 4 characters, 32/4 = 8 means we need 8 rows.
Exclusive OR (XOR)
A function within Boolean algebra used as an encryption function in which two bits are compared.
If the two bits are identical, the result is a binary 0.
If the two bits are not identical, the result is a binary 1.
Very simple to implement and simple to break; should not be used by itself when organization is transmitting/storing sensitive data.
Exclusive OR (XOR)
Function of Boolean algebra; two bits are compared and binary result generated
If two bits are identical, result is binary 0
If two bits not identical, result is binary 1
Very simple to implement and simple to break; should not be used by itself when organization is transmitting/storing sensitive data
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XOR Table
| First bit | Second bit | result |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Copyright © 2018 Cengage. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.
Example XOR Encryption
| Text value | Binary value |
| CAT as bits | 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 1 0 1 0 1 0 1 0 0 |
| VVV as key | 0 1 0 1 0 1 1 0 0 1 0 1 0 1 1 0 0 1 0 1 0 1 1 0 |
Copyright © 2018 Cengage. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.
Example XOR Encryption
| Text value | Binary value |
| CAT as bits | 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 1 0 1 0 1 0 1 0 0 |
| VVV as key | 0 1 0 1 0 1 1 0 0 1 0 1 0 1 1 0 0 1 0 1 0 1 1 0 |
| Cipher | 0 0 0 1 0 1 0 1 0 0 0 1 0 1 1 1 0 0 0 0 0 0 1 0 |
Copyright © 2018 Cengage. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.
Vernam Cipher
A cryptographic technique developed at AT&T and known as the “one-time pad.”
This cipher uses a set of characters for encryption operations only one time and then discards it.
To perform:
The pad values are added to numeric values that represent the plaintext that needs to be encrypted
Each character of the plaintext is turned into a number and a pad value for that position is added
The resulting sum for that character is then converted back to a ciphertext letter for transmission
If the sum of the two values exceeds 26, then 26 is subtracted from the total
Vernam Cipher
Developed at AT&T Bell Labs
Uses set of characters once per encryption process
To perform:
The pad values are added to numeric values that represent the plaintext that needs to be encrypted
Each character of the plaintext is turned into a number and a pad value for that position is added
The resulting sum for that character is then converted back to a ciphertext letter for transmission
If the sum of the two values exceeds 26, then 26 is subtracted from the total
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Example: Vernam Cipher
| A | B | C | D | E | F | G | H | I | J |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| K | L | M | N | O | P | Q | R | S | T |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
| U | V | W | X | Y | Z | ||||
| 20 | 21 | 22 | 23 | 24 | 25 |
Decrypt the following plain text (Hello) using the key (NCBTA).
Sol/
| H | E | L | L | O |
| 7 | 4 | 11 | 11 | 14 |
| N | C | B | T | A |
| 13 | 2 | 1 | 19 | 0 |
Add both values to each other.
| 20 | 6 | 12 | 30 -26 = 4 | 14 |
| U | G | M | E | O |
Note: if you add numbers and get a number grater then 25, then subtract it from 26
Example: Vernam Cipher
To decrypt the message:
Subtract the cipher text from the key”
| U | G | M | E | O |
| 20 | 6 | 12 | 4 | 14 |
| N | C | B | T | A |
| 13 | 2 | 1 | 19 | 0 |
| 7 | 4 | 11 | -15 +26 = 11 | 14 |
| H | E | L | L | O |
Book-Based Ciphers
Uses text from a predetermined book as a key to decrypt a message.
Book cipher: ciphertext consists of a list of codes representing page, line, and word numbers of plaintext word.
Running key cipher: uses a book for passing the key to cipher similar to Vigenère cipher; sender provides encrypted message with sequence of numbers from predetermined book to be used as an indicator block.
Template cipher: involves use of hidden message in book, letter, or other message; requires page with specific number of holes cut into it.
Book-Based Ciphers
Uses text in book as key to decrypt a message
Book cipher: ciphertext consists of list of codes representing page, line, and word numbers of plaintext word
Running key cipher: uses a book for passing key to cipher similar to Vigenère cipher; sender provides encrypted message with sequence of numbers from predetermined book to be indicator block
Template Cipher: involves use of hidden message in book, letter, or other message; requires page with specific number of holes cut into it.
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