Ciphers and codes
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This document provides an overview of different types of codes and ciphers used for encryption, including monoalphabetic ciphers like the Caesar cipher and Pigpen cipher, polyalphabetic ciphers like the Vigenère cipher, the Playfair cipher which encrypts letter pairs, and transcriptional ciphers like the rail fence cipher which writes the plaintext across multiple "rails". Examples are given for each type of cipher to demonstrate how they work.
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Ciphers and codes
- 2. Codes, Ciphers, Encryption and Cryptography • Cryptography is the discipline of using codes and ciphers to encrypt a message and make it unreadable unless the recipient knows the secret to decrypt it. Encryption has been used for many thousands of years. The following codes and ciphers can be learned and used to encrypt and decrypt messages by hand.
- 3. TYPES OF CIPHERS Monoalphabetic Cipher •Polyalphabetic Cipher Polygraphic Cipher •Transcriptonal Cipher
- 4. Monoalphabetic Ciphers CAESARS CIPHER • A Caesar cipher is one of the simplest (and easiest cracked) encryption methods. It is a Substitution Cipher that involves replacing each letter of the secret message with a different letter of the alphabet which is a fixed number of positions further in the alphabet. • Because each letter in the message has a direct translation to another letter, frequency analysis can be used to decipher the message. For example, the letter E is the most commonly used letter in the English language. Thus, if the most common letter in a secret message is K, it is likely that K represents E. Additionally, common word endings such as ING, LY, and ES also give clues. • A brute-force approach of trying all 25 possible combinations would also work to decipher the message. • Example In this example, each letter in the plaintext message has been shifted 3 letters down in the alphabet. • Plaintext: This is a secret message Ciphertext: wklv lv d vhfuhw phvvdjh
- 5. Monoalphabetic Ciphers PIGPEN / MASONIC CIPHER The pigpen cipher (sometimes called the masonic cipher or Freemason's cipher) is a simple substitution cipher exchanging letters for symbols based on a grid. The scheme was developed and used by the Freemasons in the early 1700s for record-keeping and correspondence. The example key shows one way the letters can be assigned to the grid
- 6. Polyalphabetic Ciphers Vigenère Cipher • To encipher, a table of alphabets can be used, termed a tabula recta, Vigenère square, or Vigenère table. It consists of the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar ciphers. At different points in the encryption process, the cipher uses a different alphabet from one of the rows. The alphabet used at each point depends on a repeating keyword. • For example, suppose that the plaintext to be encrypted is: • ATTACKATDAWNThe person sending the message chooses a keyword and repeats it until it matches the length of the plaintext, for example, the keyword "LEMON": • LEMONLEMONLEEach letter is encoded by finding the intersection in the grid between the plaintext letter and keyword letter. For example, the first letter of the plaintext, A, is enciphered using the alphabet in row L, which is the first letter of the key. This is done by looking at the letter in row L and column A of the Vigenere square, namely L. Similarly, for the second letter of the plaintext, the second letter of the key is used; the letter at row E and column T is X. The rest of the plaintext is enciphered in a similar fashion: • Plaintext: ATTACKATDAWN • Key: LEMONLEMONLE • Ciphertext: LXFOPVEFRNHR • Decryption is performed by finding the position of the ciphertext letter in a row of the table, and then taking the label of the column in which it appears as the plaintext. For example, in row L, the ciphertext L appears in column A, which taken as the first plaintext letter. The second letter is decrypted by looking up X in row E of the table; it appears in column T, which is taken as the plaintext letter.
- 7. tabula recta / Vigenère square Plaintext: ATTACKATDAWN Key: LEMONLEMONLE Ciphertext: LXFOPVEFRNHR
- 8. Polygraphic Ciphers Playfair Cipher • The Playfair cipher encrypts pairs of letters (digraphs), instead of single letters. This is significantly harder to break since the frequency analysis used for simple substitution ciphers is considerably more difficult. • Memorization of the keyword and 4 simple rules is all that is required to create the 5 by 5 table and use the cipher. • The Playfair cipher uses a 5 by 5 table containing a key word or phrase. To generate the table, one would first fill in the spaces of the table with the letters of the keyword (dropping any duplicate letters), then fill the remaining spaces with the rest of the letters of the alphabet in order (to reduce the alphabet to fit you can either omit "Q" or replace "J" with "I"). In the example to the right, the keyword is "keyword". • To encrypt a message, one would break the message into groups of 2 letters. If there is a dangling letter at the end, we add an X. We now take each group and find them out on the table. Noticing the location of the two letters in the table, we apply the following rules, in order. • If both letters are the same, add an X between them. Encrypt the new pair, re-pair the remining letters and continue. • If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively, wrapping around to the left side of the row if necessary. • If the letters appear on the same column of your table, replace them with the letters immediately below, wrapping around to the top if necessary.. • If the letters are on different rows and columns, replace them with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair. The order is important - the first letter of the pair should be replaced first.. • To decipher, ignore rule 1. In rules 2 and 3 shift up and left instead of down and right. Rule 4 remains the same. Once you are done, drop any extra Xs that don't make sense in the final message and locate any missing Qs or any Is that should be Js.
- 9. •Encrypting the message "Hide the gold in the tree stump": Message :HI DE TH EG OL DI NT HE TR EX ES TU MP Encryption :BM OD ZB XD NA BE KU DM UI XM MO UV IF
- 10. Transcriptional Ciphers Rail Fence Cipher • In the rail fence cipher, the plaintext is written downwards on successive "rails" of an imaginary fence, starting a new column when the bottom is reached. The message is then read off in rows. For example, if we have 3 rails and a message of "This is a secret message", you would write out: • T S A C T S G • H I S R M S E • I S E E E A • The last J is just a random letter to fill in the space. The secret message is then condensed and regrouped. • TSACT SGHIS RMSEI SEEEA • To decipher a message you must know the number of rails that were used to encipher it. You then break up the letters into equal groups for each rail. For example, if you are using 3 rails, you would break the secret message into 3 equal groups. Now you stack the groups on top of each other and read off the message vertically. If you get gibberish, then there are probably some extra letters tacked on the end of the message that are throwing off the grouping
- 11. OTHER TYPES OF CIPHERS secret message 411414 3144121 353254 2533125 The numbers are now read off horizontally and grouped into pairs. 41 14 14 31 44 12 13 53 25 42 53 31 25 The Polybius Square is used again to convert the numbers back into letters which gives us our ciphertext:qddltbcxkrxlk