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Monoalphabetic substitution

The monoalphabetic substitution takes a letter of an alphabet and substitutes it with another letter, this way a ciphertext is generated. The way of converting is fixed. A character of the plaintext will be replaced by the same ciphertext character, during the entire ciphertext. There is no additional key. The only way of security is to keep the substitution-table secret. A popular example for the monoalphabetic substitution is the caesar cipher.

The Monoalphabetic substitution can easily be cracked with a frequency-analysis.


One of the oldest of cipher types was the simple substitution or monoalphabetic substitution ciphers in which each letter of the alphabet is replaced by another letter. Each plaintext letter is substituted by a unique ciphertext letter. The earliest known example is the Atbash cipher which is found in the Old Testament and dates from around 600-500BC. It uses the Hebrew alphabet where the first letter of the Hebrew alphabet is replaced by the last letter, the 2nd letter is replaced by the 2nd to last letter and so on. Using the English alphabet the Atbash substitution is:

Atbash cipher

                Plaintext:  abcdefghijklmnopqrstuvwxyz
                Ciphertext: ZYXWVUTSRQPONMLKJIHGFEDCBA
            

Another example is the Caesar cipher which was used by the Roman general and statesman Julius Caesar (100 BC-44 BC) to exchange messages with Marcus Tullius Cicero (106 BC-43 BC) and others. In this type each plaintext letter was replaced by the letter standing three places further along in the alphabet. So plaintext letter A is replaced by ciphertext letter D, B is replaced by E and so on. The full substitution is:

Caesar cipher

                Plaintext:  abcdefghijklmnopqrstuvwxyz
                Ciphertext: DEFGHIJKLMNOPQRSTUVWXYZABC
            

Later versions of the substitution cipher used a keyword to create a keyed alphabet which can be used for the plaintext alphabet, the ciphertext alphabet or both. Where word divisions are kept the American Cryptogram Association (ACA) refers to the monoalphabet substitution cipher as the "Aristocrat" cipher. Another type of cipher, the Patristocrat, uses the same method of encryption but normal word divisions are not retained. An additional "rule" used by the ACA is that no plaintext letter can be substituted by the same ciphertext letter so the keyed alphabets can be shifted to avoid this. Four types of substitution are used depending on how the keyed alphabet is used and these are known in the ACA as keyword types K1, K2, K3 and K4.


K1 Keyword: Keyed plain alphabet, straight cipher alphabet


                pt: gcarlinbdefhjkmopqstuvwxyz
                CT: QRSTUVWXYZABCDEFGHIJKLMNOP
            

K2 Keyword: Straight plain alphabet, keyed cipher alphabet


                pt: abcdefghijklmnopqrstuvwxyz
                CT: HJKMOPQSTUVWXYZGCARLINBDEF
            

K3 Keyword: Keyed plain alphabet, keyed cipher alphabet with the same keywords


                pt: gcarlinbdefhjkmopqstuvwxyz
                CT: FHJKMOPQSTUVWXYZGCARLINBDE
            

K4 Keyword: Keyed plain alphabet, keyed cipher alphabet with different keywords


                pt: gcarlinbdefhjkmopqstuvwxyz
                CT: KLMNPRSVWXYZQUOTEABCDFGHIJ
            

The following examples encipher the George Carlin quote "To be intoxicated is to feel sophisticated but not be able to say it.” using the four keyword types and the keyed alphabets shown above.

Examples:
Type: Monoalphabetic Substitution
Keyword Type: K1
Plaintext keyword: GCARLIN


                Plaintext:  to be intoxicated is to feel sophisticated but not be able to say it
                Ciphertext: JF XZ VWJFNVRSJZY VI JF AZZU IFGBVIJVRSJZY XKJ WFJ XZ SXUZ JF ISO VJ
            

Type: Monoalphabetic Substitution
Keyword Type: K2
Ciphertext keyword: GCARLIN


                Plaintext:  to be intoxicated is to feel sophisticated but not be able to say it
                Ciphertext: LZ JO TYLZDTKHLOM TR LZ POOW RZGSTRLTKHLOM JIL YZL JO HJWO LZ RHE TL
            

Type: Monoalphabetic Substitution
Keyword Type: K3
Plaintext keyword: GCARLIN
Ciphertext keyword: GCARLIN


                Plaintext:  to be intoxicated is to feel sophisticated but not be able to say it
                Ciphertext: RZ QT OPRZBOHJRTS OA RZ UTTM AZGVOAROHJRTS QLR PZR QT JQMT RZ AJD OR
            

Type: Monoalphabetic Substitution
Keyword Type: K4
Plaintext keyword: GCARLIN
Ciphertext keyword: QUOTE


                Plaintext:  to be intoxicated is to feel sophisticated but not be able to say it
                Ciphertext: CT VX RSCTHRLMCXW RB CT YXXP BTEZRBCRLMCXW VDC STC VX MVPX CT BMI RC
            

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