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 600-500 BC. 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 monoalphabetic 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 plaintext alphabet, straight ciphertext alphabet
Plaintext: gcarlinbdefhjkmopqstuvwxyz
Ciphertext: QRSTUVWXYZABCDEFGHIJKLMNOP
K2 Keyword: Straight plain alphabet, keyed cipher alphabet
Plaintext: abcdefghijklmnopqrstuvwxyz
Ciphertext: HJKMOPQSTUVWXYZGCARLINBDEF
K3 Keyword: Keyed plain alphabet, keyed cipher alphabet with the same keywords
Plaintext: gcarlinbdefhjkmopqstuvwxyz
Ciphertext: FHJKMOPQSTUVWXYZGCARLINBDE
K4 Keyword: Keyed plain alphabet, keyed cipher alphabet with different keywords
Plaintext: gcarlinbdefhjkmopqstuvwxyz
Ciphertext: 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.
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
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
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
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
This uses a dictionary keyword search or a hill climbing algorithm.
If a Dictionary keyword search is selected the keyword type must be selected from the Setup drop down menu. The K4 keyword type is not supported by the Dictionary search.
After solving using the genetic algorithm the keyword can be sought by selecting the Keyword Finder function in the Tools tab. The default Keyword type is set from the main window but this can be changed if necessary.