Classical cipher

A cipher is a means of concealing a message, where letters of the message are substituted or transposed for other letters, letter pairs, and sometimes for many letters. In cryptography, a classical cipher is a type of cipher that was used historically but now has fallen, for the most part, into disuse. In general, classical ciphers operate on an alphabet of letters (such as "A-Z"), and are implemented by hand or with simple mechanical devices. They are probably the most basic types of ciphers, which made them not very reliable, especially after new technology came about—modern schemes use computers or other digital technology, and operate on bits and bytes. A lot of classical ciphers were used by well-respected people, such as Julius Caesar and Napoleon, who created their own ciphers which were then popularly used. A lot of ciphers found their origins in the military and were used for transporting secret messages among people on the same side. Classical schemes are often susceptible to ciphertext-only attacks, sometimes even without knowlede of the system itself, using tools such as frequency analysis. Sometimes grouped with classical ciphers are more advanced mechanical or electro-mechanical cipher machines, such as the Enigma machine.

Types of Classical ciphers

Classical ciphers are often divided into transposition ciphers and substitution ciphers.

Substitution ciphers

In a substitution cipher, letters (or groups of letters) are systematically replaced throughout the message for other letters (or groups of letters). For instance a simple (and therefore easy to crack) encryption would be to substitute each letter for the next letter in the alphabet (a to b, b to c, and so on with z being substituted by a). Using this encryption the sentence "Hello my name is Alice." would be encrypted as "Ifmmp nz obnf jt Bmjdf."

The Caesar shift is one of the most well-known types of substitution ciphers. It gained fame around the 2nd century AD when Julius Caesar used it to encipher messages. In order to apply a Caesar shift to a text, all the writer had to do was shift the alphabet down three letters and that would be the new cipher alphabet used to encode the message. D would become A, E would become B, F would become C, etc. So, for example, if someone wanted to encode the message "Wikipedia," it would become ZLNLSHGLD. When Caesar used this shift, he only used a shift of three letters, but subsitution ciphers work with shifts of any length.

Another method of substitution cipher is using a keyword. You choose a word or short phrase, remove all the spaces and repeated letters and then use this as the start of the cipher alphabet. The end of the cipher alphabet is the rest of the alphabet in order without repeating the letters in the keyword. For example, if the keyword is CIPHER, the cipher alphabet would look like this:

normal alphabet: a b c d e f g h i j k l m n o p q r s t u v w x y z

cipher alphabet: c i p h e r s t u v w x y z a b d f g j k l m n o q

The previous examples were all examples of monoalphabetic substitution ciphers, where just one cipher alphabet is used. It is also possible to have a polyaphabetic substitution cipher, where multiple cipher alphabets are used. The encoded would just make up two or more cipher alphabets using whatever techniques he or she chooses, and then encode their message, alternating what cipher alphabet is used with every letter or word. This makes the message a lot hard to decode because the codebreaker would have to figure out both cipher alphabets.

Another example of a polyaphabetic substition cipher that is a lot harder to decode is the Vigenere square, an innovative encoding method. With the square, there are 26 different cipher alphabets that are used to encrypt text. Each cipher alphabet is just another rightword Caesar shift of the original alphabet. This is what a Vigenere square looks like:

                A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
                B C D E F G H I J K L M N O P Q R S T U V W X Y Z A
                C D E F G H I J K L M N O P Q R S T U V W X Y Z A B
                D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
                E F G H I J K L M N O P Q R S T U V W X Y Z A B C D
                F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
                G H I J K L M N O P Q R S T U V W X Y Z A B C D E F
                H I J K L M N O P Q R S T U V W X Y Z A B C D E F G
                I J K L M N O P Q R S T U V W X Y Z A B C D E F G H
                J K L M N O P Q R S T U V W X Y Z A B C D E F G H I 
                K L M N O P Q R S T U V W X Y Z A B C D E F G H I J
                L M N O P Q R S T U V W X Y Z A B C D E F G H I J K 
                M N O P Q R S T U V W X Y Z A B C D E F G H I J K L
                N O P Q R S T U V W X Y Z A B C D E F G H I J K L M
                O P Q R S T U V W X Y Z A B C D E F G H I J K L M N
                P Q R S T U V W X Y Z A B C D E F G H I J K L M N O 
                Q R S T U V W X Y Z A B C D E F G H I J K L M N O P
                R S T U V W X Y Z A B C D E F G H I J K L M N O P Q
                S T U V W X Y Z A B C D E F G H I J K L M N O P Q R
                T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
                U V W X Y Z A B C D E F G H I J K L M N O P Q R S T
                V W X Y Z A B C D E F G H I J K L M N O P Q R S T U
                W X Y Z A B C D E F G H I J K L M N O P Q R S T U V
                X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
                Y Z A B C D E F G H I J K L M N O P Q R S T U V W X
                Z A B C D E F G H I J K L M N O P Q R S T U V W X Y

In order to use the Vigenere square to encrypt a message, you first choose a keyword and then repeat it is the same length as the message you wish to encode. You then would write the message underneath the repeated keyword to see which cipher alphabet you would use for each letter of the message. The first letter of the message would be encoded using the cipher alphabet that corresponds with the first letters of the keyword. The cipher alphabet that uses B for A and C for B etc. would be cipher alphabet 'B'. Each cipher alphabet is named by the first letter in it. For example if you have a keyword of WORD and the message you want to encode is I LOVE CRYPTOGRAPHY, this is what you would do:

message: I LOVE CRYPTOGRAPHY

keyword: W ORDW ORDWORDWORDW

encoded text:E ZFYA QIBLHFJNOGKU

Some subsitution ciphers involve using numbers instead of letters. An example of this is the Great Cipher, where numbers were used to represent syllables. There is also another number substitution cipher that involves having four different number pair options for a letter based on a keyword.

Instead of numbers, symbols can also be used to replace letters or syllables. One example of this is Zodiac alphabet, where signs of the zodiac were used to represent different letters, for example, the symbols for the sun stood for A, Jupiter stood for B, and Saturn stood for C. Dots, lines, or dashes could also be used, one example of this being Morse Code, which isn't really a cipher, but uses dots and dashes as letters nonetheless. The pigpen cipher uses a grid system or lines and dots to establish symbols for letters. There are various other methods that involve substituting letters of the alphabet with symbols or dots and dashes.

Transposition ciphers

In a transposition cipher, the letters themselves are kept unchanged, but their order within the message is scrambled according to some well-defined scheme. A lot of transposition ciphers are done according to a geometric design. A simple (and once again easy to crack) encryption would be to write every word backwards. For example "Hello my name is Alice." would now be "olleH ym eman si ecilA." A scytale is a machine that aids in the transposition of methods.

One specific example of a transposition is a columnar cipher. In a columnar cipher, the original message is put into a rectangle, going from left to right and top to bottom. Next, a key is chosen, in order to assign a number to each column in the rectangle to determine the order or rearranging the letters. The number corresponding to the letters in the key is determined by their place in the alphabet, i.e. A is 1, B is 2, C is 3, etc. For example, if the key word is CAT and the message is THE SKY IS BLUE, this is how you would arrange your message:

                         C A T
                         3 1 20
                         T H E
                         S K Y
                         I S B
                         L U E

Next, you take the letters in numerical order and that is how you would transpose the message. You take the column under A first, then the column under C, then the column under T, as a result your message "The sky is blue" has become: HKSUTSILEYBE

Another type of transposition cipher is the Chinese cipher. In this method of transposing, the letters of the message that need to be enciphered are written from right to left, down and up columns in order to scramble the letters. Then, starting in the first row, the letters are then taken in order to get the new ciphertext. For example, if the message needed to be enciphered was THE DOG RAN FAR, the Chinese cipher would look like this:

                           R R G T
                           A A O H  
                           F N D E  

The cipher text would then read: RRGT AAOH FNDE

A lot of transposition ciphers are similar to these two examples and usually involved rearranging the letters into rows or columns and then taking them in a systematic way in order to transpose the letters. Some other examples include the Vertical Parallel and the Double Transposition Cipher.

More complex algorithms can be formed by mixing substitution and transposition in a product cipher; modern block ciphers such as DES iterate through several stages of substitution and transposition.

Decipherment of Classical Ciphers

While classical ciphers are easy to create, they are also easy to decipher, which is probably one reason for their disuse. With substitution ciphers, especially ones where the letters are just shifted down, it is just a simple matter of figuring out the shift in order to decode the message. With keywords, it can be a little difficult to figure out the keyword used, but once that is found, decipherment is a breeze. The Vigenere square is a harder substitution cipher to crack, but if cryptanalysts use frequency analysis, that can aid them in breaking the cipher. Frequency analysis is just as its name implies—the cryptanalyst looks to see how often letters or combinations of letters appear in the ciphertext and compare that to their knowledge of the frequency of letters in the English alphabet, for example, "e" is the most common letter in the English alphabet, so if there is a large number of "m" in the ciphertext, it probably stands for "e". Transposition ciphers usually can be deciphered using frequency analysis but there are other methods that make them easy to crack. One way is to arrange the letters of the ciphertext into a square or rectangle based on the number or letters in the ciphertext and seeing if there is a trend in the square. If a trend is not seen immediately, it helps to take the square row by row or column by column and looking for a trend and then rearranging the square in a different way. Since transposition ciphers just involve rearranging letters, all of the letters need are in the ciphertext; it is just a matter of making sense of them. Decipherment takes time, patience, and trial and error, but it can be done.

The Future of Ciphers

Since the use of classical ciphers, enciphering and encoding have come a long way. One innovative new method of enciphering is public key cryptography, which does not require an exchange of the key used to encode the message. It uses an asymmetric cipher, where the key for encoding the message is different than the key needed to decode the message. An example of this is RSA, which uses a formula to calculate the key.

Symmetric ciphers are still in use today; they just involve different methods than in the past. One example is the block cipher, which involves bits (binary digits) and a mathematical formula to encode the text. Encryption today has become more complicated and mathematical, which, as a result, makes messages harder to decipher. Now, instead of being done by hand, most ciphers are computerized. This makes them more efficient and easier to create, but harder to crack. Ciphers have come far since Caeser shifts and Vigenère squares. As times evolve, so do ciphers.

See also

• History of cryptography

References

• Hand Ciphers Pencil-and-paper ciphers on Ciphermachines & Cryptology
• Trinity College Department of Computer Science: Historical Cryptography Information about many different types of encryption algorithms including substitution and transposition ciphers
• Singh, Simon. The Code Book The Science of Secrecy from Ancient Egypt to Quantum *Cryptography. New York: Anchor, 2000.
• D'Agapeyeff, Alexander. Codes and Ciphers. Oxford UP, 1939.
• Laffin, John. Codes and Ciphers: Secret Writing through the Ages. Abelard-Schuman, 1964.
• Wrixon, Fred B. Codes, Ciphers, and Secret Languages. New York: Bonanza Books, 1989.