Substitution cipher
As monoalphabetic substitution (from Greek: μόνο (mono) = "one " and αλφάβητο ( alphabeto ) = " Alphabet " and from the Latin: substituere = " replace " ) is referred to in cryptography, ie in the branch of science of cryptology, which by itself the secret writing is concerned, encryption method, in which only a single (fixed ) alphabet for encryption, so to convert the plaintext into the ciphertext, will be used.
- 4.1 Frequency Analysis
- 4.2 plaintext attack ( pattern search)
Principle
The letters or characters or groups of letters or groups of characters of the plain text will be replaced by default this one alphabet, also called the key or secret alphabet alphabet by other letters, characters or groups.
Classic examples of mono alphabetic substitution are the Caesar cipher and the Playfair method. In contrast to the mono-alphabetic substitutions are polyalphabetic substitutions, which are used to encrypt several (many ) different alphabets. Examples are the Vigenere encryption and the Enigma machine.
Examples
Simple monoalphabetic substitution
An example of a mono- alphabetic encryption is the following procedure: This individual letters of the plaintext are substituted using the key alphabet into individual characters in the ciphertext. This method is therefore precisely as " monographic monoalphabetic monopartite substitution" or simply referred to as "simple mono alphabetic substitution".
Caesar cipher
Main article: Caesar cipher
This is a special case of simple mono- alphabetic substitution, where the alphabet used for encryption is obtained by cyclically shifting each letter of the standard alphabet. The number of places in order to be moved, is the key. Caesar already used this procedure for the most part with the key "C", which corresponds to a shift by three letters.
Example of the Caesar cipher:
Secret alphabet creation
There are different methods to generate the secret alphabet required for encryption and decryption. In addition to the particularly simple ( and very uncertain ) variants, such as
Caesar shift ( only 25 different keys possible here with key e, ie shift to five characters. )
Clear: abcdefghijklmnopqrstuvwxyz Secret: FGHIJKLMNOPQRSTUVWXYZABCDE and
Atbasch ( revertiertes alphabet set, only a single fixed key available):
Clear: abcdefghijklmnopqrstuvwxyz Secret: ZYXWVUTSRQPONMLKJIHGFEDCBA is to produce a scrambled secret alphabet using a password (key) usual. Advantage of this method is that such a large number of different secret alphabets can be formed without the need to transmit the key in written form. It is sufficient to send the authorized recipient the password (key) orally or in some other (secret ) way. The password is easy to remember and thus protected well against spying. Both encryptor (transmitter ) and descrambler (receiver ) are the identical secret alphabet in the same way from the password.
For example, they agree as their secret password " Regenschirmstaender ". First, remove all of the letters occur more than once from the password. From " Regenschirmstaender " is REGNSCHIMTAD. These letters form the beginning of the secret alphabet. The rest of the alphabet, so the letters do not occur in the password are padded on the right (below in bold type ). Thus, for a secret alphabet
Clear: abcdefghijklmnopqrstuvwxyz Secret: REGNSCHIMTADBFJKLOPQUVWXYZ Better yet, the remaining characters are not alphabetic, but in reverse alphabetical order ( reverted ) fill is. This avoids the disadvantage that otherwise the secret alphabet often ( as here ) ends with ... XYZ. By reverted replenish the remaining letters of the alphabet for the password this results in a secret alphabet:
Clear: abcdefghijklmnopqrstuvwxyz Secret: REGNSCHIMTADZYXWVUQPOLKJFB Alternatively you can also the missing alphabet letters in alphabetical order to attach to the last letter of the password (progressive replenishment ) and as a scrambled possible secret alphabet produce:
Clear: abcdefghijklmnopqrstuvwxyz Secret: REGNSCHIMTADFJKLOPQUVWXYZB It is also possible to use a completely random scrambled secret alphabet. A disadvantage, however, that the two partners can not remember this in your head usually. It must therefore be noted and can then be spied policy.
Clear: abcdefghijklmnopqrstuvwxyz Secret: NKJSZWHMLAVYFCPTBQRUOGIDXE Using the above secret alphabet of the plaintext is " water is boiling in the kettle " in the ciphertext " INRRZQ VPJMU LF UZZVZRRZY " converted. Of course, you would be removed before transmission of the ciphertext for complicating the unauthorized deciphering the blanks and send the text as a "worm" " INRRZQVPJMULFUZZVZRRZY " or in groups " INRRZ QVPJM ULFUZ ZVZRR ZY ".
Security
In contrast to Caesar encryption with only 25 possibilities, there are many opportunities for scrambling of the standard alphabet: the first letter "A " can be placed at one of 26 possible positions alphabet. For the second letter "B", there are still 25 possible locations to choose from, for the third 24, and so on. Overall, calculated as 26.25.24.23 · · · 4.3.2.1 = 26! (Factorial) ways for scrambling the alphabet. That's about 4.1026 cases and corresponds to about 88 bits. Accordingly, a decipherment by trying all cases ( brute force method ) is virtually impossible. Nevertheless, the monoalphabetic substitution is unsafe and easy to "crack". Even relatively short ciphertexts that are encrypted monoalphabetically ( thirty to fifty characters are fully sufficient ) can be decoded using statistical tests ( frequency counts ) and by pattern search.
Deciphering
Frequency analysis
To decipher monoalphabetic encryption without key known to performs a frequency analysis of letters by the key text and can be as close to certain letters, from which words and thus more and more associations can be drawn to plaintext letters. (Some frequency tables can be found at the German alphabet. )
Example:
Mjjp nop cni Hzgfzqosmqgr zqo scd Gjdkqpcmucmcngf. Cm rjddp TJD ciabnogfci qis fcnoop vjcmpbngf qcucmocpyp: Vqmycb. Letter frequencies: 12.6%: c, each 6.7%: mp, 5.9%: oq, 5%: dgj From the distribution can be assumed that the e is coded as the most common letter by c. This results in the following is:
Mjjp nop cni Hzgfzqosmqgr zqo scd Gjdkqpcmucmcngf. Cm rjddp TJD Ciabnogfci qis fcnoop vjcmpbngf qcucmocpyp: Vqmycb. E e ....... ................. ...... e. e.e. ... E. ........ E. e ...... E .... ..... e ...... e ... e ..: .... e. Now is searched word contexts. 3-letter words and e in the middle are usually articles ( the, the, the, ...), especially if they occur more than once; as can be so close to the d. A word with 3 letters and e at the beginning is an often. Here it is important to try and document the steps so that you can go through backtracking in case of errors.
Mjjp nop cni Hzgfzqosmqgr zqo scd Gjdkqpcmucmcngf. Cm rjddp TJD ciabnogfci qis fcnoop vjcmpbngf qcucmocpyp: Vqmycb. I ..... ....... A d ...... de. ...... e. ... ee E. ........ En. i .. s. nd. ei ..... e .. i .. e ... e ..: .... e. This can easily be the words and and is found in:
T ... is a ..... u.d.u. .. us de. . .. .... ute .. ee ... E. .... t ... En is .. s and EISST .. eti ue.e.set.t: . u e. . From which with a little imagination and practice slightly more words and letter strings ( like, sh / ch s, etc.) and suggest Finally the plain text:
T ... is a Fachausd.u .. from de. C. .. ute .. e.e.ch.. E. .... t ... En pean and say .. etich ue.e.set.t: . U e. . Root is a technical term from the computer area. He comes from English and means literally translated: root. Deciphering the ciphertext by evaluating the letter frequencies can be made difficult or impossible by a leipogrammatischen text. The fact that can not be used in a text leipogrammatischen one or more letters (eg, failure to use words with e), the whole letter frequency shifts, and without the knowledge of / avoided the letter is absent or greatly impeded evaluation carried out.
Plaintext attack ( pattern search)
Main article: Pattern Search
Are parts of the plaintext is known ( individual terms ), so you can look in the ciphertext after their pattern, by holding, for example, by double letters out. In plain - and ciphertext in the double character should at a monoalphabetic substitution at the same positions occur. Similarly, you can also search for patterns in the ciphertext corresponding to the pattern of the suspected word.
Example:
Suspected: INTERNET Ciphertext: WXMNASXUAXSXNA ' INTERNET ' → ' NASXUAXS ' MAKE PROFIT encryption
This very simple monoalphabetic encryption of digits based on the fact that digits are replaced by their assigned letters from the easily remembered phrase "MAKE PROFIT. "
Digits: 1 2 3 4 5 6 7 8 9 0 Key: M A K E P R O F I T Examples: 3719346 87550 46025504 12892 COMEDIAN teases caught MAFIA Such encryption is less suitable as a secret code, but it uses the key to convert numbers into letters there where no digits may be used or should be used. One example is type codes in catalogs and price lists for sellers. When Siemens AG, the former Siemens manager Michael Kutschenreuter opposite the prosecutor's office in Munich, the code has been used in conjunction with instructions to bribes as a secret key.