Column transposition

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The column transposition is a cryptographic method, a plaintext to encrypt and so in a ciphertext convert. It is based on the transposition method . The individual characters of the message (mostly letters) are re-sorted according to a certain procedural rule that is controlled by a secret key . This is in contrast to the substitution method , in which each plaintext character remains in its place but is replaced ("substituted") by another character.

Procedure

In the column transposition method, a rectangular arrangement (also known as a matrix) is generally used, consisting of several lines (as many as are necessary to enter the plain text) and a number specified by the key (usually a password made up of letters, also called a key word) of columns. The number of columns corresponds to the number of letters in this keyword. The plain text is then entered into the matrix line by line.

As ciphertext, the individual letters of the plain text are now read column by column from the matrix, the order in which the columns are read out is determined by the alphabetical order of the letters of the password.

example

The following plain text should be encrypted using the column transposition. The key word is "WIKIPEDIA". It consists of nine letters. This means that the plain text must be entered line by line in a 9-width rectangle. If there are fewer than nine letters in the last line, the rest remains empty.

Plain text: 

DIESISTNUREINBEISPIELTEXTUNDERDIENTHIERINUNSERERWIKIPEDIAZURILLUSTRATIONDERSPALTENTRANSPOSITION

Rectangle: 

DIESISTNU
REINBEISP
IELTEXTUN
DERDIENTH
IERINUNSE
RERWIKIPE
DIAZURILL
USTRATION
DERSPALTE
NTRANSPOS
ITION

WIKIPEDIA
947583261

The reading order is determined by the key word, the individual letters of which are to be numbered in alphabetical order (A corresponds to 1, D corresponds to 2, etc. Identical letters are numbered in the order in which they appear, see the three "I" in positions 4, 5 and 6). WIKIPEDIA becomes 947583261. The above text is now read out column by column in this order.

Read columns: 

UPNHEELNES TITNNIIILP SEXEUKRTAS IEEEEEISETT SNTDIWZRSAO NSUTSPLOTO EILRRRATRRI IBEINIUAPNN DRIDIRDUDNI

In order not to reveal the length of the individual columns or the length of the keyword, the ciphertext is sent in groups of a specified length, usually in groups of five, i.e.:

Ciphertext (in groups of five): 

UPNHE ELNES TITNN IIILP SEXEU KRTAS IEEEE EISET TSNTD IWZRS AONSU TSPLO TOEIL RRRAT RRIIB EINIU APNND RIDIR DUDNI

Double column transposition

The "simple" column transposition shown above does not offer great security against unauthorized decipherment . However, it can be improved by a second process step for double column transposition, also called double cube . The best way to do this is to use a second independent keyword with a different length and understand the ciphertext given above only as an intermediate text, which is entered again line by line in a second matrix (with a different width) and then read out column by column according to the letter sequence of the second password becomes. This results in the ciphertext of the double column transposed plain text.

Decryption

The authorized recipient of the message is in possession of the key or keys with which he can determine the width of the rectangles. By reversing the process steps described above (now column-by-column entry and line-by-line reading), the ciphertext can be converted back into plain text.

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