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cifer - multipurpose classical cryptanalysis and code‐breaking tool


cifer [-finqs] [command]


Cifer provides many functions designed to aid in cracking classical ciphers; a group of
ciphers used historically, but which have now fallen into disuse because of their
suceptability to ciphertext‐only attacks. In general, they were designed and implemented
by hand, and operate on an alphabet of letters (such as [A‐Z]).

Cifer is implemented as an interactive shell, with support for scripting. All of its
commands are documented via the usage command. For instance, type usage load_dict for
information on the load_dict command.

Buffers and Filters
The shell uses the concept of a buffer to store a string of text, which most comands read
from as input, and write to as output. Unless run with the -n option, cifer will
automatically create 10 buffers at startup. Buffers are referred to in the form,
buffer_#, where # is substituted with the buffer's index number. For more information on
buffers, see the usage for: buffers, resize, clear, copy, load, write, read, bufferinfo,
and nullbuffer.

Filters can be used to manipulate the set of characters in a buffer, for example making
all characters uppercase, or removing all whitespace. For more information on filters, see
the usage of filter.

Some of cifer's functions require a specially formatted 'dictionary', which takes the
basic form of a list of words. The utility cifer-dict(1) can be used to create these
dictionaries. The loaddict command is used to load a dictionary for use.

Frequency Analysis
Frequency analysis is the study of the frequency of symbols, or groups of symbols in a
ciphertext. It aids in cracking monoalphabetic substition schemes. Frequency analysis
works upon the principle that, in any given sample of written language, certain characters
and groups of characters will occur more often than others. Furthermore, the distribution
of those frequencies will be roughly the same for all samples of that written language.
For instance, in any section of English language, the character 'E' appears far more often
than 'X'. Likewise, the pair of letters 'TH' is very common, whilst 'XY' is very rare. In
monoalphabetic substitution schemes, these patterns are preserved and it is possible to
determine certain mapppings of letters from ciphertext‐>plaintext from the frequencies
alone. As more and more characters are converted, it becomes easy to guess the remaining
ones to form words in the target language.

Perhaps the most tedious part of this method is the actual counting of the symbols
themselves. Thus, Cifer provides functions to count characters, digrams (pairs of
characters), and trigrams (triplets of characters). It can also use frequency analyis to
guess ciphertext‐>plaintext mappings for the English language. For more information, see
the usage for: frequency_guesses, identity_frequency_graph, frequency_analysis,
count_digrams, and count_trigrams.

Affine Ciphers
An affine cipher is a type of monoalphabetic substitution cipher. In order to implement an
affine cipher, one would assign each character of the chosen alphabet a number, for
example, a = 0; b = 1; c = 2; etc. Then for each letter of the plaintext, put it through
the encryption function:

e(x) = (ax + b) (mod m)

Where x is the plaintext character's assigned number, a and m are coprime and m is the
size of the alphabet. The ciphertext character for this plaintext character is the
character assigned to the number e(x).

Cifer provides functions to both encrypt and decrypt affine ciphers as well as crack
affine ciphers using frequency analysis or brute force. Note that cifer is currently only
able to deal with affine ciphers where m = 26. For more information, see the usage for:
affinesolve, affinebf, affineencode, affinedecode, and mmi.

Vigenere Ciphers
The Vigenere cipher is a form of polyalphabetic substitution consisting of several Caesar
ciphers in sequence with differing shift values, which vary according to a repeating
keyword. Cifer provides the function vigenere_crack, which employs a brute‐force (for each
possible keyword length) frequency analysis method in order to find the keyword, and crack
the cipher.

Keyword Ciphers
A keyword cipher is a type of monoalphabetic substitution in which the mapping of
plaintext characters to ciphertext characters is affected by the inclusion of a 'keyword'.
Cifer provides the function keyword_bruteforce which attempts to find the correct keyword
by going through a 'dictionary' of possible words and trying each one in turn, then
selecting the best solution by matching words in the solution to those in the dictionary.
If the keyword to a ciphertext is already known, it can be decoded using the
keyword_decode command.

Bacon Ciphers
A bacon cipher is a method of stenography, in which a message is concealed in the
presentation of text, rather than its content. The ciphertext consists of any message
(again, the language has no impact on the concealed plaintext) in which each character can
be categorised into one of two distinct groups, we call these 'A' and 'B'. This
distinction may be made in any number of predetermined ways, such as two typefaces, or
other indicators. In order to decode the cipher one replaces groups of 5 As and Bs with
their corresponding plaintext character, as dictated by the Baconian alphabet (however, be
aware that it would be trivial for the two communicating parties to create their own
'custom' version of the Baconian alphabet). To encode a plaintext, the reverse operation
is performed.

A Bacon cipher can be easily encoded/decoded, and cifer provides the functions
bacon_encode and bacon_decode to achieve this. They use a buffer of As and Bs as input and
output, and thus any ciphertext that needs to be decoded must first be turned into As and
Bs. Before the plaintext is loaded, it should be modified so that upper and lower case
characters belong to the A and B groups, respectively. Then, the casebacon filter can be
applied to convert the upper and lower case characters in the buffer to As and Bs. There
is also a bacon filter, which removes all characters which are not 'A' or 'B'.

Rail Fence Ciphers
The rail fence cipher is a form of transposition cipher, which gets its name from the way
the plaintext is written alternatively downwards and upwards diagonally on 'rails', before
being read off as the ciphertext in rows.

Cifer provides the function rfbf to crack rail fence ciphers using a brute force method
and checking for solutions using a dictionary.

Columnar Transposition
Columnar transposition is a relatively complex form of cipher, with many variants. The
basic process of encoding using this method involves first writing the plaintext out in a
table defined by its width (which is also the length of the keyword). Then, depending on
the variant, the ciphertext is written and read out of the table in any number of
different ways.

The keyword can be specified in numeric or alphabetic form. In the former, each digit must
only be used once and there must be enough digits to form a full key (ie. for a key length
4, all the digits [0,1,2,3] must be used). An alphabetic keyword, such as 'apple', first
has duplicate letters removed. This gives us 'aple'. If you were encrypting by hand, you
would write out 'aple' at the top of your table, and them move the columns around until
the keyword is in alphabetic order, ie. 'aelp'.

In order to decrypt a ciphertext, we first 'flip' the keyword, turning 'aelp' into 'plea'.
We can then use this keyword as if we were encrypting, and the process will reverse the
original function to give us the plaintext.

Cifer's keyword functions provide utilities to automate many variants. There are nine
commands: c2c_encode, c2c_decode, c2c_bruteforce, r2c_encode, r2c_decode, r2c_bruteforce,
c2r_encode, c2r_decode and c2r_bruteforce.

The first three letters of each command are short for: 'column to column', ´column to row'
and 'row to column'; these refer to different ways in which the ciphertext can be read off
the table. In c2c, the table is written from left to right, re‐ordered and read off left
to right again. In r2c, the table is written from top to bottom, re‐ordered and then read
off from left to right. Finally, in c2r the table is written left to right, re‐ordered and
read off from top to bottom.

´Encode' and 'decode' mode both take a keyword and work as one would expect. In
'bruteforce' mode, cifer tries all permutations of increasing key lengths in an attempt to
find the real keyword. It tests possible solutions by matching words in the dictionary.


-n Disable auto-init.

-f Execute the commands in the (script) file specified, then exit

-i Execute the script file and then go to interactive mode

-q Do not fully parse file before execution

-s Exit on soft-fails, not just hard-fails (for script execution)

Any text found after the options will be interpreted as a command to the shell; Please
note that you cannot specify a command if either -i or -f are used, and that -q and -s
only apply to -f or -i.

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