Trifid Cipher

In Trifid Cipher, encoding is done by using a table to fractionate each plaintext letter into a trigram, mixes the constituents of the trigrams, and then applies the table in reverse to turn these mixed trigrams into ciphertext letters.

Create a function that takes three arguments; a 27 letter key, a break period, and a string containing the message to be encoded.

There are some variations on the rules of encipherment. One version of the cipher rules are outlined below:

key = "EPSDUCVWYM+ZLKXNBTFGORIJHAQ"
period = 5
message = "DEFENDTHEEASTWALLOFTHECASTLE+"

trifid_cipher(key, period, message) ➞ "SUEFECPHSEGYYJIXIMFOFOCEJLBSP"

Step 1: Divide the key in the form of three squares:

Square 1   Square 2   Square 3

  1 2 3      1 2 3      1 2 3
1 E P S    1 M + Z    1 F G O
2 D U C    2 L K X    2 R I J
3 V W Y    3 N B T    3 H A Q

Step 2: Locate the plaintext letters in the squares above, D is in square 1, row 2, column 1, so D becomes 121. Similarly, E becomes 111 and so on.

Write down the numbers corresponding to each letter vertically:

D E F E N D T H E E A S T W A L L O F T H E C A S T L E +
1 1 3 1 2 1 2 3 1 1 3 1 2 1 3 2 2 3 3 2 3 1 1 3 1 2 2 1 2
2 1 1 1 3 2 3 3 1 1 3 1 3 3 3 2 2 1 1 3 3 1 2 3 1 3 2 1 1
1 1 1 1 1 1 3 1 1 1 2 3 3 2 2 1 1 3 1 3 1 1 3 2 3 3 1 1 2

Step 3: At the moment, it is still a substitution cipher and fairly easy to break. Now, you have to use period, which is usually a number between 5 - 20.

D E F E N  D T H E E  A S T W A  L L O F T  H E C A S  T L E +
1 1 3 1 2  1 2 3 1 1  3 1 2 1 3  2 2 3 3 2  3 1 1 3 1  2 2 1 2
2 1 1 1 3  2 3 3 1 1  3 1 3 3 3  2 2 1 1 3  3 1 2 3 1  3 2 1 1
1 1 1 1 1  1 3 1 1 1  2 3 3 2 2  1 1 3 1 3  1 1 3 2 3  3 1 1 2

Step 4: Group the numbers and read off the numbers in each group horizontally, and do the substitution back to the letters using the original key-squares.

113 122 111 311 111  123 112 331 113 111  312 133 133 323 322
S   U   E   F   E    C   P   H   S   E    G   Y   Y   J   I

223 322 211 311 313  311 313 123 111 323   221 232 113 112
X   I   M   F   O    F   O   C   E   J     L   B   S   P

Step 5: Return the final result:

eMessage = "SUEFECPHSEGYYJIXIMFOFOCEJLBSP"

See the below examples for a better understanding:

Examples

key="EPSDUCVWYM+ZLKXNBTFGORIJHAQ"

trifid_cipher(key, 5, "DEFENDTHEEASTWALLOFTHECASTLE+") ➞ "SUEFECPHSEGYYJIXIMFOFOCEJLBSP"

trifid_cipher(key, 15, "MUBASHIR") ➞ "+OHTW+XD"

trifid_cipher(key, 6, "ABCDEFG") ➞ "RSAMXEG"

Notes

• The key, data, and output will all contain exclusively valid uppercase letters and a plus sign(+) as the 27th letter.
• Keys will always be a complete 27-letter alphabet with no repeats, and periods will always be positive integers.