- WELCOME TO A LESSON ON
TRANSPOSITION CIPHERS. ONE APPROACH TO CRYPTOGRAPHY
IS USE OF A TRANSPOSITION CIPHER WHERE A TRANSPOSITION CIPHER
IS ONE IN WHICH THE ORDER THE CHARACTERS IS
CHANGED TO OBSCURE THE MESSAGE. AN EARLY VERSION
OF A TRANSPOSITION CIPHER WAS AN -- IN WHICH PAPER
WAS WRAPPED AROUND A STICK AND THE MESSAGE WAS WRITTEN. ONCE UNWRAPPED, THE MESSAGE
WOULD BE UNREADABLE UNTIL THE MESSAGE WAS WRAPPED AROUND A SIMILAR SIZED STICK
AGAIN. ONE MODERN TRANSPOSITION CIPHER IS DONE BY WRITING
THE MESSAGE IN ROWS, THEN FORMING THE ENCRYPTED
MESSAGE FROM THE TEXT IN COLUMNS. AS AN EXAMPLE,
LET'S ENCRYPT THE MESSAGE, "MEET AT 3:00 P.M.
TODAY AT THE USUAL LOCATION" USING ROWS OF SIX CHARACTERS. SO IF EACH ROW HAS SIX
CHARACTERS, THAT MEANS WE SHOULD FORM
A TABLE WITH SIX COLUMNS, ONE, TWO, THREE, FOUR,
FIVE, SIX. NOTICE HOW EACH ROW
WILL HAVE SIX CHARACTERS. NEXT WE'D WRITE THE MESSAGE
FROM LEFT TO RIGHT COMPLETING EACH ROW. NOTICE HERE WE HAVE MEET
AT 3:00 P.M. TODAY AT THE USUAL LOCATION. AND NOW TO ENCRYPT THE MESSAGE, WE'D WRITE THE CHARACTERS
USING THE COLUMNS. SO USING THE COLUMNS, THE ENCRYPTED MESSAGE OR ENCODED
MESSAGE WOULD BE M, T, M, "A,", S, C, WHICH WE
SEE HERE, E, H, T, T, U, "A," WE SEE HERE, E, R, O T, "A,"
T HERE, T, E, D, H, L, "I" HERE "A," E, "A," E, L, O,
AND FINALLY T, P, Y, U, O, N. BUT THERE IS ONE MORE THING. THE SPACES WOULD BE REMOVED
OR REPOSITIONED TO HIDE THE SIZE OF THE TABLE
USED SINCE THAT IS THE ENCRYPTION KEY
IN THIS MESSAGE. SO WE WOULDN'T LEAVE IT
IN THIS FORM HERE BECAUSE IF LEFT IN THIS FORM, IT COULD TELL THE MESSAGE
WAS ENCRYPTED USING ROWS OF SIX CHARACTERS. LET'S TAKE A LOOK
AT ANOTHER EXAMPLE, LET'S DECRYPT THE MESSAGE GIVEN
HERE IF IT WAS ENCRYPTED USING A TABULAR TRANSPOSITION
CIPHER WITH ROWS OF LINK FOUR. SO BECAUSE WE KNOW IT WAS FORMED WITH ROWS OF LINK
FOUR CHARACTERS, WE'D FORM A TABLE WITH FOUR
COLUMNS WHICH WE SEE HERE BUT BEFORE WE DECRYPT
THE MESSAGE, WE DO HAVE TO KNOW HOW MANY ROWS
THERE WERE. SO WE'LL FIND THE TOTAL NUMBER
OF CHARACTERS WHICH IS 3 x 10 + THESE EXTRA
TWO ARE 32 CHARACTERS. WE'RE GOING TO DIVIDE BY THE
NUMBER OF CHARACTERS PER ROW WHICH IS FOUR,
32 DIVIDED BY 4 = 8 WHICH MEANS WE NOW KNOW
WE'LL HAVE EIGHT ROWS. SO, ONE, TWO, THREE, FOUR, FIVE,
SIX, SEVEN, EIGHT. SO WHEN DECRYPTING A MESSAGE
USING THIS METHOD, IT IS IMPORTANT TO ALWAYS FIND
OUT HOW MANY ROWS WE HAVE AND NOW BECAUSE THIS WAS FORMED BY READING THE CHARACTERS OFF
IN COLUMNS, WE'LL WRITE THESE DOWN IN COLUMN
ONE, TWO, THREE AND FOUR. TO HELP US DO THIS, LET'S COUNT
OFF EIGHT CHARACTERS, SO WE HAVE THREE, SIX, SEVEN,
EIGHT, FOUR, SEVEN, EIGHT, TWO, FIVE, EIGHT,
AND THEN EIGHT AGAIN. SO THE FIRST COLUMN WILL CONSIST
OF THESE CHARACTERS. THESE WILL BE IN THE SECOND
COLUMN, THESE WILL BE IN THIRD
AND SO ON. HERE WE HAVE "A," E, S, E, "A,"
O, O, "I." HERE WE HAVE T, V, U, "I," N, N,
N, N, FOLLOWED BY E, E, R, L, C, F, T,
E AND FINALLY
L, N, V, L, E, R, L, S. NOW TO DECRYPT THIS, WE'LL READ
THIS OFF GOING ROW BY ROW. SO THE ORIGINAL MESSAGE WAS AT
11, SURVEILLANCE ON FRONT LINES. AGAIN THE MESSAGE WAS AT 11,
SURVEILLANCE ON FRONT LINES. MORE COMPLEX VERSIONS
OF THIS ROWS AND COLUMN BASED TRANSPOSITION CIPHER
CAN BE CREATED BY SPECIFYING AN ORDER IN WHICH
THE COLUMNS SHOULD BE RECORDED. FOR EXAMPLE THE METHOD
COULD SPECIFY THAT AFTER WRITING THE MESSAGE
OUT IN ROWS, THAT YOU SHOULD RECORD THE THIRD
COLUMN THEN THE FOURTH COLUMN, THEN THE FIRST, THEN THE SECOND. THIS ADDS ADDITIONAL COMPLEXITY
THAT WOULD MAKE IT HARDER TO MAKE IT A BRUTE FORCE ATTACK. TO MAKE THE ENCRYPTION KEY
EASIER TO REMEMBER, A WORD COULD BE USED. FOR EXAMPLE,
IF THE KEY WORD WAS MATH, IT WOULD SPECIFY THAT ROWS
SHOULD HAVE FOUR CHARACTERS EACH BECAUSE THERE IS FOUR LETTERS
IN THE WORD. THE ORDER OF THE LETTERS
IN THE ALPHABET WOULD DICTATE WHICH ORDER TO READ
THE COLUMNS IN. SINCE "A" THE SECOND LETTER
IN THE WORD, IS THE EARLIEST LETTER IN THE
ALPHABET FROM THE WORD MATH, THE SECOND COLUMN
WOULD BE USED FIRST FOLLOWED BY THE FOURTH COLUMN
BECAUSE THAT'S H, THEN THE FIRST COLUMN
BECAUSE OF THE M, THEN THE LAST COLUMN
WOULD BE THE THIRD COLUMN BECAUSE OF THE T. LET'S TAKE A LOOK
AT TWO MORE EXAMPLES. WE WANT TO ENCRYPT THE MESSAGE
GIVEN HERE USING A TABULAR TRANSPOSITION
CIPHER WITH ENCRYPTION KEY MAINE, IF NECESSARY PAD THE MESSAGE
WITH "A'S." NOTICE HERE THERE
ARE FIVE LETTERS IN MAINE, SO WE BEGIN BY MAKING ROWS
OF FIVE CHARACTERS SO WE'LL HAVE FIVE COLUMNS, ONE,
TWO, THREE, FOUR, FIVE. NOW WE'LL WRITE THE MESSAGE OUT
IN ROWS. SO WE HAVE AT 4, SURVEILLANCE
ON ENEMY CAMP, WE HAVE ONE EXTRA CHARACTER HERE
WE FILL WITH "A" THAT WAY EACH ROW HAS EXACTLY
FIVE CHARACTERS. BUT NOW WE'RE NOT GOING TO READ
THE ROWS OFF FROM ROW ONE TO ROW FIVE, WE'RE GOING TO BASE IT UPON
THE LETTERS IN THE WORD MAINE. IF WE ORDER THESE LETTERS
ALPHABETICALLY NOTICE "A"
WOULD BE THE FIRST LETTER WHICH REPRESENTS COLUMN TWO SO
WE'LL READ OFF COLUMN TWO FIRST. THE NEXT LETTER WOULD BE E,
THE FIFTH LETTER, SO WE'LL READ OFF COLUMN FIVE
NEXT. THE NEXT LETTER IN THE ALPHABET
WOULD BE "I" WHICH IS THE THIRD LETTER, SO WE'LL READ OFF COLUMN THREE
FOLLOWED BY THE LETTER M WHICH REPRESENTS THE FIRST
COLUMN SO THE FIRST COLUMN WOULD BE READ OFF FOURTH AND THEN FINALLY N IS THE LAST
LETTER ALPHABETICALLY SO THE LAST COLUMN
WILL BE COLUMN FOUR. SO WE'LL FIRST READ OFF COLUMN
TWO, THIS COLUMN HERE, SO OUR ENCRYPTED MESSAGE
WILL BE T, S, "I," C, N, "A," NEXT WE'LL READ OFF COLUMN FIVE, SO WE'LL HAVE U, V, "A," N, Y,
"A," NEXT COLUMN THREE, SO WE HAVE F, U, L, E, E, M,
THEN COLUMN ONE SO WE HAVE "A," R, E, N, E, C
AND FINALLY COLUMN FOUR. SO WE HAVE O, R, L, O, M, P. AND AGAIN WE,
AGAIN AS OUR LAST STEP, WE SHOULD REMOVE THE SPACES, REGROUP THESE LETTERS
DIFFERENTLY NOT TO GIVE AWAY HOW THIS WAS ENCRYPTED. AS OUR LAST EXAMPLE, LET'S
DECRYPT THE MESSAGE GIVEN HERE IF IT WAS ENCRYPTED USING A
TABULAR TRANSPOSITION CIPHER WITH ENCRYPTION KEY PLAN. SO THIS TELLS US THAT WE HAVE
FOUR CHARACTERS PER ROW, WE HAVE HERE,
SO WE HAVE FOUR COLUMNS. NEXT WE HAVE TO DETERMINE HOW
MANY ROWS WE WOULD HAVE BASED UPON
THE NUMBER OF CHARACTERS WHICH IS 20, SO WE'LL TAKE 20, DIVIDE BY THE NUMBER
OF CHARACTERS PER ROW WHICH IS FOUR,
AND THEREFORE WE CAN SEE WE'LL HAVE A TOTAL OF FIVE ROWS
EACH CONTAINING FOUR CHARACTERS. SO, ONE, TWO, THREE, FOUR, FIVE AND NOW WE'RE GOING TO COMPLETE
THE COLUMNS USING THESE CHARACTERS BASED
UPON THE LETTERS IN PLAN. SO ALPHABETICALLY "A"
WOULD COME FIRST, SO WE'LL COMPLETE COLUMN
3 FIRST. NEXT WOULD BE THE LETTER L
SO WE'LL COMPLETE COLUMN TWO, NEXT WOULD BE N,
SO WE'LL COMPLETE COLUMN FOUR, AND THEN FINALLY WE'LL COMPLETE
THE FIRST COLUMN FOR P LAST. AND AGAIN BECAUSE EACH COLUMN
HAS FIVE CHARACTERS, LET'S BLOCK THIS INTO FIVES SO WE'D HAVE FIRST FIVE
CHARACTERS HERE, THE NEXT FIVE,
THE NEXT FIVE AND THE LAST FIVE. THE FIRST COLUMN WE'LL COMPLETE WILL BE COLUMN THREE
BECAUSE OF THE "A" SO NOTICE HOW WE HAVE N, "A," C,
S, M IN COLUMN THREE. THE SECOND COLUMN WE'LL COMPLETE
WILL BE COLUMN TWO BECAUSE OF THE L, SO WE'LL HAVE
T, N, "A," "A," "A" WHICH WE SEE HERE. THE THIRD WILL BE THE FOURTH
COLUMN BECAUSE OF THE N, WE'LL HAVE O, T, K, E, P AND THEN FOURTH
WILL BE THE FIRST COLUMN BECAUSE OF THE P
WHICH WILL BE "A," O, T, B, C. AND NOW WE'LL USE THE ROWS
TO DECRYPT THE MESSAGE. THE ORIGINAL MESSAGE
WAS AT NOON, ATTACK BASE CAMP. UNFORTUNATELY,
SINCE THE TRANSPOSITION CIPHER DOES NOT CHANGE THE FREQUENCY
OF INDIVIDUAL LETTERS, IT IS STILL SUSCEPTIBLE
TO FREQUENTLY ANALYSIS THOUGH THE TRANSPOSITION
DOES ELIMINATE INFORMATION FROM LETTER PAIRS. I HOPE YOU FOUND
THIS LESSON HELPFUL.
