COMFIC- Substitution ciphers

Transposition and more complex Substitution Ciphers

Dictionary Attacks on substitution ciphers

Would scan dictionary for all candidate words in  each cipher text

Record number of times E maps to A, B, C etc and then pick and replace most popular with cipher texts

XOR ciphers

A	B	Outcome
0	0	0
0	1	1
1	0	1
1	1	1

SO ..

110101110

XOR

011011010

Is

101110100

XOR Cipher

It is essentially, encrypting the text with something else to create the cipher text

Solving

Use similar tactic to Vignere

Find the key length- index of coincidence

Crack key using freq analaysis

Transposition

Involves moving position of plaintext letters

Difficult to spot patterns unless knowing the transposition method

May be worth using python to try grid sizes to look for patterns

Vignere Example time!

Cipher Text:

PVPP WFRI YFV WHCZMGX XLBJ. MX PRWR'M JYTIFWIW KS FX KSS ARVH, ULX WMZPP T TLEECIRZV. ERRNEC, MYI FNIMIW KVITJYVX ZW LBUHIG UMVXTXPR SILBEH SNI YWNRP QXVXMGX WTHK, XLX TEJX. Z EWLLQI RFY OGFA XKVEWNII ML FYV VFHI PFVH. GFFSWP AMEC IZXI HIVICTM KLML RRCPRC, WH Z QMZYX EL NIPE KIPE PSY B CMOX KS ATKGL VYMGD WPMVBW. FR KLI PRC, AX'II FXZRK YFPPHNIH.

Putting this in to a index of coincidence showed 4 to have the highest result. Therefore we will assume that the Key is 4 letters.

Using a vignere frequency calculator, we are told the first letter for frequency is X

Knowing that E is the most comman we look up E and X in a vignere Table

Image from http://pages.mtu.edu/~shene/NSF-4/Tutorial/VIG/Vig-Base.html

So, you go from the column of E down to X, and the big letter to the left T, is the first letter of the Lets repeat but change the key letter to 1

Its not coming up so easily. This is fine as not every cipher comes up perfectly.

Now, key letters 2 and 3 both showed I as the most frequent. So we have the following letters:

1)T

2)

3)E

4)E

Now this sounds like the word Tree may be a hit. So let’s try TREE:

You should get the following;

“WELL DONE FOR SOLVING THIS. IT WASN'T SUPPOSED TO BE TOO HARD, BUT STILL A CHALLENGE. ANYWAY, THE BURIED TREASURE IS HIDDEN DIRECTLY BEHIND OUR USUAL MEETING SPOT, THE CAFE. I ASSUME YOU KNOW TREASURE IS OUR CODE WORD. NOBODY WILL EVER DECRYPT THIS ANYWAY, SO I MIGHT AS WELL TELL YOU I LIKE TO WATCH CHICK FLICKS. BY THE WAY, WE'RE BEING FOLLOWED.”