CSC 233

We talked about basic terminology in cryptography (including a discussion of Kerkhoff's principles), the Caesar cipher, and how to break it by hand, and automatically (using frequency analsyis); we then moved on to general monoalphabetic substitution ciphers, a subject we'll discuss in more depth next week. We saw some of the early, Arabic history, of this type of cipher.

**Submission**:
The homework is due by midnight (I will not accept late homeworks). You can
submit your homework through d2l
into the drop-box for this homework.

Please prepare your homework as a *single file* containing all answers
(e.g. doc, docx, or pdf,
not a zip file). For an example, see hwexample.docx
. How to take screenshots? Check out screenshots for
MAC,
Windows,
Linux.

1. (Reading Assignment) Read sections 2.1-2.6 of the textbook. We'll continue with chapter 2 next time, so if you want to read ahead, check out 2.7-2.17 (though we won't cover all of those topics).

2. (Shift Cipher, 24pt) You'll be breaking several shift ciphers in this
problem. You'll be allowed different tools for the different texts, ranging from
a *by hand *solution, to an automated solution. For each problem,
determine the original plaintext, and the shift used to encrypt it.

a) [8pt] Solve this problem by hand (you can write the solution in a texteditor, but don't use alphabet-strips to shift around, that'll be part b). Include a screenshot of your solution.

TACPQ PCHLW VTYR

OJCJK ZRVNO JZSKZ XO

*Note*: you don't need a strip for every letter, once
you have the shift you can decrypt by group. So about 5 strips should be
enough.

c) [8pt] Use the Caesar cipher spreadsheet we saw in class (available on d2l) to find the correct solution. Fill in the ciphertext, hit "analyze ciphertext", and then try all different shifts, looking for the shift that's closest to English. Include a screenshot (just the relevant parts showing the shift and the sum).

YFNHL ZTBTF DVKYV IVRJF EJWFI RGGIF MZEXN YRKNV CZB

*Note*: You need to enable macros for the spreadsheet
to work. If you cannot run the spreadsheet on your computer or on a lab
computer, use the method from b) instead.

3.
(Transposition
or Substitution, 16) You intercepted the four ciphertexts a) through d) below;
you know that the original plaintexts are in English and that either a
substitution cipher (such as the shift cipher, or the monoalphabetic
substitution cipher) or a transposition cipher (such as the skytale) was used. For each ciphertext, determine whether it
is the result of a substitution cipher or a transposition cipher **and**
explain how you reached your decision. You **do not** have to find out
which particular cryptosystem was used in each case (we haven't seen
most
the four systems yet); in particular, you do not have to break the ciphers.

a) NLNNJ PUPRN UDCCX JPFEF JVKPP FUXUR PLNUJ PVCEU VFRVT UIRVI EFUJV TCVSU VJUVT UJPCD EUVTC VPJIU NROXU

b) SNTOH OSICH TEOUD EEEPL REIIN LIPRS NHESR AOCAA HIBFO NSTSL TNCUH EEEVE RISNE DENAC NAIGR OINSS ENEDN TF

c) GGDGA GFAAF FDFDF FAFFA DAFGG GADAG FGAAA FDFDG AAFAF FFDDA GAADA DDFGD GAAAD AFGDF GAAAD FAFAA DFFGF AGDFF FDVFV VXVFX GADAA FAFXD DXDFV VVFDG DFAAF DADGD ADDAF DFGVG XFDFX DVXFA DDFAA DFDVG XAVVA DFXFA VVDX

d) UEALS GTAUO EEAIC INFEI ESSIS STDNE TDPIE OTIEL ESHND HNEES CNALR SROHN HERNN CNEON CONSR ERFHP IATVI OB

*Hint*: the frequency analysis tool in the Caesar
cipher spreadsheet (on d2l)
will be useful. If you can't run that, you can also try this
frequency analsyis tool.