SFL Prof. Dr. C. Rossow / S. Hausotte TU Dortmund WS 2021/2022 Exercise 1 (Symmetric Cryptography)
This sentence has been encoded using a caesar cipher. Can you find out the original message?
N pelcgbtencuvp flfgrz fubhyq or frpher rira vs rirelguvat nobhg gur flfgrz,
rkprcg gur xrl, vf choyvp xabjyrqtr.
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(a) Decode the message and explain how you figured out the key.
(b) Take a look at the contents of the plain text message. What does it say and why is it important?
1.2 Substitution Cipher
The following (english) text has been encrypted using a substitution cipher where every letter of the alphabet is mapped to an arbitrary but fixed other letter. Every non-letter symbol remains the same.
hun cninxh znlnybqgnxh bw lscrbjd gnhubzd bw gbzjyshrbx djiu sd qig sxz qqg
turiu nfiusxkn asxztrzhu wbc drkxsy-hb-xbrdn cshrb usd rxhnxdrwrnz hun rxhncndh
rx s knxncsy hunbcv bw ibggjxrishrbx. s asdrd wbc djiu s hunbcv rd ibxhsrxnz
rx hun rgqbchsxh qsqncd bw xvejrdh1 sxz uschynv2 bx hurd djapnih. rx hun
qcndnxh qsqnc tn tryy nfhnxz hun hunbcv hb rxiyjzn s xjganc bw xnt wsihbcd, rx
qschrijysc hun nwwnih bw xbrdn rx hun iusxxny, sxz hun dslrxkd qbddrayn zjn hb
hun dhshrdhrisy dhcjihjcn bw hun bcrkrxsy gnddskn sxz zjn hb hun xshjcn bw hun
wrxsy zndhrxshrbx bw hun rxwbcgshrbx. hun wjxzsgnxhsy qcbayng bw ibggjxrishrbx
rd hush bw cnqcbzjirxk sh bxn qbrxh nrhunc nfsihyv bc sqqcbfrgshnyv s gnddskn
dnynihnz sh sxbhunc qbrxh. wcnejnxhyv hun gnddsknd usln gnsxrxk; hush rd hunv
cnwnc hb bc scn ibccnyshnz siibczrxk hb dbgn dvdhng trhu inchsrx quvdrisy bc
ibxinqhjsy nxhrhrnd. hundn dngsxhri sdqnihd bw ibggjxrishrbx scn rccnynlsxh hb
hun nxkrxnncrxk qcbayng. hun drkxrwrisxh sdqnih rd hush hun sihjsy gnddskn rd bxn
dnynihnz wcbg s dnh bw qbddrayn gnddsknd. hun dvdhng gjdh an zndrkxnz hb bqncshn
wbc nsiu qbddrayn dnynihrbx, xbh pjdh hun bxn turiu tryy sihjsyyv an iubdnx drxin
hurd rd jxoxbtx sh hun hrgn bw zndrkx. rw hun xjganc bw gnddsknd rx hun dnh rd
wrxrhn hunx hurd xjganc bc sxv gbxbhbxri wjxihrbx bw hurd xjganc isx an cnkscznz
sd s gnsdjcn bw hun rxwbcgshrbx qcbzjinz tunx bxn gnddskn rd iubdnx wcbg hun dnh,
syy iubrind anrxk nejsyyv yronyv. sd tsd qbrxhnz bjh av uschynv hun gbdh xshjcsy
iubrin rd hun ybkscrhugri wjxihrbx. syhubjku hurd znwrxrhrbx gjdh an knxncsyrmnz
ibxdrzncsayv tunx tn ibxdrznc hun rxwyjnxin bw hun dhshrdhrid bw hun gnddskn
sxz tunx tn usln s ibxhrxjbjd csxkn bw gnddsknd, tn tryy rx syy isdnd jdn sx
nddnxhrsyyv ybkscrhugri gnsdjcn.
(a) Why is it harder to break this cipher compared to the one from task 1? Explain why such a cipher is still easy to break and briefly describe how an attack could possibly look like in this scenario.
(b) Perform the attack you chose in a and try to decrypt the given text. Who is the author of the text?
1.3 Quick Questions (Mixed Topics)
(a) One-Time-Pad is a perfect cipher (if applied correctly). What does it mean for a cipher to be perfect and why is it rarely used?
(b) The BedenkenSecond GmbH suggests to implement OTP by using a 256 bit value as a seed for a pseudo random number generator and only share the seed among the encrypting parties. How do you rate the security of this procedure?
SFL Prof. Dr. C. Rossow / S. Hausotte TU Dortmund WS 2021/2022 Exercise 1 (Symmetric Cryptography)
(c) Assume the SFL instructors have met every single student in person to exchange a personalized, random and non repetitive keys with everyone. After the exam, these keys are used to inform everyone about the outcome of the exam (passed or failed). You are able to intercept the following messages:
Exam ID 001
Outcome 0010110100001000 01111101100001 00101110000001 0101011011001101 0000101110110110 11110110101101 0010110110111010
Your own exam has the ID 007 and you have passed the exam. Which information can you gain?
(d) The BedenkenSecond GmbH makes use of AES to provide digital vaccination certificates by encrypting the name and vaccination status of their customers and handing out certificates which contain the cipher text. The company reaches out to you and asks you whether they should keep their encryption key private or make it public in order for their system to work. What’s your answer?
(e) In literature, you often find the distinction between key length and effective key length. Explain the difference using a block cipher with 2 bit blocks and various lengths of keys. You can assume ECB mode if this makes it easier to explain.
(f) Assume you found a security vulnerability in a software product. In which way would you handle the situation in order to comply with §202 StGB?
1.4 Cipher Modes
(a) CBC uses Fk(x) for encryption and Fk(x)−1 for decryption. Could we swap the order and use Fk(x)−1 for encryption and Fk(x) for decryption?
(b) Your classmate suggests to fix weaknesses of ECB and invents a CBC-like encryption mode. He defines the encryption as follows: ci = enc(mi) = Fk(mi) ⊕ Fk(mi−1). For i = 0, an IV is used instead of Fk(mi−1).
How would decryption dec(ci) look like, assuming an inverse function F−1? k
Assume m1 equals m3. Can you spot this based on the ciphertexts (and IV)? Would you prefer this mode over ECB? If so, why? If not, why not?
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