Cryptography and Computer Security - Exercise 1
Subject: Classical Encryption
Submission deadline: 27/10/2010, 16:00


Submission is via EMAIL ONLY - Read the instructions!

1. Break the following cipher. It is a mono-alphabetic substitution cipher, in the English language. Hint: compute the letter frequencies.

   JXAJQHQXQHPI PNTE B JHIROT OTQQTE - JHDVOT JXAJQHQXQHPI - GBI AT
   
   
   LTDPIJQEBQTL AS FEHQHIR PXQ QCT BOVCBATQ HI JPDT PELTE QP ETVETJTIQ
   
   
   QCT JXAJQHQXQHPI. QCHJ HJ QTEDTL B JXAJQHQXQHPI BOVCBATQ. QCT GHVCTE
   
   
   BOVCBATQ DBS AT JCHZQTL PE ETNTEJTL (GETBQHIR QCT GBTJBE BIL BQABJC
   
   
   GHVCTEJ, ETJVTGQHNTOS) PE JGEBDAOTL HI B DPET GPDVOTK ZBJCHPI, HI
   
   
   FCHGC GBJT HQ HJ GBOOTL B DHKTL BOVCBATQ PE LTEBIRTL
   
   
   BOVCBATQ. QEBLHQHPIBOOS, DHKTL BOVCBATQJ BET GETBQTL AS ZHEJQ FEHQHIR
   
   
   PXQ B UTSFPEL, ETDPNHIR ETVTBQTL OTQQTEJ HI HQ, QCTI FEHQHIR BOO QCT
   
   
   ETDBHIHIR OTQQTEJ HI QCT BOVCBATQ.
   

2. A common method of generating mono-alphabetic substitution ciphers is by using a password, as follows. Write the password horizontally, ommitting any duplicate appearance of letters. Then write under the password all the remaining letters of the alphabet. Read off the substitution alphabet vertically. E.g., if the password is FOOTBALL:

     FOTBAL
     CDEGHI
     JKMNPQ
     RSUVWX
     YZ
   

   and the resulting substitition is

    Plaintex letters:    abcdefghijklmnopqrstuvwxyz
    Ciphertext letters:  FCJRYODKSZTEMUBGNVAHPWLIQX
   

   The cipher of question 1 was generated this way. What was the password? Hint: write the substitution alphabet you broke in q.1 ordered by the plaintext letters, then look for sequences of consecutive letters in "hops". E.g., in the example above the letters C,D,E appear in positions 2, 7, 12, with a hop of 5.

3. Encrypt your name using the cipher you broke in q.1.

Submission Instructions

1. Send your results via email to crypto-netsec@eng.tau.ac.il. That's a dash ("-"), NOT an underscore ("_").
2. The subject should be: ex1
3. The body of the email should contain 4 lines, including the leading keywords and the ":=" symbols:

      TZ  := your "Teudat Zehut" number (9 digits)
      Q1  := the last line of decoded text from question 1 here ...
      Q2  := the password you broke in question 2 here ...
      Q3  := your name, encrypted as in question 3.
   

4. Send plain ASCII email. In particular:
   1. No attachments
   2. No HTML email: configure your mail program to generate "unformatted" or "Plain text" messages only (not "Rich formatting")
   3. Be extra careful with Outlook which by default sends the text encapsulated in an attachment called "winmail.dat". A recipe on how to fix this can be found in http://facstaff.gpc.edu/~jbenson/resource/winmail.htm (scroll all the way down)
   4. When in doubt, use a Unix text-based mailer like "mail" or "pine".