Tutorial Activity 3

# Tutorial Activity 3 - MH 8300 Tutorial Activity 3 Caesar...

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MH 8300 Tutorial Activity 3 Caesar Cipher, Affine Cipher and Frequency Analysis Set A Group Members Name NTU Email ID Marks : /2

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MH 8300 Tutorial Activity 3 Caesar Cipher, Affine Cipher and Frequency Analysis Set A Question 1. (Frequency Analysis of the Caesar Cipher) Consider the following ciphertext that has been encrypted by a Caesar cipher using the key k . PDA A KDG DQ HAWUD VPDOO AHQRSV ZKR SODBHG D ABORSKRQH IRU ZRUGV WKDW VWDUW ZLWK A WKH SHRSOH KLW WKH GHFNV! ZRUGV OLNH AHQRSV SODBV ABORSKRQH IRU PDA A A A A A A A A The frequency distribution of the letters are as follows: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 17 4 0 11 0 1 5 12 2 0 9 3 0 2 8 3 5 12 8 0 6 8 9 0 0 4 (a) What is your first guess for the key k ? k = (b) Decrypt the ciphertext using the key you guessed in (a). Is the key correct? Yes / No (c) If your first guess for the key was wrong, make a second guess for the key k . k = (d) Decrypt the ciphertext using the key you guessed in (c). Is the key correct? Yes / No
Question 2. (Frequency Analysis of the Affine Cipher) Consider the following message that has been encrypted using an affine cipher with the key ( a, b ) . AHGCFS VQXBZ GADG AF BHCC ADUF GX KSFDV NZ HQ GAHZ HZCDQY XS CXZF GAF BDS. HM BF RDQ ZGDQY NE GX AHJ, DCC FNSXEF JDP KF MSFF DQY GAF CHMF XM GAF BXSCY JDP JXUF MXSBDSY HQGX KSXDY, ZNQCHG NECDQYZ. KNG HM BF MDHC, GAFQ GAF BAXCF BXSCY, HQRCNYHQT GAF NQHGFY ZGDGFZ, HQRCNYHQT DCC GADG BF ADUF VQXBQ DQY RDSFY MXS, BHCC ZHQV HQGX GAF DKPZZ XM D QFB YDSV DTF JDYF JXSF ZHQHZGFS, DQY EFSADEZ JXSF ESXGSDRGFY, KP GAF CHTAGZ XM EFSUFSGFY ZRHFQRF. CFG NZ GAFSFMXSF KSDRF XNSZFCUFZ GX XNS YNGHFZ, DQY ZX KFDS XNSZFCUFZ GADG, HM GAF KSHGHZA FJEHSF DQY HGZ RXJJXQBFDCGA CDZG MXS D GAXNZDQY PFDSZ, JFQ BHCC ZGHCC ZDP, "GAHZ BDZ GAFHS MHQFZG AXNS." Frequency analysis indicate that "GAF" is the most frequent trigram in the ciphertext.

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