(4) This questions deals with affine ciphers and shift ciphers. The alphabet is consid- ered modulo...

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(4) This questions deals with affine ciphers and shift ciphers. The alphabet is consid- ered modulo 26; that is, A=0,...,Z=25. (a) The following ciphertext has been encrypted with a shift cipher with encryp- tion key (shift) 3 or 16: c₁=IUBBOEKHSQHVEHJUDJXEKIQDTTEBBQHI. This means that from the plaintext we use the key 3 or 16 to obtain c₁. Decrypt c₁ (4+4 marks). Here you need to use both keys and give the message that makes more sense (1 mark). (b) Using an affine cipher c = 11p + 17 where p is the plaintext, encrypt the plaintext: p=(STUDENT FIRST NAME), MEET ME AT THE NATIONAL GALLERY. For instance, in my case, the plaintext reads GUILLERMO, MEET ME AT THE NATIONAL GALLERY. (i) Give the ciphertext c for the plaintext p (5 marks). (ii) Show manually how to decrypt the affine cipher c (6 marks). This includes computing the function p = (c-17)* 11-¹ (mod 26). (iii) With the function you constructed in (ii), decrypt the ciphertext NWVASPFWRAQVRSYJRXBE (5 marks). (4) This questions deals with affine ciphers and shift ciphers. The alphabet is consid- ered modulo 26; that is, A=0,...,Z=25. (a) The following ciphertext has been encrypted with a shift cipher with encryp- tion key (shift) 3 or 16: c₁=IUBBOEKHSQHVEHJUDJXEKIQDTTEBBQHI. This means that from the plaintext we use the key 3 or 16 to obtain c₁. Decrypt c₁ (4+4 marks). Here you need to use both keys and give the message that makes more sense (1 mark). (b) Using an affine cipher c = 11p + 17 where p is the plaintext, encrypt the plaintext: p=(STUDENT FIRST NAME), MEET ME AT THE NATIONAL GALLERY. For instance, in my case, the plaintext reads GUILLERMO, MEET ME AT THE NATIONAL GALLERY. (i) Give the ciphertext c for the plaintext p (5 marks). (ii) Show manually how to decrypt the affine cipher c (6 marks). This includes computing the function p = (c-17)* 11-¹ (mod 26). (iii) With the function you constructed in (ii), decrypt the ciphertext NWVASPFWRAQVRSYJRXBE (5 marks).

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Croper 5051 c GITES C IUBBOEKHSQHVEHJUDJXEKIQDTTEBBQHI If key 3 p... View the full answer