Fundamentals of Cryptography.
1 CS512: Fundamentals of Cryptography Assignment ( 2 ) 1. Bob proposes the idea of using double encryption with the Hill cipher. Specifically, he proposes using two separate keys, A followed by B, for encryption. To decrypt, he says that we must first use B-1 followed by A-1. Does Bob’s idea create a cipher that is more secure than the regular Hill cipher? Why or why not? 2. The Enigma was a rather good encryption machine for its time. However, there were some weaknesses with how it was used that helped the Polish and British crack messages encrypted by it. Name and explain some of these weaknesses. 3. Break the affine cipher (i.e., find the key (k1, k2)) based on the knowledge that the cipher encrypts “E” to “L”, and “K” to “Z”; the message is written in English; and the ciphertext starts from the sequence of letters: “SLKKR”. 4. Decrypt the ciphertext “OCASJAM VG KOS OSJA DBZZJM” using the Vigenère cipher with the key “HONOR”. Then, encrypt the obtained message using Playfair cipher (following the exact specification given in the Stalling’s book) using the key “PRACTICE MAKES PERFECT”. 5. Using Hill cipher; decrypt the ciphertext “SYICHOLER” using the keyword “alphabet” and a 3×3 matrix. Hint “The first step is to create a matrix using the keyword (since the keyword is shorter than 9 letters, just start the alphabet again until the matrix is full)”. 2 CS512: Fundamentals of Cryptography 6. You are attempting to determine a key for the playfair cipher. You have some matching plaintext and ciphertext. Namely, the plaintext “TODAYISASUNNY” corresponds to the ciphertext “PDBIIDQRTPRZIZ”. (Note: The dummy letter used in the plaintext is X, if necessary.) Try to determine as much of the key as possible. (Note: You are given three working grids below, and one final grid to put your answer.) Final Grid 3 CS512: Fundamentals of Cryptography 7. 8. 4 CS512: Fundamentals of Cryptography 9. Note: Show your work in each question.