Cryptography

Using the inverse of the permutation in the previous question, decrypt the following

ciphertexts:

(a) OISWNHTIETEOCTMMFOEOWRDAR

(b) UYOTPRENPUISOLCTETHNBESLA.

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Using the permutation

1 2 3 4 5

5 4 1 3 2

Encrypt the following plaintexts:

(a) SEND MONEY TO YOUR UNCLE

(b) TRAVEL TO THE CITY NEXT MONDAY.

Another autokey cipher (also developed by Vigenere), uses the letters of the ciphertext, instead of plaintext, to form new key letters, as in

plaintext W I L L T R A V E L S O U T H B Y C A R N E X T F R I D A Y key C O D E Y W O P R N O K V Y G Y P R N Z N T N Q A X K J F 0 ciphertext Y W O P R N O K V Y G Y P R N Z N T N Q A X K J F O S M F M

Show that this is a much weaker cipher than the first autokey method, and is vulnerable to a brute-force attack.

By using a brute-force attack, decrypt the shift ciphers:

(a) XPPEXPZYHPOYPDOLJ

(b) LXVNCXVHYJACHCXWRPQC

(c) IUBBOEKHSQHVEHJUDJXEKIQDTTEBBQHI

Suppose the Hill cipher is used twice: once with an n x n matrix M1 and then further encrypting the ciphertext with an n x n matrix M2. Does this increase the security of the cipher? If not, why not?