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P1

usai si my cmiw lokngch

wasn’t that fun

P2

If Trudy knew that the words “bob” and “alice”

appeared in the text, then she would know the ciphertext for b,o,a,l,i,c,e (since “bob” is the only palindrome in the message, and “alice” is the only 5-letter word. If Trudy knows the ciphertext for 7 of the letters, then she only needs to try 19!, rather than 26!, plaintext-ciphertext pairs. The difference between 19! and 26! is 262524…*20, which is 3315312000, or approximately 10⁹.

P3

能破凯撒加密，但不能破多码代替密码

Every letter in the alphabet appears in the phrase “The quick fox jumps over the lazy brown dog.” Given this phrase in a chosen plaintext attack (where the attacker has both the plain text, and the ciphertext), the Caesar cipher would be broken - the intruder would know the ciphertext character for every plaintext character. However, the Vigenere cipher does not alway translate a given plaintext character to the same ciphertext character each time, and hence a Vigenere cipher would not be immediately broken by this chosen plaintext attack.

P4

a) The output is equal to 00000101 repeated eight

times.

b) The output is equal to 00000101 repeated seven times+ 10000101.

c) We have (ARBRCR)R = CBA, where A, B, C are

strings, and R means inverse operation. Thus:

1. For (a), the output is 10100000 repeated eight times;

2. For (b), the output is 10100001 + 10100000 repeated seven times.

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