[SOLVED] 程序代写代做代考 ## part 1

## part 1

Part 1 requires to decrypt the cipher to obtain the location of the assignment specification. The cipher consists of 16 lines, which are XORed with the same secret key. The idea is make good use of the knowledege of linguistic statistics of English.

If two ciphers are XORed from the same key, then we may be able to eliminate the secret key by XORing this two ciphers, i.e.,

$$cipher_1 = plain_1oplus key$$
$$cipher_2 = plain_2oplus key$$

then:

$$cipher_1oplus cipher_2 = plain_1oplus plain_2$$

We all know English consists of alphabet and punctuation, so I enumerate how 26 upper case characters XORed with while space (‘ ‘) will return, and I get:

0x61 ‘A’^’ ‘

0x62 ‘B’^’ ‘

0x63 ‘C’^’ ‘

:

0x79 ‘Y’^’ ‘

0x7a ‘Z’^’ ‘

Which means that if I get such character (0x61-0x7a), then it is possible that two plaintexts are 1 uppercase character with 1 white space. And I found such pair of plains is first line and last line.

I guess the first character of the key is 0x6b from above process, then I recover the all line’s first character and found two ‘T’s (line 8 and 12) follows by 3 common letters, which means that they are probably ‘The ‘, ‘This’, ‘That’, ‘They’, such common patterns. I tried out all of them and found that ‘The ‘ is the correct plaintext.

Then the 2nd plain becomes ‘Info’, I guess it should be ‘Information’, so to recover the next key. Follow such process, I successfully obtained the necessary key (length 73) to decrypt the location of assignment3. Actually, the plain text is (the last line):

> Your assignment is in https://cs.adelaide.edu.au/~yval/SP18/A3ishere.php