#46 「 Two Time Pad 」


密码学课程homework1, Instructor: Wenfei Wu

Let us see what goes wrong when a stream cipher key is used more than once. Open the file with the name of your student ID in the folder “two_time_pad”. There are 12 hex-encoded ciphertexts that are the results of encrypting 12 plaintexts with a stream cipher, all with the same stream cipher key.

The 1-11 lines are encryption of normal English, and the 12th line is an encryption of 3 big numbers (each about 20 digits) seperated by 10 spaces (format: [big number 1][10 spaces][big number 2][10 spaces][big number 3]).

Your goal is to decrypt the last line of ciphertext, and submit the last line.

Hint: XOR the ciphertexts together, and consider what happens when a space is XORed with a character in [a-zA-Z].