Cryptography Worksheet

• An organization uses same stream cipher key multiple times. An attacker who knows that, captures two ciphertexts C1 and C2 and finds that,

C1⊕C2= 0000101000011101000000110001001100010000000101000000011000000011000010100000100100011011

The attacker also knows that the plaintexts are composed of only English capital letters without any punctuations or space, and that part of the plaintexts contains the word: “PRESIDENT”. He was able to recover both the plaintexts. Show, how. (Points 25)

Hints: Slide ASCII for “PRESIDENT” over C1⊕C2 and XOR them. You can start from the left and keep sliding right, or start from the right and keep sliding left until you find a meaningful 9 letter phrase (since PRESIDENT is a 9 letter word). Once you retrieve the partial plaintexts, use your intuition and linguistic observations to complete the rest as much as possible.

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