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# regarding one time pads,can someone show with an example...

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 Question by suhas Submitted on 1/15/2004 Related FAQ: Cryptography FAQ (01/10: Overview) Rating: Rate this question: N/A Worst Weak OK Good Great regarding one time pads,can someone show with an example how, if the same pad is used  for encrypting two different plain texts, it can be broken very easily

 Answer by Anal Srivastava Submitted on 4/19/2004 Rating: Rate this answer: N/A Worst Weak OK Good Great One time pad can be broken if the attacker has atleast 1 plaintext-ciphertext pair. Its pretty simple. One time pad takes XOR of a random sequence of bits (called key) with the plaintext to produce the cipertext, so we can describe this mathematically as C = P xor K where C = ciphertext       P = plaintext       K = key if we xor with the plaintext on both sides, we get P xor C = P xor P xor K P xor P on the rhs will yield 0, and anything xored with 0 returns the same so we have effectively K = P xor C now if the attacker has even one plaintext-ciphertext pair then he can produce the key that was used for encryption. For any other cipertext encrypted using the same key, its easy to get the palintext, simply xor the key with the ciphertext, the result would be the plaintext. Following works out an example, (for simplicity only 4 bits are taken) lets assume M = 0101, K = 1101 then C = 1000 Suppose attacker knows M & C then key = 0101 xor 1000 = 1101 which is the original key. now any new ciphertext (encrypted with the same key) can be decrypted.

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