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I have eleven ciphertexts that were encrypted with the same key (which I don't know). I want to decrypt the last ciphertext. I read similar question like Many time pad attack but I can't solve my problem with their answer.

I'm new in Crypto can anyone tell what can I do?

I know when XOR two ciphertext (Let's assume that the plaintexts consist only of spaces and ASCII letters) if both of them were space the result is null. If one of them was space and another was ASCII we have a new ciphertext. If both of them were ciphertext we have new word that is XOR of orginal message but how can I reveal the plain-text?

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marked as duplicate by e-sushi, figlesquidge, DrLecter, B-Con, AFS Jan 16 at 21:58

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Have you taken a look at crypto.stackexchange.com/questions/2249/… ? –  poncho Apr 14 '13 at 18:37
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Welcome to cryptography Stack Exchange. Please note that randomly sprinkling source code highlighting through your question doesn't make it more readable, or more likely to be answered. Also, could you more clearly explain what you don't understand about the existing answers to both linked questions? –  Paŭlo Ebermann Apr 14 '13 at 20:29
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You stated that 'if one of them was space and another was ASCII we have a new ciphertext'; you might reconsider exactly what happens in that case. For example, if one plaintext had an 0x20 (ASCII space), and the other plaintext had a 0x41 (ASCII 'A'), what would the result of the exclusive or be? –  poncho Apr 14 '13 at 22:04

1 Answer 1

This sounds suspiciously like the first extra credit assignment out of Dan Boneh's Cryptography I on Coursera. Dan gives you a great hint in the assignment listing for this one. Take a look at an ASCII chart and note what happens when you XOR something with space rather than with an alphanumeric.

Once you've got that sorted out, your best bet is to XOR things together, as you suggest, and use the knowledge gained from Dan's hint on XORing with space to build a histogram of candidate plaintext characters for each position. My first cut at this assignment got about 85% of the characters, and filling in the rest was simple due to the common digraphs of english.

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But I managed to misspell one word on the first attempt :) –  owlstead Apr 17 '13 at 0:03

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