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So, i know that if there is a change in source plaintext p1, c1 and rest of cipher blocks will be affected as c is required for the calculation in cfb. What i need to know is that how can i show how many blocks will be affected when there is this error. I have tried comparing the two cipher texts(one with no change in p1, and one with a bit change in p1) using diff, but i've had no luck in showing how many blocks are affected with the change in p1. Thanks.

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    $\begingroup$ Thank you for the reply, however my question requires that I show this in linux. 'Evidence' of how many blocks are affected, when there is an error in p1. $\endgroup$ – Xace May 23 '18 at 14:36
  • $\begingroup$ Ah so the problem is experimentally verifying the (correct) theoretical analysis. That's a programming question, off-topic for crypto.SE. One idea is to hex dump then compare, like this. $\endgroup$ – fgrieu May 23 '18 at 14:40
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    $\begingroup$ Even though you may "have to" show things using hard, irrefutable proof-by-example, you may still want to follow @fgrieu's now deleted advice to first figure out on paper what to expect and then to verify / falsify said expectation using an experiment (this is also kinda how natural scientists usually work). $\endgroup$ – SEJPM May 23 '18 at 19:27
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i don't think that there is a way to check the changes on block cipher if you are using random IV here, since the IV will go to Enc first, then the output would be totally different than before,

if you are using the same IV, then try to figure out the the block length for the block you are using, if it is 16 bytes, then change the first 16 bytes will result first 16 bytes of ciphertext to be different. Since before output the ciphertext, it is just a one-time pad.

hope it helps.

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