Let Ci be the cipher text corresponding to plain text Pi using DES.

   1. What effect does a transmission error in C1 (first bit of cipher text) have on the decrypted cipher text at the reciever?. Assume CBC mode
   2. What is the effect in the cipher text block due to an error in P1 of plain text?. Assume CBC mode.
      What is the error at the reciever?
   3. If a bit error occurs in the i th transmitted byte in CFB mode, how far does the error propagate?. 
   
   
   
   Answers
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1)  The Cipher text after being recieved, the Initialisation Vector(IV) and  Decrypted first ( cypher ) block are XORed, this will give the first block of plain text and now after that we use the previous cypher text to XOR with the Decrypted messages to get the rest of the message blocks.

Thus  in this process if there is an error in the first Block of Cypher text, then it gets propagated to all the message. So no block of the Actual Mesage can be decoded
So at reciever we will not get any Part of the Actual Message.




2) In case of error in the P1 of the actual message, it wont get propagated to all the messages.
We will get the wrong Cypher texts but while decoding it wont matter much to us, as we will get the error in the first block and remaining blocks will get decryted to their actual messages.
So at reciever we will get only the P1 as error and others as it is.




3) In case there is any error in the ith transmitted byte then this along with the next ( 64 / s where s is the no of bits shifted if s=8 then ...) 8 blocks will have errors.i.e a total of 9 blocks.
Since for each block the Register is shifted by 8 bytes, so for this error bit to be cleared from the shift register we need 8 shifts.