3. Let Ci be the cipher text xorresponding 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.
	
	Ans. A transmission error in C1 (first bit of cipher text) will cause the first block of the decryptrd cipher text (plain text) to be completely garbled, as thid block will be obtained from decryption the the first block of cipher text which contains the erroneous bit C1. The second block will also have bit errors in the same position as the original erroneous block, as the second block will be obtained by the decryption of cipher text for second block and then XOR with the cipher text of first block, which is erroneous.  All the subsequent  decypted cipher texts will be error free.

					------------------------------------
					
	2. What is the effect on the cipher text of a error in block P1 of plain text?. Assume CBC mode. What is the error at the reciever?
	
	Ans. If a bit of a plain text block P1 is in error the entire cipher text block will be effected and will be erroneous. (Though, the encryption algorithm is correctly encrypting what is given to it.) All subsequent cipher blocks will also be effected each cipher text block is fed to next stage and XOR with next plain text block.
	However, at the receiver, only the block P1 of plain text recovered reproduces the same bit error. All the subsequent plain text blocks are reproduced correcltly.

					-------------------------------------

	3. If a bit error occurs in the i th transmitted byte in CFB mode, how far does the error propagate?
	
	Ans. A bit error in i th transmitted cipher byte will cause bit error at the same bit position in  i th recovered plain text byte at the receiver (As recovering ith plain text byte involves an XOR opertaion with i th cipher text byte). Th erroneous cipher text byte will then be fed to the shift register and cause bit errors in the plain text as long as the erroneous but stays in the shift register. So as I assume a 8-bit CFB, 8 bytes following the i th byte of plain text will be garbled. After that, the system will recover and all the subsequent bytes will be decrypted correctly. (Ofcourse if there are no further bit errors). Thus the error propagates for 64/s blocks after the erroneous block i if the block size of CFB is s and shift register size is 64.

	
					------ * ------- * --------- * -----------	
