610 In discussing the CTR mode it was mentioned that if any

6.10 In discussing the CTR mode, it was mentioned that if any plaintext block that is en- crypted using a given counter value is known, then the output of the encryption function can be determined easily from the associated ciphertext block. Show the calculation.

Solution

To encrypt a series of plaintext blocks let assume p1, p2, . . . pn using a block cipher \'E\' operating in Electronic Code Book [ECB] mode, then each ciphertext block c1, c2, . . . cn is computed as below::

ci = Ek(pi).

If a ciphertext block is modified or corrupted, then after decryption the corresponding plaintext block and all the following plaintext blocks will be affected.

In Electronic Code Book (ECB), altering a ciphertext block only affects a single plaintext block.