Describe a block cipher counter (CTR) mode of operation.

What will be an ideal response?

Diffie and Hellman in 1979 introduced the counter mode encryption, which is another way to turn a block cipher into a stream cipher. Counter mode requires using an n-bit string called a counter. The counter can be produced by any sequence-generation function that is guaranteed not to repeat for a long time. Since the counter generation is independent of the encryption/decryption process, random access is achievable. The ith ciphertext block, Ci, can be encrypted or decrypted in a random-access fashion. Compared with CBC mode, encrypting the ith block requires all of the i-1 prior blocks to be encrypted first.