Modes of Operation – Part 2/2

Counter Mode (CTR) is another type of Block Ciphers. CTR encrypts an increasing counter value and then XORs with plaintext block, where we must have a different counter value for every plaintext block (never reused). The figure below shows the CTR operation.

ctr-operation

 

Advantages and Limitations of CTR operation:

  • Efficiency
    • Can do parallel encryptions in both hardware and software.
    • Can pre-process Oi values in advance.
    • Good for high speed links.
  • CRT operation has a random access to encrypted data blocks.
  • Provable security (as good as other modes), but must ensure never reuse counter values (otherwise could break).

 

Important: Block Ciphers can be used to emulate Stream Ciphers.

In summary, ECB operation is weak. Thus, we used CBC operation instead. Also, the CTR operation turns a Block Cipher into a Stream Cipher.

 

The link below provides more practical work on symmetric encryption (Crypto Lab).

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