Comparison of Block Cipher Modes

Complete each entry in the following table according to the 6 situations described below. When comparing blocks of ciphertext or plaintext, if corresponding blocks differ then mention which specific blocks differ. If they differ by only 1 bit, mention that fact specifically. If they differ by more than 1 bit, simply state that they differ. If all blocks differ simply state that there is no relation. If a particular situation is not applicable to a mode write n/a. Be as precise as you can in your comparisons to illustrate that you understand the various modes.

  1. Suppose two plaintext samples P and Q are encrypted using a block cipher with the same secret key K and the same initialization vector IV (or nonce). Suppose each plaintext sample is divided into 100 blocks (including padding). If the plaintext blocks differ only by 1 bit in block 10, compare the corresponding ciphertext for each block cipher mode listed in the table.
  2. Same as #1, except use different IVs (or nonces) (IV1 [or nonce1] is used to encrypt P, IV2 [or nonce2] is used to encrypt Q
  3. Same as #1, except compare plaintext P encrypted twice using a different IV (or nonce) each time.
  4. Suppose two ciphertext samples P and Q are decrypted using key K and the same IV (or nonce). Suppose each ciphertext sample differs by 1 bit in block 25. Compare the corresponding plaintext blocks following decryption of P and Q for each block cipher mode.
  5. Comment on the expected performance for encrypting a large file using each mode. (i.e., can encryptin be done in parallel or must it be done serially, can any pre-computation take place)
  6. Same as #5, except for decryption
  7. Suppose block 50 of an encrypted file needs to be accessed. Which specific blocks of ciphertext must be accessed to perform the decryption?

Test case ECB CBC CTR CFB
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Click below for a review of a particular mode: