Cipher Encryption Modes

Encrypts each block independently, like printing each page of a book separately. Identical pages result in identical encrypted pages.

ECB

Exam Tips:

• Weak for repeated or patterned data.

• Deterministic — same plaintext = same ciphertext.

Each block depends on the previous one, like following a recipe step by step, where each step’s output affects the next. Uses an IV for the first block.

CBC

Exam Tips:

• Adds randomness via chaining.

• Slower; cannot encrypt blocks in parallel.

Converts a block cipher into a self-synchronizing stream cipher. Each new block uses part of the previous ciphertext in the encryption process, like whispering one letter at a time using the last spoken letter to decide the next.

CFB

Exam Tips:

• Good for real-time data.

• Error propagation affects a few subsequent bits.

Similar to CFB, but feedback uses the output of the cipher itself, not ciphertext. Like a metronome keeping beat regardless of mistakes, so errors don’t propagate.

OFB

Exam Tips:

• Synchronous stream cipher.

• IV reuse breaks security.

• Error in one block does not affect others.

Uses a counter and nonce to encrypt each block independently. Like giving each box in a warehouse a unique number to lock and unlock individually.

CTR

Exam Tips:

• Allows parallel processing — fast for bulk encryption.

• Stream-like behavior.

• Blocks are independent — a single error does not propagate.