Additional Context

A modern stream cipher built for both speed and security — like a finely tuned sports car designed to run fast even on low-power devices. It mixes bits efficiently to create unpredictable outputs, used often in TLS and VPNs.

ChaCha20

Exam Tips:

• Successor to Salsa20.

• Used in TLS (e.g., HTTPS) and WireGuard VPN.

• Resistant to timing attacks (unlike RC4).

A legacy block cipher from the 1980s that locks small boxes of data, like old combination locks — functional, but outdated by today’s standards.

RC2 (Rivest Cipher 2)

Exam Tips:

• 64-bit block cipher.

• Variable key size (8–128 bits).

• Obsolete due to weak key scheduling.

A stream cipher that once encrypted data quickly, like a fast-talking messenger — but with leaks that make it easy to eavesdrop on if not properly secured.

RC4 (Rivest Cipher 4)

Exam Tips:

• Stream, not block, cipher.

• Symmetric, not asymmetric.

• Used historically in WEP, WPA, SSL/TLS (now deprecated).

• Weak — vulnerable to key reuse and bias attacks.

A block cipher designed to be strong and balanced, using multiple math operations like addition, XOR, and multiplication — like a puzzle with layers that reinforce each other.

IDEA

Exam Tips:

• 64-bit block cipher, 128-bit key.

• Used in early PGP versions.

• Replaced by AES; still conceptually important for hybrid designs.

Each “round” is a full cycle of scrambling and transforming data — like repeatedly folding and shuffling a deck of cards to hide the original order.

Rounds

Exam Tips:

• More rounds = stronger but slower encryption.

• Each round adds confusion and diffusion.

• AES uses 10, 12, or 14 rounds depending on key length.

Encrypts small parts of data at a time, feeding the ciphertext output of one block into the next — like taking the last word spoken in a game of telephone to start the next phrase.

CFB

Exam Tips:

• Feedback = ciphertext output.

• Converts block cipher into self-synchronizing stream cipher.

• Bit error affects next few blocks.

Encrypts data by feeding the cipher output, not ciphertext, forward — like a metronome keeping perfect rhythm regardless of what’s being said.

OFB

Exam Tips:

• Feedback = cipher output (not ciphertext).

• Error does not propagate.

• IV reuse is catastrophic (produces same keystream).

Encrypts each block using a unique counter value, like giving each letter in a message a unique serial number before locking it.

CTR

Exam Tips:

• Supports parallel processing (independent block encryption).

• Acts like a synchronous stream cipher.

• Nonce + counter must never repeat.

Performing multiple encryption operations at once, like having many workers each lock their own chest simultaneously instead of waiting in line.

Parallel Processing

Exam Tips:

• ECB and CTR support parallel encryption/decryption.

• CBC and CFB cannot (due to chaining dependencies).

Think of two musicians playing in sequence — one copies the sound of the last note (OFB), while the other copies the echo (CFB).

OFB vs CFB

Exam Tips:

• OFB → feeds cipher output forward.

• CFB → feeds XOR result (ciphertext) forward.

• OFB errors do not propagate; CFB errors affect next block.

Stream ciphers are like live translators — encrypting data as it’s spoken, not after the sentence finishes.

Stream Cipher Applications

Exam Tips:

• Used in real-time communications (voice/video).

• Examples: RC4 (legacy), ChaCha20 (modern).

• Do not reuse keystreams or IVs.

It’s like calling a live interpreter a book editor — they both deal with language but in totally different ways.

RC4 Misconception

Exam Tips:

• False because: RC4 is a symmetric stream cipher, not block.

• Used in WEP, WPA, SSL/TLS (pre-2016) — now deprecated.

• Vulnerable to keystream reuse and bias attacks.