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Commonly confused · Security Fundamentals & Crypto

Block Cipher vs Stream Cipher: What's the Difference?

A block cipher encrypts fixed-size chunks of data (for example 128-bit blocks in AES), while a stream cipher encrypts data one bit or byte at a time by combining it with a keystream. Block ciphers can behave like stream ciphers in certain modes such as CTR and GCM.

Block Cipher

Full definition
vs

Stream Cipher

Full definition
Block CipherStream Cipher
Unit of encryptionFixed-size blocks (e.g. 64 or 128 bits)Individual bits or bytes
PaddingOften required in modes like ECB and CBCNot required
ExamplesAES, 3DES, Blowfish, TwofishChaCha20, RC4 (deprecated)
Speed and latencyEfficient for stored data and hardware accelerationLow latency, suited to real-time streams
Key riskWeak mode choice (e.g. ECB patterns)Reusing a key/nonce pair exposes plaintext
Typical useFile, disk, and TLS encryptionMobile/TLS ciphersuites, real-time communication
Remember it

Block ciphers chop data into boxes; stream ciphers pour it through a keystream.

Exam tip

Expect to identify AES as a block cipher and RC4 or ChaCha20 as stream ciphers. Questions on padding attacks point to block ciphers.

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