Block Cipher
Definition
A symmetric cipher that operates on fixed-size blocks (typically 64 or 128 bits) under a key. AES is a 128-bit-block cipher; DES and Blowfish are 64-bit. Modes of operation define how to handle messages longer than one block.
- Category
- Symmetric-key cryptographic primitive
- Common block sizes
- 64 bits (DES, Blowfish) or 128 bits (AES)
- Requires for long messages
- A mode of operation (ECB, CBC, GCM, etc.)
- Forensic relevance
- Identifying cipher and mode matters for decryption feasibility
Common questions
Why does the mode of operation matter as much as the cipher itself?+
The block cipher only defines how one fixed-size block is scrambled under a key. The mode of operation decides how successive blocks of a longer message are chained together, and a weak mode like ECB can leak structural patterns even when the underlying cipher itself is strong.
Why are older 64-bit block ciphers like DES considered weak today?+
A 64-bit block size makes a cipher vulnerable to birthday-bound collision attacks once enough data is encrypted under one key, a risk that becomes practical at the data volumes common in modern network traffic. This is a separate weakness from DES's short 56-bit key, and it is why AES was designed with a 128-bit block.
Related terms
- AEAD
- Authenticated Encryption with Associated Data. Combines confidentiality and integrity in one primitive. AES-GCM and ChaCha20-Poly1305 are the two modern standards. TLS 1.3...
- AES-NI
- AES New Instructions: hardware AES acceleration introduced on Intel Westmere (2010) and AMD Bulldozer (2011) and now ubiquitous. Single-cycle round operations make...
- Constant-Time Crypto
- An implementation whose execution time, memory access pattern and power profile do not depend on the secret key or plaintext. The defence...
- Mode of Operation
- The wrapping protocol that defines how a block cipher handles arbitrary-length data, what IV or nonce is used, and whether integrity is...
- Stream Cipher
- A symmetric cipher that produces a pseudo-random keystream which is XORed bit-by-bit or byte-by-byte with the plaintext. RC4 (legacy), ChaCha20 (current). Stream...