Mode of Operation
Definition
The wrapping protocol that defines how a block cipher handles arbitrary-length data, what IV or nonce is used, and whether integrity is included. The same block cipher under ECB and GCM has very different security properties.
- Governs
- Handling of arbitrary-length data by a block cipher
- Weak example
- ECB, no integrity, pattern leakage
- Strong example
- GCM, authenticated encryption
- Also defines
- IV or nonce usage
Common questions
Why is ECB mode considered forensically weak even with a strong cipher like AES?+
ECB encrypts identical plaintext blocks to identical ciphertext blocks, so repeating patterns in the plaintext, such as image regions, remain visible in the ciphertext, which is why examiners can sometimes recognise structure without decryption.
What does an authenticated mode like GCM add that a plain confidentiality-only mode does not?+
GCM combines encryption with a message authentication tag, so any tampering with the ciphertext is detectable, which matters when an examiner needs to establish that seized encrypted evidence has not been altered.
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...
- Block Cipher
- 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...
- 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...
- 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...