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Also known as: Feistel Round Function
A round function within Feistel networks combining substitution, permutation, and key mixing.
DESCRIPTION
The Feistel function is the core round operation in Feistel ciphers, transforming half of the state and mixing it into the other half. Feistelās structure enables invertibility regardless of whether the round function itself is invertible. Classic ciphers like DES use S-box-based Feistel round functions with subkeys from a key schedule. Good Feistel functions provide strong nonlinearity, diffusion, and avalanche across rounds. The structure contrasts with SPNs, which process the full state using layered substitution and permutation.
ALTERNATIVE DEFINITIONS
Wikipedia
In cryptography, a Feistel cipher is a symmetric structure used in the construction of block ciphers.
Defines the Feistel structure underlying Feistel functions.
Encyclopaedia Britannica
In a Feistel cipher, the plaintext is split into two halves and the round function is applied to one half, which is then combined with the other half.
Highlights the half-splitting and mixing operation characteristic of Feistel rounds.
RELATED TERMS
Symmetric Block Cipher Algorithms Substitution Transposition Data Encryption Standard Round Function Key Schedule Avalanche Effect Confusion Diffusion
COMPETING TERMS
SubstitutionāPermutation Network
RELATIONS
- Broader: Feistel Function
KNOWLEDGE AREAS
Data Security Software Engineering Mathematics
TARGET AUDIENCE
Security Architect Cybersecurity Engineer Software Developer
EXTERNAL SOURCES
- Wikipedia - Feistel cipher
- NIST - DES specification showing Feistel round functions.
- IETF - CAST-128 block cipher (Feistel) specification.
- Cloudflare - Explanation of Feistel networks and rounds.
TOP QUESTIONS
What is the Feistel Function in a Feistel cipher?
It is the round function applied to one half of the state and mixed into the other half, enabling invertible encryption even if the function is not invertible.
Category: Definition
Why are Feistel structures popular?
They simplify decryption and key schedule symmetry while allowing complex round functions and efficient implementations.
Category: Relevancy
How does a Feistel cipher compare to an SPN?
Feistel ciphers operate on halves with a mixing function; SPNs apply substitution and permutation layers across the entire state.
Category: Comparison
What properties should a Feistel function have?
High nonlinearity, strong avalanche, good diffusion, and resistance to differential and linear cryptanalysis.
Category: Requirements
What is an example of a Feistel cipher?
DES is a classic Feistel cipher with eight S-boxes in its round function.
Category: Example
Can the Feistel round function be non-invertible?
Yes. The Feistel structure guarantees invertibility of the whole cipher even if the round function isnāt invertible.
Category: Misunderstanding
What risks arise from a weak Feistel function?
Predictable differences and correlations can enable key recovery via differential or linear cryptanalysis.
Category: Risk
How is the Feistel subkey used?
A round-specific subkey is mixed into the function input or output to ensure key-dependent behavior each round.
Category: Concept
Where can I study Feistel functions in standards?
Review DES (FIPS 46-3) and IETF CAST-128 specifications for detailed Feistel round designs.
Category: Getting Started