Overview
A Feistel network splits a block into halves, transforms one half with a keyed round function, and swaps halves so decryption can reverse the same structure.
CoreSource checked
Feistel Network
Builds an invertible block cipher structure from repeated round functions.
Quick Facts
Compact metadata for implementation and discovery.
- Category
- Cryptography
- Difficulty
- Intermediate
- Family
- Block Cipher Structure
- Tier
- Core
- Status
- Verified
- Review
- Source Checked
- Sources
- 1
- Formulas
- 1
- Relationships
- 2
- Code example
- Ready
- Visualization
- Ready
Core Concepts
round functionleft-right splitxor mixinginvertible structuremisuse resistanceparameter validation
Quick facts
Complexity
Runtime and memory behavior at a glance.
Best
O(rounds)
Average
O(rounds)
Worst
O(rounds)
Space
O(1)
When to Use
- Use when protecting messages needs round function.
- Use when the input matches the assumptions behind block Cipher Structure.
When Not to Use
- Avoid when a well-reviewed library already provides the primitive for production use.
- Avoid when a simpler algorithm gives the required result with less setup.
What It Does
- Builds an invertible block cipher structure from repeated round functions
- Uses round function and left-right split to guide the work.
- Produces ciphertext, digest, proof, or derived key.
Use Cases
- Protecting messages
- Authenticating data
- Deriving or managing cryptographic material
Inputs
- Secret key or public parameters
- Message or secret data
- Nonce, salt, or context where needed
Outputs
- Ciphertext, digest, proof, or derived key
Key Formulas (1)
Feistel round
Lnext = R, Rnext = L xor F(R, keyi)
L_next = R, R_next = L xor F(R, key_i)Updates the two halves during one encryption round.
Pseudocode
- 1.Validate keys, nonces, salts, group parameters, and input ranges
- 2.Apply the primitive's block, hash, group, or reduction operation
- 3.Combine intermediate values exactly as the construction specifies
- 4.Return the protected value, proof, signature, derived key, or verification result
Canonical Example
- Input
- message = 'pay 10' with configured keys, nonces, or public parameters
- Output
- Ciphertext, digest, proof, or derived key
- Explanation
- Feistel Network applies round function to produce ciphertext, digest, proof, or derived key.
Implementation Notes
- Use constant-time library implementations for real systems instead of hand-written primitives.
- Keep the round function step separate enough to test independently.
- Use reviewed libraries and protocol specifications for production security-sensitive code.
Common Pitfalls
- Reusing nonces, salts, pads, or domain parameters can destroy the intended security property.
- Test edge cases, not only clean examples.
- Treating educational pseudocode as production crypto can hide nonce, parameter, and side-channel requirements.
Interactive Mechanism Walkthrough
Graph Frontier & Edge RelaxationShow the major cryptographic rounds or data dependencies without exposing secrets.
Step 1 of 4: Validate keys, nonces, salts, group parameters, and input ranges
cryptography-mechanismNotes
A Feistel network is a cipher construction pattern; security depends on the round function, key schedule, and number of rounds.
Sources (1)
What each reference supports for this entry.
Handbook of Applied Cryptography
BookReference text for classical cryptographic constructions, block ciphers, hash constructions, and security properties.
Open sourceExplore Next
Good next jumps from the relationship graph, ranked for useful discovery.
Relationships
View full relationship pageConceptual neighborAES
Feistel networks are a contrasting block-cipher construction to AES-style substitution-permutation networks.
Related Algorithms
Tags
CryptographySymmetric KeySecurityClassic