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Network Security: Introduction to Fiestal Structure

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This article is about introduction of Fiestal Structure that describe how to perform the encryption operation with block cipher.

Before moving ahead let’s take a look at Introduction To Stream And Block Ciphers.

Table of Contents

Fiestal Structure

Fiestal Structure is the cryptography method used in encryption and works with the block cipher. This method divides the encryption process into two equal parts, i.e., left and right. The fiestal structure involves increasing complexity as the encryption process is divided into multiple rounds.

fiestal-structure-1- codingstreets
Source: AU

Terms in the encryption process

  1. Plaintext is divided into two blocks of left and right with equal partitions.
  2. Each step of the encryption process adjusts into square boxes called Rounds (R1, R2,….., Rn)
  3. Each round has half left and half right with a key (K1, K2,…., Kn) derived from the encryption process.

Steps in the encryption process

  1. No operation is performed on R in each round, and it remains the same to become L for the next round.

  2. In each round, an XOR operation is performed on L with a function’s output denoted as F(K, R), where K is the current Key and R is the current R of that round.

  3. In the R2, the resulting output from the L  (R1) will become the R for the next round (R2), and the R of the R1 will become the L for the R2. In other words, L and R will be swapped, and the process will be repeated for the next round.

  4. After the desired rounds, the left and right halves are swapped one last time.

  5. The final output combines the right and left halves, resulting in the encrypted ciphertext.
fiestal-structure-2- codingstreets
Source: AU

Above replacement and stage steps structure a ’round’. The algorithm plan indicates the number of rounds.

When the last round is finished, the two sub-squares, ‘L’ and ‘R’, are connected in a specific order to frame the cipher text block.

Decryption Process

  1. The ciphertext will be divided into two equal parts L and R.

  2. The key used in the encryption process will be used in the reverse order.

  3.  The same number of rounds as in encryption is performed. However, the operations in each round are applied in reverse order compared to encryption. This means the right half is combined with the current round key, and the result is XOR with the left half.

  4. After all the rounds are completed, the left and right parts are swapped.

  5. The final output combines the right and left halves, resulting in the decrypted plaintext.
Source: AU

Decryption in Ciphers Based on the Feistel Structure

encryption & decryption process - codingstreets
Source: AU

Advantages and Disadvantages


  1. Confusion and Diffusion: Feistel ciphers provide a strong level of confusion and diffusion, which are essential for ensuring encrypted data security. Confusion obscures the relationship between the plaintext and the ciphertext. At the same time, diffusion disperses the influence of individual data elements, making it difficult for an attacker to deduce information about the original data.

  2. Parallelism: Feistel ciphers can be easily parallelized, enabling efficient simultaneous encryption and decryption on multiple processing units. This makes them suitable for hardware implementations, where parallel processing can significantly improve performance.

  3. Security Analysis: Feistel ciphers have been extensively studied and analyzed for security vulnerabilities. This analysis has helped identify and address weaknesses, making well-designed and properly implemented Feistel ciphers highly secure.

  4. Adaptability and Flexibility: Feistel ciphers can accommodate cryptographic operations, such as substitution and permutation functions, enabling customization and adaptability to specific security requirements. Different Feistel ciphers can be designed by varying the number of rounds, the choice of operations, and the key length.


  1. Performance Overhead: Feistel ciphers typically involve many rounds and complex operations, which can result in slower encryption and decryption speeds compared to some other encryption algorithms. The computational overhead can disadvantage resource-constrained environments or applications requiring high-speed processing.

  2. Key Management: Feistel ciphers require proper key management practices, including secure key generation, distribution, and storage. The cypher’s security heavily relies on the secrecy and integrity of the encryption key. Key management can be challenging, especially in large-scale systems or scenarios that require frequent key updates.

  3. Block Size Limitation: Feistel ciphers operate on fixed-size blocks, which can be a limitation when encrypting data that doesn’t align with the block size. Additional techniques like padding or chaining modes are required to handle data of variable lengths, which may introduce complexity and potential vulnerabilities if not implemented correctly.

  4. Cryptanalysis Advances: As with any encryption algorithm, cryptanalysis technique advancements can weaken Feistel cyphers’ security over time.


This article provides an introduction to the Feistel cipher structure, which is a widely used design in symmetric encryption algorithms. The Feistel structure offers several advantages, including strong confusion and diffusion properties, adaptability, and parallelism. It has been extensively analyzed and provides a solid foundation for building secure encryption algorithms.

Overall, the Feistel cipher structure is a valuable framework for building secure encryption algorithms, balancing security and efficiency. Understanding its principles and limitations is essential for developing robust cryptographic systems that protect sensitive data in various applications and environments.

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