{"id":1408,"date":"2020-04-25T17:16:03","date_gmt":"2020-04-25T11:46:03","guid":{"rendered":"https:\/\/binaryterms.com\/?p=1408"},"modified":"2021-01-18T11:44:50","modified_gmt":"2021-01-18T06:14:50","slug":"feistel-block-cipher","status":"publish","type":"post","link":"https:\/\/binaryterms.com\/feistel-block-cipher.html","title":{"rendered":"Feistel Block Cipher"},"content":{"rendered":"

Feistel block cipher<\/strong> is a structure used to derive many symmetric block ciphers such as DES<\/a> which we have discussed in our previous content. Feistel cipher proposed a structure which implements substitution<\/strong> and permutation<\/strong> alternately to obtain cipher text from the pain text and vice-versa.<\/p>\n

In the Feistel block cipher, each block has to undergo many rounds where each round has the same function. In this context, we will discuss the structure proposed by Feistel for developing the block ciphers.<\/p>\n

Content: Feistel Block Cipher<\/h2>\n
    \n
  1. What is Feistel Block Cipher?<\/a><\/li>\n
  2. Feistel Cipher Structure<\/a><\/li>\n
  3. Feistel Decryption Algorithm<\/a><\/li>\n
  4. Key Takeaways<\/a><\/li>\n<\/ol>\n

    <\/a><\/p>\n

    What is Feistel Cipher?<\/h3>\n

    Feistel cipher is a structure proposed by a Horst Feistel<\/strong> which was considered while developing many symmetric block ciphers. Actually, the structure proposed by Feistel is based on the Shannon structure<\/strong> which was proposed in 1945. The Shannon structure shows the implementation of confusion and diffusion alternately.<\/p>\n

    Confusion<\/strong> fabricates a complex relation between the cipher text<\/strong> and encryption key<\/strong>\u00a0by implementing a complex substitution<\/strong> algorithm. Whereas, the diffusion<\/strong> fabricates a complex relation between plain text<\/strong> and cipher text<\/strong>\u00a0by implementing more complex permutation<\/strong> algorithm.<\/p>\n

    The Shannon structure was successful in achieving a more complex block cipher and thus confusion and diffusion were adopted by the Feistel structure.<\/p>\n

    Feistel cipher proposed the structure that alternately implements substitution and permutation. Substitution<\/strong> is implemented by replacing the elements of plain text or the set of elements of plain text by the element of cipher text or set of elements of cipher text.<\/p>\n

    A permutation<\/b> is implemented by changing the order of elements of the plain text. No element here is replaced by any other element, only the order of elements is changed. Now, let us proceed towards the structure of Feistel cipher.
    \n    <\/a><\/p>\n

    Feistel Cipher Structure<\/h3>\n

    To understand the Feistel cipher in a better way observe the figure below:<\/p>\n

    \"Feistel<\/p>\n

    Step 1:<\/strong> The plain text is divided into the blocks of a fixed size and only one block is processed at a time. So, the input to encryption algorithm is a plain text block and a key K.<\/p>\n

    Step 2:<\/strong> The plain text block is divided into two equal halves which we will denote as a LE0<\/sub><\/strong> as the left half of the plain text block and RE0<\/sub><\/strong> as the right half of the plain text block. Now, both these halves of the plain text block (LE0<\/sub> & RE0<\/sub>) undergoes multiple rounds to produce ciphertext block.<\/p>\n

    In each round, the encryption function<\/strong> is applied on the right half REi<\/sub><\/strong> of the plaintext block along with the key Ki<\/sub>. The result of this encryption function is then XORed<\/strong> with the left half LE<\/strong>i<\/strong>. <\/sub>The result of XOR function becomes the new right half for next round REi+1<\/sub><\/strong>. Whereas, the old right half REi <\/sub>becomes the new left half LEi+1 <\/sub><\/strong>for the next round as you can see in the figure above.<\/p>\n

    Each round executes the same function. In each round initially, a substitution<\/strong> function is implemented by applying the round function or the encryption function on the right half of the plain text block. The result of this round function is XORed by the with the left half of the block. After this substitution function, a permutation<\/strong> function is implemented by swapping the two halves and the result of this permutation is provided to the next round.<\/p>\n

    This is how Feistel cipher structure presents the application of substitution and permutation alternately which is similar to the Shannon structure.<\/p>\n

    The design features of Feistel cipher that are considered while implementing any block cipher are as follow:<\/p>\n