How to crack a cipher if it lacks a non-linear part?

Question

Cryptography

Greetings!
Help to understand: there is a block cipher without a non-linear component. Describe a method for opening a cryptogram obtained using such a cipher, if the attacker has the choice of text-ciphertext pairs.

Answer 1

First, is the location of the decrypted part of the plaintext known? If not, it is necessary to iterate over all possible positions, including the offset within the blocks.
After that, you match the blocks with the plaintext and make part of that part of the S-box that you can. Using it, you open in addition those blocks of the cryptogram, which is possible. Next comes either enumeration of the rest of the table, or analysis of the known part of the text and supplementing the table using simple analytics.
But for this you need to know what format the S-box has. Because not everything is easy to pick up. For example, tables of the type used in DES are not at all easy to select. If there is a use of only one table for the direct conversion of the block "back and forth", then the task is really trivial