Step by Step solution about How to resolve the algorithm Main step of GOST 28147-89 step by step in the JavaScript programming language

The provided JavaScript code implements the Feistel Cipher algorithm, which is a classic encryption/decryption scheme used in various cryptographic applications. Here's a detailed explanation of the code:

1. Таблица_замен - This is a constant 8x5 lookup table containing the S-boxes used in the Feistel Cipher. Each row and column represents a different substitution box.

2. Основной_шаг:

   • Input: This function takes three parameters - блок_текста (plaintext block), элемент_ключа (subkey), and ТЗ (the table of S-boxes).
   • Initialization: It starts by creating a copy of the plaintext block, called N. It then calculates S by XORing the first element of N (N[0]) with the subkey.
   • S-Box Lookup and Mixing: It enters a loop that iterates four times (the number of rounds in the Feistel Cipher). In each round, it extracts the eight most significant bits (MSB) of S and uses them as an index into the S-boxes (ТЗ) to lookup four nibbles (4-bit values). These nibbles are then recombined to form the new value of S.
   • Cycle and XOR: The modified S is left-rotated by 11 bits and XORed with the second element of N (N[1]). The result is stored back into N[0], and N[1] becomes the new N[0].
   • Output: Finally, the encrypted/decrypted block is returned as the modified array N.

In the context of a complete Feistel Cipher implementation, this function would be used in both encryption and decryption processes, applying different subkeys in each round. The goal is to make the cipher resistant to cryptanalysis by introducing non-linearity and diffusion in the encryption process.

const Таблица_замен = [
	[ 4, 10,  9,  2, 13,  8,  0, 14,  6, 11,  1, 12,  7, 15,  5,  3],
	[14, 11,  4, 12,  6, 13, 15, 10,  2,  3,  8,  1,  0,  7,  5,  9],
	[ 5,  8,  1, 13, 10,  3,  4,  2, 14, 15, 12,  7,  6,  0,  9, 11],
	[ 7, 13, 10,  1,  0,  8,  9, 15, 14,  4,  6, 12, 11,  2,  5,  3],
	[ 6, 12,  7,  1,  5, 15, 13,  8,  4, 10,  9, 14,  0,  3, 11,  2],
	[ 4, 11, 10,  0,  7,  2,  1, 13,  3,  6,  8,  5,  9, 12, 15, 14],
	[13, 11,  4,  1,  3, 15,  5,  9,  0, 10, 14,  7,  6,  8,  2, 12],
	[ 1, 15, 13,  0,  5,  7, 10,  4,  9,  2,  3, 14,  6, 11,  8, 12]
];

const Основной_шаг = (блок_текста, элемент_ключа, ТЗ) => {
	const
		N = блок_текста.slice(0),
		S = N[0] + элемент_ключа & 0xFFFFFFFF;
	let нов_S = 0;
	for (let сч = 0; сч < 4; сч++) {
		const яч = (S >>> (сч << 3)) & 0xFF;
		нов_S +=
			ТЗ[сч * 2][яч & 0x0F]
			+ (ТЗ[сч * 2 + 1][яч >>> 4] << 4)
			<< (сч << 3);
	}
	нов_S =
		(нов_S << 11)
		+ (нов_S >>> 21)
		& 0xFFFFFFFF
		^ N[1];
	N[1] = N[0]; N[0] = нов_S;
	return N;
};