DES algorithm subkey generation process

In one execution of the DES algorithm, 16 rounds of iterative encryption will be performed;
16 round keys k are obtained from the original 56-bit key, and each round key ki is 48 bits; the
round key is also called a sub-key;

Let’s take a look at the sub-key generation process;

The general process is like this; look at the picture below,

This picture comes from 34. "Cryptozoology". Explanation of the principle of DES algorithm_Subkey generation and summary_bilibili_bilibili

For a 64-bit key, 8 check bits are removed, leaving 56 bits;

Replace according to the PC-1 table and shuffle the order; then divide it into two 28-bits; one 28-bit is C0, and the other 28-bit is D0.

Then we need to perform a circular left shift. The circular left shift needs to be shifted by 1 or 2 bits. It can be judged according to which round it is. You can check the circular left shift lookup table in the figure above;

After the shift, Ci and Di are obtained, i is the number of rounds;

Combine Ci and Di to get 56 bits;

The obtained 56 bits are replaced according to the PC-2 table, 8 bits will be removed during the replacement, and the 48-bit subkey ki of this round is obtained;

The PC-1 table is 56 bits and the PC-2 table is 48 bits;

Or look at the picture below for more conciseness;

This picture comes from, DES encryption and decryption algorithm (simple, easy to understand, super detailed)_des encryption algorithm_sunny-ll's blog-CSDN blog

The process is: 64-bit key input, PC1 replacement, divided into two 28-bit keys, circular left shift to obtain a 56-bit key, PC2 replacement, and 48-bit subkey ki;

I took a look at the DES encryption algorithm code; I haven’t fully understood it yet;

The bits in the algorithm description should be binary bits;

The operation in the code does not use bit operations in C language; it should be a binary string such as 11001010, which is used as a string. Each string is taken and operated according to the algorithm; after completion, the binary string is converted into an integer. The ASCII code of the character can be obtained from the integer type; it should be like this;

Its PC1 replacement code is like this,

int pc1Table[56] =
{
57,49,41,33,25,17,9,1,
58,50,42,34,26,18,10,2,
59,51,43,35,27,19,11,3,
60,52,44,36,63,55,47,39,
31,23,15,7,62,54,46,38,
30,22,14,6,61,53,45,37,
29,21,13,5,28,20,12,4
};

string miyao, miyaoBinary, pc1MiyaoBinary;

//Get 56bit from the 64bit key according to the PC-1 box
for (i = 0; i < 56; i++)
{ pc1MiyaoBinary += miyaoBinary[pc1Table[i] - 1]; }

Among them miyaoBinary and pc1MiyaoBinary are string types;

There are also two functions in its code that implement conversion between binary strings and integers.

int binaryToInt(string s), input a binary string and return an integer;
string intToBinary(int i), input an integer and return a binary string;

Let’s take a look at these two functions separately in MFC; I changed their string type to MFC’s CString;

int binaryToInt(CString );
CString intToBinary(int );

void CInbyView::OnDraw(CDC* pDC)
{
	CInbyDoc* pDoc = GetDocument();
	ASSERT_VALID(pDoc);
	// TODO: add draw code for native data here
	CString str1;

	CString b1 = "0101";
	int b2 = binaryToInt(b1);
	str1.Format("0101的整数：%d", b2);
	pDC->TextOut(50, 50, str1);

	CString b3 = "1100101";
	int b4 = binaryToInt(b3);
	str1.Format("1100101的整数：%d", b4);
	pDC->TextOut(50, 80, str1);

	str1 = intToBinary(12);
	pDC->TextOut(50, 110, "12的二进制：" + str1);
	
	CString str3 = intToBinary(14);
	pDC->TextOut(50, 140, "14的二进制：" + str3);

	CString str4 = intToBinary(37);
	pDC->TextOut(50, 170, "37的二进制：" + str4);
}

//二进制转整型
int binaryToInt(CString s)
{
    int i, result = 0, p = 1;
    for (i = s.GetLength() - 1; i >= 0; i--)
    {
        result += ((s[i] - '0') * p);   //数字字符转成字符
        p *= 2;
    }
    return result;
}

//整型转二进制
CString intToBinary(int i)
{
    int k = 0;
    CString result;
    while (k < 4) //此处，处理进入S盒后取出的数据转为2进制，此处最多用4bit
 
    {
        if (i)
        {
            result += ((i % 2) + '0');
            i /= 2;
        }
        else result += '0';
        k++;
    }
    result.MakeReverse();
    return result;
}

The output is as follows;

Among them, intToBinary() uses up to 4 bits according to its instructions; the binary number of 37 is as follows. When 37 is entered, only the lower 4 bits are returned;

There is time to continue;

DES algorithm subkey generation process

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