How to encrypt programs in the C language?

In C programming language, there are various methods available to encrypt programs. Here are some common encryption methods:

1. String encryption: Simple algorithms, such as shifting or substitution, can be used to encrypt strings. For example, each character in the string can be shifted forward or backward a certain number of positions, or each character can be replaced with another character.

#include <stdio.h>

void encryptString(char* str, int key) {
    int i = 0;
    while (str[i] != '\0') {
        str[i] += key; // 位移加密，将每个字符向前或向后移动key个位置
        i++;
    }
}

int main() {
    char str[] = "Hello World";
    int key = 3;
    
    encryptString(str, key);
    
    printf("Encrypted string: %s\n", str);
    
    return 0;
}

1. File encryption: You can use file input/output functions to read the content of a file, encrypt it, and then write the encrypted content back to the file.

#include <stdio.h>

void encryptFile(const char* filename, int key) {
    FILE* file = fopen(filename, "r+");
    if (file == NULL) {
        printf("Error opening file.\n");
        return;
    }
    
    char ch;
    while ((ch = fgetc(file)) != EOF) {
        ch += key; // 位移加密，将每个字符向前或向后移动key个位置
        fseek(file, -1, SEEK_CUR);
        fputc(ch, file);
    }
    
    fclose(file);
}

int main() {
    const char* filename = "test.txt";
    int key = 3;
    
    encryptFile(filename, key);
    
    printf("File encrypted.\n");
    
    return 0;
}

The encryption methods mentioned above are just simple ones. In reality, the complexity of an encryption program depends on the encryption algorithm and requirements being used. It is important to note that encryption can only provide a certain level of security and cannot completely prevent hacking.