CTF中将二进制文件每两个字节替换

今天刷BuildCTF发现有一个png文件中每两个字节都被调换，故写了一个C程序来把输入的文件进行转换.

使用方法：编译后在程序目录下放置你要转换的文件，然后改名为input.bin，运行后会生成output.bin

#include <stdio.h>  
#include <stdlib.h>  
  
void swap_bytes(unsigned char *buffer, size_t size) {  
    for (size_t i = 0; i < size; i += 2) {  
        if (i + 1 < size) {  
            unsigned char temp = buffer[i];  
            buffer[i] = buffer[i + 1];  
            buffer[i + 1] = temp;  
        }  
    }  
}  
  
int main() {  
    FILE *input_file = fopen("input.bin", "rb");  
    if (input_file == NULL) {  
        perror("Open Failed");  
        return 1;  
    }  
    fseek(input_file, 0, SEEK_END);  
    long file_size = ftell(input_file);  
    fseek(input_file, 0, SEEK_SET);  
    if (file_size <= 0) {  
        fprintf(stderr, "error\n");  
        fclose(input_file);  
        return 1;  
    }  
    unsigned char *buffer = (unsigned char *)malloc(file_size);  
    if (buffer == NULL) {  
        perror("Failed to allocate memory");  
        fclose(input_file);  
        return 1;  
    }  
    fread(buffer, 1, file_size, input_file);  
    fclose(input_file);  
    swap_bytes(buffer, file_size);  
    FILE *output_file = fopen("output.bin", "wb");  
    if (output_file == NULL) {  
        perror("Output Failed");  
        free(buffer);  
        return 1;  
    }  
    fwrite(buffer, 1, file_size, output_file);  
    fclose(output_file);  
    free(buffer);  
    printf("Successfully\n");  
    return 0;  
}