Base64 to Hexadecimal String Converter C Code: Explanation and Implementation

This C code is a program that converts a base64 encoded string into a hexadecimal string. It includes the following functions:

1. decode_base64(char c): This function decodes a single base64 character into its corresponding numerical value. It maps the characters 'A' to 'Z', 'a' to 'z', '0' to '9', '+' and '/' to their respective numerical representations used in base64 encoding.

2. base64_to_hex(const char *base64, char *hex): This function takes a base64 string and converts it into a hexadecimal string. It processes the base64 string in groups of 4 characters. Each group is decoded into 3 numerical values, which are then converted into 2 hexadecimal characters. This process results in a hexadecimal representation of the original base64 string.

3. hex_to_str(unsigned char c, char *str): This function takes a hexadecimal numerical value and converts it into a string representation. For example, 0x0a would be converted to the string "0a".

4. main(int argc, char *argv[]): The program's entry point. It receives a base64 string as a command-line argument. It then calls the base64_to_hex() function to convert it into a hexadecimal string and uses the hex_to_str() function to format each hexadecimal value into a string. Finally, it prints the converted hexadecimal string to the console.

Code Explanation:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const char *base64_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int decode_base64(char c) {
    if (c >= 'A' && c <= 'Z') return c - 'A';
    if (c >= 'a' && c <= 'z') return c - 'a' + 26;
    if (c >= '0' && c <= '9') return c - '0' + 52;
    if (c == '+') return 62;
    if (c == '/') return 63;
    return -1;
}

void base64_to_hex(const char *base64, char *hex) {
    int len = strlen(base64);
    int i, j;
    for (i = 0, j = 0; i < len; i += 4, j += 3) {
        int b1 = decode_base64(base64[i]);
        int b2 = decode_base64(base64[i+1]);
        int b3 = decode_base64(base64[i+2]);
        int b4 = decode_base64(base64[i+3]);
        int n = (b1 << 18) | (b2 << 12) | (b3 << 6) | b4;
        hex[j] = (n >> 16) & 0xff;
        hex[j+1] = (n >> 8) & 0xff;
        hex[j+2] = n & 0xff;
    }
}

void hex_to_str(unsigned char c, char *str) {
    int hi = (c >> 4) & 0xf;
    int lo = c & 0xf;
    str[0] = hi < 10 ? hi + '0' : hi - 10 + 'a';
    str[1] = lo < 10 ? lo + '0' : lo - 10 + 'a';
    str[2] = '\0';
}

int main(int argc, char *argv[]) {
    if (argc < 2) {
        printf("Usage: %s base64_string\n", argv[0]);
        return 0;
    }
    const char *base64 = argv[1];
    char hex[strlen(base64) * 3 / 4 + 1];
    base64_to_hex(base64, hex);
    char hex_str[strlen(hex) * 2 + 1];
    for (int i = 0, j = 0; i < strlen(hex); i++, j += 2) {
        hex_to_str(hex[i], &hex_str[j]);
    }
    printf("%s\n", hex_str);
    return 0;
}

This code provides a practical example of how to work with Base64 encoded data in C. It demonstrates the process of decoding, converting, and formatting the data, which are essential tasks in various applications where Base64 encoding is used.