将编码的 Base 64 字符串解码为 ASCII 字符串
先决条件：什么是 base64 编码以及为什么我们将字符串编码为 base64 格式
Base64 编码在通过网络传输比特之前在发送节点执行，接收节点将编码数据解码回原始 ASCII字符串。
Base64字符集是
// 64 characters
char_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz0123456789+/" 例子：
Input : TUVO04= // (Encoded into base64 format)
Output : MENON // (Decoded back to ASCII string)
Input : Z2Vla3Nmb3JnZWVrcw==
Output : geeksforgeeks方法：
- 这里编码字符串中的每个字符都被认为是由 6 位组成的。我们将一次从编码字符串中提取 4 个字符，即 4 * 6 = 24 位。对于编码字符串的每 4 个字符，我们将生成 3 个原始字符串字符，每个字符为 8 位，即 3 * 8 = 24 位。
- 在char_set中找到它们各自的位置，并使用 '|' 将其存储在变量(num)中用于存储位的 OR运算符和（LEFT – SHIFT）乘以 6 为另外 6 位腾出空间。
注意：我们在编码器中使用 '=' 来代替 2 个丢失的位，所以在解码器中我们必须反转这个过程。每当我们遇到 '=' 时，我们必须使用 (RIGHT – SHIFT) by 2 删除num的 2 位。 - 在我们将所有位存储在num中之后，我们将以 8 个为一组检索它们，通过使用带有 255 (11111111) 的 &运算符，这将存储来自 num 的 8 位，这将是我们来自 ASCII字符串的原始字符。
C++
// C++ Program to decode a base64
// Encoded string back to ASCII string
#include
using namespace std;
#define SIZE 100
/* char_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz0123456789+/" */
char* base64Decoder(char encoded[], int len_str)
{
char* decoded_string;
decoded_string = (char*)malloc(sizeof(char) * SIZE);
int i, j, k = 0;
// stores the bitstream.
int num = 0;
// count_bits stores current
// number of bits in num.
int count_bits = 0;
// selects 4 characters from
// encoded string at a time.
// find the position of each encoded
// character in char_set and stores in num.
for (i = 0; i < len_str; i += 4)
{
num = 0, count_bits = 0;
for (j = 0; j < 4; j++)
{
// make space for 6 bits.
if (encoded[i + j] != '=')
{
num = num << 6;
count_bits += 6;
}
/* Finding the position of each encoded
character in char_set
and storing in "num", use OR
'|' operator to store bits.*/
// encoded[i + j] = 'E', 'E' - 'A' = 5
// 'E' has 5th position in char_set.
if (encoded[i + j] >= 'A' && encoded[i + j] <= 'Z')
num = num | (encoded[i + j] - 'A');
// encoded[i + j] = 'e', 'e' - 'a' = 5,
// 5 + 26 = 31, 'e' has 31st position in char_set.
else if (encoded[i + j] >= 'a' && encoded[i + j] <= 'z')
num = num | (encoded[i + j] - 'a' + 26);
// encoded[i + j] = '8', '8' - '0' = 8
// 8 + 52 = 60, '8' has 60th position in char_set.
else if (encoded[i + j] >= '0' && encoded[i + j] <= '9')
num = num | (encoded[i + j] - '0' + 52);
// '+' occurs in 62nd position in char_set.
else if (encoded[i + j] == '+')
num = num | 62;
// '/' occurs in 63rd position in char_set.
else if (encoded[i + j] == '/')
num = num | 63;
// ( str[i + j] == '=' ) remove 2 bits
// to delete appended bits during encoding.
else {
num = num >> 2;
count_bits -= 2;
}
}
while (count_bits != 0)
{
count_bits -= 8;
// 255 in binary is 11111111
decoded_string[k++] = (num >> count_bits) & 255;
}
}
// place NULL character to mark end of string.
decoded_string[k] = '\0';
return decoded_string;
}
// Driver code
int main()
{
char encoded_string[] = "TUVOT04=";
int len_str = sizeof(encoded_string) / sizeof(encoded_string[0]);
// Do not count last NULL character.
len_str -= 1;
cout <<"Encoded string : " <<
encoded_string << endl;
cout <<"Decoded string : " <<
base64Decoder(encoded_string, len_str) << endl;
return 0;
}
// This code is contributed by
// shubhamsingh10 C
// C Program to decode a base64
// Encoded string back to ASCII string
#include
#include
#define SIZE 100
/* char_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz0123456789+/" */
char* base64Decoder(char encoded[], int len_str)
{
char* decoded_string;
decoded_string = (char*)malloc(sizeof(char) * SIZE);
int i, j, k = 0;
// stores the bitstream.
int num = 0;
// count_bits stores current
// number of bits in num.
int count_bits = 0;
// selects 4 characters from
// encoded string at a time.
// find the position of each encoded
// character in char_set and stores in num.
for (i = 0; i < len_str; i += 4) {
num = 0, count_bits = 0;
for (j = 0; j < 4; j++) {
// make space for 6 bits.
if (encoded[i + j] != '=') {
num = num << 6;
count_bits += 6;
}
/* Finding the position of each encoded
character in char_set
and storing in "num", use OR
'|' operator to store bits.*/
// encoded[i + j] = 'E', 'E' - 'A' = 5
// 'E' has 5th position in char_set.
if (encoded[i + j] >= 'A' && encoded[i + j] <= 'Z')
num = num | (encoded[i + j] - 'A');
// encoded[i + j] = 'e', 'e' - 'a' = 5,
// 5 + 26 = 31, 'e' has 31st position in char_set.
else if (encoded[i + j] >= 'a' && encoded[i + j] <= 'z')
num = num | (encoded[i + j] - 'a' + 26);
// encoded[i + j] = '8', '8' - '0' = 8
// 8 + 52 = 60, '8' has 60th position in char_set.
else if (encoded[i + j] >= '0' && encoded[i + j] <= '9')
num = num | (encoded[i + j] - '0' + 52);
// '+' occurs in 62nd position in char_set.
else if (encoded[i + j] == '+')
num = num | 62;
// '/' occurs in 63rd position in char_set.
else if (encoded[i + j] == '/')
num = num | 63;
// ( str[i + j] == '=' ) remove 2 bits
// to delete appended bits during encoding.
else {
num = num >> 2;
count_bits -= 2;
}
}
while (count_bits != 0) {
count_bits -= 8;
// 255 in binary is 11111111
decoded_string[k++] = (num >> count_bits) & 255;
}
}
// place NULL character to mark end of string.
decoded_string[k] = '\0';
return decoded_string;
}
// Driver function
int main()
{
char encoded_string[] = "TUVOT04=";
int len_str = sizeof(encoded_string) / sizeof(encoded_string[0]);
// Do not count last NULL character.
len_str -= 1;
printf("Encoded string : %s\n", encoded_string);
printf("Decoded_string : %s\n", base64Decoder(encoded_string, len_str));
return 0;
} Java
// Java Program to decode a base64
// Encoded String back to ASCII String
class GFG
{
static final int SIZE = 100;
/* char_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz0123456789+/" */
static String base64Decoder(char encoded[], int len_str)
{
char []decoded_String;
decoded_String = new char[SIZE];
int i, j, k = 0;
// stores the bitstream.
int num = 0;
// count_bits stores current
// number of bits in num.
int count_bits = 0;
// selects 4 characters from
// encoded String at a time.
// find the position of each encoded
// character in char_set and stores in num.
for (i = 0; i < len_str; i += 4)
{
num = 0; count_bits = 0;
for (j = 0; j < 4; j++)
{
// make space for 6 bits.
if (encoded[i + j] != '=')
{
num = num << 6;
count_bits += 6;
}
/* Finding the position of each encoded
character in char_set
and storing in "num", use OR
'|' operator to store bits.*/
// encoded[i + j] = 'E', 'E' - 'A' = 5
// 'E' has 5th position in char_set.
if (encoded[i + j] >= 'A' && encoded[i + j] <= 'Z')
num = num | (encoded[i + j] - 'A');
// encoded[i + j] = 'e', 'e' - 'a' = 5,
// 5 + 26 = 31, 'e' has 31st position in char_set.
else if (encoded[i + j] >= 'a' && encoded[i + j] <= 'z')
num = num | (encoded[i + j] - 'a' + 26);
// encoded[i + j] = '8', '8' - '0' = 8
// 8 + 52 = 60, '8' has 60th position in char_set.
else if (encoded[i + j] >= '0' && encoded[i + j] <= '9')
num = num | (encoded[i + j] - '0' + 52);
// '+' occurs in 62nd position in char_set.
else if (encoded[i + j] == '+')
num = num | 62;
// '/' occurs in 63rd position in char_set.
else if (encoded[i + j] == '/')
num = num | 63;
// ( str[i + j] == '=' ) remove 2 bits
// to delete appended bits during encoding.
else
{
num = num >> 2;
count_bits -= 2;
}
}
while (count_bits != 0) {
count_bits -= 8;
// 255 in binary is 11111111
decoded_String[k++] = (char)
((num >> count_bits) & 255);
}
}
return String.valueOf(decoded_String);
}
// Driver code
public static void main(String[] args)
{
char encoded_String[] = "TUVOT04=".toCharArray();
int len_str = encoded_String.length;
// Do not count last null character.
len_str -= 1;
System.out.printf("Encoded String : %s\n",
String.valueOf(encoded_String));
System.out.printf("Decoded_String : %s\n",
base64Decoder(encoded_String, len_str));
}
}
// This code is contributed by 29AjayKumarC#
// C# Program to decode a base64
// Encoded String back to ASCII String
using System;
class GFG
{
static readonly int SIZE = 100;
/* char_set = "ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz0123456789+/" */
static String base64Decoder(char []encoded, int len_str)
{
char []decoded_String;
decoded_String = new char[SIZE];
int i, j, k = 0;
// stores the bitstream.
int num = 0;
// count_bits stores current
// number of bits in num.
int count_bits = 0;
// selects 4 characters from
// encoded String at a time.
// find the position of each encoded
// character in char_set and stores in num.
for (i = 0; i < len_str; i += 4)
{
num = 0; count_bits = 0;
for (j = 0; j < 4; j++)
{
// make space for 6 bits.
if (encoded[i + j] != '=')
{
num = num << 6;
count_bits += 6;
}
/* Finding the position of each encoded
character in char_set
and storing in "num", use OR
'|' operator to store bits.*/
// encoded[i + j] = 'E', 'E' - 'A' = 5
// 'E' has 5th position in char_set.
if (encoded[i + j] >= 'A' && encoded[i + j] <= 'Z')
num = num | (encoded[i + j] - 'A');
// encoded[i + j] = 'e', 'e' - 'a' = 5,
// 5 + 26 = 31, 'e' has 31st position in char_set.
else if (encoded[i + j] >= 'a' && encoded[i + j] <= 'z')
num = num | (encoded[i + j] - 'a' + 26);
// encoded[i + j] = '8', '8' - '0' = 8
// 8 + 52 = 60, '8' has 60th position in char_set.
else if (encoded[i + j] >= '0' && encoded[i + j] <= '9')
num = num | (encoded[i + j] - '0' + 52);
// '+' occurs in 62nd position in char_set.
else if (encoded[i + j] == '+')
num = num | 62;
// '/' occurs in 63rd position in char_set.
else if (encoded[i + j] == '/')
num = num | 63;
// ( str[i + j] == '=' ) remove 2 bits
// to delete appended bits during encoding.
else
{
num = num >> 2;
count_bits -= 2;
}
}
while (count_bits != 0)
{
count_bits -= 8;
// 255 in binary is 11111111
decoded_String[k++] = (char)
((num >> count_bits) & 255);
}
}
return String.Join("",decoded_String);
}
// Driver code
public static void Main(String[] args)
{
char []encoded_String = "TUVOT04=".ToCharArray();
int len_str = encoded_String.Length;
// Do not count last null character.
len_str -= 1;
Console.Write("Encoded String : {0}\n",
String.Join("",encoded_String));
Console.Write("Decoded_String : {0}\n",
base64Decoder(encoded_String, len_str));
}
}
// This code is contributed by 29AjayKumarJavascript
输出：
Encoded string : TUVO04=
Decoded string : MENON