base64编解码的实现（golang版本和C版本）

Golang的base64编解码实现

第一种：NoPadding的

package main

import (
	"encoding/base64"
	"fmt"
)

func main() {
	data := "changeme"
	sEnc := base64.StdEncoding.WithPadding(base64.NoPadding).EncodeToString([]byte(data))
	fmt.Println(sEnc) // Y2hhbmdlbWU
	sDec, _ := base64.StdEncoding.WithPadding(base64.NoPadding).DecodeString(sEnc)
	fmt.Println(string(sDec)) // changeme
	fmt.Println()
}

第二种：StdPadding的

package main

import (
	"encoding/base64"
	"fmt"
)

func main() {
	data := "changeme"
	sEnc := base64.StdEncoding.WithPadding(base64.StdPadding).EncodeToString([]byte(data))
	fmt.Println(sEnc) // Y2hhbmdlbWU=
	sDec, _ := base64.StdEncoding.WithPadding(base64.StdPadding).DecodeString(sEnc)
	fmt.Println(string(sDec)) // changeme
	fmt.Println()
}

默认模式base64.StdEncoding.EncodeToString([]byte(data))等价于上述第二种base64.StdEncoding.WithPadding(base64.StdPadding).EncodeToString([]byte(data))。

const (
	StdPadding rune = '=' // Standard padding character
	NoPadding  rune = -1  // No padding
)

源码里可以看到，如果使用StdPadding模式，如果长度不足，则会填充=。

C语言的base64编解码实现

postgres里有一个很经典的base64编解码的实现，在postgres/scr/common目录下，我们抄过来测试一下。

#include <stdio.h> 
#include <string.h>   
#include <stdlib.h>
 
typedef signed char int8;		/* == 8 bits */
typedef unsigned int uint32;	/* == 32 bits */

static const char _base64[] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static const int8 b64lookup[128] = {
	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
	-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
	-1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1,
};

/*
 * pg_b64_encode
 *
 * Encode into base64 the given string.  Returns the length of the encoded
 * string.
 */
int
pg_b64_encode(const char *src, int len, char *dst)
{
	char	   *p;
	const char *s,
			   *end = src + len;
	int			pos = 2;
	uint32		buf = 0;

	s = src;
	p = dst;

	while (s < end)
	{
		buf |= (unsigned char) *s << (pos << 3);
		pos--;
		s++;

		/* write it out */
		if (pos < 0)
		{
			*p++ = _base64[(buf >> 18) & 0x3f];
			*p++ = _base64[(buf >> 12) & 0x3f];
			*p++ = _base64[(buf >> 6) & 0x3f];
			*p++ = _base64[buf & 0x3f];

			pos = 2;
			buf = 0;
		}
	}
	if (pos != 2)
	{
		*p++ = _base64[(buf >> 18) & 0x3f];
		*p++ = _base64[(buf >> 12) & 0x3f];
		*p++ = (pos == 0) ? _base64[(buf >> 6) & 0x3f] : '=';
		*p++ = '=';
	}

	return p - dst;
}

/*
 * pg_b64_decode
 *
 * Decode the given base64 string.  Returns the length of the decoded
 * string on success, and -1 in the event of an error.
 */
int
pg_b64_decode(const char *src, int len, char *dst)
{
	const char *srcend = src + len,
			   *s = src;
	char	   *p = dst;
	char		c;
	int			b = 0;
	uint32		buf = 0;
	int			pos = 0,
				end = 0;

	while (s < srcend)
	{
		c = *s++;

		/* Leave if a whitespace is found */
		if (c == ' ' || c == '\t' || c == '\n' || c == '\r')
			return -1;

		if (c == '=')
		{
			/* end sequence */
			if (!end)
			{
				if (pos == 2)
					end = 1;
				else if (pos == 3)
					end = 2;
				else
				{
					/*
					 * Unexpected "=" character found while decoding base64
					 * sequence.
					 */
					return -1;
				}
			}
			b = 0;
		}
		else
		{
			b = -1;
			if (c > 0 && c < 127)
				b = b64lookup[(unsigned char) c];
			if (b < 0)
			{
				/* invalid symbol found */
				return -1;
			}
		}
		/* add it to buffer */
		buf = (buf << 6) + b;
		pos++;
		if (pos == 4)
		{
			*p++ = (buf >> 16) & 255;
			if (end == 0 || end > 1)
				*p++ = (buf >> 8) & 255;
			if (end == 0 || end > 2)
				*p++ = buf & 255;
			buf = 0;
			pos = 0;
		}
	}

	if (pos != 0)
	{
		/*
		 * base64 end sequence is invalid.  Input data is missing padding, is
		 * truncated or is otherwise corrupted.
		 */
		return -1;
	}

	return p - dst;
}

/*
 * pg_b64_enc_len
 *
 * Returns to caller the length of the string if it were encoded with
 * base64 based on the length provided by caller.  This is useful to
 * estimate how large a buffer allocation needs to be done before doing
 * the actual encoding.
 */
int
pg_b64_enc_len(int srclen)
{
	/* 3 bytes will be converted to 4 */
	return (srclen + 2) / 3 * 4;
}

/*
 * pg_b64_dec_len
 *
 * Returns to caller the length of the string if it were to be decoded
 * with base64, based on the length given by caller.  This is useful to
 * estimate how large a buffer allocation needs to be done before doing
 * the actual decoding.
 */
int
pg_b64_dec_len(int srclen)
{
	return (srclen * 3) >> 2;
}

测试一下这两个函数：

int main()  
{  
    char *input = "changeme";
    char *base64_res = (char *) malloc(pg_b64_enc_len(strlen(input)) + 1);
	pg_b64_encode(input, strlen(input), base64_res);
    printf("input = %s\n", input);  // changeme
    printf("base64_res = %s\n", base64_res);  // Y2hhbmdlbWU=

    char *base64_dec_res = (char *) malloc(pg_b64_dec_len(strlen(base64_res)) + 1);
    pg_b64_decode(base64_res, strlen(base64_res), base64_dec_res);
    printf("base64_dec_res = %s\n", base64_dec_res); // changeme

    return 0;  
} 

可以看出postgres实现的是前文说的golang实现的StdPadding模式。这样C语言和golang语言的base64实现结果就对应上了。

参考博客：https://blog.csdn.net/qq_26093511/article/details/78836087