base16、base32、base64编码解码的介绍与实现

1.前言

base编码被用作数据传输编码，通过对字符进行位运算得到key，然后再从”彩虹表”中找到key对应的ascii码来表示编码后的数据，下面来看下其原理介绍。

2.原理

3.PHP的base64编码解码算法实现

<?php

$base64Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
const BASE64BIT = 6;
const BASE64GROUPSIZE = 3;

function base64Encode($str)
{
    global $base64Alphabet;
    $strLength = strlen($str);
    $strIndex = 0;
    $encode = '';
    while($strLength >= BASE64GROUPSIZE) {
        $subStr = substr($str, $strIndex, BASE64GROUPSIZE);

        $first = ord($subStr[0]) >> 2;
        $second = (ord($subStr[0]) & 3) << 4 | (ord($subStr[1]) & 0xf0) >> 4;
        $third = (ord($subStr[1]) & 0xf) << 2 | ord($subStr[2]) >> BASE64BIT;
        $fourth = ord($subStr[2]) & 0x3f;

        $encode .= $base64Alphabet{$first};
        $encode .= $base64Alphabet{$second};
        $encode .= $base64Alphabet{$third};
        $encode .= $base64Alphabet{$fourth};

        $strIndex += BASE64GROUPSIZE;
        $strLength -= BASE64GROUPSIZE;
    }

    if ($strLength % BASE64GROUPSIZE === 0) {
        return $encode;
    }

    $lastStr = substr($str, $strIndex);
    $lastFirst = ord($lastStr[0]) >> 2;
    $encode .= $base64Alphabet{$lastFirst};

    if ($strLength === 1) {
        $lastSecond =(ord($lastStr[0]) & 3) << 4;
        $encode .= $base64Alphabet{$lastSecond};
        $encode .= '==';
    } else {
        $lastSecond = (ord($lastStr[0]) & 3) << 4 | (ord($lastStr[1]) & 0xf0) >> 4;
        $lastThird = (ord($lastStr[1]) & 0xf) << 2;

        $encode .= $base64Alphabet{$lastSecond};
        $encode .= $base64Alphabet{$lastThird};
        $encode .= '=';
    }

    return $encode;
}

function base64Decode($str)
{
    global $base64Alphabet;
    $strLength = strlen($str);
    $strIndex = 0;
    $decode = '';
    while($strLength  >= BASE64GROUPSIZE + 1) {
        $subStr = substr($str, $strIndex, BASE64GROUPSIZE + 1);

        $first = strpos($base64Alphabet, $subStr[0]) << 2 | strpos($base64Alphabet, $subStr[1]) >> 4;
        $second = (strpos($base64Alphabet, $subStr[1]) & 0xf) << 4 | (strpos($base64Alphabet, $subStr[2]) & 0x3c) >> 2;
        $decode .= pack('H*' , dechex($first));

        if ($strLength === BASE64GROUPSIZE + 1) {
            switch(count(explode('=', $str))) {
                case 2:
                    $decode .= pack('H*' , dechex($second));
                case 3:
                    return $decode;
            }

        }

        $third = (strpos($base64Alphabet, $subStr[2]) & 3) << 6 | strpos($base64Alphabet, $subStr[3]);
        $decode .= pack('H*' , dechex($second));
        $decode .= pack('H*' , dechex($third));

        $strIndex += (BASE64GROUPSIZE + 1);
        $strLength -= (BASE64GROUPSIZE + 1);
    }

    return $decode;
}

$a = 'Hello PHP 你好 PHP!';
var_dump(base64_encode($a), base64_decode(base64_encode($a)));
var_dump(base64Encode($a), base64Decode(base64Encode($a)));

4.运行结果

5.参考

https://datatracker.ietf.org/doc/rfc4648/

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