Base64属于简单加密算法的一种。类似于凯撒密码【它是一种替换加密的技术】

Base64字符串由65个字符组成，

大写字母A~Z，

小写字母a~z,

数字0~9，以及三个特殊字符+、/、=  

【=“等号”用于补充字符，使Base64字符串长度变成4的倍数】

规则

考虑到初始源字符串可能是任何文本编码的【中文GBK，Unicode，ASCII等】，因此Base64字符串加密只处理字节数组【字节数组通过encoding.GetBytes(string src)获得】。

Base64编码字符串的长度一定是4的倍数。

Base64要求把每三个8Bit的字节转换为四个6Bit的字节（3*8 = 4*6 = 24），然后把6Bit再添两位高位0，组成四个8Bit的字节，因此每个Base64字节的十进制范围为0~63。也就是说，转换后的字符串理论上将要比原来的长1/3。

字节数组的长度应该是3的倍数，如果这个条件不能满足的话，具体的解决办法是这样的：原文剩余的字节根据编码规则继续单独转(1变2，2变3；不够的位数用0补全)，再用=号补满4个字节。这就是为什么有些Base64编码会以一个或两个等号结束的原因，但等号最多只有两个。因为一个原字节至少会变成两个目标字节，所以余数任何情况下都只可能是0，1，2这三个数中的一个。如果余数是0的话，就表示原文字节数正好是3的倍数（最理想的情况）。如果是1的话，转成2个Base64编码字符，为了让Base64编码是4的倍数，就要补2个等号；同理，如果是2的话，就要补1个等号。 

6Bit数字【0~63】映射Base64字符表如下

索引	对应字符	索引	对应字符	索引	对应字符	索引	对应字符
0	A	17	R	34	i	51	z
1	B	18	S	35	j	52	0
2	C	19	T	36	k	53	1
3	D	20	U	37	l	54	2
4	E	21	V	38	m	55	3
5	F	22	W	39	n	56	4
6	G	23	X	40	o	57	5
7	H	24	Y	41	p	58	6
8	I	25	Z	42	q	59	7
9	J	26	a	43	r	60	8
10	K	27	b	44	s	61	9
11	L	28	c	45	t	62	+
12	M	29	d	46	u	63	/
13	N	30	e	47	v
14	O	31	f	48	w
15	P	32	g	49	x
16	Q	33	h	50	y

 测试Base64源程序

新建WinForm应用程序Base64EncoderDemo，重命名默认的 Form1为FormBase64Encoder，

窗体FormBase64Encoder设计如图：

FormBase64Encoder.cs主要代码如下

（忽略设计器自动生成的代码）：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Base64EncoderDemo
{
    public partial class FormBase64Encoder : Form
    {
        public FormBase64Encoder()
        {
            InitializeComponent();
            //参考Convert微软源程序
            //https://referencesource.microsoft.com/#mscorlib/system/convert.cs,fc990bd1275d43d6
        }

        private void FormBase64Encoder_Load(object sender, EventArgs e)
        {
            rtxtMessage.ReadOnly = true;
            //编码格式
            cboEncoding.Items.AddRange(new string[] { "ASCII", "Unicode", "UTF-8", "GBK" });
            cboEncoding.SelectedIndex = 0;
        }

        private void btnClear_Click(object sender, EventArgs e)
        {
            rtxtSourceString.Clear();
            rtxtBase64String.Clear();
            rtxtMessage.Clear();
        }

        /// <summary>
        /// 显示提示消息
        /// </summary>
        /// <param name="content"></param>
        private void DisplayMessage(string content)
        {
            if (rtxtMessage.TextLength >= 20480)
            {
                rtxtMessage.Clear();
            }
            rtxtMessage.AppendText($"{DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff")} -> {content}\n");
            rtxtMessage.ScrollToCaret();
        }

        private void btnConvertBase64_Click(object sender, EventArgs e)
        {
            rtxtBase64String.Clear();
            if (rtxtSourceString.Text.Trim().Length == 0)
            {
                rtxtSourceString.Focus();
                DisplayMessage("源字符串不能为空");
                return;
            }
            try
            {
                Encoding encoding = Encoding.GetEncoding(cboEncoding.Text);
                byte[] buffer = encoding.GetBytes(rtxtSourceString.Text.Trim());
                rtxtBase64String.Text = Convert.ToBase64String(buffer, Base64FormattingOptions.None);
                DisplayMessage($"转换成功，Base64字符串【{rtxtBase64String.Text}】");
            }
            catch (Exception ex)
            {
                DisplayMessage($"转换为Base64时出错：【{ex.Message}】");
            }
        }

        private void btnRestore_Click(object sender, EventArgs e)
        {
            rtxtSourceString.Clear();
            if (rtxtBase64String.Text.Trim().Length == 0)
            {
                rtxtBase64String.Focus();
                DisplayMessage("Base64字符串不能为空");
                return;
            }
            try
            {
                Encoding encoding = Encoding.GetEncoding(cboEncoding.Text);
                byte[] buffer = Convert.FromBase64String(rtxtBase64String.Text);
                rtxtSourceString.Text = encoding.GetString(buffer);
                DisplayMessage($"还原成功，源字符串【{rtxtSourceString.Text}】");
            }
            catch (Exception ex)
            {
                DisplayMessage($"还原字符串时出错：【{ex.Message}】");
            }
        }
    }
}

程序运行如图：

 

参考微软源代码：Reference Source

public static unsafe String ToBase64String(byte[] inArray, int offset, int length, Base64FormattingOptions options) {
            //Do data verfication
            if (inArray==null) 
                throw new ArgumentNullException("inArray");
            if (length<0)
                throw new ArgumentOutOfRangeException("length", Environment.GetResourceString("ArgumentOutOfRange_Index"));
            if (offset<0)
                throw new ArgumentOutOfRangeException("offset", Environment.GetResourceString("ArgumentOutOfRange_GenericPositive"));
            if (options < Base64FormattingOptions.None || options > Base64FormattingOptions.InsertLineBreaks)
                throw new ArgumentException(Environment.GetResourceString("Arg_EnumIllegalVal", (int)options));
            Contract.Ensures(Contract.Result<string>() != null);
            Contract.EndContractBlock();
 
            int inArrayLength;
            int stringLength;
 
            inArrayLength = inArray.Length;
            if (offset > (inArrayLength - length))
                throw new ArgumentOutOfRangeException("offset", Environment.GetResourceString("ArgumentOutOfRange_OffsetLength"));
           
            if (inArrayLength == 0)
                return String.Empty;
 
            bool insertLineBreaks = (options == Base64FormattingOptions.InsertLineBreaks);
            //Create the new string.  This is the maximally required length.
            stringLength = ToBase64_CalculateAndValidateOutputLength(length, insertLineBreaks);
 
            string returnString = string.FastAllocateString(stringLength);
            fixed (char* outChars = returnString){
                fixed (byte* inData = inArray) {
                    int j = ConvertToBase64Array(outChars,inData,offset,length, insertLineBreaks);
                    BCLDebug.Assert(returnString.Length == j, "returnString.Length == j");
                    return returnString;
                }
            }
        }
 

Base64处理字节数组逻辑函数

internal static readonly char[] base64Table = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O',
                                                       'P','Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d',
                                                       'e','f','g','h','i','j','k','l','m','n','o','p','q','r','s',
                                                       't','u','v','w','x','y','z','0','1','2','3','4','5','6','7',
                                                       '8','9','+','/','=' };        
 
        private const Int32 base64LineBreakPosition = 76;     

[System.Security.SecurityCritical]  // auto-generated
        private static unsafe int ConvertToBase64Array(char* outChars, byte* inData, int offset, int length, bool insertLineBreaks) {
            int lengthmod3 = length%3;
            int calcLength = offset + (length - lengthmod3);
            int j=0;
            int charcount = 0;
            //Convert three bytes at a time to base64 notation.  This will consume 4 chars.
            int i;
 
            // get a pointer to the base64Table to avoid unnecessary range checking
            fixed(char* base64 = base64Table) {
                for (i=offset; i<calcLength; i+=3)
                {
                    if (insertLineBreaks) {
                        if (charcount == base64LineBreakPosition) {
                            outChars[j++] = '\r';
                            outChars[j++] = '\n';
                            charcount = 0;
                        }
                        charcount += 4;
                    }
                    outChars[j] = base64[(inData[i]&0xfc)>>2];
                    outChars[j+1] = base64[((inData[i]&0x03)<<4) | ((inData[i+1]&0xf0)>>4)];
                    outChars[j+2] = base64[((inData[i+1]&0x0f)<<2) | ((inData[i+2]&0xc0)>>6)];
                    outChars[j+3] = base64[(inData[i+2]&0x3f)];
                    j += 4;
                }
 
                //Where we left off before
                i =  calcLength;
 
                if (insertLineBreaks && (lengthmod3 !=0) && (charcount == base64LineBreakPosition)) {
                    outChars[j++] = '\r';
                    outChars[j++] = '\n';
                }
                    
                switch(lengthmod3)
                {
                case 2: //One character padding needed
                    outChars[j] = base64[(inData[i]&0xfc)>>2];
                    outChars[j+1] = base64[((inData[i]&0x03)<<4)|((inData[i+1]&0xf0)>>4)];
                    outChars[j+2] = base64[(inData[i+1]&0x0f)<<2];
                    outChars[j+3] = base64[64]; //Pad
                    j+=4;
                    break;
                case 1: // Two character padding needed
                    outChars[j] = base64[(inData[i]&0xfc)>>2];
                    outChars[j+1] = base64[(inData[i]&0x03)<<4];
                    outChars[j+2] = base64[64]; //Pad
                    outChars[j+3] = base64[64]; //Pad
                    j+=4;
                    break;
                }
            }
            
            return j;
        }
 
        private static int ToBase64_CalculateAndValidateOutputLength(int inputLength, bool insertLineBreaks) {
            long outlen = ((long)inputLength) / 3 * 4;          // the base length - we want integer division here. 
            outlen += ((inputLength % 3) != 0) ? 4 : 0;         // at most 4 more chars for the remainder
 
            if (outlen == 0)
                return 0;
 
            if (insertLineBreaks) {
                long newLines = outlen / base64LineBreakPosition;
                if ((outlen % base64LineBreakPosition) == 0) {
                    --newLines;    
                }
                outlen += newLines * 2;              // the number of line break chars we'll add, "\r\n"
            }
 
            // If we overflow an int then we cannot allocate enough
            // memory to output the value so throw
            if (outlen > int.MaxValue)
                throw new OutOfMemoryException();
 
            return (int)outlen;
        }