Base64编码学习

在使用文本编辑器打开二进制文件时，其二进制数据并不是全部可见的字符，所以会出现乱码。

为了避免这种情况，使用 Base64 编码的方式，将二进制数据编码为可见字符。注意 Base64 编码是一种编码方法并不是加密方法。

原理

将一段二进制数据进行 Base64 编码，步骤如下：

将二进制数据每三个字节一组，进行分组，最后不足三个字节的补零构成3个字节

将每组的3个字节分成4组，每组6个bit位，对每6个bit位高位补零，得到4个字节

这样二进制的数据变长了，但每个字节的值均在0到63的范围内，进行查表替换为相应的字符

字符表中共64个字符，包括42个英文字母（大写与小写字母各26个），0到9共10个数字，另外两个 ‘+’、‘/’ 字符，需要注意的是，若第一步进行了补零操作，那么，补充了几个字节，则最后的几个字节使用 ‘=’ 字符替换

最终得到编码后的二进制数据，用 UTF-8 将该二进制数据编码，得到可见的字符串

将一段经过 Base64 编码得到的字符串解码为原二进制数据，步骤如下：

将字符串转换为二进制数据

将二进制数据分为每4个字节一组

处理每个组的每一个字节，查表，找到该字节表示的字符的序号，取该序号的低6位

每个组经过处理拼接后，得到24个bit位，共计3个字节

处理最后一组4个字节时，检查最后有几个 ‘=’ 字符，若没有，正常处理，若有1个，那么丢弃3个字节中的最后一个字节，若有2个，则只保留3个字节中的第一个字节

实现

iOS 官方接口 NSData 的分类 NSDataBase64Encoding 中提供了 Base64 的相关方法，如下：

@interface NSData (NSDataBase64Encoding)/* 使用经过 Base64 编码后的字符串，初始化一个 NSData 类实例，这里就是将 Base64 编码后的字符串进行了解码，得到原始的二进制数据*/- (instancetype)initWithBase64EncodedString:(NSString *)base64String options:(NSDataBase64DecodingOptions)options NS_AVAILABLE(10_9, 7_0);/* 使用自身的二进制数据进行 Base64 编码，最后返回编码后的字符串*/- (NSString *)base64EncodedStringWithOptions:(NSDataBase64EncodingOptions)options NS_AVAILABLE(10_9, 7_0);/* 使用经过 Base64 编码后得到的二进制数据，初始化一个 NSData 类实例，这里就是将 Base64 编码后的二进制数据进行了解码，得到原始的二进制数据*/- (instancetype)initWithBase64EncodedData:(NSData *)base64Data options:(NSDataBase64DecodingOptions)options NS_AVAILABLE(10_9, 7_0);/*使用自身的二进制数据进行 Base64 编码，最后返回编码后的二进制数据*/- (NSData *)base64EncodedDataWithOptions:(NSDataBase64EncodingOptions)options NS_AVAILABLE(10_9, 7_0);@end上面4种方法，可看做两组方法，分别进行了编码和解码操作，只是前两个方法，编码的结果和解码的入参都是字符串，后两种是二进制数据。

除了调用 iOS 提供的方法，还可以自己实现 Base64 编码及解码，如下：

#import 
   
    @interface Base64 : NSObject/** 对提供的二进制数据进行Base64编码 @param data 待编码的二进制数据 */+ (NSString *)base64Encode:(NSData *)data;/** 将经过Base64编码的字符串解码为原二进制数据 @param base64String 待解码的Base64字符串 */+ (NSData *)base64Decode:(NSString *)base64String;@end

#import "Base64.h"@implementation Base64#pragma mark - base64 编码+ (NSString *)base64Encode:(NSData *)data{    NSMutableData *base64 = [NSMutableData new];    NSRange range = NSMakeRange(0, 3);    unsigned char buffer[3] = {
  0};    unsigned char base[4] = {
  0};    //三的整数倍数据    double index = data.length / 3  * 3;    while (range.location + range.length <= index) {        [data getBytes:buffer range:range];        //第一个 6 bit        base[0] = base64_table[buffer[0] >> 2];        //第二个 6 bit        base[1] = base64_table[(buffer[0] & 0b11) << 4 | (buffer[1] >> 4)];        //第三个 6 bit        base[2] = base64_table[(buffer[1] & 0b1111) << 2 | (buffer[2] >> 6)];        //第四个 6 bit        base[3] = base64_table[buffer[2] & 0b111111];        [base64 appendBytes:base length:4];        range.location +=range.length;    }    //处理最后一个字节或两个字节数据    if (data.length%3 == 1) {        [data getBytes:buffer range:NSMakeRange(index, 1)];        //第一个 6 bit        base[0] = base64_table[buffer[0] >> 2];        //第二个 6 bit        base[1] = base64_table[(buffer[0] & 0b11) << 4 | 0];        base[2] = '=';        base[3] = '=';        [base64 appendBytes:base length:4];    }else if (data.length%3 == 2){        [data getBytes:buffer range:NSMakeRange(index, 2)];        //第一个 6 bit        base[0] = base64_table[buffer[0] >> 2];        //第二个 6 bit        base[1] = base64_table[(buffer[0] & 0b11) << 4 | (buffer[1] >> 4)];        //第三个 6 bit        base[2] = base64_table[(buffer[1] & 0b1111) << 2 | 0];        base[3] = '=';        [base64 appendBytes:base length:4];    }    return [[NSString alloc] initWithData:base64 encoding:NSUTF8StringEncoding];}#pragma mark - base64 解码+ (NSData *)base64Decode:(NSString *)base64String{    NSData *data = [base64String dataUsingEncoding:NSUTF8StringEncoding];    if (data.length % 4) {        NSLog(@"Base64 decode error : %@",[NSError errorWithDomain:@"The source data is not base64 code ." code:0 userInfo:nil]);        return nil;    }    NSMutableData *srcData = [NSMutableData new];    unsigned char buffer[3] = {
  0};    unsigned char base[4] = {
  0};    int32_t temp = 0;    NSRange range = NSMakeRange(0, 4);    while (range.location + range.length < data.length) {        [data getBytes:base range:range];        temp |= deBase64_table[base[0]];        temp <<= 6;        temp |= deBase64_table[base[1]];        temp <<= 6;        temp |= deBase64_table[base[2]];        temp <<= 6;        temp |= deBase64_table[base[3]];        buffer[0] = (temp & 0xFF0000) >> 16;        buffer[1] = (temp & 0x00FF00) >> 8;        buffer[2] = (temp & 0x0000FF);        [srcData appendBytes:buffer length:3];        temp = 0;        range.location += 4;    }    [data getBytes:base range:range];    if (base[2] == '=') {        base[2] = 0;        base[3] = 0;        temp |= deBase64_table[base[0]];        temp <<= 6;        temp |= deBase64_table[base[1]];        buffer[0] = temp >> 4;        [srcData appendBytes:buffer length:1];    }else if (base[3] == '='){        base[3] = 0;        temp |= deBase64_table[base[0]];        temp <<= 6;        temp |= deBase64_table[base[1]];        temp <<= 6;        temp |= deBase64_table[base[2]];        buffer[0] = (temp >> 2 & 0xFF00) >> 8;        buffer[1] = (temp >> 2 & 0x00FF);        [srcData appendBytes:buffer length:2];    }else {        temp |= deBase64_table[base[0]];        temp <<= 6;        temp |= deBase64_table[base[1]];        temp <<= 6;        temp |= deBase64_table[base[2]];        temp <<= 6;        temp |= deBase64_table[base[3]];        buffer[0] = (temp & 0xFF0000) >> 16;        buffer[1] = (temp & 0x00FF00) >> 8;        buffer[2] = (temp & 0x0000FF);        [srcData appendBytes:buffer length:3];    }    return [NSData dataWithData:srcData];}static const char base64_table[64] ={    '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','+','/'};static const char deBase64_table[] ={    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x3E,0x00,0x00,0x00,0x3F,    0x34,0x35,0x36,0x37,0x38,0x39,0x3A,0x3B,0x3C,0x3D,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0A,0x0B,0x0C,0x0D,0x0E,    0x0F,0x10,0x11,0x12,0x13,0x14,0x15,0x16,0x17,0x18,0x19,0x00,0x00,0x00,0x00,0x00,    0x00,0x1A,0x1B,0x1C,0x1D,0x1E,0x1F,0x20,0x21,0x22,0x23,0x24,0x25,0x26,0x27,0x28,    0x29,0x2A,0x2B,0x2C,0x2D,0x2E,0x2F,0x30,0x31,0x32,0x33,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,    0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00};@end

测试代码如下：

- (IBAction)btnClickedOfBase64:(UIButton *)sender {    NSString *test = @"this is a test string !";    NSLog(@"origin string : %@",test);    NSData *data = [test dataUsingEncoding:NSUTF8StringEncoding];    NSLog(@"origin data : %@",data);    //编码    NSString *baseString = [Base64 base64Encode:data];    NSLog(@"baseString : %@",baseString);    NSString *baseString1 = [data base64EncodedStringWithOptions:0];    NSLog(@"baseString1 : %@",baseString1);    //解码    NSData *decodeData = [Base64 base64Decode:baseString];    NSLog(@"decodeData : %@",decodeData);    NSData *decodeData1 = [[NSData alloc]initWithBase64EncodedString:baseString options:0];    NSLog(@"decodeData1 : %@",decodeData1);    //编码    NSData *base64Data = [data base64EncodedDataWithOptions:0];    NSLog(@"base64Data : %@",base64Data);    //解码    NSData *base64DataDecode = [[NSData alloc]initWithBase64EncodedData:base64Data options:0];    NSLog(@"base64DataDecode : %@",base64DataDecode);    NSString *decodeString = [[NSString alloc]initWithData:base64DataDecode encoding:NSUTF8StringEncoding];    NSLog(@"decodeString : %@",decodeString);}

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