iOS

iOS安全之路--Base64

iOS安全之路

Posted by CatchZeng on May 11, 2016

一、Base64简介

Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一。Base64编码可用于在HTTP环境下传递较长的标识信息。例如,在Java Persistence系统Hibernate中,就采用了Base64来将一个较长的唯一标识符(一般为128-bit的UUID)编码为一个字符串,用作HTTP表单和HTTP GET URL中的参数。在其他应用程序中,也常常需要把二进制数据编码为适合放在URL(包括隐藏表单域)中的形式。此时,采用Base64编码具有不可读性,即所编码的数据不会被人用肉眼所直接看到。做过XMPP开发的朋友可能使用过Base64将图片编码后再传输。另外迅雷的“专用地址”也是用Base64”加密”的,其过程是在地址的前后分别添加AA和ZZ,然后对新的字符串进行Base64编码。

需要说明的是Base64 可以对任意的文件进行加密,但是Base64 过程是可逆的. 可以反向解密。所以,严格地讲Base64只是一种编码方式,并非加密算法。另外Base64将原来8个bit为一个字节的数据,6个bit为一个单位处理,会导致数据量会增大1/3[(8-6)/6]。

二、算法描述

描述出处 转码过程例子: 3*8=4*6

内存1个字符占8位 转前: s 1 3 先转成ascii:对应 115 49 51 2进制: 01110011 00110001 00110011 6个一组(4组) 011100110011000100110011 然后才有后面的 011100 110011 000100 110011 然后计算机是8位8位的存数 6不够,自动就补两个高位0了 所有有了 高位补0 科学计算器输入 00011100 00110011 00000100 00110011 得到 28 51 4 51 查对下照表 c z E z

三、算法实现

@implementation CATSecurity

#pragma mark --
#pragma mark -- Base64

static const char base64EncodingTable[64] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static const short base64DecodingTable[256] = {
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -1, -1, -2,  -1,  -1, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -1, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, 62,  -2,  -2, -2, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -2, -2,  -2,  -2, -2, -2,
    -2, 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, -2,  -2,  -2, -2, -2,
    -2, 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, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2,
    -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,  -2,  -2, -2, -2
};

/**
 *  get data's base64 encoded string
 *
 *  @param data source data
 *
 *  @return base64 encoded string
 */
+(NSString *)base64EncodedStringWithData:(NSData *)data{
    NSUInteger length = data.length;
    if (length == 0)
        return @"";
    
    NSUInteger out_length = ((length + 2) / 3) * 4;
    uint8_t *output = malloc(((out_length + 2) / 3) * 4);
    if (output == NULL)
        return nil;
    
    const char *input = data.bytes;
    NSInteger i, value;
    for (i = 0; i < length; i += 3) {
        value = 0;
        for (NSInteger j = i; j < i + 3; j++) {
            value <<= 8;
            if (j < length) {
                value |= (0xFF & input[j]);
            }
        }
        NSInteger index = (i / 3) * 4;
        output[index + 0] = base64EncodingTable[(value >> 18) & 0x3F];
        output[index + 1] = base64EncodingTable[(value >> 12) & 0x3F];
        output[index + 2] = ((i + 1) < length)
        ? base64EncodingTable[(value >> 6) & 0x3F]
        : '=';
        output[index + 3] = ((i + 2) < length)
        ? base64EncodingTable[(value >> 0) & 0x3F]
        : '=';
    }
    
    NSString *base64 = [[NSString alloc] initWithBytes:output length:out_length encoding:NSASCIIStringEncoding];
    free(output);
    return base64;
}

/**
 *  get source data from base64 encoded string
 *
 *  @param base64EncodedString base64 encoded string
 *
 *  @return source data
 */
+(NSData *)dataWithBase64EncodedString:(NSString *)base64EncodedString{
    NSInteger length = base64EncodedString.length;
    const char *string = [base64EncodedString cStringUsingEncoding:NSASCIIStringEncoding];
    if (string  == NULL)
        return nil;
    
    while (length > 0 && string[length - 1] == '=')
        length--;
    
    NSInteger outputLength = length * 3 / 4;
    NSMutableData *data = [NSMutableData dataWithLength:outputLength];
    if (data == nil)
        return nil;
    if (length == 0)
        return data;
    
    uint8_t *output = data.mutableBytes;
    NSInteger inputPoint = 0;
    NSInteger outputPoint = 0;
    while (inputPoint < length) {
        char i0 = string[inputPoint++];
        char i1 = string[inputPoint++];
        char i2 = inputPoint < length ? string[inputPoint++] : 'A';
        char i3 = inputPoint < length ? string[inputPoint++] : 'A';
        
        output[outputPoint++] = (base64DecodingTable[i0] << 2)
        | (base64DecodingTable[i1] >> 4);
        if (outputPoint < outputLength) {
            output[outputPoint++] = ((base64DecodingTable[i1] & 0xf) << 4)
            | (base64DecodingTable[i2] >> 2);
        }
        if (outputPoint < outputLength) {
            output[outputPoint++] = ((base64DecodingTable[i2] & 0x3) << 6)
            | base64DecodingTable[i3];
        }
    }
    
    return data;
}

/**
 *  get str's base64 encoded string
 *
 *  @param str source string
 *
 *  @return base64 encoded string
 */
+(NSString *)base64EncodedStringWithString:(NSString *)str{
    NSData* data = [str dataUsingEncoding:NSUTF8StringEncoding];
    return [self base64EncodedStringWithData:data];
}

/**
 *  get source string from base64 encoded string
 *
 *  @param base64EncodedString base64 encoded string
 *
 *  @return source string
 */
+ (NSString *)stringWithBase64EncodedString:(NSString *)base64EncodedString{
    NSData *data = [self dataWithBase64EncodedString:base64EncodedString];
    return [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
}

@end

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