越狱开发初级之UUID

元始天尊 发表于 2021-8-4 21:41:55

本帖最后由元始天尊于 2021-8-4 21:45 编辑

## 什么是UUID

&emsp;&emsp;UUID即Unique Device Identifier，唯一设备识别符。iOS上的IDFA/IDFV/OpenUDID/macho-uuid，都采用UUID4格式。UUID是RFC定义的高随机性的标识字符串，结构说明在<https://www.ietf.org/rfc/rfc4122.txt>

```
struct rfc_uuid {
uint32_ttime_low;
uint16_ttime_mid;
uint16_ttime_hi_and_version;       // v=(v&0xfff)|0x400
uint8_t clock_seq_hi_and_reserved; // v=(v&0x3f)|0x80
uint8_t clock_seq_low;
uint8_t node;
};
```

&emsp;&emsp;UUID目前有多个版本，包括UUID1/UUID3/UUID4/UUID5，一个典型的UUID4串`ef9cbf1e-3560-4555-b1aa-81bf4773cd9e`。注意uuid4的结构和其他类型不同，有些bit位固定为0或1，其他bit位完全随机(`********-****-4***-(8|9|a|b)***-************`)，因此如果用上面的结构体解析UUID4串是没有意义的。正如rfc所述:

```
4.4.Algorithms for Creating a UUID from Truly Random or
   Pseudo-Random Numbers
The version 4 UUID is meant for generating UUIDs from truly-random or
pseudo-random numbers.
The algorithm is as follows:
oSet the two most significant bits (bits 6 and 7) of the
   clock_seq_hi_and_reserved to zero and one, respectively.
oSet the four most significant bits (bits 12 through 15) of the
   time_hi_and_version field to the 4-bit version number from
   Section 4.1.3.
oSet all the other bits to randomly (or pseudo-randomly) chosen
   values.
```

&emsp;&emsp;使用ObjC和Python生成uuid4

```
// Objective-C生成UUID
NSString* myuuid = NSUUID.UUID.UUIDString;
// Python生成UUID
import uuid
myuuid = str(uuid.uuid4())
```

&emsp;&emsp;讲一个趣事，之前一个朋友不知道IDFA用UUID4格式生成，用Hook修改系统IDFA获取的部分，改为纯随机，结果跑起来莫名其妙就是崩。。。。改机软件模拟的UUID均为上述方式生成。

## 怎样生成相对固定的uuid4

&emsp;&emsp;此需求看似有点奇怪，但是其实是有用的。在模拟参数的时候如果uuid需要和其他参数配套，那么固定一个参数去生成uuid就十分必要了。这里提供一个使用时间戳生成uuid4的实例

```
void uuid4_format(unsigned char* buf) { // bufsize=16
struct rfc_uuid* uuid = (struct rfc_uuid*)buf;
uuid->time_hi_and_version = (uuid->time_hi_and_version & 0xff0f) | 0x0040;
uuid->clock_seq_hi_and_reserved = (uuid->clock_seq_hi_and_reserved & 0x3f) | 0x80;
}

NSString* uuid4_init_with_ts(time_t ts) {
unsigned char uuid;
if (ts == 0) {
   ts = time(0);
}
((uint32_t*)uuid) = crc32_for_byte(ts & 0xff);
((uint32_t*)uuid) = crc32_for_byte((ts >> 8) & 0xff);
((uint32_t*)uuid) = crc32_for_byte((ts >> 16) & 0xff);
((uint32_t*)uuid) = crc32_for_byte((ts >> 24) & 0xff);
uuid4_format(uuid);
NSUUID* nsuuid = [ initWithUUIDBytes:uuid];
return nsuuid.UUIDString;
}
```

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越狱开发初级之UUID