LLVM IR 深入研究分析
前置知识LLVM是C++编写的构架编译器的框架体系，可用于优化以任意程序语言编写的程序。
LLVM IR可以理解为LLVM平台的汇编语言，以是官方也是以语言参考手册(Language Reference Manual)的情势给出LLVM IR的文档说明。既然是汇编语言，那么就和传统的CUP类似，有特定的汇编指令集。但是它又与传统的特定平台相干的指令集(x86,ARM,RISC-V等)不一样，它定位为平台无关的汇编语言。也就是说，LLVM IR是一种相对于CUP指令集高级，但是又是一种低级的代码中间表示（比抽象语法树等高级表示更加低级)。
LLVM IR即代码的中间表示，有三种情势：
[*].ll 格式：人类可以阅读的文本(汇编码) -->这个就是我们要学习的IR
[*].bc 格式：适合呆板存储的二进制文件
[*]内存表示
下面给出.ll格式和.bc格式生成及相互转换的常用指令清单：
.c -> .ll：clang -emit-llvm -S a.c -o a.ll
.c -> .bc: clang -emit-llvm -c a.c -o a.bc
.ll -> .bc: llvm-as a.ll -o a.bc
.bc -> .ll: llvm-dis a.bc -o a.ll
.bc -> .s: llc a.bc -o a.s那么我们以一道CTF赛题来分析实验，学习LLVM IR
实验剖析
标题附件直接给出了中间表示.II文件
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618479.png
打开查看一下汇编码，毕竟.II文件是人类可以阅读的文本，这边笔者使用的是Sublime Text（使用VScode查看即可）代码量不多，大概600行
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618199.png
标题初步分析
我们直接寻找一下main函数
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618569.png
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618544.png
我们可以看出标题经历了两次RC4，然后Base64，我们从上面可以看到密文，RC4_key,我们直接一把锁，cyberchef启动，会发现解不出来，那么程序应该做了其他的操纵，最质朴的，我们可以想到把RC4魔改了，base64魔改等等。
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618553.png
So！继续学习研究ing
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.II详细分析
以是本着学习的态度，我们这时候应该掏出LLVM Language Reference Manual(官方文档)来简朴了解学习一些常见指令、符号标识以及特性。这边给出一些分析 .ll 中间文件的算法流程
@ - 全局变量
% - 局部变量
alloca - 在当前执行的函数的堆栈帧中分配内存，当该函数返回到其调用者时，将自动释放内存
i32 - 32位4字节的整数
align - 对齐
load - 读出，store写入
icmp - 两个整数值比较，返回布尔值
br - 选择分支，根据条件来转向label，不根据条件跳转的话类型goto
label - 代码标签
call - 调用函数
首先看到一些全局变量，知道了RC4_key = llvmbitccipher = "TSzkWKgbMHszXaj@kLBmRrnTxsNtZsSOtZzqYikCw="
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618578.png
我们继续分析，重点分析各个function
b64encode
b64encode 魔改
[*]每三个字符，24位，切分成4断，每段6位。
[*]将6位对应的值 (value+ 59)&0xff 则是编码后的值。
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618312.png
%22 = getelementptr inbounds i8, i8* %19, i64 %21 // 取出当前处理字符
%23 = load i8, i8* %22, align 1
%24 = zext i8 %23 to i32 // 类型强制转化
%25 = ashr i32 %24, 2 // 算数右移两位 input>>2
%26 = add nsw i32 %25, 59 // input+59
%27 = trunc i32 %26 to i8 // 强制转化相当于 &0xff
%28 = load i8*, i8** %6, align 8
%29 = load i32, i32* %9, align 4
%30 = sext i32 %29 to i64
%31 = getelementptr inbounds i8, i8* %28, i64 %30 // 存储base64 编码串
store i8 %27, i8* %31, align 1
%32 = load i8*, i8** %4, align 8
%33 = load i32, i32* %7, align 4
%34 = sext i32 %33 to i64
%35 = getelementptr inbounds i8, i8* %32, i64 %34
%36 = load i8, i8* %35, align 1
%37 = zext i8 %36 to i32
%38 = and i32 %37, 3 // 获取第一个字符 低两位
%39 = shl i32 %38, 4 // 左移四位RC4_init
RC4_init 正常，无魔改
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618416.png
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618452.png
define dso_local void @Rc4_Init(i8*, i32) #0 { //RC4_init function
%3 = alloca i8*, align 8
%4 = alloca i32, align 4
%5 = alloca i32, align 4
%6 = alloca i32, align 4
store i8* %0, i8** %3, align 8
store i32 %1, i32* %4, align 4 //初始化S，T盒
call void @llvm.memset.p0i8.i64(i8* align 16 getelementptr inbounds (, * @s, i64 0, i64 0), i8 0, i64 256, i1 false)
call void @llvm.memset.p0i8.i64(i8* align 16 getelementptr inbounds (, * @t, i64 0, i64 0), i8 0, i64 256, i1 false)
store i32 0, i32* %5, align 4
br label %7
7: ; preds = %26, %2
%8 = load i32, i32* %5, align 4
%9 = icmp slt i32 %8, 256
br i1 %9, label %10, label %29 //如果 %9 为真（即 %8 小于 256），跳转到标签 %10；否则跳转到标签 %29，根据t打乱s盒
10: ; preds = %7
%11 = load i32, i32* %5, align 4
%12 = trunc i32 %11 to i8
%13 = load i32, i32* %5, align 4
%14 = sext i32 %13 to i64
%15 = getelementptr inbounds , * @s, i64 0, i64 %14
store i8 %12, i8* %15, align 1
%16 = load i8*, i8** %3, align 8
%17 = load i32, i32* %5, align 4
%18 = load i32, i32* %4, align 4
%19 = urem i32 %17, %18
%20 = zext i32 %19 to i64
%21 = getelementptr inbounds i8, i8* %16, i64 %20
%22 = load i8, i8* %21, align 1
%23 = load i32, i32* %5, align 4
%24 = sext i32 %23 to i64
%25 = getelementptr inbounds , * @t, i64 0, i64 %24
store i8 %22, i8* %25, align 1
br label %26
26: ; preds = %10
%27 = load i32, i32* %5, align 4
%28 = add nsw i32 %27, 1
store i32 %28, i32* %5, align 4
br label %7
29: ; preds = %7
store i32 0, i32* %6, align 4
store i32 0, i32* %5, align 4
br label %30
30: ; preds = %54, %29
%31 = load i32, i32* %5, align 4
%32 = icmp slt i32 %31, 256
br i1 %32, label %33, label %57
33: ; preds = %30
%34 = load i32, i32* %6, align 4
%35 = load i32, i32* %5, align 4
%36 = sext i32 %35 to i64
%37 = getelementptr inbounds , * @s, i64 0, i64 %36
%38 = load i8, i8* %37, align 1
%39 = zext i8 %38 to i32
%40 = add nsw i32 %34, %39
%41 = load i32, i32* %5, align 4
%42 = sext i32 %41 to i64
%43 = getelementptr inbounds , * @t, i64 0, i64 %42
%44 = load i8, i8* %43, align 1
%45 = zext i8 %44 to i32
%46 = add nsw i32 %40, %45
%47 = srem i32 %46, 256
store i32 %47, i32* %6, align 4
%48 = load i32, i32* %5, align 4
%49 = sext i32 %48 to i64
%50 = getelementptr inbounds , * @s, i64 0, i64 %49
%51 = load i32, i32* %6, align 4
%52 = sext i32 %51 to i64
%53 = getelementptr inbounds , * @s, i64 0, i64 %52
call void @swap(i8* %50, i8* %53) //call swap function
br label %54RC4_enc
RC4_enc 魔改 多了一层xor 89
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618511.png
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618616.png
define dso_local void @Rc4_Encrypt(i8*, i32) #0 { //RC4_enc function
%3 = alloca i8*, align 8
%4 = alloca i32, align 4
%5 = alloca i8, align 1
%6 = alloca i8, align 1
%7 = alloca i8, align 1
%8 = alloca i8, align 1
store i8* %0, i8** %3, align 8
store i32 %1, i32* %4, align 4
store i8 0, i8* %6, align 1
store i8 0, i8* %7, align 1
store i8 0, i8* %8, align 1
br label %9
9: ; preds = %14, %2
%10 = load i8, i8* %8, align 1
%11 = zext i8 %10 to i32
%12 = load i32, i32* %4, align 4
%13 = icmp ult i32 %11, %12
br i1 %13, label %14, label %64
14: ; preds = %9
%15 = load i8, i8* %6, align 1
%16 = zext i8 %15 to i32
%17 = add nsw i32 %16, 1
%18 = srem i32 %17, 256
%19 = trunc i32 %18 to i8
store i8 %19, i8* %6, align 1
%20 = load i8, i8* %7, align 1
%21 = zext i8 %20 to i32
%22 = load i8, i8* %6, align 1
%23 = zext i8 %22 to i64
%24 = getelementptr inbounds , * @s, i64 0, i64 %23 //生成密钥流
%25 = load i8, i8* %24, align 1
%26 = zext i8 %25 to i32
%27 = add nsw i32 %21, %26
%28 = srem i32 %27, 256
%29 = trunc i32 %28 to i8
store i8 %29, i8* %7, align 1
%30 = load i8, i8* %6, align 1
%31 = zext i8 %30 to i64
%32 = getelementptr inbounds , * @s, i64 0, i64 %31
%33 = load i8, i8* %7, align 1
%34 = zext i8 %33 to i64
%35 = getelementptr inbounds , * @s, i64 0, i64 %34 //经典Swap了再加
call void @swap(i8* %32, i8* %35)
%36 = load i8, i8* %6, align 1
%37 = zext i8 %36 to i64
%38 = getelementptr inbounds , * @s, i64 0, i64 %37
%39 = load i8, i8* %38, align 1
%40 = zext i8 %39 to i32
%41 = load i8, i8* %7, align 1
%42 = zext i8 %41 to i64
%43 = getelementptr inbounds , * @s, i64 0, i64 %42
%44 = load i8, i8* %43, align 1
%45 = zext i8 %44 to i32
%46 = add nsw i32 %40, %45
%47 = srem i32 %46, 256
%48 = sext i32 %47 to i64
%49 = getelementptr inbounds , * @s, i64 0, i64 %48
%50 = load i8, i8* %49, align 1
store i8 %50, i8* %5, align 1
%51 = load i8, i8* %5, align 1
%52 = zext i8 %51 to i32
%53 = xor i32 %52, 89 //xor 89
%54 = load i8*, i8** %3, align 8
%55 = load i8, i8* %8, align 1
%56 = zext i8 %55 to i64
%57 = getelementptr inbounds i8, i8* %54, i64 %56
%58 = load i8, i8* %57, align 1
%59 = zext i8 %58 to i32
%60 = xor i32 %59, %53 //xor k
%61 = trunc i32 %60 to i8
store i8 %61, i8* %57, align 1
%62 = load i8, i8* %8, align 1
%63 = add i8 %62, 1
store i8 %63, i8* %8, align 1
br label %9
64: ; preds = %9
ret void
}main
main函数逻辑cipher -->RC4_init-->RC4_enc-->RC4_enc-->b64encode需要注意一下在RC4_enc的参数中，传入的数据块长度是固定的16，以是说程序进行两次RC4_enc的原因也就确定了，是为了分两次对程序进行加密，也算是一点点小本领，总之，即使让你好好分析.II代码，观察对软件分析的细节，耐心，嘻嘻。
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618234.png
https://m-1254331109.cos.ap-guangzhou.myqcloud.com/202408191618152.png
OK,理清晰逻辑，就可以试着敲代码解密啦。
解密
逆向分析过程明白之后，那么写代码就简朴多了
#includeunsigned char s,t;void b64decode(unsigned char * enc,unsigned char* dec);void Rc4_dec1(int len, unsigned char *enc);void Rc4_Init(char *key,int len);void Rc4_dec2(int len, unsigned char *enc);int main() { unsigned char enc="TSz`kWKgbMHszXaj`@kLBmRrnTxsNtZsSOtZzqYikCw="; unsigned char dec1={0x00}; char key ="llvmbitc"; unsigned char a; int i=0; b64decode(enc,dec1); Rc4_Init(key,8); Rc4_dec1(16,&dec1); for(i=0;i
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