string - Delphi:将字符串放入编辑框中时出现访问冲突？

Question

嗯，我正在 Delphi 中研究一些内联汇编，汇编加密例程一切进展顺利，直到我尝试将 ShortString 解析到文本框中。

我收到的违规信息如下:

完整代码在这里:

procedure TForm2.Button1Click(Sender: TObject);

var
len,keylen:integer;
name, key:ShortString;

begin

name :=  ShortString(Edit1.Text);
key := '_r <()<1-Z2[l5,^';
len := Length(name);
keylen := Length(key);

nameLen := len;
serialLen := keyLen;

asm

  XOR EAX,EAX
  XOR ESI,ESI
 XOR EDX,EDX
 XOR ECX,ECX


  @loopBegin:

        MOV EAX,ESI
        PUSH $019
        CDQ
        IDIV DWORD PTR DS:[serialLen]
        MOV EAX,ESI
        POP EBX
        LEA ECX,DWORD PTR DS:[key+EDX]
        CDQ
        IDIV DWORD PTR DS:[nameLen]
        LEA EAX,DWORD PTR DS:[name]
        MOVZX EAX,BYTE PTR DS:[name+EDX]
        MOVZX EDX,BYTE PTR DS:[ECX]
        XOR EAX,EDX
        CDQ
        IDIV EBX
        ADD DL,$041
        INC ESI
        CMP ESI,DWORD PTR DS:[serialLen]
        MOV BYTE PTR DS:[ECX],DL

        JL @loopBegin


end;

edit2.Text:= TCaption(key);


end;

如果我在“edit2.Text:= TCaption(key);”行上放置一个断点我可以看到 ShortString“key”确实已经被正确加密，但后面也有很多奇怪的字符。

前16个字符是真正的加密。

encryption http://img831.imageshack.us/img831/365/29944312.png

bigger version: http://img831.imageshack.us/img831/365/29944312.png

谢谢!

Answer 1

代码的作用

对于那些不懂汇编语言的人来说，这就是 Pascal 代码应该做的事情。 “可能”是因为原始版本包含一些错误:

procedure TForm14.Button1Click(Sender: TObject);
var KeyLen:Integer;
    Name, Key:ShortString;
    i:Integer;
    CurrentKeyByte:Byte;
    CurrentNameByte:Byte;
begin
  Name := ShortString(Edit1.Text);
  Key := '_r <()<1-Z2[l5,^';
  keyLen := Length(key);

  asm int 3 end; // This is here so I can inspect the assembler output in the IDE
                 // for the "Optimised" version of the code

  for i:=1 to Length(Name) do
  begin
    CurrentKeyByte := Byte(Key[i mod KeyLen]);
    CurrentNameByte := Byte(Name[i]);
    CurrentNameByte := ((CurrentKeyByte xor CurrentNameByte) mod $019) + $041;
    Name[i] := AnsiChar(CurrentNameByte);
  end;

  Caption := Name;

end;

打开优化后，由此生成的汇编代码实际上比建议的代码更短，不包含冗余代码，而且我敢打赌更快。以下是我在 Delphi 生成的代码中注意到的一些优化(与 OP 提出的汇编代码相比):

• Delphi 反转了循环(降至 0)。这节省了一条“CMP”指令，因为编译器可以简单地“DEC ESI”并在零标志上循环。
• 使用“XOR EDX”和“DIV EBX”进行第二次除法，节省了一些周期。

为什么提供的汇编代码失败？

这是原始汇编代码，带有注释。该错误位于例程末尾的“CMP”指令处 - 它将 ESI 与 KEY 的长度进行比较，而不是与 NAME 的长度进行比较。如果 KEY 比 NAME 长，则“加密”会在 NAME 之上进行，覆盖一些内容(被覆盖的内容包括字符串的 NULL 终止符，导致调试器在正确的字符后面显示有趣的字符)。

虽然不允许覆盖 EBX 和 ESI，但这并不是导致代码为 AV 的原因，可能是因为周围的 Delphi 代码没有使用 EBX 或 ESI(刚刚尝试过)。

asm

 XOR EAX,EAX ; Wasteful, the first instruction in Loop overwrites EAX
 XOR ESI,ESI
 XOR EDX,EDX ; Wasteful, the first CDQ instruction in Loop overwrites EDX
 XOR ECX,ECX ; Wasteful, the first LEA instruction overwrites ECX


 @loopBegin:
       ; Etering the loop, ESI holds the index for the next char to be
       ; encrypted.

       MOV EAX,ESI ; Load EAX with the index for the next char, because
                   ; we intend to do some divisions (setting up the call to IDIV)
       PUSH $019   ; ? pushing this here, so we can pop it 3 lines later... obfuscation
       CDQ         ; Sign-extend EAX (required for IDIV)
       IDIV DWORD PTR DS:[serialLen] ; Divide EAX by the length of the key.
       MOV EAX,ESI ; Load the index back to EAX, we're planning on an other IDIV. Why???
       POP EBX     ; Remember the PUSH $019?
       LEA ECX,DWORD PTR DS:[key+EDX] ; EDX is the result of "ESI mod serialLen", this
                                      ; loads the address of the current char in the
                                      ; encryption key into ECX. Dividing by serialLen
                                      ; is supposed to make sure we "wrap around" at the
                                      ; end of the key
        CDQ ; Yet some more obfuscation. We're now extending EAX into EDX in preparation for IDIV.
            ; This is obfuscation becasue the "MOV EAX, ESI" instruction could be written right here
            ; before the CDQ.
        IDIV DWORD PTR DS:[nameLen] ; We divide the current index by the length of the text
                                    ; to be encrypted. Once more the code will only use the reminder,
                                    ; but why would one do this? Isn't ESI (the index) always supposed to
                                    ; be LESS THEN nameLen? This is the first sign of trouble.
        LEA EAX,DWORD PTR DS:[name] ; EAX now holds the address of NAME.
        MOVZX EAX,BYTE PTR DS:[name+EDX] ; EAX holds the current character in name
        MOVZX EDX,BYTE PTR DS:[ECX]      ; EDX holds the current character in Key
        XOR EAX,EDX ; Aha!!!! So this is an obfuscated XOR loop! EAX holds the "name[ESI] xor key[ESI]"
        CDQ         ; We're extending EAX (the XOR result) in preparation for a divide
        IDIV EBX    ; Divde by EAX by EBX (EBX = $019). Why????
        ADD DL,$041 ; EDX now holds the remainder of our previous XOR, after the division by $019;
                    ; This is an number from $000 to $018. Adding $041 turns it into an number from
                    ; $041 to $05A (ASCII chars from "A" to "Z"). Now I get it. This is not encryption,
                    ; this is a HASH function! One can't un-encrypt this (information is thrown away at
                    ; the division).
        INC ESI     ; Prep for the next char


        ; !!! BUG !!!
        ;
        ; This is what's causing the algorithm to generate the AV. At this step the code is
        ; comparing ESI (the current char index) to the length of the KEY and loops back if
        ; "ESI < serialLen". If NAME is shorter then KEY, encryption will encrypt stuff beyond
        ; then end of NAME (up to the length of KEY). If NAME is longer then KEY, only Length(Key)
        ; bytes would be encrypted and the rest of "Name" would be ignored.
        ;
        CMP ESI,DWORD PTR DS:[serialLen]


        MOV BYTE PTR DS:[ECX],DL ; Obfuscation again. This is where the mangled char is written
                                 ; back to "Name".

        JL @loopBegin            ; Repeat the loop.

我的 2 美分建议

汇编器应该用于速度优化，而不是其他。在我看来，OP 似乎试图使用汇编器来混淆代码正在执行的操作。没有帮助，我只花了几分钟就弄清楚代码到底在做什么，而且我不是汇编专家。