, , , , , , . "", :
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;
, , , , , . , , Delphi ( , OP):
- Delphi ( 0). "CMP", "DEC ESI" .
- "XOR EDX" "DIV EBX" , .
?
. "CMP" - ESI KEY, NAME. KEY , NAME, "" NAME, ( , , NULL- , ).
EBX ESI , , AV, , , Delphi EBX ESI ( ).
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 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.
2
SPEED . , OP Assembler, , . , , , , .