为什么嵌入式CRC和当前CRC不同？

时间：2014-01-13 09:19:15

标签： delphi delphi-xe5 crc32

我找到了this德尔福考试。它应该嵌入CRC并检查当前的CRC。两者都应该匹配，但我会得到不同的结果。怎么解决？以及如何加快速度？

CRC32Calc.pas 

unit CRC32Calc;

interface

uses Classes, SysUtils, windows, messages;

type
  Long = record
    LoWord: Word;
    HiWord: Word;
  end;

const
  CRCPOLY = $EDB88320;

procedure BuildCRCTable;
function RecountCRC(b: byte; CrcOld: LongWord): LongWord;
function GetCRC32(FileName: string; Full: boolean): string;

function SetEmbeddedCRC(FileName: string): string;
function GetEmbeddedCRC(FileName: string): string;

function BytesToHexStr(pB: PByte; BufSize: LongWord): String;
function HexStrToBytes(Str: String): String;

implementation

var
  CRCTable: array [0 .. 512] Of LongWord;

  // A helper routine that creates and initializes
  // the lookup table that is used when calculating a CRC polynomial
procedure BuildCRCTable;
var
  i, j: Word;
  r: LongWord;
begin
  FillChar(CRCTable, SizeOf(CRCTable), 0);
  for i := 0 to 255 do
  begin
    r := i shl 1;
    for j := 8 downto 0 do
      if (r and 1) <> 0 then
        r := (r Shr 1) xor CRCPOLY
      else
        r := r shr 1;
    CRCTable[i] := r;
  end;
end;

// A helper routine that recalculates polynomial relative to the specified byte
function RecountCRC(b: byte; CrcOld: LongWord): LongWord;
begin
  RecountCRC := CRCTable[byte(CrcOld xor LongWord(b))
    ] xor ((CrcOld shr 8) and $00FFFFFF)
end;

// A helper routine that converts Word into String
function HextW(w: Word): string;
const
  h: array [0 .. 15] Of char = '0123456789ABCDEF';
begin
  HextW := '';
  HextW := h[Hi(w) shr 4] + h[Hi(w) and $F] + h[Lo(w) shr 4] + h[Lo(w) and $F];
end;

// A helper routine that converts LongWord into String
function HextL(l: LongWord): string;
begin
  with Long(l) do
    HextL := HextW(HiWord) + HextW(LoWord);
end;

// Calculate CRC32 checksum for the specified file
function GetCRC32(FileName: string; Full: boolean): string;
var
  f: TFileStream;
  i, CRC: LongWord;
  aBt: byte;
begin
  // Build a CRC table
  BuildCRCTable;

  CRC := $FFFFFFFF;
  // Open the file
  f := TFileStream.Create(FileName, (fmOpenRead or fmShareDenyNone));

  // To calculate CRC for the whole file use this loop boundaries
  if Full then
    for i := 0 to f.Size - 1 do
    begin
      f.Read(aBt, 1);
      CRC := RecountCRC(aBt, CRC);
    end
  else
    // To calculate CRC for the file excluding the last 4 bytes
    // use these loop boundaries
    for i := 0 to f.Size - 5 do
    begin
      f.Read(aBt, 1);
      CRC := RecountCRC(aBt, CRC);
    end;

  f.Destroy;
  CRC := Not CRC;

  Result := HextL(CRC);
end;

// Calculate CRC and writes it to the end of file
function SetEmbeddedCRC(FileName: string): string;
var
  f: TFileStream;
  CRCOffset: LongWord;
  CRC: string;
begin
  f := TFileStream.Create(FileName, (fmOpenReadWrite or fmShareDenyNone));
  CRCOffset := f.Size;

  // Append a placeholder for actual CRC to the file
  f.Seek(CRCOffset, TSeekOrigin.soBeginning);
  f.Write(PByte(HexStrToBytes('FFFFFFFF'))^, 4);

  // Obtain CRC
  CRC := GetCRC32(FileName, True);

  // Write CRC to the end of file
  f.Seek(CRCOffset, TSeekOrigin.soBeginning);
  f.Write(PByte(HexStrToBytes(CRC))^, 4);
  f.Destroy;
  Result := CRC;
end;

// Extract the CRC that was stored at last 4 bytes of a file
function GetEmbeddedCRC(FileName: string): string;
var
  f: TFileStream;
  CRCOffset: LongWord;
  pB: PByte;
begin
  GetMem(pB, 4);

  // Open file
  f := TFileStream.Create(FileName, (fmOpenRead or fmShareDenyNone));

  // Proceed upto the end of file
  CRCOffset := f.Size - 4;
  f.Seek(CRCOffset, TSeekOrigin.soBeginning);

  // Read the last four bytes where the CRC is stored
  f.Read(pB^, 4);
  f.Destroy;
  Result := BytesToHexStr(pB, 4);
end;

// A helper routine that converts byte value to string with hexadecimal integer
function BytesToHexStr(pB: PByte; BufSize: LongWord): String;
var
  i, j, b: LongWord;
begin
  SetLength(Result, 2 * BufSize);

  for i := 1 to BufSize do
  begin
    for j := 0 to 1 do
    begin
      if j = 1 then
        b := pB^ div 16
      else
        b := pB^ - (pB^ div 16) * 16;
      case b of
        0:
          Result[2 * i - j] := '0';
        1:
          Result[2 * i - j] := '1';
        2:
          Result[2 * i - j] := '2';
        3:
          Result[2 * i - j] := '3';
        4:
          Result[2 * i - j] := '4';
        5:
          Result[2 * i - j] := '5';
        6:
          Result[2 * i - j] := '6';
        7:
          Result[2 * i - j] := '7';
        8:
          Result[2 * i - j] := '8';
        9:
          Result[2 * i - j] := '9';
        10:
          Result[2 * i - j] := 'A';
        11:
          Result[2 * i - j] := 'B';
        12:
          Result[2 * i - j] := 'C';
        13:
          Result[2 * i - j] := 'D';
        14:
          Result[2 * i - j] := 'E';
        15:
          Result[2 * i - j] := 'F';
      end;
    end;

    Inc(pB);
  end;
end;

// A helper routine that converts string with hexadecimal integer to byte value
function HexStrToBytes(Str: String): String;
var
  b, b2: byte;
  lw, lw2, lw3: LongWord;
begin
  lw := Length(Str) div 2;
  SetLength(Result, lw);

  for lw2 := 1 to lw do
  begin
    b := 0;

    for lw3 := 0 to 1 do
    begin
      case Str[2 * lw2 - lw3] of
        '0':
          b2 := 0;
        '1':
          b2 := 1;
        '2':
          b2 := 2;
        '3':
          b2 := 3;
        '4':
          b2 := 4;
        '5':
          b2 := 5;
        '6':
          b2 := 6;
        '7':
          b2 := 7;
        '8':
          b2 := 8;
        '9':
          b2 := 9;
        'a':
          b2 := 10;
        'b':
          b2 := 11;
        'c':
          b2 := 12;
        'd':
          b2 := 13;
        'e':
          b2 := 14;
        'f':
          b2 := 15;
        'A':
          b2 := 10;
        'B':
          b2 := 11;
        'C':
          b2 := 12;
        'D':
          b2 := 13;
        'E':
          b2 := 14;
        'F':
          b2 := 15;
      else
        b2 := 0;
      end;

      if lw3 = 0 then
        b := b2
      else
        b := b + 16 * b2;
    end;

    Result[lw2] := char(b);
  end;
end;

end.

AppendCRC 

program AppendCRC;

{$APPTYPE CONSOLE}

uses
  SysUtils, Classes,
  CRC32Calc in '..\CRC32Checker\CRC32Calc.pas';

var
  FileName: string;

begin
  { TODO -oUser -cConsole Main : Insert code here }
  if ParamCount = 1 then
  begin
    FileName := ParamStr(1);
    // Verify whether a file exists
    if not FileExists(FileName) then
    begin
      WriteLn('The specified file does not exist.');
      Exit;
    end;
    WriteLn('Full checksum (before): ' + GetCRC32(FileName, True));
    SetEmbeddedCRC(FileName);
    WriteLn('Half checksum: ' + GetCRC32(FileName, False));
    WriteLn('Full checksum (after): ' + GetCRC32(FileName, True));
    WriteLn('GetEmbeddedCRC: :' + GetEmbeddedCRC(FileName));
    WriteLn('The checksum was successfully embedded.')
  end
  else
  begin;
    WriteLn('Wrong parameters.');
    WriteLn('Parameter1 - Full path to file.');;
  end;

end.

我的结果是：

AppendCRC.exe Hello_Delphi_World.exe
Full checksum (before): 1912DA64
Half checksum: 1912DA64
Full checksum (after): B3F0A43E
GetEmbeddedCRC: :4400A000
The checksum was successfully embedded.

我正在使用Delphi XE5。

1 个答案:

答案 0 :(得分：1)

您应该了解此代码的工作原理。 总体思路是将CRC作为EXE结构中的额外4个字节附加到文件末尾。 （更好的想法是在开始时将CRC放入EXE标题内的特殊字段中。）

然而，这引起了母鸡和蛋的问题：在我们计算CRC并嵌入它之后 - CRC文件被更改（附加了CRC的值）并且更改文件的CRC也改变了。

所以你基本上必须实现CRC计算的两种模式/功能：对于整个文件和没有最后4个字节的文件。你应该使用后一种模式来计算附加后的CRC（你称之为嵌入），而前一种模式在vanilla刚编译的程序之前计算CRC。

你的GetCRC32函数总是从文件中删除最后4个字节，因此在嵌入之前它只计算文件某些部分的CRC，而不是整个文件的CRC。但是有两种不同的模式。

PS：您也可以将CRC“嵌入”到NTFS备用流中，就像将MyApp.exe程序和CRC存储为MyApp.exe:CRC一样。

PPS。我认为在GetCRC32中逐字节使用无缓冲读取应该非常慢。如果可能的话，最好使用TBytesStream将文件作为整体读入内存，然后在通常的数组循环中扫描。或者通过4096字节的块而不是字节变量读取它。 对于最后一个非完整缓冲区，您可以使用零来清理缓冲区的其余部分。

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