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Another CRC check question but this time no second file to compare with.

Posted on 2003-03-01
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I am related to production biz and I download important big files about 2-30MB, sent me over by my affiliates and clients. I need to get them checked using a CRC check over Delphi to check their integrity, and to verify not a bit is changed during the download. Ofcourse I don't have anyother file to compare the bytes, therefore, a brief explanation with code will be greatly appreciated.

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Question by:mdadnan
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LVL 7

Accepted Solution

by:
Motaz earned 200 total points
ID: 8048718
You need to check CRC in server, and then check it again in client after download:

Use this unit:

UNIT CRC32;

 (* Comment below is the development history of this unit *)

 {CRC32 calculates a cyclic redundancy code (CRC), known as CRC-32, using
  a byte-wise algorithm.

  (C) Copyright 1989, 1995-1996 Earl F. Glynn, Overland Park, KS.
  All Rights Reserved.

  This UNIT was derived from the CRCT FORTRAN 77 program given in
  "Byte-wise CRC Calculations" by Aram Perez in IEEE Micro, June 1983,
  pp. 40-50.  The constants here are for the CRC-32 generator polynomial,
  as defined in the Microsoft Systems Journal, March 1995, pp. 107-108

  This CRC algorithm emphasizes speed at the expense of the 512 element
  lookup table.}

 { In 1999, This unit updated by Motaz Abdel Azim, motaz1@yahoo.com }
 { The update include ability of getting CRC of any part of file by }
 { adding BytesFromStart and BytesBeforEnd, if both of them are 0, the }
 { CRC code of entire file will be returned }


Interface

Function GetFileCRC (FileName: string;var CRCvalue:  LongInt;
         BytesFromStart, BytesBeforEnd: LongInt): integer;
procedure CalcCRC32 (var Buf: array of byte; nbyte: word;
          var CRCvalue: LongInt);

Implementation

const
    table:  array[0..255] of LongWord =
   ($00000000, $77073096, $EE0E612C, $990951BA,
    $076DC419, $706AF48F, $E963A535, $9E6495A3,
    $0EDB8832, $79DCB8A4, $E0D5E91E, $97D2D988,
    $09B64C2B, $7EB17CBD, $E7B82D07, $90BF1D91,
    $1DB71064, $6AB020F2, $F3B97148, $84BE41DE,
    $1ADAD47D, $6DDDE4EB, $F4D4B551, $83D385C7,
    $136C9856, $646BA8C0, $FD62F97A, $8A65C9EC,
    $14015C4F, $63066CD9, $FA0F3D63, $8D080DF5,
    $3B6E20C8, $4C69105E, $D56041E4, $A2677172,
    $3C03E4D1, $4B04D447, $D20D85FD, $A50AB56B,
    $35B5A8FA, $42B2986C, $DBBBC9D6, $ACBCF940,
    $32D86CE3, $45DF5C75, $DCD60DCF, $ABD13D59,
    $26D930AC, $51DE003A, $C8D75180, $BFD06116,
    $21B4F4B5, $56B3C423, $CFBA9599, $B8BDA50F,
    $2802B89E, $5F058808, $C60CD9B2, $B10BE924,
    $2F6F7C87, $58684C11, $C1611DAB, $B6662D3D,

    $76DC4190, $01DB7106, $98D220BC, $EFD5102A,
    $71B18589, $06B6B51F, $9FBFE4A5, $E8B8D433,
    $7807C9A2, $0F00F934, $9609A88E, $E10E9818,
    $7F6A0DBB, $086D3D2D, $91646C97, $E6635C01,
    $6B6B51F4, $1C6C6162, $856530D8, $F262004E,
    $6C0695ED, $1B01A57B, $8208F4C1, $F50FC457,
    $65B0D9C6, $12B7E950, $8BBEB8EA, $FCB9887C,
    $62DD1DDF, $15DA2D49, $8CD37CF3, $FBD44C65,
    $4DB26158, $3AB551CE, $A3BC0074, $D4BB30E2,
    $4ADFA541, $3DD895D7, $A4D1C46D, $D3D6F4FB,
    $4369E96A, $346ED9FC, $AD678846, $DA60B8D0,
    $44042D73, $33031DE5, $AA0A4C5F, $DD0D7CC9,
    $5005713C, $270241AA, $BE0B1010, $C90C2086,
    $5768B525, $206F85B3, $B966D409, $CE61E49F,
    $5EDEF90E, $29D9C998, $B0D09822, $C7D7A8B4,
    $59B33D17, $2EB40D81, $B7BD5C3B, $C0BA6CAD,

    $EDB88320, $9ABFB3B6, $03B6E20C, $74B1D29A,
    $EAD54739, $9DD277AF, $04DB2615, $73DC1683,
    $E3630B12, $94643B84, $0D6D6A3E, $7A6A5AA8,
    $E40ECF0B, $9309FF9D, $0A00AE27, $7D079EB1,
    $F00F9344, $8708A3D2, $1E01F268, $6906C2FE,
    $F762575D, $806567CB, $196C3671, $6E6B06E7,
    $FED41B76, $89D32BE0, $10DA7A5A, $67DD4ACC,
    $F9B9DF6F, $8EBEEFF9, $17B7BE43, $60B08ED5,
    $D6D6A3E8, $A1D1937E, $38D8C2C4, $4FDFF252,
    $D1BB67F1, $A6BC5767, $3FB506DD, $48B2364B,
    $D80D2BDA, $AF0A1B4C, $36034AF6, $41047A60,
    $DF60EFC3, $A867DF55, $316E8EEF, $4669BE79,
    $CB61B38C, $BC66831A, $256FD2A0, $5268E236,
    $CC0C7795, $BB0B4703, $220216B9, $5505262F,
    $C5BA3BBE, $B2BD0B28, $2BB45A92, $5CB36A04,
    $C2D7FFA7, $B5D0CF31, $2CD99E8B, $5BDEAE1D,

    $9B64C2B0, $EC63F226, $756AA39C, $026D930A,
    $9C0906A9, $EB0E363F, $72076785, $05005713,
    $95BF4A82, $E2B87A14, $7BB12BAE, $0CB61B38,
    $92D28E9B, $E5D5BE0D, $7CDCEFB7, $0BDBDF21,
    $86D3D2D4, $F1D4E242, $68DDB3F8, $1FDA836E,
    $81BE16CD, $F6B9265B, $6FB077E1, $18B74777,
    $88085AE6, $FF0F6A70, $66063BCA, $11010B5C,
    $8F659EFF, $F862AE69, $616BFFD3, $166CCF45,
    $A00AE278, $D70DD2EE, $4E048354, $3903B3C2,
    $A7672661, $D06016F7, $4969474D, $3E6E77DB,
    $AED16A4A, $D9D65ADC, $40DF0B66, $37D83BF0,
    $A9BCAE53, $DEBB9EC5, $47B2CF7F, $30B5FFE9,
    $BDBDF21C, $CABAC28A, $53B39330, $24B4A3A6,
    $BAD03605, $CDD70693, $54DE5729, $23D967BF,
    $B3667A2E, $C4614AB8, $5D681B02, $2A6F2B94,
    $B40BBE37, $C30C8EA1, $5A05DF1B, $2D02EF8D);


procedure CalcCRC32 (var Buf: array of byte; nbyte:
  word; var CRCvalue: LongInt);
   {The following is a little cryptic (but executes very quickly).
    The algorithm is as follows:
      1.  exclusive-or the input byte with the low-order portion of
          the CRC register to get an INDEX
      2.  shift the CRC register eight bits to the right
      3.  exclusive-or the CRC register with the contents of
          Table[INDEX]
      4.  repeat steps 1 through 3 for all bytes}
var
  i: LongInt;
begin
  for i:= 0 to nBYTE-1 do
    CRCvalue := (LongWord(CRCvalue) shr 8)  xor
      Table[Buf[i] xor (CRCvalue and $000000FF)]
end;

(*********************  Get file CRC  *******************)

Function GetFileCRC(FileName: string; var CRCvalue:  LongInt;
            BytesFromStart, BytesBeforEnd: LongInt): integer;
var
  BytesRead: integer;
  F: file;
  Buf: array [0..1023] of byte;
  FSize, TotalBytes, MustRead: integer;
  Error: integer;
begin
  FileMode := 0;  (*** Read only access ***)
  CRCValue := -1;
  AssignFile(F, FileName);
  {$I-}
  Reset(F, 1);
  {$I+}
  Error := IOResult;
  IF Error <> 0 then
  begin
    GetFileCRC:= Error;
    exit;
  end;
  TotalBytes := 0;
  FSize:= FileSize(F)-BytesFromStart-BytesBeforEnd;

  Seek(F, BytesFromStart); (*** Bypass file header ***)
  MustRead:= SizeOf(Buf);

  repeat
    if TotalBytes+MustRead > FSize then
      MustRead:= FSize-TotalBytes;
    {$i-}
    BlockRead(F, Buf, MustRead, BytesRead);
    {$i+}
    Error:= IOResult;
    if Error <> 0 then
    begin
      CloseFile(F);
      GetFileCRC:= Error;
      Exit;
    end;
    CalcCRC32(Buf, BytesRead, CRCvalue);
    Inc(TotalBytes, BytesRead)
  until TotalBytes >= FSize;

  CloseFile(F);
  CRCvalue := NOT CRCvalue;
  GetFileCRC :=0; (*** Successed ***)
end;

end.


To use this function:

Var
  Err: integer;
  CRC: integer;
begin
  Err:= GetFileCRC('c:\command.com', CRC,
         0, 0);
  if Err <> 0 then
   ShowMessage('Error while reading CRC code')
  else
    ShowMessage('File CRC code is: '+IntToStr(CRC));
end;
0
 

Author Comment

by:mdadnan
ID: 8051049
Thank you Motaz for a very brief response and I can see it IS your modified script, and you are very good at it.

But I can't compare the bytes with the server file and the files after download. There are some restrictions that I have to follow before or after download, lets say the download is encrypted, therefore your solution will not work on these terms.

Let me be brief a little on what I am looking for. Lets say I download an Mpeg file sizing about 40MB somewhere with encrypted download or from Kaaza, and there is no second file to compare with online or offline. What I am looking for is a CRC or anyother check maybe specific for file types so that I can verify its integrity and to make sure it isn't corrupted during download. A production file like a large WAV or MP3 file with a changed bit or even byte will introduce a large noise during playback.

But still you did very good with your logic. I have increased points to 100. Thanks again for all your help!
0
 
LVL 1

Assisted Solution

by:SimesA
SimesA earned 200 total points
ID: 8051737
Generally, this can't be done, because you need something to compare against.

You need to know the CRC of the un-corrupted file, i.e. the CRC of the file on the server, or on the PC that it was sent from, and compare this against the CRC you calculate after you've received the file. If they match, the file is almost certainly uncorrupted. If they don't match, the file is corrupt.

Some file formats include a CRC actually within the file. If MPEGs do this (and I don't know whether they do or not), then you can compare the calculated CRC with this one.

Here's a link that may tell you more about the MPEG format: http://www.wotsit.org/search.asp?page=2&s=animated
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Expert Comment

by:CleanupPing
ID: 9317000
mdadnan:
This old question needs to be finalized -- accept an answer, split points, or get a refund.  For information on your options, please click here-> http:/help/closing.jsp#1 
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by:snehanshu
ID: 9999566
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Split between Motaz and SimesA

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