Zip And CRC

Posted on 2001-07-06
Last Modified: 2010-08-05
Does anyone know how Zip works, and his CRC.
I need details...

Question by:Yevgen
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions

Expert Comment

ID: 6261300
I have CRC code:



 (* 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, }
 { 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 }


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


    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
      4.  repeat steps 1 through 3 for all bytes}
  i: LongInt;
  for i:= 0 to nBYTE-1 do
    CRCvalue := (LongWord(CRCvalue) shr 8)  xor
      Table[Buf[i] xor (CRCvalue and $000000FF)]

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

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

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

    if TotalBytes+MustRead > FSize then
      MustRead:= FSize-TotalBytes;
    BlockRead(F, Buf, MustRead, BytesRead);
    Error:= IOResult;
    if Error <> 0 then
      GetFileCRC:= Error;
    CalcCRC32(Buf, BytesRead, CRCvalue);
    Inc(TotalBytes, BytesRead)
  until TotalBytes >= FSize;

  CRCvalue := NOT CRCvalue;
  GetFileCRC :=0; (*** Successed ***)



This is an example of using CRC32.pas unit:

- Add CRC32 to Uses clause.
- Write this code:

  Err: integer;
  CRC: integer;
  Err:= GetFileCRC('c:\', CRC,
         0, 0);
  if Err <> 0 then
   ShowMessage('Error while reading CRC code')
    ShowMessage('File CRC code is: '+IntToStr(CRC));

Very simple! isn't it ?

GetFileCRC function declared as:

Function GetFileCRC(FileName: string;
   var CRCvalue:  LongInt;
   BytesBeforEnd: LongInt): integer;
BytesFromStart and BytesBeforEnd enables you get file CRC code for any part of a file.  For example you can get file CRC without first 10 bytes:

  GetFileCRC('c:\', CRC, 10, 0);

If you want to get file CRC except last 4 bytes you can write:

  GetFileCRC('c:\', CRC, 0, 4);


Author Comment

ID: 6263300
I didnt understand a thing, i asked how does WinZip work and WHAT is CRC...

Accepted Solution

felonious earned 100 total points
ID: 6263569
Not getting technical (because I really don't know much beyond what I write here) WinZip and most other compression programs have 'tables' that contain strings of bytes that seem to happen a lot in data files (or whatever it is you're compressing).  If winzip finds one of these patterns it can then substitute in a predefined code that is smaller (byte wise) then the original string of bytes.  On uncompression, winzip can look for these codes and put the strings back in.  There is definately more to it then this, but I am no compression expert.  

CRC is a calculated value that will be different for all combinations of bytes.  so CRC for 'AVA' is different then CRC for 'ABA' and even the CRC for 'aba'.  WinZip, on compressing a file, can then calculate a CRC for it, and then put the CRC in the ZIP file.  On uncompression WinZip will uncompress the file, generate a CRC for that file and compair it to the orginal CRC.

Hope this helps.

Independent Software Vendors: We Want Your Opinion

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!


Expert Comment

ID: 6263571
by the way Motaz, very nice CRC32 unit :)

LVL 49

Expert Comment

ID: 7085584
Hi Yevgen,
You've requested to delete this question, but its status remains as 'Pending Delete' because one or more comments have been added.  Normally, the only way to fully delete such a Question is to post a message to Community Support and ask for assistance.

EE is making a one-time database sweep to purge the Pending Delete Questions automatically.  During this sweep:

    Yevgen -- To allow the deletion to proceed:  Do nothing.
    EXPERTS -- Please DON'T POST a comment except to contest this deletion.

In the future, please refer to for instruction on deleting questions.

DanRollins -- EE database cleanup volunteer

Expert Comment

ID: 7104968
Moondancer - EE Moderator

Featured Post

Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Objective: - This article will help user in how to convert their numeric value become words. How to use 1. You can copy this code in your Unit as function 2. than you can perform your function by type this code The Code   (CODE) The Im…
Introduction The parallel port is a very commonly known port, it was widely used to connect a printer to the PC, if you look at the back of your computer, for those who don't have newer computers, there will be a port with 25 pins and a small print…
There are cases when e.g. an IT administrator wants to have full access and view into selected mailboxes on Exchange server, directly from his own email account in Outlook or Outlook Web Access. This proves useful when for example administrator want…
Add bar graphs to Access queries using Unicode block characters. Graphs appear on every record in the color you want. Give life to numbers. Hopes this gives you ideas on visualizing your data in new ways ~ Create a calculated field in a query: …
Suggested Courses
Course of the Month10 days, 17 hours left to enroll

631 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question