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Adler-32 Checksum Calculation

By , 29 Oct 2007
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Introduction

Checksums are a common way to ensure data integrity. Adler-32 checksum may be used for revealing the damaged files or to compare two files for identity.

Background (from en.wikipedia.org)

Adler-32 is a checksum algorithm which was invented by Mark Adler. It is almost as reliable as a 32-bit cyclic redundancy check for protecting against accidental modification of data, such as distortions occurring during a transmission.

An Adler-32 checksum is obtained by calculating two 16-bit checksums A and B and concatenating their bits into a 32-bit integer. A is the sum of all bytes in the string; B is the sum of the individual values of A from each step. At the beginning of an Adler-32 run, A is initialized to 1, B to 0. The sums are done modulo 65521 (the largest prime number smaller than 216). The bytes are stored in network order (big endian), B occupying the two most significant bytes. The function may be expressed as A = 1 + D1 + D2 + ... + DN (mod 65521) B = (1 + D1) + (1 + D1 + D2) + ... + (1 + D1 + D2 + ... + DN) (mod 65521) = N×D1 + (N-1)×D2 + (N-2)×D3 + ... + DN + N (mod 65521) Adler-32(D) = B * 65536 + A where D is the string of bytes for which the checksum is to be calculated, and N is the length of D.

Using the Code

Here are the steps needed to add Adler-32 checksum into your program without worrying about how it works:

  • Add the source code AdlerChecksum.cs to your project and add to your source file using FileHelper;
  • Create an instance of the AdlerChecksum class.
  • Call MakeForFile method for checksum calculation.
  • Get Adler-32 checksum value by ChecksumValue property. For printing method ToString can be used.

See example code below:

// Adler-32 checksum using
 AdlerChecksum acs = new AdlerChecksum();
 if (acs.MakeForFile(textPath.Text))
    textVal.Text = acs.ToString(); //success
 else
    textVal.Text = "Unable to get checksum!"; //failure

Conclusion

I hope this code will be useful! Thanks all for the comments and critique. New suggestions and ideas are welcome.

History

  • Version 1.0.0.0, 29th October 2007 - First initial release

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

jukov
Software Developer (Senior) Elmo Motion Control
Israel Israel
Software developer since 1992

Comments and Discussions

 
Questionwhat type of hashing is used in it Pinmemberzameer990915-Apr-10 13:00 
GeneralAccuracy Pinmemberseandillon12-Nov-07 1:47 
GeneralApalling. PinmemberGZero5-Nov-07 1:01 
GeneralRe: Apalling. Pinmembermerlin9817-Nov-07 4:37 

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