Adler-32 Checksum Calculation

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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, 29^th October 2007 - First initial release