13,141,429 members (23,462 online)
alternative version

#### Stats

175.3K views
67 bookmarked
Posted 13 Apr 2009

# How to calculate a CRC in C#

, 13 Apr 2009
 Rate this:
How to calculate CRC in C#?

CRC – Cyclic Redundancy Check is an algorithm which is widely used in different communication protocols, packing and packaging algorithms for assuring robustness of data. The idea behind it is simple – calculate a unique checksum (frame check sequence) for each data frame, based on its content and stick it at the end of each meaningful message. Once data is received, it’s possible to perform the same calculation and compare results – if results are similar, message is ok.

There are two kinds of CRC – 16 and 32 bit. There are also less used checksums for 8 and 64 bits. All this is about appending a string of zeros to the frame equal in number of frames and modulo two device by using generator polynomial containing one or more bits, then checksum to be generated. This is very similar to performing a bit-wise XOR operation in the frame, while the remainder is actually our CRC.

In many industries, the first polynomial is in use to create CRC tables and then apply it for performance purposes. The default polynomial, defined by IEEE 802.3 is 0xA001 for 16 bit and 0×04C11DB7 for 32 bit. We're in C#, thus we should use its inversed version which is 0×8408 for 16 bit and 0xEDB88320 for 32 bit. We're going to use those polynomials also in our sample.

So let’s start. Because CRC is `HashAlgorithm `after all, we can derive our classes from `System.Security.Cryptography.HashAlgorithm` class.

```public class CRC16 : HashAlgorithm {
public class CRC32 : HashAlgorithm {```

Then, upon first creation we'll generate hashtables with CRC values to enhance future performance. It’s all about values table for bytes from 0 to 255 , so we should calculate it only once and then we can use it statically.

```[CLSCompliant(false)]
public CRC16(ushort polynomial)
{
HashSizeValue = 16;
_crc16Table = (ushort[])_crc16TablesCache[polynomial];

if (_crc16Table == null) {
_crc16Table = CRC16._buildCRC16Table(polynomial);
}

Initialize();
}

[CLSCompliant(false)]
public CRC32(uint polynomial)
{
HashSizeValue = 32;
_crc32Table = (uint[])_crc32TablesCache[polynomial];

if (_crc32Table == null) {
_crc32Table = CRC32._buildCRC32Table(polynomial);
}

Initialize();
}```

Then let’s calculate it:

```private static ushort[] _buildCRC16Table(ushort polynomial)
{
// 256 values representing ASCII character codes.
ushort[] table = new ushort[256];
for (ushort i = 0; i < table.Length; i++)
{
ushort value = 0;
ushort temp = i;

for (byte j = 0; j < 8; j++) {
if (((value ^ temp) & 0×0001) != 0) {
value = (ushort)((value >> 1) ^ polynomial);
} else {
value >>= 1;
}

temp >>= 1;
}

table[i] = value;
}

return table;
}

private static uint[] _buildCRC32Table(uint polynomial)
{
uint crc;
uint[] table = new uint[256];
// 256 values representing ASCII character codes.
for (int i = 0; i < 256; i++)
{
crc = (uint)i;
for (int j = 8; j > 0; j–)
{
if ((crc & 1) == 1)
crc = (crc >> 1) ^ polynomial;
else
crc >>= 1;
}

table[i] = crc;
}

return table;
}```

The result will look like this for 32 bits:

```0x00, 0x31, 0x62, 0x53, 0xC4, 0xF5, 0xA6, 0x97,
0xB9, 0x88, 0xDB, 0xEA, 0x7D, 0x4C, 0x1F, 0x2E,
0x43, 0x72, 0x21, 0x10, 0x87, 0xB6, 0xE5, 0xD4,
0xFA, 0xCB, 0x98, 0xA9, 0x3E, 0x0F, 0x5C, 0x6D,
0x86, 0xB7, 0xE4, 0xD5, 0x42, 0x73, 0x20, 0x11,
0x3F, 0x0E, 0x5D, 0x6C, 0xFB, 0xCA, 0x99, 0xA8,
0xC5, 0xF4, 0xA7, 0x96, 0x01, 0x30, 0x63, 0x52,
0x7C, 0x4D, 0x1E, 0x2F, 0xB8, 0x89, 0xDA, 0xEB,
0x3D, 0x0C, 0x5F, 0x6E, 0xF9, 0xC8, 0x9B, 0xAA,
0x84, 0xB5, 0xE6, 0xD7, 0x40, 0x71, 0x22, 0x13,
0x7E, 0x4F, 0x1C, 0x2D, 0xBA, 0x8B, 0xD8, 0xE9,
0xC7, 0xF6, 0xA5, 0x94, 0x03, 0x32, 0x61, 0x50,
0xBB, 0x8A, 0xD9, 0xE8, 0x7F, 0x4E, 0x1D, 0x2C,
0x02, 0x33, 0x60, 0x51, 0xC6, 0xF7, 0xA4, 0x95,
0xF8, 0xC9, 0x9A, 0xAB, 0x3C, 0x0D, 0x5E, 0x6F,
0x41, 0x70, 0x23, 0x12, 0x85, 0xB4, 0xE7, 0xD6,
0x7A, 0x4B, 0x18, 0x29, 0xBE, 0x8F, 0xDC, 0xED,
0xC3, 0xF2, 0xA1, 0x90, 0x07, 0x36, 0x65, 0x54,
0x39, 0x08, 0x5B, 0x6A, 0xFD, 0xCC, 0x9F, 0xAE,
0x80, 0xB1, 0xE2, 0xD3, 0x44, 0x75, 0x26, 0x17,
0xFC, 0xCD, 0x9E, 0xAF, 0x38, 0x09, 0x5A, 0x6B,
0x45, 0x74, 0x27, 0x16, 0x81, 0xB0, 0xE3, 0xD2,
0xBF, 0x8E, 0xDD, 0xEC, 0x7B, 0x4A, 0x19, 0x28,
0x06, 0x37, 0x64, 0x55, 0xC2, 0xF3, 0xA0, 0x91,
0x47, 0x76, 0x25, 0x14, 0x83, 0xB2, 0xE1, 0xD0,
0xFE, 0xCF, 0x9C, 0xAD, 0x3A, 0x0B, 0x58, 0x69,
0x04, 0x35, 0x66, 0x57, 0xC0, 0xF1, 0xA2, 0x93,
0xBD, 0x8C, 0xDF, 0xEE, 0x79, 0x48, 0x1B, 0x2A,
0xC1, 0xF0, 0xA3, 0x92, 0x05, 0x34, 0x67, 0x56,
0x78, 0x49, 0x1A, 0x2B, 0xBC, 0x8D, 0xDE, 0xEF,
0x82, 0xB3, 0xE0, 0xD1, 0x46, 0x77, 0x24, 0x15,
0x3B, 0x0A, 0x59, 0x68, 0xFF, 0xCE, 0x9D, 0xAC```

Now, all we have to do upon request is to lookup into this hash table for related value and XOR it:

```protected override void HashCore(byte[] buffer, int offset, int count)
{
for (int i = offset; i < count; i++)
{
ulong ptr = (_crc & 0xFF) ^ buffer[i];
_crc >>= 8;
_crc ^= _crc32Table[ptr];
}
}

new public byte[] ComputeHash(Stream inputStream)
{
byte[] buffer = new byte[4096];

{
}

return HashFinal();
}

protected override byte[] HashFinal()
{
byte[] finalHash = new byte[4];
ulong finalCRC = _crc ^ _allOnes;

finalHash[0] = (byte)((finalCRC >> 0) & 0xFF);
finalHash[1] = (byte)((finalCRC >> 8) & 0xFF);
finalHash[2] = (byte)((finalCRC >> 16) & 0xFF);
finalHash[3] = (byte)((finalCRC >> 24) & 0xFF);

return finalHash;
}```

We're done. Have a good time and be good people. Also, I want to thank Boris for helping me with this article. He promised to write here some day…

## Share

 Architect Better Place Israel
Hello! My name is Tamir Khason, and I am software architect, project manager, system analyst and [of course] programmer. In addition to writing big amount of documentation, I also write code, a lot of code. I used to work as a freelance architect, project manager, trainer, and consultant here, in Israel, but recently join the company with extremely persuasive idea - to make a world better place. I have very pretty wife and 3 charming kids, but unfortunately almost no time for them.

To be updated within articles, I publishing, visit my blog or subscribe RSS feed. Also you can follow me on Twitter to be up to date about my everyday life.

## You may also be interested in...

 First Prev Next
 [My vote of 2] Doesn't work jefffhaynes29-Apr-16 11:25 jefffhaynes 29-Apr-16 11:25
 My vote of 1 zzfima23-Apr-16 22:15 zzfima 23-Apr-16 22:15
 How many characters of result CRC32Hash BuiLuongTruong2-Aug-15 21:25 BuiLuongTruong 2-Aug-15 21:25
 fff melbin mp6-Jul-15 6:25 melbin mp 6-Jul-15 6:25
 Is it unique always. Member 103097022-Oct-14 11:07 Member 10309702 2-Oct-14 11:07
 Re: Is it unique always. Tim Arheit15-Nov-14 15:47 Tim Arheit 15-Nov-14 15:47
 crc16 bug? Alberto Molero17-Sep-13 22:24 Alberto Molero 17-Sep-13 22:24
 Re: crc16 bug? Alberto Molero17-Sep-13 22:26 Alberto Molero 17-Sep-13 22:26
 My vote of 1 Member 92037495-Sep-13 3:50 Member 9203749 5-Sep-13 3:50
 Small bug when calculating crc over part of byte array Andrew Pearson29-Jul-13 14:31 Andrew Pearson 29-Jul-13 14:31
 Re: code is functional but is trash Andrew Pearson29-Jul-13 14:28 Andrew Pearson 29-Jul-13 14:28
 I dont get it to work for a message with crc16..... is the message coded by another algorithm? ConnyWesth17-Jun-12 21:01 ConnyWesth 17-Jun-12 21:01
 I have done some research with the CRC, and i dont think the crc in my specific problem (AMBrc)is compliant to the CRC16-CCITT standard. I downloaded the crc code to carlculate CRC16-CCITT and CRC32-CCITT but none of them works for calculating the AMBrc messages. I'm convinced it is a 16-bit CRC algorithm but dont know wich, there is so many different algorithms. I added som test-code like this: ```////////////////////////////////////////////////////////////////////////////////// // This is file: Program.cs, it is the main startingpoint for the testproject. ////////////////////////////////////////////////////////////////////////////////// // Author: Conny Westh, 2012 // E-Mail: conny.westh@verimentor.se ////////////////////////////////////////////////////////////////////////////////// // To compile, you only need to include the file CRC16.cs in the project from // http://www.codeproject.com/Articles/35134/How-to-calculate-CRC-in-C?msg=4283673#xx4283673xx ////////////////////////////////////////////////////////////////////////////////// using System; using System.Collections.Generic; using System.Text; using System.IO;   namespace crctest { class Program { static void Main(string[] args) { //testCRC16(); testRigg(); Console.WriteLine("Press [ENTER] to continue..."); Console.ReadLine();   }   public static void testRigg() { //byte[] bytes = new byte[10] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; //byte[] bytes1 = new byte[10] { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 }; byte[] bytes13a = new byte[13] { 0x8e, 0x00, 0x00, 0x00, 0x74, 0xb8, 0x00, 0x00, 0x24, 0x00, 0x01, 0x00, 0x8f }; byte[] bytes13b = new byte[13-8] { 0x24, 0x00, 0x01, 0x00, 0x8f }; byte[] bytes13c = new byte[13-4] { 0x8e, 0x00, 0x00, 0x00, 0x24, 0x00, 0x01, 0x00, 0x8f }; byte[] bytes13d = new byte[13 - 7] { 0x00, 0x00, 0x24, 0x00, 0x01, 0x00 }; byte[] bytes13e = new byte[13 - 9] { 0x24, 0x00, 0x01, 0x00};   byte[] bytes13ar = new byte[bytes13a.Length]; byte[] bytes13br = new byte[bytes13b.Length]; byte[] bytes13cr = new byte[bytes13c.Length]; byte[] bytes13dr = new byte[bytes13d.Length]; byte[] bytes13er = new byte[bytes13b.Length];   for (byte i = 0; i < bytes13a.Length; i++) { bytes13ar[bytes13a.Length - i - 1] = bytes13a[i]; }   for (byte i = 0; i < bytes13b.Length; i++) { bytes13br[bytes13b.Length - i - 1] = bytes13b[i]; }   for (byte i = 0; i < bytes13c.Length; i++) { bytes13cr[bytes13c.Length - i - 1] = bytes13c[i]; }   for (byte i = 0; i < bytes13d.Length; i++) { bytes13dr[bytes13d.Length - i - 1] = bytes13d[i]; }   for (byte i = 0; i < bytes13e.Length; i++) { bytes13er[bytes13e.Length - i - 1] = bytes13e[i]; }     try {   Console.WriteLine(""); Console.WriteLine("Test of transponder message 13 characters!"); test("74b80000", "13a", bytes13a); test("74b80000", "13b", bytes13b); test("74b80000", "13c", bytes13c); test("74b80000", "13d", bytes13d); test("74b80000", "13e", bytes13e);   Console.WriteLine(""); Console.WriteLine("Test of transponder message 13 characters, reverse order!"); test("74b80000", "13ar", bytes13ar); test("74b80000", "13br", bytes13br); test("74b80000", "13cr", bytes13cr); test("74b80000", "13dr", bytes13dr); test("74b80000", "13er", bytes13er);   } catch { } }   public static void test(string expectedCRC, string arrayName, byte[] bytes) { Console.WriteLine("CRC-16 ({0}) for bytearray {1} is {2:x4}", expectedCRC, arrayName, getCRC16(bytes)); //Console.WriteLine("Sum-16 ({0}) for bytearray {1} is {2:x4}", expectedCRC, arrayName, getSum16(bytes)); //Console.WriteLine("CRC-32 ({0}) for bytearray {1} is {2:x8}", expectedCRC, arrayName, getCRC32(bytes)); //Console.WriteLine("Sum-32 ({0}) for bytearray {1} is {2:x8}", expectedCRC, arrayName, getSum32(bytes));   }   public static UInt16 getCRC16(byte[] bytes) { CRC16 crc16 = new CRC16(); byte[] newBytes = new byte[bytes.Length]; byte[] retArr = new byte[2]; for (int i=0; i
 My vote of 5 ShaneMcDonald27-Jan-11 11:55 ShaneMcDonald 27-Jan-11 11:55
 My vote of 3 Reuben200529-Aug-10 0:03 Reuben2005 29-Aug-10 0:03