I need CRC Reverse Code for my CRC64 Checksum Coding

Question

Can someone please code me the CRC64 Reverse Algorithm in C#? I am unable to code it, can't understand anything. Thanks,

I have copied the CRC64 Checksum code from C++ and converted it into C# .NET. The entire code is displayed below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CRC64
{
    public class cCRC64
    {
        public UInt64[] crc64_table = {
             0x0000000000000000, 0x42F0E1EBA9EA3693,
             0x85E1C3D753D46D26, 0xC711223CFA3E5BB5,
             0x493366450E42ECDF, 0x0BC387AEA7A8DA4C,
             0xCCD2A5925D9681F9, 0x8E224479F47CB76A,
             0x9266CC8A1C85D9BE, 0xD0962D61B56FEF2D,
             0x17870F5D4F51B498, 0x5577EEB6E6BB820B,
             0xDB55AACF12C73561, 0x99A54B24BB2D03F2,
             0x5EB4691841135847, 0x1C4488F3E8F96ED4,
             0x663D78FF90E185EF, 0x24CD9914390BB37C,
             0xE3DCBB28C335E8C9, 0xA12C5AC36ADFDE5A,
             0x2F0E1EBA9EA36930, 0x6DFEFF5137495FA3,
             0xAAEFDD6DCD770416, 0xE81F3C86649D3285,
             0xF45BB4758C645C51, 0xB6AB559E258E6AC2,
             0x71BA77A2DFB03177, 0x334A9649765A07E4,
             0xBD68D2308226B08E, 0xFF9833DB2BCC861D,
             0x388911E7D1F2DDA8, 0x7A79F00C7818EB3B,
             0xCC7AF1FF21C30BDE, 0x8E8A101488293D4D,
             0x499B3228721766F8, 0x0B6BD3C3DBFD506B,
             0x854997BA2F81E701, 0xC7B97651866BD192,
             0x00A8546D7C558A27, 0x4258B586D5BFBCB4,
             0x5E1C3D753D46D260, 0x1CECDC9E94ACE4F3,
             0xDBFDFEA26E92BF46, 0x990D1F49C77889D5,
             0x172F5B3033043EBF, 0x55DFBADB9AEE082C,
             0x92CE98E760D05399, 0xD03E790CC93A650A,
             0xAA478900B1228E31, 0xE8B768EB18C8B8A2,
             0x2FA64AD7E2F6E317, 0x6D56AB3C4B1CD584,
             0xE374EF45BF6062EE, 0xA1840EAE168A547D,
             0x66952C92ECB40FC8, 0x2465CD79455E395B,
             0x3821458AADA7578F, 0x7AD1A461044D611C,
             0xBDC0865DFE733AA9, 0xFF3067B657990C3A,
             0x711223CFA3E5BB50, 0x33E2C2240A0F8DC3,
             0xF4F3E018F031D676, 0xB60301F359DBE0E5,
             0xDA050215EA6C212F, 0x98F5E3FE438617BC,
             0x5FE4C1C2B9B84C09, 0x1D14202910527A9A,
             0x93366450E42ECDF0, 0xD1C685BB4DC4FB63,
             0x16D7A787B7FAA0D6, 0x5427466C1E109645,
             0x4863CE9FF6E9F891, 0x0A932F745F03CE02,
             0xCD820D48A53D95B7, 0x8F72ECA30CD7A324,
             0x0150A8DAF8AB144E, 0x43A04931514122DD,
             0x84B16B0DAB7F7968, 0xC6418AE602954FFB,
             0xBC387AEA7A8DA4C0, 0xFEC89B01D3679253,
             0x39D9B93D2959C9E6, 0x7B2958D680B3FF75,
             0xF50B1CAF74CF481F, 0xB7FBFD44DD257E8C,
             0x70EADF78271B2539, 0x321A3E938EF113AA,
             0x2E5EB66066087D7E, 0x6CAE578BCFE24BED,
             0xABBF75B735DC1058, 0xE94F945C9C3626CB,
             0x676DD025684A91A1, 0x259D31CEC1A0A732,
             0xE28C13F23B9EFC87, 0xA07CF2199274CA14,
             0x167FF3EACBAF2AF1, 0x548F120162451C62,
             0x939E303D987B47D7, 0xD16ED1D631917144,
             0x5F4C95AFC5EDC62E, 0x1DBC74446C07F0BD,
             0xDAAD56789639AB08, 0x985DB7933FD39D9B,
             0x84193F60D72AF34F, 0xC6E9DE8B7EC0C5DC,
             0x01F8FCB784FE9E69, 0x43081D5C2D14A8FA,
             0xCD2A5925D9681F90, 0x8FDAB8CE70822903,
             0x48CB9AF28ABC72B6, 0x0A3B7B1923564425,
             0x70428B155B4EAF1E, 0x32B26AFEF2A4998D,
             0xF5A348C2089AC238, 0xB753A929A170F4AB,
             0x3971ED50550C43C1, 0x7B810CBBFCE67552,
             0xBC902E8706D82EE7, 0xFE60CF6CAF321874,
             0xE224479F47CB76A0, 0xA0D4A674EE214033,
             0x67C58448141F1B86, 0x253565A3BDF52D15,
             0xAB1721DA49899A7F, 0xE9E7C031E063ACEC,
             0x2EF6E20D1A5DF759, 0x6C0603E6B3B7C1CA,
             0xF6FAE5C07D3274CD, 0xB40A042BD4D8425E,
             0x731B26172EE619EB, 0x31EBC7FC870C2F78,
             0xBFC9838573709812, 0xFD39626EDA9AAE81,
             0x3A28405220A4F534, 0x78D8A1B9894EC3A7,
             0x649C294A61B7AD73, 0x266CC8A1C85D9BE0,
             0xE17DEA9D3263C055, 0xA38D0B769B89F6C6,
             0x2DAF4F0F6FF541AC, 0x6F5FAEE4C61F773F,
             0xA84E8CD83C212C8A, 0xEABE6D3395CB1A19,
             0x90C79D3FEDD3F122, 0xD2377CD44439C7B1,
             0x15265EE8BE079C04, 0x57D6BF0317EDAA97,
             0xD9F4FB7AE3911DFD, 0x9B041A914A7B2B6E,
             0x5C1538ADB04570DB, 0x1EE5D94619AF4648,
             0x02A151B5F156289C, 0x4051B05E58BC1E0F,
             0x87409262A28245BA, 0xC5B073890B687329,
             0x4B9237F0FF14C443, 0x0962D61B56FEF2D0,
             0xCE73F427ACC0A965, 0x8C8315CC052A9FF6,
             0x3A80143F5CF17F13, 0x7870F5D4F51B4980,
             0xBF61D7E80F251235, 0xFD913603A6CF24A6,
             0x73B3727A52B393CC, 0x31439391FB59A55F,
             0xF652B1AD0167FEEA, 0xB4A25046A88DC879,
             0xA8E6D8B54074A6AD, 0xEA16395EE99E903E,
             0x2D071B6213A0CB8B, 0x6FF7FA89BA4AFD18,
             0xE1D5BEF04E364A72, 0xA3255F1BE7DC7CE1,
             0x64347D271DE22754, 0x26C49CCCB40811C7,
             0x5CBD6CC0CC10FAFC, 0x1E4D8D2B65FACC6F,
             0xD95CAF179FC497DA, 0x9BAC4EFC362EA149,
             0x158E0A85C2521623, 0x577EEB6E6BB820B0,
             0x906FC95291867B05, 0xD29F28B9386C4D96,
             0xCEDBA04AD0952342, 0x8C2B41A1797F15D1,
             0x4B3A639D83414E64, 0x09CA82762AAB78F7,
             0x87E8C60FDED7CF9D, 0xC51827E4773DF90E,
             0x020905D88D03A2BB, 0x40F9E43324E99428,
             0x2CFFE7D5975E55E2, 0x6E0F063E3EB46371,
             0xA91E2402C48A38C4, 0xEBEEC5E96D600E57,
             0x65CC8190991CB93D, 0x273C607B30F68FAE,
             0xE02D4247CAC8D41B, 0xA2DDA3AC6322E288,
             0xBE992B5F8BDB8C5C, 0xFC69CAB42231BACF,
             0x3B78E888D80FE17A, 0x7988096371E5D7E9,
             0xF7AA4D1A85996083, 0xB55AACF12C735610,
             0x724B8ECDD64D0DA5, 0x30BB6F267FA73B36,
             0x4AC29F2A07BFD00D, 0x08327EC1AE55E69E,
             0xCF235CFD546BBD2B, 0x8DD3BD16FD818BB8,
             0x03F1F96F09FD3CD2, 0x41011884A0170A41,
             0x86103AB85A2951F4, 0xC4E0DB53F3C36767,
             0xD8A453A01B3A09B3, 0x9A54B24BB2D03F20,
             0x5D45907748EE6495, 0x1FB5719CE1045206,
             0x919735E51578E56C, 0xD367D40EBC92D3FF,
             0x1476F63246AC884A, 0x568617D9EF46BED9,
             0xE085162AB69D5E3C, 0xA275F7C11F7768AF,
             0x6564D5FDE549331A, 0x279434164CA30589,
             0xA9B6706FB8DFB2E3, 0xEB46918411358470,
             0x2C57B3B8EB0BDFC5, 0x6EA7525342E1E956,
             0x72E3DAA0AA188782, 0x30133B4B03F2B111,
             0xF7021977F9CCEAA4, 0xB5F2F89C5026DC37,
             0x3BD0BCE5A45A6B5D, 0x79205D0E0DB05DCE,
             0xBE317F32F78E067B, 0xFCC19ED95E6430E8,
             0x86B86ED5267CDBD3, 0xC4488F3E8F96ED40,
             0x0359AD0275A8B6F5, 0x41A94CE9DC428066,
             0xCF8B0890283E370C, 0x8D7BE97B81D4019F,
             0x4A6ACB477BEA5A2A, 0x089A2AACD2006CB9,
             0x14DEA25F3AF9026D, 0x562E43B4931334FE,
             0x913F6188692D6F4B, 0xD3CF8063C0C759D8,
             0x5DEDC41A34BBEEB2, 0x1F1D25F19D51D821,
             0xD80C07CD676F8394, 0x9AFCE626CE85B507
        };

        /* Initialize a CRC accumulator */
        public void Init_crc(ref UInt64 crc)
        {
            crc = 0xffffffffffffffff;
        }

        /* Finish a CRC calculation */
        public void Fin_crc(ref UInt64 crc)
        {
            crc ^= 0xffffffffffffffff;
        }

        /* Accumulate some (more) bytes into a CRC */
        public void Calc_crc(ref UInt64 crc, Byte[] pbData, uint len)
        {
            /* Constant table for CRC calculation */
            uint __len = len;
            uint pos = 0;
            byte byteInput = 0;

            while (__len-- > 0)
            {
                uint __tab_index = ((uint) (crc >> 56) ^ pbData[pos++]) & 0xFF;
                crc = crc64_table[__tab_index] ^ (crc << 8);
            }
        }
    }
}

Waiting for help!

There are bugs in the `Init_crc` and `Fin_crc` method. The parameters have to be by reference, or the methods have no effect. — Guffa, Aug 21 '10 at 21:38

score 9 · Answer 1 · answered Aug 21 '10 at 21:21

9

No.

Hashing algorithms are not reversible. You can't get back the original data, but you can get all possible sets of a specific length by calculating the hash for all possible combinations and compare to the hash that you have.

This is of course imensely calculation intensive. Just ten bytes for example gives you 1 208 925 819 614 629 174 706 176 combinations to test...

answered Aug 21 '10 at 21:21

Guffa

687,336
108
737
1,005

1

CRC32 can be reversed. I have the working code. I am able to get the text when the text is only 4 bytes, no more, no less. – Tush Aug 21 '10 at 21:29
Yes I missed "ref" attribute to the parameters. I just found it! /* Initialize a CRC accumulator */ public void Init_crc(ref UInt64 crc) { crc = 0xffffffffffffffff; } /* Finish a CRC calculation */ public void Fin_crc(ref UInt64 crc) { crc ^= 0xffffffffffffffff; } – Tush Aug 21 '10 at 21:46
3

@Tush: Not entirely correct: If you have a CRC32 checksum, you can find (by brute force, i.e. by making CRC32 of every possible string) a string which has this checksum; however, there is no guarantee that this is the original string. Many other strings exist which have the same checksum. – Piskvor left the building Aug 22 '10 at 11:18
If the string is made of random bytes, even sequential but not same, the chances of identical crc generation is very rare. – Tush Aug 24 '10 at 12:18
1

@Tush: I don't know about the four-byte case, specifically, but in the general case, the improbability of an accidental collision does not imply that a randomly discovered match is likely to be the original string. For any given CRC value, there is still a vast number of possible matches, of which only one is the original document. – Marcelo Cantos Jun 09 '11 at 07:22
1

Even if hash collisions are rare, they are still very common compared to the number of possible files. For files four bytes long there is by average one possible file per hash code. For files five bytes long there is by average 256 possible files per hash code. For files ten bytes long, there is by average 281474976710656 possible files per hash code. – Guffa Jun 09 '11 at 08:16
4

CRC 32 bit is bijective for a 32 bit input. Therefore, if the input is exactly 32 bits, the CRC can be reversed. Same for 64 bit CRC where the input is 64 bits. – IamIC Sep 12 '12 at 16:08
1

@IanC: Interresting. That of course means that you also have to know the length of the original. – Guffa Sep 12 '12 at 17:28
1

@Guffa the length must be identical to the CRC checksum. Otherwise it's not reversible. – IamIC Sep 12 '12 at 18:05
@Guffa Could you amend the answer to highlight the reversibility in the special case? – mafu Jun 22 '16 at 13:34

I need CRC Reverse Code for my CRC64 Checksum Coding

1 Answers1

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