 Cyclic redundancy check

A cyclic redundancy check (CRC) is an errordetecting code designed to detect accidental changes to raw computer data, and is commonly used in digital networks and storage devices such as hard disk drives. Blocks of data entering these systems get a short check value attached, derived from the remainder of a polynomial division of their contents; on retrieval the calculation is repeated, and corrective action can be taken against presumed data corruption if the check values do not match.
CRCs are so called because the check (data verification) value is a redundancy (it adds no information to the message) and the algorithm is based on cyclic codes. CRCs are popular because they are simple to implement in binary hardware, are easy to analyze mathematically, and are particularly good at detecting common errors caused by noise in transmission channels. Because the check value has a fixed length, the function that generates it is occasionally used as a hash function. The CRC was invented by W. Wesley Peterson in 1961; the 32bit polynomial used in the CRC function of Ethernet and many other standards is the work of several researchers and was published in 1975.
Contents
Introduction
CRCs are based on the theory of cyclic errorcorrecting codes. The use of systematic cyclic codes, which encode messages by adding a fixedlength check value, for the purpose of error detection in communication networks was first proposed by W. Wesley Peterson in 1961.^{[1]} Cyclic codes are not only simple to implement but have the benefit of being particularly well suited for the detection of burst errors, contiguous sequences of erroneous data symbols in messages. This is important because burst errors are common transmission errors in many communication channels, including magnetic and optical storage devices. Typically, an nbit CRC, applied to a data block of arbitrary length, will detect any single error burst not longer than n bits, and will detect a fraction 1−2^{−n} of all longer error bursts.
Specification of a CRC code requires definition of a socalled generator polynomial. This polynomial resembles the divisor in a polynomial long division, which takes the message as the dividend and in which the quotient is discarded and the remainder becomes the result, with the important distinction that the polynomial coefficients are calculated according to the carryless arithmetic of a finite field. The length of the remainder is always less than the length of the generator polynomial, which therefore determines how long the result can be.
In practice, all commonly used CRCs employ the finite field GF(2). This is the field of two elements, usually called 0 and 1, comfortably matching computer architecture. The rest of this article will discuss only these binary CRCs, but the principles are more general.
The simplest errordetection system, the parity bit, is in fact a trivial 1bit CRC: it uses the generator polynomial x+1.
Application
A CRCenabled device calculates a short, fixedlength binary sequence, known as the check value or improperly the CRC, for each block of data to be sent or stored and appends it to the data, forming a codeword. When a codeword is received or read, the device either compares its check value with one freshly calculated from the data block, or equivalently, performs a CRC on the whole codeword and compares the resulting check value with an expected residue constant. If the check values do not match, then the block contains a data error and the device may take corrective action such as rereading or requesting the block be sent again, otherwise the data is assumed to be errorfree (though, with some small probability, it may contain undetected errors; this is the fundamental nature of errorchecking).^{[2]}
CRCs and data integrity
CRCs are specifically designed to protect against common types of errors on communication channels, where they can provide quick and reasonable assurance of the integrity of messages delivered. However, they are not suitable for protecting against intentional alteration of data. Firstly, as there is no authentication, an attacker can edit a message and recalculate the CRC without the substitution being detected. This is even the case when the CRC is encrypted, leading to one of the design flaws of the WEP protocol.^{[3]} Secondly, the linear properties of CRC codes even allow an attacker to modify a message in such a way as to leave the check value unchanged,^{[4]}^{[5]} and otherwise permit efficient recalculation of the CRC for compact changes. Nonetheless, it is still often falsely assumed that when a message and its correct check value are received from an open channel then the message cannot have been altered in transit.^{[6]}
Cryptographic hash functions, while still not providing security against intentional alteration when used in this manner, can provide stronger error checking in that they do not rely on specific error pattern assumptions.^{[citation needed]} However, they are much slower than CRCs, and are therefore commonly used to protect offline data, such as files on servers or databases.
When stored alongside the data, CRCs and cryptographic hash functions by themselves do not protect against intentional modification of data. Any application that requires protection against such attacks must use cryptographic authentication mechanisms, such as message authentication codes.
Computation of CRC
Main article: Computation of CRCTo compute an nbit binary CRC, line the bits representing the input in a row, and position the (n+1)bit pattern representing the CRC's divisor (called a "polynomial") underneath the lefthand end of the row.
Start with the message to be encoded:
11010011101100
This is first padded with zeroes corresponding to the bit length n of the CRC. Here is the first calculation for computing a 3bit CRC:
11010011101100 000 < input left shifted by 3 bits 1011 < divisor (4 bits) = x³+x+1  01100011101100 000 < result
If the input bit above the leftmost divisor bit is 0, do nothing. If the input bit above the leftmost divisor bit is 1, the divisor is XORed into the input (in other words, the input bit above each 1bit in the divisor is toggled). The divisor is then shifted one bit to the right, and the process is repeated until the divisor reaches the righthand end of the input row. Here is the entire calculation:
11010011101100 000 < input left shifted by 3 bits 1011 < divisor 01100011101100 000 < result 1011 < divisor ... 00111011101100 000 1011 00010111101100 000 1011 00000001101100 000 1011 00000000110100 000 1011 00000000011000 000 1011 00000000001110 000 1011 00000000000101 000 101 1  00000000000000 100 <remainder (3 bits)
Since the leftmost divisor bit zeroed every input bit it touched, when this process ends the only bits in the input row that can be nonzero are the n bits at the righthand end of the row. These n bits are the remainder of the division step, and will also be the value of the CRC function (unless the chosen CRC specification calls for some postprocessing).
The validity of a received message can easily be verified by performing the above calculation again, this time with the check value added instead of zeroes. The remainder should equal zero if there are no detectable errors.
11010011101100 100 < input with check value 1011 < divisor 01100011101100 100 < result 1011 < divisor ... 00111011101100 100 ...... 00000000001110 100 1011 00000000000101 100 101 1  0 < remainder
Mathematics of CRC
Main article: Mathematics of CRCMathematical analysis of this divisionlike process reveals how to pick a divisor that guarantees good errordetection properties. In this analysis, the digits of the bit strings are thought of as the coefficients of a polynomial in some variable x—coefficients that are elements of the finite field GF(2) instead of more familiar numbers. This binary polynomial is treated as a ring. A ring is, loosely speaking, a set of elements somewhat like numbers, that can be operated on by an operation that somewhat resembles addition and another operation that somewhat resembles multiplication, these operations possessing many of the familiar arithmetic properties of commutativity, associativity, and distributivity. Ring theory is part of abstract algebra.
Designing CRC polynomials
The selection of generator polynomial is the most important part of implementing the CRC algorithm. The polynomial must be chosen to maximize the error detecting capabilities while minimizing overall collision probabilities.
The most important attribute of the polynomial is its length (largest degree(exponent) +1 of any one term in the polynomial), because of its direct influence on the length of the computed check value.
The most commonly used polynomial lengths are:
 9 bits (CRC8)
 17 bits (CRC16)
 33 bits (CRC32)
 65 bits (CRC64)
The design of the CRC polynomial depends on the maximum total length of the block to be protected (data + CRC bits), the desired error protection features, and the type of resources for implementing the CRC as well as the desired performance. A common misconception is that the "best" CRC polynomials are derived from either an irreducible polynomial or an irreducible polynomial times the factor (1 + x), which adds to the code the ability to detect all errors affecting an odd number of bits.^{[7]} In reality, all the factors described above should enter in the selection of the polynomial. However, choosing a nonirreducible polynomial can result in missed errors due to the ring having zero divisors.
The advantage of choosing a primitive polynomial as the generator for a CRC code is that the resulting code has maximal total block length; in here if r is the degree of the primitive generator polynomial then the maximal total blocklength is equal to 2^{r} − 1, and the associated code is able to detect any single bit or double errors.^{[8]} If instead, we used as generator polynomial g(x) = p(x)(1 + x), where p(x) is a primitive polynomial of degree r − 1, then the maximal total blocklength would be equal to 2^{r − 1} − 1 but the code would be able to detect single, double, and triple errors.
A polynomial g(x) that admits other factorizations may be chosen then so as to balance the maximal total blocklength with a desired error detection power. A powerful class of such polynomials, which subsumes the two examples described above, is that of BCH codes. Regardless of the reducibility properties of a generator polynomial of degree r, assuming that it includes the "+1" term, such error detection code will be able to detect all error patterns that are confined to a window of r contiguous bits. These patterns are called "error bursts".
Specification of CRC
The concept of the CRC as an errordetecting code gets complicated when an implementer or standards committee turns it into a practical system. Here are some of the complications:
 Sometimes an implementation prefixes a fixed bit pattern to the bitstream to be checked. This is useful when clocking errors might insert 0bits in front of a message, an alteration that would otherwise leave the check value unchanged.
 Sometimes an implementation appends n 0bits (n being the size of the CRC) to the bitstream to be checked before the polynomial division occurs. This has the convenience that the remainder of the original bitstream with the check value appended is exactly zero, so the CRC can be checked simply by performing the polynomial division on the received bitstream and comparing the remainder with zero.
 Sometimes an implementation exclusiveORs a fixed bit pattern into the remainder of the polynomial division.
 Bit order: Some schemes view the loworder bit of each byte as "first", which then during polynomial division means "leftmost", which is contrary to our customary understanding of "loworder". This convention makes sense when serialport transmissions are CRCchecked in hardware, because some widespread serialport transmission conventions transmit bytes leastsignificant bit first.
 Byte order: With multibyte CRCs, there can be confusion over whether the byte transmitted first (or stored in the lowestaddressed byte of memory) is the leastsignificant byte or the mostsignificant byte. For example, some 16bit CRC schemes swap the bytes of the check value.
 Omission of the highorder bit of the divisor polynomial: Since the highorder bit is always 1, and since an nbit CRC must be defined by an (n+1)bit divisor which overflows an nbit register, some writers assume that it is unnecessary to mention the divisor's highorder bit.
 Omission of the loworder bit of the divisor polynomial: Since the loworder bit is always 1, authors such as Philip Koopman represent polynomials with their highorder bit intact, but without the loworder bit (the x^{0} or 1 term). This convention encodes the polynomial complete with its degree in one integer.
These complications mean that there are three common ways to express a polynomial as an integer: the first two, which are mirror images in binary, are the constants found in code; the third is the number found in Koopman's papers. In each case, one term is omitted. So the polynomial x^{4} + x + 1 may be transcribed as:
 0x3 = 0011b, representing
x^{4} +0x^{3} + 0x^{2} + 1x^{1} + 1x^{0} (MSBfirst code)  0xC = 1100b, representing 1x^{0} + 1x^{1} + 0x^{2} + 0x^{3}
+ x^{4}(LSBfirst code)  0x9 = 1001b, representing 1x^{4} + 0x^{3} + 0x^{2} + 1x^{1}
+ x^{0}(Koopman notation)
In the table below they are shown as:
Representations: normal / reversed / reverse of reciprocal 0x3 / 0xC / 0x9 Commonly used and standardized CRCs
Numerous varieties of cyclic redundancy check have been incorporated into technical standards. By no means does one algorithm, or one of each degree, suit every purpose; Koopman and Chakrabarty recommend selecting a polynomial according to the application requirements and the expected distribution of message lengths.^{[9]} The number of distinct CRCs in use have however led to confusion among developers which authors have sought to address.^{[7]} There are three polynomials reported for CRC12,^{[9]} thirteen conflicting definitions of CRC16, and six of CRC32.^{[10]}
The polynomials commonly applied are not the most efficient ones possible. Between 1993 and 2004, Koopman, Castagnoli and others surveyed the space of polynomials up to 16 bits,^{[9]} and of 24 and 32 bits,^{[11]}^{[12]} finding examples that have much better performance (in terms of Hamming distance for a given message size) than the polynomials of earlier protocols, and publishing the best of these with the aim of improving the error detection capacity of future standards.^{[12]} In particular, iSCSI and SCTP have adopted one of the findings of this research, the CRC32C (Castagnoli) polynomial.
The design of the 32bit polynomial most commonly used by standards bodies, CRC32IEEE, was the result of a joint effort for the Rome Laboratory and the Air Force Electronic Systems Division by Joseph Hammond, James Brown and ShyanShiang Liu of the Georgia Institute of Technology and Kenneth Brayer of the MITRE Corporation. The earliest known appearances of the 32bit polynomial were in their 1975 publications: Technical Report 2956 by Brayer for MITRE, published in January and released for public dissemination through DTIC in August,^{[13]} and Hammond, Brown and Liu's report for the Rome Laboratory, published in May.^{[14]} Both reports contained contributions from the other team. In December 1975, Brayer and Hammond presented their work in a paper at the IEEE National Telecommunications Conference: the IEEE CRC32 polynomial is the generating polynomial of a Hamming code and was selected for its error detection performance.^{[15]} Even so, the Castagnoli CRC32C polynomial used in iSCSI or SCTP matches its performance on messages from 58 bits–131 kbits, and outperforms it in several size ranges including the two most common sizes of Internet packet.^{[12]} The ITUT G.hn standard also uses CRC32C to detect errors in the payload (although it uses CRC16CCITT for PHY headers).
The table below lists only the polynomials of the various algorithms in use. Variations of a particular protocol can impose preinversion, postinversion and reversed bit ordering as described above. For example, the CRC32 used in both Gzip and Bzip2 use the same polynomial, but Bzip2 employs reversed bit ordering, while Gzip does not.
CRCs in proprietary protocols might use a nontrivial initial value and final XOR for obfuscation but this does not add cryptographic strength to the algorithm. An unknown errordetecting code can be characterized as a CRC, and as such fully reverseengineered, from its output codewords.^{[16]}
Name Polynomial Representations: normal / reversed / reverse of reciprocal CRC1 x + 1 (most hardware; also known as parity bit) 0x1 / 0x1 / 0x1 CRC4ITU x^{4} + x + 1 (ITUT G.704, p. 12) 0x3 / 0xC / 0x9 CRC5EPC x^{5} + x^{3} + 1 (Gen 2 RFID^{[17]}) 0x09 / 0x12 / 0x14 CRC5ITU x^{5} + x^{4} + x^{2} + 1 (ITUT G.704, p. 9) 0x15 / 0x15 / 0x1A CRC5USB x^{5} + x^{2} + 1 (USB token packets) 0x05 / 0x14 / 0x12 CRC6ITU x^{6} + x + 1 (ITUT G.704, p. 3) 0x03 / 0x30 / 0x21 CRC7 x^{7} + x^{3} + 1 (telecom systems, ITUT G.707, ITUT G.832, MMC, SD) 0x09 / 0x48 / 0x44 CRC8CCITT x^{8} + x^{2} + x + 1 (ATM HEC), ISDN Header Error Control and Cell Delineation ITUT I.432.1 (02/99) 0x07 / 0xE0 / 0x83 CRC8Dallas/Maxim x^{8} + x^{5} + x^{4} + 1 (1Wire bus) 0x31 / 0x8C / 0x98 CRC8 x^{8} + x^{7} + x^{6} + x^{4} + x^{2} + 1 0xD5 / 0xAB / 0xEA^{[9]} CRC8SAE J1850 x^{8} + x^{4} + x^{3} + x^{2} + 1 0x1D / 0xB8 / 0x8E CRC8WCDMA x^{8} + x^{7} + x^{4} + x^{3} + x + 1^{[18]} 0x9B / 0xD9 / 0xCD^{[9]} CRC10 x^{10} + x^{9} + x^{5} + x^{4} + x + 1 (ATM; ITUT I.610) 0x233 / 0x331 / 0x319 CRC11 x^{11} + x^{9} + x^{8} + x^{7} + x^{2} + 1 (FlexRay^{[19]}) 0x385 / 0x50E / 0x5C2 CRC12 x^{12} + x^{11} + x^{3} + x^{2} + x + 1 (telecom systems^{[20]}^{[21]}) 0x80F / 0xF01 / 0xC07^{[9]} CRC15CAN x^{15} + x^{14} + x^{10} + x^{8} + x^{7} + x^{4} + x^{3} + 1 0x4599 / 0x4CD1 / 0x62CC CRC16IBM x^{16} + x^{15} + x^{2} + 1 (Bisync, Modbus, USB, ANSI X3.28, many others; also known as CRC16 and CRC16ANSI) 0x8005 / 0xA001 / 0xC002 CRC16CCITT x^{16} + x^{12} + x^{5} + 1 (X.25, V.41, HDLC, XMODEM, Bluetooth, SD, many others; known as CRCCCITT) 0x1021 / 0x8408 / 0x8810^{[9]} CRC16T10DIF x^{16} + x^{15} + x^{11} + x^{9} + x^{8} + x^{7} + x^{5} + x^{4} + x^{2} + x + 1 (SCSI DIF) 0x8BB7^{[22]} / 0xEDD1 / 0xC5DB CRC16DNP x^{16} + x^{13} + x^{12} + x^{11} + x^{10} + x^{8} + x^{6} + x^{5} + x^{2} + 1 (DNP, IEC 870, MBus) 0x3D65 / 0xA6BC / 0x9EB2 CRC16DECT x^{16} + x^{10} + x^{8} + x^{7} + x^{3} + 1 (cordless telephones)^{[23]} 0x0589 / 0x91A0 / 0x82C4 CRC16Fletcher Not a CRC; see Fletcher's checksum Used in Adler32 A & B CRCs CRC24 x^{24} + x^{22} + x^{20} + x^{19} + x^{18} + x^{16} + x^{14} + x^{13} + x^{11} + x^{10} + x^{8} + x^{7} + x^{6} + x^{3} + x + 1 (FlexRay^{[19]}) 0x5D6DCB / 0xD3B6BA / 0xAEB6E5 CRC24Radix64 x^{24} + x^{23} + x^{18} + x^{17} + x^{14} + x^{11} + x^{10} + x^{7} + x^{6} + x^{5} + x^{4} + x^{3} + x + 1 (OpenPGP) 0x864CFB / 0xDF3261 / 0xC3267D CRC30 x^{30} + x^{29} + x^{21} + x^{20} + x^{15} + x^{13} + x^{12} + x^{11} + x^{8} + x^{7} + x^{6} + x^{2} + x + 1 (CDMA) 0x2030B9C7 / 0x38E74301 / 0x30185CE3 CRC32Adler Not a CRC; see Adler32 See Adler32 CRC32 x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} + x^{11} + x^{10} + x^{8} + x^{7} + x^{5} + x^{4} + x^{2} + x + 1 (ISO 3309, ANSI X3.66, FIPS PUB 71, FEDSTD1003, ITUT V.42, Ethernet, SATA, MPEG2, Gzip, PKZIP, POSIX cksum, PNG^{[24]}, ZMODEM) 0x04C11DB7 / 0xEDB88320 / 0x82608EDB^{[12]} CRC32C (Castagnoli) x^{32} + x^{28} + x^{27} + x^{26} + x^{25} + x^{23} + x^{22} + x^{20} + x^{19} + x^{18} + x^{14} + x^{13} + x^{11} + x^{10} + x^{9} + x^{8} + x^{6} + 1 (iSCSI & SCTP, G.hn payload, SSE4.2) 0x1EDC6F41 / 0x82F63B78 / 0x8F6E37A0^{[12]} CRC32K (Koopman) x^{32} + x^{30} + x^{29} + x^{28} + x^{26} + x^{20} + x^{19} + x^{17} + x^{16} + x^{15} + x^{11} + x^{10} + x^{7} + x^{6} + x^{4} + x^{2} + x + 1 0x741B8CD7 / 0xEB31D82E / 0xBA0DC66B^{[12]} CRC32Q x^{32} + x^{31} + x^{24} + x^{22} + x^{16} + x^{14} + x^{8} + x^{7} + x^{5} + x^{3} + x + 1 (aviation; AIXM^{[25]}) 0x814141AB / 0xD5828281 / 0xC0A0A0D5 CRC40GSM x^{40} + x^{26} + x^{23} + x^{17} + x^{3} + 1 (GSM control channel^{[26]}^{[27]}) 0x0004820009 / 0x9000412000 / 0x8002410004 CRC64ISO x^{64} + x^{4} + x^{3} + x + 1 (HDLC — ISO 3309, SwissProt/TrEMBL; considered weak for hashing^{[28]}) 0x000000000000001B / 0xD800000000000000 / 0x800000000000000D CRC64ECMA182 x^{64} + x^{62} + x^{57} + x^{55} + x^{54} + x^{53} + x^{52} + x^{47} + x^{46} + x^{45} + x^{40} + x^{39} + x^{38} + x^{37} + x^{35} + x^{33} + x^{32} + x^{31} + x^{29} + x^{27} + x^{24} + x^{23} + x^{22} + x^{21} + x^{19} + x^{17} + x^{13} + x^{12} + x^{10} + x^{9} + x^{7} + x^{4} + x + 1 (as described in ECMA182 p. 51) 0x42F0E1EBA9EA3693 / 0xC96C5795D7870F42 / 0xA17870F5D4F51B49 See also
 Computation of CRC
 Error correcting code
 Cyclic code
 Redundancy check
 List of checksum algorithms
 Parity
 Information security
 Simple file verification
 cksum
 Header Error Correction
 Adler32
 Fletcher's checksum
 Mathematics of CRCs
References
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External links
 MathPages  Cyclic Redundancy Checks: overview with an explanation of errordetection of different polynomials.
 Clean C++ CRC32 Source Code; OS and Library Independent
 The CRC Pitstop  home of A Painless Guide to CRC Error Detection Algorithms
 Black, R. (199402) Fast CRC32 in Software; algorithm 4 is used in Linux and infozip's zip and unzip.
 Kounavis, M. and Berry, F. (2005). A Systematic Approach to Building High Performance, Softwarebased, CRC generators, Slicingby4 and slicingby8 algorithms
 pycrc  a free C/C++ source code generator for various CRC algorithms
 CRC32: Generating a checksum for a file, C++ implementation by Brian Friesen
 The CRC++ Project Implementation in C++ which uses template classes to deal with different bit order
 'CRCAnalysis with Bitfilters'.
 Cyclic Redundancy Check: theory, practice, hardware, and software with emphasis on CRC32. A sample chapter from Henry S. Warren, Jr. Hacker's Delight.
 CRCs Code for CRC32 and CRC16 calculation in BASIC, 6502, Z80, PDP11, 6809, 80x86, ARM and C.
 ReverseEngineering a CRC Algorithm
List of Ecma standards Categories: Checksum algorithms
 Finite fields
 Hash functions
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