# apktool

# Cyclic redundancy check

##
A **cyclic redundancy check** (**CRC**) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short *check value* attached, based on the remainder of a polynomial division of their contents. On retrieval, the calculation is repeated and, in the event the check values do not match, corrective action can be taken against data corruption. CRCs can be used for error correction (see bitfilters).^{[1]}
CRCs are so called because the *check* (data verification) value is a *redundancy* (it expands the message without adding information) and the algorithm is based on *cyclic*codes. CRCs are popular because they are simple to implement in binary hardware, easy to analyze mathematically, and 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 32-bit CRC function, used in Ethernet and many other standards, is the work of several researchers and was published in 1975.

A

**cyclic redundancy check**(**CRC**) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short*check value*attached, based on the remainder of a polynomial division of their contents. On retrieval, the calculation is repeated and, in the event the check values do not match, corrective action can be taken against data corruption. CRCs can be used for error correction (see bitfilters).^{[1]}
CRCs are so called because the

*check*(data verification) value is a*redundancy*(it expands the message without adding information) and the algorithm is based on*cyclic*codes. CRCs are popular because they are simple to implement in binary hardware, easy to analyze mathematically, and 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 32-bit CRC function, used in Ethernet and many other standards, is the work of several researchers and was published in 1975.

## Contents

## Introduction[edit]

##
CRCs are based on the theory of cyclic error-correcting codes. The use of systematic cyclic codes, which encode messages by adding a fixed-length check value, for the purpose of error detection in communication networks, was first proposed by W. Wesley Peterson in 1961.^{[2]} 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 *n*-bit CRC applied to a data block of arbitrary length will detect any single error burst not longer than *n* bits and the fraction of all longer error bursts that it will detect is (1 − 2^{−n}).
Specification of a CRC code requires definition of a so-called generator polynomial. This polynomial becomes 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. The important caveat is that the polynomial coefficients are calculated according to the arithmetic of a finite field, so the addition operation can always be performed bitwise-parallel (there is no carry between digits).
In practice, all commonly used CRCs employ the Galois field of two elements, GF(2). The two elements are usually called 0 and 1, comfortably matching computer architecture.
A CRC is called an *n*-bit CRC when its check value is *n* bits long. For a given *n*, multiple CRCs are possible, each with a different polynomial. Such a polynomial has highest degree *n*, which means it has *n* + 1 terms. In other words, the polynomial has a length of *n* + 1; its encoding requires *n* + 1 bits. Note that most polynomial specifications either drop the MSB or LSB, since they are always 1. The CRC and associated polynomial typically have a name of the form CRC-*n*-XXX as in the table below.
The simplest error-detection system, the parity bit, is in fact a 1-bit CRC: it uses the generator polynomial *x* + 1 (two terms), and has the name CRC-1.

CRCs are based on the theory of cyclic error-correcting codes. The use of systematic cyclic codes, which encode messages by adding a fixed-length check value, for the purpose of error detection in communication networks, was first proposed by W. Wesley Peterson in 1961.

^{[2]}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*n*-bit CRC applied to a data block of arbitrary length will detect any single error burst not longer than*n*bits and the fraction of all longer error bursts that it will detect is (1 − 2^{−n}).
Specification of a CRC code requires definition of a so-called generator polynomial. This polynomial becomes 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. The important caveat is that the polynomial coefficients are calculated according to the arithmetic of a finite field, so the addition operation can always be performed bitwise-parallel (there is no carry between digits).

In practice, all commonly used CRCs employ the Galois field of two elements, GF(2). The two elements are usually called 0 and 1, comfortably matching computer architecture.

A CRC is called an

*n*-bit CRC when its check value is*n*bits long. For a given*n*, multiple CRCs are possible, each with a different polynomial. Such a polynomial has highest degree*n*, which means it has*n*+ 1 terms. In other words, the polynomial has a length of*n*+ 1; its encoding requires*n*+ 1 bits. Note that most polynomial specifications either drop the MSB or LSB, since they are always 1. The CRC and associated polynomial typically have a name of the form CRC-*n*-XXX as in the table below.
The simplest error-detection system, the parity bit, is in fact a 1-bit CRC: it uses the generator polynomial

*x*+ 1 (two terms), and has the name CRC-1.## Application[edit]

##
A CRC-enabled device calculates a short, fixed-length binary sequence, known as the *check value* or *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 CRC values do not match, then the block contains a data error.
The device may take corrective action, such as rereading the block or requesting that it be sent again. Otherwise, the data is assumed to be error-free (though, with some small probability, it may contain undetected errors; this is inherent in the nature of error-checking).^{[3]}

A CRC-enabled device calculates a short, fixed-length binary sequence, known as the

*check value*or*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 CRC values do not match, then the block contains a data error.

The device may take corrective action, such as rereading the block or requesting that it be sent again. Otherwise, the data is assumed to be error-free (though, with some small probability, it may contain undetected errors; this is inherent in the nature of error-checking).

^{[3]}## Data integrity[edit]

##
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 recompute the CRC without the substitution being detected. 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 or digital signatures (which are commonly based on cryptographic hash functions).
Secondly, unlike cryptographic hash functions, CRC is an easily reversible function, which makes it unsuitable for use in digital signatures.^{[4]}
Thirdly, CRC is a linear function with a property that^{[5]}

as a result, even if the CRC is encrypted with a stream cipher that uses XOR as its combining operation (or mode of block cipher which effectively turns it into a stream cipher, such as OFB or CFB), both the message and the associated CRC can be manipulated without knowledge of the encryption key; this was one of the well-known design flaws of the Wired Equivalent Privacy (WEP) protocol.^{[6]}

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 recompute the CRC without the substitution being detected. 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 or digital signatures (which are commonly based on cryptographic hash functions).
Secondly, unlike cryptographic hash functions, CRC is an easily reversible function, which makes it unsuitable for use in digital signatures.

^{[4]}
Thirdly, CRC is a linear function with a property that

^{[5]}
as a result, even if the CRC is encrypted with a stream cipher that uses XOR as its combining operation (or mode of block cipher which effectively turns it into a stream cipher, such as OFB or CFB), both the message and the associated CRC can be manipulated without knowledge of the encryption key; this was one of the well-known design flaws of the Wired Equivalent Privacy (WEP) protocol.

^{[6]}## CRC-32 algorithm[edit]

##
This is the algorithm for the CRC-32 variant of CRC.^{[7]} The CRCTable is a memoization of a calculation that would have to be repeated for each byte of the message.

^{[7]}The CRCTable is a memoization of a calculation that would have to be repeated for each byte of the message.

## Computation[edit]

##
This section **does not cite any sources**. *(July 2016)** (Learn how and when to remove this template message)*

To compute an *n*-bit 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 left-hand end of the row.
In this example, we shall encode 14 bits of message with a 3-bit CRC, with a polynomial *x*^{3} + *x* + 1. The polynomial is written in binary as the coefficients; a 3rd-degree polynomial has 4 coefficients (1*x*^{3} + 0*x*^{2} + 1*x* + 1). In this case, the coefficients are 1, 0, 1 and 1. The result of the calculation is 3 bits long.
Start with the message to be encoded:
This is first padded with zeros corresponding to the bit length *n* of the CRC. Here is the first calculation for computing a 3-bit CRC:

This section
does not cite any sources. (July 2016) (Learn how and when to remove this template message) |

To compute an

*n*-bit 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 left-hand end of the row.
In this example, we shall encode 14 bits of message with a 3-bit CRC, with a polynomial

*x*^{3}+*x*+ 1. The polynomial is written in binary as the coefficients; a 3rd-degree polynomial has 4 coefficients (1*x*^{3}+ 0*x*^{2}+ 1*x*+ 1). In this case, the coefficients are 1, 0, 1 and 1. The result of the calculation is 3 bits long.
Start with the message to be encoded:

This is first padded with zeros corresponding to the bit length

*n*of the CRC. Here is the first calculation for computing a 3-bit CRC: