Checksum

Information about Checksum

This article is about checksums calculated using addition. The term "checksum" is sometimes used in a more general sense to refer to any kind of redundancy check. Checksums on decimal numbers are discussed under check digit.

A checksum is a form of redundancy check, a simple way to protect the integrity of data by detecting errors in data that are sent through space (telecommunications) or time (storage). It works by adding up the basic components of a message, typically the asserted bits, and storing the resulting value. Anyone can later perform the same operation on the data, compare the result to the authentic checksum, and (assuming that the sums match) conclude that the message was probably not corrupted.

An example of a simple checksum:

Given 4 bytes of data (can be done with any number of bytes): 0x25, 0x62, 0x3F, 0x52
Step 1: Adding all bytes together gives 0x118.
Step 2: Drop the Carry Nibble to give you 0x18.
Step 3: Get the two's complement of the 0x18 to get 0xE8. This is the checksum byte.
To Test the Checksum byte simply add it to the original group of bytes. This should give you 0x100.
Drop the carry nibble again giving 0x00. Since it is 0x00 this means no error was detected (although an undetectable error could have occurred).

The simplest form of checksum, which simply adds up the asserted bits in the data, cannot detect a number of types of errors. Such a checksum, for example, is not changed by:

Reordering of the bytes in the message.
Inserting or deleting zero-valued bytes.
Multiple errors which sum to zero.

More sophisticated types of redundancy check, including Fletcher's checksum, Adler-32, and cyclic redundancy checks (CRCs), are designed to address these weaknesses by considering not only the value of each byte but also its position. The cost of the ability to detect more types of errors is the increased complexity of computing the redundancy check value.

These types of redundancy check are useful in detecting accidental modification such as corruption to stored data or errors in a communication channel. However, they provide no security against a malicious agent as their simple mathematical structure makes them trivial to circumvent. To provide this level of integrity, the use of a cryptographic hash function, such as SHA-256, is necessary. (Collisions have been found in the popular MD5 algorithm and finding collisions in SHA-1 seems possible, but there is no evidence as of 2006 that SHA-256 suffers similar weaknesses.)

On Unix, there is a tool called "cksum" that generates both a 32-bit CRC and a byte count for any given input file.

External links

Additive Checksum Algorithms as Error Detection Codes

In telecommunication, a redundancy check is extra data added to a message for the purposes of error detection and correction.

Any hash function can be used as a redundancy check.
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A check digit is a form of redundancy check used for error detection, the decimal equivalent of a binary checksum. It consists of a single digit computed from the other digits in the message.
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In mathematics, computer science, telecommunication, and information theory, error detection and correction has great practical importance in maintaining data (information) integrity across noisy channels and less-than-reliable storage media.
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Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process typically involves the sending of electromagnetic waves by electronic transmitters, but in earlier times telecommunication may have involved the use of
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Computer data storage, computer memory, and often casually storage or memory refer to computer components, devices and recording media that retain digital data used for computing for some interval of time.
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BIT is an acronym for:

Bannari amman Institute of Technology
Bangalore Institute of Technology
Beijing Institute of Technology
Benzisothiazolinone
Bilateral Investment Treaty
Bhilai Institute of Technology - Durg

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nibble (or less commonly, nybble) is the computing term for a four-bit aggregation^[1], or half an octet (an octet being an 8-bit byte). As a nibble contains 4 bits, there are sixteen (2⁴
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The two's complement of a binary number is defined as the value obtained by subtracting the number from a large power of two (specifically, from 2^N for an N-bit two's complement).
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Fletcher's checksum is one of several types of checksum algorithms, which are relatively simple processes used by computers to check the integrity of data.

The implementation is best described on the page for Adler-32 (a very similar algorithm).
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Adler-32 is a checksum algorithm which was invented by Mark Adler. Compared to a cyclic redundancy check of the same length it trades reliability for speed.

History

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A cyclic redundancy check (CRC) is a type of function that takes as input a data stream of any length and produces as output a value of a certain fixed size. The term CRC is often used to denote either the function or the function's output.
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Complexity in general usage is the opposite of simplicity. Complexity in specific usage is the opposite of independence, while complication is the opposite of simplicity.
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In mathematics, a structure on a set, or more generally a type, consists of additional mathematical objects that in some manner attach to the set, making it easier to visualize or work with, or endowing the collection with meaning or significance.
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In cryptography, a cryptographic hash function is a transformation that takes an input and returns a fixed-size string, which is called the hash value. Hash functions with this property are used for a variety of computational purposes, including cryptography.
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MD5

General
Ronald Rivest
April 1992
MD, MD2, MD3, MD4, MD5

Detail
128 bits

4

In cryptography, MD5 (Message-Digest algorithm 5) is a widely used cryptographic hash function with a 128-bit hash value.
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SHA hash functions are five cryptographic hash functions designed by the National Security Agency (NSA) and published by the NIST as a U.S. Federal Information Processing Standard. SHA stands for Secure Hash Algorithm.
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20th century - 21st century - 22nd century
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2006 by topic:
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Unix (officially trademarked as UNIX®) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy.
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cksum is a POSIX command that reads the files specified by the File parameter and calculates a checksum, cyclic redundancy check (CRC) and the byte count for a file or files. If no files are specified, the cksum command reads standard input.
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File verification is the process of using an algorithm for verifying the integrity or authenticity of a computer file. This can be done by comparing two files bit-by-bit, but requires two copies of the same file, and may miss systematic corruptions which might occur to both files.
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In telecommunication, a Hamming code is a linear error-correcting code named after its inventor, Richard Hamming. Hamming codes can detect and correct single-bit errors. In other words, the Hamming distance between the transmitted and received code-words must be zero or one for
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In computer science, the integrity check value (ICV) is a checksum or message footprint that allows an information technology system to detect changes or errors in data, thus ensuring data integrity.
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This is a list of hash functions, including cyclic redundancy checks, checksum functions, and cryptographic hash functions.

Cyclic redundancy checks

Main article: Cyclic redundancy check

Name Length Type
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The Luhn algorithm or Luhn formula, also known as the "modulus 10" or "mod 10" algorithm, is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.
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A parity bit is a binary digit that indicates whether the number of bits with value of one in a given set of bits is even or odd. Parity bits are used as the simplest error detecting code.

There are two types of parity bits: even parity bit and odd parity bit.
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A frame check sequence (FCS) refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction.

The sending host computes a checksum on the entire frame and sends this along.
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