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). In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital It works by adding up the basic components of a message, typically the asserted bits, and storing the resulting value. A bit is a binary digit, taking a value of either 0 or 1 Binary digits are a basic unit of Information storage and communication 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 most likely 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. A nibble (often nybble) is the Computing term for a four- Bit aggregation or half an octet (an octet being an 8-bit Byte
• Step 3: Get the two's complement of the 0x18 to get 0xE8. 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 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. 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 Adler-32 is a Checksum Algorithm which was invented by Mark Adler. 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 space commonly a 32-bit integer The cost of the ability to detect more types of errors is the increased complexity of computing the redundancy check value. In general usage complexity often tends to be used to characterize something with many parts in intricate arrangement

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. In Mathematics, a structure on a set, or more generally a type, consists of additional Mathematical objects that in some manner attach to the To provide this level of integrity, the use of a cryptographic hash function, such as SHA-256, is necessary. A cryptographic Hash function is a transformation that takes an input (or 'message' and returns a fixed-size string which is called the hash value (sometimes (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. In Cryptography, MD5 ( Message-Digest algorithm 5) is a widely used partially insecure Cryptographic hash function with a 128- Bit hash value Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. )

On Unix, there is a tool called "cksum" that generates both a 32-bit CRC and a byte count for any given input file. Unix (officially trademarked as UNIX, sometimes also written as Unix with Small caps) is a computer 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

See also

• Check digit
• File verification
• Hamming code
• List of checksum algorithms
• Luhn algorithm
• Parity bit
• Frame check sequence

A check digit is a form of Redundancy check used for Error detection, the decimal equivalent of a binary Checksum. File verification is the process of using an Algorithm for verifying the integrity or authenticity of a Computer file. In Telecommunication, a Hamming code is a linear Error-correcting code named after its inventor Richard Hamming. This is a list of Hash functions including Cyclic redundancy checks Checksum functions and Cryptographic hash functions Cyclic redundancy checks The Luhn algorithm or Luhn formula, also known as the " modulus 10" or "mod 10" Algorithm,is a simple Checksum formula used Error detection If an odd number of bits (including the parity bit are changed in transmission of a set of bits then parity bit will be incorrect and will thus indicate A frame check sequence (FCS refers to the extra Checksum characters added to a frame in a communication protocol for Error detection and correction

© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic