Checksums are fixed blocks of data used to verify and maintain data integrity during storage and transmission. They are used in file transfer protocols and can detect and repair minor problems with data integrity. CRCs are a type of checksum that can identify accidental alterations of the original data format. Checksums are useful for detecting errors in files downloaded over the Web or via peer-to-peer networks. They are generally only useful for verifying and repairing small blocks of data, while CRCs can be applied to larger blocks of data.
Checksums are fixed blocks of information, or data, used to verify and maintain data integrity during the storage and transmission of computer information. Using mathematical functions, a checksum algorithm calculates the number of bits in a particular block of information and generates a number that represents accurate data. When data is duplicated by any means, the duplicate is then compared to the original number, via the calculated value. If the numbers match, the data is considered complete and accurate.
Most valid file transfer protocols (FTP) require some sort of data verification, and many features are capable of not only detecting, but also repairing minor problems with data integrity. Some types of checksums, also known as hash sums, include md5 and Cyclic Redundancy Check (CRC). CRCs are a type of checksum known as a polynomial code checksum, which can identify accidental alterations of the original data format.
While useful for detecting errors in files downloaded over the Web or via peer-to-peer (P2P) networks, checksums are also used in any function where data can become corrupted when transferred over a network or storage medium. Data errors are often caused by missing, duplicated, or bad bits. Because the value of a checksum is calculated based on each bit in a block of data, even one missing bit can cause a checksum error.
Most files encode checksums directly into their data, usually appended to the end of the file. When the file is transferred to another system or device, the recipient reads the checksum and uses its algorithm to verify that the entire file has been received without errors. Files with missing, corrupted, or repeating bits may not work properly or at all. Others may appear to work fine despite the checksum failure. Incomplete or corrupted data can continue to degrade over time, eventually becoming unusable or causing errors.
Due to their nature, basic checksums are generally only useful for verifying and repairing small blocks of data. As the amount of information increases, so does the likelihood of an error that the checksum can neither detect nor correct. This can cause the corrupted data to repeatedly fail the checksum, generate more errors, and even corrupt the checksum data.
Cyclic redundancy checks, due to the greater complexity of their algorithms, have a much lower failure rate than a standard checksum and can be applied to larger blocks of data. While CRCs are not secure, they still provide greater reliability when checking and maintaining data integrity. Some software also provides checksum and error repair capabilities based on custom functions.
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