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The effect of a checksum algorithm that yields an n-bit checksum is to map each m-bit message to a corner of a larger hypercube, with dimension m + n. The 2 m + n corners of this hypercube represent all possible received messages. The valid received messages (those that have the correct checksum) comprise a smaller set, with only 2 m corners.
The Internet checksum, [1] [2] also called the IPv4 header checksum is a checksum used in version 4 of the Internet Protocol (IPv4) to detect corruption in the header of IPv4 packets. It is carried in the IPv4 packet header , and represents the 16-bit result of the summation of the header words.
cksum is a command in Unix and Unix-like operating systems that generates a checksum value for a file or stream of data. The cksum command reads each file given in its arguments, or standard input if no arguments are provided, and outputs the file's 32-bit cyclic redundancy check (CRC) checksum and byte count. [1]
To calculate the checksum, each digit in order is multiplied by a weight in the ordered set [7 9 10 5 8 4 2 1 6 3 7 9 10 5 8 4 2] and summed together. The sum modulus 11 is used as an index into the ordered set [1 0 X 9 8 7 6 5 4 3 2], with the first index being zero. The indexed value is the checksum digit.
The first weakness of the simple checksum is that it is insensitive to the order of the blocks (bytes) in the data word (message). If the order is changed, the checksum value will be the same and the change will not be detected. The second weakness is that the universe of checksum values is small, being equal to the chosen modulus.
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A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'-complement operation prior to transmission to detect unintentional all-zero messages. Checksum schemes include parity bits, check digits, and longitudinal redundancy checks.
Among other features, it can calculate various checksums, compare files, or shred files. [1] HxD is distributed as freeware and is available in multiple languages of which the English version is the first in the category of coding utilities on Download.com. [2] The c't magazine has featured HxD in several issues and online-specials. [3] [4]