查看“Checksum”的源代码

{{From|https://help.ubuntu.com/community/Checksum}}
{{Languages|Checksum}}

== Definition ==

A checksum is like a signature of data given in a standard fashion. This representation has a fixed length, and is unique for different strings of data.
'''Files like this usually ends with md5, md5sum, sha, checksum, sum, or something similar'''

The idea is that it is virtually impossible to modify the data without affecting the signature so that a package cannot be corrupted or manipulated without affecting this signature and once it is corrupted or manipulated, you can know about it.

== Normal Usage ==
This feature can be used to verify file downloads. If the owner/maintainer/provider of a file creates a checksum, eg an MD5 or SHA file. You can download the file and also download this checksum file. And once the downloads are finished, you can natively calculate your own checksum using the checksum program. These downloaded and calculated values can be verified, thus telling you that the file downloaded is an exact copy of the original.
Another use is to verify larger automatic backup solutions, such as common mirroring applications as rsync or wget.

== History ==
This system has been used almost since the dawn of computers. And it is used even today without you knowing it, for example there is CRC checking as you transfer TCP/IP packages to view this page.

 how relevent is this???
=== xmodem ===
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Xmodem, a popular file-transfer protocol, uses a 1-byte checksum, which is calculated by adding all the ASCII values for all 128 data bytes and ignoring any numeric overflow. The checksum is added to the end of the Xmodem data packet. This type of checksum does not always detect all the errors. In the later versions of the Xmodem protocol, cyclical redundancy check (CRC) is used instead of more rigorous error control.
''(Ref: Dyson, Dictionary of Networking)''
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== Links ==
[http://freshmeat.net/browse/19/ Freshmeat software repository. Category System Archiving]

[http://ee.tut.fi/~heikki/foss/ckmd5/ chkmd5, a popular md5 checksum verifier]

[http://cfv.sourceforge.net/ cfv, a general checksum creator/checker. This can even verify .torrent files!]

[http://winmd5sum.solidblue.ca/ winmd5sum is a free, open source, easy to use md5 checker for Windows]