md5sum is a computer program that calculates and verifies 128-bit MD5 hashes, as described in RFC 1321. The MD5 hash (or checksum) functions as a compact digital fingerprint of a file. It is extremely unlikely that any two non-identical files existing in the real world will have the same MD5 hash (although as with all such hashing algorithms, in theory there is an unlimited number of files that will have any given MD5 hash).

Because almost any change to a file will cause its MD5 hash to also change, the MD5 hash is commonly used to verify the integrity of files (i.e., to verify that a file has not changed as a result of file transfer, disk error, meddling, etc.). The md5sum program is installed by default in most Unix, Linux, and Unix-like operating systems or compatibility layers. BSD variants (including Mac OS X) have a similar utility called md5. Versions for Microsoft Windows do exist (see external links).

Note that a cryptanalytic attack on the MD5 algorithm has been found , which means a method has been found to calculate a file that will have a given md5sum in less than the time required for a brute force attack. Although it would still be quite computationally expensive to construct such a file, md5sum should not be used in situations where security is important (such as cryptographic hashing). It is still useful for general-purpose file integrity verification, such as protecting against random bit flips.

Checking for validity

To see if a given file is valid, pass the output of a previous md5sum run with the switch -c. Thus, the following two steps occur:

  1. Compute the MD5 sum and store it in a file using md5sum file > file.md5
  2. Check for validity with md5sum -c file.md5

If file is not present, omitting the file parameter or passing a - reads from standard input.


See also

External links

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