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{{Short description|Operation of checking the integrity of file}}
{{Refimprove|date=September 2014}}
'''File verification''' is the process of using an [[algorithm]] for verifying the integrity of a [[computer file]], usually by [[checksum]]. This can be done by [[file comparison|comparing two files]] bit-by-bit, but requires two copies of the same file, and may miss systematic corruptions which might occur to both files. A more popular approach is to generate a [[Cryptographic hash function|hash]] of the copied file and comparing that to the hash of the original file.

== Integrity verification ==
[[Computer file|File]] integrity can be compromised, usually referred to as the file becoming [[Data corruption|corrupted]]. A file can become corrupted by a variety of ways: faulty [[storage media]], errors in transmission, write errors during copying or moving, [[software bug]]s, and so on.

[[hash function|Hash]]-based verification ensures that a file has not been corrupted by comparing the file's hash value to a previously calculated value. If these values match, the file is presumed to be unmodified. Due to the nature of hash functions, [[hash collision]]s may result in [[false positive]]s, but the likelihood of collisions is often negligible with random corruption.

== Authenticity verification ==
It is often desirable to verify that a file hasn't been modified in transmission or storage by untrusted parties, for example, to include malicious code such as [[virus]]es or [[Backdoor (computing)|backdoor]]s. To verify the authenticity, a classical hash function is not enough as they are not designed to be [[collision resistance|collision resistant]]; it is computationally trivial for an attacker to cause deliberate hash collisions, meaning that a malicious change in the file is not detected by a hash comparison. In cryptography, this attack is called a [[preimage attack]].

For this purpose, [[cryptographic hash function]]s are employed often. As long as the hash sums cannot be tampered with — for example, if they are communicated over a secure channel — the files can be presumed to be intact. Alternatively, [[digital signature]]s can be employed to assure [[tamper resistance]].

== File formats ==
A '''checksum file''' is a small file that contains the checksums of other files.

There are a few well-known checksum file formats.<ref>
[https://ant.apache.org/manual/Tasks/checksum.html "Checksum"].
</ref>

Several utilities, such as [[md5deep]], can use such checksum files to automatically verify an entire directory of files in one operation.

The particular hash algorithm used is often indicated by the file extension of the checksum file.

The ".sha1" file extension indicates a checksum file containing 160-bit [[SHA-1]] hashes in [[sha1sum]] format.

The ".md5" file extension, or a file named "MD5SUMS", indicates a checksum file containing 128-bit [[MD5]] hashes in [[md5sum]] format.

The ".sfv" file extension indicates a checksum file containing 32-bit CRC32 checksums in [[simple file verification]] format.

The "crc.list" file indicates a checksum file containing 32-bit CRC checksums in brik format.

As of 2012, best practice recommendations is to use [[SHA-2]] or [[SHA-3]] to generate new file integrity digests; and to accept MD5 and SHA-1 digests for backward compatibility if stronger digests are not available.
The theoretically weaker SHA-1, the weaker MD5, or much weaker CRC were previously commonly used for file integrity checks.<ref>NIST.
[http://csrc.nist.gov/groups/ST/hash/policy.html "NIST's policy on hash functions"] {{Webarchive|url=https://web.archive.org/web/20110609064344/http://csrc.nist.gov/groups/ST/hash/policy.html |date=2011-06-09 }}.
2012.</ref><ref>
File Transfer Consulting.
[http://www.filetransferglossary.com/category/integrity/ "Integrity"].
</ref><ref>[http://www.sans.org/security-resources/idfaq/integrity_checker.php "Intrusion Detection FAQ: What is the role of a file integrity checker like Tripwire in intrusion detection?"] {{Webarchive|url=https://web.archive.org/web/20141012055343/http://www.sans.org/security-resources/idfaq/integrity_checker.php |date=2014-10-12 }}.</ref><ref>
Hacker Factor.
[http://fotoforensics.com/tutorial-digest.php "Tutorial: File Digest"].
</ref><ref>
Steve Mead.
[http://www.nsrl.nist.gov/Documents/analysis/draft-060530.pdf "Unique File Identification in the National Software Reference Library"]
p. 4.
</ref><ref>
Del Armstrong.
[http://www.giac.org/paper/gcux/188/introduction-file-integrity-checking-unix-systems/104739 "An Introduction To File Integrity Checking On Unix Systems"].
2003.
</ref><ref>
[http://www.cisco.com/web/about/security/intelligence/iosimage.html "Cisco IOS Image Verification"]
</ref><ref>
Elizabeth D. Zwicky, Simon Cooper, D. Brent Chapman.
[https://books.google.com/books?id=Q0ErhHGxNWcC "Building Internet Firewalls"].
p. 296.
</ref><ref>
Simson Garfinkel, Gene Spafford, Alan Schwartz.
[https://books.google.com/books?id=50maN7VmpusC "Practical UNIX and Internet Security"].
p. 630.
</ref>

CRC checksums cannot be used to verify the authenticity of files, as CRC32 is not a collision resistant hash function -- even if the hash sum file is not tampered with, it is computationally trivial for an attacker to replace a file with the same CRC digest as the original file, meaning that a malicious change in the file is not detected by a CRC comparison.{{Citation needed|date=March 2021}}

== See also ==
* [[Checksum]]
* [[Data deduplication]]

== References ==
<references />

{{Computer files}}

[[Category:Computer files]]
[[Category:Error detection and correction]]

File verification - Revision history

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