Simple file verification: Difference between revisions

Revision as of 00:37, 9 December 2014

Revision as of 22:35, 31 December 2014

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== Checksum ==

Files can become corrupted for a variety of reasons including faulty [[Computer Storage|storage media]], errors in [[Transmission (telecommunications)|transmission]], write errors during [[copying]] or moving, and [[software bug]]s. SFV verification ensures that a file has not been corrupted by comparing the file's [[cyclic redundancy check|CRC]] [[Hash function|hash]] value to a previously calculated value. Due to the nature of hash functions, [[hash collision]]s may result in [[false positive]]s, but the likelihood of collisions is usually negligible with random corruption. (The number of possible checksums is limited though large, so that with any checksum scheme many files will have the same checksum,. althoughHowever, the probability of a corrupted file having the same checksum as its original is exceedingly small, unless deliberately constructed to maintain the checksum.)

SFV cannot be used to verify the authenticity of files, as CRC32 is not a [[collision resistance|collision resistant]] hash function; even if the hash sum file is not tampered with, 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 [[collision attack]]. For this reason, the [[md5sum]] and [[sha1sum]] utilities are often preferred in [[Unix]] operating systems, which use the [[MD5]] and [[SHA-1]] [[cryptographic hash function]]s respectively.