Simple file verification (SFV) is a file format for storing

CRC32 A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to digital data. Blocks of data entering these systems get a short ''check value'' attached, based on t ...

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...

s of files to verify the integrity of files. SFV is used to verify that a file has not been corrupted, but it does not otherwise verify the file's

authenticity Authenticity or authentic may refer to: * Authentication, the act of confirming the truth of an attribute Arts and entertainment * Authenticity in art, ways in which a work of art or an artistic performance may be considered authentic Music * A ...

. The

file extension A filename extension, file name extension or file extension is a suffix to the name of a computer file (e.g., .txt, .docx, .md). The extension indicates a characteristic of the file contents or its intended use. A filename extension is typically d ...

is usually used for SFV files.

Checksum

Files can become corrupted for a variety of reasons, including faulty

storage media Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, phonographic recording, magnetic tape, and optical discs are all examples of storage media. Biological molecules such as RNA and DNA are consi ...

, errors in

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission *** ...

, write errors during

copying Copying is the duplication of information or an artifact based on an instance of that information or artifact, and not using the process that originally generated it. With analog forms of information, copying is only possible to a limited degree o ...

or moving, and

software bug A software bug is an error, flaw or fault in the design, development, or operation of computer software that causes it to produce an incorrect or unexpected result, or to behave in unintended ways. The process of finding and correcting bugs i ...

s. SFV verification ensures that a file has not been corrupted by comparing the file's CRC hash value to a previously calculated value. Due to the nature of hash functions,

hash collision In computer science, a hash collision or hash clash is when two pieces of data in a hash table share the same hash value. The hash value in this case is derived from a hash function which takes a data input and returns a fixed length of bits. Al ...

s may result in

false positive A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test result ...

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. However, 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 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 In cryptography, a collision attack on a cryptographic hash tries to find two inputs producing the same hash value, i.e. a hash collision. This is in contrast to a preimage attack where a specific target hash value is specified. There are roughl ...

. For this reason, the

md5sum is a computer program that calculates and verifies 128-bit MD5 hashes, as described in RFC 1321. The MD5 hash functions as a compact digital fingerprint of a file. As with all such hashing algorithms, there is theoretically an unlimited number ...

and

sha1sum is a computer program that calculates and verifies SHA-1 hashes. It is commonly used to verify the integrity of files. It (or a variant) is installed by default on most Linux distributions. Typically distributed alongside are , , and , whic ...

utilities are often preferred in

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...

operating systems, which use the MD5 and

SHA-1 In cryptography, SHA-1 (Secure Hash Algorithm 1) is a cryptographically broken but still widely used hash function which takes an input and produces a 160-bit (20-byte) hash value known as a message digest – typically rendered as 40 hexadecima ...

cryptographic hash function A cryptographic hash function (CHF) is a hash algorithm (a map of an arbitrary binary string to a binary string with fixed size of n bits) that has special properties desirable for cryptography: * the probability of a particular n-bit output re ...

s respectively. Even a single-bit error causes both SFV's CRC and md5sum's cryptographic hash to fail, requiring the entire file to be re-fetched. The

Parchive Parchive (a portmanteau of parity archive, and formally known as Parity Volume Set Specification) is an erasure code system that produces par files for checksum verification of data integrity, with the capability to perform data recovery operatio ...

and

rsync rsync is a utility for efficiently transferring and synchronizing files between a computer and a storage drive and across networked computers by comparing the modification times and sizes of files. It is commonly found on Unix-like operat ...

utilities are often preferred for verifying that a file has not been accidentally corrupted in transmission, since they can correct common small errors with a much shorter download. Despite the weaknesses of the SFV format, it is popular due to the relatively small amount of time taken by SFV utilities to calculate the CRC32 checksums when compared to the time taken to calculate cryptographic hashes such as MD5 or SHA-1. SFV uses a

plain text In computing, plain text is a loose term for data (e.g. file contents) that represent only characters of readable material but not its graphical representation nor other objects (floating-point numbers, images, etc.). It may also include a limit ...

file containing one line for each file and its checksum in the format ''FILENAMECHECKSUM''. Any line starting with a semicolon ';' is considered to be a comment and is ignored for the purposes of file verification. The delimiter between the filename and checksum is always one or several spaces; tabs are never used. A sample SFV file is: ; This is a comment file_one.zip c45ad668 file_two.zip 7903b8e6 file_three.zip e99a65fb

Command-line utility

An example of an

open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

cross-platform In computing, cross-platform software (also called multi-platform software, platform-agnostic software, or platform-independent software) is computer software that is designed to work in several computing platforms. Some cross-platform software r ...

command-line A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...

utility that outputs crc32 checksums is

7-Zip 7-Zip is a free and open-source file archiver, a utility used to place groups of files within compressed containers known as "archives". It is developed by Igor Pavlov and was first released in 1999. 7-Zip has its own archive format called 7z, ...

References

{{reflist, refs= {{citation, url=https://sevenzip.osdn.jp/chm/cmdline/commands/hash.htm, title=h (Hash) command, work=

, date=May 23, 2016 {{cite book, title=Steal this file sharing book: what they won't tell you about file sharing, first=Wallace, last=Wang, year=2004, ISBN=9781593270940

External links

Online SFV Calculator

QuickSFV
SFV checksum verifier (Windows Vista or earlier and Linux)

Opensource Windows/Linux application
Check SFV - SFV software for UNIX systems

Mac OS X, MD5 compatible, free
isfv
sfv checksum verifier for Mac OS X

Windows only

RapidCRC
Freeware application
RekSFV
- SFV, MD5, SHA1 utility (Multi-Language, Unicode, with batch mode for checking a huge amount of folders)
RapidCRC Unicode
RapidCRC with Unicode support (v0.3.4 as of 05/27/2012 supports UTF-8 with or without BOM and UTF-16 LE)
AmoK SFV Utility
- CRC32 and MD5 Compatible
SFV Ninja
- SFV, MD5, SHA-1/256/384/512 utility (Freeware for personal use)

SFVManager

SlavaSoft FSUM
- Fast File Integrity Checker
HashCheck Shell Extension
- SFV, MD4, MD5, SHA-1 (Multi-Language)
Total Commander
- supports creation and verification of SFV files
hkSFV
- supports creation and verification of SFV files (crashes on massive SFV files check)
DySFV
- Open Source (free) application for large files
ilSFV
- free and open-source SFV, MD5 and SHA-1 file verification utility. Computer file formats Checksum algorithms

Checksum

Command-line utility

See also

References

Further reading

External links

Windows only