關於SFV的小知識

[psac]

在下載Warez時, ftp下傳都經常可以看到一些.sfv的文件檔，到底這種文件是這麼回事呢，這裡有比較詳細的說明：

sfv是simple file verification（簡單文件校驗）的簡寫，
可以以文書編輯或筆記本打開本文編寫............
但通常網上已程式, quicksfv , flashsfv...等可創建
它雖說是文字檔,但不成文並沒規範...
有時創建的sfv ,被其他程式來檢 驗檔案使用,會有錯誤出現....
本文判斷錯誤,並影響crc校驗......只需改正回來...

但原理是一樣!
它採用的是crc32的校驗算法。
crc32校驗產生32bit（8位十六進制數）校驗值，所以一般大家看見的文件「sfv值」是一個8位的16進制數。
由於crc32產生校驗值時源數據塊的每一個bit位都參與了計算，所以數據塊中即使一位bit發生變化，也會得到不同的crc32值（但對於兩個完全不相干的文件是有可能有相同校驗值的，因為校驗值的取值空間有限——2^32，而現實中文件數量一定是大大超過這個值了）。

關於crc32校驗的具體算法，大致的過程是（沒興趣的可以忽略這一段）：
採用多項式除法，將一個數據塊（不論大小）中的每一位（bit）作為各項係數來產生一個很長的多項式（例如對一個16進制的數F0h，可以產生多項式1*X^7 + 1*X^6 + 1*X^5 + 1*X^4 + 0*X^3 + 0*X^2 + 0*X^1 + 0*X^0，化簡一下，就變成X^7 + X^6 + X^5 + X^4），然後將產生的多項式除以如下被稱為「發生器多項式」的特殊多項式（X^32+X^26+X^23+X^22+X^16+X^12+X^11+X^10+X^8+X^7+X^5+X^4+X^2+X^1+1），取結果的餘數部分，就是我們需要的crc32（也就是sfv）值了。至於除法的具體算法，有硬體實現也有很多軟件實現，這裡就不再贅述。

md5是一個消息摘要算法（也叫單向散列函數）的名稱。它通過一定的算法（任何有關應用密碼學的書裡都有具體的算法表述）根據源數據的每一位bit最終產生一個128bit（32位十六進制數）的校驗值。由於長度達到一定的要求，所以在密碼學領域中有一定應用（用它來校驗文件理論上講由於有更大的取值空間而肯定比sfv更安全，但實際上似乎sfv就夠用了）。


Quick Verification
QuickSFV was designed with one goal ... to be quick! Existing SFV file verification programs were slow and cumbersome to use. Most users need to verify files and the quicker, the better. QuickSFV integrates into the Windows Explorer shell and makes it very easy to verify files. Just double-click on the .SFV file and QuickSFV does the rest. QuickSFV also understands several other file verification formats including .CRC, .CSV and .CKZ.

For even quicker verification of .SFV files, QuickSFV remembers which files have been previously verified and will skip them in future checks. You can always tell QuickSFV to go back and verify all files again just to be sure.

Quick Creation

The same fast CRC engine inside QuickSFV can also be used to create .SFV files. From Windows Explorer, just select the files you wish to include in the .SFV files, right-click on them and select Create .SFV File.

Quick Features

QuickSFV has many other features to help minimize the work in verifying files. From the line-by-line analysis, to the ending summary information, to hitting ESC to close the window, QuickSFV was designed to eliminate all of the bottlenecks in verifying files so you can get to the important stuff.

Quick Download

QuickSFV is small. At approximately 133,000 bytes, it is one of the smallest, yet most useful utilities you may ever see on the Internet. Use the download link to the left and try it for yourself.




home page:
http://www.geocities.com/SiliconVall.../overview.html
download file:
http://zarius.com/quicksfv/qsfv222.exe
QSFV222.EXE - QuickSFV version 2.22 for Windows9x/NT/2000/XP.

[boss]

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