Tag: crc

Amazon S3 支援 MD5 以外的檢查演算法了

Amazon S3 宣佈支援 MD5 以外的檢查演算法了：「New – Additional Checksum Algorithms for Amazon S3」。

多支援了 SHA-1、SHA-256 以及 CRC-32 與 CRC-32C：

In particular, you can specify the use of any one of four widely used checksum algorithms (SHA-1, SHA-256, CRC-32, and CRC-32C) when you upload each of your objects to S3.

雖然有拿 cryptographic hash function 來用，但其實是當作 checksum algorithm 在用，拿來檢查檔案正確性的，而不是防中間被竄改之類 (這個部份是靠 HTTPS)，本來支援的 MD5 應該算是夠用，只是現在多了不少選擇。

然後全商用區都可以用了：

The four additional checksums are now available in all commercial AWS Regions and you can start using them today at no extra charge.

PHP 的 crc32、crc32b 以及 hash_file

查 PHP 上 CRC32 的資料時，查到讓人噴飯的 comment：

For those who are wondering, there appears to be no fundamental difference between hash_file('md5')/hash_file('sha1') and md5_file()/sha1_file(). They produce identical output and have comparable performance.

There is, however, a difference between hash_file('crc32') and something silly like crc32(file_get_contents()).

crc32(file_get_contents())'s results are most similar to those of hash_file('crc32b'), just with the octets reversed:

<?php
$fname = "something.png";

$hash = hash_file( 'crc32', $fname );
echo "crc32  = $hash\n";

$hash = hash_file( 'crc32b', $fname );
echo "crc32b = $hash\n";

$hash = sprintf("%x",crc32(file_get_contents($fname)));
echo "manual = $hash\n";
?>

crc32  = f41d7f4e
crc32b = 7dafbba4
manual = a4bbaf7d

不只是 hash_file() 抓出來不一樣，連 algorithm 都來亂...