linux-ext4 - Re: Question on ext4 directory hashes in combination with file name encryption

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Message-Id: <CAF99CF6-E26D-4696-9F12-B3DFBC3C7561@dilger.ca>
Date:   Fri, 30 Sep 2016 14:20:00 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Richard Weinberger <richard@....at>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>,
        Eric Biggers <ebiggers@...gle.com>,
        David Gstir <david@...ma-star.at>
Subject: Re: Question on ext4 directory hashes in combination with file name encryption

On Sep 30, 2016, at 8:09 AM, Richard Weinberger <richard@....at> wrote:
> 
> Hi,
> 
> if I read the ext4 code correctly, you pass encrypted filenames to ext4fs_dirhash().
> These filenames are not encoded and therefore binary gibberish.
> Isn't this a problem for the ext4 hash functions? My fear is that these hashes are optimized
> for ASCII strings and produce more collisions when binary data is used as input.

The default hash function (half-md4) is an (old) crypto hash and works
fine with binary data.  Some of the other hash functions are less strong,
but I don't think anyone changes the hash function for ext4.

Cheers, Andreas






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