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

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Message-ID: <20160930220533.x4cbv2g2ynsetqhn@thunk.org>
Date:   Fri, 30 Sep 2016 18:05:33 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Richard Weinberger <richard@....at>
Cc:     linux-ext4@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        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 Fri, Sep 30, 2016 at 04:09:09PM +0200, Richard Weinberger wrote:
> 
> if I read the ext4 code correctly, you pass encrypted filenames to ext4fs_dirhash().
> These filenames are not encoded and therefore binary gibberish.

That's correct.

> 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.

I'm not particularly worried.  In general, while the converse is true;
that hashes that assume that they are used for binary data might not
work as well for ASCII strings, the ext4 hash functions should work
fine for binary data.

					- Ted