phc-discussions - A little nit which bothers me...

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Date: Wed, 02 Apr 2014 05:39:15 +0000
From: Poul-Henning Kamp <phk@....freebsd.dk>
To: discussions@...sword-hashing.net
Subject: A little nit which bothers me...

There's one little nit which bothers me, and I've spotted it a
couple of times already so far:

	for some criteria:
		entropy = HASH( static || entropy )

When the static value goes into the HASH before the entropy,
it can be integrated into the initialization of the hash function,
both in hardware and software, trading space for time.

This bothers me because I specifically addressed and commented on
this little detail in my md5crypt 20 years ago:

        /* The password first, since that is what is most unknown */
        MD5_Update(&ctx, pw, strlen(pw));

        /* Then our magic string */
        MD5_Update(&ctx, magic, strlen(magic));

        /* Then the raw salt */
        MD5_Update(&ctx, sp, sl);

And yes, I realize it's all my own fault for not publishing an article
about my thinking back then, but as far as I am concerned this is a
firm rule now:

	Feed data into hash functions in order of decreasing entropy.

If your loop feeds in fixed size data, less than the block size of
the hash, then the order doesn't matter at all:  You can always
integrate the static part into the initialization of the hash.

The corollary rule therefore is:

	Feed at least a block-size worth of entropy before static data.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@...eBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.