Date: Sat, 7 May 2016 11:51:24 -0400
From: Stephen Frost <sfrost@snowman.net>
To: Nagy =?iso-8859-1?B?TOFzemzz?= Zsolt <gandalf@shopzeus.com>
Cc: pgsql-admin@postgresql.org
Subject: Re: md5 auth procotol - can it be replayed?
Message-ID: <20160507155124.GP10850@tamriel.snowman.net>
References: <572E0BFE.7040709@shopzeus.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
	protocol="application/pgp-signature"; boundary="V0qMWm28XkADNbH3"
Content-Disposition: inline
In-Reply-To: <572E0BFE.7040709@shopzeus.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Precedence: bulk
Sender: pgsql-admin-owner@postgresql.org


--V0qMWm28XkADNbH3
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

Greetings,

* Nagy L=E1szl=F3 Zsolt (gandalf@shopzeus.com) wrote:
> How the md5 hashed authentication method works? Is it protected against
> replay attacks? Here is what I have in mind:
>=20
> * If the server stores salted hashed passwords, then I do not see how
> the server could authenticate the users without getting the password in
> clear text?

If you're interested in how these things can be addressed, you should
review the SCRAM protocol.

> * If the server stores (unsalted) password hash values, then basically
> there is almost no difference between a clear text password and an md5
> hash, because anyone can replay the send the same hash value and log in
> again.

If the hash was sent in the clear, then that would be true, but it
isn't.

> Am I missing something?

There is a challenge/response compoent, so the md5 hash which is stored
is not what is sent across the wire.  That prevents replay attacks when
the attacker is simply sniffing the network.  If the attacker is able to
acquire the as-stored hash then, yes, that can be used to authenticate.

Thanks!

Stephen

--V0qMWm28XkADNbH3
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: Digital signature

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1
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=Byo+
-----END PGP SIGNATURE-----

--V0qMWm28XkADNbH3--