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Re: kwallet crypto misuse
From: Simon McVittie <smcv () debian org>
Date: Fri, 03 Jan 2014 18:13:26 +0000

On 03/01/14 17:44, Daniel Kahn Gillmor wrote:
what kind of hashing and salting are you talking about?  i don't
think hashing and salting makes sense in the context that you were
quoting above.  Are you aware that kwallet stores a database of
passwords that need to be able to be produced back for the user (or
the user's applications) in the clear?

My understanding was that kwallet is like gnome-keyring or the Firefox
password store: it contains a large number of stored passwords, all
encrypted with a (key derived from a) master password. (Terminology in
this email is borrowed from Firefox, and might not match KWallet.)
It's important to distinguish between the stored passwords and the
master password.

Issue 1[1] described in that blog post: the *master* password is
passed through a key derivation function (which does not need to be
reversible) in order to turn it into an encryption key, but that KDF
is not very good (hashing, but no salt), making it vulnerable to
dictionary or brute-force attacks assisted by precomputation (rainbow
tables), particularly if the password used is relatively weak. As far
as I can see, MITRE has not allocated a CVE ID for this. (Or is it
considered to be part of CVE-2013-7252?)

You're right that the *stored* passwords cannot be hashed/salted in a
non-reversible way for this particular use case, because the user
needs to be able to recover the original stored password.

Issue 2 described in that blog post: when the stored passwords are
encrypted using the key derived from the master password, KWallet uses
ECB, which is bad at hiding patterns in data. For instance, if a
password is stored more than once, an attacker can determine that this
is likely to have been done, by noticing the corresponding pattern in
the output. As far as I can see, this is now CVE-2013-7252.

Issue 3 (which exacerbates issue 2 rather than being something
separate, AIUI?) is that the encoding of the stored passwords is
relatively low-entropy: if the stored password happens to be entirely
Latin-1 (which is quite likely in practice), then it has a pattern,
namely "odd-numbered bytes are zero".

    S

[1] I don't know where the boundary between vulnerability and lack of
hardening is, so I'm deliberately using a more neutral term


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