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Posted to dev@spamassassin.apache.org by bu...@bugzilla.spamassassin.org on 2018/02/21 14:43:12 UTC
[Bug 7559] New: DKIM default minimum key size should be 1024 bits
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
Bug ID: 7559
Summary: DKIM default minimum key size should be 1024 bits
Product: Spamassassin
Version: SVN Trunk (Latest Devel Version)
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P2
Component: Plugins
Assignee: dev@spamassassin.apache.org
Reporter: davej@apache.org
Target Milestone: Undefined
Current 3.4 branch and trunk DKIM code is allowing a 768 bit key size.
OpenDKIM header:
================
Authentication-Results: smtp3i.ena.net;
dkim=policy reason="signing key too small" (768-bit key)
header.d=mails-express.com header.i=@mails-express.com header.b="Mv82gS9m"
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed;
d=mails-express.com; s=default;
That message is hitting DKIM_VALID but shouldn't be considered valid these days
since a key size less than 1024 can be cracked trivially.
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[Bug 7559] DKIM default minimum key size should be 1024 bits
Posted by bu...@spamassassin.apache.org.
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
Bill Cole <bi...@apache.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |billcole@apache.org
--- Comment #4 from Bill Cole <bi...@apache.org> ---
(In reply to Giovanni Bechis from comment #2)
> AFAIK there is no standard that requires 1024 bit as minimum key size, is
> just good practice having bigger keys.
https://tools.ietf.org/html/rfc6376#section-3.3.3:
3.3.3. Key Sizes
Selecting appropriate key sizes is a trade-off between cost,
performance, and risk. Since short RSA keys more easily succumb to
off-line attacks, Signers MUST use RSA keys of at least 1024 bits for
long-lived keys. Verifiers MUST be able to validate signatures with
keys ranging from 512 bits to 2048 bits, and they MAY be able to
validate signatures with larger keys. Verifier policies may use the
length of the signing key as one metric for determining whether a
signature is acceptable.
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[Bug 7559] DKIM default minimum key size should be 1024 bits
Posted by bu...@spamassassin.apache.org.
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
Giovanni Bechis <gi...@paclan.it> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |giovanni@paclan.it
--- Comment #2 from Giovanni Bechis <gi...@paclan.it> ---
AFAIK there is no standard that requires 1024 bit as minimum key size, is just
good practice having bigger keys.
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[Bug 7559] DKIM default minimum key size should be 1024 bits
Posted by bu...@spamassassin.apache.org.
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
Giovanni Bechis <gi...@paclan.it> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |INVALID
--- Comment #6 from Giovanni Bechis <gi...@paclan.it> ---
The minimum key size applies to DKIM whitelisting and DKIM_VALID_AU and it's
managed by the "dkim_minimum_key_bits" configuration parameter.
RFC6376 says that the verifiers "MUST be able to validate signatures with keys
ranging from 512 bits to 2048 bits", so this is not an issue.
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[Bug 7559] DKIM default minimum key size should be 1024 bits
Posted by bu...@spamassassin.apache.org.
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
RW <rw...@googlemail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rwmaillists@googlemail.com
--- Comment #3 from RW <rw...@googlemail.com> ---
The minimum key size applies to DKIM whitelisting and DKIM_VALID_AU. The
documentation explicitly says that is doesn't apply to DKIM_VALID which seen as
just informational.
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[Bug 7559] DKIM default minimum key size should be 1024 bits
Posted by bu...@bugzilla.spamassassin.org.
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
Kevin A. McGrail <km...@apache.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |kmcgrail@apache.org
--- Comment #1 from Kevin A. McGrail <km...@apache.org> ---
Is there a standard that requires 1024 bit? Do you have a suggested fix such
as perhaps adding a tag that can trigger a rule to alert admins?
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[Bug 7559] DKIM default minimum key size should be 1024 bits
Posted by bu...@spamassassin.apache.org.
https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7559
--- Comment #5 from RW <rw...@googlemail.com> ---
(In reply to Bill Cole from comment #4)
Verifier policies may use the
> length of the signing key as one metric for determining whether a
> signature is acceptable.
Which is controlled in the DKIM plugin by
dkim_minimum_key_bits n (default: 1024)
The smallest size of a signing key (in bits) for a valid signature
to be considered for whitelisting. Additionally, the eval function
check_dkim_valid() will return false on short keys when called with
explicitly listed domains, and the eval function
check_dkim_valid_author_sig() will return false on short keys
(regardless of its arguments). Setting the option to 0 disables a
key size check.
Note that the option has no effect when the eval function
check_dkim_valid() is called with no arguments (like in a rule
DKIM_VALID). A mere presence of some valid signature on a message
has no reputational value (without being associated with a
particular domain), regardless of its key size - anyone can prepend
its own signature on a copy of some third party mail and re-send it,
which makes it no more trustworthy than without such signature. This
is also a reason for a rule DKIM_VALID to have a near-zero score,
i.e. a rule hit is only informational.
The bug is reporting documented intentional behaviour. I don't see any reason
not to close it.
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