[ldns-users] ldns-signzone ECDSA random failure

W.C.A. Wijngaards wouter at nlnetlabs.nl
Tue Aug 18 10:12:02 CEST 2015


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Hi Matt

On 17/08/15 22:05, Matt Smith wrote:
> On Aug 17 20:49, Matt Smith wrote:
>> If it helps I have saved copies of the syslog, KSK and ZSK keys
>> along with the original zone file and the signed zone file if
>> anybody wants to examine them.
> 
> Actually I've just noticed something that stands out. In a record
> which works fine the RRSIG looks like this:

Thank you for the details, I have worked out that the shorter RRSIG is
wrong.  It is generated because ldns is omitting leading zeroes when
generating the signature encoding, but the RFC mandates equal length
parts (of length curvebits / 8).

The fix is in git and basically adds leading zeroes to the created RRSIG
.

Because ecdsa signatures have a randomised component, this only
happens when the leading bytes randomly happen to be zero.

The ldns_convert_ecdsa_rrsig_asn12rdf is therefore not capable of
generating good signatures, and is removed, replaced with
ldns_convert_ecdsa_rrsig_asn1len2rdf that takes the curve length as a
function call parameter.

Best regards, Wouter

> 
> host1.example.com.        3600    IN      RRSIG   A 13 3 3600 
> 20150914191810 20150817191810 57320 xtaz.uk. 
> ot+ASP55jXoBrNNqxT5yr3KIO/n+YazEc4NEq0/IpwhB4BucBRiBAiKihAdELzSf+CDTr2
X7v8TiqE59mNBeSg==
>
> 
> 
> In one that fails to validate it looks like this:
> 
> host2.example.com.        3600    IN      RRSIG   A 13 3 3600 
> 20150914191810 20150817191810 57320 xtaz.uk. 
> T0zvO7h5yAxTg5TqtGUAZqdsbj3T4EsvoWDzYOe4QaD/QJKs4eCvBwlLQ2DaQpxNIhd9oO
qTWgLeeGL7aRwA
>
> 
> 
> It looks like the signature has been truncated and doesn't have ==
> on the end of it?
> 
> Forgot to say as well, I'm using ldns-tools 1.6.17.
> 

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