dscverify: nanomsg_0.4~beta+dfsg-3.dsc failed signature check

I'm trying to backport libnanomsg-dev from jessie to squeeze. And dget says:

$ dget -x http://ift.tt/15EZsq9
dget: retrieving http://ift.tt/15EZsq9
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                             Dload  Upload   Total   Spent    Left  Speed
100  2016  100  2016    0     0  47558      0 --:--:-- --:--:-- --:--:-- 96000
dget: using existing nanomsg_0.4~beta+dfsg.orig.tar.gz
dget: using existing nanomsg_0.4~beta+dfsg-3.debian.tar.xz
nanomsg_0.4~beta+dfsg-3.dsc:
dscverify: nanomsg_0.4~beta+dfsg-3.dsc failed signature check:
gpg: Signature made Fri 29 Aug 2014 09:49:58 PM EEST using RSA key ID 864CC8BF
gpg: Can't check signature: public key not found
Validation FAILED!!

As far as I can tell, I've got to install jessie's keyring somehow. But how do I do that?