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://ftp.de.debian.org/debian/pool/main/n/nanomsg/nanomsg_0.4~beta+dfsg-3.dsc dget: retrieving http://ftp.de.debian.org/debian/pool/main/n/nanomsg/nanomsg_0.4~beta+dfsg-3.dsc % 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?