"yum update" yields many "Public key not available . . ."

Folks,

In the past "yum update" has worked well for me. Now it gives:

<snip>
.
.
libquicktime-0.9.4-2.1.fc 100% |=========================| 414 kB  00:01
gwenhywfar-1.7.2-0.fc3.i3 100% |=========================| 373 kB  00:01
gnet2-2.0.5-1.1.fc3.rf.i3 100% |=========================| 149 kB  00:01
lame-3.96.1-2.1.fc3.rf.i3 100% |=========================| 648 kB  00:04
warning: rpmts_HdrFromFdno: V3 DSA signature: NOKEY, key ID 6b8d79e6
public key not available
for //var/cache/yum/dag/packages/rsync-2.6.3-1.1.fc3.rf.i386.rpm
public key not available
for //var/cache/yum/dag/packages/epic-2.0-1.1.fc3.rf.i386.rpm
public key not available
for //var/cache/yum/dag/packages/pan-0.14.2.91-2.1.fc3.rf.i386.rpm
public key not available
for //var/cache/yum/dag/packages/lftp-3.0.12-1.1.fc3.rf.i386.rpm
public key not available
for //var/cache/yum/dag/packages/bzflag-2.0.0-1.1.fc3.rf.i386.rpm
.
.
<snip>

What is the problem? How can I acquire these "public keys"?

Thanks . . .

Bob St. John