# cat b1.bak 
ssh-rsa CCCC/DDDD/knMQ== user@host2.f.q.d.n
ssh-rsa AAAA/BBBB/kPW+Yi9yZ7Kh0mL/knMQ== user@host.f.q.d.n

 # perl -w -n -e 'my $str = sprintf( qq(%s), q(ssh-rsa AAAA/BBBB/kPW+Yi9yZ7Kh0mL/knMQ== user@host.f.q.d.n) ); print $_ unless $_ =~ /$str/ '  /root/user/b1.bak
ssh-rsa CCCC/DDDD/knMQ== user@host2.f.q.d.n
ssh-rsa AAAA/BBBB/kPW+Yi9yZ7Kh0mL/knMQ== user@host.f.q.d.n