If you follow my previous post about ssh/scp without password, but you got this error when you try to ssh to B on A, then you need to add RSA or DSA identities to the authentication agent. A ssh-add command on host A will solve your pain.
$ ssh-add
# Sample output
Identity added: /home/jack/.ssh/id_rsa (/home/jack/.ssh/id_rsa)
Reference
http://www.cyberciti.biz/faq/unix-appleosx-linux-bsd-agent-admitted-failuretosignusingkey/