Comment 8 for bug 414743

More about SSH keys and paramiko. There is code in bzrlib/transport/ssh.py

        server_key = t.get_remote_server_key()
        server_key_hex = paramiko.util.hexify(server_key.get_fingerprint())
        keytype = server_key.get_name()
        if host in SYSTEM_HOSTKEYS and keytype in SYSTEM_HOSTKEYS[host]:
            our_server_key = SYSTEM_HOSTKEYS[host][keytype]
            our_server_key_hex = paramiko.util.hexify(
                our_server_key.get_fingerprint())
        elif host in BZR_HOSTKEYS and keytype in BZR_HOSTKEYS[host]:
            our_server_key = BZR_HOSTKEYS[host][keytype]
            our_server_key_hex = paramiko.util.hexify(
                our_server_key.get_fingerprint())
        else:
            trace.warning('Adding %s host key for %s: %s'
                          % (keytype, host, server_key_hex))
            add = getattr(BZR_HOSTKEYS, 'add', None)
            if add is not None: # paramiko >= 1.X.X
                BZR_HOSTKEYS.add(host, keytype, server_key)
            else:
                BZR_HOSTKEYS.setdefault(host, {})[keytype] = server_key
            our_server_key = server_key
            our_server_key_hex = paramiko.util.hexify(
                our_server_key.get_fingerprint())
            save_host_keys()

So actually bzrlib checks host keys and therefore can ask user for confirmation.