Impossible to use bzr

Binary package hint: bzr

I'm trying to download but everytime i've got errors.
njin@Lucid:~$ bzr branch lp:a4
Permission denied (publickey).
bzr: ERROR: Connection closed: Unexpected end of message. Please check connectivity and permissions, and report a bug if problems persist.
or by root
njin@Lucid:~$ sudo bzr branch lp:a4
[sudo] password for njin:
The authenticity of host 'bazaar.launchpad.net (91.189.90.11)' can't be established.
RSA key fingerprint is 9d:38:3a:63:b1:d5:6f:c4:44:67:53:49:2e:ee:fc:89.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'bazaar.launchpad.net,91.189.90.11' (RSA) to the list of known hosts.
Permission denied (publickey).

The key is up on launchpad, but i can't understand why i've gotr these errors.

ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: bzr 2.2.1-0ubuntu1
Uname: Linux 2.6.37-020637rc1-generic x86_64
Architecture: amd64
CheckboxSubmission: da0a9053457cd51534683f0d3b1afbb3
CheckboxSystem: 4ed15c40009aa6f7770f606350a390a2
Date: Sun Nov 14 17:59:51 2010
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
ProcEnviron:
 LANG=it_IT.utf8
 SHELL=/bin/bash
SourcePackage: bzr