Failed to install steam via Ubuntu Software Center (ubuntu 12.04.2 full updated)

[dupontd]

Hello.
My system :Ubuntu 12.04.2 64 bit.

There's some weeks than I want install skype ONLY via Ubuntu Software Center, I follow instructions :
"The Steam Client is available through the Ubuntu Software Center (USC). Run the USC, search for Steam, and install it"
Source : https://wiki.ubuntu.com/Valve

USC failed and show always this message :
W:Failed to fetch https://private-ppa.launchpad.net/commercial-ppa-uploaders/steam/ubuntu/dists/precise/main/binary-amd64/Packages The requested URL returned error: 401
, W:Failed to fetch https://private-ppa.launchpad.net/commercial-ppa-uploaders/steam/ubuntu/dists/precise/main/binary-i386/Packages The requested URL returned error: 401
, E:Some index files failed to download. They have been ignored, or old ones used instead

[dupontd (dupontdrf-deactivatedaccount-deactivatedaccount)]

The added sources looks like :
Added by software-center; credentials stored in /etc/apt/auth.conf
https://private-ppa.launchpad.net/commercial-ppa-uploaders/steam/ubuntu

[dupontd (dupontdrf-deactivatedaccount-deactivatedaccount)]

Same problem with Ubuntu 64 bit 13.04

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[zouloufr (zoulou45)]

Hello, I have a problem with the launch of steam following a update, I give you back the console

"Running Steam on ubuntu 12.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1379035603_client)
Installing breakpad exception handler for appid(steam)/version(1379035603_client)
Installing breakpad exception handler for appid(steam)/version(1379035603_client)
unlinked 0 orphaned pipes
removing stale semaphore last operated on by process 3665 with name 0eBlobRegistryMutex_C5C45596D505A58B9AD8B08F77EEA318
removing stale semaphore last operated on by process 3665 with name 0eBlobRegistrySignal_C5C45596D505A58B9AD8B08F77EEA318
removing stale semaphore last operated on by process 3665 with name 0emSteamEngineInstance
removing stale semaphore last operated on by process 3665 with name 0eSteamEngineLock
Installing breakpad exception handler for appid(steam)/version(1379035603_client)
[0913/170954:WARNING:proxy_service.cc(958)] PAC support disabled because there is no system implementation"