no public key available for ID 7638D0442B90D010

[llcfree <llcfree@gmail.com>]

Greetings,

apt-get update now returns

W: There is no public key available for the following key
IDs:7638D0442B90D010

for two of the archives in /etc/apt/sources.list:

deb http://ftp.de.debian.org/debian/ squeeze-updates main
deb http://ftp.de.debian.org/debian/ squeeze-lts main

Same result if I use a different mirror (ftp.au)

>From here:

https://wiki.debian.org/SecureApt

I understand this kind of message is a warning related to an old key but
the archive is signed with a new valid key, so it should be safe to use.
It also says that the warning should disappear once the keys are
updated.

Therefore, I tried:

apt-key update 

but the warning persist.

Am I doing something wrong?

Can I safely install updates?

Thanks, Loredana