debian wheezy(virtualbox)にpostgresql 9.2をインストールする

Debian Wheezyを含むvirtualboxがあります。 Postgresql 9.2をインストールしようとしています。私がこれをしようとしたときこれ次の指示があります。

Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 postgresql-9.2 : Depends: postgresql-common (>= 135~) but it is not going to be installed
                  Depends: ssl-cert but it is not installable
E: Unable to correct problems, you have held broken packages.

postgresql-commonをインストールしようとすると：

apt-get install postgresql-common

私は次のことを得た:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 postgresql-common : Depends: ssl-cert (>= 1.0.11) but it is not installable
E: Unable to correct problems, you have held broken packages

SSL-certをインストールしようとすると：

apt-get install ssl-cert

私は次のことを得た:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package ssl-cert is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'ssl-cert' has no installation candidate

私はLinuxを初めて使用します。 Postgresql 9.2のインストールを手伝ってください。

私のものがあります。sources.list

#

# deb cdrom:[Debian GNU/Linux 7.0.0 _Wheezy_ - Official i386 CD Binary-1 201305$

deb cdrom:[Debian GNU/Linux 7.0.0 _Wheezy_ - Official i386 CD Binary-1 20130504$

deb http://security.debian.org/ wheezy/updates main
deb-src http://security.debian.org/ wheezy/updates main

# wheezy-updates, previously known as 'volatile'
# A network mirror was not selected during install.  The following entries
# are provided as examples, but you should amend them as appropriate
# for your mirror of choice.
#
# deb http://ftp.debian.org/debian/ wheezy-updates main
# deb-src http://ftp.debian.org/debian/ wheezy-updates main