Error Message in apt-get update

While using sudo apt-get install it is displaying the following message:

sudo apt-get install build-essential gcc-arm-linux-gnueabi
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package gcc-arm-linux-gnueabi

Also while using:sudo apt-get update

 Get:1 http://security.ubuntu.com trusty-security InRelease                     
    100% [1 InRelease gpgv 4,110 B] [Waiting for headers]Splitting up /var/lib/apt/lists/partial/security.ubuntu.com_ubuntu_dists_trusty-security_InRelease into datIgn http://security.ubuntu.com trusty-security InRelease
    E: GPG error: http://security.ubuntu.com trusty-security InRelease: Clearsigned file isn't valid, got 'NODATA' (does the network require authentication?)

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