sudo apt-get update
.
.
.
W: GPG error: file:  Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 431A4671ECDACA52
W: Failed to fetch bzip2:/var/lib/apt/lists/partial/us-east-1.ec2.archive.ubuntu.com_ubuntu_dists_precise-updates_main_source_Sources  Hash Sum mismatch

W: Failed to fetch bzip2:/var/lib/apt/lists/partial/us-east-1.ec2.archive.ubuntu.com_ubuntu_dists_precise-updates_universe_source_Sources  Hash Sum mismatch

E: Some index files failed to download. They have been ignored, or old ones used instead.