megatools is shit so...

#!/bin/bash
id=$( echo "$1"| awk -F '!' '{print $2}')
key=$(echo "$1"| awk -F '!' '{print $3}'|tr _- /+|tr -d ,)

if ! (test "$id"&&test "$key");then
    echo "usage example:"
    echo "$0 'https://mega.nz/#!gx5giRLT!y-fuezB5pFWwZ8tGRMNdLI_QeLWQK7b1nsHkIAAHCkA'"
    exit 1
fi

key64=$(echo -n "$key"|base64 -di 2>/dev/null|xxd -p|tr -d '\n')
(( key[0] = 0x${key64:00:16} ^ 0x${key64:32:16} ))
(( key[1] = 0x${key64:16:16} ^ 0x${key64:48:16} ))
key=$(printf "%016x" ${key[*]})
iv=${key64:32:16}0000000000000000

json=$(curl -sX POST --data-binary '[{"a":"g","g":1,"p":"'$id'"}]' https://eu.api.mega.co.nz/cs)
if ((${#json} < 20));then
    echo "got this instead of json: $json"
    exit 1
fi
# file url:
url1=$(echo "$json"|python -c "import json,sys;sys.stdout.write(json.loads(sys.stdin.read())[0]['g'])")

filenamejson=$(echo "$json"|\
  python -c "import json,sys;sys.stdout.write(json.loads(sys.stdin.read())[0]['at'])"|\
  tr _- /+|tr -d ,|base64 -di 2>/dev/null|\
  openssl enc -d -aes-128-cbc -K $key -iv 0 -nopad)
# the output seems to begin with "MEGA", followed by json, thus the sed
filename=$(echo "$filenamejson"|sed 's/^.[^{]*{/{/'|\
  python -c "import json,sys;sys.stdout.write(json.loads(sys.stdin.read())['n'])")

if ! test -e "$filename";then
    echo "saving to: $filename"
    curl "$url1"|openssl enc -d -aes-128-ctr -K $key -iv $iv -out "$filename"
else
    echo "already exists: $filename"
fi