Unhash the hostnames hashed in your known_hosts file by passing their names.

ssh-hostname-unhash.sh

#!/bin/bash
# based on http://unix.stackexchange.com/questions/175071/how-to-decrypt-hostnames-of-a-crypted-ssh-known-hosts-with-a-list-of-the-hostna/175199#175199

function replace {
  host="$1"
  found=$(ssh-keygen -F "$host" 2>/dev/null | grep -v '^#' | sed "s/^[^ ]*/$host/")
  if [ -n "$found" ]; then
    ssh-keygen -R "$host" &>/dev/null
    echo "$found" >>~/.ssh/known_hosts
    echo "Found and replaced: $host"
  else
    echo "Not found: $host"
  fi
}

if [ "$#" -eq 0 ]; then
  echo "Enter one hostname per line, or input a filename ( < file )."
  while read host comment; do
	replace "$host"
  done
  exit
fi

while (( "$#" )); do
  replace "$1"
  shift
done