Shell script to manage keypairs. You can add an user and establish a new keypair, reset an existing keypair or repair ownerships and permissions.

keypair.sh

#!/bin/bash
#
# Developed by: Javier Sianes - jsianes@gmail.com
#

BITS="4096"

username(){
  USERNAME=`echo ${USERNAME} | tr '[:upper:]' '[:lower:]' | sed 's/ /_/g'`

  id ${USERNAME} >/dev/null 2>&1
  
  if [ $? -eq 0 ]
  then
    echo "--- error: \"${USERNAME}\" username exists"
    exit 2
  fi
}

usage(){
  echo ""
  echo "Usage: $0 [-r | -p] username"
  echo ""
  echo "  Without options, add a new username and generate a new keypair for this username"
  echo "  -r : Reset keypair for an existing username"
  echo "  -p : Reset ownership and permissions associated to keypair"
  echo ""
  exit 1
}

verify_sudo(){
  sudo ls / >/dev/null 2>&1

  if [ $? -ne 0 ]
  then
    echo "--- error: \"$(whoami)\" username is unable to run sudo commands"
    exit 3
  fi
}

add_user(){
  verify_sudo
  username
  echo "--- adding user \"${USERNAME}\" ..."
  sudo useradd -d /home/${USERNAME} -c "${USERNAME}" -s /bin/bash -m ${USERNAME} >/dev/null 2>&1
  
  if [ $? -eq 0 ]
  then
    echo -n "--- user added: "
    id ${USERNAME}
    echo "--- generating keypair..."
    sudo rm -f /home/${USERNAME}/${USERNAME}.pem /home/${USERNAME}/${USERNAME}.pem.pub >/dev/null 2>&1
    sudo ssh-keygen -b ${BITS} -f /home/${USERNAME}/${USERNAME}.pem -t rsa -N '' >/dev/null 2>&1
    if [ $? -eq 0 ]
    then
      if [ -d /home/${USERNAME} ] 
      then
        sudo mkdir /home/${USERNAME}/.ssh >/dev/null 2>&1
        sudo mv -f /home/${USERNAME}/${USERNAME}.pem.pub /home/${USERNAME}/.ssh/authorized_keys >/dev/null 2>&1
        if [ $? -eq 0 ]
        then
          sudo chmod 700 /home/${USERNAME}/.ssh >/dev/null 2>&1
          sudo chmod 600 /home/${USERNAME}/.ssh/authorized_keys >/dev/null 2>&1
          sudo chown -R ${USERNAME}: /home/${USERNAME} >/dev/null 2>&1
          echo "--- keypair generated and configured. New private key for \"${USERNAME}\" username: /home/${USERNAME}/${USERNAME}.pem"
        else
          echo "--- error: unable to configure new keypair"
          exit 252
        fi
      else
        echo "--- error: unable to configure new keypair"
        exit 253
      fi
    else
      echo "--- error: unable to generate a new keypair"
      exit 254
    fi
  else
    echo "--- error: unable to add \"${USERNAME}\" username"
    exit 255
  fi
}

reset_permissions(){
  verify_sudo
  id ${USERNAME} >/dev/null 2>&1
  if [ $? -eq 0 ]
  then
    HOMEDIRECTORY=`sudo cat /etc/passwd | grep "^${USERNAME}:" | cut -d : -f 6`
    if [ -d ${HOMEDIRECTORY} ]
    then
      sudo chmod 700 ${HOMEDIRECTORY}/.ssh >/dev/null 2>&1
      sudo chmod 600 ${HOMEDIRECTORY}/.ssh/authorized_keys ${HOMEDIRECTORY}/${USERNAME}.pem >/dev/null 2>&1
      sudo chown ${USERNAME}: ${HOMEDIRECTORY} ${HOMEDIRECTORY}/${USERNAME}.pem >/dev/null 2>&1
      sudo chown -R ${USERNAME}: ${HOMEDIRECTORY}/.ssh >/dev/null 2>&1
      echo "--- keypair ownership and permissions for \"${USERNAME}\" username with home directory ${HOMEDIRECTORY} repaired"
    else
      echo "--- error: unable to find home directory (${HOMEDIRECTORY}) associated to \"${USERNAME}\" username"
      exit 246
    fi
  else
    echo "--- error: \"${USERNAME}\" username doesn't exists"
    exit 247
  fi
}

reset_keypair(){
  verify_sudo
  id ${USERNAME} >/dev/null 2>&1
  if [ $? -eq 0 ]
  then
    HOMEDIRECTORY=`sudo cat /etc/passwd | grep "^${USERNAME}:" | cut -d : -f 6`
    sudo rm -f ${HOMEDIRECTORY}/${USERNAME}.pem ${HOMEDIRECTORY}/${USERNAME}.pem.pub >/dev/null 2>&1
    sudo ssh-keygen -b ${BITS} -f ${HOMEDIRECTORY}/${USERNAME}.pem -t rsa -N '' >/dev/null 2>&1
    if [ $? -eq 0 ]
    then
      if [ -d ${HOMEDIRECTORY} ]
      then
        sudo mkdir ${HOMEDIRECTORY}/.ssh >/dev/null 2>&1
        sudo mv -f ${HOMEDIRECTORY}/${USERNAME}.pem.pub ${HOMEDIRECTORY}/.ssh/authorized_keys >/dev/null 2>&1
        if [ $? -eq 0 ]
        then
          sudo chmod 700 ${HOMEDIRECTORY}/.ssh >/dev/null 2>&1
          sudo chmod 600 ${HOMEDIRECTORY}/.ssh/authorized_keys ${HOMEDIRECTORY}/${USERNAME}.pem >/dev/null 2>&1
          sudo chown ${USERNAME}: ${HOMEDIRECTORY} ${HOMEDIRECTORY}/${USERNAME}.pem >/dev/null 2>&1
          sudo chown -R ${USERNAME}: ${HOMEDIRECTORY}/.ssh >/dev/null 2>&1
          echo "--- keypair generated and configured. New private key for \"${USERNAME}\" username: ${HOMEDIRECTORY}/${USERNAME}.pem"
        else
          echo "--- error: unable to configure new keypair"
          exit 251
        fi
      else
        echo "--- error: unable to configure new keypair"
        exit 250
      fi
    else
      echo "--- error: unable to find home directory (${HOMEDIRECTORY}) associated to \"${USERNAME}\" username"
      exit 249
    fi
  else
    echo "--- error: \"${USERNAME}\" username doesn't exists"
    exit 248
  fi
}

case "$#" in
  1)
        USERNAME="$1"; add_user
        ;;
  2)
        if [ "$1" = "-r" ]; then USERNAME="$2"; reset_keypair; fi
        if [ "$1" = "-p" ]; then USERNAME="$2"; reset_permissions; fi
        ;;
  *)
        usage
        ;;
esac