Fritzbox Fritz!Box AVM SSL Letsencrypt automatically update

fritzbox-cert-update.sh

#!/bin/bash

set -euo pipefail
IFS=$'\n\t'

# parameters
USERNAME="maybe empty"
PASSWORD="fritzbox-password"
CERTPATH="path to cert eg  /etc/letsencrypt/live/domain.tld/"
CERTPASSWORD="cert password if needed"
HOST="http://fritz.box"
unset RESPONSE

# make and secure a temporary file. Arrange for automatic cleanup
TMP=""
trap 'rm -f "$TMP"' exit
TMP="$(mktemp -t XXXXXX)"
chmod 600 "$TMP"

# login to the box and get a valid SID
CHALLENGE=$(wget -q -O - "$HOST/login_sid.lua" | sed -e 's/^.*<Challenge>//' -e 's/<\/Challenge>.*$//')
if [ -z "$CHALLENGE" ]
  then
    RESPONSE="Is HOST-name pointing to a Fritz!BOX?"
  else
    # continue with the script on success
    HASH="$(echo -n "$CHALLENGE-$PASSWORD" | uconv -f ASCII -t UTF16LE |md5sum|awk '{print $1}')"
    SID=$(wget -q -O - "$HOST/login_sid.lua?sid=0000000000000000&username=$USERNAME&response=$CHALLENGE-$HASH"| sed -e 's/^.*<SID>//' -e 's/<\/SID>.*$//')

    if [[ $SID == "0000000000000000" ]]
      then
        RESPONSE="Failed to authenticate."
      else
        # generate our upload request
        BOUNDARY="---------------------------"$(date +%Y%m%d%H%M%S)
        (
        printf -- "--%s\r\n" "$BOUNDARY"
        printf "Content-Disposition: form-data; name=\"sid\"\r\n\r\n%s\r\n" "$SID"
        printf -- "--%s\r\n" "$BOUNDARY"
        printf "Content-Disposition: form-data; name=\"BoxCertPassword\"\r\n\r\n%s\r\n" "$CERTPASSWORD"
        printf -- "--%s\r\n" "$BOUNDARY"
        printf "Content-Disposition: form-data; name=\"BoxCertImportFile\"; filename=\"BoxCert.pem\"\r\n"
        printf "Content-Type: application/octet-stream\r\n\r\n"
        cat "$CERTPATH"/privkey.pem
        cat "$CERTPATH"/fullchain.pem
        printf "\r\n"
        printf -- "--%s--" "$BOUNDARY"
        ) >> "$TMP"
        
        # upload the certificate to the box
        RESPONSE=$(wget -q -O - "$HOST/cgi-bin/firmwarecfg" --header="Content-type: multipart/form-data boundary=$BOUNDARY" --post-file "$TMP" | grep SSL)
    fi
fi

if [ -z "$RESPONSE" ]
  then
    echo $HOST ": Certificate import failed."
  else
    echo $HOST ": " "$RESPONSE"
fi

• Untested changes: Needs testing
• Ran through shellcheck
• strict-mode enabled
• error handling from FvdLaar
• installed trap for script unnormal script termination from joba-1