Fritzbox Fritz!Box AVM SSL Letsencrypt automatically update

fritzbox-cert-update.sh

#!/bin/bash

# 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

# make and secure a temporary file
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 | iconv -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
       # continue with the script on success

      # generate our upload request
      BOUNDARY="---------------------------"`date +%Y%m%d%H%M%S`
      printf -- "--$BOUNDARY\r\n" >> $TMP
      printf "Content-Disposition: form-data; name=\"sid\"\r\n\r\n$SID\r\n" >> $TMP
      printf -- "--$BOUNDARY\r\n" >> $TMP
      printf "Content-Disposition: form-data; name=\"BoxCertPassword\"\r\n\r\n$CERTPASSWORD\r\n" >> $TMP
      printf -- "--$BOUNDARY\r\n" >> $TMP
      printf "Content-Disposition: form-data; name=\"BoxCertImportFile\"; filename=\"BoxCert.pem\"\r\n" >> $TMP
      printf "Content-Type: application/octet-stream\r\n\r\n" >> $TMP
      cat $CERTPATH/privkey.pem >> $TMP
      cat $CERTPATH/fullchain.pem >> $TMP
      printf "\r\n" >> $TMP
      printf -- "--$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

# clean up
rm -f $TMP

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

Improved version with error handling.