gpg-quorum/gpg-quorum_3of5.sh

#!/usr/bin/env bash

#Scipt to encrypt a file with some known keys,
#the script will combin the encryptions to ensure that a minimum quorum defined in "combinate" variable is needed to open the file.
#the script will not auto-adjust to an arbitrary change of "combinate" since one or more loops in the main loop should be tuned,
#right now it's adjusted ofr a quorum of 3 recipients.
#
#usage: gpg-quorum_3of5.sh filename_to_encrypt.ext

#retrieve the email list
readarray -t emails < emails.txt
file=$1
#debug email if you want to self-decrypt, you can also leave it blank but don't populate it without "-r":
debug="-r youermail@provider.net"
#counters:
counter=0
firstrun=0
#minimum numbers of recipients that needs to agree to decypher the file:
combinate=3
#work directory:
workdir="_WORKDIR"
#lenght of array:
max=${#emails[@]}                                  # Take the length of that array


#-----
#CALCULATE number of combinations for our emails

#function for factorial calculation
function fun_fact {
  num=$1
  fact=1
  for((i=2;i<=num;i++))
  {
    fact=$((fact * i))  #fact = fact * i
  }
  echo $fact
}

#n is the lenght of the array
n=$max
#r is the number of recipients needed to open the file
r=$combinate

a=$( fun_fact $n )
b=$( fun_fact $r )
c=$(( $n - $r ))
d=$( fun_fact $c )
t=$(( $b * $d ))
ans=$(( $a / $t ))

#echo "max number of combinations: "$ans
#-----


#create our workdir if it does not exist:
if [ ! -d "$workdir" ]
then
    echo "creating dir"
    mkdir $workdir
else
    echo "Directory $workdir exists, exiting"
    exit
fi


#main loop:
for ((idxA=0; idxA<max; idxA++)); do              # iterate idxA from 0 to length
  for ((idxB=idxA; idxB<max; idxB++)); do         # iterate idxB from idxA to length
    for ((idxC=idxB; idxC<max; idxC++)); do         # iterate idxC from idxB to length

      if [ "${emails[$idxA]}" == "${emails[$idxB]}" ] || [ "${emails[$idxB]}" == "${emails[$idxC]}" ] || [ "${emails[$idxA]}" == "${emails[$idxC]}" ]; then

        #echo "A, B or C are the same"
        continue

      else
        #custom execution for first run:
        if [ "$counter" == 0 ]; then
          gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" "$debug" --output $workdir/$file.step$counter $file

        #custom execution for last run:
        elif [ "$counter" == $((ans-1)) ]; then
          gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" "$debug" --output $file.ENCRYPTED $workdir/$file.step$((counter-1))

        #normal execution:
        else
          gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" "$debug" --output $workdir/$file.step$counter $workdir/$file.step$((counter-1))

        fi
          #increase loop counter:
          ((counter++))

      fi
    done
  done
done

#remove work directory:
rm -rf $workdir


#the combination matrix for a quorum of 3 recipients on 5:
#recipients: A,B,C,D,E
#
#A,B,C
#A,B,D
#A,B,E
#A,C,D
#A,C,E
#A,D,E
#B,C,D
#B,C,E
#B,D,E
#C,D,E


#SOURCES:
#http://www.anonhack.in/2018/05/program-to-calculate-combination-ncr-in-bash-shell-scripting/
#https://www.log2base2.com/shell-script-examples/loop/shell-script-to-find-factorial-of-a-number.html
#https://www.dcode.fr/combinations