added to readme

README.md

@@ -16,7 +16,11 @@ I looked into "Shamir secret sharing" but I find that generating a new secret th
 
 ### output:
 
-this bash script
+this bash script is set to take N recipients and encrypt four file to make sure only with the quorum of at least 3 recipients the file will be opened
+
+the logic behind that is really simple:
+
+
 
 ### requirements:
 
@@ -33,3 +37,87 @@ edit the debug variable to add your email address (if you want) for debugging pu
 your output will be a file named
 
  your_file_to_encrypt.ext.ENCRYPTED
+
+### customization:
+
+if you want to increase the number of people needed to decrypt the file to 4 for example,
+edit this block of code from:
+
+```
+#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$((counter-1))
+
+        fi
+        #increase loop counter:
+        ((counter++))
+
+      fi
+    done
+  done
+done
+
+```
+
+```
+#main loop:
+for ((idxA=0; idxA<max; idxA++)); do              # iterate idxA from 0 to length
+  for ((idxB=idxA; idxB<max; idxB++)); do         # iterate idxB from idxA to length
+    for ((idxC=idxB; idxC<max; idxC++)); do         # iterate idxC from idxB to length
+      for ((idxD=idxC; idxD<max; idxD++)); do         # iterate idxD from idxC to length
+
+        if [ "${emails[$idxA]}" == "${emails[$idxB]}" ] || [ "${emails[$idxB]}" == "${emails[$idxC]}" ] || [ "${emails[$idxA]}" == "${emails[$idxC]}" ] || [ "${emails[$idxA]}" == "${emails[$idxD]}" ] || [ "${emails[$idxB]}" == "${emails[$idxD]}" ] || [ "${emails[$idxC]}" == "${emails[$idxD]}" ]; then
+
+          #echo "A, B, C or D are the same"
+          continue
+
+        else
+          #custom execution for first run:
+          if [ "$counter" == 0 ]; then
+            echo "gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" -r "${emails[$idxD]}" "$debug" --output $workdir/$file.step$counter $file"
+
+          #custom execution for last run:
+          elif [ "$counter" == $((ans-1)) ]; then
+            echo "gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" -r "${emails[$idxD]}" "$debug" --output $file.ENCRYPTED $workdir/$file.step$((counter-1))"
+
+          #normal execution:
+          else
+            echo "gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" -r "${emails[$idxD]}" "$debug" --output $workdir/$file.step$counter $workdir/$file.step$((counter-1))"
+
+          fi
+          #increase loop counter:
+          ((counter++))
+
+        fi
+      done
+    done
+  done
+done
+
+```
+
+and then adjust the variable "combinate" to 4:
+```
+combinate=4
+```
+
+

file.txt

@@ -0,0 +1 @@
+banana

gpg-quorum_4of5.sh

@@ -0,0 +1,123 @@
+#!/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=$1
+#debug email if you want to self-decrypt, you can also leave it blank but don't populate it without "-r":
+debug="-r youermail@provider.net"
+#counters:
+counter=0
+firstrun=0
+#minimum numbers of recipients that needs to agree to decypher the file:
+combinate=4
+#work directory:
+workdir="_WORKDIR"
+#lenght of array:
+max=${#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 $workdir exists, exiting"
+    exit
+fi
+
+
+#main loop:
+for ((idxA=0; idxA<max; idxA++)); do              # iterate idxA from 0 to length
+  for ((idxB=idxA; idxB<max; idxB++)); do         # iterate idxB from idxA to length
+    for ((idxC=idxB; idxC<max; idxC++)); do         # iterate idxC from idxB to length
+      for ((idxD=idxC; idxD<max; idxD++)); do         # iterate idxD from idxC to length
+
+        if [ "${emails[$idxA]}" == "${emails[$idxB]}" ] || [ "${emails[$idxB]}" == "${emails[$idxC]}" ] || [ "${emails[$idxA]}" == "${emails[$idxC]}" ] || [ "${emails[$idxA]}" == "${emails[$idxD]}" ] || [ "${emails[$idxB]}" == "${emails[$idxD]}" ] || [ "${emails[$idxC]}" == "${emails[$idxD]}" ]; then
+
+          #echo "A, B, C or D are the same"
+          continue
+
+        else
+          #custom execution for first run:
+          if [ "$counter" == 0 ]; then
+            echo "gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" -r "${emails[$idxD]}" "$debug" --output $workdir/$file.step$counter $file"
+
+          #custom execution for last run:
+          elif [ "$counter" == $((ans-1)) ]; then
+            echo "gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" -r "${emails[$idxD]}" "$debug" --output $file.ENCRYPTED $workdir/$file.step$((counter-1))"
+
+          #normal execution:
+          else
+            echo "gpg -ea -r "${emails[$idxA]}" -r "${emails[$idxB]}" -r "${emails[$idxC]}" -r "${emails[$idxD]}" "$debug" --output $workdir/$file.step$counter $workdir/$file.step$((counter-1))"
+
+          fi
+          #increase loop counter:
+          ((counter++))
+
+        fi
+      done
+    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
+