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bashencryptionxorencryption-symmetricbitwise-xor

How to XOR two hex numbers in bash script? (XOR Encryption)

I write a bash script who manipulate hex values and i need to do XOR operation between two hexa numbers. My problem is when i try in bash prompt it's work and return right value but in script this value is false.

When XOR variable $ExtendAuthKey and $IPAD the result must be : 181ad673a5d94f0e12c8894ea26381b363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636

But in fact i get this value : 3906369333256140342

I dont understand this behavior, if you have explanation or solution i take it, thank

see my script : `

#!/bin/bash

AuthID=80001f8880e9bd0c1d12667a5100000000

IPAD=0x36363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636363636
OPAD=0x5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c5c
Ext0=0x000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

WholeMsgIn=0x3081800201033011020420dd06a7020300ffe30401050201030431302f041180001f8880e9bd0c1d12667a5100000000020105020120040475736572040c00000000000000000000000004003035041180001f8880e9bd0c1d12667a51000000000400a11e02046b4c5ac20201000201003010300e060a2b06010201041e0105010500

line=test

#Calcule AuthKey
  AuthKey=$(snmpkey md5 $line $AuthID | grep auth | cut -d ' ' -f 2)

#Concat AuthKey + Ext0
  ExtendAuthKey=$AuthKey${Ext0:2}

#Calcule de K1 = ExtendAuthKey XOR IPAD
  K1=$(( $ExtendAuthKey ^ $IPAD ))

#Calcule de K2 = ExtendAuthKey XOR OPAD
  K2=$(( $ExtendAuthKey ^ $OPAD ))

#Concat K1 + WholeMsgIn
  Concat1=$K1$WholeMsgIn

#Calcul Hash Concat1
  echo ${Concat1:2} > tempH.hex
  cat tempH.hex | xxd -r -p > tempB.bin
  HashConcat1=$(md5sum tempB.bin | cut -d ' ' -f 1)

#Concat K2 + Concat1
  Concat2=$K2$HashConcat1

#Calcul Hash Concat1
  echo ${Concat2:2} > tempH.hex
  cat tempH.hex | xxd -r -p > tempB.bin
  HashConcat2=$(md5sum tempB.bin | cut -d ' ' -f 1)

`

Solution

  • If you do echo $((IPAD)) you will get 3906369333256140342. The problem is that once you perform arithmetic operation in shell, your inputs get truncated to size of int of your platform (in this case 64b). I suspect you will have to reach out beyond shell to perform the bitwise XOR (or process it in smaller chunks, but the md5 digest alone is already twice the size).