Cipher AES, How IV and CBC mode works

[walbus]

How IV and CBC mode works - alterable code - full working

I see again and again that it is not understood how an IV or the CBC mode works.

This almost inevitably results in a wrong application, which can have fatal consequences.

Unfortunately, the PB manual is not helpful here

I have therefore broken down the way in which it works here

This codes works primary with any block chiffres
Also you can looking how this works, for creating other, not from PB supported crypt modes
The handling of these little tricky codes is extremely simple

Make a try, it is as sample available for creating CFB and OFB mode with this base codes, be creative !
Its only a little brain jogging

https://en.wikipedia.org/wiki/Block_cip ... _.28CFB.29

Code: Select all

; Author  W. Albus - www.nachtoptik.de
; No warranty whatsoever
; Use at your own risk

;----------------- CBC Crypting - base code ------------------

Procedure AES_Encoder(*buffer_in.quad, *buffer_out.quad, bytes, *key, *iv )
  
  Protected i, rounds=bytes>>4 -1
  Static.q Dim iv(1)
  CopyMemory(*iv, @iv(0), 16)
  
  For i=0 To rounds
    *buffer_in\q ! iv(0) : *buffer_in+8
    *buffer_in\q ! iv(1) : *buffer_in-8
    result=AESEncoder(*buffer_in ,*buffer_out ,16 ,*key ,256 ,0 , #PB_Cipher_ECB)
    CopyMemory(*buffer_out, @iv(0), 16)
    *buffer_in+16 : *buffer_out+16
  Next i
  
  ProcedureReturn result 
EndProcedure
;---------------------------------------------

Procedure AES_Decoder(*buffer_in.quad, *buffer_out.quad, bytes, *key, *iv )
  
  Protected i, rounds=bytes>>4 -1
  Static.q Dim iv(1)
  MoveMemory(*iv, @iv(0), 16)
  
  For i=0 To rounds
    result=AESDecoder(*buffer_in ,*buffer_out ,16 ,*key ,256 ,0 , #PB_Cipher_ECB)
    *buffer_out\q ! iv(0) : *buffer_out+8
    *buffer_out\q ! iv(1) : *buffer_out+8
    CopyMemory(*buffer_in, @iv(0), 16)
    *buffer_in+16
  Next i
  
  ProcedureReturn result 
EndProcedure

;---------------------------------------------
string$="aaaaaaaaaaaaaaaa"+"bbbbbbbbbbbbbbbb"+"cccccccccccccccc"+"dddddddddddddddd"
len_string=StringByteLength(string$)
*buffer_0=AllocateMemory(len_string)
string_result$=Space(len_string)

; --------------------------------------------
AES_Encoder(@string$, *buffer_0, len_string, ?KEY, ?IV )
ShowMemoryViewer(*buffer_0,len_string)

; --------------------------------------------
AES_decoder(*buffer_0, @string_result$, len_string, ?KEY, ?IV )
; ShowMemoryViewer(@string_result$,len_string)
Debug string_result$

; --------------------------------------------
DataSection ; Sample key and initialization vector for coder test demo
  KEY:
  Data.b $09 , $a9 , $20 , $40 , $35 , $b8 , $a1 , $5b , $52 , $2e , $03 ,$d5 , $34 , $11 , $00 , $08
  Data.b $11 , $b8 , $31 , $61 , $26 , $c3 , $32 , $64 , $d9 , $f3 , $01 ,$a4 , $27 , $61 , $56 , $29
  IV:
  Data.b $3d , $ae , $ba , $43 , $9d , $9e , $b5 , $30 , $b4 , $23 , $da ,$80 , $2d , $9f , $ac , $45
EndDataSection

[the.weavster]

How IV and CBC mode works - alterable code - full working

I see again and again that it is not understood how an IV or the CBC mode works.

This almost inevitably results in a wrong application, which can have fatal consequences.

Hi Walbus,

It's my understanding that the IV should be different every time so that if you encrypt the same data twice you get different outcomes so I'd suggest if you are going to make a post to demonstrate the right way to do encryption that really should be part of the demo.

[walbus]

Hi the.weavster

So far :
The IV should always be different
The IV must not depend on the password (key)
IV does not provide protection against brute forcing
The encryption is not stronger with a used IV - regardless of whether one or a thousands different IV uses
For many things the CBC mode is not the best solution, or it is even unsuitable
It is available, add the IV directly to the data
There's a lot of crap being written about it