2017-12-15

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Character Frequency Counter

Algorithm creator(s)

n/a


PB author(s)

Steve Hutchesson


Description

A tool to assist with cryptanalytical frequency distribution analysis attacks.


Note


Source

https://forum.powerbasic.com/forum/user-to-user-discussions/powerbasic-for-windows/6393-inline-asm-byte-frequency-counter


See also

n/a


Source Code

Download source code file frequency.bas (Right-click -> "Save as ...")

' GLOBAL array declaration, could be local within the scope of the calling function
GLOBAL cnt&()

' array must be 256 LONG or DWORD members
redim cnt&(0 to 255)

' calling code for frequency counter
bfcounter a$, VarPtr(cnt&(0))

' Parameters are,
' 1. string to have the byte frequency counted
' 2. address of the 256 member array

' Result is stored in the 256 member array which is the count of each
' character in the array.

  '##########################################################################

  SUB bfcounter(a$,ByVal lpArr as DWORD)

      #REGISTER NONE
    ' -----------------------------
    ' fill array members with zero
    ' -----------------------------
      ! mov ecx, 256
      ! xor eax, eax
      ! mov edi, lpArr
      ! rep stosd
    ' ------------------------------------------------
    ' put string length in ECX and its address in ESI
    ' ------------------------------------------------
      ! mov ecx, a$
      ! mov ecx, [ecx]            ; dereference string address
      ! mov esi, ecx              ; copy address to ESI
      ! sub ecx, 4                ; get string length
      ! mov ecx, [ecx]            ; put it in ECX
      ! add esi, ecx              ; add length to ESI
      ! neg ecx                   ; reverse sign
      ! mov ebx, lpArr            ; base address in EBX
      ! xor eax, eax              ; prevent stall
    fcStart:
      ! mov al, [esi+ecx]         ; get byte
      ! inc DWORD PTR [ebx+eax*4] ; increment value at memory location
      ! inc ecx
      ! jnz fcStart

  END SUB

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