Question

Encryption Using Rotate Operations Write a procedure that performs simple encryption by rotating each plaintext byte a varying number of positions in different directions. For example, in the following array that represents the encryption key, a negative value indicates a rotation to the left and a positive value indicates a rotation to the right. The integer in each position indicates the magnitude of the rotation: key BYTE -2, 4, 1, 0, -3, 5, 2, -4, -4, 6

Answer 1

Answer:

Check the explanation

Explanation:

1) Assembly language code for Encryption using rotate operations.

INCLUDE Irvine32.inc

INCLUDE Macros.inc

BufSize = 80

.data

  key BYTE 6, 4, 1, 2, 7, 5, 2, 4, 3, 6

  myText BYTE "This text is going to be encrypted.", 0

  buffer BYTE BufSize DUP(?),0,0

  stdInHandle HANDLE ?

  bytesRead DWORD ?

.code

main PROC

  ;Pass the pointer to the text string in EDX,

  MOV EDX, OFFSET myText

 

  ;the array size to ECX

  MOV ECX, SIZEOF myText

 

  ;pointer to the key array in ESI,  

  MOV ESI, OFFSET key

  ;the direction value (0 or 1) in EBX

  MOV EBX, 0 ; rotate left for encryption

  call WriteString

  call Crlf

  call encDecText

  call WriteString

  call Crlf

  MOV EBX, 1

  call encDecText

  call WriteString

  call Crlf

  ;bonus - get string from console and encrypt

  mWriteln "Write a text to be encrypted."

  ; Get handle to standard input

  INVOKE GetStdHandle, STD_INPUT_HANDLE

  mov stdInHandle,eax

  ; Wait for user input

  INVOKE ReadConsole, stdInHandle, ADDR buffer,

  BufSize, ADDR bytesRead, 0

  ;encrypt and output to console

  MOV EDX, OFFSET buffer

  MOV ECX, BufSize

  MOV EBX, 0

  call encDecText

  call WriteString

  call Crlf

  ;decrypt and output to console

  MOV EBX, 1

  call encDecText

  call WriteString

  INVOKE ExitProcess,0 ; end the program

main ENDP

encDecText PROC

  ;Receives EDX - OFFSET of the text

  ; ECX - SIZE of the text

  ; ESI - OFFSET of the key

  ; EBX - rotation direction 0 for left 1 for right

  PUSHAD

  CMP EBX, 0

  JE equals

      MOV EBX, ESI

      ADD EBX, 9 ;the length of key

      loopNotEquals:

      MOV AL, [EDX] ; value of the text

      PUSH ECX

      MOV CL, [ESI] ; value of the key

      ROR AL, CL ; ror the text by the key

      MOV [EDX], AL

      POP ECX

      CMP ESI, EBX ; if all the keys are used, reset the offset so it uses the beginning

      JE reset1

     

      INC ESI

     

      JMP endReset1

      reset1:

      SUB ESI, 9

      endReset1:

     

      INC EDX

      loop loopNotEquals

      mWriteln "Input decrypted."

  JMP endCMP

  equals:

      MOV EBX, ESI

      ADD EBX, 9 ; the length of key

      loopEquals:

      MOV AL, [EDX] ; value of the text

      PUSH ECX

      MOV CL, [ESI] ; value of the key

      ROL AL, CL ; rol the text by the key

      MOV [EDX], AL

      POP ECX

      CMP ESI, EBX ; if all the keys are used, reset the offset so it uses the beginning

      JE reset2

      INC ESI

     

      JMP endReset2

      reset2:

      SUB ESI, 9

      endReset2:

      INC EDX

      loop loopEquals

      mWriteln "Input encrypted."

  endCMP:

  POPAD

  RET

encDecText ENDP

END main

Save the files with .asm extension.