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

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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.

a variable of array type holds a reference of object

array are commonly used in computer program to organize data so that a related set of values

array are using zero base indexing it start with 0.

all component of array have same type, two different types doesn't use in single array

if array are store 10 quantities its index value varies from 0 to 9.

it stores 1st quantity value in index value 0 and this process will same till index value 9.