;----------------- Solution to Project 5 --------------------
; Author: Aneesh Akella
; Last Modified Date: 12/2/2021
; Summary: This program decrypt and encrypt a message (input) with a key using Vigenere Cipher, the results will be stored in output.
.386
.model flat, stdcall
.stack 4096
ExitProcess PROTO, dwExitCode:DWORD
.data
input byte "JPUESZSBAPANNHTXRTLBVLL"
key byte "ABCXYZ"
options BYTE 0 ; Variable to determine what procedure to execute. 1 for Encryption, otherwise for Decryption
d byte 26 ; variable to hold the devisor that will be used for division ( mod )
output byte lengthof input dup(0)
.code
main proc
cmp options, 1 ; compare value of options to 1
je En ; if value equal to 1 go to encryption rotten
jne De ; if not go to Decryption
De:
call Decrypt
jmp MOVEON ; after returning from Decrypt procedure jump to "moveon" so we don't call Decrypt accidentally
En:
call Encrypt
jmp MOVEON ; There is no need use jump but just in case is.
MOVEON:
mov eax, 0
; OPTIONAL: Print Output to console: uncomment the next 2 lines to activate (Require irvine32.inc)
; lea EDX, output
CALL writestring
invoke ExitProcess, 0
main endp
Decrypt proc
mov esi, 0 ; index of both input and output
mov edi, 0 ; index of the key
mov ecx, lengthof input ; (1) loop L (LENGTHOF input) times
L:
mov eax, 0 ; clear EAX
mov al, input[esi] ; move cypher character ASCII value into al
sub al, 65 ; get the "order" of the character ( A=0, B=1, C=2, ... , Z=25)
mov bl, key[edi] ; (2) move the key ASCII character into bl
sub bl, 65 ; (3) get the "order" of the character ( A=0, B=1, C=2, ... , Z=25)
sub al, bl ; (4) subtract the indices to "decrypt" the character
add al, 26 ; add 26 in case subtraction result is a negative value
div d ; (5) use divide to get "mod 26". AX will be divided, quotient in AL and reminder in AH.
add ah, 65 ; (6) add 65 to convert reminder back from order to the proper ASCII value
mov output[esi], ah ; write ASCII value to output
inc esi
inc edi
cmp edi, lengthof key
jne Next
mov edi, 0 ; (7)if key index reached the end, reset the key index
Next:
loop L
ret
Decrypt endp
Encrypt proc
mov esi, 0
mov edi, 0
mov ecx, lengthof input ; loop L (LENGTHOF input) times
L:
mov eax, 0
mov al, input[esi] ; move the input character into al
sub al, 65 ; get the order of the character ( A=0, B=1, C=2, ... , Z=25)
mov bl, key[edi] ; move the key character into bl
sub bl, 65 ; get the "index" of the character ( A=0, B=1, C=2, ... , Z=25)
add al, bl ; (8) add the indices to "encrypt" the character
div d ; divide so we can "mod 26"
add ah, 65 ; add 65 to convert from order back to the proper ASCII value
mov output[esi], ah ; write reminder into the output
inc esi
inc edi
cmp edi, lengthof key ; if we have reached the end of the key ...
jne Next
mov edi, 0 ; ...reset the key index
Next:
loop L
ret
Encrypt endp
end main
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Assembly language(asm) is a low-level programming language, where the language instructions will be more similar to machine code instructions.
Every assembler may have it's own assembly language designed for a specific computers or an operating system.
Assembly language requires less execution time and memory. It is more helful for direct hardware manipulation, real-time critical applications. It is used in device drivers, low-level embedded systems etc.
Assembly language usually consists of three sections,
Data section
To initialize variables and constants, buffer size these values doesn't change at runtime.
bss section
To declare variables
text section
_start specifies the starting of this section where the actually code is written.
There are various define directives to allocate space for variables for both initialized and uninitialized data.
variable-name define-directive initial-value
| Define Directive | Description | Allocated Space |
|---|---|---|
| DB | Define Byte | 1 byte |
| DW | Define Word | 2 bytes |
| DD | Define Doubleword | 4 bytes |
| DQ | Define Quadword | 8 bytes |
| DT | Define Ten Bytes | 10 bytes |
| Define Directive | Description |
|---|---|
| RESB | Reserve a Byte |
| RESW | Reserve a Word |
| RESD | Reserve a Doubleword |
| RESQ | Reserve a Quadword |
| REST | Reserve a Ten Bytes |
Constants can be defined using
CONSTANT_NAME EQU regular-exp or value
%assign constant_name value
%define constant_name value
Loops are used to iterate a set of statements for a specific number of times.
mov ECX,n
L1:
;<loop body>
loop L1
where n specifies the no of times loops should iterate.
Procedure is a sub-routine which contains set of statements. Usually procedures are written when multiple calls are required to same set of statements which increases re-usuability and modularity.
procedure_name:
;procedure body
ret