Caesar Cipher in Arm Assembly Assignment Sample

The Caesar Cipher is one of the simplest encryption schemes imaginable, and is very to implement in a high level language. To encrypt a string in C you would write code similar to the following.

void encrypt(char* text, int shift) {

while (*text) {

char ch = *text;

if (isAlpha(ch)) ch = ‘a’ + ((ch – ‘a’ + shift) % 26);

*text++ = ch;

}

}

This modifies the string in place, and only affects alphabetical characters. Since the Raspberry PI uses an ARM processor, you may write the program to run on a Raspberry PI. For assembly language assignment help contact us, no matter the processor as we have experts in a wide range of processors.

Solution:

PRESERVE8

AREA vectors, CODE, READONLY

EXPORT __Vectors

__Vectors

DCD 0x20008000 ; stack pointer value when stack is empty

DCD Reset_Handler ; reset vector

ALIGN

;————————————————–

; Data area

;————————————————–

AREA MyData, DATA, READWRITE

EXPORT Ciphertext, Deciphertext

Ciphertext SPACE 100 ; space to save cipher text

Deciphertext SPACE 100 ; space to save deciphered text

ALIGN

AREA MainProgram, CODE, READONLY

;————————————————–

; Key to use for the cipher

;————————————————–

Key DCD 33

;————————————————–

; String to cipher

;————————————————–

Plaintext DCB “This is a sample text to be used with the Caesar’s Cipher.\n”,0

ALIGN

ENTRY

EXPORT Reset_Handler

;————————————————–

; Start of code

;————————————————–

Reset_Handler

; Cipher plaintext using Caesar’s cipher

ldr R0,=Key ; load cipher key in R0

ldr R0,[R0]

ldr R1,=Plaintext ; load plaintext address in R1

ldr R2,=Ciphertext ; load ciphertext address in R2

bl Caesar_Cipher ; cipher plaintext

; here, variable Ciphertext will have the plaintext cipher

;Now test the deciphering using the negative key

ldr R0,=Key ; load cipher key in R0

ldr R0,[R0]

neg R0, R0 ; convert to negative

ldr R1,=Ciphertext ; load ciphertext address in R1 (use as plaintext argument)

ldr R2,=Deciphertext ; load deciphertext address in R2 (use as ciphertext argument)

bl Caesar_Cipher ; cipher ciphertext

; here, variable Deciphertext will have the original plaintext

endloop ; infinite loop to end program

B endloop

;————————————————–

; Procedure to cipher a text using Caesar’s cipher

; On entry:

; R0 = Key

; R1 = address of zero-terminated plain text

; R2 = address of space to save ciphertext

;————————————————–

Caesar_Cipher PROC

push {R0-R3, LR}

_loop

ldrb R3, [R1], #1 ; load character from plaintext

cmp R3, #0 ; see if we reached the end of text

beq _endcipher ; if so, end cipher loop

cmp R3, #32 ; see if character is < 32

blt _save ; if so, don’t cipher, only save

cmp R3, #127 ; see if character is >= 127

bge _save ; if so, don’t cipher, only save

add R3, R3, R0 ; else, use key to cipher the character

cmp R3, #32

addlt R3, #95 ; if after ciphering is < 32, wrap around

cmp R3, #127

subge R3, #95 ; if after ciphering is >=127, wrap around

_save

strb R3, [R2], #1 ; save character in ciphertext

b _loop ; repeat

_endcipher

pop {R0-R3, LR}

bx lr ; return to caller

ENDP

ALIGN

END