i have a project where i am suppose to encrypt or decrypt a message typed in by the user using the ceaser encryption algorithm and printing it back, i need some help in as to how to write the encryption alg, (ill figure the dec out from there), heres the code im still fixing parts of it so, thanks

.orig x3000

main JSR read_input

read_input LEA R0, prompt1

PUTS

TRAP x20

ADD R1, R0, #-68 ;will need to sub 68 to dec value of ascii

RET

LEA R1, enckey

again LDR R2, R1, #0

BRz next

ADD R1, R1, #1

BR again

next LEA R0, enckey

PUTS

LS R3, negenter

again2 GETC

OUT

ADD R2, R0, R3

BRz cont

STR R0, R1, #0

ADD R1, R1, #1

BR again2

cont AND R2, R2, #0

STR R2, R2, #0

LEA R0, hello

PUTS

HALT

negenter .FILL xFFF6

prompt1 .FILL x000a

.STRINGZ "E(ncrypt)/D(ecrypt): "

enckey .STRINGZ "Encryption Key: "

.BLKW #2

;

asciitobinary AND R0, R0, #0

ADD R1, R1, #0

BRz doneatob

;

LD R3, negasciioffset

LEA R2, asciibuff

ADD R2, R2, R1

ADD R2, R2, #-1

;

LDR R4, R2, #0

ADD R4, R4, R3

ADD R0, R0, R4

;

ADD R1, R1, #-1

BRz doneatob

ADD R2, R2, #-1

;

LDR R4, R2, #0

ADD R4, R4, R3

LEA R5, lookup10

ADD R5, R5, R4

LDR R4, R5, #0

ADD R0, R0, R4

doneatob ret

negasciioffset .FILL xFFD0

asciibuff .BLKW 4

lookup10 .FILL #0

.FILL #10

.FILL #20

.FILL #30

.FILL #40

.FILL #50

.FILL #60

.FILL #70

.FILL #80

.FILL #90

;

encrypt ;God knows

decrypt ;???

print_output

length .FILL #41

string .BLKW #41

.END