ADD R$1,$ R$1,$ #$1$ ; Sets R$1$ to x$3101($memory location to store

key$)$

STR R$0,$ R$1,$ #$0$ ; Stores Encryption Key $(0-9)$ character at

memory location indicated by R$1$

LEA R$0,$ PROMPT$3$ ; Load third prompt Encryption Message into R$0$

PUTS ; Output Prompt$3$ same as TRAP $22.$ Write null$-$

terminated string to console.

;$-$ The key is mapped to the line feed character on the LC$-3($ASCII x$0$A$)$

ADD R$1,$ R$1,$ #$1$ ; Sets R$1$ to x$3102$ for message storage

ADD R$2,$ R$2,$ #$-10$

ADD R$2,$ R$2,$ #$-10$ ; Sets R$2$ to $-20$ for counter. Encrypt message

limited to $20$ characters

ADD R$3,$ R$3,$ #$-10$ ; Sets R$3$ to $-10$ for comparison with "ENTER"

key LC$-3($ASCII x$0$A$)$

InputLoop GETC ; Begin of InputLoop, gets characterinput

OUT ; Write one character to console same as TRAP

$21$

;Add code here ; Compares input character in R$0$ with R$3$

BRz ExitInput ; Exits loop if "ENTER" key is detected

;Add code here ; Stores input character in R$0$ into memory

location in R$1$

;Add code here ; Increments R$1$ memory location for next

character in input encrypt message

;Add code here ; Increments counter

BRn InputLoop ; Loops if counter is still negative

ExitInput ;

AND R$3,$ R$3,$ #$0$ ; Clears R$3$ for later use

AND R$4,$ R$4,$ #$0$ ; Clears R$4$ for later use

;These lines prepare for encryption$/$decryption

LD R$1,$ STORE ; Stores x$3100$ in R$1$

;Add code here ; Load encryption key $($ASCII$)$ from the memory

address which the R$1$ point to into R$2$

ADD R$2,$ R$2,$ #$-16$ ; These three lines convert R$2$ from ASCII to

Decimal. $"0-9"$ are $"48-57"$ ASCII code

ADD R$2,$ R$2,$ #$-16$ ;

ADD R$2,$ R$2,$ #$-16$ ;

;Add code here ; Makes R$3$ point to first char in input messge

for encryption from the memory location in R$1$

LD R$5,$ NEW ; Loads memory location NEW to store

encrypted message

LDI R$1,$ STORE ; Reloads R$1$ with x$3100$

LD R$6,$ N$68$ ; Loads R$6$ with $-68$ to check for D input

;Add code here ; Adds R$1$ and R$6$ to check if D was first input

;BRz DECRYPT ; Goes to DECRYPT if D was first input, else it runs

encyption in next instruction

;

;Encryption algorithm:

;The encryption algorithm is as follows. Each ASCII code in the message will be

transformed as follows:

;

; $1.$The low order bit of the code will be toggled. That is$,$ if it is a $1,$

; it will be replaced by a $0,$ if it is a $0,$ it will be replaced by a $1.$

;

; $2.$ The key will be added to the result of step $1$ above.

; For example, if the input $($plain text$)$ is A and the encryption