Procedure:

Implement, in LC$-3$ assembly language, an encryption and a decryption programs that meet the

following requirements:

$1.$ The key is $6$ and stored at x$3100.$

$2.$ Your message is restricted to a maximum of $32$ characters. They are stored at x$3110$ to

x$311$F$.$ The last ASCII code of the text string must be EOT $($x$04).$

$3.$ The encryption algorithm is as follows. Each ASCII code in the message will be

transformed as follows:

$$ The lower $4-$bits of the code will be toggled. That is$,$ if it is a $1,$ it will be replaced

by $0$ and vice verse.

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

$$ The result should be stored at location x$3120$ to x$312$F$.$

$$ For example, if the input $($plain text$)$ is A$,$ toggles lower $4$ bits, and the encryption

key is $6,$ the program should take the ASCII value of A$,01000001,$ toggle bit $[3$:$0],$

producing $01001110$ and then add the encryption key, $6.$ The final output

character would be $01010100,$ which is the ASCII value T$.$

$4.$ Decryption algorithm is as follows. Each ASCII code in the message will be transformed

as follows:

$$ Subtract the encryption key from the ASCII code.

$$ Toggle the low order bit of the result of step $1.$

The result should be stored back to location x$3110$ to x$311$F$.$