Cryptography

Description:

An encryption algorithm transforms "plain text" into "cipher text." Just like a door needs a key to lock and unlock it, an encryption algorithm often requires a key to encrypt and decrypt a message. For example, suppose the plain text message is HELLO WORLD. An encryption algorithm consisting of nothing more than replacing each letter with the next letter in the alphabet would produce the cipher text IFMMP XPSME. The decryption algorithm simply replaces each letter with the one before it, and presto: the plain text HELLO WORLD is produced.

Procedure:

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

The key is 6 and stored at x3100.

Your message is restricted to a maximum of 32 characters. They are stored at x3110 to x311F. The last ASCII code of the text string must be EOT (x04).

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 x3120 to x312F.

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 01000110, which is the ASCII value F.

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 x3110 to x311F.