Coding Games and Programming Challenges to Code Better
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I noticed something when playing around with my own code. I’ll explain by example.
Consider the following board:
0 1 2 3 4 5 6 7 8 9
0 A B C D E F G H
1 I J K L M N O P Q R
2 / . S T U V W X Y Z
key = 1
plaintext = A
ciphertext (doesn’t exist)
When/if the last message character keys to one of the blanks, there is no next message character to combine with it, so the last character is lost. I know no one uses this cipher in real life, but it might be worth adding a note to the description.
The easiest solution would probably be to select a key that doesn’t cause a problem with the last character. Alternatively, a program could append a “/” automatically as the last character. It should drop on decryption. (I’m not asking for the puzzle to be changed.)
EDIT: A more elegant solution (I think)
Add a space to the end of the string to be encrypted. If you need a digit, you have it. If you don’t need it, you can discard it when you get to it. Using the example above (with _ for space):
Encrypt: A_ (which needs an additional character)
A _
0 1
1 1 (key)
1 2
^ (blank as next to last char, use last char)
K <= ciphertext
Decrypt: K
K
1 2
1 1 (key)
0 1
^ (blank as last char, discard)
A <= plaintext
Encrypt: C_ (which doesn't need an additional char, but we don't know that yet)
C _
4 1
1 1 (key)
5 2
(no blank as next to last char, discard last char)
D <= ciphertext