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Step 7Deciphering ciphertext
Now let's suppose someone just got a message encrypted with Solitaire. How does one decrypt it ? As you might have already figured out, Solitaire is a symmetric algorithm : what you did to encrypt your message, is what you're going to do to decrypt it - the other way around.
Step 1 - You're going to need the keystream values the sender used to encrypt the message. This is pretty simple, all things considered : all one needs to do is follow the process described in Step 6 - Generating keystream values using the correct deck, and generate as many keystream values as there are of characters in the message, as seen before.
Step 2 - We're going to use the example from the previous step :
U D V M W Q I K Z M,
and convert it into numbers :
21 4 22 13 23 17 9 11 25 12.
Step 3 - If the recipient of the message followed the keystream values generation process correctly, starting from the deck that was agreed upon with the sender, the values generated are the same as those the sender used :
17 15 8 24 3 10 20 13 1 14 ;
Step 4 - From there, all one needs to do is substract the generated keystream values from the numbers that came up when we converted the message, still using modulo 26. This gives us :
4 15 14 15 20 7 15 24 24 24,
Which we convert into letters, obtaining :
D O N O T G O X X X.
As I told you, nothing too difficult if you've understood how the encryption process works - the decryption process is just the same, only reversed. Don't hesitate to train and work on it until it makes sense ! Better make mistakes now than when time will actually be of the essence !
If you're clear on that part, let's move on to the conclusion.
Step 1 - You're going to need the keystream values the sender used to encrypt the message. This is pretty simple, all things considered : all one needs to do is follow the process described in Step 6 - Generating keystream values using the correct deck, and generate as many keystream values as there are of characters in the message, as seen before.
Step 2 - We're going to use the example from the previous step :
U D V M W Q I K Z M,
and convert it into numbers :
21 4 22 13 23 17 9 11 25 12.
Step 3 - If the recipient of the message followed the keystream values generation process correctly, starting from the deck that was agreed upon with the sender, the values generated are the same as those the sender used :
17 15 8 24 3 10 20 13 1 14 ;
Step 4 - From there, all one needs to do is substract the generated keystream values from the numbers that came up when we converted the message, still using modulo 26. This gives us :
4 15 14 15 20 7 15 24 24 24,
Which we convert into letters, obtaining :
D O N O T G O X X X.
As I told you, nothing too difficult if you've understood how the encryption process works - the decryption process is just the same, only reversed. Don't hesitate to train and work on it until it makes sense ! Better make mistakes now than when time will actually be of the essence !
If you're clear on that part, let's move on to the conclusion.
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