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Step 6Generating ciphertext
We're almost there now ! On the one hand we have a series of numbers and on the other we have a slightly tweaked message. Let's get working...And get familiar with the wonderful world of modulo !
This word "modulo" might sound scary and strange, but that is actually something people do all the time when, for example, we figure out what time it is - namely modulo 12. Let's suppose it's 11 in the morning and you have to meet someone 3 hours later - that'll be 2 in the afternoon. Easy, is it not ? These operations are called modular arithmetics. Now let us apply this to our cleartext and keystream values.
Let's go back to the cleartext we prepared earlier so that it'd turn into DONOT GOXXX. We're going to convert every letter to a numeric equivalent :
D O N O T G O X X X transforms into :
4 15 14 15 20 7 15 24 24 24
To which we had the keystream values (these are completely arbitrary, and are only here as an example) :
17 15 8 24 3 10 20 13 1 14
Which gives us :
21 30 22 39 23 1735 37 25 38
Do you remember the modulo I mentioned earlier ? If watches are modulo 12, then the alphabet is modulo 26. Converting these values accordingly, we have now :
21 4 22 13 23 17 9 11 25 12
Which once converted back to letters becomes :
U D V M W Q I K Z M.
This word "modulo" might sound scary and strange, but that is actually something people do all the time when, for example, we figure out what time it is - namely modulo 12. Let's suppose it's 11 in the morning and you have to meet someone 3 hours later - that'll be 2 in the afternoon. Easy, is it not ? These operations are called modular arithmetics. Now let us apply this to our cleartext and keystream values.
Let's go back to the cleartext we prepared earlier so that it'd turn into DONOT GOXXX. We're going to convert every letter to a numeric equivalent :
D O N O T G O X X X transforms into :
4 15 14 15 20 7 15 24 24 24
To which we had the keystream values (these are completely arbitrary, and are only here as an example) :
17 15 8 24 3 10 20 13 1 14
Which gives us :
21 30 22 39 23 1735 37 25 38
Do you remember the modulo I mentioned earlier ? If watches are modulo 12, then the alphabet is modulo 26. Converting these values accordingly, we have now :
21 4 22 13 23 17 9 11 25 12
Which once converted back to letters becomes :
U D V M W Q I K Z M.
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