Making text unreadable, 1 sample code works, 2 do not work

[Dale Yarker]

dale y. second sample get ".This is my secret mes" only

Noted in code opening comments. Try a different key. (note key needs changing in 2 places)

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dale y. third sample get "This is my secret message. 5"

changing the seed to RANDOMIZE, changes the "extra" character.
The dot dat file has one character too many. I don't see how.

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 It is essentially a One-Time Pad.

No it isn't. It is key stream. 

Both 2 and 3 are XOR. One with a single fixed modifier value (VERY unsecure), the other with a stream from a PRNG. note: RND is not a cryptographically secure PRNG.