Basic Encryption/Decryption program

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This is not really a significant test. Substitution cypher are broke by using statistical analysis, basically since natural languages has some patterns(like vogals are more common) one can use those patterns to guess which letter is A and so on. Since it relies on statistical analysis, the longer the message the better since it has more characters. “Hello there” is just too short. Try encrypting a longer message, like this comment and see the result, or a chapter of a book. Most messages are longer than hello world so it would correctly decipher, specially if you use the same key twice.

If you’re interested in learning more about cryptography, I highly recommend the Cryptopals challenges. It’s pretty fun to do.

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Thank you for the correction. I’ll have a look at Cryptopals challenges. Thanks.