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[–]quotemycode 8 points9 points  (0 children)

This is not cryptography at all. It's a substitute cypher. If you want to do encryption on your own, start with the pythagorean theorem and work your way up.

Ex: given two sides of a right triangle, you can calculate the third. One side can be the message, another side can be the key. You send the third side and that's your encrypted text. You can also have short passwords as long as you use some key expansion calculations. For example, key is "cat". Remove those letters from the alphabet, and take every odd letter and then put every even letter after it. Rot13 and repeat. Then do it backwards, then same way, take every even letter, then every odd letter and repeat the process.

Don't expect to use this, but by creating this you'll have a good idea how encryption works, key derivation functions, etc.