don't roll your own crypto
expressioncryptography
The first commandment of applied cryptography: use the vetted algorithms and libraries, never your homemade ones.
Designing crypto that others cannot break is one of the hardest problems in the field, and every homemade cipher looks unbreakable to its author - Schneier's law in action. Rolling your own means your mistakes ship without ever meeting the years of public attack that hardened AES and TLS.