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Quote: Modern Cryptography: Applied Mathematics for Encryption and Information Security @ Amazon.com[^]

Kerckhoffs’s Principle/Shannon’s Maxim
Kerckhoffs’s principle is an important concept in cryptography. Auguste Kerckhoffs’s first articulated this in the 1800s, stating that “the security of a cipher depends only on the secrecy of the key, not the secrecy of the algorithm.”

Shannon rephrased this, stating that “One ought to design systems under the assumption that the enemy will ultimately gain full familiarity with them.”3 This is referred to as Shannon’s maxim and states essentially the same thing Kerckhoffs’s principle states.
Let me attempt to restate and expound this in terms you might find more verbose, but hopefully easier to understand. Both Kerckhoffs’s principle and Shannon’s maxim state that the only thing that you must keep secret is the key. You don’t need to keep the algorithm secret. In fact, in subsequent chapters, this book will provide intimate details of most modern algorithms, and that in no way compromises their security. As long as you keep your key secret, it does not matter that I know you are using AES 256 bit, or Serpent, or Blowfish, or any other algorithm you could think of.
I would add to Kerckhoffs’s principle/Shannon’s maxim something I will humbly call Easttom’s corollary:

You should be very wary of any cryptographic algorithm that has not been published and thoroughly reviewed. Only after extensive peer review should you consider the use of any cryptographic algorithm.