Applications
In accordance with Kerckhoffs's principle, the majority of civilian cryptography makes use of publicly known algorithms. By contrast, ciphers used to protect classified government or military information are often kept secret (see Type 1 encryption). However, it should not be assumed that government/military ciphers must be kept secret to maintain security. It's possible that they are intended to be as cryptographically sound as public algorithms, and the decision to keep them secret is in keeping with a layered security posture.
Eric Raymond extends this principle in support of open source security software, saying, "Any security software design that doesn't assume the enemy possesses the source code is already untrustworthy; therefore, never trust closed source."
Read more about this topic: Kerckhoffs's Principle