To expound slightly on burgerbrain's comment, this phrase is universally used to refer to schemes whose security relies critically on the suppression of informa

To expound slightly on burgerbrain's comment, this phrase is universally used to refer to schemes whose security relies critically on the suppression of information about the system. The term simply does not apply to strong encryption with configurable keys. The term doesn't apply to door locks either, at least not in practice. If I were forever prevented from changing the locks on my house, then yes my security would depend critically on you never knowing about the key. In practice though, I can change the locks at will, and this is an effective remedy against the persistent threat posed by the release of the key. A good example of security though obsurity is the CSS system used on DVDs, which was designed such that once the key was discovered, the system became forever and irretrievably broken.