classified vs in (strict) confidence

classified

adj
  • Sorted into classes or categories. 

  • Formally assigned by a government to one of several levels of sensitivity, usually (in English) top secret, secret, confidential, and, in some countries, restricted; thereby making disclosure to unauthorized persons illegal. 

  • Not meant to be disclosed by a person or organization. 

noun
  • A classified advertisement in a newspaper or magazine. 

in (strict) confidence

How often have the words classified and in (strict) confidence occurred in a corpus of books? (source: Google Ngram Viewer )