(c) an entrusted or committed secret, which is one that a person promised (and before he learned it) to keep from others. The promise here is either implicit or explicit. An implicit promise of secrecy is one that is demanded by the confidential nature of communications between two parties (professional secret), as when physicians, lawyers, priests, parents, or friends are told of private matters on account of their position or relationship. An explicit promise is one that is given in express terms, as when A says to B: “I have a matter of great importance to tell you, but you must first promise that you will keep it secret”; and on B promising, A confides to him the secret.
2409. Sinfulness of Violating a. Secret.—A secret is the property of its owner, and to it he has a strict right; for if it is a good secret (such as an original idea or discovery), it is the product of his labor or at least a possession which he has lawfully come by; if it is an evil secret (such as a crime of which he has been guilty), it may not be made known without infringing on his right of reputation. It is no more lawful to violate the right to a secret than to violate the right to property, and, as there are three kinds of injuries to property, so there are three kinds of injuries to a secret.
(a) Thus, the right of possession is injured by those who by fraud or force or other illegal means deprive another of his secret (e.g., by secretly intercepting private letters, by making a person drunk in order to learn a secret).
(b) The right of use is injured by those who on acquiring knowledge of a secret guide themselves or others by it to the detriment of the owner’s rights.
(c) The right of disposition is injured by those who reveal a secret which they were obliged not to reveal.
2410. Prying Into Others’ Secrets.—To seek to discover the secrets of others is not lawful unless the following conditions are present:
(a) one must have a right to the knowledge. Hence, if there is question about a crime that has been committed or that is about to be committed, one has a right to investigate in order to prevent harm to public or private good; in war one may try to discover the plans of the enemy. But it is not lawful to pry into purely personal matters, to fish from others natural or confidential secrets which they are bound to keep, to steal from another the thoughts, plans, inventions, etc, which are his own;
(b) one must use only honest means to discover secrets to which one has a right (1504). Thus, it does not seem lawful generally to inebriate another in order to learn his secret, and it is certainly sinful to resort to lies or simulation or immorality.
2411. Reading Another’s Letters or Papers.—When is it lawful to read the letters or other papers of another person?
(a) This is lawful when the writings are not intended to be secret to anyone, as when a circular is meant for public use, when greetings are written on a postcard which all may read, and when a letter is left open and thrown away or otherwise abandoned. But a sealed letter, or one left open in a private room, or one lost in a public place, is secret. If a letter or manuscript has been torn up by its owner and thrown away on the street or other public place, it does not seem lawful to piece the fragments together and read the writing, for, though the paper has been abandoned, the owner by destroying it has indicated his will to keep the contents secret.