## Chapter 16 — The signing key stays with the platform ### Lesson map Lesson 16. Map, example, EX062. ```mermaid flowchart TD A["The platform holds the signing key"] --> B["Receivers can check the sender"] B --> C{"Does signing hide the newsletter?"} C -->|No| D["Authenticity stays, secrecy does not"] C -->|Yes| E["That promise was not made"] ``` The key's home is the supplier. Its promise is about who sent the mail. ### Logical drawing ```text newsletter platform holds the email signing key signs outgoing mail promise receivers can tell the mail comes from the library not the promise the body is secret forbidden paste the key into email, chat, or a shared document ``` ### Learning outcomes After this chapter, you should be able to: 1. Leave the signing key with the newsletter platform. 2. Describe the check as authenticity. 3. Refuse a request to copy the key into a message. ### 16.1 Who holds the key decides who must not be asked for it. Chapter 3 says the newsletter platform signs outgoing email so receivers can tell it really comes from the library, and that the platform holds the signing key. The library depends on that supplier and cannot change the setting directly. A signature lets someone check who signed and that the signed bytes were not altered. It does not, by itself, hide the newsletter's words. No key value, no public key, and no screenshot of a signer appears in the file. A classmate who asks the platform to paste the key into chat breaks the rule against secrets in email, chat, and shared documents, and also asks for a control the library does not hold. Leo names the holder and the promise. He does not request the key. ### 16.2 Worked example | What someone cites | What the file holds | Decision | |---|---|---| | Platform holds the key | Chapter 3 | Keep the holder | | Receivers can check the sender | The signing purpose | Keep the purpose | | "The body is therefore secret." | Not the stated promise | Refuse | | "Email me the key." | Secrets stay out of mail | Refuse | > **Synthetic case.** No key characters are requested or stored. ### Errors and limits Naming the platform as holder does not give the library the key, and it does not make the newsletter confidential. ### Apply - **EX062 — Where is the key?** The newsletter platform signs mail so receivers can tell it comes from the library. A colleague asks Leo to have the key sent by email for the summary. What do you answer? *Rubric:* 3 = the platform keeps the key; signing here is about the sender, not secrecy of the body; do not ask for the key by email, chat, or a shared document; no key value is in the file;; 2 = the key stays with the platform;; 1 = one promise;; 0 = none. ### Continue studying — lesson 16 (chapter 16 of 20) [← Chapter 15](#ch-15) · [Course chapter index](../SUBJECTS.md) · [Chapter 17 →](#ch-17) Continue to the next authored lesson. ## Worked answers **EX062.** The newsletter platform keeps the signing key. The check concerns who sent the mail, not secrecy of the body. Do not ask for the key by email, chat, or a shared document.