split from {https://dev.gnupg.org/T6702}
**Usecase**:
- Users should be able to use invalid/expired/~~disabled~~ certificates for both signature/encryption
- Invalid: users might have an S/MIME cert for a recipient, but not the root/intermediate certificate, and still need to send information
- Expired: especially renewal of S/MIME certificates might take a while, but also pgp recipients might have forgotten to extend or publish their extended cert
~~- Disabled: could be temporarily enabled, but as it's easy to implement, it can be easily used once that way without the need to use the CLI~~ There is (and will be) no gpg option for this.
**Current state** of offered pgp/smime certs:
- GpgOL (security dialog)
- "sign as": expired/revoked/disabled not offered, invalid forbidden (OK button disabled)
- "encrypt to": all offered, invalid/disabled/expired/revoked forbidden (OK button disabled)
- Kleopatra (tested with sign/encrypt file, probably the same for notepad/etc)
- all offered (via "show certificate list"), invalid/disabled/expired/revoked forbidden (OK button disabled)
**Implementation**:
- gpg
- a "force/ignore-expiration" flag is needed, which also permits using expired certs
- Kleopatra and GpgOL
- add a checkbox "Lower Security" ("Sicherheit verringern") next to the disabled OK button
- should only be displayed, if a forbidden cert (only invalid/expired, not revoked or disabled) is selected
- tooltip should show more explanation for the implications (especially vs-nfd compliance)
- if checked
- the OK button should be re-enabled (which adds the "always-trust or ignore-expiration" flag; needs the latter flag implemented in `gpg`) -> {T8195}
- on OK, an additional warning/confirmation dialog with an explanation of the implication should be displayed
- add a registry setting flag to disable this checkbox (as some admins might not want to enable their users to do this)
- Kleopatra
- the difficulty to add forbidden certs is intentional (to incentivise the proper fix of the problem, e.g. cert renewal)
- GpgOL
- add the forbidden certs (only invalid/expired, not revoked/disabled) to "sign as" as well
- ensure, that the security dialog is shown, if a forbidden cert is chosen (probably already works that way)
- ideally on "reply all", the forbidden certs should be preselected in the security dialog