keyfork-derive-openpgp: document KEYFORK_OPENPGP_EXPIRE

crates/derive/keyfork-derive-openpgp/src/lib.rs

@@ -59,8 +59,12 @@ pub enum Error {
 #[allow(missing_docs)]
 pub type Result<T, E = Error> = std::result::Result<T, E>;
 
-/// Create an OpenPGP Cert with derived keys from the given derivation response, keys, and User
-/// ID.
+/// Create an OpenPGP Cert with private key data, with derived keys from the given derivation
+/// response, keys, and User ID.
+///
+/// Certificates are created with a default expiration of one day, but may be configured to expire
+/// later using the `KEYFORK_OPENPGP_EXPIRE` environment variable using values such as "15d" (15
+/// days), "1m" (one month), or "2y" (two years).
 ///
 /// # Errors
 /// The function may error for any condition mentioned in [`Error`].
@@ -109,7 +113,7 @@ pub fn derive(xprv: XPrv, keys: &[KeyFlags], userid: &UserID) -> Result<Cert> {
     let cert = cert.insert_packets(vec![Packet::from(userid.clone()), binding.into()])?;
     let policy = sequoia_openpgp::policy::StandardPolicy::new();
 
-    // Set certificate expiration to one day
+    // Set certificate expiration to configured expiration or (default) one day
     let mut keypair = primary_key.clone().into_keypair()?;
     let signatures =
         cert.set_expiration_time(&policy, None, &mut keypair, Some(expiration_date))?;

crates/keyfork/src/cli/derive.rs

@@ -20,8 +20,12 @@ pub enum DeriveSubcommands {
     /// Derive an OpenPGP Transferable Secret Key (private key). The key is encoded using OpenPGP
     /// ASCII Armor, a format usable by most programs using OpenPGP.
     ///
-    /// The key is generated with a 24-hour expiration time. The operation to set the expiration
-    /// time to a higher value is left to the user to ensure the key is usable by the user.
+    /// Certificates are created with a default expiration of one day, but may be configured to
+    /// expire later using the `KEYFORK_OPENPGP_EXPIRE` environment variable using values such as
+    /// "15d" (15 days), "1m" (one month), or "2y" (two years).
+    ///
+    /// It is recommended to use the default expiration of one day and to change the expiration
+    /// using an external utility, to ensure the Certify key is usable.
     #[command(name = "openpgp")]
     OpenPGP {
         /// Default User ID for the certificate, using the OpenPGP User ID format.