Prevent changing the default PIN to itself

Closes str4d/age-plugin-yubikey#120.

CHANGELOG.md

@@ -11,6 +11,8 @@ to 0.3.0 are beta releases.
 - When `age-plugin-yubikey` assists the user in changing their PIN from the
   default PIN, it no longer tells the user that PINs shorter than 6 characters
   are allowed, and instead loops until the user enters a PIN of valid length.
+  It also now prevents the user from setting their PIN to the default PIN, to
+  avoid creating a cycle.
 - More kinds of SmartCard readers are ignored when they have no SmartCard
   inserted.
 

i18n/en-US/age_plugin_yubikey.ftl

@@ -135,6 +135,7 @@ mgr-enter-current-puk = Enter current PUK (default is {$default_puk})
 mgr-choose-new-pin    = Choose a new PIN/PUK
 mgr-repeat-new-pin    = Repeat the PIN/PUK
 mgr-pin-mismatch      = PINs don't match
+mgr-nope-default-pin  = You entered the default PIN again. You need to change it.
 
 mgr-changing-mgmt-key =
     ✨ Your {-yubikey} is using the default management key.

src/key.rs

@@ -284,20 +284,27 @@ pub(crate) fn manage(yubikey: &mut YubiKey) -> Result<(), Error> {
         let current_puk = Password::new()
             .with_prompt(fl!("mgr-enter-current-puk", default_puk = DEFAULT_PUK))
             .interact()?;
-        let new_pin = request_pin(
-            |prev_error| {
-                if let Some(err) = prev_error {
-                    eprintln!("{}", err);
-                }
-                Password::new()
-                    .with_prompt(fl!("mgr-choose-new-pin"))
-                    .with_confirmation(fl!("mgr-repeat-new-pin"), fl!("mgr-pin-mismatch"))
-                    .interact()
-                    .map(|pin| Result::<_, Infallible>::Ok(SecretString::new(pin)))
-            },
-            yubikey.serial(),
-        )?
-        .unwrap();
+        let new_pin = loop {
+            let pin = request_pin(
+                |prev_error| {
+                    if let Some(err) = prev_error {
+                        eprintln!("{}", err);
+                    }
+                    Password::new()
+                        .with_prompt(fl!("mgr-choose-new-pin"))
+                        .with_confirmation(fl!("mgr-repeat-new-pin"), fl!("mgr-pin-mismatch"))
+                        .interact()
+                        .map(|pin| Result::<_, Infallible>::Ok(SecretString::new(pin)))
+                },
+                yubikey.serial(),
+            )?
+            .unwrap();
+            if pin.expose_secret() == DEFAULT_PIN {
+                eprintln!("{}", fl!("mgr-nope-default-pin"));
+            } else {
+                break pin;
+            }
+        };
         let new_pin = new_pin.expose_secret();
         yubikey.change_puk(current_puk.as_bytes(), new_pin.as_bytes())?;
         yubikey.change_pin(pin.as_bytes(), new_pin.as_bytes())?;