Merge pull request #128 from str4d/115-yubikey-4-no-pin-cache

Document and warn that PIN policy "Once" doesn't work for YubiKey 4

CHANGELOG.md

@@ -11,7 +11,7 @@ to 0.3.0 are beta releases.
 - MSRV is now 1.60.0.
 - The YubiKey PIV PIN and touch caches are now preserved across processes in
   most cases. See [README.md](README.md#agent-support) for exceptions. This has
-  several usability effects:
+  several usability effects (not applicable to YubiKey 4 series):
   - If a YubiKey's PIN is cached by an agent like `yubikey-agent`, and then
     `age-plugin-yubikey` is run (either directly or as a plugin), the agent
     won't request a PIN entry on its next use.

README.md

@@ -123,9 +123,10 @@ age client as normal (e.g. `rage -d -i yubikey-identity.txt`).
 ### Agent support
 
 `age-plugin-yubikey` does not provide or interact with an agent for decryption.
-It does however preserve the PIN cache by not soft-resetting the YubiKey after a
-decryption or read-only operation, which enables YubiKey identities configured
-with a PIN policy of `once` to not prompt for the PIN on every decryption.
+It does however attempt to preserve the PIN cache by not soft-resetting the
+YubiKey after a decryption or read-only operation, which enables YubiKey
+identities configured with a PIN policy of `once` to not prompt for the PIN on
+every decryption. **This does not work for YubiKey 4 series.**
 
 The session that corresponds to the `once` policy can be ended in several ways,
 not all of which are necessarily intuitive:
@@ -133,6 +134,8 @@ not all of which are necessarily intuitive:
 - Unplugging the YubiKey (the obvious way).
 - Using a different applet (e.g. FIDO2). This causes the PIV applet to be closed
   which clears its state.
+  - This is why the YubiKey 4 series does not support PIN cache preservation:
+    their serial can only be obtained by switching to the OTP applet.
 - Generating a new age identity via `age-plugin-yubikey --generate` or the CLI
   interface. This is to avoid leaving the YubiKey authenticated with the
   management key.

i18n/en-US/age_plugin_yubikey.ftl

@@ -76,6 +76,14 @@ cli-setup-name-identity       = 📛 Name this identity
 cli-setup-select-pin-policy   = 🔤 Select a PIN policy
 cli-setup-select-touch-policy = 👆 Select a touch policy
 
+cli-setup-yk4-pin-policy =
+    ⚠️ Your {-yubikey} is a {-yubikey} 4 series. With ephemeral applications like
+    {-age-plugin-yubikey}, a PIN policy of "Once" behaves like a PIN policy of
+    "Always", and your PIN will be requested for every decryption. However, you
+    might still benefit from a PIN policy of "Once" in long-running applications
+    like agents.
+cli-setup-yk4-pin-policy-confirm = Use PIN policy of "Once" with {-yubikey} 4?
+
 cli-setup-generate-new = Generate new identity in slot {$slot_index}?
 cli-setup-use-existing = Use existing identity in slot {$slot_index}?
 

src/main.rs

@@ -487,6 +487,8 @@ fn main() -> Result<(), Error> {
                     .report(true)
                     .interact_text()?;
 
+                let mut displayed_yk4_warning = false;
+                let pin_policy = loop {
                     let pin_policy = match Select::new()
                         .with_prompt(fl!("cli-setup-select-pin-policy"))
                         .items(&[
@@ -512,6 +514,29 @@ fn main() -> Result<(), Error> {
                         None => return Ok(()),
                     };
 
+                    // We can't preserve the PIN cache for YubiKey 4 series, because to
+                    // retrieve the serial we switch to the OTP applet.
+                    match (pin_policy, yubikey.version().major) {
+                        (PinPolicy::Once, 4) => {
+                            if !displayed_yk4_warning {
+                                eprintln!();
+                                eprintln!("{}", fl!("cli-setup-yk4-pin-policy"));
+                                eprintln!();
+                                displayed_yk4_warning = true;
+                            }
+
+                            if Confirm::new()
+                                .with_prompt(fl!("cli-setup-yk4-pin-policy-confirm"))
+                                .report(true)
+                                .interact()?
+                            {
+                                break pin_policy;
+                            }
+                        }
+                        _ => break pin_policy,
+                    }
+                };
+
                 let touch_policy = match Select::new()
                     .with_prompt(fl!("cli-setup-select-touch-policy"))
                     .items(&[