mgm: extract MgmAlgorithmId::default_for_version (#633)

Adds a private method for determining the MGM key algorithm to use for a
given YubiKey `Version`

src/mgm.rs

@@ -141,6 +141,26 @@ impl From<MgmAlgorithmId> for u8 {
 }
 
 impl MgmAlgorithmId {
+    /// Get the default MGM key algorithm for the given YubiKey version.
+    fn default_for_version(version: Version) -> Self {
+        match version {
+            // Initial firmware versions default to 3DES.
+            Version { major: ..=4, .. }
+            | Version {
+                major: 5,
+                minor: ..=6,
+                ..
+            } => Self::ThreeDes,
+            // Firmware 5.7.0 and above default to AES-192.
+            Version {
+                major: 5,
+                minor: 7..,
+                ..
+            }
+            | Version { major: 6.., .. } => Self::Aes192,
+        }
+    }
+
     /// Looks up the algorithm for the given Yubikey's current management key.
     fn query(txn: &Transaction<'_>) -> Result<Self> {
         match txn.get_metadata(crate::piv::SlotId::Management(ManagementSlotId::Management)) {
@@ -204,22 +224,8 @@ impl MgmKey {
     /// Generates a random MGM key using the preferred algorithm for the given Yubikey's
     /// firmware version.
     pub fn generate_for(yubikey: &YubiKey, rng: &mut impl TryCryptoRng) -> Result<Self> {
-        match yubikey.version() {
+        let alg = MgmAlgorithmId::default_for_version(yubikey.version());
-            // Initial firmware versions default to 3DES.
+        Self::generate(alg, rng)
-            Version { major: ..=4, .. }
-            | Version {
-                major: 5,
-                minor: ..=6,
-                ..
-            } => Self::generate(MgmAlgorithmId::ThreeDes, rng),
-            // Firmware 5.7.0 and above default to AES-192.
-            Version {
-                major: 5,
-                minor: 7..,
-                ..
-            }
-            | Version { major: 6.., .. } => Self::generate(MgmAlgorithmId::Aes192, rng),
-        }
     }
 
     /// Parses an MGM key from the given byte slice.
@@ -242,21 +248,10 @@ impl MgmKey {
     ///
     /// Returns an error if the Yubikey's default algorithm is unsupported.
     pub fn get_default(yubikey: &YubiKey) -> Result<Self> {
-        match yubikey.version() {
+        match MgmAlgorithmId::default_for_version(yubikey.version()) {
-            // Initial firmware versions default to 3DES.
+            MgmAlgorithmId::ThreeDes => Ok(Self(MgmKeyKind::Tdes(DEFAULT_MGM_KEY.into()))),
-            Version { major: ..=4, .. }
+            MgmAlgorithmId::Aes192 => Ok(Self(MgmKeyKind::Aes192(DEFAULT_MGM_KEY.into()))),
-            | Version {
+            _ => Err(Error::NotSupported),
-                major: 5,
-                minor: ..=6,
-                ..
-            } => Ok(Self(MgmKeyKind::Tdes(DEFAULT_MGM_KEY.into()))),
-            // Firmware 5.7.0 and above default to AES-192.
-            Version {
-                major: 5,
-                minor: 7..,
-                ..
-            }
-            | Version { major: 6.., .. } => Ok(Self(MgmKeyKind::Aes192(DEFAULT_MGM_KEY.into()))),
         }
     }