Improve SetManagementKey example in readme, fix ykAuthenticate metada…

…ta cmd error handling

README.md

@@ -95,16 +95,16 @@ newPUKInt, err := rand.Int(rand.Reader, big.NewInt(100_000_000))
 if err != nil {
 	// ...
 }
-var newKey [24]byte
-if _, err := io.ReadFull(rand.Reader, newKey[:]); err != nil {
+newKey := make([]byte, 24)
+if _, err := io.ReadFull(rand.Reader, newKey); err != nil {
 	// ...
 }
 // Format with leading zeros.
 newPIN := fmt.Sprintf("%06d", newPINInt)
 newPUK := fmt.Sprintf("%08d", newPUKInt)
 
 // Set all values to a new value.
-if err := yk.SetManagementKey(piv.DefaultManagementKey, newKey[:]); err != nil {
+if err := yk.SetManagementKey(piv.DefaultManagementKey, newKey); err != nil {
 	// ...
 }
 if err := yk.SetPUK(piv.DefaultPUK, newPUK); err != nil {
@@ -114,8 +114,8 @@ if err := yk.SetPIN(piv.DefaultPIN, newPIN); err != nil {
 	// ...
 }
 // Store management key on the YubiKey.
-m := piv.Metadata{ManagementKey: &newKey[:]}
-if err := yk.SetMetadata(newKey[:], m); err != nil {
+m := piv.Metadata{ManagementKey: &newKey}
+if err := yk.SetMetadata(newKey, m); err != nil {
 	// ...
 }
 

v2/piv/piv.go

@@ -382,9 +382,10 @@ func ykAuthenticate(tx *scTx, key []byte, rand io.Reader, version *version) erro
 			param2:      keyCardManagement,
 		}
 		resp, err := tx.Transmit(cmd)
-		if err == nil {
-			managementKeyType = resp[2:][0]
+		if err != nil {
+			return fmt.Errorf("determining key management type: %w", err)
 		}
+		managementKeyType = resp[2:][0]
 	}
 
 	// set challengeLength based on managementKeyType