Improve parsing of serial numbers (#466)

Checks the length of the data returned when querying the serial number,
returning an error if it's longer than 4 bytes, and left-padding with
zeroes if it's too short.

This fixes some potential panics due to incorrect slice lengths as were
experienced in #465

src/transaction.rs

@@ -105,7 +105,8 @@ impl<'tx> Transaction<'tx> {
 
     /// Get YubiKey device serial number.
     pub fn get_serial(&self, version: Version) -> Result<Serial> {
-        let response = if version.major < 5 {
+        match version.major {
+            4 => {
                 // YK4 requires switching to the yk applet to retrieve the serial
                 let sw = Apdu::new(Ins::SelectApplication)
                     .p1(0x04)
@@ -118,12 +119,12 @@ impl<'tx> Transaction<'tx> {
                     return Err(Error::GenericError);
                 }
 
-            let resp = Apdu::new(0x01).p1(0x10).transmit(self, 0xFF)?;
+                let response = Apdu::new(0x01).p1(0x10).transmit(self, 0xFF)?;
 
-            if !resp.is_success() {
+                if !response.is_success() {
                     error!(
                         "failed retrieving serial number: {:04x}",
-                    resp.status_words().code()
+                        response.status_words().code()
                     );
                     return Err(Error::GenericError);
                 }
@@ -140,26 +141,24 @@ impl<'tx> Transaction<'tx> {
                     return Err(Error::GenericError);
                 }
 
-            resp
+                response.data().try_into()
-        } else {
+            }
+            5 => {
                 // YK5 implements getting the serial as a PIV applet command (0xf8)
-            let resp = Apdu::new(Ins::GetSerial).transmit(self, 0xFF)?;
+                let response = Apdu::new(Ins::GetSerial).transmit(self, 0xFF)?;
 
-            if !resp.is_success() {
+                if !response.is_success() {
                     error!(
                         "failed retrieving serial number: {:04x}",
-                    resp.status_words().code()
+                        response.status_words().code()
                     );
                     return Err(Error::GenericError);
                 }
 
-            resp
+                response.data().try_into()
-        };
+            }
-
+            _ => Err(Error::NotSupported),
-        response.data()[..4]
+        }
-            .try_into()
-            .map(|serial| Serial::from(u32::from_be_bytes(serial)))
-            .map_err(|_| Error::SizeError)
     }
 
     /// Verify device PIN.

src/yubikey.rs

@@ -98,6 +98,20 @@ impl From<Serial> for u32 {
     }
 }
 
+impl TryFrom<&[u8]> for Serial {
+    type Error = Error;
+
+    fn try_from(bytes: &[u8]) -> Result<Self> {
+        if bytes.len() > 4 {
+            return Err(Error::SizeError);
+        }
+
+        let mut arr = [0u8; 4];
+        arr[(4 - bytes.len())..].copy_from_slice(bytes);
+        Ok(Self(u32::from_be_bytes(arr)))
+    }
+}
+
 impl FromStr for Serial {
     type Err = Error;