Drop YubiKey NEO support (closes #18)

YubiKey NEOs are legacy YubiKey devices, most of which contain
unpatchable security vulnerabilities.

They have smaller buffer sizes than YK4 and YK5, which necessitates a
whole bunch of conditional gating and buffer size calculations.

Getting rid of them simplifies this logic and allows us to assume
consistent buffer sizes everywhere.

We never tested on NEOs anyway, and looking at the deleted code it seems
it may have been miscalculating the NEO's buffer size!

If someone *really* wants to support NEOs, it shouldn't be that hard to
restore, but the codebase is definitely cleaner without it.

src/transaction.rs

@@ -32,15 +32,6 @@ impl<'tx> Transaction<'tx> {
         })
     }
 
-    /// Get an attribute of the card or card reader.
-    pub fn get_attribute<'buf>(
-        &self,
-        attribute: pcsc::Attribute,
-        buffer: &'buf mut [u8],
-    ) -> Result<&'buf [u8], Error> {
-        Ok(self.inner.get_attribute(attribute, buffer)?)
-    }
-
     /// Transmit a single serialized APDU to the card this transaction is open
     /// with and receive a response.
     ///
@@ -66,7 +57,7 @@ impl<'tx> Transaction<'tx> {
     pub fn select_application(&self) -> Result<(), Error> {
         let response = APDU::new(Ins::SelectApplication)
             .p1(0x04)
-            .data(&AID)
+            .data(&PIV_AID)
             .transmit(self, 0xFF)
             .map_err(|e| {
                 error!("failed communicating with card: '{}'", e);
@@ -102,13 +93,11 @@ impl<'tx> Transaction<'tx> {
 
     /// Get YubiKey device serial number.
     pub fn get_serial(&self, version: Version) -> Result<Serial, Error> {
-        let yk_applet = [0xa0, 0x00, 0x00, 0x05, 0x27, 0x20, 0x01, 0x01];
-
         let response = if version.major < 5 {
-            // get serial from neo/yk4 devices using the otp applet
+            // YK4 requires switching to the yk applet to retrieve the serial
             let sw = APDU::new(Ins::SelectApplication)
                 .p1(0x04)
-                .data(&yk_applet)
+                .data(&YK_AID)
                 .transmit(self, 0xFF)?
                 .status_words();
 
@@ -130,7 +119,7 @@ impl<'tx> Transaction<'tx> {
             // reselect the PIV applet
             let sw = APDU::new(Ins::SelectApplication)
                 .p1(0x04)
-                .data(&AID)
+                .data(&PIV_AID)
                 .transmit(self, 0xFF)?
                 .status_words();
 
@@ -141,7 +130,7 @@ impl<'tx> Transaction<'tx> {
 
             resp
         } else {
-            // get serial from yk5 and later devices using the f8 command
+            // YK5 implements getting the serial as a PIV applet command (0xf8)
             let resp = APDU::new(Ins::GetSerial).transmit(self, 0xFF)?;
 
             if !resp.is_success() {