get_assertion: Skip attStmt unless requested

For makeCredential, a missing attestation format preference list
means that we should use the default format (packed).  For getAssertion,
it means that we should skip the attestation statement entirely.
Previously, we implemented the makeCredential algorithm for both cases.
This caused an incompatibility with firefox because it fails on
unexpected fields in the response (in this case, the attestation
statement).  This patch fixes this issue and applies the correct default
for getAssertion requests.

Fixes: #98

src/ctap2.rs

@@ -472,8 +472,12 @@ impl<UP: UserPresence, T: TrussedRequirements> Authenticator for crate::Authenti
 
         let serialized_auth_data = authenticator_data.serialize()?;
 
-        let att_stmt_fmt =
-            SupportedAttestationFormat::select(parameters.attestation_formats_preference.as_ref());
+        // select attestation format or use packed attestation as default
+        let att_stmt_fmt = parameters
+            .attestation_formats_preference
+            .as_ref()
+            .map(SupportedAttestationFormat::select)
+            .unwrap_or(Some(SupportedAttestationFormat::Packed));
         let att_stmt = if let Some(format) = att_stmt_fmt {
             match format {
                 SupportedAttestationFormat::None => {
@@ -1673,8 +1677,11 @@ impl<UP: UserPresence, T: TrussedRequirements> crate::Authenticator<UP, T> {
         .to_bytes()
         .unwrap();
 
-        let att_stmt_fmt =
-            SupportedAttestationFormat::select(data.attestation_formats_preference.as_ref());
+        // select preferred format or skip attestation statement
+        let att_stmt_fmt = data
+            .attestation_formats_preference
+            .as_ref()
+            .and_then(SupportedAttestationFormat::select);
         let att_stmt = if let Some(format) = att_stmt_fmt {
             match format {
                 SupportedAttestationFormat::None => {
@@ -2016,11 +2023,7 @@ enum SupportedAttestationFormat {
 }
 
 impl SupportedAttestationFormat {
-    fn select(preference: Option<&AttestationFormatsPreference>) -> Option<Self> {
-        let Some(preference) = preference else {
-            // no preference, default to packed format
-            return Some(Self::Packed);
-        };
+    fn select(preference: &AttestationFormatsPreference) -> Option<Self> {
         if preference.known_formats() == [AttestationStatementFormat::None]
             && !preference.includes_unknown_formats()
         {