osnma: refactor handling of chain in force in key storage

This uses a different approach to keeping track of the key in force
in the key storage. The new approach fixes failures in the OSNMA
test vector Chain Revocation Step 2.

Instead of keeping track of which is the key of the chain in force,
we keep track of what is the CID of the chain in force. This simplifies
keeping track of the start of applicability, as we can simply set
it whenever the CID changes value. It also handles the case in
Chain Revocation Step 2, when the DSM-PKR is transmitting a KROOT
for a chain that will be in force. In that case, the receiver never
gets a KROOT from the first chain, and so it would never set the
start of applicability for the key of the new chain correctly.

src/osnma.rs

@@ -108,16 +108,14 @@ struct PubkeyStore {
 #[derive(Debug, Clone)]
 struct KeyStore {
     keys: [Option<Key<Validated>>; 2],
-    in_force: Option<KeyInForce>,
+    chain_in_force: Option<ChainInForce>,
 }
 
 #[derive(Debug, Clone)]
-struct KeyInForce {
-    index: bool, // false for the 1st position, true for then 2nd position
-
-    // This is None if we haven't seen the key entering through a chain renewal
-    // or revocation, in which case we don't know when the key starts being
-    // applicable (and don't care, since we don't have the previous key).
+struct ChainInForce {
+    cid: u8,
+    // This is None if the current CID has never replaced a previous different
+    // CID. Otherwise, it is set to the Gst in which the replacement happened.
     start_applicability: Option<Gst>,
 }
 
@@ -664,7 +662,7 @@ impl KeyStore {
     fn empty() -> KeyStore {
         KeyStore {
             keys: [None; 2],
-            in_force: None,
+            chain_in_force: None,
         }
     }
 
@@ -699,32 +697,17 @@ impl KeyStore {
                 }
             }
         }
-        // update self.in_force
-        match (&self.keys[0], &self.keys[1]) {
-            (Some(k), _) if k.chain().chain_id() == cid => {
-                let index = false;
-                let start_applicability = match &self.in_force {
-                    Some(in_force) if in_force.index != index => Some(gst),
-                    _ => None,
-                };
-                self.in_force = Some(KeyInForce {
-                    index,
-                    start_applicability,
-                });
-            }
-            (_, Some(k)) if k.chain().chain_id() == cid => {
-                let index = true;
-                let start_applicability = match &self.in_force {
-                    Some(in_force) if in_force.index != index => Some(gst),
-                    _ => None,
-                };
-                self.in_force = Some(KeyInForce {
-                    index,
-                    start_applicability,
-                });
-            }
-            _ => self.in_force = None,
-        }
+        // update chain in force
+        self.chain_in_force = Some(ChainInForce {
+            cid,
+            start_applicability: self.chain_in_force.as_ref().and_then(|cif| {
+                if cif.cid != cid {
+                    Some(gst)
+                } else {
+                    None
+                }
+            }),
+        });
     }
 
     fn store_key(&mut self, key: Key<Validated>) {
@@ -742,24 +725,30 @@ impl KeyStore {
     }
 
     fn current_key(&self) -> Option<&Key<Validated>> {
-        self.in_force
-            .as_ref()
-            .and_then(|in_force| self.keys[usize::from(in_force.index)].as_ref())
+        self.chain_in_force.as_ref().and_then(|cif| {
+            self.keys
+                .iter()
+                .flatten()
+                .find(|&&k| k.chain().chain_id() == cif.cid)
+        })
     }
 
     // Similar to current_key but returns a key from the other chain if the
     // requested GST is before the start of applicability of the current
     // chain. This is used to get the key for MACK validation for Slow MAC.
     fn key_past_chain(&self, gst: Gst) -> Option<&Key<Validated>> {
-        self.in_force
+        self.chain_in_force
             .as_ref()
-            .and_then(|in_force| match in_force.start_applicability {
+            .and_then(|cif| match cif.start_applicability {
                 Some(gst0) if gst0 > gst => {
                     // Requested time is before the start of the applicability.
                     // Get the key from the other slot (if occupied).
-                    self.keys[usize::from(!in_force.index)].as_ref()
+                    self.keys
+                        .iter()
+                        .flatten()
+                        .find(|&&k| k.chain().chain_id() != cif.cid)
                 }
-                _ => self.keys[usize::from(in_force.index)].as_ref(),
+                _ => self.current_key(),
             })
     }