Replace references to "counter" with "index".

There is still a bit of a mix between the terms left in Megolm, but
those should be cleaned up in a separate PR.

src/olm/messages/message.rs

@@ -266,8 +266,11 @@ impl Debug for Message {
 pub struct InterolmMessage {
     pub(crate) version: u8,
     pub(crate) ratchet_key: Curve25519PublicKey,
-    pub(crate) counter: u32,
-    pub(crate) previous_counter: u32,
+    /// The current chain index
+    pub(crate) index: u32,
+    /// The chain index at the point in time when the DH ratchet was previously
+    /// ratcheted
+    pub(crate) previous_index: u32,
     pub(crate) ciphertext: Vec<u8>,
     pub(crate) mac: InterolmMessageMac,
 }
@@ -278,15 +281,15 @@ impl InterolmMessage {
 
     pub(crate) fn new(
         ratchet_key: Curve25519PublicKey,
-        counter: u32,
-        previous_counter: u32,
+        index: u32,
+        previous_index: u32,
         ciphertext: Vec<u8>,
     ) -> Self {
         Self {
             version: Self::VERSION,
             ratchet_key,
-            counter,
-            previous_counter,
+            index,
+            previous_index,
             ciphertext,
             mac: InterolmMessageMac([0u8; Mac::TRUNCATED_LEN]),
         }
@@ -313,18 +316,11 @@ impl InterolmMessage {
             } else {
                 let mac = InterolmMessageMac(mac_slice.try_into().expect("Can never happen"));
                 let ratchet_key = Curve25519PublicKey::from_slice(&decoded.ratchet_key)?;
-                let counter = decoded.counter;
-                let previous_counter = decoded.previous_counter;
+                let index = decoded.index;
+                let previous_index = decoded.previous_index;
                 let ciphertext = decoded.ciphertext;
 
-                Ok(InterolmMessage {
-                    version,
-                    ratchet_key,
-                    counter,
-                    previous_counter,
-                    ciphertext,
-                    mac,
-                })
+                Ok(InterolmMessage { version, ratchet_key, index, previous_index, ciphertext, mac })
             }
         }
     }
@@ -348,8 +344,8 @@ impl InterolmMessage {
     pub fn to_mac_bytes(&self) -> Vec<u8> {
         InterolmProtoBufMessage {
             ratchet_key: self.ratchet_key.to_interolm_bytes().to_vec(),
-            counter: self.counter,
-            previous_counter: self.previous_counter,
+            index: self.index,
+            previous_index: self.previous_index,
             ciphertext: self.ciphertext.clone(),
         }
         .encode_manual()
@@ -368,13 +364,13 @@ impl<'a> From<&'a InterolmMessage> for AnyNormalMessage<'a> {
 
 impl Debug for InterolmMessage {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let Self { version, ratchet_key, counter, previous_counter, ciphertext: _, mac: _ } = self;
+        let Self { version, ratchet_key, index, previous_index, ciphertext: _, mac: _ } = self;
 
         f.debug_struct("InterolmMessage")
             .field("version", version)
             .field("ratchet_key", ratchet_key)
-            .field("counter", counter)
-            .field("previous_counter", previous_counter)
+            .field("index", index)
+            .field("previous_index", previous_index)
             .finish()
     }
 }
@@ -395,9 +391,9 @@ struct InterolmProtoBufMessage {
     #[prost(bytes, tag = "1")]
     ratchet_key: Vec<u8>,
     #[prost(uint32, tag = "2")]
-    counter: u32,
+    index: u32,
     #[prost(uint32, tag = "3")]
-    previous_counter: u32,
+    previous_index: u32,
     #[prost(bytes, tag = "4")]
     ciphertext: Vec<u8>,
 }
@@ -410,8 +406,8 @@ impl InterolmProtoBufMessage {
     const CIPHER_TAG: &'static [u8; 1] = b"\x22";
 
     fn encode_manual(&self) -> Vec<u8> {
-        let counter = self.counter.to_var_int();
-        let previous_counter = self.previous_counter.to_var_int();
+        let index = self.index.to_var_int();
+        let previous_index = self.previous_index.to_var_int();
         let ratchet_len = self.ratchet_key.len().to_var_int();
         let ciphertext_len = self.ciphertext.len().to_var_int();
 
@@ -421,9 +417,9 @@ impl InterolmProtoBufMessage {
             &ratchet_len,
             &self.ratchet_key,
             Self::INDEX_TAG.as_ref(),
-            &counter,
+            &index,
             Self::PREVIOUS_INDEX_TAG.as_ref(),
-            &previous_counter,
+            &previous_index,
             Self::CIPHER_TAG.as_ref(),
             &ciphertext_len,
             &self.ciphertext,

src/olm/messages/mod.rs

@@ -67,7 +67,7 @@ impl AnyNormalMessage<'_> {
     pub(crate) fn chain_index(&self) -> u64 {
         match self {
             AnyNormalMessage::Native(m) => m.chain_index,
-            AnyNormalMessage::Interolm(m) => m.counter.into(),
+            AnyNormalMessage::Interolm(m) => m.index.into(),
         }
     }
 }

src/olm/session/double_ratchet.rs

@@ -70,13 +70,13 @@ impl DoubleRatchet {
         config: &SessionConfig,
         session_creator: SessionCreator,
         session_keys: &SessionKeys,
-        previous_counter: u32,
+        previous_index: u32,
         plaintext: &[u8],
     ) -> InterolmMessage {
         self.next_message_key(config).encrypt_interolm(
             session_keys,
             session_creator,
-            previous_counter,
+            previous_index,
             plaintext,
         )
     }

src/olm/session/message_key.rs

@@ -92,7 +92,7 @@ impl MessageKey {
         self,
         session_keys: &SessionKeys,
         session_creator: SessionCreator,
-        previous_counter: u32,
+        previous_index: u32,
         plaintext: &[u8],
     ) -> InterolmMessage {
         let cipher = Cipher::new_interolm(&self.key);
@@ -102,7 +102,7 @@ impl MessageKey {
         let mut message = InterolmMessage::new(
             *self.ratchet_key.as_ref(),
             self.index.try_into().expect("Interolm doesn't support encrypting more than 2^32 messages with a single sender chain"),
-            previous_counter,
+            previous_index,
             ciphertext,
         );
 

src/olm/session/mod.rs

@@ -124,13 +124,13 @@ impl ChainStore {
     }
 
     #[cfg(feature = "interolm")]
-    fn previous_counter(&self) -> u32 {
+    fn previous_index(&self) -> u32 {
         match self.previous_chain() {
             Some(chain) => {
                 if chain.hkdf_ratchet.chain_index() > 0 {
                     (chain.hkdf_ratchet.chain_index() - 1)
                         .try_into()
-                        .expect("Interolm counter should fit into u32")
+                        .expect("Interolm chain index should fit into u32")
                 } else {
                     0
                 }
@@ -388,7 +388,7 @@ impl Session {
                     &self.config,
                     self.session_creator,
                     &self.session_keys,
-                    self.receiving_chains.previous_counter(),
+                    self.receiving_chains.previous_index(),
                     plaintext.as_ref(),
                 ),
             ),