feat: generate 144-bit group IDs

src/chat.rs

@@ -6292,11 +6292,10 @@ mod tests {
         // Alice has an SMTP-server replacing the `Message-ID:`-header (as done eg. by outlook.com).
         let sent_msg = alice.pop_sent_msg().await;
         let msg = sent_msg.payload();
-        assert_eq!(msg.match_indices("Message-ID: <Mr.").count(), 2);
-        assert_eq!(msg.match_indices("References: <Mr.").count(), 1);
-        let msg = msg.replace("Message-ID: <Mr.", "Message-ID: <XXX");
-        assert_eq!(msg.match_indices("Message-ID: <Mr.").count(), 0);
-        assert_eq!(msg.match_indices("References: <Mr.").count(), 1);
+        assert_eq!(msg.match_indices("Message-ID: <").count(), 2);
+        assert_eq!(msg.match_indices("References: <").count(), 1);
+        let msg = msg.replace("Message-ID: <", "Message-ID: <X.X");
+        assert_eq!(msg.match_indices("References: <").count(), 1);
 
         // Bob receives this message, he may detect group by `References:`- or `Chat-Group:`-header
         receive_imf(&bob, msg.as_bytes(), false).await.unwrap();
@@ -6313,7 +6312,7 @@ mod tests {
         send_text_msg(&bob, bob_chat.id, "ho!".to_string()).await?;
         let sent_msg = bob.pop_sent_msg().await;
         let msg = sent_msg.payload();
-        let msg = msg.replace("Message-ID: <Mr.", "Message-ID: <XXX");
+        let msg = msg.replace("Message-ID: <", "Message-ID: <X.X");
         let msg = msg.replace("Chat-", "XXXX-");
         assert_eq!(msg.match_indices("Chat-").count(), 0);
 

src/tools.rs

@@ -272,30 +272,26 @@ async fn maybe_warn_on_outdated(context: &Context, now: i64, approx_compile_time
 
 /* Message-ID tools */
 
-/// Generate an ID. The generated ID should be as short and as unique as possible:
-/// - short, because it may also used as part of Message-ID headers or in QR codes
-/// - unique as two IDs generated on two devices should not be the same. However, collisions are not world-wide but only by the few contacts.
+/// Generate an unique ID.
 ///
-/// IDs generated by this function are 66 bit wide and are returned as 11 base64 characters.
+/// The generated ID should be short but unique:
+/// - short, because it used in Message-ID and Chat-Group-ID headers and in QR codes
+/// - unique as two IDs generated on two devices should not be the same
 ///
-/// Additional information when used as a message-id or group-id:
-/// - for OUTGOING messages this ID is written to the header as `Chat-Group-ID:` and is added to the message ID as `Gr.<grpid>.<random>@<random>`
-/// - for INCOMING messages, the ID is taken from the Chat-Group-ID-header or from the Message-ID in the In-Reply-To: or References:-Header
-/// - the group-id should be a string with the characters [a-zA-Z0-9\-_]
+/// IDs generated by this function have 144 bits of entropy
+/// and are returned as 24 Base64 characters, each containing 6 bits of entropy.
+/// 144 is chosen because it is sufficiently secure
+/// (larger than AES-128 keys used for message encryption)
+/// and divides both by 8 (byte size) and 6 (number of bits in a single Base64 character).
 pub(crate) fn create_id() -> String {
     // ThreadRng implements CryptoRng trait and is supposed to be cryptographically secure.
     let mut rng = thread_rng();
 
-    // Generate 72 random bits.
-    let mut arr = [0u8; 9];
+    // Generate 144 random bits.
+    let mut arr = [0u8; 18];
     rng.fill(&mut arr[..]);
 
-    // Take 11 base64 characters containing 66 random bits.
-    base64::engine::general_purpose::URL_SAFE
-        .encode(arr)
-        .chars()
-        .take(11)
-        .collect()
+    base64::engine::general_purpose::URL_SAFE.encode(arr)
 }
 
 /// Returns true if given string is a valid ID.
@@ -311,7 +307,7 @@ pub(crate) fn validate_id(s: &str) -> bool {
 /// - the message ID should be globally unique
 /// - do not add a counter or any private data as this leaks information unnecessarily
 pub(crate) fn create_outgoing_rfc724_mid() -> String {
-    format!("Mr.{}.{}@localhost", create_id(), create_id())
+    format!("{}@localhost", create_id())
 }
 
 // the returned suffix is lower-case
@@ -925,7 +921,7 @@ DKIM Results: Passed=true";
     #[test]
     fn test_create_id() {
         let buf = create_id();
-        assert_eq!(buf.len(), 11);
+        assert_eq!(buf.len(), 24);
     }
 
     #[test]
@@ -961,7 +957,7 @@ DKIM Results: Passed=true";
     #[test]
     fn test_create_outgoing_rfc724_mid() {
         let mid = create_outgoing_rfc724_mid();
-        assert!(mid.starts_with("Mr."));
+        assert_eq!(mid.len(), 34);
         assert!(mid.ends_with("@localhost"));
     }