feat: Add create_broadcast_shared_secret()

src/chat.rs

@@ -43,9 +43,9 @@ use crate::smtp::send_msg_to_smtp;
 use crate::stock_str;
 use crate::sync::{self, Sync::*, SyncData};
 use crate::tools::{
-    IsNoneOrEmpty, SystemTime, buf_compress, create_id, create_outgoing_rfc724_mid,
-    create_smeared_timestamp, create_smeared_timestamps, get_abs_path, gm2local_offset,
-    smeared_time, time, truncate_msg_text,
+    IsNoneOrEmpty, SystemTime, buf_compress, create_broadcast_shared_secret, create_id,
+    create_outgoing_rfc724_mid, create_smeared_timestamp, create_smeared_timestamps, get_abs_path,
+    gm2local_offset, smeared_time, time, truncate_msg_text,
 };
 use crate::webxdc::StatusUpdateSerial;
 use crate::{chatlist_events, imap};

src/tools.rs

@@ -300,6 +300,25 @@ pub(crate) fn create_id() -> String {
     base64::engine::general_purpose::URL_SAFE.encode(arr)
 }
 
+/// Generate a shared secret for a broadcast channel, consisting of 64 characters..
+///
+/// The string generated by this function has 384 bits of entropy
+/// and is returned as 64 Base64 characters, each containing 6 bits of entropy.
+/// 384 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).
+// TODO ask someone what a good size would be here - also, not sure whether the AES-128 thing is true
+pub(crate) fn create_broadcast_shared_secret() -> String {
+    // ThreadRng implements CryptoRng trait and is supposed to be cryptographically secure.
+    let mut rng = thread_rng();
+
+    // Generate 384 random bits.
+    let mut arr = [0u8; 48];
+    rng.fill(&mut arr[..]);
+
+    base64::engine::general_purpose::URL_SAFE.encode(arr)
+}
+
 /// Returns true if given string is a valid ID.
 ///
 /// All IDs generated with `create_id()` should be considered valid.