url-safe id generation (#3231)

* test dc_create_id() for invalid characters

* create url-save ids (as documented)

src/dc_tools.rs

@@ -214,7 +214,10 @@ pub(crate) fn dc_create_id() -> String {
     rng.fill(&mut arr[..]);
 
     // Take 11 base64 characters containing 66 random bits.
-    base64::encode(&arr).chars().take(11).collect()
+    base64::encode_config(&arr, base64::URL_SAFE)
+        .chars()
+        .take(11)
+        .collect()
 }
 
 /// Function generates a Message-ID that can be used for a new outgoing message.
@@ -762,6 +765,15 @@ Hop: From: hq5.example.org; By: hq5.example.org; Date: Mon, 27 Dec 2021 11:21:22
         assert_eq!(buf.len(), 11);
     }
 
+    #[test]
+    fn test_dc_create_id_invalid_chars() {
+        for _ in 1..1000 {
+            let buf = dc_create_id();
+            assert!(!buf.contains('/')); // `/` must not be used to be URL-safe
+            assert!(!buf.contains('.')); // `.` is used as a delimiter when extracting grpid from Message-ID
+        }
+    }
+
     #[test]
     fn test_dc_extract_grpid_from_rfc724_mid() {
         // Should return None if we pass invalid mid