First try empty passphrase. In case no passphrase works notify SYNC_PASSPHRASE_REQUIRED.

src/message_api.c

@@ -5334,12 +5334,32 @@ PEP_STATUS try_encrypt_message(
 
     // https://dev.pep.foundation/Engine/MessageToSendPassphrase
 
+    if (session->curr_passphrase) {
+        // first try with empty passphrase
+        char *passphrase = session->curr_passphrase;
+        session->curr_passphrase = NULL;
+        status = encrypt_message(session, src, extra, dst, enc_format, flags);
+        session->curr_passphrase = passphrase;
+        if (!(status == PEP_PASSPHRASE_REQUIRED || status == PEP_WRONG_PASSPHRASE))
+            return status;
+    }
+
     do {
+        // then try passphrases
         status = encrypt_message(session, src, extra, dst, enc_format, flags);
-        if (status == PEP_PASSPHRASE_REQUIRED || status == PEP_WRONG_PASSPHRASE)
+        if (status == PEP_PASSPHRASE_REQUIRED || status == PEP_WRONG_PASSPHRASE) {
             status = session->messageToSend(NULL);
-        else
+            if (status == PEP_PASSPHRASE_REQUIRED || status == PEP_WRONG_PASSPHRASE) {
+                pEp_identity *me = identity_dup(src->from);
+                if (!me)
+                    return PEP_OUT_OF_MEMORY;
+                session->notifyHandshake(me, NULL, SYNC_PASSPHRASE_REQUIRED);
+                break;
+            }
+        }
+        else {
             break;
+        }
     } while (!status);
 
     return status;

src/sync_api.h

@@ -35,6 +35,9 @@ typedef enum _sync_handshake_signal {
     // forming group
     // SYNC_NOTIFY_FORMING_GROUP = 10,
 
+    // message cannot be sent, need passphrase
+    SYNC_PASSPHRASE_REQUIRED = 128,
+
     // notification of actual group status
     SYNC_NOTIFY_SOLE = 254,
     SYNC_NOTIFY_IN_GROUP = 255
@@ -53,7 +56,7 @@ typedef enum _sync_handshake_signal {
 //      PEP_STATUS_OK or any other value on error
 //
 //  caveat:
-//      ownership of self and partner go to the callee
+//      ownership of me and partner go to the callee
 
 typedef PEP_STATUS (*notifyHandshake_t)(
         pEp_identity *me,