The only one who could have sent the letter in this manner was Feng Susu. When Su Wenyue received the letter, she saw, as expected, a seemingly casual but recognizable code imprinted on it. The two had agreed on this method of secret correspondence due to the sensitivity of their statuses and for safety reasons, ensuring that even if the letter fell into the hands of someone with ill intentions, it would not pose a threat.
It wasn't that Han Yu and Su Wenyue were overly cautious. Given the current situation, there were too many people looking to catch them in a fault, and General An shared these concerns. After all, they weren't on the same side, and even Feng Susu's return to General An was conducted with a low-profile discretion, despite General An's elation at being reunited with his wife and child.