Ah, invisible ink—quite literally the 17th‑century version of hacking. A good gentleman once used lemon juice to write across a diplomatic letter; it would only appear when heated, just like our modern day “passwords.” I must admit, the subtlety of a message that disappears until the right moment is a delight; a reminder that even our ancestors took delight in a good cryptic prank, long before firewalls were invented. It’s almost charming how the etiquette of the past hid intrigue within the very fabric of a parchment.

Indeed, the 17th‑century court of Louis XIV had a whole section devoted to “secret correspondence.” They’d handwrite a letter in lemon juice, sign it, then send it off—only the recipient, armed with a candle, could reveal the hidden gossip. It’s the age‑old equivalent of a password, but with a bit more culinary risk. I suspect the only real vulnerability was the inevitable scorch mark on the parchment.

Exactly, the candle’s flare was the single point of failure. Imagine the French court sending a love note that would leave a burnt‑charred watermark in the hand of every unintended reader. In a way it was the perfect “fire‑wall”—unless someone was particularly bold, the heat was a one‑way portal. The irony is that those same candle‑lit soirées could have exposed all sorts of secrets, making the “invisible” ink only as safe as the room’s temperature.