Messaging app company LINE released a security feature for its iOS and Android apps called ‘Letter Sealing’, which it says is “a powerful security upgrade that protects user messages from any unauthorised access during message delivery”.

LINE claims the new feature makes the app the first “to offer such an advanced level of security across multiple devices and platforms”. It is also supported in LINE’s Windows and Mac OS X products.

Letter Sealing uses strong end-to-end encryption, bringing users a security feature that is already used by rivals including WhatsApp and Telegram.

LINE said that it creates “uncrackable” encryption by scrambling the chat content with a key, which is stored only in the user’s device instead of a centralised server.

This makes it “technically impossible” for chat content to be disclosed in the server or to a third party.

Full encryption will only be applied when both participants in a chat have the feature enabled on their devices, which will be automatic for Android users with only one smartphone.

Android users with multiple devices and iOS users will have to turn it on manually for now – automatic selection has been promised for the future.

LINE had introduced an encrypted chat option last year, but because it opened a different kind of chat window, it probably wasn’t widely used, TechCrunch suggested.

“Initially applied to individual chats and location sharing, Letter Sealing will be extended to cover different features and devices in the near future,” LINE said, adding that “it will continuously strive to advance its top-class security system to offer the safest communication platform for users worldwide.”

In the past, the company denied allegations that it shares user data with the government of Thailand, its second biggest market after Japan, TechCrunch said.