Skype has introduced Private Conversations, an end-to-end encrypted chat mode that uses the Signal Protocol. For now, Private Conversations is only available on Skype’s main apps for Android, iOS, macOS, and Windows. It is not available on Skype Lite.
The Signal Protocol is also used by messaging services like WhatsApp, which introduced encrypted messaging in 2014 and encrypted calls and group messages in 2016. Skype’s feature is similar to Facebook Messenger’s and Telegram’s Secret/Private Conversation feature. It is built on the same concept of end-to-end encryption on a separate chat, and is restricted to two devices. On Messenger, Skype and Telegram, it’s not possible for a user to read a conversation from another device, like a desktop. Normal conversations on Skype, which are by default encrypted by Transport Layer Security (TLS), will continue to be available across all devices.
End-to-end encryption ensures that a message’s contents are not readable by any intermediary, even by the company that is providing the service. For instance, even if WhatsApp wanted to, it wouldn’t be able to read any messages sent by its users — the only way to do so would be to get physical access to one of the two devices participating in the conversation.
On its website, Skype says that all calls and messages are encrypted. “For instant messages, we use TLS (transport-level security) to encrypt your messages between your Skype client and the chat service in our cloud, or AES (Advanced Encryption Standard) when sent directly between two Skype clients,” the company says. “Most messages are sent both ways, but in the future it will only be sent via our cloud to provide the optimal user experience.” It adds, “User public keys are certified by the Skype server at login using 1536 or 2048-bit RSA certificates.” Calls that are made from Skype to a regular telephone network are not encrypted.
Further reading (from 2016): The Legal Position of Encryption in India