The difference between S0 and S2 Z-Wave Protocols is that S2 is more secure than S0. There are many advanced technical features supported by S2 that aren't utilized by S0. Using S2 on your Z-Wave network requires both the Z-Wave devices and the central Z-Wave hub to support the S2 Protocol.
Most modern security systems will support Z-Wave home automation devices, including lights, locks, thermostats, and more. Updates to Z-Wave technology in recent years have shown various security enhancements. The transition from the S0 Protocol to the S2 Protocol represents one of the biggest improvements for Z-Wave in terms of security. The S2 Protocol is supported by every Z-Wave Plus V2 700-Series device, as well as many Z-Wave Plus devices.
Security 2 (S2) uses various technologies to make it more secure than the older S0 Protocol. It isn't just one single aspect that makes S2 more secure. It is multiple factors coming together to make an enhanced security protocol and offer better protection overall. For starters, the encryption protocol for S2 consists of an asymmetric key exchange, which represents the standard for other secure protocols, including VPN networks. By using a cryptographic system that involves the use of public keys and private keys, communication sent using S2 is kept extremely secure. And the network key within this exchange is protected using Elliptic Curve Diffie-Hellman (ECDH) technology, which makes it virtually impossible for an outsider to decipher. Overall, the encryption level of S2 is rated at 128-bit AES, which is frequently used by many military applications to this day.
Prior to S2, Z-Wave software developers were responsible for implementing their own security. Often, developers would skip out on providing protection, and they just left their automation devices vulnerable. But now, the secured S2 protocol is made readily available within the Z-Wave framework. All developers must do is implement the security protocol into their offerings. Not only does this make it much easier for a developer to provide strong security for their products, this also ensures that all Z-Wave devices using the S2 Protocol meet the same excellent security standard. Remember, the Z-Wave Alliance requires the S2 Protocol for all 700-Series devices, but it is optional for 500-Series.