Simple Secure Pairing: a security architecture introduced supported in Bluetooth >= 2.1. Service-level enforced, similar to other modes. Mandatory when both devices are Bluetooth >= 2.1. There are four association models currently supported by mode 4. Security requirements for services must be classified as one of the following: authenticated link key required, unauthenticated link key required, or no security required. SSP improves security through the addition of ECDH public key cryptography for protection against passive eavesdropping.