OPC UA Secure Conversation (UASC) allows secure communication using binary encoded Messages.

UASC is designed to operate with different TransportProtocols that may have limited buffer sizes. For this reason, OPC UA Secure Conversation will break OPC UA Messages into several pieces (called ‘MessageChunks’) that are smaller than the buffer size allowed by the TransportProtocol. UASC requires a TransportProtocol buffer size that is at least 8 192 bytes when using RSA based Asymmetric Cryptography This minimum buffer size drops to 1 024 when using ECC based Asymmetric Cryptography (see 6.8) is supported.

All security is applied to individual MessageChunks and not the entire OPC UA Message. A Stack that implements UASC is responsible for verifying the security on each MessageChunk received and reconstructing the original OPC UA Message.

All MessageChunks will have a 4-byte sequence assigned to them. These sequence numbers are used to detect and prevent replay attacks.

UASC requires a TransportProtocol that will preserve the order of MessageChunks, however, a UASC implementation does not necessarily process the Messages in the order that they were received.