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

UASC is designed to operate with different TransportProtocolsthat may have limited buffer sizes. For this reason, OPC UA Secure Conversation will break OPC UA Messagesinto 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 CryptographyThis minimum buffer size drops to 1 024 when using ECC based Asymmetric Cryptography (see 6.8) is supported.

All security is applied to individual MessageChunksand not the entire OPC UA Message. A Stackthat implements UASC is responsible for verifying the security on each MessageChunkreceived and reconstructing the original OPC UA Message.

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

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