The existing IP multimedia subsystem (IMS) network security mechanism mainly focus on the SIP signaling plane protection and lack effective security mechanism for media plane. According to the characteristics of file transferring in IMS and potential security threats, this paper proposes a file safety transmission method based on secure/multipurpose Internet mail extensions (S/MIME). The scheme uses message session relay protocol (MSRP) to realize the IMS file transfer and achieves end-to-end security by S/MIME. It can effectively protect the transmission file entity and decrease the delay by avoiding traverse each call session control function in the IMS. Simulation results prove that the scheme can realize file safety transmission in the IMS network.% 针对现有IMS网络中文件传输时仅能保护SIP信令层，缺乏面向媒体层的有效安全机制的问题，提出一种基于S/MIME的文件安全传输方案。通过分析IPsec、TLS等安全机制的适用范围和在文件传输时的安全缺陷，依托MSRP协议实现IMS中文件传输，基于S/MIME实现端到端传输安全。仿真测试结果表明：该方案能有效保护传输过程中的文件实体，避免了通过IMS中各呼叫会话控制实体带来的延时，实现了IMS网络中的文件安全传输。
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