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Media Security
• Real Time Transport Protocol (RTP)
– delivering media content after session has been negotiated by SIP
– Marking and Synchronising of sent and received
packets by timestamping
– Unique Sequence Numbers in RTP to ensure correct
packet order
– Identification of packet initiator
Media Security
• RTP Control Protocol (RTCP)
– Adaption to variable quality of service and bandwiths. Feedback comes via RTCP
– Support Multicast-Sessions
Media Security
Security Protocols for SIP
• SIP Digest:
Authentication analogous to HTTP Digest Authentication Method
• SIPS (SIP over TLS): can be deployed if TCP is transport protocol for SIP
Media Security
Goals of SRTP – Secure RTP
– Confidentiality
– Integrity Protection
– Replay Protection
– High Throughput, Low Packet Expansion
– High tolerance against packet loss / packet reorder
Media Security
Key Management in SRTP
• Master Keys are agreed upon independently
• A Master Key Index in the packets points to the Master Key currently in use
• After r packets, new session keys for authentication, and encryption are derived from MK
• The same Master Key is used for max. 248 RTP-packets per session
• Different Streams (Audio/Video) of one session may use the same Master Key
• Different Sessions must use different Master Keys
Media Security
MIKEY Variants
• MIKEY_PS
– Password based
– Password has to be securely exchanged in beforehand
• MIKEY_PK
– Based on RSA-Public Keys and certificates
• MIKEY_DHSIGN
– signed Diffie-Hellmann-Key Exchange
• MIKEY_DHHMAC
– DH-key exchange authenticated with HMAC
Media Security
MIKEY_PS Advantages/Disadvantages
• Advantages
– fast
– computationally lightweight
– short messages
=>Suitable for wireless scenarios and thin devices
• Disadvantages
– Need to exchange secret K in beforehand
– Bob must use Alice‘s choice of TGK
Media Security
MIKEY_PK Advantages/Disadvantages
• Advantages
– No secret exchange in beforehand needed
– Envelope key may be used for further key updates
• Disadvantages
– PKI necessary
– verification of certificates may make further messages necessary
– Expensive Message Generation for Initiator
– Bob must use Alice‘s choice of TGK
Media Security
MIKEY_DHSIGN Advantages/Disadvantages
• Advantages
– No secret exchange in beforehand needed
– Both parties take part in generating TGK
• Disadvantages
– PKI needed
– verification of certificates may make further messages necessary
– Expensive Message Generation for both
– pure P2P protocol, no way to generate a group key
Media Security
MIKEY_DHHMAC Advantages/Disadvantages
• Advantages
– Both parties take part in generating TGK
– No signatures needed
– No PKI needed
• Disadvantages
– Need to exchange secret K in beforehand
– pure P2P protocol, no way to generate a group key
Media Security
Multicast Security Issues
Data Confidentiality
Group Key Management
Data Source Authentication
Security Policies
– Policies concerning group membership
– Policies concerning security enforcement
Media Security
Requirements for Group Key Management
• Requirements
– Security
– Efficiency
– Scalability
– Support removing/adding members
• Forward Secrecy
• Backward Secrecy
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