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Enhancing Security in Packetcable Telephony with RTP Payload Encryption Techniques

This document discusses the security specifications related to Packetcable, the telephony over cable initiative by North American cable operators. It covers signaling, provisioning, and media integrity focusing on two main goals: privacy and integrity. Key encryption methods, such as the RC-4 stream cipher and MMH for MACs, are evaluated for their efficiency and performance. The text highlights concerns regarding potential attacks and emphasizes the need for optimizing data transmission while ensuring security. Suggestions for improvement and future drafts related to SDP and codec performance are also explored.

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Enhancing Security in Packetcable Telephony with RTP Payload Encryption Techniques

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  1. RTP Payload Crypto Profile Michael Thomas mat@cisco.com

  2. Packetcable Crypto Profile • Packetcable is the telephony over cable effort of NA cable operators • Set of specs for signaling, provisioning, security, etc • http://www.packetcable.com in interim section • Security specification for media had two main goals: • privacy and integrity for media • large PSTN gateway friendly algorithms • Two current choices: RC-4 and MMH

  3. Packetcable RTP Packet Covered by MMH MAC 2 or 4 byte MAC RTP payload RTP header MAC Covered by RC-4 encryption

  4. MMH MAC • 2 or 4 byte MAC appended to end of payload • 16 byte MAC would be strong, but expensive • Actual barge-in threat of occasional random guessing attack acceptable • Applied over entire RTP packet, not just headers • eliminates man in the middle attack on payload • Signaled in SDP via ciphersuite exchange • Optimal use of bytes on wire (no need for pad byte) • Feedback that a new profile may be better • DSP friendly

  5. RC-4 Payload Encryption • RC-4 is a cheap, strong stream cipher • RTP is close to a stream, except it’s unreliable • requires RTP headers in the clear • timestamps for replay protection • with timestamps and maximized packetization size we can always reconstruct the stream state • pretty optimal for fixed voice, needs more work for variable rate coders • Very fast, CPU-wise • ~10MB/sec on a P150

  6. Conclusion • Together, RC4/MMH are very fast, and cheap • ~ 1kb code, 300bytes/DS0, ~.5 MIPS/DS0 • Very DSP friendly, other transforms break budgets on one or more of the parameters • Could still use work for rationalizing SDP and variable rate coders • Nothing fundamental about the spec which cannot add larger MAC’s (SHA-1…) or different encryption (3-DES, AES, SEAL…) • Hopefully submit this as a draft for Pittsburgh • same IP tangle as DCS group in SIP

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