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Collaboration, Voice & AV · Advanced

SDP Security Descriptions (SDES)

A method (RFC 4568) of exchanging SRTP keys in SDP a=crypto attributes within SIP signaling.

Definition

SDES places the master key and cipher suite, such as AES_CM_128_HMAC_SHA1_80, in the SDP, so anyone who can read the signaling can decrypt the media. It is therefore secure only when SIP is protected by TLS. It is widely used by CUCM, CUBE and SIP trunks, while WebRTC uses DTLS-SRTP instead.

Example

a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:... appears in the SDP of an encrypted CUCM call.

Exam tip

SDES requires TLS signaling; DTLS-SRTP negotiates keys on the media path.

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