User's Manual 216 Document #: LTRT-10532
Mediant 500L Gateway & E-SBC
14.7 Configuring Media (SRTP) Security
The device supports Secured RTP (SRTP) according to RFC 3711. SRTP is used to
encrypt RTP and RTCP transport for protecting VoIP traffic. SRTP requires a cryptographic
key exchange mechanism to negotiate the keys. To negotiate the keys, the device
supports the Session Description Protocol Security Descriptions (SDES) protocol
(according to RFC 4568) or Datagram Transport Layer Security (DTLS) protocol for SBC
calls. For more information on DTLS, see SRTP using DTLS Protocol. The key exchange is
done by adding the 'a=crypto' attribute to the SDP. This attribute is used (by both sides) to
declare the various supported cipher suites and to attach the encryption key. If negotiation
of the encryption data is successful, the call is established.
SRTP supports the following cipher suites (all other suites are ignored):
AES_CM_128_HMAC_SHA1_32
AES_CM_128_HMAC_SHA1_80
ARIA_CM_128_HMAC_SHA1_80
ARIA_CM_192_HMAC_SHA1_80
When the device is the offering side (SDP offer), it can generate a Master Key Identifier
(MKI). You can configure the MKI size globally (using the SRTPTxPacketMKISize
parameter) or per SIP entity (using the IP Profile parameter, IpProfile_MKISize). The length
of the MKI is limited to four bytes. If the remote side sends a longer MKI, the key is ignored.
Note:
• Gateway application: The device only initiates the MKI size.
• SBC application: The device can forward MKI size transparently for SRTP-to-
SRTP media flows or override the MKI size during negotiation (inbound or
outbound leg).
The key lifetime field is not supported. However, if it is included in the key it is ignored and
the call does not fail. For SBC calls belonging to a specific SIP entity, you can configure the
device to remove the lifetime field in the 'a=crypto' attribute (using the IP Profile parameter,
IpProfile_SBCRemoveCryptoLifetimeInSDP).
For SDES, the keys are sent in the SDP body ('a=crypto') of the SIP message and are
typically secured using SIP over TLS (SIPS). The encryption of the keys is in plain text in
the SDP. The device supports the following session parameters:
UNENCRYPTED_SRTP
UNENCRYPTED_SRTCP
UNAUTHENTICATED_SRTP
Session parameters should be the same for the local and remote sides. When the device is
the offering side, the session parameters are configured by the following parameter -
'Authentication On Transmitted RTP Packets', 'Encryption On Transmitted RTP Packets,
and 'Encryption On Transmitted RTCP Packets'. When the device is the answering side,
the device adjusts these parameters according to the remote offering. Unsupported
session parameters are ignored, and do not cause a call failure.
Below is an example of crypto attributes usage:
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PsKoMpHlCg+b5X0YLuSvNrImEh/dAe
a=crypto:2 AES_CM_128_HMAC_SHA1_32
inline:IsPtLoGkBf9a+c6XVzRuMqHlDnEiAd
The device also supports symmetric MKI negotiation, whereby it can forward the MKI size
received in the SDP offer 'a=crypto' line in the SDP answer. You can enable symmetric
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