A single srtp_policy_t struct represents the policy for a single SRTP stream, and a linked list of these elements represents the policy for an entire SRTP session. Each element contains the SRTP and SRTCP crypto policies for that stream, a pointer to the SRTP master key for that stream, the SSRC describing that stream, or a flag indicating a `wildcard' SSRC value, and a `next' field that holds a pointer to the next element in the list of policy elements, or NULL if it is the last element.
The wildcard value SSRC_ANY_INBOUND matches any SSRC from an inbound stream that for which there is no explicit SSRC entry in another policy element. Similarly, the value SSRC_ANY_OUTBOUND will matches any SSRC from an outbound stream that does not appear in another policy element. Note that wildcard SSRCs &b cannot be used to match both inbound and outbound traffic. This restriction is intentional, and it allows libSRTP to ensure that no security lapses result from accidental re-use of SSRC values during key sharing.
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