The a=crypto attribute in RFC 4568 has a separate section 9.2. for SRTP “Crypto” Attribute Grammar. What it basically includes is a list of attribute values required for encrypting media (crypto suite, method, session params, keys, MKI…).
However, DTLS-SRTP also does the same (RFC 5764 – SRTP Extension for DTLS). So, is it correct to say that where DTLS-SRTP is used, the a=crypto: attribute is not used. For example, does webRTC offer-answer SDP use the “a=crypto:” attribute as DTLS-SRTP is a must for webRTC?
Informational RFC “SDP for webRTC” also does not throw any light on this issue.